Note: Descriptions are shown in the official language in which they were submitted.
7~
This invention relates to methods
of preparing environmental:Ly stable bonded
aluminum structure and more particularly relates
to methods of preparing bonded aluminum structures
in which the aluminum surface is rendered
especially well adapted to receive the adhesive
resin and is resistant to subsequent delamination
and failure of the adhesive bond at the adhesive
resin-aluminum interface.
The structural bonding of metal to
metal and composite
.~ ~
`; ~
~ 8~7a8
1 ¦type assembly widely used in the aircraft industry and elsewhere
2 ¦frequently require a resultant structure which is reasonably re-
3 ¦sistant to the extremes of atmospheric conditions found in use. For
4 ¦example, in aircraft construction the wing structure utilized in
5 ¦manufacture of large passenger, cargo, and military aircraft, utili-
6 ¦zes adhesively bonded structures which are subjected to extremes of
7 ¦temperature varying from substantially below zero Farenheit in
8 ¦ Arctic areas to temperatures in excess of 150F. in tropical areas
~ ¦ when the aircraft must be exposed to the tropical sun. Aircraft are
10 ¦ also exposed to marine environments and other highly corrosive at-
11 ¦ mospheres. To avoid failures o~ the aircraft structures as well as
12 ¦ to meet the stringent requirements of the military aircraft
13 ¦ standards and the standards established by the aircraft industry for
1~ commercial passenger and cargo aircraft, bonded metal to metal and
composite type assemblies must be able to withstand the environment-
1~ al conditions to be encountered. Of particular importance is re-
17 sistance to corrosion and delamination of composite structures
18 1 occasioned by hu~mid warm environments which attack prior art
19 ¦ materials. Heretofore, the adhesively bonded metal-to-metal and
20 ¦ composite type assemblies have performed less than satisfactorily
21 ¦ due to adhesive failure at the interface beteen the polymeric
22 ¦ adhesive and the aluminum surface, frequently necessitating field
23 ¦ repairs and occasionally removal of the aircraft from service so
24 j that extensive repairs may be undertaken.
25 1 It is welI known that aluminum or aluminum alloy surfaces
26 ¦ exhibit unpredictable and unreliable adherence to bonding media
27 ¦ particularly in moist and salt laden atmospheres. It has been pro-
28 ¦ posed to increase adherence of surface coating such as electroplated
29 ¦ metal on aluminum base by means of an anodic treatment in an acid
30 ¦ bath and then dissolving a portion of the oxide film in an acid
~ 7~
1 ¦or alkaline bath prior to electroplating. See U.S. patent
2 ¦1,971,761. It has also been proposed to electroplate directly over
3 ¦an oxide film produced by anodizing aluminum or aluminum alloys in
4 ¦chromic acid or phosphoric acid solution without intermediate
5 ¦treatment of the oxide film such as is taught in U.S. patents
6 ¦1,947,981 2,036,962 and 2,095,519. In each of the above-noted
7 ¦patents the aluminum surface is being prepared for electroplating.
8 ¦ Similarly, it has been proposed in U.S. Patent 3,672,972
9 ¦ to form anodic coatings having improved adhesive properties on alum-
10 ¦ inum surfaces by depositing coatings on the aluminum substrate by
11 ¦ subjecting the aluminum article to electrolytic treatment in an
12 ¦ aqueous solution of various acids such as phosphoric acid, oxalic
13 ¦ acid, sulphuric acid, malonic acid and the like at elevated tem-
14 ¦ peratures for a very short treatment period. Similarly, it is known
15 ¦ to treat oxides already formed on an aluminum surface by other means
1~ ¦ with a phosphate bath electrolysis to render the oxide surface hy-
17 ¦ dration resistant. The elevated temperature phosphoric acid anod-
18 1 ization process results in the deposition of an oxide surface
19 ¦ characterized as "pseudoboehmite," a highly active form of aluminum
20 ¦ oxide deposited in a very thin, nonporous and uniform layer on an
21 aluminum surface. The characteristics o~ this form of aluminum
22 ¦ oxide apparently permit failure within the oxide structure when high
23 ¦ stressed under humid conditions. In addition, lag time after ces-
24 ¦ sation of anodizing current encountered in commercial processing of
2~ ¦ aluminum surfaces at the elevated temperatures t95-122F.) of this
26 ¦ patent cause dissolution of the aluminum surface by the phosphoric
27 ¦ acid electrolyte. Poor bonding results wherever the aluminum
28 ¦ surface is excessively dissolved.
29 1--
30 1--
7~
This invention is directe~d at preparing adhesively
bonded aluminum or aluminum alloy structures wherein the adhesive-
aluminum interface exhibits environmental stability in an aqueous
environment.
