CA1249494A

CA1249494A - Surface preparation for aluminum parts

Info

Publication number: CA1249494A
Application number: CA000486657A
Authority: CA
Inventors: Thomas E. Baker; Herbert J. Tufts; Jack N. Arnold
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1984-08-06
Filing date: 1985-07-11
Publication date: 1989-01-31
Also published as: GB2162865A; JPS6148574A; GB8519539D0; GB2162865B

Abstract

Abstract of the Disclosure A process is disclosed for preparing a surface of a part comprising aluminum for subsequent application of a structural bonding adhesive. A coating of chromate of predetermined thickness is first applied to the surface of the part. A
primer layer having predetermined thickness is applied over the chromate coating and the chromate-coated and primed part heated at a predetermined temperature for a predetermined time. The part is subsequently cooled to substantially room temperature. The thusly treated part may be adhesively bonded to a similarly prepared part by applying an adhesive to at least one of the treated surfaces of the parts, bonding the parts together, and curing the adhesive.

Description

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SURFACE PREPARATION FOR ALUMINUM PAR~S
Background of the Invention This invention relates generally to the art of struc-tural adhesive bonding of metals and more particularly to preparation of the surfaces of aluminum, or aluminum alloy, parts for structural adhesive bonding of such parts As is known, proper preparation of the surfaces of the aluminum parts to be adhesively bonded is necessary if a du-rable bond between them is to be achieved. One surface preparation technique is Phosphoric Acid Anodizing (PAA), a process described in U.S. Patent No. 4,085,012, issued to Marceau et al in which the aluminum parts are immersed in a phosphoric acid solution while an electric current i5 passed through the parts. This anodizing produces a film of aluminum oxide on the surface of the parts which facilitates adhesive bonding thereto. While the PAA process has yielded satisfac-tory results, the produced aluminum oxide film is quite fragile, making the PAA treated surface very sensitive to handling.
Another method of preparing the surface of aluminum parts for structural adhesive bonding is chromate conversion coating, wherein a layer of chromate material is applied to the sur-faces of the aluminum parts to be bonded. The applied chromate coating converts each such surface to a superficial layer containing a complex mixture of chromium compounds to provide ,'. ~

he subsequently bonded joint with increased corrosion pro-tection. An article entitled "Effect of Surface Preparation on Stressed Aluminum Joints in Corrosive Saltwater Exposure", appearing in ~ in October, 1980, noted that when a coating of chromate is applied to the surfaces of aluminum parts followed by a coating of high temperature curing adhe-sive, and the adhesive cured by heating the thus coated parts for a predetermined time, a durable adhesive bond between them is obtained. However, high temperature curing adhesives are generally inconvenient in a manufacturing environmentc The article's author postulated that the high bond durability was possibly due to a thermally induced struc-tural change in the chromate. The author further theorized that the use of a room temperature curing adhesive in place lS of the high temperature curing adhesive would lead to different results; however, he did not pursue this theory further.

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Summary of the Invention In accordance with the present invention, a process is described for preparing a surface of a metal part comprising alumi-num for subsequent applicat.ion of a structural bonding adhesive, comprising the steps of:
applying a coating of chromate to the surface of the part;
heating the chromate-coated part; and subsequently cooling the chromate-coated part to substan-tially room temperature for application of the bonding adhesive.
By the use of such process, an extremely durable structural bond to t~e~. part may be achieved using a room temperature curing adhesive.
According to one aspect of the present inventio.n there is provided a process for preparing a surface of a metal part compris-ing aluminum for subsequent application of a structural bonding adhesive, comprising the steps of:
applying a coating of chromate to the surface of the part;
heating the chromate-coated part at between substantially 250F and substantially 350F for approximately 2 hours; and subsequently cooling the chromate-coated part to substan-tially room temperature.
According to a further aspect of the present invention there is provided a process for preparing a surface of a first part comprising aluminum for adhesive ~onding to a second part comprising aluminum, said process comprising the steps of:
applying a coating of chromate to the surface of the first part;

.

