CHEMICAL ENGINEERING DIVISION
.a Hb 19.I 4
CHEMICAL ENGINEERING EDUCATION
Chemical Engineering Division
American Society for Engineering Education
Materials and Chemical Engineering,
by Lawrence H. Van Vlack - - - - - - - - - - - - - - - - 1
The Chemical Engineer and Materials Processing,
by Stephen F. Urban -- - - - - --- - - - - - ----- 3
The Materials Engineer in the Chemical Industry,
by Carl H. Samans - - - - - - - - - - - - - - - - - - - - 6
Materials and process Design Celculptions,
by C. M. Sliepoevich - - - - - - - - - - - - - - - - - - 15
The Reduction of Corrosion Through Materials Design and
Selection, by C. A. Siebert - - - - - - - - - - - - - - - 24
Materials Instruction for Chemical Engineers,
by Premo Chiotti - - - - - - - - - - - - - - - - - - - -28
What Should Be Taught By the Engineer?,
by M. J. Sinnott - - - - - - - - - - - - - - - - - - - - 30
Chemical Engineering Division
American Society for Engineering Education
Joseph J. Martin (Michigan) Chairman
George Burnet (Iowa State) Vice Chairman
J. B. West (Oklahoma State) Secretary-Treasurer
CHEMICAL ENGINEERING EDUCATION �, Journal of the Chemical
Engineering Division, American Society for Engineering Education.
Published Quarterly, in March, June, September and December, by
Albert H. Cooper, Editor.
Publication Office: University of Connecticut
P.O. Box 445, Storrs, Connecticut 06268
Subscription price, $2.00 per year.
MATERIALS AND CHEMICAL ENGINEERING
Lawrence H. Van Vlack
University of Michigan
Ann Arbor, Michigan
Our materials of engineering have undergone a major technological ev-
olution within the last five or ten years. Although the rapid strides have
not involved the chemical engineer as extensively as it has engineers from
other disciplines, the changes which have occurred are of interest to the
chemical engineer because he is responsible for a number of the processing
steps which are required in materials production. Furthermore, the chem-
ical engineer has received new materials with which he may build his equip-
ment and base his designs.
The topics of the next six papers will be concerned with the role of
the chemical engineer in materials processing, the role of the materials
engineer in the chemical processing industry, the considerations which must
be given to materials in chemical process design calculations, and the sig-
nificant problem of corrosion and its control through materials design and
selection. Each of these will lead to the pedagogical problem of how do we
teach the required topics to our chemical engineering students. Two approach
may be taken, viz. through physical chemistry and through engineering. Ob-
viously the optimum approach is probably some combination of the two; how-
ever, we shall hear opinions on the contributions of the two academic areas.
The rapid evolution which has occurred in the general area of materials
during recent years is a result of two situations: (1) There has been a de-
mand for improved materials for the more complicated designs of present
engineering. Previously available materials have not met these demands.
(2) Science has provided the engineer with a better understanding of the
nature of materials as a consequence of advances in solid state physics and
crystal chemistry. These advances have permitted general underlying prin-
ciples to be formulated. These principles present a better means of extra-
polating our knowledge for use with the development of new and improved
materials. Simply stated, the basic underlying principle in the science of
materials is "properties are controlled by the internal structure." Thus
(1) if we know the internal structure of a material, we can be more specific
about the material's properties, or (2) if the processing or service condition,
which are encountered by the material alter the internal structure, then we
can anticipate a change in the accompanying properties.
The significance of structure on the properties and behavior of mater-
ials has changed the teaching and engineering considerations from previous
descriptive approaches to approaches that take into consideration (1) the
atomic structure, (2) the arrangement of atoms into molecular and crystal
structure, (3) the arrangement of phases into microstructures, and ( ) var-
iation of microstructures within macrostructures. This systemization has
permitted the establishment of principles which are usuable with various
types of materials whether they be metallic, ceramic, or polymeric. In
general, the materials specialist is concerned with solid materials; how-
ever, the principles which are developed may also be used for liquids if
In general, there are two types of processing which are encountered in
the manufacture of any engineering material. These include (1) chemical
processing, and (2) mechanical processing. In addition, consideration could
be given thermal processing; however, thermal treatments are generally used
to facilitate the chemical and mechanical processing through changes in the
equilibrium, kinetics, or properties of the materials.
CHEMICAL ENGINEERING EDUCATION
The chemical engineer has participated extensively in the processing of
polymeric materials and less so in the processing of metallic and ceramic
materials. If the chemical engineer is involved in the processing of mater-
ials, he must be fully conversant with the specialist who specifies proper-
ties and applications. Conversely, if the metallurgical or ceramic engineer
is to have his full effect in the manufacture of materials, he must be cog-
nizant of the processing principles which the chemical engineer and the
mechanical engineer have developed.
The Materials Engineer
The materials engineer may bear the label of metallurgist, ceramist, or
plastics engineer. In any case he is a specialist who is concerned with (1)
the development of new materials, and (2) the critical application of mater-
ials in engineering design. As such he fills the gap between processing and
engineering design. The materials engineer must understand the basic nature
of materials so that he may develop real materials from the chemists' and
physicists' ideas. Furthermore, he must understand the service conditions
which prescribe the designed requirements. To this end he must be conversant
with the mechanical, electrical, structural, and chemical engineer. The
procedure for training men to fill this category is a subject of active dis-
cussion in many academic circles.
The engineering future as measured by the present research and devel-
opment indicates that several areas will receive emphasis with respect to
materials processing by chemical means. Foremost among these is the em-
phasis upon greater purity. This emphasis ranges from the beneficiation of
existing raw materials so that they have more consistent and desirable com-
position, to the demand for extreme purity in the manufacture of many of the
newer electronic and higher temperature materials. A second area of pro-
jected activity is into the processing of new materials with compositions
and structures which have not been utilized heretofore on a commercial basis.
Often this means high temperature or special pressure and catalytic require-
ments. A third area of future expansion is that of the production of single
crystals. Currently we think of single crystals as laboratory specialties.
The demands for single crystals have only been scratched and almost cer-
tainly the variety and tonnage requirements will be sufficient to build sev-
eral special industries.
There has been a rapid evolution in the area of materials science and
materials development. The chemical engineer along with engineers from
several other disciplines are deeply involved. Furthermore, we should ex-
pect significant changes to occur in the foreseeable future.
THE CHEMICAL ENGINEER AND MATERIALS PROCESSING
Stephen F. Urban
Titanium Alloy Mfg. Co.
Niagara Falls, N. Y.
Before getting on with subject given in the title, it is well to say
something about engineers in general before dealing with the chemical en-
gineers in particular.
Engineers deal with things and people as they are related to things.
Granted an engineer deals with ideas also, but nowhere to the extent of say,
a researcher. Things are difficult enough, but people are even more complex.
Because of this large intermeshing with people the engineer becomes involved
in broad non-technical generalizations which definitely deal with philosophy.
Therefore, considerable of what I shall say will deal with philosophical as-
pects of an engineer's work, which are as important, in my mind, as the more
commonly accepted technical aspects.
Frequently an engineer must listen carefully to the lowest man on the
totem pole to get clues toward the solution of problems. At the other end
of the spectrum he has to sell his solution to his superiors, who may or may
not be technically trained. If his supervisors are technical by training
his selling is perhaps a bit less difficult, but he still must sell his solu-
tion. In the latter instance of a technically trained superior, it is my
experience that he thinks more as a manager than engineer per se, and this is
as it should be. The engineer should realize that all his solutions will not
be accepted, because upon comparison to many other things that management has
on mind his solution is not important. Just because of this it can be help-
ful to be selective in choice of problems, both from the standpoint of rela-
tive importance to the whole endeavor, as well as to available resources.
The question can be put to me "Were you always aware of this?" The answer is
in the negative and it can be said that osmosis is a slow process. But then
learning people is a slow process.
Having said something about an engineer in general we shall deal with
our subject the chemical engineer. At first glance it is sound to say he has
to do with engineering aspects of chemical manufacture, for example sulfuric
acid. Most people would not consider titanium dioxide pigment manufacturing
as a chemical business but it is just as much so as petro-chemicals. Thus,
a chemical engineer is a person who in school has had much formal chemistry,
more than other engineers, has quite a grasp for fluid heat flow and-has a
good knowledge of a wide variety of unit processes.
As an illustration of what has just been said I should like to mention
a particular session on metallurgical education, held a number of years ago
at a national AIME meeting. Some of the professors wanted suggestions how to
improve or strengthen the teaching of process metallurgy. Dr. 0. Ralston, at
that time head of Metallurgy Branch of the Bureau of Mines, and now retired,
gave an answer that did not sit well. He stated that after many years of ex-
perimenting he found that it is best to teach chemical engineers some metal-
lurgy and turn them loose than to try to teach metallurgist (physical for most
part) unit processes. At the mentioned meeting a number of persons, including
myself, voiced the same opinion as Dr. Ralston.
The chemical engineer, like all engineers has problems to solve, without
creating too many in the process. Before getting into the matter of problem
solving it is desirable to again talk about people. First there is the mat-
ter of receptiveness. When collecting opinions of description of events it
is important to remember that you may have to ask your question a number of
times and in different ways. This is necessary so that the other person
understands your question and repetition causes the other person to think more
actively. This can be construed as a form of harassment and the approach
should be used only in the most important cases. A second version of recep-
tiveness is when an associate or subordinate is trying to come through. The
first time around you may not take to the idea discussed. This can be a re-
sult of poor presentation by the other person or lack of attentiveness on your
part. Repetition by the other party may at times bring out good ideas because
of your improved attentiveness or because of a better presentation by the other
party. The same reasoning applies to one's superiors.
March 19614 CHEMICAL ENGINEERING EDUCATION 4
In general problems are related with new processes for old or new products.
Then we have trouble shooting or fire fighting. In between there may be very
time consuming problems having to do with causes and corrective measures for
Before solutions are sought it is important to know what the problem is.
I shall give examples later to show that at times a problem appears on the
scene because of a lack of meeting of minds on a specification and details
of testing procedures contained therein.
In school we are taught to study literature before embarking on solutions.
I agree with this concept. Having established the nature of the problem one
finds that people are pained by the notion that they should struggle with the
other chap's work before starting active work themselves. This may shock
those of you in teaching, however, this is par for engineers a few years out
of school. The few that do search the literature before suggesting solutions
overdo it at times. For example, since messrs. X and Y wrote an article or
book they must be right in all they say, a form of author worship. If this
were the case there would be less need for most research. The difficulty is
that publications are too frequently read and too infrequently studied.
Assuming that the literature and other background information has been
studied we turn to suggested solutions. A typical case is where A makes a
suggestion and B knocks it down as unworkable. Does B feel that A's sug-
gestion is technically unsound or uneconomical or both? One never knows un-
less one inquires. The best approach is to confine the first discussions to
technical evaluation of suggestions and then to evaluate the many aspects. To
carry both at the same time creates friction, and is often a waste of time.
In any discussion of solutions to a problem it is nice to recognize pre-
cedence, provided this does not lead to a continued status quo. At times there
is much more than meets the eye about a process. This is particularly true
for a minor product that has been made for years without much complaint from
customers. The rub is when a customer wants a lot of the stuff, made to a
spelled out specification and a much lower price. This involves some sort of
changes. The particular case is difficult, in that the process is not well
understood, in that Joe and Mike have made the stuff for years without the
benefit of a written or up-to-date operating procedure. They know how to make
it at present scale, the engineers do not know, but must know before they make
scale-up changes. This takes us back to the process of careful interrogation.
This must be done most carefully because Joe and Mike have their guards up.
Did they do something wrong and are you trying to eliminate their jobs. Gen-
tlemen, this is a real problem to my mind.
The solution that chemical engineers come out with can require consider-
able effort by supporting research people. Sometimes the projected effort can
be made brief by reasoning. After discussion with the research group the pro-
jected process should be understood. If we now assume that we have a stack of
reports answering most of your questions we can make calculations just as if
we actually had the reports. On infrequent occasions this approach will show
that the suggested solution does not have merit. It is important to mention
that some people have a mental block unless they have reams of data.
If the solution involves purchase of new equipment that you have not had
experience with and cannot benefit from the experience of acquaintances In
other plants, put in large safety factors. Most equipment manufacturers are
carried away by their enthusiasm. Therefore, literature per se is usually
very troublesome. To narrow the gray area, telephone calls or visits and con-
ferences are very much in order. I well recall the time we decided to replace
an old and tired jaw crusher. Literature from seven manufacturers was studied.
Many words were printed by all of them, but not a word about type of bearings
or nature of jaws. Phone calls revealed that only one distributor could an-
swer the critical question. One wonders, how as a nation we can compete with
other countries if this example is not a rare one.
What one runs into in the processing field can best be illustrated by
random examples that I have been exposed to. This does not mean that I have
been directly associated with each experience, for some of them have been
called to my attention by friends in other plants A few examples are as fol-
a) A specification on a powdered material gave a distribution of particle
sizes with a part being "not more than 15% thru a 325 mesh". The customer got
65% thru a 325 mesh screen which was difficult to understand in that we wet
washed on this one sieve and he dry screened only. Exchange of samples used
5 CHEMICAL ENNIEEING EDUCATION March 1964
for testing yielded the same results. There were letters and there were tele-
phone calls and finally a meeting of four people - two from each side. After
about a half a day it became clear that our super assumed assumption about
ASTM standards were not valid. We assumed that the sample was ro-tapped for
15 min. per standard, but no, the customer used 30 min. In any event, when
both labs used either time they checked. What is grain size?
b) Again dealing with particle size and philosophy I remember well a case
of milled rutile that was a bit finer than specified. Since it went into a
weld rod coating we saw no harm and shipped the product. Customer rejected
product. At first this appeared harsh. Finally it turned out he had a case.
