US3790453A - Corrosion protected anodized aluminum surfaces - Google Patents

Abstract

AN ALUMINUM SURFACE CAN BE PROTECTED AGAINST COROSION BY INITIALLY ANODIZING SUCH A SURFACE SO AS TO PLACE UPON IT AN OXIDE SURFACE FILM AND THE SEALING SUCH A SURFACE COVERING BY FIRST LOCATING COBALT IONS WITHIN IT AND THEN SUBSEQUENTLY EXPOSING SUCH COBALT IONS IN THE SURFACE COVERING TO CHROMATE IONS IN ORDER TO REACT THE COBALT AND CHROMATE IONS TO PRODUCE A COBALTOUS CHROMATE COMPOSITION WITHIN THE OXIDE SURFACE. SUCH A COMPOSITION IS CONSIDERED TO BY A MIXTURE OF COBALTOUS OXYCHROMATE COMPOUNDS.

Description

United States Patent 3,790,453 CORROSION PROTECTED ANODIZED ALUMINUM SURFACES John L. Wanamaker, Burbank, Kenneth E. Weber, L os Angeles, and Geraldine M. Hoch, Chatsworth, Cahf., assignors to Lockheed Aircraft Corporation, Burbank, Calif. No Drawing. Filed Mar. 22, 1971, Ser. No. 126,932 Int. Cl. C23b 9/02; C23f 17/00 US. Cl. 204-35 N 12 Claims ABSTRACT OF THE DISCLOSURE An aluminum surface can be protected against corrosion by initially anodizing such a surface so as to place upon it an oxide surface film and then sealing such a surface covering by first locating cobalt ions within it and then subsequently exposing such cobalt ions in the surface covering to chromate ions in order to react the cobalt and chromate ions to produce a cobaltous chromate composition within the oxide surface. Such a composition is considered to be a mixture of cobaltous oxychromate compounds.

CROSS-REFERENCES TO RELATED APPLICATIONS US. patent application entitled, Production of Dicobaltous Oxychromate and Percobaltous Oxychromates, filed Mar. 30, 1970, Ser. No. 24,024 by Weber et al., now US. Pat. 3,664,808, issued May 23, 1972.

US. patent application entitled, Use of Cobalt Chromates in Inhibiting Corrosion, filed Apr. 13, 1970, Ser. No. 28,008 by Weber et al., now abandoned.

BACKGROUND OF THE INVENTION Corrosion of aluminum is extremely important in many industries. High strength aluminum alloys are extensively utilized in many different structures such as in aircraft, missile systems and the like. When unprotected from corrosion except for the oxides which develop upon such alloys in air, these alloys are undesirably susceptible to corrosion.

The recognition of this has led to the investigation of a number of different systems for inhibiting corrosion on aluminum or aluminum alloy surfaces. Paint or polymer systems are frequently utilized to provide such protection. It is known to utilize within such paint or polymer systems various compounds which will inhibit corrosion by mechanisms which are comparatively complex. An understanding of the present invention is not considered to require a detailed understanding of such mechanisms. As an example of this type of corrosion protection using an inhibitor in an appropriate carrier vehicle reference is made to the co-pending US. patent application entitled, Use of C0- balt Chromate in Inhibiting Corrosion cross-referenced in the preceding section of this specification. The entire disclosure of this co-pending application is incorporated herein by reference.

On many occasions it is not desired to utilize paint or polymer systems to provide corrosion protection, it is often believed that effective corrosion protection requires more direct, intimate protection at an aluminum surface than can be obtained utilizing a paint or polymer coating for such a surface. In the past, such more direct corrosion protection has been provided by creating on aluminum surfaces anodized aluminum oxide films and frequently by sealing such films with one or more different compounds. It is recognized that such anodized aluminum surfaces and in particular such sealed, anodized aluminum surfaces provide reasonable protection against corrosion.

The literature with respect to such anodized and sealed,

anodized aluminum surfaces is extremely extensive. In this connection reference is made to the report entitled, Optimization and Evaluation of Aluminum Sealing, by W. M. Fassell, Jr. dated March 1970, published by the Air Force Materials Laboratory, Wright-Patterson AFB, Ohio, 45433, and the literature references cited therein. Reference is also made to the article entitled, Practical Implications of Research on Anodic Coatings on Aluminum, by J. F. Murphy apparing at page 1241 in the November 1957 issue of Plating and the references cited therein.