- 3a - 2901-671 heating the chromate-coated first part at between substan-tially 250F and subs-tantially 350F for approximately 2 hours;
subsequently cooling the chromate-coated first part; and applying a layer of adhesive to the cooled, chromate-coated first part.
According to another aspect of the present invention there is provided a process for preparing a surface of a part for subse-quent application of a structural bonding adhesive, said part being a metal selected from the group consisting of aluminum and alloys 0 of aluminum, said process comprising the steps of:
applying a coating of chromate to the surface of the part;
applying a layer of primer over the chromate coating;
heating the chromate coated and primed part at between substantially 250F and substantially 350F for approximately 2 hours; and subsequently cooling the part to substantially room tem-perature.
According to a s-till further aspect of the present inven-tion there is provided a process for bonding a first part comprising aluminum with a second part comprising aluminum, comprising the steps of:
applying a coating of chromate having a predetermined thickness to a surface of each of the parts;
heating the chromate-coated parts at between substantially 250F and substantially 350F for approximately 2 hours and subse-quently cooling said parts to substantially room temperature;

' '~^ ~ ' 3a ~Z~'-3~

-3b- 6290l-671 applying an adhesive to the chromate-coated surface of at least one of the cooled parts; and bonding the par-ts together.
According to another aspect of the present invention there is provided a process for bonding a first metal part comprising aluminum with a second metal part comprising aluminum, comprising the steps of:
applying a coating of chromate comprising a nominal per-centage of hexavalent chromium to a surface of each of said metal parts;

heating the coated metal parts at a predetermined tempera-ture for a predetermined time selected to substantially increase the percentage o~ hexavalent chromium in the coating from the nomi-nal percentage thereof;
applying an adhesive to said surface of at least one of the metal parts; and bondin~ said parts together.
According to a further aspect of the present invention there is provided a process for bonding a first part comprising aluminum with a second part comprising aluminum, such process com-prising the steps of:
applying a coating of chromate comprising a nominal amount of hexavalent chromium to a surface of each of the first and second parts;
heating the chromate-coated first and second parts at a predetermined temperature for a predetermined time, such predeter-mined temperature and time being selected to substantially increase -3b-."f 9~
- 3c ~ 2901~671 the amount oE hexavalent chromium in the coating from the nominal amount thereof;
subsequently cooling the first and second parts to sub-stantially room temperature;
applying a bonding adhesive to the chromate-coated surface of one of the cooled Eirst and second parts; and bonding the first and second parts together and curring the adhesive.
In a preferred embodiment of -the present invention, the process for preparing a sur:Eace of a part comprising aluminum for subsequent application of a structural bonding adhesive first com-prises the step of applying a coating of chromate to the surface of the part. A layer of primer is then applied over the chromate layer. The chromate-coated and primed aluminum part is heated to thermally "age" the chromate and cure the primer. The part is sub-sequently cooled to substantially room temperatuxe. The thusly treated surface of the part may then have a layer of a room tempera-ture curing adhesive applied thereto for structural bonding to a similarly treated surEace of another part. In the interim, and un-like Phosphoric Acid Anodizing (P~A) treated aluminum, the surfaceof the parts treated by the process of the present invention 3~9'~

is quite insensitive to handling.
Additionally, the present invention provides a process for bonding a first part comprising aluminum with a second part comprising aluminum~ The flrst step in such process is applying a coating of chromate having a predetermined thick-ness to a surface of each of the parts. The chromate-coated parts are then heated at a predetermined temperature for a predetermined time and are subsequently cooled to substan-tially room temperature. An adhesive is applied to the chro-mate-coated surface of at least one of the cooled parts, and the parts bonded together.

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Brief Description of the Drawings The foregoing features of the present invention and the advantages thereof may be more fully understood from the following detailed description when read in conjunction with the accompanying drawings wherein:
FIG. 1 is a flow diagram of a first embodiment of the aluminum sur~ace preparation process of the present invention; and FIG. 2 is a flow diagram of a second embodiment of the aluminum surface preparation process of the present invention.

