A STUDY CF METHODS FOR !PREPARING
CLAD 24S-T3 ALUMINUM-ALLOY SHEET
SURFACES FOR ADHESIVE BONDING
May 1950
This Report is One of a Series
Issued In Cooperation with the
AM FORCE-NAVY-CIVIL SUBCOMMITTEE
on
AIRCRAFT DESIGN CRITERIA
Under the Supervision of the
AIRCRAFT COMMITTEE
of the
MUNITIONS BOARD
No. 1813
UNITED STATES /DEPARTMENT OF AGRICULTURE
- ,,FOREST SERVICE
FORESY- PRODUCTS LABORATORY
tt-
Madison 5, Wisconsin
- In Cooperation with the University of Wisconsin
A , STUDY OF„METE016 FOR,PREPARING CLAD 24S-T3 ALUMINUM-ALLOY
SHEET SURFACES FOR ADRESivi BONDINGBy
H. W. EICKNER, Chemical Engineer
and W. E. SCHOWALTER, Technologist
Forest Products Laboratory, 2- Forest Service
U. S. Department of Agriculture
SUMMAR Y
Comparisons were made of the effectiveness of 22 different cleaning
methods in preparing 0.064-inch 24S-T3 clad aluminum-alloy sheet surfaces
for bonding into lap-joint specimens with four adhesive bonding processes
A, B, C, and D. The cleaning methods used included solvent treatment,
abrasion, alkaline cleaning, acid etching, electrolytic cleaning, and combinations of alkaline and acid cleaning methods. The adhesive bonding
processes used were considered to be representative of the types of adhesives capable of producing high-strength joints between metals.
In general, preparing clad 24S-T3 aluminum-alloy sheet surfaces for
bonding joints of high-strength with any of the four bonding processes was
best accomplished when using a warm sulfuric acid-sodium dichromate solution for cleaning.
With bonding process A, the best results were usually obtained when
an acid cleaning solution such as hydrofluosilicic acid or sulfuric acidsodium dichromate, or the acid deoxidizing cleaner L, was used in preparing
the clad 24S-T3 aluminum-alloy sheet. Abrasion cleaning was also
moderately effective in preparing the clad 24S-T3 sheet for bonding with
adhesive A, but solvent cleaning or immersion in alkaline solutions was
usually found to result in low-strength joints.
1This progress report is one of a series prepared and distributed by the
Forest Products, Laboratory under U. S. Navy Bureau of Aeronautics No.
NBA-PO-NAer 00619, Amendment No. 2, and U. S. Air Force No. USAF-PO(33-038)49-4696%. Results here reported are preliminary and may be
revised as additional data become available.
-Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
Report No, 1813 Agriculture-Madison
With bonding process B, several of either acid or alkaline cleaning
methods were found to have prepared the surfaces so that high-strength
joints were obtained. Alkaline cleaning methods were generally found to
result in better cleaning of the clad 24S-T3 sheets for bonding with
process C or D than did acid or other cleaning methods, but several of the
acid etching processes, including sulfuric acid-sodium dichromate solution,
also gave very efficient surface preparation.
With the three bonding processes, B, C, and D, merely wiping the
clad 24S-T3 sheet surfaces with a cloth saturated with benzene, although
not resulting in as efficient cleaning as that obtained with some of the
other cleaning methods, did result in bonds with joint strengths exceeding
2,000 pounds per square inch.
The efficiency of the several types of cleaning processes in preparing for bonding clad 24S-l3 sheet surfaces having various types of contamination was compared. Surfaces having sodium metasilicate stain were found
to be the most difficult to bond. The acid etching process with sulfuric
acid-sodium dichromate solution gave the best results in preparing the
metasilicate-stained surface. Fairly good preparation of the surface for
bonding was also obtained by 'washing this surface with benzene and then
abrading it thoroughly with aluminum wool. When surfaces coated with oil
and paraffin to simulate a surface contaminated with grease were washed with
benzene and abraded with aluminum wool, good-quality joints were obtained,
and cleaning the paraffin-oil surface with sulfuric acid-sodium dichromate
solution was found to result in even better bonding. Cleaning this oilparaffin surface with benzene and sodium metasilicate was also found to
result in good joint strength when bonding with processes B or C.
Bonding directly to the side of the clad 24S-T3 sheet containing the
identification lettering without prior treatment of the sheet was also
investigated. Joints made to these surfaces with bonding processes B or C
were found to have strengths of about 2,000 pounds per square inch, which
was 65 to 80 percent of the strength of joints made with these adhesives to
clad 24S-T3 sheets that were cleaned by the more elaborate cleaning
processes. Joints made with bonding process A on untreated clad 24S-T3
sheets were only about half as strong as those obtained with the other two
bonding processes under the same conditions.