A rather dark substance like rutile will have a progressively lighter shade
as particle size is reduced. The customer agreed that the welding character-
istics would not be impaired but were uncertain about a welder in the field,
who sometimes makes up his mind he has a different rod and he will manage to
prove it does not work. So you are technically right, but are you going to
suggest to a foreman in that field that he prove to the welder you are right?
c) Take the case of an operating procedure that stated "Pump A Into tank M
then pump B from tank N into tank M and heat to boil and agitate". In some
instances this type of instruction may be valid, but in case of precipitation
of hydrates it is not. The time to boil, and type of agitation must be
spelled out. Again if you want the same results today as yesterday you had
better make your instruction at least concise enough to have assurance that
your desire is met. If you do this much you have an easier time of tracing
trouble when it arises.
d) There is the case of a frit manufacturer who decided that it would be
less expensive to have the vendor calcine borax (Na2B207.lOH20), since about
half is water and save on freight. Then too it seemed to make sense since he
would have to supply BTU's to remove water in his fritting furnace if the
vendor did not. The dehydrated product did not produce good frit. After
much to-do one lad woke up and realized that things were not the same as they
had been. So dehydrated material was moistened with proper amount of water
and satisfactory frit was produced. The assumption made was that at the frit
making temperature, (say 13000�C), the water could not be part of the melt.
A very plausible notion but not a valid one.
e) Another example of the role of water in high temperature reactions is
preparation of titanium diboride by reacting titanium dioxide, carbon and boric
acid. At 1600/18000�C very nearly theoretical product is obtained. When an-
hydrous boron trioxide is used a very poor product is obtained.
f) A customer requested a fair quantity of a zirconium soap dissolved in
mineral spirits. To do the job safely meant that operations had to be placed
in an explosion proof building, etc. Since the quantity was not large the cost
came out fairly high. Whereupon one staff member suggested we make a solid
soap and let the customs dissolve it in solvent. This drastically reduced coat
not only because a special building was not needed but also because a given
size kettle can produce more pounds per hour. All of which demonstrate that
requests are not unalterable, as we are lead to believe at times.
g) Zirconium metal, which by laboratory tests is superb in concentrated
boiling hydrochloric acid,was made into a valve that failed after three weeks
service at room temperature. In this instance reagent acid was used for lab-
oratory testing while service was with commercial acid that had a few hun-
dredths of a percent of sulfates and iron.
h) In another instance a product we make was off specification on small
amounts of iron. After much discussion it became clear that despite changes
our raw materials should be analyzed. It turned out that our supplier of
hydrochloric acid was not adhering to the specification. This is a particu-
larly knotty area in that cost is added in analysis of raw materials, yet you
have to have a cross check even though the vendor is supposedly adhering to
In the process of closing this delivery I would like to add one final
word of caution. This has to do with the biggest curse of all - the concept
of an average - it should be outlawed in engineering certainly.
THE MATERIALS ENGINEER IN THE CHEMICAL INDUSTRY
Carl H. Samans
Research and Development Department
American Oil Company
The materials engineer should start evaluating possible constructional
materials, to minimize capital investment, with the earliest decision to com-
mercialize a process. This evaluation continues, with suitable modifications
because of equipment and process changes, as the design develops. Specifica-
tions and procedures are drawn up so materials can be purchased and fabricated
to give the results expected. Finally, plant operations are followed closely
to assure that specified corrosion controls are maintained, that new controls
are added or modifications made as needed, and that the materials used deteri-
orate in service only as predicted. With such a broad scope of activities,
the materials engineer can function properly only if he is an integral part of
the process team from inception to ultimate plant operation.
Some Ground Rules
Two terms should be defined at once to minimize possible misunderstanding.
First, the materials engineer discussed here is not trained in the field of
"materials science" which is receiving so much attention and publicity from the
government and from many of our colleges and universities. He is not supposed
to develop new materials but is to utilize, most effectively and economically,
existing materials that can be purchased and fabricated with definite schedules
and lead times. The basic constructional materials still are metallic, so a
good grounding in metals and alternate engineering materials, and the ways in
which they can be modified and can deteriorate in service, is essential. How-
ever, even the best men trained academically in materials science will be of
little direct value to the chemicals industry that needs a materials engineer.
Second, the term "chemical industry" means a plant which must operate at
a satisfactory profit under a civilian economy in which urgency of plant com-
pletion is not necessarily the key factor and taxes are paid and capital in-
vestment is recovered without artificial incentives. The capital requirements
and operating costs of a proposed plant are the major factors that determine
whether or not it will be built. These costs, in turn, depend largely on the
materials of construction. Therefore, the materials engineer, who has the re-
sponsibility for their choice, plays an important role in the commercializa-
tion of new processes or products.
What Has To Be Considered
Consideration will be given, first, to the over-all picture and how the
materials engineer should fit into it; second, to certain practical questions
which must be answered, usually by Management; third, to the metallic and non-
metallic materials available for use, either as constructional materials or
as liners or coatings, with some of the limitations they impose; and fourth,
to the information secured from the operating plant to control materials deter-
The Over-All Picture
,'hen e decision is made to build a new plant, the engineering and process
designs usiua.lly have to be reasonably well fixed in order to estimate capital
investment. The major factors rffeeting this investment, therefore, are mater-
ials selection and servicabillty. Service deterioration usually results from
corrosion or some combination of it and another factor, such as mechanical wear,
cyclic stressing (known as fatigue), or over stressing (leading to premature
failure by cracking). When plant operation is at elevated pressures or temper-
atures, such deterioration is particularly hazardous because either the raw
materials or the products, intermediate or final, frequently are flammable and
may be noxious as well. It also can be costly because leakage or spillage may
cause loss of valuable raw material or product while the plant is trying to
operate. Even when there is no hazard or direct loss, a plant not operating
because of an unplanned shutdown is not profitable.
The materials engineer must select and specify (a) materials of construc-
tion, (b) thermal or mechanical treatments, and (c) methods of fabrication to
satisfy the process requirements. He also must have assurance that the mater-
ials supplied and the methods of fabrication used actually are as specified.
After the plant is built he must be completely conversant with all service
experience and field failures.
In many plants this work is done successfully by several different en-
gineering groups with the materials engineer acting largely in a liaison and
coordinating capacity. However, even to coordinate all of these functions, he
must be part of the project team from the beginning. He should be conversant
with bench scale developments and must be part of the development team during
the pilot plant and process and engineering design stages. His responsibilities
should be well recognized and his knowledge and experience be utilized to the
7 EMIAL EHGIMREINGE EDUCATION March 1964
Before a materials engineer can function effectively, certain practical
questions must be answered. These answers usually are given by company man-
agement on the basis of specific cases or alternatives presented to it.
First, how much contamination can be tolerated at various stages of the
chosen process design? Both competitive products and the economic consider-
ations Incident to establishing or breaking into a market are involved. Con-
taminants Introduced with raw materials usually are recognized but those re-
sulting from corrosion or wear of mechanical or process equipment often are
overlooked. These impurities, usually metallic, may deactivate existing cata-
lysts or catalyze undesirable side reactions as well as contaminate or decrease
the yield of the final product. Contaminants from corrosion also have im-
proved the yield or quality (e.g color) of the final product. This has been
overlooked when the pilot plant group did not understand the nature of the cor-
rosion fully. Also, in many processes, the build-up or corrosion products
during the earlier stages or in recycle streams is ignored because care Is
taken to purify the product by distillation, crystallization, or extraction
as a final step. These are some of the reasons why the pilot plant, if possi-
ble, should be built of the materials projected for the final plant design. If
this is done, any materials weakness or disadvantages may be shown up early
when they are correctable. To overdesign the pilot plant so no corrosion occurs
merely postpones the day of decision or trouble.
Second, must the engineering design conform to the requirements of ap-
plicable construction codes? These Codes may be local, state or national like
the ASME Boiler and Pressure Vessel Code or the ASA Code for Pressure Piping.
Third, where will the plant be built? This can affect materials selec-
tion because It determines the type of worker who will run the plant and may
affect safety considerations as well. Labor may be a significant factor, ex-"
pecially in foreign or previously nonindustrialized areas. The ability to se-
cure replacement materials readily when needed, also might occur in foreign
locations and might have a profound effect.
Fourth, what are the economics of materials selection? Although the
materials engineer should be knowledgeable in this respect, the basic decision
often is made by company management as a general policy. A plant may be built
of corrosion-resistant (usually more expensive) materials to decrease or elim-
inate product contamination and maintenance expense,at the cost of a larger
capital investment. Alternatively, less costly (usually less corrosion resist-
ant) materials can be used, at the cost of a larger operating expense, which is
immediately deductible for tax purposes. All chemicals plants are designed to
be shut down periodically for Inspection and rehabilitation. The lost produc-
tion is part of the cost of doing business. A materials failure frequently
results in an unplanned shutdown or an extension of a planned shutdown. If so,
the added cost of this loss of production, which is properly charged against
the materials failure, frequently overbalances completely the savings resulting
from using lower cost materials in the first place. The best course might ap-
pear to be to use the lowest cost material that will last Indefinitely. How-
ever, from a materials engineering viewpoint two other factors should be con-
(1) There always is a possibility of something unexpected heppen'ng to
cause serious deterioration or failure in the supposedly superior materials.
This is particularly true in a new process about which not too much is known.
(2) Proper attention must be paid to the time value of money. As a rel-
atively simple example, if $100 is borrowed today (even if it is borrowed from
yourself) it must be repaid in, say, one year by a larger amount, for example
$115. Thus, for this simple example, $100 today is worth $115 after one year,
or the present worth of the $115 a year hence is $100. This $15 increase also
is known as the earnings rate of the $100. Therefore, money which is not spent
today can be earning a return in some other investment until it does have to
be spent. It may cost considerably more cash in the long run to make a large
investment initially and to have low operating costs (i.e., to have no repairs)
than to spend less capital money and repair at scheduled intervals.
Fifth, how will existing business practices affect materials selection?
Currently, the capitalized cost of the original installation is depreciated
over its life at a rate set by the Federal Government, whereas the costs of
subsequent replacements, so long as they do not upgrade the equipment, are
looked upon as repairs to help the equipment reach that life and are expensed,
(i.e., are deductible, for tax purposes, in the year incurred), even though
they last more than a year. Thus current laws on taxes and rates can influ-
ence materials selection profoundly. As an alternative to a high initial capi-
tal investment, annual operating expenses often can be increased much more than
would be expected. If such replacements can be made during regularly scheduled
shutdowns, only labor and materials need be charged against the replacement,
but not the loss of production which usually is much larger. A treatment dev-
eloped to assist in decisions involving long and short-life components is given
March 1964 CHEMICAL ENGINEERING EDUCATION 8
in the typical charts in Fig.l. With these charts, if any three of the four
variables: (1) earnings rate, (2) life of long-life alternative, (3) life of
short-life alternative, and (4) ratio of the costs of the two alternatives, are
known, the fourth can be read directly. Also, when the life of the second al-
ternative is unknown, as usually is the case, the same charts can be used to
determine what the life of this alternative will have to be in order to break
even. For example, a given piece of equipment can be built either of carbon
steel with a relatively high corrosion rate so it has an expected life of two
years or of a low alloy steel which costs twice as much. If the desired earn-
ings rate is 15%, the right-hand chart in Fig. 1 indicated that the low-alloy
steel will have to last more than six years to be economical. However, if the
earnings rate is 25%, the low-alloy steel would have to last considerably more
than 15 years, on the same basis. Thus, unless there are overriding consider-
ations such as safety or preventing contamination; it often is not economical
to use a more expensive material even if it last forever.
Only alloys based on iron, nickel, copper, and aluminum find much use in
the chemical industries. The properties of the other four basic engineering
metals: magnesium, lead, tin, and zinc, and their alloys, usually are not suit-
Relative costs and other pertinent data for some common industrial alloys
used for process equipment because of their combination of strength and corro-
sion resistance are given in Table I. Carbon or low alloy steel is the pre-
ferred material, because of low cost and ease of fabrication into process equip-
ment. Stainless steels, usually'of the higher alloy austenitic types of which
18;8 (18% chromium, 8% nickel, remainder iron)is the best known, are most wide-
ly used of the higher alloyed iron. The simpler nickel alloys are used where
necessary but are about as expensive as can be used in process units today and
still leave a reasonable capital investment. The Hastelloys (complex alloys
containing nickel and molybdenum with some iron, chromium, tungsten, etc.)
cost much more than the simpler nickel alloys and usually can be justified only
for special applications. A large number of alloys based on these engineering
metals are available for use. Selection between them depends on the particular
application and usually is determined by factors such as the strength, both
static and dynamic, and corrosion resistance.
producers of materials plan on potential consumers doing most of the devel-
opment work, particularly when the potential sale of product is relatively
small, as it usually is in the chemical industries. Also, chemical processes
requiring materials for corrosion protection, if not completely new, often have
been revised in such a way that previous experience is limited or of little
value. The consumer must find out for himself, or with the aid of experienced
engineering contractors or vessel fabricators what strength, ductility and re-
sistance to service deterioration are needed and how materials with these prop-
erties can be assembled into operating equipment. Sometimes thermal conductiv-
ity and thermal expansion also may be important.
New developments, both in materials and in the methods of using them are
being announced continually, but a high percentage of these announcements are
premature from the viewpoint of the chemical engineer and the materials engineer
trying to make an operating plant from a process design. These new materials
are largely a direct outgrowth of the extreme temperature, strength, and other
requirements of the nuclear missile and space programs, and the tremendous a-
mounts of money being spent to develop materials. Some of these new metals were
a little more than scientific curiosities not too long ago. Initially, such
materials are produced in sufficient quantity to permit some of their properties
to be determined and some times to have a limited special application. An
effort then is made to extend their use in the hope that quantity production
can be achieved at correspondingly lower prices. During this period, adver-
tising, technical papers, personal visits and technical seminars are used to a-
lert and arouse the interest of potential users so it is easy to be oversold by
this convincing flood of information.