This literature makes it clear although the anodization of aluminum has been extensively investigated that the character of anodized aluminum oxide films is not clearly understood. From these references it will be apparent that these films are normally considered to consist of an extremely thin aluminum oxide dielectric or barrier layer and of a great deal thicker porous aluminum oxide layer in which ions resulting from the anodization are present in a vaguely defined structure more or less corresponding to the structures of various alumina gels and minerals. It is clear that this porous alumina containing layer is capable of taking up various ions and/or compounds and is capable of providing internal reactive sites which can be utilized in order to seal or close up the pores of this porous layer.

To a 'large degree the mechanism of such sealing is considered to be as vaguely defined as the structure of a porous layer in an anodized aluminum oxide film. It has commonly been considered that such sealing involves the formation of hydration products between the porous layer and water. However, it is also known that any of a variety of a large number of different compositions can be located within porous alumina containing layers by immersing anodized surfaces in baths, containing such compositions prior to these layers being otherwise sealed. Thus, for example, a freshly anodized aluminum film can be impregnated with a large number of different ions and/or compounds by being "immersed in a solution containing the material to be placed within the porous layer of such an anodized film before this porous layer is otherwise sealed.

The sealing process can be utilized with various materials which will increase protection against corrosion over that provided by an anodized aluminum oxide film alone on an aluminum or aluminum alloy. In utilizing sealing for protecton against corrosion the porous oxide layer in an anodized oxide film is contacted With an appropriate material of a type commonly referred to as an inhibitor so that the porous layer on the oxide surface is impregnated with such an inhibitor. Frequently this scaling is carried out in two stages so that the porous oxide layer is first impregnated with one inhibitor and then, before it is completely sealed, is impregnated with another inhibitor. Such use of two different sealing agents is commonly referred to as duplex sealing.

In spite of the work which has been done in providing corrosion protected, sealed, anodized aluminum surfaces it is considered that there is still a need for providing better corrosion protection on such surfaces than has been obtained in the past. It has been recognized that cobalt compounds such as dicobaltous oxychromate and percobaltous oxychromate provide a very effective degree of protection against corrosion when utilized as set forth in the previously noted application, Use of Cobalt Chromates in Inhibiting Corrosions. Cobalt chromate compounds as noted can be produced as indicated in the pending US. patent application entitled, Production of Dicobaltous Oxychromate and Percobaltous Oxychromate indicated in the preceding section of this specification. The entire disclosure of this co-pending application is incorporated herein by reference. In the past the possibility of using cobalt chromate compounds of this type to provide for corrosion inhibition an anodized surface has not been recognized, probably because of the complex nature of cobalt chromate chemistry.

BRIEF SUMMARY OF THE INVENTION An objective of the present invention is to provide for better corrosion protected or inhibited anodized aluminum surfaces than have been known in the past. A more specific objective of the present invention is to provide corrosion protected, sealed anodized aluminum surfaces in which the corrosion protection is obtained utilizing complex cobaltous chromate type compositions created in situ within the porous aluminum oxide layer of an anodized aluminum oxide film. A further objective of the present invention is to provide a process for providing such a corrosion protected, anodized surface which is relatively inexpensive, which is relatively easy to carry out and which is effective for its intended purpose.

In accordance with this invention the aforegoing and other related objectives of the invention as will be apparent from a detailed consideration of the remainder of this specification are achieved by anodizing an aluminum or aluminum alloy surface in accordance with conventional practice so as to provide an adherent aluminum oxide film on such a surface and subsequently impregnating the oxide surface with cobalt ions, and then subsequently impregnating the oxide surface with chromate ions so that such ions will react with the cobalt ions present, producing a cobaltous chromate type composition within the aluminum oxide surface layer.

DETAILED DESCRIPTION In spite of the simplicity with which the essential features of the invention are indicated and summarized in the preceding the invention itself involves some comparatively complex and detailed considerations. The invention is equally applicable to pure aluminum surfaces and to the surfaces of common aluminum alloys containing a sufiicient amount of aluminum so that the reactions on the surfaces of .such alloys during anodizing are essentially the reactions of aluminum itself. Such alloys are well-known in the industry and are designated by various designations such as 7057-T6 and 2024-T3. All of such alloys are commonly referred to as aluminum or aluminum alloys. Whenever the term aluminum is used herein it is to be understood that it designates not only pure aluminum but such alloys.

