Adhesive bonding properties of various metals as affected by chemical and anodizing treatments of the surfaces

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Title:
Adhesive bonding properties of various metals as affected by chemical and anodizing treatments of the surfaces
Physical Description:
Book
Creator:
Eickner, H. W
United States -- Munitions Board. -- Aircraft Committee
Forest Products Laboratory (U.S.)
Publisher:
Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture ( Madison, Wis )
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oclc - 18720592
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Table of Contents
    Front Cover
        Page i
        Page ii
    Main body
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Back Cover
        Page 11
        Page 12
Full Text




L1 VL I ilLRUli I'RiK
ITALS AS A4I CHIU IY CUIHMICAL ANU

ULIIINC TRIATMENTS OF TllE SUIIFACS

(Part A Additional tests on Anodized Aluminum and on
Zinc-Chromate-Primed Maenesium)
F*hreiarv 1 VifiA


This Ieport is One of a SeriO
Issued in Cooperation with
AIl rOeICE-NAVY-CIVIL SUBCOMMITT[E
.on
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ADHESIVE BONDING PROPERTIES OF VARIOUS METALS AS AFFECTED BY
1
CHEMICAL AND ANODIZING TREATMENTS OF THE SURFACES-

(Part A--Additional tests on Anodized Aluminum and on
Zinc-Chromate-Primed Magnesium)


By

H. W. EICKNER, Chemical Engineer
2
Forest Products Laboratory,-- Forest Service
U. S. Department of Agriculture




Summary


The concentration of chromic acid (5 or 10 percent) and the length of
the anodizing period (20 or 40 minutes) were not found to influence the
adhesive bonding properties of the chromic-acid-anodized, bare 75S-T6
aluminum alloy. However, seal treatment of the anodized surface by
heating in water greatly interfered with bonding except for one type of
adhesive (high-temperature formulation of neoprene, nylon, and phenol
resins on fabric carrier). This same adhesive also gave unusually good
bonding to zinc-chromate-primed magnesium surfaces with bond strengths
averaging as high as 2,770 pounds per square inch on the standard 1/2-
inch overlap specimens of O.064-inch-thick FS1-H magnesium.


Introduction


The following tests were made to supplement the previous work on adhesive
bonding of various metal surfaces reported in Forest Products Laboratory


1
-This progress report is one of a series prepared and distributed by the
Forest Products Laboratory under U. S. Navy Bureau of Aeronautics
Order No. NAer 01628 and U. S. Air Force Order No. AF 18(600)-102,
Amendt. No. A6(55-286). Results here reported are preliminary and may
be revised as additional data become available.
2
-Maintained at Madison, Wis., in cooperation with the University of







of chromic acid concern tractionn (5 or 10 percent by weight), length of
treatments (20 or 40 minutes), and hot-water sealing in anodizing bare
75S-T6 aluminum, on the initial bond strength of joints to this metal.
Adhesive bonding tests were also made on magnesium surfaces that had
be treated and zinc-chromate-primed by two aircraft fabricators using
th ir standard surface treatments. The zinc-chromate-prime coatings
used with these treatments were thinner than those used on magnesium
surfaces in the previous work in Report No. 1842, and were reported to
improve boniIng to this metal.


Procedure


Type and Number of Specimens

Small 4- by 5-1/2-inch test panels were prepared by overlapping two 3-
by 4-inch metal sheets, 0,,064-inch in thickness, for 1/2 inch. Metal
sheets of 2 types, bare 75S-T6 aluminum alloy and FS1-H24 magnesium
alloy, were included in these tests. Three test panels were bonded for
each of the variables studied with three 1-inch-wide test specimens cut
from each of the panels.


Preparation of Metal Surfaces

Aluminumo--The bare 75S-T6 aluminum surfaces were prepared for bonding
by first wiping with a cloth saturated with acetone, and then immersing
for 5 minutes at 170 to 1900 F. in an alkaline solution (pH approxi-
mately 12.0) of the composition:

5.0 ounces sodium metasilicate
0.5 ounce Nacconal NR
1.0 gallon water

The alkaline solution was rinsed from the surfaces in hot water and
then cold water. The sheets were then anodized in a chromic acid
solution (5 or 10 percent by weight) at 90 to 100 F. by first apply-
ing a small direct current potential (3 to 5 volts) between the alumi-
num pieces suspended as anodes and the cathode, which was the lead con-
tainer for the solution. This voltage was gradually increased (within
5 minutes) to 40 volts. The higher voltage was applied at a current
density of 2.5 to 3.0 amperes per square foot of anode surface for
periods of 20 or 40 minutes.



