US3832239A - Production of clear,sealed anodized films - Google Patents

Abstract

IT IS CONVENTIONAL TO SEAL ANODIZED FILMS PRODUCED ON ALUMINUM SURFACES BY ELECTROYLSIS IN A SULFURIC ACID ELECTROLYTE BY NEUTRALIZING SUCH FILMS, WASHING THEM AND THEN CONTACTING THEM WITH A SEALING SOLUTION. AN EFFECTIVE SOLUTION FOR SEALING SUCH FILMS SO THAT THEY ARE CORROSION RESISTANT AND SO THAT THEY ARE SUBSTANTIALLY CLEAR IN APPEARANCE CAN CONSIST ESSENTIALLY OF 0.1 TO ABOUT 5% BY WEIGHT OF CHROMIC OXIDE PRESENT IN THE FORM OF A COMPOUND SELECTED FROM THE GROUP CONSISTING ESSENTIALLY OF CHROMIC ACID AND AMMONIUM AND ALKALI METAL CHROMATE AND DICHROMATE SALTS AND MIXTURES THEREOF AND FROM ABOUT (1.5 TO ABOUT 10% BY WEIGHT OF TUNGSTATE IONS PRESENT IN THE FORM OF A COMPOUND SELECTED FROM THE GROUP CONSISTING ESSENTIALLY OF AMMONIUM AND ALKALI METAL TUNGSTATE AND METATUNGSTATE SALTS AND MIXTURES THEREOF. THE SOLUTION USED MAY CONTAINS AN ALKALI METAL HYDROXIDE OR SIMILAR PH ADJUSTING AGENT AND SHOULD HAVE A PH OF FROM ABOUT 4.5 TO ABOUT 8 AND SHOULD BE AT A TEMPERATURE OF FROM ABOUT 130*F. TO IMMEDIATELY BELOW THE BOILING POINT OF THE SOLUTION.

Description

United States Patent Oflice 31,832,239 Patented Aug. 27, 1974 3,832,239 PRODUCTION OF CLEAR, SEALED ANODIZED FILMS Geraldine M. Hoch, Chatsworth, and Kenneth E. Weber, Granada Hills, Calif., assignors to Lockheed Aircraft Corporation, Burbank, Calif.

No Drawing. Continuation-impart of application Ser. No. 881,884, Dec. 3, 1969. This application Dec. 26, 1972, Ser. No. 318,522

Int. Cl. C23c /50 US. Cl. 204-35 N 2 Claims ABSTRACT OF THE DISCLOSURE It is conventional to seal anodized films produced on aluminum surfaces by electroylsis in a sulfuric acid electrolyte by neutralizing such films, washing them and then contacting them with a sealing solution. An effective solution for sealing such films so that they are corrosion resistant and so that they are substantially clear in appearance can consist essentially of 0.1 to about 5% by weight of chromic oxide present in the form of a compound selected from the group consisting essentially of chromic acid and ammonium and alkali metal chromate and dichromate salts and mixtures thereof and from about 0.5 to about by weight of tungstate ions present in the form of a compound selected from the group consisting essentially of ammonium and alkali metal tungstate and metatungstate salts and mixtures thereof. The solution used may contain an alkali metal hydroxide or similar pH adjusting agent and should have a pH of from about 4.5 to about 8 and should be at a temperature of from about 130 F. to immediately below the boiling point of the solution.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of a copending application Ser.. No. 881,884, filed Dec. 3, 1969 entitled Clear Sealing of Anodized Aluminum, and now abandoned. The entire content of this co-pending application is incorporated herein by reference.

This application contains subject matter which is related to the subject matter of the co-pending application Ser. No. 286,595, filed Sept. 5, 1972, entitled Production of Stain Resistant, Clear, Sealed Anodized Films, (Lockheed Docket No. P-01-1370). The entire content of this co-pending application is incorporated herein by reference.

BACKGROUND OF THE INVENTION The invention set forth in this specification pertains to the sealing of anodized aluminum films so as to improve the corrosion resistance of such films and so as to provide films which are substantially clear in appearance.