The initial step in providing corrosion protection on an aluminum surface in accordance with this invention involves the anodization of such a surface. The anodization carried out in practicing this invention is conventional and is carried out using a conventional electrolyte in a conventional manner. To a degree any type of known anodization will provide a degree of corrosion protection in accordance with this invention if the procedure indicated in this specification is followed.

However, it is preferred that the anodization used in practicing this invention be carried out in a comparatively strong acid electrolyte since the oxide film produced in a strong electrolyte tends to have a more porous layer at the surface of the oxide film than an oxide film produced by anodization utilizing a comparatively weak acid. Further, an oxide film produced in a comparatively weak acid is somewhat limited in its thickness because of factors which are unimportant to this invention, whereas the oxide films produce din comparatively strong acids can be grown to a comparatively thick depth.

It is considered that this latter is advantageous since it is desired to protect an aluminum surface in accordance with this invention to a comparatively thick depth in order to obtain the maximum advantages of the invention. Because of the nature of the oxide film produced by anodization in sulfuric acidthat is, a porous comparatively thick film, it is preferred to utilize this acid in anodizing an aluminum surface for treatment as explained in this specification. However, it is considered that reasonably effective products can be obtained by anodization in other acids such as oxalic, phosphoric or the like.

In anodizing an aluminum surface for use with this invention it is considered preferable to utilize an electrolyte in which the acid concentration is sufliciently low so that there is no tendency for the oxide film produced to be dissolved in the electrolyte as it is produced. It is also considered preferable to use an acid concentration which is sufficiently great so that the rate of oxide film production is sufficiently rapid to avoid undue time delay. Also, the acid concentration should be sufficiently high so that there is no tendency for the aluminum oxide coating produced to be patchy or spotty.

Because of the properties of sulfuric acid, suitable acid concentrations for use in anodizing with this acid are considered to be within the range of from about 7 to about 25 by weight sulfuric acid to avoid the consequences indicated in this discussion. In general the higher the acid concentration used in the anodizing the greater the porosity of the anodized film. Such porosity is desirable with the invention. Particularly suitable results can be obtained utilizing an aqueous sulfuric acid anodizing solution containing from about 11 to 17% by weight of the acid.

During the anodization, the temperature at which the anodization is carried out should be regulated in accordance with conventional practice so that the alumina film produced has as great a degree of absorption as reasonably possible. In general if too high a temperature is used during anodization the oxide produced will tend to have a comparatively soft, powdery type of character making it undesirable for use in situations where abrasion may be encountered or may be significant. In general if too low a temperature is used the oxide produced will tend to have a comparatively hard surface structure which does not possess desired absorption characteristics. Further, an oxide layer produced at too low a temperature may tend to crack and haze during subsequent processing. For these reasons with a sulfuric acid electrolyte as indicated in the preceding the anodization should be carried out at a temperature of from about 60 to F. Preferred results are achieved using a temperature of from about 68 to 72 F. since within this range an absorptive, reasonably hard oxide film is produced.

It is well-recognized that the higher the current density used during the anodization, the greater the soft, powdery character of the alumina coating produced on aluminum. On the other hand, if too low a current density is used during anodization, the rate at which alumina is formed is undesirably slow for practical reasons and the aluminum surface being anodized may tend to be attacked by the acid electrolyte. It is considered that acceptable results can be achieved utilizing current densities within the range of from 7 to 20 amps. per square foot without the oxide created being of an undesired character and without the process being uneconomically slow. With sulfuric acid electrolytes as indicated it is preferred to utilize a current density during anodization of from 10 to 15 amps. per square foot since current densities within this range produce a structurally desirable coating at a reasonable rate.

In accordance with conventional practice it is preferred to use voltages during anodization which result from the current density applied. Usually voltages of from 15 to 20 volts are utilized. It is, however, possible to hold the voltage constant so that the current density used varies. It is not considered that this produces oxide films having as desirable properties from a physical standpoint as are produced by holding the current density constant. Such films can, however, be used in accordance with this invention.

The time required to anodize an aluminum surface so that such a surface can be treated in accordance with this invention can be varied within comparatively wide limits.