Eickner, H. W. Adhesive Bonding Properties of Various Metals as
Affected by Chemical and Anodizing Treatments of the Surfaces.
Forest Products Laboratory Report No. 1842.


Reit. No. 1842-A


n'<






moval from the anodizini


(unsealed), or sealed by heating the metal sheets in distilled water
for i hour at 180 F. before drying.

With the use of 2 concentrations of anodize solution, at 2 anodizing
periods for both sealed and unsealed surfaces, 8 types of anodized
aluminum surfaces were therefore prepared for investigation in this study

Magnesium.--The surfaces of the magnesium sheets were prepared for bond-
ing by two aircraft fabricators by using their established procedures.

A. One method, involving electrolytic treatments, was reported to con-
sist essentially of the following steps:

(1) Vapor degreasing surfaces in stabilized trichloroethylene to
remove oils and contaminants

(2) Cathodic cleaning (4.0 to 6.0 volts direct current at 5 to
25 amperes per square foot of cathode surface) for 3 to 10
minutes at 180 to 200 F. in a solution of the composition:

10.0 ounces sodium hydroxide
1.0 gallon water

(3) Rinse in cold water

(4) Pickling for 3 to 5 seconds at room temperature in a solution
of the composition:

4.3 ounces concentrated sulfuric acid (sp. gr.-1.84)
2.8 ounces concentrated nitric acid (sp. gr.-1.42)
1.0 gallon water

(5) Rinse in cold water

(6) Electrolytic treatment for approximately 15 minutes with a
4.0- to 8.0-volt, 60-cycle alternating current at a current
density of 20 to 30 amperes per square foot of magnesium
surface in a solution of the composition:

40.0 ounces sodium hydroxide
0.5 ounce phenol
5.2 ounces sodium silicate (410 Be.)
Water to make 1 gallon
Operating temperature of the solution was 180 to 200 F.

(7) Rinse in hot water


J






(8) Neutralizing for 1 to 5 minutes at 135 to 145 F. in solution
(pH-2.4) of the composition:

0.07 ounce chromic acid
1.0 gallon water

(9) Force drying in hot air

(10) Priming by dipping in a zinc chromate primer (MIL-P-6889A,
Type I) thinned to result in a dry film thickness of
0.00020 to 0.00035 inch

(11) Air drying prime coat for at least 24 hours prior to bonding.

B. The other method, involving only chemical treatments, was reported to
consist essentially of the following steps: (Metal pieces with this
method were slightly larger, 5- by 6-inches, than the standard size.)

(1) Vapor degreasing surfaces in stabilized trichloroethylene to
remove oils and contaminants

(2) Cleaning surfaces by immersion for 10 minutes at 170 to 190-
F. in a commercial phosphoric-silicate cleaner for magnesium
(5 ounces per gallon)

(3) Rinse in cold water

(4) Removing scale by immersion for 1 to 10 minutes at room temp-
eratures in a solution of the composition:

24.0 ounces chromic acid
4.0 ounces sodium nitrate
1.0 gallon water

(5) Rinse in hot water

(6) Pickling for 5 minutes at room temperature in a solution of
the composition:

1 part by volume hydrofluoric acid (50 percent)
2 parts by volume water

(7) Rinse in cold water

(8) Sealing by boiling for 30 minutes in a solution of the
composition:

16.0 ounces sodium dichromate
n c1nr'o cn rP 1 mii, f1 ii r1 rla








(10) Forced drying in hot air

(11) Priming by dipping in zinc chromate primer, thinned with
toluene (1 part primer to 3 parts thinner) to a consis-
tency to result in a dry film thickness of 0.00015 to
0.00025 inch

(12) Air drying prime coat 24 hours, and then curing for 30
minutes at 180 to 200 F.

iese two methods were essentially the same as methods M-5 and M-3,
!spectively, in the original work of Report No. 1842, except that the
ime coat in the earlier work was much thicker, and different pro-
iletary primers were used.


hesive Bonding Processes

Le following 4 adhesive bonding processes were used in bonding the
*p-joint panels prepared with the 2 metals having several surface
conditions.