The word aluminum is used in this specification in a generic manner corresponding to the way this word is conventionally employed industrially. This word is conventionally used to not only designate the metal aluminum but is commonly employed to designate various alloys which predominately contain aluminum. Such alloys are commonly referred to by various trade designations such as 6061, 7075, 2024 and the like used to indicate the compositions of these alloys.

Various aluminum articles or bodies are wellwknown to be highly susceptible to corrosion. It is also well-known that such susceptibility may be decreased by anodizing them in an appropriate electrolyte so as to provide upon their surfaces adherent aluminum oxide coatings or films. Such coatings or films are normally referred to as anodized films because of the manner in which they are produced. Although they can be produced by anodizing aluminum articles or bodies in a wide number of different electrolytes, it is recognized in the industry that it is desireable to form such films using sulfuric acid electrolytes.

The reasons for this are comparatively unimportant to the present invention. They primarily relate to the physical properties and characteristics of the films produced by anodization in this particular electrolyte. These properties or characteristics differ from those obtained by anodization in other electrolytes in various manners and to various extents. Although the use of sulfuric acid anodization is recognized to be highly desirable there is a drawback associated with this type of procedure. The films produced by this type of anodization are of a porous, complex character. As the result of this, they do not provide the degree of corrosion protection desired in many applications of aluminum structures.

In order to increase the resistance of anodized aluminum articles and bodies to corrosion and similar attack it has become customary to seal anodized aluminum films by immersing such films either in hot water or in a solution containing one or more different ions capable of entering into the porous structure of an anodized film so as to tend to fill up the pores within such a film. Those familiar with anodization will realize that the mechanism of such scaling is extremely complex and involves considerations which are not well understood at the present time. It can be theorized that the sealing action involved concerns the formation of complex alumina gels or hydrates when water alone is used as a sealant. Probably zeolitic type compounds are formed in addition to such gels or hydrates when various ions are used for sealing purposes. It is possible, however, that the sealing action with various ions is primarily an action involving the mere physical blocking of the pores within an anodized film.

It is well-established that various different sealing procedures used with anodized aluminum films are more desirable than others for any of a variety of different reasons. Films sealed in accordance with certain procedures are more corrosion resistant than fihns sealed with other procedures. Certain of such film procedures are known to create a loose or smutty type of surface structure. Various known sealant procedures are well-recognized to discolor an anodized aluminum surface so that such a surface does not have essentially the appearance of the base aluminum metal.

Certain other sealing procedures are considered disadvantageous inasmuch as they locate or tend to locate within an anodized film an organic material or composition which may react under some circumstances so as to detrimentally affect either the corrosion resistance and/or the color of an anodized coating. Thus. it is known to seal an anodized film with a parafiin type of composition.

In some cases various organic agents such as relatively reactive nitrogen compounds are used with various inorganic ions in sealing. Such compounds can decompose when subject to actinic light for a prolonged period and/ or can react with various reagents coming in contact with an anodized aluminum surface. Because of this type of consideration it is considered that no sealing procedure utilizing an organic material is desirable in providing an efiectively sealed anodized aluminum article or body which can be used for a wide variety of application.

The importance of these factors is best considered with reference to the utilization of anodized aluminum articles and bodies. Such articles and bodies may be used in a wide variety of dilferent ways where they are subject to a wide variety of different corrosive and related type influences. To be acceptable a sealed, anodized film must hold up for prolonged periods so as to protect against corrosion over such periods. For aesthetic and related considerations in many types of uses such films also must have the initial appearance of the aluminum metal and must preserve such an appearance even after prolonged use. This type of aesthetic factor is considered to be particularly important when aluminum is used in buildings and similar structures.

SUMMARY OF THE INVENTION A broad objective of the present invention is to provide a new and improved process for providing clear, sealed anodized films on aluminum bodies and articles. More specifically the invention is intended to provide a process which will produce clear, sealed films which are capable of withstanding a wide variety of conditions for prolonged periods and which will preserve their initial appearance throughout such periods of use. Another ob.- jective of the present invention is to provide a process for producing such clear, sealed films which is essentially very simple and which may be easily and conveniently carried out at a comparatively nominal cost with a minimum of difficulty.