This invention provides a method of forming adhesively
bonded aluminum composite type structures in which adhesive failures
at the aluminum-adhesive interface are minimized.
This invention provides an adhesively bonded aluminum
structure wherein the aluminum surface is subjected to a low temper-
ature anodic electrolysis in a dilute phosphoric acid bath under
conditions which enhance the formation of an anodic porous coher-
ent oxide while minimizing or controlling the dissolution of alu-
minum oxide from the surface after termination of the anodization
current.
Furthermore, this invention provides a process for
phosphoric acid anodization of aluminum surfaces at low temper-
atures and under conditions which form oxide coatings of 500 to : :
8,000 Angstroms in thickness and having a porous structure wherein
the pores have a diameter in the range of 300 to 1,000 Angstroms
and a depth of about 400 to 7,500 Angstroms extending into the
film and wherein the aluminum film is not removed by dissolution
in the phosphor:ic acid electrolyte during the necessary lag time
before removal of the electrolyte by rinsing.
~:i
One specific object of this invention is to provide an anodization
process which may be used to prepare aluminum alloys containing copper for
bonding into a structure which is environmentally stable.
The present invention contemplates the formation of an environ-
mentally stable, porous oxide coating on the surface of an aluminum object
which is well-suited to adhesion by known polymeric adhesives and resulting
in an adhesively bonded structure which, UpOII exposure to severe environ-
ments, resists hydration and thus maintains its structural integrity. When
highly stressed under severe test conditions, the resultant structure pre-
dominantly exhibits cohesive failure within the adhesive layer rather than
adhesive failure within the oxide coating or at the adhesive-oxide interface.
The aluminum is prepared by a surface treatment to form a porous anodic
oxide coating using a phosphoric acid electrolyte maintained at a temper-
ature in the range of 50 to ~5F., and preferably from 65 to 80F., while
imposing a potential of from about 1 to about 50 volts for a period of about
lO to 30 minutes.
According to the present invention, there is provided a method
of forming a porous columnar aluminum oxide coating on an aluminum alloy ar-
ticle, said aluminum alloy article containing about 1.6 to about 4.5% by
20 weight copper, comprising anodizing said article in an aqueous solution com-
prising phosphoric acid, the anodizing potential being from about 1.0 to
about 50 volts, the phosphoric acid concentration being about 1.5 to about
50% by weight and the temperature of said solution being from about 50F.
to about 85F. and rinsing said article to remove said solution within a
period of time no more than 2.5 minutes from cessation of anodizing current.
In another aspect, the invention provides a process for preparing
a copper-containing aluminum alloy adherendfor adhesive bonding to another
surface wherein said aclherend has a polymer receptive aluminum oxide sur-
face thereon having a porous columnar structure with a thickness of from about
30 500 to about 8,000 Angstroms, having pores from about 300 to about 1,000
Angstroms in diameter cmd from about 400 to about 7,500 Angstroms in depth
extending into said oxide surface, said surface prepared by the steps of:
,fA. i
38
cleaning and deoxidizing said adherend;
anodizing said adherend in an aqueous phosphoric acid solution
containing from about 1.5 to about 50% by weight H3P04 at a temperature of
from about 50 to about ~5F~ for about 10 to about 30 minutes at a potential
of about 1.0 to about 50 volts; and
rinsing said adherend to remove said aqueous pllosphoric acid
solution within a period of time no more than 2.5 minutes from cessation of
anodizing current.
In yet another aspect, the inventin provides an aluminum alloy
adherend containing from about 1.6 to about 4.5% by weight copper and having
a polymer receptive aluminum oxide surface thereon having a porous columnar
structure with a thickness of from about 500 to about 8,000 Angstroms, having
pores from about 300 to about 1,000 Angstroms in diameter and from about
400 to about 7,500 Angstroms in depth extending into said oxide surface,
said surface prepared by anodizing the surface of said adherend in an
aqueous phosphoric acid solution containing from about 1.5 to about 50% by
weight H3P04 at a temperature of from about 50Fto about S5F. for about
10 to about 30 minutes at a potential of from about 1.0 to about 50 volts
and then rinsing said adherend to remove said solution with a period of
time no more than 2.5 minutes from cessation of anodizing current.
The above-noted processing parameters for the anodizing step are
suitable for anodization of both aluminum alloys and relatively pure alu-
minum metal commonly employed in adhesive bonded structures. Various alloys
and nearly pure aluminum have been processed at the same time using the
following parameters for the process:
Temperature: 70 - 75F.