However, the use of these new metals develops slowly in either government-
financed or business-financed industries, usually because they cost too much.
Table I includes titanium, probably the best developed and lowest cost of these
new metals. It's cost still is comparable with that of the Hastelloys, even on
a volume basis. Consequently, titanium becomes competitive today, only where
corrosion resistance is the main factor and strength is minor.
In Table II several of the newer exotic metals are compared with carbon
steels. The comparison is based on the simplest commercial form, frequently
a metal sponge, rather than on a fabricated shape. Next to titanium the best
developed probably is tantalum, but it is so costly that its use can be justi-
fied only when nothing else is available. Furthermore, although practical meth-
ods suitable for fabricating vessels and other plant equipment from these metals
may have been worked out, it usually has been only for simple constructions and
9 CHEMICAL ENGINEERING EDUCATION March 1964
on a laboratory scale. All of them are extremely susceptible to- contamination
by such elements as oxygen and carbon and usually by nitrogen and hydrogen as
well. Hence, the scale-up factor from laboratory or small-scale fabrication
to full-size process reactors or distillation columns may be far more diffi-
cult than would be expected. The materials engineer must know these facts and
the state of the art at any time because, if the need for an alloy is known
far enough in advance, fabrication procedures suitable for a specific applica-
tion can be developed. Mqny metal producers and fabricating companies are
any xious to work with potential users if the demand is great enough or the po-
tential user will pay the costs.
TYPICAL METAT.T.C COS'TRoCTIOnAL MATERIALS NOW EsIM USD
0.2% Offset A.S.M.E. Relative Cost
Yield Strength Allowable Stress Per Unit Per Unit
1000 psi 1000 psi Weight Volume
Carbon Steels 4o0-8 15 1 1
Aluminum Alloys 4-11 1.6-7.3 6-9 2-3
Stainless Steels 30-40 17.5-18.75 10-15 11-16
Nickel and Monel 15-40 10-19 21 24
Hastelloys 50-58 20 82-100 90-125
Titanium 40-65 12.5 125-225 75-135
PROMISING EXOTIC METAIS
0.2% Offset Relative Cost
Yield Strength Elasti9 Modulus Per Unit Per Unit
1000 psi Weight Volume
Carbon Steels o0-48 30 1 1
Titanium 40-65 15.5 o0-42 24-26
Zirconium 29-61 13.7 170 156
Molybdenum 70-80 47 210 290
Columbium 20-30 15 950-1450 1100-1700
Tantalum 30-40 27 920 2100
CHARTS FOR SELECTING BETWEEN A LONG-LIFE ALTERNATIVE AND
A SHORT-LIFE ALTERNATIVE THE RATIO OF WHOSE COSTS IS r.
S'8I EARNINGS RATE 25% - EARNINGS RATE 15I
O - I-
S iSFlS 61 3 2 -- 2 - IO � 4� 3 2 1 i
LIFE OF LONG-LIFE ATEINATIVE IN TEAM
Mareh 1964 - OHMICL IWGIUNRING EDUCATION 10
Sometimes the more expensive corrosion resistant materials can be used as
liners in process equipment. Many unexpected problems can arise with liners,
however, especially in the initial application to a given process when elevated
temperatures and elevated pressures are involved. Cyclic conditions are par-
ticularly troublesome. Leaks may develop which can be quite serious under oper-
ating conditions, particularly when the low cost backing material, e.g., carbon
steel, has little corrosion resistance, and yet these leaks may not be detecta-
ble at the normal atmospheric conditions at which repairs would have to be made.
Three classes of metallic liners are in common use.
(1) Loose liners, either of the bladder type constructed externally for
use inside a flanged steel vessel (which provides backing strength but not
necessarily pressure tightness), or of the more conventional type constructed
inside the vessel and supported primarily at the flange of each manway or noz-
(2) Welded liners, attached to the vessel either as fairly large panels
using isolated or continuous spot welds or plug welds on a square or diamond
pattern designed to take care of thermal expansion differences. Liners of this
type also are made by continuous deposition of stringers of weld metal.
(3) Integrally-bonded liners, attached to the plates of which the vessel
is made as completely as possible by roll welding, by casting and rolling, or
by vacuum brazing.
With lined construction, however, savings on materials costs seldom ex-
ceed 25% and some of this saving may be used up by the special procedures re-
quired during vessel fabrication. For example, one factor which frequently is
overlooked is that the clad plates themselves must be joined together to make
the vessel and this may give considerable trouble. The welds joining these
plates together generally have to be of carbon steel, but the process side
either has to have corrosion-resistant weld metal, or be capped with a strip of
Nonmetallic linings may be used successfully if materials serviceable
under process conditions can be found. The carbon steel vessel itself may
have adequate resistance to process corrosion if its temperature is low enough
so the lining may only have to provide internal insulation to give a sufficient-
ly low shell temperature at a low cost. The refractory concrete linings used
to do this must be properly designed to avoid tensile stress-cracking and
must have sufficient thickness of well-installed low-permeability material
to provide good protection against corrosive fluids that might attack the
shell but do not attack the concrete. Proper liner stress conditions seldom
if ever can be obtained without decreasing the metal temperature substantially.
Refractory linings intended primarily for corrosion control usually are made
in one layer, supported by V- or T- studs. Two layer linings using light-
weight insulating concrete against the shell and a thinner layer of dense con-
crete on the surface to resist erosion and corrosion are used frequently in
vapor-state processes. In the two-layer lining, welding studs are used for
support. If wear resistance is needed, hexagonal steel grating welded to these
studs supports the dense concrete. Two-layer construction protects against
corrosive vapors but is not reliable for liquid immersion. External insula-
tion on the vessel must be eliminated or kept to a minimum if low shell temp-
eratures are to result. Usually, shell temperatures are kept above the con-
densation range, however, to avoid both internal and external condensation cor-
Glass linings have been used in the past, within the limits imposed by
their corrosion resistance and susceptibility to thermal shook and mechanical
damage. In addition to the limitations obviously imposed by the training and
habits of plant personnel, e.g., dropped wrenches or carelessly used hobnailed
boots, glass linings can be used only with specially designed vessels. Mod-
ernized versions, partially crystallized borosilicate glasses (Pfaudler Com-
pany's "Nueerite" and Corning Glass Works' "Pyroceram" are examples. See J.R.
Little and D.H. Hall, Crystallized Glass Coatings, Materials Protection,
June 1962, 40-44) have greater resistance to both corrosion, thermal shock,
and mechanical damage, and apparently higher temperature limits, than the older
linings. They appear to hold considerable promise, but experience still is
Likewise, the modern prestressed brick lining (C.A. Honigsberg and G.P.
Eschenbrenner, Prestressed Nonmetallic Linings for Process Vessels -- Structur-
al Design Considerations, presented at National A.I.Ch.E. Meeting, Los Angeles,
California, Feb. 6, 1962; also 3. Reys and A. Koeppel, Chemical Engineering
Progress, June 1962, 92-94.), originally developed in Europe, appears to have
definite advantages over the conventional acid-proof brick linings (Donald
11 CHMaCAL ENGINENRING EDUCATION Ioh4 s -*
Thompson, Brick-Lined Process Equipment, Chemical Engineering, Feb.
129-134; March 21, 1960, 161-166). The prestressing is secured by use of an
expanding thermoatting organic resin or potassium silicate cement. This resin
or cement is used both as a bedding material and as a mortar with either acid-
resistant clay or carbon brick. Expansion of the mortar during a curing cycle
imposes elastic compressive stresses on the liner. With proper design, these
liner stresses are always elastic and are held within the compressive or, at
worst, the very low tensile range, under all foreseeable conditions of opera-
tion. Each lining must be designed specifically for the vessel and for the
process and other (e.g., cleaning and shutdown) conditions under which it must
operate. Design is not difficult if operating conditions are nearly constant
but it becomes much more complicated for cyclic conditions whether these are
imposed by the normal operating cyclic or, for example, by steaming before shut'
The use of plastic coatings or linings for protecting steel vessels and
piping against corrosion is growing. Usually these are applied best in the
shop, preferably on new vessels or lines, rather than in the field because of
the many cleaning and application problems involved in securing an adherent
coating free of defects. If the surface already has corroded and must be
cleaned thoroughly before coating, these problems are magnified greatly. Plas-
tics for this purpose have definite temperature limitations. The older vinyl,
epoxy, chlorinated rubbers and polyesters, and epoxy-modified phenolic coat-
ing materials seldom can be used above 220F. The newer chlorinated polyethers
(e.g., Penton) after fusing at 425 F to 450 P, can be used to at least 250 F
both as coatings and as solid components. The fluorinated polyolefine can be
used to even higher temperature; PEP Teflon for example, is usable to 400 F.
However, the coatings are expensive (about 68 to $12 per sq ft as applied, 20
mil thick) and both application and service experience still are somewhat lim-
ited. Shop application of some of the new materials often is in the form of
cemented sheet linings. However, for thermal plastic materials considerable
progress has been made in plasma jet sprayed coatings on a developmental basis.
The older TFE Teflon still has the highest potential service temperature (550F)
of any plastic material but it is not thermal plastic so it cannot be sprayed or
applied by fusion. Sheet linings of TFE Teflon can be applied by cementing
with PEP Teflon but the cement then limits the useful temperature to, roughly,
400 F and the life of these linings still is somewhat questionable because of
lack of service experience. The very high thermal expansion of Teflon (about
10 times that of steel) also may be a serious disadvantage. A successful solu-
tion to the TFE Teflon application problem would be a significant breakthrough.
Natural and synthetic rubber sheet linings are used widely at temperatures
up to 200 F or slightly higher. They are not usually satisfactory in contact
with organic solvents but provide reliable and durable protection against many
aqueous solutions. Their cost is high compared to that of coatings, but sheet
linings entail less risk of pinholes and have better resistance to mechanical
damage and permeation by liquids than coatings. They are applied by using ad-
hesives, almost always by specialists. Usual thicknesses are 3/16 to 1/4 in.
Neoprene and soft or semiliquid natural rubber are used most commonly but ni-
trile and butyl rubbers and chlorosulfonated polyethylene (Hypalon)are better
for some purposes.
Reinforced plastics, usually of the epoxy, or polyester types, also are
much less used for linings or for primary constructional materials in chemical
plants than might be expected from advertising literature. Both reinforced
plastic pipe and plastic-lined pipe are finding increased usage, however. Ac-
ceptability by engineers and by plant operators has not been good if there is
any other solution, and constructional Codes usually have not considered the
question. Although reinforced plastics often have definite advantages over
metals in corrosion resistance and light weight, they also may have definite
disadvantages in low elastic modulus, which gives them poor resistance to in-
ternal pressure and to buckling and collapse, and in low flexural fatigue re-
sistance, in poor flame resistance, and frequently in cost. Objection to the
use of plastic piping has been particularly great if there is any possibility
of fire. Resistance to overheating by fire is no more than a few minutes un-
less external fireproofing is used. However, service experience with reinforced
plastic pipe, selected for corrosion resistance, generally has been good. Both
cemented joints and flanged joints are used.
At the present time almost any vessels made of reinforced plastics have
to be of standard shapes and sizes and usually have been used at substantially
atmospheric pressures and temperatures. There are definite limitations re-
garding the number, size, and location of openings which can be used. For el-
evated pressures, proper reinforcement requires winding on specially prepared,
relatively expensive mandrels which must be reused many times to be economical.
For example, reinforced epoxy vessels have been mAde as large as 12 ft in diame-
ter by 20 ft long, with a capacity of 17,000 gal. Even with a 1/8 in. wall
thickness these tanks are more expensive than steel. In addition, because of
the poor flexural fatigue strength of the reinforced plastic, a factor of safe-
ty of 6 to 8 must be used, as compared to 4 for steel, so a vessel of this size
for 200 psi service would have to be 1 in. thick, about the same as steel with-
out the corrosion allowance. Service would be limited to 200 F. The unfabri-
cated materials cost of the plastic vessel would be about three times that of
a carbon steel vessel; the fabricated cost much more.
CHEMICAL ENGINEERING EDUCATION
Repair and Maintenance
The use of n'w materials in the chemicals industries is much freer when
maintenance and repair are involved than for new construction. In these cases
the requirements of the various constructional Codes either do not apply or
apply less stringently than to new construction so the ingenuity of the en-
gineers involved govern more than any arbitrary rules.
For example, although there are many objections to the use of glass-
reinforced plastics as basic constructional materials, they are finding ap-
plication for replacement and repair within the limitations of their chemical
and thermal stability. For repairing a storage tank bottom (J.F. Wygant, Rein-
forced Plastic Replacement Tank Bottoms, A.S.M.E., Petroleum Division, Dallas
Meeting, 1962.), for example, where the deteriorated steel can be used as a
construction form, a glass-reinforced isophthalic polyester plastic bottom
costs about half as much as a replacement steel bottom. Isopolyesters adhere
well to sand-blasted steel and are appreciably less expensive and more adherent
than the epoxies which also have been used.
CONTROL OF MATERIALS DETERIORATION
Two methods of protecting metal vessels are electrochemical in nature.
The first of these is cathodic protection, Fig. 2. In this method another
material is made anodic with reference to the material being protected. The
anodic material may be more active electrochemically and thus corrode instead
of the metal being protected or it may be relatively inactive chemically and
be made anodic by use of an impressed external voltage.
The second method is just the reverse, namely, anodic protection (C.