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Description of the Preferred Embodiments Referring now to FIG. 1, the aluminum surface preparation process of the present invention is illustrated. The first step in this process consists of applying a coating of chromate having a predetermined thickness to the sur-Eace of the alu-minum, or aluminum alloy, parts which are to be adhesively bonded together. As hereinafter used, the term aluminum encompasses the metal aluminum and alloys thereof. One method of applying the chromate coating is to immerse the parts in a c~romate solution, although the chromate film may also be applied by spraying, brushing or swabbing the parts.
As i5 known, chromate is a complex mi~ture of chromium with additional compounds, and is cornmercially available under several trade ~ , such as Iridite, manufactured by the Allied-Kelite Division of Witco Chemical Corporation of Des Plaines, Iowa, and Alodine, manufactured by Amchem Products Inc. of Ambler, Pennsylvania.
Proper chromate thickness is necessary to pro~ide an optirnum bond. Sufficient chromate is needed to ~reYent mo~sture from affecting the bonded joint and thus corroding the bond and reducing durability. However, if the chromate coating is too thick, the bonded joint may fail in the chromate coating itself. The concentration of the chromate solution, its degree of acidity (pH) and temperature, the irnmersion time, and the alloy composition of the part being chromated ~ ;~ 4 ~ . 9 L"k all influence the thickness of the chromate coating. An acceptable concentration range for the chromate solution, using Iridite inaterial as the chromate, i5 between 1.0 and 1.5 ounces of Iridite material per gallon of solvent (here, water)O This yields a chromate solution with a pH of between 1.3 and 1.6. The temperature of the solution should be between 60F and 100F. Proper immersion time is between 25 and ~5 seconds~
Proper chromate thickness is on the order of a few millionths of an inch; therefore, it is impractical to measure absolutely the thickness of the chromate coating. However, it is possible to determine whether the chromate coating thickness of a given chromated part is within an acceptable range, that is, neither too thin nor too thick. This can be done by comparing the color of such chromated part with color standards. The color of a chromate-coated part ranges from clear, for a very thin chromate coating, to brown for a much thicker coating. Thus, color standards representing a color range corresponding to a thickness range can be obtained.
The first color standard, representing the thinnest acceptable chromate coating, is obtained by immersing sample sections (called coupons) of aluminum into the weakest chromate solu-tion to be used (here 1.0 oz. of Iridite chromate/gal. of water) for the minimum practlced time (here 25 sec.). The second color standard, representing the thickest acceptable chromate `3'~

coating, is produced by dipping aluminum coupons into the strongest chromate solution to be used (here 1.5 oz. of Iridite chromate/gal.) for the maximum practiced time (here 45 sec.).
Thus, a standard color range is found, to which the color of the parts chromated for bonding are compared to determine if the chromate coating of such parts is of the proper thickness.
By thermally "aging" the chromate a strong and durable adhesive bond may be made to the chromated aluminum parts.
Thermally "aging" the chromate is the second step of the surface preparation process of the present invention and is performed by heating the chromate-coated aluminum parts at a predetermined temperature, for example, 3S0F, for a predetermined time, for example, two hours. The chromated aluminum parts are then cooled to substantially room tempera-ture. The "aged" chromated aluminum parts were examined usiny Electron Spectroscopy for Chemical Analysis (ESCA) (also called photoelectron spectroscopy), a generally known surface analysis technique in which x-rays having known energy are directed at the surface of the "aged" chromated parts and the energy of electrons emitted therefrom in response to the x-rays analyzed to identify the element ~rom which the elec-trons came and the molecular configuration of such element.
The use of ESCA on the "aged" chromàted parts reveals that the thermal "aging" converts the normally trivalent chro~ium atom (Cr~3) of the chromate to hexavalent chromium (Cr~6).

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As is known, hexavalent chromium resists corrosion better than does trivalent chromium; thus, a more durable bond can be obtained by bondlng to the "aged" chromate surfaces of the parts. As the table below shows, the higher the aging tem-perature, the more complete the conversion. However, care should be taken not to expose the aluminum parts to excessive temperatures, as this may mechanically weaken the metal.
The table also shows that the conversion process will take place in an inert atmosphere of nitrogen, as well as in air. This indicates that the conversion reaction is internal to the chromium and is not part of an oxidation process.
Chromium Oxidation State Percentages Chromium 2P
~ Hexavalent Triv Non-thermally aged 36.6~ 63.4~

2 hrs. at 250F in air 41.8~ 54.2%
2 hrs. at 300F in air 48,5~ 45.3~
2 hrs. at 350F in air 55.9% 30.2%
2 hrs. at 250F in N2 44 7% 52.6%
2 hrs. at 300F in N2 47O3% 41.8%
2 hrs. at 350F in N2 52.4~ 31.6~
The next step in the aluminum surface preparation process is to apply a layer of primer to the chromated aluminum parts.
As is known, primer is an epoxy-based material containing hexavalent chromium and assists the chromate layer in resisting _g_ ~4~9~