Anodization potential: 10 - 15 volts
H3P04 concentration: 10 - 12 ~
Anodization time: 20 - 25 minutes
Lag time before rinse: 1-1/2 to 2--1/2 minutes
Part to solution potential: 4 - 12 volts
_ 5a -
7~
As is noted above, the processing parameters described herein
produce an adherent porous aluminum oxide coating securely bonded to a
barrier layer of aluminum oxide which in turn is tightly adhered to the
aluminum metal surface. Attempts at producing aluminum oxide coatings
suitable for bondings at temperatures below about 50F. resulted in small
diameter pore structure or no observable pores at all in the surface of the
aluminum oxide. As a consequence, poor bonding results were obtained by
comparison to the adhesive bonds obtained upon aluminum oxide coated
substrates prepared in H3P04 electrolytes maintained at temperatures of
50-85F.
Temperatures above about 85F. cause increasingly detrimental
dissolution of the oxide coating by the phosphoric acid electrolyte,
especially during the time period from cessation of the anodization current
flow until the phosphoric acid is rinsed off the aluminum part with water.
In commercial processes, lag times of 1-1/2 to 2-1/2 minutes are usually
unavoidable and, as a consequence, the electrolyte bath temperature must
be kept at a level which minimizes dissolution of aluminum oxide, yet
permits formation of the essential porous structure.
Anodizing under the conditions disclosed herein consistently
produces a surface superior in performance to that produced by conventional
industry standard methods, such as chromic acid anodizing or sulphuric
acid-sodium dichromate etch. This superior performance is clearly demon-
strated by the bond stability test shown in Figure 7 while exposing the
speci~en to different water and salt environments. Conventionally processed
7075-T6 aluminum clad bondements prepared by chromic acid anodization fail
at the oxide-primer interface within two to three days when exposed under
stress to hot humid conditions. The same alloy, phosphoric acid anodized
prior to bonding under the preferred anodization parameters set forth below
~,~
7~8
1 ¦ does not show any evidence of interfacial failure after exposure t
2 ¦ the same environmental conditions for more than 7 months. Typical
3 ¦ failure modes of specimens prepared with the production parameters
4 ¦ noted above are cohesive, i.e., the specimens fail in the adhesive
5 ¦ zone rather than at the adhesive-metal interface. Thus, interfacial
6 ¦ failure modes which typify service failures are eliminated or at
7 ¦ least minimized with this method of aluminum prebond phosphoric
8 1 acid anodization surface preparation.
9 ¦ Hydration resistance of oxides formed by anodization i
10 ¦ phosphoric acid appear to be a significant factor associated with th
1~ ¦ improvement in bond stability and their low reactivity to water. Th
12 ¦ applicants postulate that bonds of aluminum to adhesives which ar
13 ¦ exposed to water and then torn apart at the adhesive-metal interfac
14 ¦ are in reality cohesive failures within the oxide suggesting tha
15 ¦ most bond failures exhibiting adhesive failure after exposure to
16 ~ater are due to weakening in the oxide by hydration. The applicants
17 further postulate that the failure mechanism associated with adhesive
18 appearing failures of bonded structure are due to weakening of the
19 oxide by hydration resulting in delamination when the bond is
20 ¦ stressed. Once delamination occurs, corrosion can then take place in
21 ¦ the delaminated area causing additional damage to the bonded
22 ¦ structure. The applicants have found that phosphoric acid
23 ¦ anodization of the surface of aluminum metal and alloys using
24 ¦ relatively low temperatures and dilute phosphoric acid electrolytes
25 ¦ provides a hydration resistant oxide coating well adapted to prevent
26 ¦ delamination and subsequent corrosion.
27 ¦ The rnost significant aspects of low voltage, low tempera-
28 ¦ ture anodization in phosphoric acid of aluminum surfaces prior to
29 ¦ adhesive bonding are that the process provides positive control of
30 ¦ the oxide formation and therefore high reliability, thus producing a
~ 37a8
1 ¦porous oxide with desirable physical characteristics and which i
2 ¦more stable in the presence of water than are other anodically formec
3 lor deposited oxides including phosphoric anodized coatings producec
4 ¦at elevated temperatures. The process provides a range of anodizinc
5 ¦conditions in which both relatively pure aluminum metal and aluminun
¦alloys commonly used for bonding can be anodized, i.e., 2024-T3
7 ¦aluminum alloy and 7075-T6 aluminum alloy as well as those alloys of c
8 ¦ higher aluminum content. The process is also well suited to treat-
9 ¦ ment of clad aluminum material.
lO ¦ Temperatures in excess of about 85F. in solutions of
11 ¦ phosphoric acid cause the dissolution rate of the oxide layer to
12 ¦ approach or exceed the rate at which it is formed so that the oxide
13 ¦ surface is removed, especially following termination of the
1~ 1 anodization current. In the commercial production of phosphoric
1~ ¦ anodized aluminum surfaces, it is necessary to have a process which
16 accommodates lag time of up to approximately 2 to 2-1/2 minutes from
17 the time the power supply is turned~ off until the parts can be
IB removed from the phosphoric acid bath and rinsed to remove the phos-
19 phoric acid. During this time, the dissolution of oxide surface at
elevated temperatures becomes excessive and it is, therefore, neces-
21 sary to maintain the temperature below 85F. and usually in the
22 range of 65 - 80F. in order to obtain the desired results.