Edeleanu, Anodic Protection, Chemistry and Industry, M1rch 11, 1961, 301-308),
Fig. 3. Many metals are resistant to corrosion in certain media because of
the formation of a passive layer on their surface. For example, stainless
steels have excellent corrosion resistance in the presence of oxidizing agents
such as sodium dichromate or nitric acid but corrode rapidly in the presence
of reducing environments. This corrosion resistance is a type of passivity,
and, in stainless steels at least, probably results from the formation of a
continuous oxide layer or monomolecular layer of oxygen on the surface. Pas-
sivity can be induced and maintained by keeping the metal anodic in the cor-
rosion system, although this may sound paradoxical because, as mentioned above,
the anode usually is the corroding electrode In such a system. In this new
system of anodic protection the anodic potential must be kept under proper con-
trol so the metal remains passive. If it becomes active, it will corrode.
Some sort of stable reference electrode must be used in the system, along with
a cathode. The necessary relationship between electrode potential and log cur.
rent or current density (proportional to corrosion rate) on the metal being
protected is shown in Fig. 4.
In systems to which anodic protection is applicable (D.A. Shock, 0.L.
Riggs and J.D. Sudbury, Application of Anodic Protection in the Chemical In-
dustry, Corrosion, 16, February 1960, 55t-58t.), as the potential first in-
creases to a maximum-(at point B). With a further increase in potential the
current density decreases to point A -nd then remains substantially constant
over a fairly wide range before it starts to increase again. This minimum-
current range is the passive range. In it the current density is a minimum
so the corrosion rate is a minimum.
In many cases, the first real test of constructional materials under ac-
tual operating conditions comes only when the new plant goes on stream. Pilot
plants often are used largely to outline feasible operating ranges. The op-
timum operating conditions then are determined by mathematical analyses. Even
DETERMINATION OF PIPE WALL
THICKNESS BY RADIOGRAPHY-SCHEMATIC
/SO URCE IMAGE
-/� _- T _OF WALL
if pilot plant runs are made under the expected full-scale plant conditions
the scale-up to a full-size unit frequently introduced unanticipated variables
which affect materials deterioration. Such things as localized process con-
ditions or velocity effects, or differences in material characteristics re-
sulting from construction techniques often cannot be predicted from either the
process design or the pilot plant studies. Also, plant maintenance practices
may introduce new corrosive conditions such as washing procedures or the for-
mation of corrosive solid deposits or condensates in piping.
Four general procedures are available to give the materials engineer in-
formation on the service durability of the materials selected. Two of these:
corrosion coupons and off-steam inspection, have been used for many years, but
the other two: corrosion probes and on-stream inspection, are relatively recent
developments. In addition, considerable insight often is needed to know where
to look for possible deterioration in the time available. Any corrosion eval-
uation device is, in a sense, only a statistical tool because it can give in-
formation only on what is happening at a particular location.
In exposing corrosion coupons of various materials to operating condi-
tions, the only real problems are the selection of materials and of a repre-
sentative exposure location. Coupons customarily are evaluated by a combina-
tion of weight-loss measurements and visual and metallographic examination.
They usually are assembled on racks, which are either bolted or welded
to the vessel in the location selected. For liquid or condensation conditions,
especially aqueous, some sort of insulators, e.g., Teflon, customarily are used
to prevent electrical connection and eliminate possible galvanic effects. Se-
lection of test specimens depends largely on the application but these speci-
mens should be in a condition, e.g., welded or stressed, comparable to their
use in the process equipment. Specimen thicknesses of the order of 0.1 to 0.3
in. frequently are used for mechanical durability, so long exposures are desir-
able to secure accurately measurable weight losses. Exposures usually are de-
termined by the length of run between unit turnarounds. However, coupon chang-
ers now are available, which permit a limited number of coupons to be
inserted or recovered during a run, even with pressurized systems. The chief
disadvantage of coupons is that only average corrosion rates can be secured
from them end even these require certain assumptions, of which the simplest and
most cornornly used is that corrosion occurs at a uniform rate. The main ad-
vantage of coupons is that they evaluate corros'an over a sizable period of
time as well as giving some information on pitting or other localized attack.
Off-stream inspection (A.J. Freedman, A. Dravnieks and B. Ostrofsky, Cor-
rosion Measurement Short Course, Petroleum Refiner, May, June, July, 1960.)
largely consists of visual inspection and thickness measurements of any parts
of the plant that are opened during the shutdown. This seldom includes all
vessels, so, if there is no past experience record, locations of important mat-
erials deterioration may not be inspected. The likelihood of this is minimized
if the materials engineer is part of the inspection team, if only on a con-
The corrosion probe (A.J. Freedman, E.S. Troscinski and A. Dravnieks, An
Electrical Resistance Method of Corrosion Monitoring in Refinery Equipment,
Corrosion, V11, April 1958, 175t), uses a very thin strip, wire, or tube
specimen and evaluates corrosion rates by a series of measurements of electri-
cal resistance (proportional to the loss in thickness). The slope of the
CATHODIC PROTECTION SCHEMATIC FIGURE 3
CHEMICAL ENGINEERING EDUCATION
CHEMICAL ENGINEERING EDUCATION
thickness-time plot then gives a corrosion rate, Fig. 8. The specimen is al-
ways in a selected location in the corroding medium so little if any information
can be secured on localized corrosion such as pitting, etc. Probes are availa-
ble which can be removed or inserted while the unit is in operation. Mechan-
ical devices have been developed for doing this easily and safely at relative-
ly high pressures. When inhibitor or other injections are being used to con-
trol corrosion, probes will give a continuous record 'of effectiveness. They
also may be useful as operating tools to evaluate the effect on corrosion of
changing operating conditions.
On-stream inspection now is being used more and more extensively, even
at elevated temperatures. This has been a logical development. What a unit
operator really wants to know is how much corrosion allowance is left at any
time. This information should be available far enough in advance of a scheduled
shutdown to have everything in readiness for immediate repair. In that way down
time is minimized, sometimes enabling more expensive and longer life materials
to be used effectively. On-stream measurements give the actual thickness of
the material at one spot and can be made rapidly enough to permit numerous read-
ings. Radiography (X-ray or isotope) and pulse-echo ultrasonic are now the
most common methods of doing this. Radiography (L.A. White and T. Arnesen,
Radiographic Techniques as Applied to Onstream Inspection Methods, Procedings,
American Petroleum Institue -- III Refining, 40, 1960, 190-195.)is effective
only where the source and film can be placed on opposite sides of the part be-
ing inspected, Fig. 9. This usually means only piping or small vessels. Pulse-
echo ultrsonic thickness measurements can be used whenever there is access to
the spot to be measured. This presents no problem if the line is below, rough-
ly, 100 F. At high temperatures, up to at least 1000 F, specially sealed and
watercooled transducers must be used (B. Ostrofsky and C.B. Parrish, Ultrason-
ic inspection at Elevated Temperature, Society for Nondestructive Testing, New
York Meeting, November, 1962.) Lead metaniobate crystals are sat-
isfactory if protected properly. The cooling water also serves as a couplant
thus simplifying or eliminating any requirement for surface preparation. Ex-
perience with on-stream inspection still is somewhat limited but it has proven
to be strikingly effective in some instances by detecting severe and unexpected
corrosion which undoubtedly would have resulted in a serious failure if the
unit had continued on stream.
Any engineering design, whether for a chemicals plant on the ground or a
rocket engine, is only useful to the extent that it can b6 translated into re-
ality by using available materials. The selection of materials often does not
seem to be complicated. However, this discussion may serve to emphasize the
breadth of the problem and the large number of factors which must be considered
if capital investment is to be kept to a minimum without paying for it by hav-
ing a low operating factor. It usually is unanticipated equipment and mater-
ials problems that make a chemicals process unsuccessful.
FIGURE 4 TYPICAL PLOT OF CORROSION-PROBE DATA SHOWING
PLOT OF ELECTRODE POTENTIAL AGAINST VARIATION IN CORROSION RATE WITH TYPE
CORROSION RATE TO ILLUSTRATE PRINCIPLE AND CONCENTRATION OF INHIBITOR
OF ANODIC PROTECTION 3.01
2.5 "0 22 20 10 .P
I 2.0. 0.001 IPY
PASSIVE 2F 0010 IPY
o *A L 0.020 IPY
P MASSIVE _J 1.0
O CORROSION 0T
RATE L C O
0.5 ;_0,250 IPY
STEADY STATE. RATE
POTENTIAL-I STEAD -STATEI
LOG CORROSION RATE
MATERIALS AND PROCESS DESIGN CALCULATIONS
C. M. Sliepoevioh
University of Oklahoma, Norman
As implied by the title, the scope of this presentation could easily en-
compass such a vast range of topics it would seem discreet not to proceed any
further and to state simply that in all process designs the best materials of
construction should be used, consistent with sound engineering economics. To
avoid speaking in such broad generalities it is expedient to concentrate on a
specific design problem since the principles illustrated are applicable in a
variety of situations.
Historically, chemical engineers have been primarily concerned in process
design with the specification of materials for high temperature and/or cor-
rosive service. Once a list of materials which will satisfy these service re-
quirements has been prepared, it is then necessary to narrow the selection to
materials which can accommodate the stress levels to be encountered in actual
operation. The final choice of a particular material will then be dictated by
purely economic factors which are measured in terms of life expectancy versus
installation and maintenance costs. The use of the singular term, material,
does not preclude combinations such as protective coatings, linings, and clad-
ding which become evident during the cost analysis.
The increasing trend toward higher and higher pressures in commercial op-
eration has pushed requirements beyond the metallurgical limit of available
materials of construction as measured by conventional design standard and code.
For the most part designers are guided by certain rules which limit working
stresses to * of the ultimate tensile strength, the stress to produce a creep
rate of less than one-tenth of a per cent in 10,000 hours, 60 per cent of the
creep to rupture stress at 100,000 hours, etc. Fortunately, with improved
techniques for producing materials, particularly in quality control, these de-
sign limits or factors of safety are gradually being relaxed.
In addition our increased understanding of the behavior -- most important
the reliability -- of materials and our greater ability to predict theoretical-
ly stress levels at all points in complex configurations (attributed to the ad-
vent of digital and analog computers) has atoned somewhat our inability to In-
vent exotic, yet economical, materials as new service requirements arise. De-
signers have learned to live with the shortcomings of the properties of mater-
ials by suitable provisions in design details; in some instances advantage can
even be taken of these peculiar characteristics. For example, industry was
plagued for many years by the unpredictable failure of boiler and furnace tubes
The tendency had been to make these tubes of sufficient wall thickness to elim-
inate creep at the expense of aggravating localized thermal stresses. Tube
failures involved not only their cost of replacement, but also repair of dam-
age to the furnace itself. By striking a compromise between the creep rate
and thermal stresses, and by making periodic visual observations of the tube
deformations, it is now possible to predict when the tubes should be taken out
of service short of imminent failure. In this case, empirical laboratory data
on creep and rupture life have been invaluable.
Another illustration can be drawn from Air Force experience. Technical
regulations called for the replacement of a rubber 0-ring in a critical com-
ponent of a hydraulic system in a jet-bomber after so many hours of flying ser-
vice. The concern here was not simply one of replacing an 0-ring valued at
less than 50-cents, but rather the cost of gaining access to the 0-ring by dis-
mantling the hydraulic system. The total direct replacement costs were esti-
mated at over 700 dollars, not including the cost of bringing the aircraft to
a maintenance base from some remote point or the cost of maintaining addition-
al aircraft to compensate for such down-time so as not to weaken our military
defenses. To make matters even worse, the 0-ring was invariably replaced by
one that had been sitting in a storage bin for several months during which it
had deteriorated more than the 0-ring in actual service. The solution to this
problem is rather obvious, and is as simple as packaging a gumdrop, but appar-
ently not convincing enough to penetrate bureaucratic red-tape as yet.
Chemical engineers are accustomed to service conditions which diminish
the properties of the materials. For example, metals have less strength at
high temperatures than at moderate temperatures. Within the last decade, chem-
ical engineers have become involved in large scale operations at very low temp-
eratures, and in so doing, have become confronted with a host of new problems
in materials. Although it is well-known that, in general, all materials Im-
prove in strength with decreasing temperatures, it does not follow that mater-
ial problems are necessarily lessened. In fact, at the present time advantage
can not be taken of any improvement in properties with temperature simply be-
cause regulatory codes won't allow it.
March 1964 CHEMICAL ENGIUEZRING EDUCATION 14
In order to illustrate the physical and economic variables that must be
considered in specifying materials for cryogenic service, the transportation
of liquefied natural gas at minus 250 degrees Farenheit and at atmospheric
pressure by ocean-going tankers will be reviewed. In the presentation that
follows it is to be understood that it will not be possible to cover all as-
pects in detail. Consequently sweeping generalizations, conclusions, and ex-
trapolations of the information presented herein would not be prudent.
Suppose you are confronted with the problem of fitting a tanker in the
20,000-ton class with the special equipment needed to load, contain, and dis-
charge a cargo of 170,000 barrels of liquefied natural gas (equivalent to 500-
million standard cubic feet of gas). The naval architect has specified the
general arrangements for a conventional tanker with the exception of the tank
storage compartments and auxiliary piping, fittings, and instrumentation for
handling the liquefied natural gas. You also note that he has provided dual-
fuel steam boilers for propulsion so that the boil-off vapors from the stor-
age tanks during transit can be fed to the boilers if economical.
Selection of Material for Tanks
The first step in the design is to determine the shape of the storage
containers. At first it would appear that these tanks should be cylindrical
to minimize cost of fabrication. But, is the fabrication cost of the tank the
only economic factor to be considered in this instance? The cross-section of
the hull of the ship is essentially square, and the plan view is rectangular
as shown in Figure 1.