In accordance with this invention this process involves contacting an anodized aluminum surface prior to the oxide film on the surface being sealed with a sealing bath containing from about 0.1 to about 5% by weight of chromic oxide and from about 0.5 to about by weight of tungstate ions and having a pH of from about 4.5 to about 8 at a temperature of from about 130 F. to immediately below the boiling point of the solution for a time period sufficient to at least partially seal the oxide coating.

DETAILED DESCRIPTION Unfortunately a summary such as the preceding is of a limited character and cannot indicate many facets and features of an invention such as the invention set forth in this specification. As an example of this, the invention can be used with a degree of effectiveness with an aluminum body or article which has been anodized in a number of different known ways. The term aluminum is used herein a broad sense as indicated earlier in this specification. However, it is considered that a body or article for use with the invention should preferably be anodized in accordance with standard sulfuric acid anodizing procedures so that it will have a physically strong, adherent oxide coating which is sufiiciently porous so that it can be effectively sealed.

Such conventional sulfuric acid anodizing involves immersing an aluminum body or article in an aqueous electrolyte containing of from about 7 to about 25%, and preferably of from about 11 to about 17%, by weight of sulfuric acid at a temperature of from about 60 to about 90 F. and preferably of from about 68 to about 72 F., and applying a current at a current density of from about 7 to about 20 amperes per square foot, and preferably from about 10 to about 16 amperes per square foot, at a voltage normally from about to volts until an oxide film is produced which is sufficiently thick to with stand normal physical abrasion.

After such a film is produced the aluminum article or body formed so as to be covered with the oxide film is normally neutralized by immersion in a weak alkaline solution such as the aqueous solution containing 5% by weight sodium bicarbonate and then is normally rinsed with distilled or deionized water at room temperature. Preferably these steps should be carried out immediately, one after another, so as to avoid any chance of intervening sealing of the anodized surface produced and should be immediately followed by the sealing step of the present invention for the same reason.

This sealing step involves some relatively complex factors relative to the reactive ions present in this sealing bath or solution. One of these is a recognition of the complex forming tendency of tungstate ions. Such tungstate ions tend to react with one another so as to form tungstate dimers, trimers and other comparatively longer and more complex inorganic complexes or polymers. They also tend to react quite rapidly with alumina as it is present on an unreacted anodized film so as to form related inorganic complexes or polymers.

It is considered that tungstate ions are somewhat unique in their ability to easily and readily form such inorganic complexes or polymers. This is considered to be evidenced by the fact that molybdate ions cannot be effectively utilized in accordance with this invention in obtaining sealed, anodized films which are as satisfactory in providing corrosion protection as tungstate ions. This is considered to be somewhat surprising in view of the close relationship of tungsten and molybden in the periodic classification of the elements.

The inorganic complex or polymer-forming characteristics of the tungstate ion is considered to make these ions particularly useful in. sealing anodized aluminum films. It is considered that effective sealing in such films requires the location of ions as indicated herein in the pores of such films at all possible reactive sites where such ions can be trapped or held in order to physically block or occupy such pores so that water and other corrosive cannot move through the anodized film. Because of the reactivity of tungstate ions with themselves and with alumina it has been considered that tungstate complexes could be effectively formed so as to physically fill up the pores in anodized films so as to seal such films.

Unfortunately however tungstates are not considered satisfactory for use in sealing anodized aluminum films because of the fact that a conventional solution of a tungstate salt or salts will tend to pit these films during sealing. Such pitting is of course detrimental in increasing the tendency for such a sealed film to corrode during service and use conditions, As nearly as it is known this tendency of tungstates to pit anodized films has not previously been recognized. It is however possible that it may have been observed.