23 Attempts to produced the anodized aluminum parts in phosphoric acid
24 at temperatures exceeding 85F. results in erratic oxide coating and
frequent failure in the resultant adhesively bonded structure. Sub-
26 stantial amounts of aluminum present in the phosphoric acid electro-
27 lyte solution, may cause a deposition of aluminum oxide in another
28 form such as that designated "pseudoboehmite" in U.S. Patent No.
29 3,672,972 and U.S.Patent No. 3,714,001. The conditions under which
this "pseudoboehmite" deposition occurs and under which the
~ ~
1 applicants' discovery of the environmentally stable oxide film
2 formed under the conditions taught herein varies with the tempera-
3 ture, acid concentration and aluminum concentration. Generally,
4 phosphoric acid anodization at temperatures above about 95F. ac-
~ cording to the teaching of U.S. Patents 3,672,972 and 3,71~,001
6 result in the deposition of "pseudoboehmite." Such temperatures
7 also result in undue amount of dissolution of porous aluminum oxide,
8 rendering such prior art processes unworkable for the applicants'
9 intended purpose. It is essential to hold the temperatures below
lO ~about 85F. to obtain consistent and reproduceable aluminum oxide
11 ¦surfaces described herein, At lower temperatures substantial quan-
12 ¦tities of aluminum may be present in the phosphoric acid electrolyte
13 ¦without causing deposition of "pseudoboehmite" and any aluminum
14 ¦oxide film is not in the form of "pseudoboehmite" but rather the
15 ¦porous, hydration resistant structure suitable for laminating alu-
16 ¦minum articles together by adhesive bonding. Under the processing
17 ¦conditions set forth below a columnar-type closely adherent aluminum
~18 ¦ oxide film is formed by oxidation of the surface of the aluminum or
19 ¦aluminum alloy. This film has a thickness varying from 500 to 8rO00
2~ ¦ Angstroms with pores having a diameter in the range of 300 to 1,000
21 Angstroms and a depth of about ~00 to 7,500 Angstroms extending into
22 the film. These pores provide many additional locations for bonding
23 by providing more surface area and a mechanical interlock between
24 the adhesive and the aluminum oxide.
The above-noted objectives of this invention and the
26 features discussed briefly in the summary of this invention will
27 become more readily apparent from a detailed examination of the fol-
28 lowing discussion of the preferred embodiments with reference to the
29 attached drawings and tabular data.
__
11187~)~
¦ 1 ¦ BRIEF DESCRIPTION OF TllE DRAWI~GS
q ¦ FIGURE l is a schematic flow diagram of two widely used
3 ¦prior art processes for prepar.ing aluminum s~rfaces for adhesive
4 ¦ bonding;
5 ¦ FIGURE 2 is a schematic flow diagram of the process oE
6 ¦ this invention;
¦ FIGURE 3 is a graph showing sustained stress lap shear
8 ¦ test data for bonded structures prepared by one process of FIGURE l
9 ¦ as compared to bonded structures prepared by the process of
10 ¦ FIGURE 2;
11 ¦ FIGURE 4 is a graph similar to FIGURE 3 for tests
12 ¦ conducted at a lower temperature;
13 ¦ FIGURE 5 is a graph showing crack propagation data for
1~ ¦ various bonded structures treated by prior art processes and by the
15 ¦ process of this invention;
16 FIGURE 6 is a graphical representation oE test results of
t7 varia~us bonded structures using prior art surface treatments and
18 variations in the ~rocess taught herein;
19 FIGURE 7 is a schematic representation of the crack pro-
pagation test utilized in evaluating the laminates formed using the
21 process of this invention;
22 1 FIGURE 8 is a graphical representation of the sustained
23 ¦ stress lap shear test used in evaluating the bonded structures
24 ¦ formed according to this invention;
2~ ¦ FIGURE 9 is a graphical representation of test results
~6 ¦ comparing sustained stress lap shear test data for various methods
27 ¦ of pretreatment of the aluminum surface and the resultant effect on
28 ¦ adhesive versus cohesive failure;
29 1 DETAILED DESCRIPTION OF THE DRAWINGS
I , ,_
30 1 Re~erring specifically to FIGURE l, two of the well known
~ 8
1 prior art processes are set forth in a step-by-step ~ashion in which
2 aluminum material as received is first sub~ected to a degreasing and
3 cleaning process in preparation for the surface treatment. The al-
4 kaline cleaner utilized is removed in a hot water rinse and the
surface is then deoxidized by exposure to a suitable etchant such as
6 sodium dichromate-sulphuric acid deoxidizer. One widely used
7 deoxidizer or etchant for aluminum is sold by Amchem Products, Inc.,
` 8 Ambler, Pennsylvania, under the trade name "Amchem No.6-16" to which
9 nitric acid is added. A suitable etchant for aluminum at room tem-
10 ¦perature has the following composition: 4 to 9 percent by volume
11 ¦Amchem No. 6, 10 to 20 oz/gal nitric acid in an aqueous solution.