It is evident from Figure 2 that cylindrical tanks can utilize only /774
or less than 80 per cent of the cargo-carrying space. An analysis of the cost
of the ship, operating costs, and value of the cargo reveals that the entire
project is unattractive if even 10 per cent, much less 20 per cent, of the space
,s not utilized. Preliminary estimates indicate, however, that if the stor-
age tanks are shaped to conform to the cargo space, prismatical or rectangular
parallelopipeds, the project might be salvaged depending on what the installed
costs of the tanks finally are.
The design of these tanks is unconventional for the chemical engineer be-
cause in addition to the static hydrostatic pressure loads, provision must be
(1) Stresses caused by cyclical dynamic loads due to rolling,
pitching, and heaving of the ship.
(2) Stresses caused by key forces restraining the tank's movements
relative to the ship.
(3) Stresses caused by thermal gradients, particularly the sharp
vertical temperature profiles in a tank that is only partially
Other factors such as maximum permissible deflections and overall and local
instability (buckling of tank walls) must be considered.
The next step is to establish the general size of the tanks in order to
make maximum use of the cargo space. Obviously, one large tank filling the
entire cargo space would be cheaper than several smaller ones. However, the
size of any one tank will be limited by two factors:
FAE :mUKOFTm HU
CHEMICAL ENGINEERING EXICATION
(1) Regulatory bodies for ships have established limits for indi-
vidual compartments subjected to dynamic loadings, free-surface
liquid effects, and safety under collision conditions.
(2) The maximum capacity of equipment to lift a completely fabricated
tank (on the dock) approximately 75-100 feet into the ship's hold.
(Alternatives would include assembly of tank in situ or installa-
tion during early stages of ship construction.
Once the overall dimensions of each tank are established, shown schemat-
ically in Figure 3, design details such as sizes of plates and stiffeners must
be determined. To do so requires a knowledge of the stresses which in turn
are dependent on the elastic modulus of the material of construction. There-
fore, the next step is to select the material.
It is common knowledge that materials having a body-centered cubic lat-
tice generally tend to become brittle at low temperatures, whereas face-center-
ed cubics or hexagonal close-packed do not. In this connection laboratory
tests such as the Charpy impact at various temperatures, which is qualitative-
ly represented in Figure 4, are useful. It will be noted that around zero de-
grees, carbon steel shows a sharp drop in impact energy denoting a transition
from ductile to brittle behavior. Nine per cent nickel steel shows a more
gradual transition whereas stainless steel and aluminum alloy show no transi-
tion at all. The absolute values of the impact energy have no known theoret-
ical significance although as a rule of thumb in design practice, values of
impact energy of at least 10 to 15 foot pounds are considered minimal. Alum-
inum alloys have a value of about 30 foot pounds or roughly j of the value for
stainless steel. Nine per cent nickel steel, although tapering off at low
temperatures, is considered suitable down to about -320�F (liquid nitrogen).
Care must be taken in drawing final conclusions from Figure 4 for any materials
which undergo a transition. Considerable variations can occur for the same
material from different heats; data in the literature on carbon steel for ex-
ample indicate transition temperatures that vary as much as between minus 200
and plus 2000F. Furthermore, the ductile to brittle transition temperature
can be some 200 degrees lower for uniaxial loading as compared to triaxial load
ing; the source of the data is therefore important to know.
From Figure 4, three candidates for possible materials of construction
emerge. Since stainless steel costs about 60 per cent more than aluminum it
is tentatively eliminated (copper and copper-based alloys are suitable for low
temperatures, but their costs rule them out for large-scale tanks). Nine per-
cent nickel steel has an allowable design stress of approximately twice that of
aluminum alloy; however, since the tank walls are flat plates rather than cy-
lindrical, the advantage gained by nickel steel is only the square root of 2
or 1.4 (stresswise) over aluminum alloy. On the other hand nickel steel has a
density of about 2.8 times that of aluminum. Therefore, the cost of the nickel
steel will have to be somewhat less than the aluminum to be competitive for
Consideration must next be given to other properties which might influ-
ence the final design and material selection. Figure 5 show that the tensile
or yield strength of all materials of construction increases with decreasing
temperature. For metals this increase amounts to a factor of 1.5-2.5 between
ambient and liquid nitrogen temperatures, whereas for plastics (Mylar, poly-
vinyl chloride, Teflon) it varies between 1.5-7.0.
FIGURE 3: PLACEMENT OF PRISMATICAL TANKS IN SHIP'S HOLDS.
m n 1964 CHEMICAL ENGIERING EDUCATION 18
The elongation, like the impact value, gives a convenient, qualitative
indication of the degree of ductility. There does not appear to be any way
of incorporating numerical values of ductility into design equations any more
than impact values. However, the greater the ductility the less tendency there
is for build-up of stress concentrations. Theoretically, a ductility of only
2-3 per cent is more than adequate to relieve localized stresses; nevertheless
experienced designers prefer not to work with materials having an elongation
less than 10 to 15 per cent in general construction.
The cargo tanks will be subjected to cyclical loadings due to motion of
the ship and vibrations. Fatigue life is thus another important variable.
The stress to cause fatigue in a million cycles increases from about 1 to 3
times as the temperature is decreased from ambient to minus 3000F. Specifi-
cally in the case of aluminum alloys this improvement is about 1.4.
The next question that arises is whether any of the properties or combi-
nations thereof will influence the design. The thermal stresses that can be
developed in a bar of material with the ends fully restrained during cooling
is given by
where s = stress
- thermal coefficient of contraction
E H modulus of elasticity
(AT)= difference in temperature
If the yield stress of the material is not to be exceeded then the ratio
SypAE(AT) will determine whether the fully restrained specimen will yield in-
elastically. If the ratio is less than one, then provision must be made in
the design to permit sufficient freedom of movement so that the yield stress
will not be exceeded. If the ratio is greater thrn one, then the material will
not yield even if fully restrained. In the case of the tank, which is only
keyed to the ship's structure and not rigidly attached, this ratio criterion is
not particularly important since the tank is essentially free to contract in
all directions. However, as will be clarified later, this ratio criterion is
significant in selecting the insulation for the tank.
Actually the ratios should be computed for the two-dimensional model
(plate) rather than the one-dimensional (bar) to include the Poisson ratio ef-
fect. On this basis the values reported above would be reduced roughly 10 per
cent for the metals.
It is interesting to compare values of this ratio for several common mat-
Wood 2.2 - 9.2
Cast iron 2.3
9% Nickel steel 1.2
Stainless steel (304 annealed) 0.28
Aluminum alloy (5000 series, annealed) 0.22
9% NICKEL STE - /___ STAINLESS STEEL
S -300 SERIESANNEALED)
S / CARBON STEEL
/ (5000 SERIES,ANNEALED)
MINUS 0 PLUS
FIGURE 4: IMPACT STRENGTH OF VARIOUS METALS, EFFECT OF TEMPERATURE
19 CHEMICAL ENGINEERING EDUCATION aeh 1%
The above values are computed on the basis of properties measured at ambient-
temperature and (AT) is taken as (60 + 320) or 380oF for service at liquid ni-
trogen temperatures. According to the table, the only metal that can with-
stand liquid nitrogen temperatures when fully restrained is 9% nickel steel and
cast iron. Cast iron of course is ruled-out for cryogenic service because of
its exceedingly brittle nature. On the basis of this ratio criterion, wood is
the "strongest" material of construction known for cryogenic serti6e.
The ratios given above are conservative. Since a( decreases with decreas-
ing temperature and E increases (about 10 to 15 per cent) with decreasing temp-
erature, these two variables tend to counterbalance each other. The yield
point as noted in Figure 5 will increase by a factor of 1.5 to 2.0. Therefore,
the ratios will be greater at lower temperatures than those reported above.
So far the properties examined and the conclusions reached were based on
data in the unwelded condition. It is important before a final selection of
material is made, that information is obtained on the welded condition since
the properties in the heat affected zone will be limiting. If the material to
be used is initially in the annealed condition, then the properties of the welc
ments will not differ significantly from the properties of the parent metal prc
viding quality control and specifications for the welds are rigidly observed.
Stress Analysis of Tanks
In any project for which the economics are fairly tight, there is no room
for sloppy or ultra-conservative design. Since the ship's tanks constitute not
only the critical part of the entire design, but also a major cost item, they
will have to be optimized to the highest, possible degree. Two problems must
be faced. First the tank configuration and details must be established to car.
ry the dynamic, mechanical loadings. This particular design must then be
checked for thermal stresses. If these are not satisfactory, the mechanical
design must be modified and the entire process of analysis repeated.
Preliminary calculations reveal that:
(1) The tanks must be braced at the corners with diagonal struts.
Depending on the reinforcement achieved, additional tie rods
extending across the tank may have to be used.
(2) In order to avoid excessive plate thickness, the tank walls
will have to be stiffened. These members will have to be attached
in successive horizontal planes which are spaced at variable
distances apart, decreasing in spacing with depth in the tank.
(The other alternative would be to use a constant spacing between
the stiffeners and to vary the plate thickness with depth).
Stiffeners running in the vertical direction aggravate the thermal
stress problem and therefore are not considered further.
With these limitations, surprisingly few alternative designs are permissible.
Of the several possible, the one shown in Figure 7 appears most promising.
The most severe loading condition will occur at the point of maximum roll of
the ship. It can be seen from the loading diagram in Figure 8 that the ship
is "tipped" to the right. This loading condition will be reversed every ten
seconds. The design point will then be the condition of maximum roll and
should be such that stresses in the tank members are at or near the allowable
design value. To achieve this idealized result, a digital computer analysis ii
The next step is to incorporate certain assumptions which will make the
stress analysis more tractable but at the same time will yield realistic re-
MINUS 0 PLUS MINUS 0 PLUS
FIGURE 5.EFFECT OF TEMPERATURE ON FIGURES: EFFECT OF TEMPERATURE
March 1964 CHEMICAL EWGIERIMNG EDUCATION 20
sults. By assuming that the load-carrying capacity of the tank walls is repre-
sented by a system of statically equivalent beams, the tank may be interpreted
as a three-dimensional truss consisting of a finite number of rigidly connected
bars. This approach would yield a rather precise solution via a "transfer-
matrix" procedure, but the cost of computer time would be unreasonable. The
problem, can however, be simplified without serious loss in precision into a
two-dimensional one by assuming that the pressure loads are carried by a series
of mutually, independent, horizontal slices of the tank. The simultaneous solu-
tion of the differential equations representing the pressure loads and the sub-
sequent matching of the boundary conditions is carried-out numerically on a
digital computer by means of a '"stiffness-matrix" method of analysis.
FIGURE 8: RESULTANT LOADING DIAGRAM (STATIC PLUS DYNAMIC)
ON VERTICAL TANK WALLS.
The final step is to check this design for thermal stresses. From a know-
ledge of the heat leak the vertical temperature gradients, and therefore the
thermal stresses, can be determined from a numerical solution of the differen-
tial equations involved with the aid of a digital computer. The most severe
thermal stresses occur when the tank is nearly empty at which time the hydro-
static stresses are a minimum. Therefore, it is only necessary to establish
that the thermal stresses do not exceed the allowable design stress.
The thermal stresses will be dependent on the material of construction.
In this case, aluminum alloys show lower thermal stresses than stainless steel
or 9 per cent nickel, although in the latter cases excessive stresses can be
avoided by designing around them.
Before the final selection of the material, consideration must be given to
the feasibility of maintaining quality control on the welds:
(1) Stainless steel is the most desirable from the standpoint of
ease of welding and 100 per cent X-Rey is not mandatory al-
though it is desirable.
(2) Aluminum alloy welding requires special precautions against
excessive humidity and dust. If porosity specifications are
to be assured, 100 per cent X-R-y is dictated. Fortunately,
these alloys can be welded repeatedly without any deleterious
(3) Until recently, 9 per cent nickel was not acceptable for ser-
vice unless the welds were stress-relieved, which eliminated
its use on large structures for all practical purposes. How-
ever, recent changes in the codes (ASME Code Case 1306) permit
9 per cent nickel, which is quenched at 1475�F and tempered at
1050� to 11250, to be used in welded cryogenic pressure vessels
without stress relieving the completely fabricated vessel.
Taking all factors into consideration, the use of aluminum alloys (5000
series) is preferred for prismatical, ship's tanks. Specifically, alloys 5083
or 5456 containing from 4.5 to 5.5 per cent magnesium and up to 0.8 per cent
manganese are superior for this application.
Safeguards in Materials Manufacture
Once the material is selected the design engineer must be on guard for
inexcusable, sloppy mill practice:
(1) The design engineer should specify precisely what test specimens
must be taken and how many. A chemical analysis of each run is
21 CHEMICAL ENGINEERING EDUCATION March 1964
- ~. )- In annealed material, it is not uncommon practice to "stretch"
the plates that come-out of the annealing furnace bowed (because
of inadequate support). This practice should not be acceptable
under any circumstance where annealed temper is specified.
(3) Rolled-in metal resulting from failure to keep the rolls clean can-
not be tolerated in an optimized structure.
(4) Ultrasonic inspection around the edges of the plate should be
(5) More uniform properties can be achieved in extrusions if a larger
'capacity press is used so that the extrusion can be carried out at
a lower temperature.
(6) Suppliers that are willing to guarantee properties above the min-
imum standards for the industry invariably expect compromise on
the preceding items.
The mere statement from a supplier that the material will be manufactured ac-
cording to "tight" aircraft standards is in the author's experience not suf-
ficient to guarantee the quality that can be obtained by exercising a little
more care. Apparently aircraft standards are not as tight as they sound.