'It has previously been proposed to use tungstates and other similar acid ions in sealing anodized films in a solution with various nitrogen-containing organic compounds. As pointed out in the preceding the use of such organic compounds is considered to be disadvantageous because of the possibility of such compounds entering into an anodized film. Any organic compound as indicated located within an anodized film may in time be leached from such a film to a sufficient extent as to make the films susceptible to corrosion. Further any such organic compound may decompose in time when subjected to various ambient conditions such as heat, actinic radiation or the like. Such decomposition is considered to be capable of affecting the appearapce of an anodized film and/or to make such a film somewhat susceptible to corrosion.

In accordance with this invention this tendency of tungstate salts to cause pitting during scaling is effectively combated with the use of a sealing solution containing chromate ions. The chromate ions in a sealing solution of this invention acts as an inhibitor tending to limit and prevent the Water molecules within such a solution from attacking the anodized surface. Further if a sealing as herein described contains some chromate ions, to a degree these ions will tend to be taken up into an anodized film during sealing and will remain in the sealed film where they will serve to inhibit a surface and galvanic corrosion.

In functioning in this manner in a final sealed, anodized film chromate ions operate in substantially the same manner as these ions operate when used in a known manner as a galvanic corrosion inhibitor. Chromate compounds such as dicobaltous oxychromate have been and are currently being utilized as galvanic corrosion inhibitors by merely being located where galvanic corrosion is anticipated. As they are so used these compounds are considered to supply chromate ions when contacted by water so as to stifle or inhibit normal galvanic reaction between two dissimilar metals. When a sealed film in accordance with this invention is contacted by water it is considered that there is a tendency for the entrapped chromate ions to be leached out of the anodized coating so as to function to inhibit any galvanic corrosion resulting from the water tending to electrically couple the aluminum beneath the anodized film and a dissimilar metal.

The tendency for tungstate ions to form complexes as indicated gradually increases as the pH of a sealing solution as herein described decreases. If the pH of the treatment solution drops below about 4.5 tendency of the tungstate ions in the solution to react with one another forming inorganic polymer type complexes becomes significantly pronounced. Although a limited amount of such complex formation is not detrimental if a pH of less than this value is used this complex formation can lead to a loss of tungstate material in the form of precipitates. Further, if the pH of the treatment bath is lower than this 4.5 figure it is considered that there may be a tendency for acid attack on the film on aluminum being treated.

If the pH of the treatment solution increases so as to be significantly alkaline the tungstate ions tend either not to form complexes as indicated or to form such complexes very slowly. As a consequence of this it is considered the pH used should be no higher than about 8 since at higher pHs the reactivity of the tungsten ions with an anodized film will be comparatively limited. In practice these pH values are best correlated with the pH range at which chromate ions will enter into an anodized film during sealing. Such correlation is necessary to provide the presence of suflicient chromate ions during the scaling to prevent pitting and to assure the formation of a uniform, sealed film without there being such a quantity of chromate ions present as will discolor the anodized coating.

It is well-established that anodized films sealed with chromates will normally have a yellow-type of surface coloration as the result of the action of the chromate ions. Sealed anodized films having such a coloration are considered disadvantageous and undesirable for many applications where appearance is a factor. By controlling the pH of the solution used in sealing it is possible to regulate the entrance of chromate ions from a sealing solution as herein described into an anodized coating.

In general the higher the pH of the sealing solution used the greater the quantity of chromate ions that will enter into a sealed film from a sealing solution as herein described. If a sealing solution in accordance with this invention has a pH of about 8 or higher there is a significant chance that sufficient chromate ions will enter into an anodized film as it is sealed so as to tend to discolor such a film. If the pH of the solution is below about 4.5 the tendency of such chromate ions to enter into an anodized film is so limited that there is some danger that these ions will not be effective in assisting in the formation of a uniform, sealed film of a desired character.

It will be recognized that what is involved here is essentially a trade-off situation. From the point of view of the tendency of tungstate ions to seal an anodized film it is preferable to use a sealing solution having a pH which approaches 4.5. On the other hand, from the point of view of achieving the benefit of the use of the chromate ions it is preferable to use a sealing solution having a pH approaching 8. Because of these factors it is considered preferable to use a sealing solution having a pH of within the range of from about 5 to 7, although a solution having a pH of from about 4.5 to about 8 may be effectively employed.