corrosion of the subsequently bonded joint. ~ere, the primer i5 EA-9228, manufactured by the Hysol Division of the Dexter Corp. of Pittsburg, California, although other primers may be used. The thickness of the primer layer must be controlled for the same reason as for chromate thickness: sufficient primer must be applied to provide, along with the chromate coating, corrosion protection to prevent moisture from a~fecting the bonded joint; however, the primer layer must not be ~o thick that it becomes weaker than the bonded joint itsel. An acceptable range of primer thickness is between 0.2 mil to Or4 mil (i.e., 0.0002 to 0.0004 inches). The primer can be applied either by spraying or by dipping the chromated aluminum parts in a primer solution and conventional techniques may be used to measure primer thickness.
The primer must be cured. Conventionally, this is done by heating the primed aluminum parts at about 250F for about one hour, although other temperatures and times may be used.
While this extra heating cycle will not adversely affect the "aged" chromate (in fact, it may convert more trivalent chromium to hexavalent chromium), the cycle is redundant given the prior step of thermally aging the chromate. There-fore, the preferred method, depicted in the flow diagram of FIG. 2, is to apply the primer layer to the chromated alu-minum parts prior to thermally aging the chromate. Thus, heating the chromated and primed aluminum parts for two ~ ~ -10-~2~ 9~

hours at 350F, for example, both ages the chromate (converts trivalent chromium to hexavalent chromium) and cure~ the primer. The chromated and primed aluminum parts are cooled to room temperature and are ready to be bonded to~ether. In fact, this process yields even higher adhesive bond strengths than does ~he process of chromating, aging, priming and primer curing.
Whichever of the two methods is used, once the primer is cured the parts can be stored for long periods of time (up to at least one year) and handled without special sanitary precautions without contaminating the treated surfaces. The surfaces need only be cleaned with, for example, an ordinary solvent prior to bonding.
The surfaces of the aluminum parts having been prepared lS by the process of the present invention, a room temperature curing adhesive is applied to at least one of the prepared surfaces of the par~s, and the surfaces of the parts placed together and held in place by conventional means, such as clamps, while the adhesive cures. Here, the adhesive is EA-93Z0NA, manufactured by the Hysol Division of the Dexter Corporation of Pittsburg, California, although other adhesives may be used. Typically, a room temperature curing adhesive obtains its maximum adhesive quality after about seven days at room temperature. Once the adhesive has cured, the clamps are removed and the parts durably held together by the adhesive ~2~ ~9'~

bond.
The parts need not be bonded with room -temperature curing adhesive, however. Higher temperature curing (i.e. oven-cured) adhesives may be used as well, with equivalent or better bond strength and durabi.lity being achieved, although high temperature curing adhesives are generally inconvenient in a manufacturing environment.
It is noted that the processes of the present invention are non-electrolytic processes, that is, such processes do not require an anodizing step, such as phosphoric acid anodizing (PAA) discussed in the above-identified Marceau Patent No. 4,085,012.
Having described preferred embodiments of the present invention, minor modifications may beccme apparent to those skilled in the art without departing from the spi.rit of the invention. I-t is therefore lmderstood that the scope of the present inven-tion is to be limited only by the scope of the appended claims.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a surface of a metal part com-prising aluminum for subsequent application of a structural bonding adhesive, comprising the steps of:
applying a coating of chromate to the surface of the part;
heating the chromate-coated part at between substantially 250°F and substantially 350°F for approximately 2 hours; and subsequently cooling the chromate-coated part to substan-tially room temperature.

2. The process recited in claim 1 wherein the heating step is performed in an atmosphere of air.

3. The process recited in claim 1 wherein the heating step is performed in a substantially inert atmosphere.

4. The process recited in claim 3 wherein said substantially inert atmosphere comprises nitrogen.

5. The process recited in claim 1 wherein the heating step is performed at between substantially 300°F and 350°F.

6. The process recited in claim 1, comprising the additional steps of:
applying a layer of primer over the coating of chromate after the chromate-coated part has been cooled to substantially room temperature; and curing the primer.

7. The process recited in claim 6 wherein the applied primer layer has a predetermined thickness.

8. The process recited in claim 7 wherein said predetermined thickness is substantially 0.2 to 0.4 mil.

9. A process for preparing a surface of a first part compri-sing aluminum for adhesive bonding to a second part comprising alu-minum, said process comprising the steps of:
applying a coating of chromate to the surface of the first part;
heating the chromate-coated first part at between substan-tially 250°F and substantially 350°F for approximately 2 hours;
subsequently cooling the chromate-coated first part; and applying a layer of adhesive to the cooled, chromate-coated first part.

10. The process recited in claim 9 comprising the additional step of:
applying a layer of primer to the chromate-coated first part before said heating step.