INTRODUCTION
Experience in bonding a metal such ks aluminum to itself or to other
materials has shown that occasionally bonds result that fail considerably
in adhesion to the metal. Frequently these faulty bonds have been attributed to improper cleaning of the metal. It has been aCknowledged for some
time that cleaning the metal is an important factor in bonding highstrength joints, but present knowledge of the many variables involved does
riot permit fabricators to select simple methods that will produce metal
surfaces that are properly and uniformly cleaned for bonding. Available
Report No. 1813
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information on currently used methods for cleaning metal prior to bonding
was collected from various manufacturers and processors and assembled.
Results of this survey showed that a number of procedures for cleaning
metals prior to bonding had been adopted, but that opinions varied as to the
effectiveness and limitations of some of these methods. Some of the cleaning methods that were described for certain adhesives required excessive
changes in procedure when shifting from one bonding process to another.
Often there was no agreement among manufacturers of one chemical type of
adhesive as to the best method for preparing a given metal for bonding.
Included among the recommended cleaning processes were abrasion, solvent,
alkaline, acid, or electrolytic cleaning. These methods may be used
separately or in combination.
The aluminum alloys, 24S-T3 (bare), clad 24S-T3, 75S-T6 (bare), and
clad 75S-T6 are the principal alloy sheet metals now used in aircraft. Of
these, the clad alloys are used for the most part. It was therefore the
purpose of this study to investigate the effectiveness of different cleaning methods, in preparing one of these sheet alloys, clad 24S-T3, for ,
bonding with several adhesive processes in order to determine the need for
changing cleaning processes when replacing one bonding process with
another. These results should then be applicable to other aluminum alloys,
both clad and bare, but since the cladding with 24S-15 is pure aluminum
and the cladding with 75S-T6 is aluminum alloy containing approximately
1 percent zinc, tests with these other alloys should be made to verify the
results before doing any extensive bonding.
The study of cleaning methods was divided into two parts. In the
first part the clad 24S-T3 sheets were used as received, and in the second
part certain contaminants were purposely deposited on the metal before
cleaning it by some of the methods that gave most promise in part I. Bonding processes are described under part I, and some of these same processes
were also used in part II.
Report No. 181
PART I. STUDIES OF VARIOUS METHODS OF CLEANING CLAD
24S-T3, ALUMINUM,.ALLOY. SHEETS AS NORMALLY RECEIVED
Procedure
Type and Number of Test Specimens
Test panels -were prepared from two 0.064-inch clad 24S-T3 aluminumalloy sheets, 4 by 6 inches in size, by overlapping and bonding them for
1 inch along the 6-inch dimension. The clad 24S-T3 sheets used throughout
the study were from one manufacturer and conformed. to Army-Navy Aeronautical
Specff iC ation AN-A -13. Two ;panels, were prepared for each bonding variable
investigated. Five test specimens, each 1 inch wide, were cut from each
panel, with allowance for at least 1/4 inch along each edge of the panel
for trimming. A total of 10 specimens was thus available for each variable
studied.
Bonding-Processes:
Four adhesive bonding ,processes, considered representative of the
types capable of producing high-strength joints between metals, were used
in preparing the lap-joint specimens. The bonding processes included two,
A end B, in which were used. adhesives A and B, believed to consist of
polyvinyl resins modified with phenol resins; a process, C, in which was
used a phenol-synthetic rubber adhesive, C; and a process, D, in which were
used the adhesive C as a primer on each metal face and an intermediate-temperature-setting phenol-resin adhesive, D, as the secondary adhesive.
Bonding Conditions
The bonding conditions (amount of spread, assembly, precure, and
cure) used were either those recommended by the adhesive manufacturer or •
those that had been found satisfactory in preliminary tests at the Forest
Products Laboratory. The conditions for each bonding process were kept
constant throughout the investigation of the various cleaning methods in
order to eliminate, insofar as possible, the effect of variables other than
the condition of the surface and the cleaning method used.
The conditions for the various bonding processes were as follows:
Process A.--Adhesive A was supplied as a liquid resin and as a
separate powder. One light coat of adhesive A liquid was brushed on each
clad 24S-T3 aluminum-alloy sheet surface, the adhesive A powder was then •
immediately sprinkled on the wet surface, aid the excess powder was removed
by shaking. The adhesive layer was air-dried for approximately 20 hours,
and then the panels were assembled and pressed for 15 minutes at a temperature of 300° F. under a pressure of 200 pounds per square inch.