12 ¦ The aluminum is subjected to the above-noted solution at
13 165O to 90F. for a period of time sufficient to deoxidize the surface
14 ¦of the aluminum.
15 ¦ In the event that the aluminum as received is reasonably
1~ ¦elean and has a thin adherent oxide coating, the above-noted steps
17 ¦may be unnecessary prior to the anodization step.
18 ¦ After the surface has been deoxidized, if necessary, the
19 ¦surface is rinsed with cold water and then subjected to an acid
20 ¦anodization step utilizing chromic acid as the electrolyte. The
21 ¦ chromie acid is suitably of a concentration of about 5~ by weight
22 ¦ chromic acid in water.
~3 ¦ The aluminum surface is subjected to the anodization at
24 ¦ 95F. with an applied voltage in the range of 40 volts for a period
25 ¦ of time sufficient to form an oxide coating of about 20t000 to about
26 ¦ 30,000 Angstom thickness. The chromic acid is rinsed from the sur-
27 ¦ face of the aluminum and the aluminum surface is dried in prepara-
28 ¦ tion for the application of the adhesive materials.
29 1 __
30 1 __
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1 Similarly, when the sulfuric acid-sodium dichromate etc~
2 process is elected, an aqueous solution containing about 4.1 tc
3 about 12 ounces of sodium dichromate dihydrate per gallon oE
4 solution and about 38.5 to 41.5 ounces H2SO~ per gallon is used. The
etching process takes place at a temperature of about 140F. to
6 160F.
7 A suitable epoxy or other primer is used such as a cor-
8 rosion inhibiting epoxy primer designated as BR127 manufactured and
9 sold by American Cyanamide Corporation. This epoxy primer is a
250F. cure epoxy resin suitable as a corrosion inhibiting primer
11 for bare metal surfaces.
12 ¦ An adhesive material such as a modified epoxy resin having
13 ¦ suitable curing characteristics is then applied to the primed alu-
14 ¦ minum surface. Several modiEied epoxy resins are readily available
and are suitable for use in this invention including a product de-
16 signated at FM123-2 manufactured and sold by the Bloomingdale
17 Division of American Cyanamide; a product designated as AF126 mod-
~1~ ified epoxy resin having a 250F. cure manufactured and sold by
19 Minnesota Mining and Manufacturing Corporation and the modified
epoxy adhesive designated as Hysol 9628 manufacuted and sold by
21 Hysol Division of Dexter Corporation. Many other resins are work-
22 able as adhesives for this invention. The primed and taped aluminum
~3 surfaces are then placed into engagement under pressure and cured at
24 an elevated temperature to effect the joint or bond between the
surfaces.
26 FIGURE 2 shows a flow diagram of the process of this in-
27 vention in which aluminum as received is subjected to similar clean-
28 ing and deoxidizing steps as those outlined above for FIGURE 1 if
29 they are found to be necessary due to the condition of the aluminum
surface. When the preliminary cleaning steps are completed, the
~,~,J. ~q r /; --12 -
37~8
aluminum surface is subjected to a low temperature anodization process in a
solution of phosphoric acid, removed from the H3P04 electrolyte and rinsed
with water within one to two-and-one-half minutes of the time the power
supply is turned off and dried. The following process par~meters have been
found to give exemplary results in the performance of the resulting bonded
laminate in use:
TABLE I
Power Supply
Temperature Potential Time H3PO4
F (Volts~ (h~in.)Concentration
Usable
range 50-85 1-50 5-60 1.5-50%
Preferred
range 65-80 3-25 10-30 3-20%
~lost
preferred
range 70-75 10-15 20-25 10-12%
Anodizing under the conditions set forth above consistently
produces a surface superior in performance to that produced by conventional
industry standard methods, i.e., chromic acid anodizing or sulfuric acid-
sodium dichromate etch. This superior performance is clearly demonstrated
by the bond stability test technique shown in Figures 7 and 8, and the
resulting test data presented in Figures 3, 4, 5, 6 and 9.