.oomingdale FM-47.--A high-temperature-setting formulation of the
nyl-phenolic type supplied by Bloomingdale Rubber Company, Delaware
Ld Flower St., Chester, Pa.

!dux E, Type R.--A high-temperature-setting, two-component formulation
a phenol-resin solution and vinyl-polymer powder supplied by Ciba
impany, 627 Greenwich St., New York 14, N. Y.

tlbond MN3C Nylon Tape.--A high-temperature-setting adhesive formula-
on of neoprene, nylon, and phenol resins, supported as a film on
Ion-fabric tape, supplied by NARMCO Resins and Coatings Company, 600
,ctoria St., Costa Mesa, Calif.

otchweld Bonding Film AF-6.--A high-temperature-setting formulation
acrvonitrile-butadiene rubber and -nhenol resin in the form of an


nt.- No. 18hP-A






a i t m tal with 1-hour air drying between coats, and overnight
ai (r i the final coat. The adhesive film was then precured
for lhur at 0 OF. in an oven. Following the procure, the joint was
aos lId n I placed in a hot press, where it was preheated without
euefr o 9 minutes at 300 F. before it was given the final
cur o 25 minutes at 300 F. and 200 pounds of pressure per
squa re I ch.

RI&du TFype R.--One medium coat of the liquid component was brushed
(on th rItal, n ithe powdered component was sprinkled immediately into
h v a oIf adhesive. Any excess powder was brushed from the
surf TrI a(desive film was air dried overnight, and the joint was
assebled pressedI at 200 pounds per square inch of pressure for 25
minute in a hot press at a temperature of 300 F. Approximately 10
minutes of the pressing period were required to bring the temperature
of the l fne to that of the press platen.

Wellb nd MN3C Nylon Tape.--Four spray coats of the priming component
j wy< 're applied to the metal pieces to result in a 0.001- to 0.002-
inch fi of adhesive. The adhesive film was dried for 30 minutes be-
tween coats and 2-1/2 to 4 hours after the final coat. The joint was
assembled with a single layer of tape adhesive and pressed at 50 pounds
per square inch pressure for 42 minutes in a hot press at a temperature
of 35 F. Approximately 12 minutes of the pressing period were required
to bring the temperature of the glue line to that of the press platen.

Scotchweld Bonding Film AF-6.--The single film of adhesive was assembled
In the joint. The assembly was pressed for 45 minutes at 150 pounds per
square inch of pressure in a hot press at a temperature of 325 F.
Approximately 10 minutes of the pressing period were required to bring
the temperature of the glue line to that of the press platen.

To insure uniform pressure distribution, all joints were pressed by
using cauls of 0.027-inch-thick chipboard between the press platen
and the metal pieces.


Tea ti 1;

he 3 lap-jo nt panels prepared with each metal, surface treatment, and
~bond r process were sawn into individual 1-inch-wide specimens. Cutting
ado e wth a metal-cutting bandsaw using a slow rate of feed and a
holdi jig to minximize any mechanical damage or overheating of the
Ji t. IThe lap-joint specimens were tested to failure by loading them
in t at' a rate of 300 pounds per minute. The ends of the speci-
ImeL. were held in 1-inch-wide Templin-tqype grips that extended down
fr e of the specimens to within 1-inch of the edgJe of the
oiw dote at a temperature of 72 to 76 F. The
f a i (i related as unit stress on the measured test area)

-6-







and estimated areas (expressed as percentage of the total area) of
adhesion, cohesion, and primer and coating failures were recorded.


Test Results


The results of the bonding tests made to the chromic-acid-anodized
aluminum, unsealed and sealed, are given in table 1, and to the zinc-
chromate-primed magnesium in table 2.

The results in table 1 definitely show that the hot-water sealing of
the aluminum oxide film formed during chromic-acid anodizing interferes
with bonding. This interference is much greater for types of adhesives
represented by Bloomingdale FM-47, Redux E, and Scotchweld Bonding Film
AF-6 than it is for the type of adhesive represented by Metlbond MN3C
Tape. The concentration of the chromic-acid anodizing solution (5 or
10 percent) and the length of the anodizing period (20 or 40 minutes)
were not found to influence the bonding properties of the anodized
surfaces significantly.