Report No. 1813
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Process B.--Adhesive B was sprayed on each faying surface to produce
film thickness of approximately 0.004 inch.. Eight coats were generally
required, depending on the adjustment of the spray gun. Following an openassembly period of 20 hours, the adhesive film was force-dried for 45
minutes at a temperature of 200° F., precured in the hot press for 9
minutes at 200° F. without pressure, and then the joints were pressed for
15 minutes at 300° F. under a pressure of 200 pounds per square inch.
a
Process C.--Eight coats of adhesive C were sprayed on each faying
surface with an elapsed time of 3 minutes between coats. After an open-•
assembly period of 18 to 24 hours, the panels were pressed for 25 minutes
at a temperature of 325° F. under a pressure of 200 pounds per square inch.
Process D.--Eight coats of adhesive C were sprayed on each faying
surface with an elapsed time of 3 minutes between coats. After an openassembly period of 18 to 24 hours, the panels were cured in an oven for
25 minutes at a temperature of 325° F. One medium heavy coat of adhesive
D was brushed on both primed metal surfaces, which had been cooled to room
temperature following the curing of the primer. Following an openassembly period of 16 to 20 hours for adhesive D, the panels were pressed
for 1 hour at a temperature of 240° F. under a pressure of 200 pounds per
square inch.
Cleaning Methods
In this part of the study all clad 24S-T3 aluminum-alloy sheets
were from one source (E) and were used as received, except that the 4- by
6-inch sheets were washed on both sides with a clean cloth saturated in
benzene to remove the identification lettering and any other contaminatiox
that might be on the sheet. Four of the 4- by 6-inch sheets were cleaned,
and two bonded panels were prepared from them for each' cleaning method and
bonding process. The cleaning methods investigated were as follows:
Method A--Benzene Wipe.--In method A no treatment was given the
clad 24S-T3 sheet surfaces other than the benzene wipe used on all sheets
included in this part of the study. This method was used for the controls.
Method B--Abrasion Cleaning.--In method B the clad 24S-T3 aluminumalloy sheet surfaces were abraded in one of the following ways:
(1) By light sandblasting with clean sand fcr . approximately 1/2
minute. The surfaces were then wiped With' 6 clean, dry cloth. Surfaces
outside of the bonding area were masked to prevent destruction of the clad
layer.
(2) By hand rubbing in parallel strokes with No. 00 aluminum wool
(dry) and then wiping with a clean, dry cloth.
(3) By hand rubbing with an iron-free abrasive cloth (grit No. 80)
and then wiping with a clean, dry cloth.
Note: Iron-free abrasives were selected in order to avoid use of materials
known to promote corrosion of clad 24S-T3 aluminum alloy in service.
Report No. 1813
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Method C--Solvent Cleaning.--In method C the metal was - cleaned in
one of the following ways:
(1) By immersing for 3 minutes in a solution made by mixing 5 percent by weight of the adhesive as received with 95 percent of the same
solvent as used in the original adhesive.
(2) By vapor degreasing with stabilized trichloroethylene (product
F), meeting the requirements of Army-Navy Aeronautical Specification
AN-T-37a, in a commercial degreasing apparatus.
Method D--Alkaline Cleaning.--In method D the metal was cleaned in
one of the following ways:
(1) By scouring with a fiber-bristled brush and a hot borax powder
solution (product G) (2.0 ± 0.5 ounces per gallon of water at 130° F.).
(2) By immersing for 5 minutes in a hot (160° to 180° F.) solution
of the following composition:
3.6 t 0.5 ounces of sodium metasilicate
0.4 t 0.1 ounce of wetting agent (product H)
1 gallon water
(3) By immersing for 5 minutes in a hot (160° to 180° F.) solution
of the following composition: .