EXAMPLE I
Comparative data for the aluminum surface preparation techniques
shown in Figures 1 and 2 are presented in Figure 3 for various epoxy resin
adhesives used in preparation of a composite structure. All samples were
prepared by cleaning as follows prior to anodizing:
(1) The surface was vapor degreased by exposure to
trichloroethylene for 3 minutes at 190F.
- 13 -
~,~
7~8
(2) The surfaces were then subjected to an alkaline
cleaning agent such as Wyandotte Altrex*, manufactured by
Wyandotte Chemicals Corporation, Wyandotte, Michigan; Pennsalt
A31*, manufactured by Pennsalt Chemical Corporation of
Philadelphia, Pennsylvania; or any of the other well-known
equivalent aluminum cleaners available and known to the industry.
The aluminum surface is exposed to the alkaline cleaner for
a period of about ~0 minutes.
(3) The aluminum surface is then rinsed with hot
water for 5 minutes to remove the alkaline cleaning agent.
(4) A prebond etch in the sodium dichromate--sul-
furic acid deoxidizer noted above for 10 minutes at 150F.
(5) The surface is then immersed in cold tap water
rinse for 5 minutes to remove the prebond e-tchant material.
One-half of the samples were then dried and primed
with BR 127*, an epoxy corrosion resistant primer, 250F. cure,
manufactured by American Cyanamide. The remaining samples
were subjected to an anodization in 3 percent phosphoric acid
at 75F. for 10 minutes with an imposed voltage of 5 volts. The
surfaces were then washed with a water rinse, dried and primed
with BR 127* as noted above.
The 2 groups of samples were then divided into 3 sub-
groups each and coated with the following adhesive materials:
TABLE II
Designation Material
-
FM123-2* Modified epoxy resin adhesive, 250F
cure, manufactured by American
Cyanamide, Bloomingdale Division.
AF 126 * Modified epoxy resin adhesive, 250 F.
cure, manufactured by Minnesota
Mining and Manufacturing.
*Trade Mark
_ 14 -
."~ ~
)8
Hysol 9628* Modified epoxy resin adhesive,
250F. cure, manufactured by
Hysol Division, Dexter Corporation.
The samples were then assembled in a orm suitable for
use in the test schematically shown in FIGURE 8 and subjected
to endwise stress of 1,750 psi while immersed in 3.5 percent
sodium chloride solution at 140F. In all cases the samples
anodized in phosphoric acid presented substantially superior
results to those prepared in the prior art process. Of par-ticular
interest is the nature of the failure, those samples prepared
with the prior art process having predominantly adhesive failure
at the interface between the adhesive and the metal, while those
manufactured utilizing the process of this invention had
sukstantially less adhesive failure, with the failure being
predominantly cohesive in the resin itself.
EXAMPLE II
Tests results for sustained stress lap shear tests at
2,750 psi while the sample was immersed in 3.5 percent sodium
chloride at 75F. are presented in FIGURE 4 for samples prepared
in a manner corresponding to those described above for FIGU~E 3.
Specimens prepared by prior art etching process failed within
one day of the start of the tests. Those samples prepared using
a phosphoric acid anodization in 3% H3PO4 at 70F. for 10
minutes at 5 volts demonstrated superior resistance to failure.
Four out of five specimens bonded with AF126* and all specimens
bonded with Hysol 9628* survived 30 days test without failure.
EXAMPLE III
Table III shows the results of 120 F., 100 percent
relative humidity test for a test specimen prepared as shown in
FIGURE 7 and indicate the effect of solution temperature on bond
stability for 8% and 12~ phosphoric acid solutions. Excellent
- 15 -
~ 7~
1 results were obtained indicating that less than 3/10 of an inch of
2 crack growth was encountered Eor both 8 and 12 percent solutions
3 after 60 days of exposure. Specimen F-l and F-5 showed a higher
4 degree of adhesive Eailure Eor anodization at 60F. suggesting that
60F. is a marginal temperature for the anodization process when the
6 substrate is pure or nearly pure aluminum or clad aluminum.
7 EXAMPLE IV
8 In order to determine the optimum process condition,
9 numerous samples of 7075-T6 aluminum clad panels, 6 inches square,
having a thickness of 0.063 inches were prepared using a preanod-
11 ization process in which the surfaces of the aluminum panels were
12 exposed to a solution of Amchem 7-17 (a proprietary solution
13 containing nitric acid sold by Amchem Products, Inc. Ambler,
14 Pennsylvania). This solution is a room temperature etchant for
aluminum. After surface etching with the Amchem 7-17 solution, 4
16 panels per condition noted in Table IV were anodized and prepared
17 for bonding by spray rinsing the anodization solution from the sur-
18 face and drying the surface at 140F. for 10 minutes. BR127 primer
19 was applied to the prepared surface and the panels were bonded with
Hysol 9628. The epoxy was applied in a 10 mil thick layer. Ten 1-
21 inch wide fracture specimens were saw cut from each pair of as-
22 semblies. Six specimens from each assembly were exposed to boiling
23 water and the amount of crack growth was measured after 1, 4 and 24
24 hours for specimens prepared as shown in FIGURE 7. The remaining 4
specimens weré exposed to 5 percent salt spray at 90F. and the
26 amount of crack growth was measured. The test results are presented
27 in Table V for the water boil test and Table VI for the 5 percent
28 salt spray at 90F. test.