It will be recognized that time involved essentially relates to or governs the thickness of the anodized film. However, excessively thick anodized films should be avoided so as to circumvent excessively large degradation of the fatigue properties of the aluminum metal upon which such film is located. The anodization used must, however, be carried out for a sufficient time so that a porous oxide layer is located on a dielectric or carrier oxide layer in order to provide some degree of corrosion protection on an article processed as herein described.

With sulfuric acid electrolytes indicated and at current densities as indicated it is considered that useable products can be obtained with the anodization carried out for periods about 5 minutes to 1 hour. Preferably times of from about 5 to 15 minutes are used in accordance with this invention since such periods provide oxide films of adequate thickness to provide a desired degree of corrosion protection. It will be recognized that the times used relate to the thickness of a final film such a way as to involve what may be considered as continuous variables.

After an aluminum surface has been anodized in accordance with the preceding before further processing the anodized film should be treated so as to remove entrained surface solution. This may be conveniently done by washing or rinsing in tap water, then washing or rinsing in demineralized water and then washing or rinsing in av diluted acid-neutralizing solution such as a 5% sodium bicarbonate solution for a brief period of such as from 1 to 5 minutes. During this treatment so as to remove entrained material the temperature of the oxide surface should be held below about 90 F. and preferably within the range of from about 65 to 75 F. so as to avoid any tendency of the pores within the surface to become sealed. Lower temperatures above the freezing point of water can of course be used. It is preferred however to use temperatures within the 65 to 75 F. range indicated since it is considered that at temperatures within this range comparatively rapid efficient removal of entrained material can be obtained by a leaching type of mechanism Without significant or noticeable pore sealing occurring.

In practicing the invention the anodized article, preferably, but not necessarily, after removal of the entrained material it is partially sealed to a degree by being immersed in a solution containing cobaltous ions so as to partially impregnate the pores of the anodized film with such ions. The anodized article may be immersed in an aqueous solution of any conveniently available soluble cobaltous salt. Because of the availability and ease of solution it is preferred to utilize cobaltous acetate but other cobalt salts such as cobaltous oxalate, cobaltous sulphate or the like may be used.

The concentration of the aqueous solution employed at this stage in creating a corrosion inhibited or protected oxide film may be varied between comparatively wide limits. It is considered that if the solution contains less than about a quarter of a percent by weight of cobalt ions that insufiicient cobalt will be taken up into the porous oxide layer on an anodized film to react during the subsequent step in practicing the process of this invention, although there will be some cobalt ions taken up into this layer which will react when less than this amount of cobalt is used in solution.

In order to provide for adequate cobalt ion impregnation so as to obtain in the final product desired inhibiting properties it is considered that the aqueous cobalt solution used should contain at least one percent by weight cobalt ions. It is also considered that if this solution contains in excess of about 5% by weight cobalt ions that the final product created as herein described will be undesir able because it will tend to have a powdery type of surface not having the desired structural characteristics for handling and use without damage. It is considered that this powdery type of surface resulting from the use of in excess of 5% by weight cobalt ions indicates what amounts to a waste of cobalt ions. For economic reasons this is to be avoided.

It is important that this first partial sealing step be carried out at a sufficiently low temperature such that the pores within an anodized oxide layer are not completely sealed up by contact with the solution used. However, it is important that during this initial sealing step that the solution employed be hot enough so that an adequate amount of cobalt ions are taken up into or absorbed by the porous layer of oxide film created as described so that the final product will have a desired degree of corrosion protection. It is considered that acceptable results can be achieved by holding the sealing bath or solution used in this step of the process at a temperature of from about 60 to 150 F. Presently preferred results are achieved using a temperature from about F. to F. since within this range cobalt absorption is pronounced and since within this range there is little tendency for the anodized film to become completely sealed.

The absorption of cobalt ions by the anodized film during this first partial sealing step is, of course, related to the time that the anodized film is held in the sealing solution used. In general the longer the time the greater the quantity of cobalt ions taken up into the anodized film. Unless adequate time is used insufiicient cobalt ions will be taken up into the oxide film to react during the subsequent sealing step so as to provide the desired corrosion protection achieved with the invention. If the time period is too long, an excess of the amount of cobalt needed to achieve such protection will be taken up into the oxide and there may be a tendency for the anodized film to become completely sealed. It is considered that useable results can be obtained by immersion in a solu tion as described for a period of from about 1 to about 15 minutes. Preferably in order to provide for adequate, but not excessive cobalt ion absorption the anodized article being treated should be immersed for a period of from about 8 to 12 minutes in an initial sealing solution as described.