Finally, the design engineer should participate in all aspects of the
fabrication, including the qualification of the welders and ultimately the hy-
drostatic pressure tests. Even though the welds are 100-per cent X-rayed, the
final structure should be dye-checked. Because neither the X-ray nor the dye-
check will indicate the presence of small crater cracks, a visual inspection
with a magnifying glass should be made of all welds end repairs made where in-
dicated before the tank is put into service.
Design for the Insulation System
The design of the insulation system will be described only briefly, par-
ticularly with respect to its effect on the design of the tanks. Figure 10
shows the general arrangement for the tank and insulation. It will be noted
that the insulation is attached to the inner hull of the ship. The following
requirements have to be met:
(1) The owner specified that the insulation system must be liquid-tight
in the remote event that the inner-liquid containing tanks should spring a
leak. The owner has a considerable investment in the ship, and he is not will-
ing to endanger the ship's structure by its coming in contact with the cold
liquid. In other words the insulation should serve as a secondary barrier, and
therefore must be inert to the liquid.
Tnamls, rml O
Fmas i:C0MI-8EC-IN OF A WVEL
lHOmA PAIrcAL. TAMS.
March 1964 CHEMICAL ENGINEERING EDUCATION 4
(2) The bottom insulation should be of sufficient strength tottransi -."
the enormous stresses (due to the ships motion) at the bottom key of the
tank into the ships structure.
(3) In the event of a fire on board ship, it is desirable that the
insulation be able to maintain structural integrity when its outer face is
exposed to a temperature of 1200�F for at least four hours. Considering that
the inner face will be at -250oF, the requirements are very severe.
So long as all of the above requirements have to be met, most insulating
materials within economic reach are immediately eliminated. Foamglas offers
a possibility except that It tends to become friable at low temperatures (not
desirable in the case of reeking in the ship). Furthermore, since the ratio
of its yield stress to thermal stress is only 0.45, provision will have to be
made for expansion joints between the panels of foamglas such that it is free
to move in all directions. These expansion joints, like all other joints, are
potential sources of leaks. Various foamed-in-place or sprayed-on resins fail
to meet the three requirements specified above in addition to not having yield
stress to thermal stress ratios of greater than one.
As was stated before, wood is the "strongest" material of construction
for cryogenic service. Furthermore, balsa wood is an excellent insulator, and
when faced with a thin skin of maple plywood, it is impervious to liquefied
natural gas. The biggest disadvantage to the use of wood is that it possesses
strong directional properties. For materials like metals, having uniform prop-
erties in all directions, stress analyses are relatively simple. On the other
hand, in wood one must deal with 3 yield stresses, 3 moduli of elasticity, and
9 Poisson's ratios. Nevertheless the situation is far from hopeless, because
here again it is possible to design around the difficulty. If the insulation
panels are built-up cross laminations(bonded together with adhesive) wherein
the grain orientation of the wood is.varied, it is possible to achieve "bal-
anced" construction such that the properties of the finished panel are essen-
tially uniform in two directions. In the case of balsa, there is no alterna-
tive since it comes in carefully hand-selected pieces of specified density
which average 2 to 4-feet in length, 3/16 to 1�-inches in thickness, and 2
to 4-inches in width. From these "toothpicks" panels up to 16-feet long, 4-
feet wide, and 8 to 12-inches thick are shop-fabricated. These panels are then
mounted on the inner hull of the ship, and the individual panels are joined to-
gether by plywood scabs.
The thickness of the insulation required is determined by economic fao-
(1) With increasing thickness the insulation costs increase while the
rate of boil-off in transit decreases. A certain amount of boil-
off is required if the gas is used to generate steam in the boilers
which in turn propels the ship. Therefore, a "controlled-rate" of
boiled-off is indicated, and it must be balanced against the cost
of other fuels. If the boil-off is not to be burned, it must be re-
liquefied. A three-way economic optimization of insulation costs,
fuel costs, and reliquefier costs are required. Unfortunately the
problem is not so simple.
(2) The optimum thickness as determined in item (1) must be further an-
alyzed. As the insulation thickness increases, the cargo capacity of
the ship decreases with negligible savings in smaller tank costs. At
first it would appear that this decrease in cargo capacity is triv-
ial. However, one-inch of insulation decreases the cargo capacity
DIAGONAL TANK BULKHEAD
FIGURE 7; HORIZONTAL ANGLE STIFFENERS AND DIAGONAL STRUTS IN SHIPS TANKS.
23 CHEMICAL ENGINEERING EDUCATION Marsh 1964
by more than 1000 barrels, and in terms of profit margins measured In
barrels of liquid carried, this figure could be very signifloiat.
(3) To-add further complexities, the thickness of the insulation will af-
fect the amount of heat leak and therefore the thermal stresses in
the tanks. Figure 11 shows qualitatively this relation. It is ap-
parent that the minimum thickness of the insulation will be deter-
mined by the maximum allowable design stress. Beyond this point the
other factors mentioned above come into play.
In summary the thickness of insulation will be influenced by insulation
costs, fuel and/or reliquefier costs, cargo capacity, and thermal stresses.
It will have to suffice to say that the cargo-handling equipment, piping,
valves, fittings, pumps, gaskets, bearings, instruments, etc. present some very
exciting problems. For example, even though aluminum piping would be cheaper,
one is almost forced into using stainless steel simply because valves, expansion
joints, etc. are only available in the sizes required in stainless steel. Of
course, one could consider using aluminum piping and stainless steel fittings,
but the junction between these two dissimilar metals which are constantly bathed
by a sea spray constitute an almost perfect galvanic cell and the result needs
no further elaboration. Of course there are ways to design.around this situa-
tion but again the factor of economics enters the picture. Aside from this
problem, picture a massive super-structure on deck constantly pounded by the
sea. Even the piping supports can tax the designer's imagination. The prob-
lems associated with cargo-handling should not be depreciated since the total
costs here exceed those of either the tanks or the insulation system.
No pretense is made that the foregoing does anything more than to scratch
the surface of some of the major problems that confront the design engineer in
specifying materials of construction for transportation of liquefied natural
gas in ocean-going tankers. Solutions to these problems, other than those pre-
sented above, are being studied and development is in some cases at very ad-
The most significant point to be made is that the present status of devel-
opment would not have been possible without extensive laboratory data and pilot-
scale tests. The cost of the development program alone is estimated in excess
of 15-million dollars. Despite recent, promising advances in the theory of the
solid state, none of this information was of any value, even the plausible
theories of rates of crack propagation. In the present state of learning, and
in the foreseeable future, there does not seem to be any hope for avoiding the
use of whet to some may seem as pretty "dirty" numbers to grind out some clean
answers which are reliable within a matter of the required few per cent. Re-
member: the economics are pretty tightly
TtKKNCOF I*aasI o at
an IIt: EFFECT OF THICKNESS OF INUATION
ON THEM STREMES IN TANM
DESIGN AND SELECTION
University of Michigan
In order to effect a reduction of corrosion through materials design
and selection one must understand the underlying principles of corrosion,
and the many ramifications which may influence the rate of corrosion of a
piece of equipment, or component parts of the equipment.
It is an accepted fact that all corrosion in aqueous media is an elec-
trochemical process, that is, metal passes into solution as metal ions giv-
ing up electrons which can be consumed by some other reaction. A metal im-
mersed in an aqueous medium will exhibit a potential which can be measured
against a hydrogen electrode or any other half cell such as a calomel or a
copper sulfate cell. This potential is referred to as the open circuit solu-
tion potential of the metal in that particular environment, and is only a
thermodynamic function indicating the tendency for corrosion to occur but is
no indication of the rate of corrosion in that environment. The standard
E.M.F. series as published in various handbooks and electrochemical textbooks
is of little value to the evaluation of a corrosion problem. The potentials
listed are for a specific concentration of metal ions of the metal in ques-
tion, namely one molal concentration. One can compute the solution potential
at some other concentration by the equation:
( E RT ln a (Mn4)
(1) EM T E
EM - the potential at unit activity of the ions
R - the gas constant
T s the absolute temperature
F - Faraday's constant
n s number of electrons involved in the electrode reaction.
At this time I should like to call your attention to a difference in
sign convention which exists between European scientists and many American
scientists. The European scientist places gold at the top of the E.M.F.
series with a potential of+ 1.50 volts. The corrosion engineer places it at
the bottom as -1.50 volts. This can lead to confusion if one is not careful
to determine the sign convention that an author is using in presenting his
work. Fortunately, both agree to call the metal of a galvanic couple which
is corroding the anode.
If a strip of iron and a strip of copper are placed in a neutral salt
solution they will each exhibit an individual open circuit solution potential.
If the two metals are placed in electrical contact with each other, a current
will flow in the circuit, or more specifically, electrons will flow from the
iron to the copper. (Iron is the anode). A measurement of the individual
solution potentials of the iron and copper under these conditions would show
that the potential of the iron moved toward the copper and that of the copper
toward the iron. This is due to polarization and is a function of the cur-
rent density on the anode and cathode. At the same time depolarization is
taking place, that is a reaction to consume electrons at the cathodic areas.
These reactions are believed to be
(2) H ++ e-- H followed by H + H -- H2
(3) 2H20 + 0 2 + 4e -- (OH)"
Reaction (2) is the main depolarizing action in acid solution where the pH
is less than 4.5, and reaction (3) is the major depolarizing action in near
neutral salt solutions. Obviously hydrogen overvoltage plays an important
part when reaction (2) is involved.
The anodic reaction cannot proceed at a faster rate than the cathodic
reaction; therefore in a near neutral solution which depends on dissolved
oxygen for the depolarizing action, the rate of corrosion is decreased mater-
5 CHEMICAL ENGINEERING EDUCATION March 1964
ially by removing the oxygen from the solution. However, anerobic bacteria
can depolarize by removing adsorbed atomic hydrogen from the surface of the
metal. In water systems of limited oxygen content this can be controlled by
the addition of a few parts per million of chlorine. It is more difficult to
control in underground structures, cathodic protection offering some possibil-
Since current density is the controlling factor on the rate of galvanic
corrosion, it is highly important that in the use of dissimilar metals in
electrical contact with each other the ratio of anodic to cathodic areas be
as large as possible.
We have discussed the case of two dissimilar metals in electrical contact
with each other. The same condition exists on a single pure metal in that
many local anodes and cathodes occur on the surface. These may be due to
grain boundaries, impurities, dislocations in the lattice, etc. If one meas-
ures the solution potential of the metal with the half cell probe somewhat re-
moved from the surface of the metal, one gets a somewhat integrated average
reading for the surface. However, if a fine probe is used close to the sur-
face, then differences of potential will be observed over the surface.
Galvanic action can occur for a number of reasons, some of these being:
(a) Dissimilar metals in electrical contact with each other. This has
been discussed above. However, it must be remembered that this same condition
can exist in multiphase alloys. Pure metals and single phase solid solution
alloys are best for resisting corrosion and multiphase alloys are good if the
polarized potentials of the various phases are nearly the same.
(b) Oxygen concentration cells can produce galvanic action. Since dis-
solved oxygen is necessary to promote the main depolarizing action at the
cathodic sites, a lowering of the oxygen content at a localized spot will caus.
this area to become anodic to the areas which have a readily available oxygen
supply. This often causes severe pit type corrosion. Lap joints not proper-
ly sealed can suffer accelerated corrosion in the crevices due to the oxygen
(c) Ion concentration cells will exhibit galvanic effects as predicted
by equation (1). The area where a higher ion concentration exists will be
cathodic to the areas where the ion concentration is low. This can help to
minimize the effect of the oxygen concentration cell in pit type corrosion,
but it does not counteract it.
(d) Localized cold working of a metal will generate anodic and cathodic
areas with the cold worked area being the anode. This can be readily demon-
strated by placing a cold headed nail in a solution composed of 20 g. NaCI,
2 g. ferricyanide, and 2 cc of phenolphthalein per liter of water. The head
and tip will turn blue and the body of the nail will turn pink. Cold riveted
joints and cold bent corners are illustrations of this condition.
(e) The velocity of the corroding medium has an influence on the de-
polarization of the electrodes. Changes in pipe sizes in a system with the
resultant changes in velocity can set up a galvanic action. This mpy be
even more noticeable if the flow changes from stream line to turbulent flow.
The above points out some of the factors which can produce Celvanic ac-
tion and therefore should be kept in mind when designing a piece of equipment.
The question of whether or not the process can tolerate the use of in-
hibitors should always be considered when selecting a material as the use of
inhibitors can often effectively reduce the rate of corrosion. Inhibitors
are classified as anodic or cathodic depending upon which electrode is af-
fected by the inhibitor. The polarized potentials of the electrodes are al-
tered by the presence of the inhibitor. Inhibitors are further divided into
inorganic and organic substances; some require the presence of dissolved oxy-
gen to be effective and others do not. The majority of the inhibitors used
today are of the anodic type. The films formed on the surface of the metal
are thin and usually invisible and are believed to be either an oxide, or an
adsorbed film in the case of some of the organic compounds. Therefore the
selection of an inhibitor must take into account whether or not dissolved oxy-
gen is present. It is imperative that a sufficient amount of anodic inhib-
itor be maintained at all times to get the optimum result, as a breakdown of
inhibition at localized spots results in creating small anodes surrounded by
large oathodic .areas which may be more detrimental than if no inhibitor had
ctcta to r McrH
t3'P � (B.P O "- Mc+
." M ct c tCo 0)
(a M 0 ct -"
pa m a tB
0P0 $CtO H3 0
S0 i0 00 1
SPDia 1 1
0 1-1 0
i - (D 0 0 0 Ca
M a� o him c
t-. OB ^fC+ 0 m
M -P, w C
F4 cr <( C+[yh)
0 0 -
Son be ca c
0 C+10 0
5 t.4 U<4a.(
W ~Oti <
b0 %.0 %*0 0 '0 %0
O + OO
c+ NO> 0 0 t-3
�D OQ Ev4 3i *
01-* W II-
y 0� &Loi4
" ' rd p
c+ 1- ca � .4 N
on- 0 H�
� l-'h H
0 1i-b H
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March 1964 CHEMICAL ENGINEERING EDUCATION 26
Some metals develop passivity when exposed to air which is maintained in
many solutions providing dissolved oxygen or other oxidizing media are pres-
ent. In some respects the oxygen appears to act as an inhibitor. Chromium,
nickel, and molybdenum are among the metals which exhibit this phenomenon.