6.0 + 1.0 ounces of an inhibited alkaline detergent cleaner
for aluminum (product I)
1 gallon water
(4) By immersing for 5 minutes in a hot (180° to 200° F.) solution
of the following composition:
6.0 + 1.0 ounces of an alkaline -etching 4eaner for aluminum
(product 4
1 gallon water
(5) By imraersing for 10 minutes at roam temperature in a solution
of the following composition:
4.0 + 1.0 ounces of sodium hydroxide
1 gallon water
(6) By immersing for 5 minutes in a hot (185° to 212° F.) solution
of the following.composition:
5.0 ± 1.0 ounces of a soap meeting Navy Aeronautical
Specification C-152 (product K)
1 gallon water
Report No. 1813
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Method E--Acid Cleaning.--In method E the metal was cleaned in the
following ways:
(1) By immersing for 10 minutes in a warm (140° to 160° F.) solution of the following composition:
45.0 ± 1.0 ounces concentrated sulfuric acid (specific
gravity, 1.84)
4.5 ± 0.5 ounces sodium dichromate
1 gallon water
(2) By immersing for 3 minutes in a warm (140° to 160° F.) solution
of the following composition:
7.0 + 1.0 ounces chromic acid
1 gallon water
(3) By immersing at room temperatures for 3 minutes in a solution
of the following composition:
10 percent by volume concentrated phosphoric acid
185 percenty04)
40 percent by volume butyl alcohol
30 percent by volume isopropyl alcohol
20 percent by volume water
(4) By immersing for 5 minutes at 140° to 160° F. in a solution of
the following composition:
10.0 ± 0.5 ounces of commercial phosphoric acid (85 percent H3PO4)
3.0 ± 0.5 ounces of chromic acid
0.20 ± 0.05 ounces of wetting agent (product H)
1 gallon water
(5) By'immersing for 5 minutes in a hot 1160° to 180° F,) solution
of- the following composition:
6.0 + 1.0 ounces of a deoxidizing cleaner (product L)'
1 gallon water
(6) By immersing for 8 to 10 minutes at room temperature in a
solution of the following composition:
'
5.0 ± 0.5 ounces of` commercialhydrofluosilicic acid
(27 percent H2SiF6)
2.20 + 0.05 ounce's of, *tting agent (product H)
1 gallon water
Report No. 1813
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Method F-- Electrolytic Cleaning.,.,The,me:tal:was cleaned, electrolytically by immersion for 5 minutes at a temperature of 130° to 190° :F• in a
solution of the following composition:
40 percent by volume sulfuric acid (specific gravity, 1.84)
40 percent by volume phosphoric acid (85 percent H3PO4)
20 percent by volume water
A current density of 0.3 to 0.5 amperes per square inch of anode
surface was applied during the immersion in the solution, with the clad
24S-T3 aluminum-alloy sheet as the anode and a copper sheet as the cathode.
Method G--Combinations of Cleaning Methods.--In method G the metal
was cleaned by one of the following combinations of methods:
(1)
D(1) (borax wash), followed by D(2) (sodium metasilicate),
and then by E(2) (chromic acid).
(2)
D(2) (sodium metasilicate), followed by t(2) (chromic acid).
(3)
D(3) (detergent cleaner I), followed by E(6) (hydrofluosilicic
acid).
In all of the methods by which the metal was cleaned by abrasion
or by solvents, the metal was bonded without any further treatment. In
the cleaning methods where water solutions were used, care was taken to
wash the clad 24S-T3 aluminum-alloy sheets thoroughly with tap water upon
removal from the cleaning baths in order to remove all the chemicals
before they could dry on the sheets. In combination treatments, the clad
24S-T3 sheets were rinsed in water after being removed from each aqueous
bath. Following removal from the last cleaning bath in each process,
the metal was thoroughly washed with hot water and was then, quickly airdried. Throughout the study, observations were made and recorded of the
condition of the water film on the surface when the metal was removed from
the rinsing water. The clean clad 24S-T3 sheets were dried at normal room
temperatures for 1 hour before application of the first coat of adhesive
to the surface. In no instance were the clean sheets allowed to remain
for more than 4 hours before application of the adhesive.
All solutions were used in vessels of nonreactive materials. .
Precautions were taken to see that none of the solutions were contaminated, that the cleaning solutions were agitated to prevent , concentration
or temperature stratification, that the surfaces to be cleaned did not
touch each other or the containing vessel, and that the clean clad
24S-T3 aluminum-alloy sheet surfaces did not become contaminated during
Report No, 1813
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the interval between cleaning the metal and the application of: the adhesive.
Determinations were made of the pH values of the various solutions used in
the cleaning.
Equipment limitations prevented preparation of specimens by all of
the cleaning methods for a particular bonding process at one time. It
was therefore necessary to repeat each bonding process on each of several
days with use of only a few of the cleaning processes on each day. In
order to insure that the quality of the joints in the several series of
runs made with each bonding process was the same, a control group of two
panels cleaned with method E(1) (sulfuric acid-sodium dichromate) was
prepared with each bonding process on each day that this bonding process
was being used in investigating methods of cleaning.
Testing
The two lap-joint panels prepared with each bonding process and
cleaning method were sawed into a total of 10 individual test specimens
in such a manner that the joints were not overheaW or subjected to
excessive mechanical damage in cutting. The specimens were then loaded
to failure in tension at a rate of approximately 600 pounds per minute in
a machine in which the ends of the specimen were held in testing grips
of the type shown in figure 1. Jaws in the grips were offset in such a
'way that the center of both grips and the adhesive line were in alinement.
Testing was done at a temperature of 80° + 5° F. The failing load and
the estimated areas (expressed as percentages of total area of joint) of
adhesion failure, of cohesion failure, and of lack of contact were
recorded.