After such a time interval, the partially sealed oxide surface is preferably again treated so as to remove entrained surface material ions. This may be accomplished as before by Washing or rinsing in tap water followed by washing or rinsing in de-mineralized water and then if desired by washing or rinsing in a sodium bicarbonate bath as previously described. Preferably the temperature of the oxide film during this step or stage should be maintained as previously described between freezing and 90 F. and preferably between about 65 to 75 F. so as to avoid any tendency for further sealing during this removal of entrained material.

An article which has been so processed is preferably then further sealed by being immersed in a sealing solution containing soluble chromate ions. Because of economic considerations, ease of solution and the like it is preferred that this second sealing solution be a solution of sodium dichromate or sodium chromate. Other soluble chromate salts can, however, be employed.

This second solution should be maintained at an elevated temperature such as to promote reactions between the cobalt ions in the anodized film leading to the formation of cobaltous chromate compounds as hereinafter indicated so that these reactions take place as rapidly as possible without cobaltous ions being leached from the oxidized film. In general the higher the temperature used the faster these reactions and the more completely an oxide film as described is sealed to what may be regarded as a complete extent by them. It is considered useable results can be achieved if the second sealing bath is held at a temperature of at least about F. to its boiling point. Preferably it is held at a temperature of at least F.

The maximum temperature useable during the second sealing operation is, of course, determined by the concentration of the solute within the bath. This concentration may be varied within comparatively wide limits. If the chromate concentration within the second sealing solution is too low desired reaction products will form too slowly and to only a limited extent, resulting in the final product having inadequate corrosion protection. If, on the other hand, the concentration of the chromate ion is too great it is considered that there is an uneconomic use of chromate materials. It is considered that acceptable results can be achieved utilizing a chromate solution containing from about 2 to 10% by weight chromate ions, but that preferred results are achieved by using a chromate solution having a concentration of from about 4 to 5% by Weight of chromate ions. Solutions within this latter range have an adequate concentration of these ions so that the desired reactions can proceed rapidly and yet do not utilize what may be regarded as an excess of chromate material.

These concentrations are related to the time when an anodized, treated article is immersed in the second sealing solution as described so as to be impregnated with chromate ions and so that the desired reactions can transpire. This second sealing step should be carried out for a period adequate to react substantially all cobalt ions in the oxide film which are held so as to be capable of reacting with the chromate ions in the second sealing solution. For purely practical reasons this period should not he so long that the anodized article is held in the second sealing solution for a period past a time when significant absorption and reaction takes place. It is considered because of these limits the immersion in this second, final sealing should be for a period of from about /2 to minutes. It is preferred, however, to use a period of from about 1 to 4 minutes in this second sealing step to make sure that the desired reactions take place without wasting time.

Following this final sealing step the resultant product can, if desired, be directly used. Normally, however, this product will be rinsed in any convenient manner and dried at a temperature in excess of 200 F. so as to remove any surface water. It should not, however, be dried at a temperature so high that there is any reasonable chance of decomposition within the anodized film. It is considered that a maximum safe drying temperature is about 250 F.

It is not to be assumed from the preceding that the two sealing steps described have to be carried out by immersing an anodized film in solutions as indicated for times and temperatures as indicated. An anodized film as described may be impregnated with cobalt and chromate ions so as to achieve the results of this invention by spraying solutions of the concentrations and at the temperatures indicated containing these ions on the anodized film for time periods as discussed. The sprays of the solutions used should be sufficiently intense so that in effect any anodized film being treated is saturated with solutions to nearly the same extent that it would be saturated if it Were immersed in a bath.

Such use of spraying instead of physical immersion permits the process of this invention to be carried out with a spray-type system on a substantially continuous basis. Spray treatment during processing of the type referred to here is considered to be well-established in the industry as shown by the article, Igloos House an Automated Chemical Processing Facility by Ross commencing on page 136 of the November 1968 issue of the magazine Production. From this discussion it will be apparent that both immersing and spraying are procedures for impregnating anodized films as indicated with cobalt and chromate ions by contacting such films with aqueous solutions containing such ions.