The concentration of the sealing solution may also be utilized to a degree to regulate the sealing action achieved so that the chromate ions serve their intended function in preventing pitting and in promoting the formation of a uniform clear, sealed anodized surface. It is presently considered that the sealing solution used should contain from about 0.1 to about 5% by Weight of chromic oxide and from about 0.5 to about 10% by weight of tungstate ions. If greater amounts of chromate material are used than indicated in this range there is danger that a sealed, anodized film will not have a desired colorless appearance even if the pH of the sealing solution is regulated so as to minimize the entrance of the chromate ions into an anodized surface. On the other hand, if the quantity of chromate material is lower than within this range it is considered impossible to achieve the desired benefits of the invention even if the pH of the sealing solution is regulated towards the upper end of the range indicated so as to promote the activity of the chromate ions.

Similarly, if the amount of the tungstate ions is less than within the range indicated there will be too little tungstate material present to accomplish effective sealing even if the pH of the sealing solution is at the lower end of the range noted so as to promote tungstate sealing. It is not considered that any significant beneficial results are achieved utilizing a greater quantity of tungstate material than indicated. Indeed, with greater quantities of tungstate ions than specified, there may be a tendency for the tungstate ions to preferentially seal an anodized surface in such a way that the benefits of the use of chromate ions are not achieved even if the pH of the solution is regulated so as to minimize the complex forming tendencies of the tungstate ion.

The chromate ions used in the sealing bath can be conviently obtained through the use of chromic acid or ammonium or alkali metal chromate or dichromate salts and mixtures of any of these. Similarly the tungstate ions used in a sealing solution of the invention may be obtained from ammonium or alkali metal tungstate and tungstate salts or mixtures of any of these. The use of ammonium and alkali metal salts such as lithium, sodium and potassium salts is not considered detrimental inasmuch as the positive ions obtained from such salts do not interfere with the sealing action achieved with the invention.

Normally, it is unnecessary to regulate the pH of the sealing solution as herein described through the use of separate compounds for this purpose inasmuch as a significant degree of pH regulation may be achieved through the use of chromic acid or various salts as indicated in the preceding discussion. Also, it is normally not necessary to use a reagent to regulate the pH of such a solution since tungstate salts as indicated will tend to a degree to exercise a buffering effect to stabilize the pH of these solutions. Although tungstates are not particularly effective buffering compounds the fact that they serve to a limited extent in this regard is considered beneficial with the invention.

On those occasions where it is desirable to regulate the pH of a sealing solution through the use of a separate reagent for this purpose, such pH control can be achieved through the use of an alkali metal hydroxide or salt such as potassium, sodium or lithium hydroxide carbonate or bicarbonate. The carbonate ion does not appear to affect the sealing action achieved with the invention. Neither does minor amounts of sulfuric acid appear to afifect this action. This is significant since if some sulfuric acid should be carried over by surface entrainment to a sealing bath of the present invention after anodization of an article such a minor quantity of the acid will not detrimentally afiect the sealing action achieved.

In order to avoid any possibility of either corrosive attack during sealing in accordance with this invention or of interference with the deactions achieved resulting from the presence of unnecessary ions it is preferred that a sealing solution in accordance with this invention contain only chromic acid or chromate, dichromate, tungstate, and metatungstate salts as indicated in the preceding discussion and only if necessary a very limited amount of a pH adjusting compound or compounds as indicated in the preceding discussion. Normally it is considered that the amount of such pH controlling material should be no 7 greater than about A: by weight of the sealing solution.

The reactions achieved in scaling in accordance with this invention are essentially time-temperature reactions. In other words, the more elevated the temperature the more rapid these reactions and the less time required to achieve effective sealing. Because the sealed films in accordance with this invention are somewhat susceptible to dye staining it is normally preferred that the sealed films produced in accordance with the invention are not sealed to the maximum possible extent so that they can be further dyed and sealed in accordance with a specialized procedure.