Report No. 1813
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FART
STUDIES OF SOME OF THE ; MORE PROMISING CLEANING
METHODS IN TREATMENT OF CONTANaNAIED SURFACES OF
CLAD 24S-T3 ALUNINUMALLOY.•SHEETS
Procedure
Surface
Conditions
As a second part of, this study, several of the cleaning methods
representative of the types used in part I were used in cleaning clad
24S-T3 aluminum-alloy sheets having several types of surface contamination.
The surface conditions were: (1) sheets as normally received, (2) sheets
as received and then coated with a greasy layer to simulate a surface condition such as might be encountered after long storage, and (3) sheets
having , stains of the type obtained when alkaline cleaning solutions are
allowed to dry on the surface.. The greasy layer (2) was obtained by
dipping the sheets, as received, for .1 minute in a solution of 50 parts
by weight of carbon tetrachloride, 10 parts petroleum paraffin, and 25
parts clean S,A.E. No. 10 oil, and by then air-drying the solution. The
stained sheets (3) were prepared by dipping the sheets, as received, in a
solution of sodium ,metasilicate as used in method D(2) of rart I, and by
then allowing the,solution to dry on the surface without rinsing after
removal from the bath. In the preparation of each of the contaminated surfaces, the sheet was washed with 'benzene before any contaminant was applied.
Cleaning Methods
The clad 24S-T3 sheets (4 by 6 inches) of each kind of surface condition were then cleaned, before being bonded with one of the three bonding
processes, A, B, or C, by methods that were the same as methods A, B (2),
C (2), D (2), and E (1) in part I, except that a new method, H, was added.
In method H the surface was not cleaned prior to bonding, and when the clad
24S-T3 sheet was bonded as received, the bonding was done on the lettered
side.
The rinsing, drying, and general precautions taken in part I of this
study were also used in cleaning specimens for part II.
Testing
Two bonded panels, providing a total of 10 specimens, were usually
prepared for each surface condition, cleaning method, and bonding process.
Because of equipment limitations a series of bonding runs was made on different days in the same manner as in part I of this study, and on each day
Report No. 1813
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a control: group Of specimens was prepared from clad 24S-T3 aluminum alloy
sheets, as received, that were washed with benzene and then cleaned with
sulfuric-acid-sodium dichromate.
The procedure for testing was as described in part I.
Results
In table I are given the results and the types of failures obtained
in tests of the clad 24S-T3 aluminum-alloy joints cleaned by the 22 cleaning methods and bonded with the 4 gluing processes described in part I.
The pH of the fresh solutions used in cleaning the clad 245-13 sheets, the
appearance of the sheet surfaces after cleaning, and the appearance of the
water film obtained when water was rinsed over the sheet surface after
cleaning, are also given.
Of the 22 cleaning processes investigated for cleaning aluminum, the
sulfuric acid-sodium dichromate cleaning method, E (1), generally resulted
in joints having the highest strengths. This cleaning method was included
as a control in each of the several series of bonding runs that were found
necessary in completing the evaluation of all of the cleaning methods used
with each of the bonding processes. The series numbers for each bonding
process are designated in table 1. The average shear strength for several
series of 1-inch lap joints cleaned with the sulfuric acid-sodium dichromate process was 3,300 pounds per square inch for bonding process A,
2,940 for bonding process B, 2,690 for bonding process C, and 2,590 for
bonding process D. If the results with each bonding process on clad 24S-T3
sheets cleaned by method E (1) may be considered as a standard, the relative
joint quality may be expressed, for each of the same adhesives on metal
cleaned by the other methods, as a percentage of the value obtained for
method E (1) in this same bonding series. Results are expressed in this
manner in table 2.
When no treatment other than a wash with benzene was used in preparing the surfaces of the clad 24S-T3 sheets as received, bonding processes B,
C, and D gave joint strengths that exceeded 2,000 pounds per square inch.
These strengths were approximately 70 to 90 percent of the strengths
Obtained when the sheets were cleaned with the sulfuric acid-sodium
dichromate method, E (1). When the metal was cleaned with a benzene wash
only and was then bonded with process A, an average joint strength of
1,632 pounds per square inch was obtained, which was somewhat lower than
that obtained with the other three bonding processes, and which was approximately 50 percent of the strength obtained when using this bonding process
to bond clad 24S-T3 sheets cleaned with sulfuric acid and sodium dichromate.
Since it is of interest to know, for each adhesive process, how much better
or poorer results may be with other cleaning methods when compared with
this simple benzene-wash method (A), these comparisons are presented in
table 3 as percentages of the average joint strength obtained when the
sheets were cleaned by merely washing with benzene.