11. A process for preparing a surface of a part for subsequent application of a structural bonding adhesive, said part being a metal selected from the group consisting of aluminum and alloys of aluminum, said process comprising the steps of:
applying a coating of chromate to the surface of the part;

applying a layer of primer over the chromate coating;
heating the chromate-coated and primed part at between substantially 250°F and substantially 350°F for approximately 2 hours; and subsequently cooling the part to substantially room temperature.

12. The process recited in claim 11 wherein said chromate coating has a first predetermined thickness and said layer of primer has a second predetermined thickness.

13. The process recited in claim 12 wherein the second pre-determined thickness of the primer layer is substantially between 0.2 and 0.4 mil.

14. A process for bonding a first part comprising aluminum with a second part comprising aluminum, comprising the steps of:
applying a coating of chromate having a predetermined thickness to a surface of each of the parts;
heating the chromate-coated parts at between substantially 250°F and substantially 350°F for approximately 2 hours and sub-sequently cooling said parts to substantially room temperature;
applying an adhesive to the chromate coated surface of at least one of the cooled parts; and bonding the parts together.

15. The process recited in claim 14 further comprising the step of:
applying a layer of primer over the coating of chromate before said heating step.

16. A process for bonding a first metal part comprising alu-minum with a second metal part comprising aluminum, comprising the steps of:
applying a coating of chromate comprising a nominal per-centage of hexavalent chromium to a surface of each of said metal parts;
heating the coated metal parts at a predetermined tempera-ture for a predetermined time selected to substantially increase the percentage of hexavalent chromium in the coating from the nominal percentage thereof;
applying an adhesive to said surface of at least one of the metal parts; and bonding said parts together.

17. A process for bonding a first part comprising aluminum with a second part comprising aluminum, such process comprising the steps of:
applying a coating of chromate comprising a nominal amount of hexavalent chromium to a surface of each of the first and second parts;
heating the chromate-coated first and second parts at a predetermined temperature for a predetermined time, such predeter-mined temperature and time being selected to substantially increase the amount of hexavalent chromium in the coating from the nominal amount thereof;
subsequently cooling the first and second parts to sub-stantially room temperature;

applying a bonding adhesive to the chromate-coated surface of one of the cooled first and second parts; and bonding the first and second parts together and curing the adhesive.

18. The process of claim 17 further comprising the step of applying a layer of primer over the coating of chromate on the first and second parts prior to said heating step.

19. The process of claim 17 wherein the adhesive curing step is performed at substantially room temperature.

20. The process of claim 17 wherein the predetermined tempera-ture and time are selected to increase the amount of the hexavalent chromium in the coating by between substantially 5% and substantial-ly 19% from the nominal amount thereof.

21. The process of claim 20 wherein said predetermined tem-perature is between substantially 250°F and substantially 350°F and said predetermined time is approximately 2 hours.

22. The process of claim 20 wherein said predetermined tem-perature is between substantially 300°F and substantially 350°F and said predetermined time is approximately 2 hours.

23. The process of claim 20 wherein said predetermined tem-perature is substantially 300°F and said predetermined time is approximately 2 hours.

24. The process of claim 20 wherein said predetermined tem-perature is substantially 350°F and said predetermined time is approximately 2 hours.

25. The process recited in claim 5 wherein the heating step is performed at substantially 300°F.

26. The process recited in claim 5 wherein the heating step is performed at substantially 350°F.

27. The process of claim 16 wherein the predetermined tem-perature and time are selected to increase the percentage of the hexavalent chromium in the coating by between substantially 5% and substantially 19% from the nominal percentage thereof.

28. The process of claim 27 wherein said predetermined tem-perature is between substantially 250°F and substantially 350°F and said predetermined time is approximately 2 hours.

29. The process of claim 28 wherein said predetermined tem-perature is substantially 250°F.

30. The process of claim 28 wherein said predetermined tem-perature is substantially 300°F.

31. The process recited in claim 28 wherein said predetermined temperature is substantially 350°F.

32. A process for preparing a surface of a metal part com-prising aluminum for subsequent application of a structural bonding adhesive, comprising the steps of:
applying a coating of chromate to the surface of the part;
heating the chromate-coated part to a temperature significantly above room temperature; and subsequently cooling the chromate-coated part to sub-stantially room temperature.

33. The process according to claim 32 wherein said chromate comprises trivalent chromium and hexavalent chromium.

34. The process according to claim 33 wherein said heating step is performed at a temperature and for a time sufficient to sub-stantially increase the amount of said hexavalent chromate of said chromate coating.