29 The average crack propagation rate and failure mode of the
specimens subjected to boiling water indicated less than 8/10 inch
~ 37~i~
1 of crack growth after 24 hours exposure. Sorne specimens that were
2 anodized in 3 percent phosphoric acid at 65F. for 10 minutes (see
3 specimen Al and A2) showed adhesive failures. All other failures
4 were center-of-the-bond or cohesive.
The oxide coating weight varied from 15 mg/ft2 to 47 mg/ft2. No
6 ¦ correlation between the coating weight and the bond stability was
7 ¦ found.
8 ¦ The test results for the 5 percent salt spray at 95F. crack
9 ¦ growth data is shown in Table VI. Extended exposure to the salt
lO ¦ environment induced failures of more anodized conditions than did
11 ¦ the water boil test. However, since the adherent was clad aluminum
12 ¦ alloy the cladding is sacrificial in a corrosive environment and it
13 I is uncertain if these adhesive failure modes were the result of
14 ¦galvanic corrosion, less than optimum surface preparation or a com-
15 ¦ bination of both.
16 ¦ EXAMPLE V
17 ¦ Variations of the anodization process parameters were explored
18 ¦ and the results shown in Table VII. The initial room temperature lap
19 ¦ shear strength was S200 + 200 psi and the mode of failure 100 percent
20 ¦ cohesive for all specimens. Under sustained stress of 1750 psi,
21 ¦most of the specimens failed in 20 to 200 hours. The specimens
22 ¦ prepared in 17 percent H3PO4 at 100F. and 3 volts anodizing poten-
23 ¦ tial (Test A6) showed poor bond stability and failed in less than 23
24 ¦ hours with 40 to 50 percent cohesive failure. The processing
25 ¦ conditions of this test would appear to cause excessive oxide dis-
~6 ¦ solution during anodization and not permit the build-up of the
27 ¦desired type of oxide coating. The high temperature thus results in
28 la poor oxide film formation and resultant poor bond performance.
29 ¦ Corresponding specimens to Test A6 when tested in a sus-
30 ¦tained stress/fracture test had complete separation of adhesive from
~ 3 7~8
1 the aluminum surface in less than 24 hours. ~he specimens of Test Al
2 failed after 2~ days exposure and all of the test specimens prepared
3 by processes A3, A4, and A7 showed excellent stability with less
4 than 2/10 inch of crack growth after 125 days of salt spray exposure.
These tests indicate that the process of this invention is capabl
6 of producing a stable bond surface using wide ranges of acid concen-
7 trations, potential and temperature. An upper and lower temperature
8 range is shown at which decreasing bond performance results when
9 temperatures in excess of about 85F. are used and when temperatures
below about 65F. are used. The optimum parameters appear to be the
11 following:
12 ¦ Orthophosphoric acid 10~ by weight
13 ¦ Potential 10 volts
14 ¦ Time 20 minutes
15 1 Temperature 75F.
16 Rinse lag time 1.0 - 2.5 minutes
17 EXAMPLE VI
18 Samples of 2024-T3 bare aluminum alloy plate (an alloy contain
19 ing about 4.5% copper, about 0.6% manganese and about 1.5~ magnesium)
were prepared for bonding in a 10% phosphoric acid anodization, usin
21 10 volts potential for 20 minutes at 70F. The surfaces were prime~
22 with BR 127 and samples bonded together with AF126 epoxy resin.
23 Identical specimens were prepared using the H2S04-Na2Cr207-2H20 etc~
24 discussed above. Both sets of samples were exposed to 5~ salt spra~
at 95F. while the bond was placed under an initially high stress an
26 maintained under stressed conditions Eor an extended period of time.
27 At the end of 70 days the samples prepared with H2S04-Na2Cr207-2H
28 failed adhesively over the entire length of the stressed bond. The
29 samples prepared using H3P04 anodization exhibited no adhesive fail-
ure at the end of 18 months exposure. A cohesive failure crack
-18-
-
extended about 1/2 inch along the bond, exclusively with the adhesive
material.