Report No. 1813
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When clad 24S-T3 sheets were benzene-washed and then abraded by sandblasting, rubbing with aluminum wool, or rubbing with abrasive, cloth, the
joints bonded with processes A and D were somewhat stronger than those
cleaned only with benzene; whereas no significant differences between these
methods of cleaning were noted with the other tv.m bonding processes.
Although joints prepared from the sheets cleaned by these mechanical abrasion methods were not so good as those cleaned,by the sulfuric acid-sodium
dichromate method or by some of the other acid and combinational methods,
they were often superior to joints in which the sheets were cleaned by
some of the alkaline or solvent cleaning methods.
Dipping the clad 24S-T3 sheets in a dilute solution of the adhesive
after, the benzene wash and before bonding (C(1)), as was recommended by one
adhesive manufacturer, was found to have no significant advantage over the
benzene wash, and for three of the bonding methods (B, C, and D) lower bond
strengths resulted.
Vapor degreasing with stabilized trichloroethylene (C(2)) had no
marked advantage over the benzene wash with, bonding processes A and B, had
some moderate advantage with bonding , process C, and resulted in weaker
joints with bonding process D. Note: Good cleaning, of metals, for bonding
has been reported using vapor degreasing methods. The inconsistent results
obtained in this study might be attributed to the small size and lower
efficiency of the vapor degreasing unit used in this investigationWhen clad 24S-T3 sheets cleaned by the two solvent cleaning methods, C(1)
and,C(2), were tested by allowing water to run over the dry surface, waterfilm breaks were obtained that indicated imperfect cleaning.
The six alkaline cleaning.solutions used in preparing clad 24S-T3
aluminum-alloy sheets for bending gave widely varying results with the
four bonding processes. Of those investigated, immersion in a solution of
sodium metasilicate (D(2)) or in a soap conforming to Navy Specification.
C-152 (D(6)) prior to bonding with process B i C I or D gave an improvement
over the results obtained when the surfaces were merely washed with benzene,
and in several instances the joint quality was equal to that obtained when
sulfuric acid-sodium dichromate cleaning (E(1)) was used. In , bonding with
process A, scrubbing with borax powder (D(1)) or immersion in sodium
metasilicate solution (D(2)) gave some improvement in Joint strength over
that obtained by washing, with benzene, but joints of clad 24S-T3 sheets
h
cleaned, with the soap, Navy Specification C-152 (D( 6)), were poor, although.
results with sheets, cleaned in this manner and bonded with the other three
bonding processes were good. Two of the more alkaline cleaning methods
D(4) and D(5) gave consistently poor results with all four bonding
processes. The cleaning, process D-4 was found to result in a loose black
film deposit after rinsing in water. High pH values are apparently not
directly correlated with poor cleaning behavior, however, since the Navy
soap in method D(6) had the highest pH measured and, yet gave good results
with three of the four bonding processes. The alkaline detergent cleaner I
(D(3)) resulted in low-quality joints when bonding with process A, and with
the other three bonding processes it gave strengths approximately the same
Report NO. 1813
as those obtained on surfaces that were merely washed with benzene. It is
worth noting, however, that joints made with . cleaning method G(3), in which
treatment with the alkaline cleaner I (D(3)) was followed by rinsing in
water and by an acid treatment (E(6)), were consistently high in strength.
This improvement, after an acid rinse, was also noted with bonding process
A and cleaning method G(2), in which chromic acid treatment followed the
sodium metasilicate bath.
Of the acid cleaning methods investigated, several gave very good
results with certain bonding processes. For adhesive A, both the deoxidizing cleaner L (E(5)) and hydrofluosilicic acid (E(6)) solutions gave
results nearly equivalent to the average results obtained with sulfuric
acid-sodium dichromate solution (E(1)); and phosphoric acid (E(3)) gave
results equal to those obtained with abrasive cleaning methods and better
than those obtained with any of the solvent or alkaline cleaning methods
investigated. For adhesive B, hydrofluosilicic (E(6)) or phosphoricchromic acid (E(4)) cleaning resulted in joint strengths that were nearly
equivalent to those obtained with the sulfuric acid-sodium dichromate
method or with any of the better alkaline cleaning methods, and phosphoric
acid (E(3)) and deoxidizing cleaner L (E(5)) gave only slightly lower
strengths. In both bonding processes C and D, phosphoric acid cleaning
(E(3)) gave an improvement in joint strength over the strengths obtained
with benzene washing (A), and process C gave equally good results with the
chromic acid method E(2). In these cases the strengths were nearly the
same as that obtained with sulfuric acid-sodium dichromate cleaning.
Generally, chromic acid (E(2)) and phosphoric-chromic acid (E(4)) were the
least effective of the acid treatments.