The results in table 2 again confirm the results indicated in the
original work and given in Forest Products Laboratory Report No. 1842;
namely, that adhesives of the type of Metlbond MN3C Tape produce bonds
to zinc-chromate-primed magnesium sheets that usually have higher bond
strengths than those obtained with the Redux E and Bloomingdale FM-47-
type adhesives. The latter two types are normally judged by lap-joint
tests with clad aluminum to have higher bond strengths than the first
type. The type of solvent in an adhesive, and its reaction with the
primer, might be expected to influence the bond strengths to the zinc-
chromate-primed magnesium but this possible effect was not investigated.
The average shear strength of 2,770 pounds per square inch for the bonds
made with Metlbond Tape to the zinc-chromate-primed magnesium, treated
by method B, is considered by present standards to be unusually good
bonding to magnesium. The Metlbond Tape adhesive, and also Scotchweld
AF-6 film, gave bond strengths of about 1,950 pounds per square inch to
the magnesium treated by method A.












Rept. No. 1842-A -7-




1
Table 1.--Results obtained in adesive bonding of sealed and unsealed chruoc-&d
anodized surfaces of bare 75S-T7 aluminum alloy


Anodize treatment


Concentra-:Anodize: Seal :
tion of :period :treatment:
chronic : :


1
Test results- for --


Bloomingdale : Metlbond MN3C : Redux E, : Scotchwed film
FM-47 : nylon tape : 2e : AF-6
typ ;


acid : : : Average :Adhesion: Average :Adhesion: Average :Adhesion: Average :Adhesion
: : : joint failuree: joint failuree: joint :failure.: joint :failure
: : : strength: : strength: : strength: : strength:
--- ------- ------ -- ---- -- --I- -------- ---- ----- -------- --------- ------- ---------- -- -------


: P.s.i. : Percent:


P.s.i. : Percent:


P.s.i. : Percent: P.s.i. : Percent


20 : Boiling
water-
None
40 : Boiling :
Swater-4
None


20 :


40


Boiling
water-
None
Boilin.
water-
None


-lEach value given is the average result
panels, 4 inches wide, with 1/2-inch


for a total of 9 test specimens, 3 cut from each of 3 bonded
overlap of 0.064-inch-thick bare 75S-T6 aluminum alloy sheet.


-Redux E, type R, used in this part of the study was used with coarse-mesh powder supplied with the
adhesive.
4Average estimate of the amount of bonded area in which failure was in adhesion between the adhesive
and anodized metal.
-Seal treatment consisting of heating the anodized aluminum panels for 1 hour at 1900 F. in distilled
water. The Reynolds Metals Co. states on page 48 of its handbook, "Finishes for Aluminum,"
copyrighted 1949, that immersion in hot water of the porous oxide film produced in anodizing
converts the oxide to the monohydrate that seals the pores of the film.


Percent :


Min. :


10 :


783
4,406

869
4,443


655
4,182

554
4,483


100
89

100
88


100
77

100
91


100
18

99
0


2,611
3,474

2,799
3,213


2,641
2,996

2,542
3,251


64
18

71
26


77
28 :

86 :
22 :


1,013
4,326

776
4,319


1,246
4,216

788
4,171


100
25

100
10


1,284
3,635

899
3,757


100
54

100
43






W
M Table 2.--Results obtained in adhesive bonding of z:
(d
C+ magnesium sheets

0
Adhesive : Method A Electrolytic treatment : Method B
C ------------------------------------- -----------
ro Average : Type of failure. : Average
: joint : ------------------------------ joint
: strength: Cohesion : Adhesion : Primer : strength
to : or
: primer : coating:

: P.s.i. : Percent : Percent : Percent: P.s.i.

Bloomingdale : :
FM-47 : 769 : 0 : 100 : 0 : 1,070

Io Metlbond MN3C: :
Nylon tape : 1,934 : 15 : 85 : 0 : 2,770

Redux E,
type R : 167 : 0 : 100 : 0 : 835

Scotchweld. :
AF-6 film : 1,948 : 3 : 97 : 0 : 633


-Test values given are the average for a total of 9 test specimens
panels, 4 inches wide, with a 1/2-inch overlap of 0.064-inch-tl
2
-Test values given are the average for a total of 12 to 15 test s-
of 3 bonded panels, 6 inches wide, with a 1/2-inch overlap of (
3
-Average estimates of the type of failure in the areas of the bon(
columns are for cohesion and adhesion failures of the adhesive:
indicating failures of the primer or coating of the metal.




























































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