IL43118A - Production of stain resistant clear sealed anodized films - Google Patents

Description

43118/3 PRODUCTION OF STAIN RESISTANT, CLEAR, SEALED, ANODIZED FILMS , D'&'UNI D'S1|7B» ,α'θηο '-ΙΙΟΙ The present invention pertains to the sealing of ^ anodized aluminum films so as to improve the corrosion and stain resistance of such films .

It is conventional to anodize aluminum bodies and articles in order to provide upon them adherent , protective aluminum oxide coatings or films . Such coatings or films are normally referred to as anodi zed coatings or films inasmuch as they are produced by electrolytic processes in appropriate electrolytes at the anodes of electrolytic cells or systems . Normally a sulfuric acid electrolyte is utilized in the production of these anodized films . The oxide coatings produced in this manner are somewhat porous in character and are somewhat subject to various types of corrosion . Further , these anodized films are well recognized ( to be susceptible to stain-ing as a result of various types of materials contacting these films .

A number of efforts have been made at "sealing" anodized films so as to improve both their corrosion and stain resistance . One such method involves contacting a rinsed, anodized aluminum surface with a ;solution preferably containing from about 1.5 to about 7% by weight of a soluble tung-state salt and from about 0.5 to about 5% by weight of chromic oxide either present as chromic acid or as a soluble salt of this acid having a pH ,of from about 3.2 to about 8.5 and preferably of from about 5.5 to about 7 at a temperature of from about 150 °F. to about 212 °F .

Sealed anodized coatings produced in accordance with the above described proces!s are considered to be highly desirable because they are substantially clear in character. their resis ance to corrosion. Parts produced in accordance with the above described process have withstood salt spray testing for significant periods without signs of the usual salt caused corrosion. However, in spite of these advantageous characteristics, sealed anodized coatings produced in accordance with the above described process are not considered to be as desirable as reasonably possible for certain commercial applications because they tend to lack adequate resis-. tance to staining.

Normally such resistance to staining is tested on an anodized aluminum article by applying to the surface of such an article a quantity of anthr quinone violet R dye, removing this dye and then determining visually if any of the dye has been absorbed in the anodized coating so as to stain this film or coating. Although a test of this type is subjected to criticism because of its empirical nature, nevertheless this type, of test is practical for determining whether or not an anodized aluminum surface is effectively stain resistant.

The use of this and other related tests has estab- i . ' ' , , . lished a need for improvement in the.stain resistant character of sealed anodized films produced utilizing a tungstate- chromate bath as indicated. More specifically there is the need for improving anodized coatings sealed with chromates and tungstates so as to retain the advantageous corrosion resistant properties or ;characteris ics produced by such combined sealing and so as to; achieve in such coatings an effective, substantially clear stain resistant appearance.

The primary purpose 'pf the present invention is to is in end d to provide a new and improved process for sealing anodized coatings which retain the benefits of sealing anodized coatings with tungstate-chromate solutions as indicated, but which also provides in such coatings a degree of stain resistance not achievable with such sealing alone. The present invention also teaches a process which may be easily and conveniently carried out at a comparatively minimal cost so that the articles produced by this process can be economically com- . petitive.

In accordance with the present invention these and various related advantages of it are achieved by partially sealing a conventionally anodized aluminum body or article by contacting the anodized film on such a body or article with a solution of a soluble tungstate salt and chromate oxide present either as chromic acid or as a soluble salt of it and then subsequently and separately sealing the partially sealed oxide film produced by contacting it with a soluble nickel or cobalt salt of a weak acid. It will be realized that the practice1 of this invention involves further details than are indicated in this 1summary.

The word "aluminum" employed in this specification is used herein in a broad sense corresponding to the manner in which this word is conventionally utilized industrially. Thus, the word "aluminum" as used herein does not only design-nate the met&l known by this word and/or articles or bodies created from this metal * but it is also utilized to designate alloy compositions and/or articles , or bodies created from this metal h ch are conventionally designated by the word "aluminum" since they predominantly jcontain this metal. A number of -uch alloys are common'ly known and used. They are frequently designated by trade designations such as 7075-Ί 7075-T76 and 2024-T3 sed to designate their composition.