Electrolytic cleaning was tigated3 and was found to result in
low and inconsistent joint strengths, except for bonding process D, in
which results were equal to those obtained when the clad 24S-T3 sheets
were cleaned by the sulfuric acid-sodium dichromate method E(1).
Several combinational cleaning methods were also investigated in
which an alkaline cleaning was followed by cleaning in an acid solution.
The combinational methods used with each of the four bonding processes
generally gave joint strengths that were essentially equal to or only
slightly lower than the strength of joints cleaned with sulfuric acid and
sodium dichromate (E(1)). Generally, the combination of two cleaning
methods gave better results than either method used alone.
In tables 4, 5, and 6 are given the shear-test results and the types
of failures obtained in part II on joints made to clad 24S-T3 aluminumalloy sheets having three types of surface condition, the normal surface as
received, a surface contaminated with paraffin and oil, and a surface contaminated with metasilicate stain. Six types of cleaning are compared for
preparing these surfaces for bonding with three processes. Series numbers
of the bonding runs made on different days are given as in table 1.
3
–The investigation of electrolytic cleaning was limited to the use of one
cleaning method. The use of other electrolytic cleaning methods, or a
variation in the conditions of the method used, might give improved
results.
Report No. 1813 -13-
Generally, the quality of joints on clad 24S-T5 sheets, as received
and then cleaned by the various methods, aeQd quite closely with those
results obtained under the same Obnditions in part I (table 1). When
joints.were prepared with clad 24S -T3 sheets as received and the bonding
,was done tbY the side of the sheet having identification printing, good results were obtained with process C'(2,380 p.s.i.) and with process B
(1,954 p.s.i.). Process A showed poor bonding to this surface. In
general, throughout the work of part II, process A was also found to be
somewhat more exacting as to . the method of surface preparation than the
other two bonding processes.:
The surfaces having metasilicate stain were found to be the most
difficult to clean and bond properly. Acid cleaning with sulfuric acidsodium dichromate solution E(1) gave the best cleaning of this type of
surface, and the joint s strengths obtained closely approached those obtained
when this cleaning method was used in preparing clad 24S-T5 sheets, as
received, for bonding. The use of a benzene wash followed by abrasion with
aluminum wool was found to give good results in cleaning the sodiUmmetasilicate-stained metal, but it was not quite so efficient as the use of
sulfuric acid-sodium dichromate cleaning.
The sulfuric acid-sodium dichromate cleaning following a benzene
wash was found to 'be the most consistently effective method of preparing
clad 24S-T5 aluminum-alloy sheet surfaces contaminated with an oilparaffin film for bonding with any of the three bonding processes investigated. Sodium metasilicate also produced reasonably efficient cleaning of
this type of surface for bonding with processes B or C, but poor bonding
was obtained when the clad 248-T5 sheets were cleaned by this method for
bonding with process A. Good results were also obtained when surfaces
having oil-paraffin contamination were cleaned by washing with benzene and
abrading with aluminum wool (D(2)) and were then bonded with adhesives A
or B. The use of a benzene wash alone or of a vapor-degreasing process was
found, in some instances, to improve the bonding of the clad 24S-T5 sheets
having oil-paraffin contamination more than the bonding obtained when these
contaminated surfaces were not cleaned at all. It was noted throughout
that results were often so
this study, however, in both parts I and
inconsistent when using these solvent methods that it was difficult to
predict whether or not solvent treatment would improve the bonding properties of a surface.
The conclusions on the effectiveness of various cleaning methods
used in parts I and II were based on the results obtained in these present
tests with cleaning methods used in the manner described under the test
procedures. If the concentration or temperature of the solution, or the
time of immersion or degree of treatment were changed, some of the
results might be expected to vary from those reported.
Report No. 1813
-14-
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•
Table 2.--Comparison of shear strengths of lap shear specimens when clad
aluminum-alloy sheets were cleaned by various methods and
when cleaned by the sulfuric acid-sodium dichromate process E (1)
:Percentage of control strength obtained?
Cleaning method-.
: Bonding : Bonding : Bonding : Bonding
:process A:process B:process C:process D
A. Benzene wash, no other treatment...:
48
••
B. Abrasion cleaning
(1) Sandblasting (2) Aluminum wool (3) Iron-free abrasive cloth...
72
67
;
:
73
:
•
C. Solvent cleaning
:
(1) Five percent solution of adhe- : sive in solvent •
(2) Vapor degreasing :
:
D. Alkaline cleaning
(1) Borax powder. •
(2) Sodium metasilicate (3) Detergent cleaner I (4) Alkaline cleaner J :
.