As a result of the procedure indicated in the preceding, a corrosion inhibited or protected anodized aluminum surface can be created in which a cobalt chromate corrosion inhibitor reaction products are formed in situ in an anodized coating. Such a surface will normally have a goldishyellow type of color. Such color is considered to be somewhat significant in suggesting the content of the oxide film in the final article produced. Such content is considered to include cobalt chromate reaction products of a partially understood character. It is considered that such reaction products are chemically combined with or held within alumina in the porous area or layer of the anodized oxide surface in some presently un-understood manner more or less corresponding to the manner in which various salts are held in an aluminum hydroxide gel or mineral type structure.

These views are held because of the complex nature of the chemistry of both aluminum and cobalt chromate compounds. As is pointed out in the pending U.S. patent application entitled, Production of Dicobaltous oxychromate and Percobaltous Oxychromate and Use of Cobalt Chromates in Inhibiting Corrosion the chemistry of cobaltous chromates is comparatively complex. Under conditions as indicated in these applications cobalt hydroxide and chromaic acid do not react to produce the neutral salt cobaltous chromate. Depending upon pH these reactants will react so as to produce dicobaltous oxychromate or corresponding percobaltous oxychromate having an excess of cobalt oxide over that found in dicobaltous oxychromate.

Because of the complex nature of the porous oxide layer within an anodized aluminum oxide film, it is considered substantially impossible to determine exactly what the pH will be at the various reactive sites in the porous oxide, particularly after the initial, partial sealing with the cobaltous ion. It is considered apparent, however, that the reaction products created are some sort of cobaltous oxychromate compounds which are probably loosely chemically attached to complex alumina structures. It is believed that such compounds are particularly eifective in an article produced as herein described because of the varying rates at which they tend to slowly go into solution during various conditions of use.

The following specific examples are given as an aid to understanding the invention disclosed. It will be understood that the parameters set forth in these examples may be changed in accordance with the preceding disclosure in this specification without departing from the essential character of the invention.

EXAMPLE 1 Bare 7075-T6 aluminum sheet can be sulfuric acid anodized in a 17% by weight sulfuric. acid solution at a temperature of 70 F. for a period of 20 minutes at a current density of 12 amperes per square foot. After such anodizing the sheet can be rinsed with tap water, then de-mineralized water and then immersed in a 5% aqueous sodium bicarbonate solution for 3 minutes all at 70 F. The resulting anodized sheet can then be immersed in an aqueous cobaltous acetate solution containing 3% by weight cobalt at a temperature of F. for a period of 10 minutes. Then the partially sealed sheet resulting can be rinsed with tap water, then de-mineralized water and then put in a bicarbonate solution as before. Next the so-processed sheet can be immersed in an aqueous bath of sodium dichromate containing a 4% by weight chromate ions maintained at a temperature of 210 F. for a period of two minutes. All of these operations should be performed one after another as rapidly as possible to minimize the chances of undesired scaling. The product resulting from this will have a goldish-yellow color and if desired can be rinsed with tap water and air dried until dry at temperature of 200 F.

be altered so as to spray the solutions of cobaltous acetate and SOdllJlTl dichromate on the sheet being processed instead of immersing this sheet in these solutions. The solutions used should be at the temperatures indicated in the preceding example and the sheet treated should be sprayed for the time periods indicated in this example.

What is claimed is: 1. A process for producing a sealed, anodized film on an aluminum surface which contains an inhibitor against galvanic corrosion between two dissimilar metals, said process including the steps of anodizing said surface so as to produce a porous aluminum oxide film on said surface and thereafter sealing said surface, in which the improvement comprises:

sealing the pores in said film by the steps consisting essentially of, placing cobalt ions in said film by impregnating said film in an aqueous solution containing cobalt ions so as to allow such cobalt ions to be taken up into said film without completely sealing the pores in said film, and thereafter reacting the partially sealed film produced by impregnating said partially sealed film in an aqueous solution containing chromate ions in order to react the cobalt ions taken up into said film with said chromate ions so as to form in situ within said film a cobalt-chromate composition sealing said film against corrosion and being located in said film where it can serve to inhibit galvanic corrosion occurring between two dissimilar metals. 2. A process as claimed in claim 1 wherein: said placing is accomplished by contacting said film with an aqueous solution containing 1 to 15% by weight cobalt ions at a temperature of from 60 to 150 F. for a period of from 1 to 5 minutes. 3. A process as claimed in claim 1 wherein: said placing is accomplished by contacting said film with an aqueous solution containing from 1 to 5% by weight cobalt ions at a temperature of from 110 to 130 F. for a period of from 8 to 12 minutes. 4. A process as claimed in claim 1 wherein: said reacting is accomplished by contacting said partially sealed film with a solution containing from 2 to by weight chromate ions at a temperature of from 150 F. to the boiling point of this solution for a period of from /2 to 10 minutes. 5. A process as claimed in claim 1 wherein: said reacting is accomplished by contacting said partially sealed film With a solution containing from 4 to 5% by weight chromate ions at a temperature of from 180% F. to the boiling point of said solution for a period of from 1 to 4 minutes. 6. A process as defined in claim 1 wherein: said placing is accomplished by immersing said film in an aqueous solution containing 1 to 5% by weight cobalt ions at a temperature of from 60 to 150 F. for a period of from 1 to 15 minutes, said reacting is accomplished by immersing said partially sealed film in a solution containing from 2 to 10% by weight chromate ions at a temperature of from 150 F. to the boiling point of this solution for a period of from /2 to 10 minutes. 7. A process as defined in claim 1 wherein: said placing is accomplished by spraying said film in an aqueous solution containing 1 to 5% by Weight cobalt ions at a temperature of from 60 to 150 F. for a period of from 1 to 5 minutes, said reacting is accomplished by spraying said partially sealed film in a solution containing from 2 to 10% by weight chromate ions at a temperature of from 150 F. to the boiling point of this solution for a period of from /2 to 10 minutes.

8. A process as claimed in claim 1 wherein:

said surface is anodized in an aqueous sulfuric acid electrolyte containing 7 to 25% by weight sulfuric acid at a temperature of from 60 to F. and at a current density of from 7 to 20 amperes per square foot for a period of from 5 minutes to 1 hour.

9. A process as claimed in claim 1 wherein:

said surface is anodized in an electrolyte containing from 11 to 17% sulfuric acid for a period of from 5 to 15 minutes at a temperature of from 68 to 72 F. at a current density of 10 to 15 amperes per square foot.

10. A process as claimed in claim 1 wherein:

said placing is accomplished by contacting said film in an aqueous solution containing 1 to 5% by weight cobalt ions at a temperature of from 60 to 150 F. for a period of from 1 to 15 minutes,

said reacting is accomplished by contacting said partially sealed film with a solution containing from 2 to 10% by weight chromate ions at a temperature of from 150 F. to the boiling point of this solution for a period of from /2 to 10 minutes,

said surface is anodized in an aqueous sulfuric acid electrolyte containing 7 to 25 by weight sulfuric acid at a temperature of from 60 to 90 F. and at a current density of from 7 to 20 amperes per square foot for a period of from 5 minutes to 1 hour.

11. A process as claimed in claim 1 wherein:

said placing is accomplished by contacting said film with an aqueous solution containing from 1 to 5% by Weight cobalt ions at a temperature of from to F. for a period of from 8 to 12 minutes,

said reacting is accomplished by contacting said partially sealed film with a solution containing from 4 to 5% by weight chromate ions at a temperature of from F. to the boiling point of said solution for a period of from 1 to 4 minutes,

said surface is anodized in an aqueous sulfuric acid electrolyte containing 11 to 17% by weight sulfuric acid at a temperature of from 60 to 90 F. and at a current density of from 7 to 20 amperes per square foot for a period of from 5 minutes to 1 hour.

12. A corrosion protected anodized aluminum surface which comprises:

an anodized aluminum oxide coating on said surface, said coating having pores sealed by a cobalt-chromate reaction composition containing cobaltous oxychromate compounds, said coating containing said reaction composition having a goldish-yellow color.

References Cited UNITED STATES PATENTS 3,616,298 10/ 1971 Fassell, Ir. 204-35 N 3,099,610 7/1963 Cybriwsky et a1. 20435 N 3,468,766 9/1969 Lang 204-35 N OTHER REFERENCES Cohn and Sosson: Plating, August 1960, pp. 917-925.

Pearlstein: Metal Finishing, August 1960, pp. 40-43.

Wernick and Pinner: Aluminum Draper Ltd., Teddington, Great Britain, 1964, pp. 337, 340, 492-3.

HOWARD S. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204-38 A, 58