EXAMPLE VII
In order to determine the relationship of higher temperature
operation and lag time from cessation of power applied at the supply to
the time of rinsing of the phosphoric acid from the surface of the anodized
aluminum, several tests were run at 95F. and 100F. Marginal results were
obtained in the idealized laboratory conditions utilized for this test with
many of the tests exhibiting failure when rinse delay exceeded 30 seconds.
Data is presented in Table VIII.
EXAMPLE VIII
Tests were conducted in a production facility at a temperature
of 85~., acid concentration 14%, at a power supply potential of 15 volts
for 20 minutes. A lag time of 2-1/2 minutes from the time the power supply
was turned off until the parts were rinsed to remove the H3PO4 electrolyte
occurred. Parts subjected to the above processing parameters exhibited
occasional failures and the process was adjudged to be inade~uate for
commercial processing.
The failures were apparently due to excessive dissolution of the
aluminum oxide from the surface before the phosphoric acid electrolyte
could be removed by rinsing.
- 19 -
r"
3'7~8
EXAMPLE IX
Production runs of phosphoric acid anodization of aluminum
surfaces for bonding were carried out at the following processing parameters:
Temperature 70-75F.
Phosphoric acid
concentration 10-12%
Power supply
potential 10-15 volts
Part to solution
potential ~-12 volts
Time 20-25 minutes
Lag time before rinse 2 to 2-1/2 minutes
Excellent bonding characteristics are obtained on aluminum and
aluminum alloys processed for bonding in the above anodi~ation procedure.
Various modifications and improvements can be made to the
present invention without departing from the spirit thereof and from
the scope of the claims set forth below.
. ~
-- .
37~38
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¦ TABLE IV
¦ANODIZE PROCESS OPTIMIZATION TEST MATI~IX
. ~ 2
r~ ~ I _ _
. ~ 9 4 ¦ Factor ll3PO~ Potential rll~mp. Time
, i `~ 5 Studied Conc. F (~1in.1
-~,~ 6 I ~ase
71 Level10%10 volts 75F ?0
81 Unit 7~5 volts 10F 10
' 1 91
. ~ ~ h
. 1' ~ 10 I Level17~15 volts 85F 30
11 ¦ I,ow
13 ¦ Level3~5 volts 65F 10
, ~ 14¦ Test Al 3 5 65 10
. .. ~ A2 3 15 65 10
-r 1
. 16 A3 3 5 65 30
17 ~4 3 15 65 ' 30
\ `~ 18 ~5 ~3 5 85 10
t ~ -`' . 19 A6 3 15 85 10
" A73 5 85 30
, ~ ~ 21 A83 15 85 30
22 A917 5 85 10
j 23 A10 17 15 85 10
~ 24 All 17 5 ` 85 30
:~! 25 A12 17 15 85 30
r'- A13 17 5 65 10
. 2~ A14 17 15 65 10
1 28 A15 17 5 65 30
29 A16 17 15 65 30
.''
. ! 2 3
~,i-..
~ 7~18 -
1 TAE3LE V
2WEDGE TEST RESUI,TS, WATER r3OII.--7075-'1`6 CLI~D*, primecl
with Epoxy resln prim~r (Epoxy resi~ ) AND coated
4with ~poxy adhesive (Epoxy resin C)
.. _ _
Coating Initial Water Boil Test **
Test wei9ll2k Crack lElr. 411r. 241lr. I'ailure
6 No. mg/ft inch inch inch inch Mode
8 A1 16.8 .71 1.15 1.30 1.64 50%Adh.
9 A2 28.0 .78 1.15 1.34 1.59 50oAdh.
A3 21.2 .78 1.16 1.30 1.56 loooocoh.
11 A4 46.8 .78 1.17 1.32 1.55 100oCOIl. .
A5 15.6 .78 1.15 1.28 1.54 100~Col- .
12
13 A6 30.8 .78 1.20 1.35 1.52 100oCOh.
14 A7 16.0 .78 1.16 1.31 1.55 100%Coh.
I~8 33.2 .78 1.17 1.33 1.51 100%Coll.
16 A9 14.4 .78 1.17 1.34 1.51 100%Coh.
17 A10 35.2 .78 1.17 1.33 1.50 100%Coh .
18 A11 18.4e ' .78 1.131.33 1.52 100%Coh.
l9 A12 17.2 .78 1.16 1.31 1.56 100%Coh.
A13 21.6 .78 1.13 1.29 1.48 lOOsLoCOl-~
21 A14 38.8 .78 1.15 1.36 1.51 100%Coh.
2 A15 45.2 .78 1.14 1.29 1.54 100!'aCoh.
23 A16 29.2 .78 1.16 1.34 1.55 1û0%Coh.
24
* 0.063 inches thick
26 ** Average o~ 6 specimens
27
28
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