It is considered that these time-temperature reactions are too slow for practical purposes if the sealing solution used is held at a temperature below about 130 P. On the other hand if the sealing solution is at a temperature approximating the boiling point of a solution a desired uniform sealing may not be achieved because of localized formation of vapor pockets from the water within the solution along a surface being sealed. Since a sealing solution as herein described will boil above 212 F. at a temperature dependent upon its concentration it is considered that at a maximum the scaling solution used should be employed at a. temperature of no greater than about 210 F.

The time required to achieve the reactions which take place on sealing with a solution as herein indicated can be varied within comparatively wide limits. Even when a scaling solution as described is at a temperature at the maximum of the range indicated it is considered that insufiicicnt sealing for practical purposes takes place if an anodized body or article is in a sealing solution as described for less than about 1 minute. It is considered that the maximum time normally required to obtain adequate sealing of an anodized surface is about 30 minutes even if the solution used is about 130 F.

The process of the present invention may be easily and conveniently carried out by merely immersing an anodized article or body into a sealing solution as described. Because of the simplicity of the process, no elfort is made herein to specifically delineate any manner of carrying it out. After an article or body has been contacted with a sealing solution as described, normally it would be rinsed to remove any entrained solution and dried in accordance with conventional practice. Such a body or article may be conveniently air dried or may be dried at an elevated temperature to drive oif any entrained surface moisture.

The following specific examples are set forth so as to clearly indicate the nature of the invention. It is to be understood that these examples are given for this purpose and no other.

EXAMPLE 1 Test panels of 6061, 7075, 2024 aluminum alloys which had been anodized in a 15% by Weight sulfuric acid electrolyte in accordance with conventional practice and then rinsed were immersed in a sealing solution of 1% by weight chromium trioxide and 4 /2% by weight sodium tungstate at 200 F. for minutes. These panels were then taken from the solution, rinsed and air-dried. They had a clear appearance corresponding to their appearance prior to scaling. As of the drafting of this specification (Dec. 15, 1972), these test panels had been subjected to standard salt spray testing since June 16, 1972 and ex- 8 hibited no sign of the pitting characteristic of salt spray corrosion.

EXAMPLE 2 Test panels of 7075-T6 and 2024-T3 aluminum were anodized in accordance with conventional practice and then were sealed with an aqueous solution of 1% by weight chromium oxide and 5% by weight sodium tungstate at a temperature of 200 F. for about 10 minutes. These panels after this sealing had a clear appearance which was substantially the same as they had prior to this sealing.

It would be possible to set forth in this specification further examples giving the extreme values of the various ranges indicated in the preceding discussion. It is thought that such use of further examples is unnecessary to enable those skilled in the art of sealing anodized aluminum to practice the invention.

We claim:

1. In a process for sealing a porous aluminum oxide film electrolytically formed on an aluminum surface in which the film is contacted by a scaling solution after it has been formed and before it is otherwise sealed, the improvement which comprises:

said solution consisting essentially of from about 0.1%

to about 5% by weight chromic oxide present in the form of a composition selected from the group consisting of chromic acid, and ammonium, lithium, potassium and sodium chromates and dichromates and mixtures thereof and from about 0.5% to about 10% by weight of tungstate ions present in the form of a composition selected from the group consisting of ammonium, lithium, potassium and sodium tungstates and metatungstates and mixtures thereof and from about 0 to about /2% by Weight of a pH adjusting agent selected from the group consistin of lithium, potassium and sodium hydroxide, carbonates, bicarbonates and mixtures thereof,

said solution having a pH of from about 4.5 to about 8 and being at a temperature of from about F. to immediately below the boiling point of said solution, said film is contacted by said sealing solution for a period of from about 1 to about 30 minutes,

said contact resulting in reactions sealing said film with a mixture of chromate and tungstate ions so that said film is resistant to corrosion and has substantially the same appearance as said film prior to contact to said sealing solution.

2. A process as claimed in claim 1 wherein:

said solution has a pH of from about 5 to about 7 and contains no pH adjusting agent.

References Cited UNITED STATES PATENTS 10/1971 Fassell 204-35 N 8/1959 Spooner 20435 N U.S. Cl. X.R. 20435 N