The stainless steels containing 11.5% or more of chromium have been studied
very extensively and many words have been written regarding the nature of the
film which causes passivation. The thin films produced by exposure to the air
have not been isolated and electron diffraction methods have not shown any
definite pattern as to the structure of the film. However, films produced by
electrochemical treatment which are thicker indicate a chromic oxide structure
Tests on an annealed 18 Cr 8 Ni type 304 stainless steel in 50% HRS0 solu-
tions at 3000C showed a loss of 0.01 mils per month in an aerated solution and
62.5 mils per month in an air free solution. This and other experiments would
lead one to believe that the passivating film must be due to an adsorption
phenomenon rather than a chromic oxide film. This is very different from the
case of exposing aluminum to air; here a definite aluminum oxide film does
form. The passivating film on stainless steels is subject to breaking down in
isolated spots in the presence of some ions such as Cl- and SO * ions. The
passive areas are cathodic to the non-passivated areas, and thE small anodic
areas result in severe pit type corrosion. Under these circumstances a molyb-
denum bearing stainless steel is desirable such as an 18 Cr, 12Ni, and 2-3
Mo, as the Mo additions greatly improve the resistance of the passive film to
such localized breakdown.
Stress Corrosion Cracking
Stress corrosion cracking must always be considered when selecting a
material for a given application. Stress corrosion cracking in general is
accompanied by very little overall corrosion and is difficult to detect by
visual inspection of equipment. It occurs when high tension stresses exist
in some types of corroding media. Stress corrosion cracking has been recog-
nized in brasses and low carbon steels and called season cracking and caustic
embrittlement respectively for the two alloy types. The cracking of 70 Cu,
30 Zn cartridge cases while in storage was eliminated when it was determined
that residual stresses introduced in the last cold forming operation was re-
sponsible for the cracking. A stress relief heat treatment as a final oper-
ation reduced the residual stresses to such an extent that cracking was elimi-
inated. The above Indicates that externally applied design stresses are not
necessary to produce cracking. Residual stress levels in excess of the yield
strength of a material are frequently encountered in welded joints. However,
the externally applied tension stresses added to localized residual tension
stresses may often be the cause of failure. Stress corrosion cracking does
not occur when the stresses are in compression.
A large number of investigations have been carried out in the past 15
to 20 years on stress corrosion cracking in other alloys than brasses and low
carbon steel. It occurs in alloys where the base metal is Mg, Al, Cu, Ni,
Ag-Au alloys, etc. Austenitic stainless steels have received a great deal of
attention and some rather interesting facts have been uncovered. For instance
they will undergo stress corrosion cracking in the presence of the chloride
ion if the solution contains dissolved oxygen but do not when the oxygen is
removed. However, they will crack in NaOH solutions regardless of whether or
not dissolved oxygen is present.
A few thoughts to keep in mind when selecting a material for a highly
stressed application are:
(a) Residual stresses may produce a highly stressed condition where the
design stress may be relatively low. Adequate stress relief should be carried
out whenever possible to minimize this condition.
(b) Increasing the temperature of the corroding medium will usually in-
crease the susceptibility to cracking.
(c) Shot peening has been suggested as a means of putting the residual
surface stresses in compression and therefore preventing the initiation of
cracking. This means should be approached with caution as the compressive
stresses introduced by the process do not extend to any great depth below the
surface, and if general overall corrosion occurs to a limited extent the com-
pressive layer may corrode away. Under these circumstances the start of
cracking would only be delayed.
(d) As far as is possible do not use an alloy which is known to be
highly susceptible to cracking in the corroding medium which is to be con-
tained by the alloy.
(e) Proper control of solution to insure oxygen or inhibitor control
where these have proven effective in preventing cracking.
OHENICAL ENGIESnING EDUCATION
(f) Stress corrosion cracking has been shown to be due to a localized
galvanic action. Therefore the possibility exists of preventing it by the
proper application of a reverse current through the use of sacrificial anodes
or from a source of D.C. current.
Intergranular corrosion occurs when the grain boundary or area in the im-
mediate vicinity of the boundary is anodic to the body of the grain. This may
result from a difference in the polarized solution potential of the grains and
boundaries due to precipitated compounds, or to a localized stress existing
at the boundary. The grain boundaries of high purity aluminum (99.986% Al)
water quenched from 62000C are anodic to the grain bodies in hydrochloric acid,
but cathodic when furnace cooled. It is difficult to picture this as due to
a precipitated compound at the boundary. On the other hand, there is no doubt
the austenitic stainless steels are made susceptible to intergranular corro-
sion by the precipitation of chromium carbide or sigma phase (iron-chromium
compound) at the boundaries. Whether the susceptibility is due to chromium
depletion or stresses accompanying the precipitation at the boundary is still
open to debate. The essential point is that the material is rendered suscep-
tible to intergranular corrosion by this precipitation process. A 304 stain-
less steel may be checked by the ASTM standard boiling nitric acid test in the
as received condition to determine whether or not it meets the necessary stan-
dard prescribed by this test. The steel is normally supplied in the annealed
condition, that is, water quenched from 1850-19500F. However, this steel be-
comes susceptible to intergranular corrosion when heated in the temperature
range of 750-1650�F due to carbide precipitation at the boundary. Therefore
welding will produce susceptibility in the heat affected zones of the weld, and
stress relieving in the temperature range required by the ASME code (1100-
1200�F) makes the entire structure susceptible to intergranular corrosion. In
this condition the same material will not pass the boiling nitric acid tests.
Susceptibility can be minimized by using Cb, Ti, or Ta stabilized steels, or
extra low carbon (.03 C max.), but the use of these steels is no assurance that
intergranular corrosion will not be encountered under all conditions. It is
somewhat ironical that stress relieving to eliminate stress corrosion cracking
in these steels makes them highly susceptible to intergranular corrosion. Any
alloy having a precipitated compound at the grain boundary offers potential
danger of susceptibility to intergranular corrosion.
Cathodic protection is used quite extensively to reduce corrosion of en-
gineering structures. This is accomplished by the use of sacrificial anodes
such as Mg-Zn alloys, or a reverse current from a source of direct current.
Protection of this type is used on underground pipe lines and other underground
structures, water tanks, sea-going vessels, offshore petroleum operations, etc.
The type and amount of protection needed is a function of the corroding environ-
ment and metal to be protected and is only determined by a careful survey of
the conditions prevailing. The possibility of using this means of reducing cor-
rosion should be considered in the selection and design of equipment.
Severe damage may occur to the surface of a metal when subjected to fluids
at varying velocities and pressures. Euler, in 1754, predicted the loss in
efficiency of turbines when operating at high speed due to the formation of
bubbles in the low pressure areas. At that time he did not foresee the re-
ultinjadamage that can occur when the fluid enters a high pressure area and
the DuDbles collapse. The calculated pressures released by the collapsing bub-
bles are -from several hundred to several thousand atmospheres depending upon
the assumptions made for the calculations. Needless to say, the forces are
great enough to produce severe pitting and loss of metal in the areas where
the bubbles are collapsing. Slip lines, indicative of plastic deformation,
have been observed in the microstructure of the metal at the base of these pits,
and cracks, which may be due to fatigue, have also been observed in severely
damaged metals. Pumps, valves, turbines, propellers, etc. are all subject to
this kind of failure when the proper flow conditions are encountered to pro-
duce cavitation in the fluid. It is impossible to describe cavitation resist-
ance in terms of some mechanical property of the metal at the present time.
For case histories on the subject the reader is referred to "Cavitation Dam-
age," A.S.M.E. 1956. In this paper an effort was made to point out some of the
factors which have an influence on corrosion rates.
In conclusion may I point out that the reduction of corrosion through
materials design and selection depends on many factors, and the selection of
a material for a given application based upon corrosion data reported in some
compilation of corrosion data can lead to sad results if the test conditions
did not duplicate the conditions encountered in service.
MATERIALS INSTRUCTION FOR CHEMICAL ENGINEERS
What Should Be Taught By The Physical Chemist?
Iowa State University
In the design and development of new processes, the chemical engineer
must make a choice from various possible construction materials. Twenty or
thirty years ago the problem of suitable construction materials was relatively
simple compared to some present day problems. The growth of our technology
is continually requiring new and better construction materials, particularly
in the areas of nuclear energy development, the direct conversion of heat
into electrical energy, and space flight development. Where once a relative-
ly few metals such as iron, nickel, aluminum, copper, lead, zinc and some of
the noble metals were of primary commercial importance, today practically all
the elements have taken on a new importance in various phases of commercial
development. The chemical engineer may be concerned with processes operating
at liquid helium temperatures, a reactor core which hopefully can operate at
1500 to 28000C, the reprocessing of reactor fuels using a fused salt and
liquid metal as solvents instead of the more usual aqueous and organic sol-
vents, or processes involving ultra high vacuum or extreme pressures. Many
potentially useful concepts cannot be developed because suitable container or
construction materials are not available. Conversely, the availability of
better construction materials can open up new fields and permit development
of new and more economical processes.
The service behavior of a material involves an environment, and its ser-
vicability will be determined by its interaction with the environment as well
as its mechanical properties, both of which will depend on chemical kinetic
and physical factors. From a chemical point of view, containers or construct-
16anmaterials are additional components in a heterogeneous system which must
not take part in a deleterious way with the reactions or processes of inter-
est. From a mechanical point of view, the construction materials are mem-
bers or units of a structure or device designed to withstand various mechanical
stresses and perform some useful purpose. The chemist or physical chemist is
more interested in the chemistry, chemical kinetics, surface phenomena, and
phase equilibria involved. The mechanical engineer is more interested in the
static and dynamic characteristics of the unit, how the various pieces fit
together and the elastic, creep, fatigue, wear-resistance and other proper-
ties of the construction materials. The chemical engineer should have some
appreciation of both of these areas. It is in the first area that the physical
chemist can contribute most to the chemical engineer's training. The wide
range of available metals, alloys, refractories, cermets, composite materials,
etc., and possible environments precludes emphasis of any one type of mater-
tal or environment. The training program which will be most effective in
preparing the chemical engineer to cope with the materials problem is, of
course one which emphasizes basic scientific and engineering principles. The
basic principles or theories involved cut across many branches of science or
engineering. With the growing complexity of our technology, new branches are
added and there is always a great deal of controversy as to what the scope of
the new branch should be and whether it will grow or eventually wither away.
Chemical engineering has been established for a sufficiently long period of
time that it is perhaps safe to say that the primary concerns of the chemi-
cal engineer are the unit operations, chemical reactions and kinetics and the
physical processes such as heat and mass transfer for the particular process
to be developed. The selection of suitable construction materials, although
very important, is only one of his problems. The development of new and bet-
ter alloys, ceramics, refractories, semiconductors, etc., is more appropri-
ately a problem for the metallurgist, ceramist, or the material scientist, and
the development of better plastics, fluorocarbon resins such as teflon and
and related materials is more appropriately a problem for the organic chemist.
Nevertheless the chemical engineer should have some knowledge of the utili-
zation and limitations of construction materials and some understanding of
their service behavior.
Chemical engineering curricula require basic courses in chemistry, phys-
ical chemistry, physics, and mathematics-which form a basis for understanding
the behavior of matter and a background upon which a more detailed knowledge
of materials can be developed. Even without exposure to materials problems
in chemical engineering courses or a special course in materials the student
has the background to enable him to make various broad generalizations in
this area. He is well aware from his training in chemistry as well as ex-
perience that some materials react vigorously with water or air while others
are relatively inert. From his study of free energy and the equilibrium con-
stant, he is aware that the reaction of water or atmospheric oxygen with most
metals, alloys, and many other materials is thermodynamically favorable. His
introduction to chemical kinetics and the effect of temperature on reaction
rates serves as a basis for understanding why many of these materials are
stable in air at ordinary temperatures. Similarly, he can conclude that iron
is able to form a suitable container for a dry alkali metal chloride since the
displacement of the alkali metal by iron is thermodynamically unfavorable and
the iron will remain in the reduced state. His study of heterogeneous equili-
bria - , phase diagrams enables him to conclude that although metals A and B
CHEMICAL ENGINEERING EDUCATION
have the requisite strength for service at a given temperature, a strUcture
with A and B in contact will fail if these metals form a eutectic which melts
below-the proposed operating temperature. Molten salts as well as molten al-
loys have been proposed as nuclear fuels. The selection of a suitable metallic
container requires knowledge of the thermal conductivity, nuclear properties,
mechanical properties, the alloying behavior and reactivity of possible me-
tallic container materials. The same problems would have to be considered in
the case of a nonmetallic container.
There is probably little or no disagreement that the material covered
in the usual one-year course in basic physical chemistry is pertinent to the
materials problem as well as other phases of chemical engineering. Examina-
tion of undergraduate physical chemistry texts will show that the material
covered is rather extensive. In many cases, such as the text, "Physical
Chemistry", by W.J. Moore, Prentice Ball, the material included in the text is
admittedly more than can be profitably discussed in the usual one-year course.
Incorporation of additional material would mean a less advanced or less thor-
ough discussion of other important topics. Further training in thermodynamics,
theory of rate processes, alloy theory, corrosion, etc., would certainly be
desirable. If two semester courses could be devoted to this area in the under-
graduate curricula, a number of possibilities exist. An additional semester
devoted to basic physical chemistry, or a semester course in chemical ther-
modynamics, thermodynamics of solids or physics of solids followed by a one-
semester course in physical metallurgy would be very helpful in strengthening
the students background.