(5) Sodium hydroxide
(6) Soap K, Navy Specification C-152:
E. Acid cleaning
:
(1) Sulfuric acid-sodium dichromate.: (2) Chromic acid (3) Phosphoric acid . :
(4) Phosphoric and chromic acid •
(5) Deoxidizing cleaner.l. (6) Hydrofluosilicic acid.... F. Electrolytic cleaning .. :
G. Combinations of cleaning
:
(1) D (1) (borax), D (2) (sodium
•.
metasilicate), and
E (2) (chromic acid) :
(2) D (2) (sodium metasilicate) and : E (2) (chromic acid) :
(3) D (3) (detergent solution I)
and E (2) (chromic acid) :
•
68
80
• 65
66
.•
..
93
:71
91
82
:
:
• • .
98
97
:
:
80 :
69
48
55
:
:
37
70
102
:
52
67
:
:
85
53
31
13
:
:
70
102
77
1
64
:
98
90
21
:
:
:
8
88
65
77
:
:
100
91
98
.
100
62
.
66
74
77
64
.
•
:
96
53
81
69
40 :
21
:
.
:
100 50
72
:
41 :
82
•.
100
•.
75..
81
.
92 •
83•.
88
94
47
48
:
:
.•
•.
•.
92
93
:
:
:
•
'
:
102
102
:
:
90
89
75
4
63
82
100
72
91
98
:
95
:
82
88
:
97
1The metal surfaces were washed with benzene before being cleaned with each of
the other cleaning methods. The solution temperatures and concentrations,
and the procedures involved in each of the cleaning methods, are described
in the text of the report. It should be noted that both concentration and
temperature variations between different solutions of one type make direct
comparison of components tnadvisable.
?In this table, control strength is the average strength of joints prepared at
the same time and with the same bonding processes to clad 24S-T3 sheets
cleaned by method E (1) (sulfuric acid and sodium dichromate).
Rept. No. 1813
Table 3.--Comparison of shear strengths of lap shear specimens when clad
243-T3 aluminum-alloy sheets were cleaned by various methods
and when cleaned by a simple washing in benzene (Process A)
:Percentage of control strength obtained?
:
.
: Bonding : Bonding : Bonding : Bonding
:process A:process B:process C:process D
Cleaning method 1-
•
A. Benzene wash, no other treatment
B. Abrasion cleaning
(1) Sandblasting 40 0000 (2) Aluminum wool
oo (3) Iron-free abrasive cloth... :
C. Solvent cleaning
(1) Five percent solution of
adhesive in solvent (2) Vapor degreasing.
......
D. Alkaline cleaning
(1) Borax powder •
142
(2) Sodium metasilicate 122
... :
(3) Detergent cleaner I. (4) Alkaline cleaner J (5) Sodium hydroxide •
(6) Soap IC, Navy Specification C-152:
E. Acid cleaning
•
(1) Sulfuric acid-sodium dichromate.:
(2) Chromic acid. (3) Phosphoric acid •
(4) Phosphoric and chromic acid (5) Deoxidizing cleaner L.....
(6) Hydrofluosilicic acid 187
F. Electrolytic cleaning. G. Combinations of cleaning
•
(1) D(1) (borax), D(2) (sodium
•
metasilicate), and
E(2) chromic acid) ..;
(2) D(2) (Sodium metasilicate), and
E(2) (chromic acid) (3) D(3)(Detergent solution I), and
E(2) (chromic acid)....... • •• 0:
100
:
100
151
139
•
152
117
94.
:
108
96
106
146
109
2
72
29
87
36
2147
93
183
116
139
125
143
109
155
172
109
114
93
5
70
142
3128
76
134
67
110
77
74
90
101
86
67
73
1 68
88
64
2108
102
106
..
125
124
109
100
21
66
98
: 100
87 :
112 :
70
12
133
2C0
100
90
88
53 :
99
:
115
80
:
137
:
119
•.
110
122
:
124
100
125
•.
.
:
1 73
136
:
123
•.
97
2The metal surfaces were washed with benzene before being cleaned with each of
the cleaning methods. The solution temperatures and concentrations, and the
procedures involved in each of the cleaning methods, are described in the
text of the report. It should be noted that both concentration and temperature variations between different solutions of one type make direct comparison of components inadvisable.
•
?In this table, control strength is the average strength obtained when metal
was cleaned only by a benzene washing process (method A).
-Based. on average of all values shown in table 1 for method E(1) for that
adhesive.
Rept. No. 1813
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Figure 1.--Test machine and grips used in applying
tension loading on bonded lap-joint specimens of
0.064-inch clad 24S-15 aluminum-alloy sheets.
(zM 83567 F)
Report No. 1813
,Z N S1%7 F