Since metals and alloys are the most commonly used construction mater-
ials in chemical processing, an introductory course in physical metallurgy
would be very worthwhile. There are various texts available that could serve
as a basis for such a course. An outline of material to be covered might be
1. Electronic structure of the elements.
2. Types of bonding, ionic, covalent, metallic; introduction
to the free electron theory and band theory of solids;
classification of solids as conductors, semiconductors and
3. Crystalline nature of solids, crystal geometry and classi-
fication of crystals; x-ray diffraction techniques.
4. Correlation of physical properties with crystal structure
and type of bonding.
5. Imperfections in crystals.
6. Diffusion in solids.
7. Interpretation of phase diagrams and their correlation with
the microstructure of alloys; effects of heat treatment with
special emphasis on iron-carbon alloys.
6. Work hardening and recrystallization, nucleation and growth,
age hardening, martensitic transformations, brittleductile
transitions, radiation hardening, dispersion hardening.
9. Introduction to theories of creep, fatigue and fracture.
10. Cermets and high temperature refractories.
11. Corrosion and corrosion mechanisms.
The degree of emphasis to be given to the various topics and the extent
to which nonmetallic materials should enter into the general discussion will
depend on the background of the student and the time allotted for the course.
Various texts can serve as a guide to the extent of coverage and level of
treatment that may be considered suitable for an advanced undergraduate
course. Some texts that may be considered for this purpose are: "Elements
of Physical Metallurgy", by A.G. Guy; "Physical Chemistry of Metals", by C.S.
Darken and R.W. Gurry; "Physical Metallurgy", by Bruce Chalmers; "Physical
Metallurgy", by C. Ernest Birchenall; "Theoretical Structural Metallurgy", by
A.H. Cottrell; "Solid State Physics", by A.J. Decker; "The Solid State for
Engineers", by Maurice J. Sinnott; "Elements of Material Science", by Lawrence
H. Van Vlack; "Thermodynamics of Solids", by Richard A. Swalin; and "Mechanical
Metallurgy", by George E. Dieter, Jr. A recent review of theories for creep,
fatigue, fracture and the mechanical behavior of materials is given in "Mech-
anical Behavior of Materials at Elevated Temperatures", edited by John E.
Dorn. Another possibility is a two-semester course based on a text such as
"The Solid State For Engineers", by Maurice J. Sinnott. This text has been
written for the specific purpose of introducing to the engineer the basic
principles which underlie the behavior of solids.
Whether such courses are to be taught be a physical chemist, metallur-
gist, chemical engineer or other qualified persons is not important. The
primary emphasis should be on general principles. This is not intended to
imply that training in basic principles is all that is necessary for the
practical application of engineering materials. As already indicated the
problem is complex and it is not yet possible on the basis of first principles
to predicate in detail the behavior of a material under various service con-
ditions. Knowledge of what materials or classes of materials are available
or have been used successfully for various types of service, methods of testing
and evaluating materials for a specific use, and economic factors are equally
important. Training in this area is more appropriately covered in engineer-
What Should Be Taught
By The Engineer?
University of Michigan
Ideally, the teaching of Materials Science or Engineering Materials by
engineers is unnecessary but in the same sense that all of engineering is un-
necessary. Engineering has been defined in many ways, but the essential fea-
tures of any definition generally incorporate some words dealing with the laws
of nature, economics, the benefit of mankind and judgment. Materials obey
certain physical laws, economics is involved in their use, benefits are de-
rived from their use, and judgments have to be made in their use so, by def-
inition then, Materials does fall within the scope of engineering. Given con-
ditions where the laws of nature are known, economics is fully understood, and
the benefits to mankind are clearly evident, there is not much left to the ex-
ercising of judgment, and the need for engineers would disappear. This is not
likely to happen in the foreseeable future, so we can dismiss the ideal case
and get to the case at point.
If engineering consists of a mixture of mathematics, science, economics,
utilitarian ends and judgments, can't this be simply broken into its compo-
nent parts, be taught by specialists, and then be brought together to produce
engineers? The answer is that this is essentially what we do in our various
engineering curriculums. The mathematics and sciences are taught in the ear-
ly years, their economic application, the development of judgment in making
alternate choices occur later in the programs. In terms of ECPD accreditation,
the sciences, engineering sciences, then the analysis, syntheses and design.
Where does Engineering Materials fit in these curriculums? Not in the
early years with the sciences since it is not a science, but an engineering
science. It is different from the usual engineering sciences in that instead
of being based only on physics, is also based on chemistry. It is this dual
base that causes so much difficulty in teaching Engineering Materials. Most
engineers, with the exception of the Chemical, Metallurgical and Ceramic
groups, elect only one year of chemistry while the chemistry that is needed
is Physical Chemistry and this, in most schools, is not a first year chem-
istry course. The Chemical Engineers then as a group have a decided advan-
tage over most other engineers in that they have the science base on which a
good materials course can be taught. A further advantage in most chemical
engineering curriculums is that they invariably have course work on Thermo-
dynamics and Rate Operations and both of these courses provide an additional
set of foundations for the materials course work.
Given then a person with training in mathematics, physics, chemistry,
thermodynamics, possibly rate-operations, and engineering science training
in mechanics and strength of materials, it isn't a particularly difficult task
to put together a Materials Science or Engineering Materials course that has
real depth and meaningfulness.
One of the first things to be done before discussing what to teach chem-
ical engineers is to determine the scope of the materials field. To many en-
gineers the word materials means the selection of a substance for a particular
application. To an electrical engineer it is the electronic properties of
conduction, semi-conduction, insulating or magnetic properties. To a mechan-
ical engineer it is a structural material with the emphasis on the elastic,
plastic or shock resistance. To chemical engineers it is a structural mater-
ial but subjected to corrosive environments. So it goes on down to a civil
engineer to whom materials means concrete, asphalt and reinforcing bars. These
engineers are primarily concerned with the specification of materials for var-
ious end uses.
Another group of engineers, equally as large or even larger, are inter-
ested in the manufacture and production of materials. These are primarily
chemical, metallurgical and ceramic engineers whose principal concern is to
prepare or take given raw materials and to convert these into intermediate
products which are to later appear in some finished form. Steel, petroleum,
plastics, textiles, paints, cements, solvents, etc., are but a few of what
might be termed the materials industries but are more commonly known as the
A third group, usually chemical, metallurgical or mechanical engineers,
are concerned with taking materials in bulk form and in fabricating them
to finished shapes for direct use by other engineers. Casting, extruding,
forming, blending, treating and in general processing them for ultimate use.
" a O iCOGTCAL E lURI.eNG EDMCATIOR March 19614.
* last two groups, in addition to .knowing enough about materials to
the for use, must also know how to manufacture, process and treat
I materials so they will have the required properties for ultimate usage.
Sis a much meore severe requirement than simply knowing what the ultimate
Pl*es are, and as you will note, chemical engineers are predominantly
Te in these second and third groups. It is for this reason that I believe
the materials book given to a chemical, metallurgical or materials en-
r should be pitched to a higher level than one would give to civil,
oetrioal or mechanical engineer. These other fields would certainly profit
men a more intensive treatment of materials science, but their lack of prep-
tion, principally in chemistry and thermodynamics, makes this an extremely
L.fieult teaching assignment.
What should chemical engineers be taught about materials? My viewpoint
Is that they should be taught from as basic a standpoint as is practicable
with a view to understanding why materials have certain properties, rather
*than attempting to teach them how to produce, process or specify materials.
oewing the why, one can usually deduce the how, while the converseis not
true. One occasionally gets into arguments with scientists about this point.
Some feel that it is their ob. to teach the why, and the engineers' job to
teaeh the how. This may have been true in some cases in the past but is not
true of many engineering fields today. There are differences, of course, be-
tween scientists and engineers but the teaching of why's and how's is not the
distinguishing feature. To me, the principal difference between the two is
this: a scientist generalizes while an engineer must particularize. Solid
state physicists are quite content with order-of-magnitude agreement between
theory and fact, and chemists are practically always concerned with thermo-
dynamio equilibrium. As engineers, however, we can't design processes to
orders of magnitude and we are more concerned with kinetics or the rate of
approach to equilibrium than we are with where it is. True, we need the sci-
ence to tell where we are going, but how we get there is our problem. There
is a vast difference between knowing about materials from a physics and chem-
istry standpoint and knowing about them from an engineering standpoint. We
must be familiar with the sciences but the scientists can and do get along
very well without the engineering.
Materials have a chemical constitution and therefore chemistry is cer-
tainly important. They do exist in various environments: electrical, mag-
netic, mechanical and thermal fields, and they will obey certain physical
laws, therefore, physics is important. If crystalline, structure is impor-
tantl if non-crystalline, the degree of lack of structure is important, etc.
It is quite fruitless to argue which is the more important since if they
yield useful information, the engineer can use them; maybe not in the way the
scientists think they should be used, but nevertheless, he uses them. Another
way of stating this I heard from the late G.G. Brown: "A scientist is one who
solves the problems he can solve while an engineer is one who solves the prob-
lems he must solve." TETs is certainly true in regard to the materials field.
LephysIrcsts work almost entirely in those fields where properties are
determined by electrons such as conduction, semi-conduction and magnetism.
Ohemists are a little broader and more apt to be experimentalists, but their
efforts are more along the lines of analysis, studies of non-stoichiometry,
phase equilibria, etc. Important studies it is true, but not important to the
engineer until they become useful.
The principal reason then engineers should teach materials science is that
they are in a better position to judge what are the useful parts of the var-
ious sciences that are applicable to this field. For example, many materials
are classified as crystalline solids, therefore a knowledge of crystallography
is necessary. The whole load such as one would get in a typical two-course
sequence in Mineralogy or Geology? Obviously not. There is nothing wrong
with these elections; one can even make a strong case for such a program. It
is just that there are large portions of this work that are of no direct use-
fulness, now or in the near future, for the engineer. The parts we need, we
need badly but not to the extent that we would elect the entire package.
Since orystallographers are usually adamant about modifying the existing course
1 extract the applicable parts and teach it ourselves. This same extractive
operation is carried ott on various portions of chemistry, physics, and even
ineering courses, to yield a body of information which can be called mater-
Chemical engineering students certainly obtain a surfeit of thermody-
,a in their training. It occurs in the early heat and material balance
es, in the physical chemistry, in the regular chemical engineering ther-
amies courses. The difficulty in using this in materials science in-
tion is that for the most part all this prior work is in the liquid or
CHEMICAL ENGIHE]RIPNG EDUCATION
gaseous state, and the student has to be shown Its applicability to the solid
state. The extensive training most engineers receive in engineering mechanics
has to be reoriented from the macroscopic scale to the microscopic. In many
respects a course In materials science consists In nothing more than taking
material previously presented In a different context and reorienting It, adding
new application information and welding the mixture into an engineering course.
To Insist, as many scientists do, trat this can't be done Is to deny reality.
It's done because it has to be done. We can't afford the time for the unhur-
ried, comprehensive, historical, and rigorous approach that they Insist upon.
The fields of metallurgical and ceramic engineering are two wnich were formed
largely because they did not constitute a respectable part of either chemistry
or physics since their subject matter didn't obey the rules. Because of in-
dustrial and engineering pressures for improved materials of these types, they
became engineering disciplines and were forced to develop their own science
along with the engineering applications. Their success In accomplishing this
Is largely responsible for the current enthusiasm of solid state physicists
and chemists for working in these fields.
As In all engineering courses, the syllabus of a course on materials
silence f'or chemical engineers changes from semester to semester as new de-
velopments occur. Of the order of one-half of toe material doesn't change and
this is the basic science. Topics such as atomic and molecular structure,
crystallography, phase equilibria, thermodynamics, Kinetics, bonding and elec-
tron theory make up the bulk of this material. The teaching of this material
does not introduce any new material to the chemical engineer; be has touched
on much of this in several courses In physics, chemistry and engineering. The
transition from the macroscopic to the microscopic scale in elasticity and
plasticity and the introduction to lattice defects opens up the whole concept
of defect structures which is so important to the understanding of many solid
state phenomena. The extension of electron theory to zone theory and the con-
cept of electronic defects leads to electrical and magneticte phenomena. Having
discussed the Influence of electrical, thermal, magnetic and stress fields on
solids, It's an easy step then to discuss the tensorial relationships between
these to pick up tne cross-effects such as tnermoelectrlolty, piezoelectrici-
ty, magnetostriction, etc. In the development of this work the emphasis sla
always on the usefulness relative to engineering with engineering applica-
tions stressed wherever possible.
More recently empnasia has shifted towards the structure of liquids and
its applicability to polymeric materials ana amorphous materials such as the
glasses. An area which is fast developing is tast of surface phenomena. Its
relevance to engineering problems in oxidation, corrosion, catalytic activity,
lubrication, wear, etc., are treated although this subject matter is nowhere
near as well developed a body of Information as the bulk properties, yet it
is probably of more immediate significance to the engineer than the bulk prop-
You will note from this brief description of the course that Is taught
to chemical engineers that tnia is not a course In how to specify materials,
how to make them or how to process them. It is aimed instead at trying to
understand why they have certain properties and why processing or treating
them can modify these properties. Subsequent course work builds on this course
and deals more specifically with metals, ceramics, plastics, etc. While I see
no reason why this course could not be taught by non-engineers, it is my feel-
ing tnat In order to bridge the gap between the science and the engineering it
would take someone with an engineering background since the principal gain Is
to engineering, not to science.
�LIJ.ab.. .*t" -~ t.;~. *)5*:.. J~.x~Jr!!:~~1