Conventionally such aluminum articles and bodiei are anodized in an electrolytic cell or system containing ό comparatively strong mineral acid such as sulfuric acid to provide on them a comparatively thick porous film which is physically strong .enough to resist normally encountered abrasion and the like. Inasmuch as such anodizing is conver. ional in the industry it is not considered necessary to describe it in detail in this specification.

It is considered that normally such anodizing is carried out utilizing an aqueous electrolyte containing from about 7 to about 25% and preferably from about 11 to about 17% by weight of sulfuric acid at a temperature of from about 60 °F. to about 90 °F. and preferably from about 68 °F. to about 72°F. at a current density within the range of from about 7 to about 20 amperes per square foot and preferably from about 10 to about 15 amperes per square foot at a voltage normally from about 15 to about 20 volts until an oxide film is produced which is sufficiently thick to withstand the physical abrasion or the like to which it is apt to be subject.

Following such anodization an anodized film may be neutralized in a weak alkaline solution such as a solution containing 5% by weight sodium bicarbonate and then is rinsed. An anodized surface film produced in this manner is well recongnized to be somewhat porous in character and to be somewhat susceptible to corrosion of various types and to be susceptible to being stained by contact with a wide variety of different materials.

Such a film can be effectivel sealed a ainst corrosion by contacting it with an aqueous solution containing from about 1.5 to about 7% by weight of a soluble tungstate salt and from about 0.5 to about 5% by weight of chromic oxide either present as chromic acid or as a soluble salt of this acid having a pH of from about 3.2 to about 8.5 and preferably of from about 5.5 to 7 at a temperature of from about 150 °P. to about 212°F. This process is referred to as "clear sealing" because it produces a clear, corrosion reses-tant oxide film.

The exact physical and chemical characteristics of a sealed oxide film produced in this manner are not completely known or understood. However, it is believed that in a film of this character tungstate and chromate ions are absorbed into a porous aluminum oxide matrix and are bonded together in the matrix in such a manner as to form a type of an amorphous gel in which all of the oxides are linked together by hydrogen bridges in the form of what may be loosely regarded as a zeolytic type structure in which there are various reactive sites located along an essentially inorganic polymer type structure. Although such sites are considered to be present in an anodized coating which has 1 I been "clear-sealed" as indicated with a tungstate-chromate bath, such a "clear sealed" coating is advantageous because of economic considerations pertaining to its manufacture, because of its desirable corrosion resistance and because it is of a substantially clear character.

In accordance with this invention it ras been found that the corrosion resistance! and the stain resistance of such a coating may be improved ,or enhanced by contesting such a coating with a solution of a soluble nickel or coi.alt -salt as hereinafter indicated. Such a salt is considered to locate either the nickel or cobalt ion at the reactive sites in an inorganic complex as indicated so as to in effect block these sites in such a manner that materials which would normally tend to stain an anodized aluminum coating cannot be absorbed or held by such a coating, presumably at these same sites.

The effectiveness of the present invention in producing stain resistant as well as corrosion resistant sealed, clear or substantially clear anodized films or coatings is considered to be indicated by what is regarded as a rather significant discovery. This discovery relates to the fact that effective stain and corrosion resistant sealed films can be created in which this step of treatment with a nickel or cobalt ion is preceded by a "clear sealing" process as indicated carried Out utilizing a solution containing smaller quantities of both tungstate and chromate ions than previously considered necessary in "clear sealing". This is considered important because it permits lesser quantities of materials to be used in making up solutions to be utilized with a two-step process as herein described than one would normally consider necessary than if the present invention merely added an additional step to prior "clear sealing".

In accordartce ! with this invention an initial "clear sealing" operation can be carried out utilizing an aqueous solution containing; from about 0.1% to about 5% by weight of chromic oxide either present as a chromic acid or as a soluble salt of this acialiand from about 0.5% to ab 10% b w i ht of soluble tun state salt havin a H of from about 4.5 to about 8.0 at a temperature of from about 130 °F. to a temperature immediately below the boiling point of the solution. Certain specific aluminum compositions are considered to be most effectively sealed using ranges and proportions within the limits given which are determined empirically with reference to the corrosion resistance achieved.

It is noted that the proportions of such ingredients given in these ranges are lower than those given in the preceding discussion relative to "clear sealing". It will also be noted that the temperature range useable with this invention during this initial partial sealing step is somewhat different than the temperature range indicated in the preceding as required for effective "clear sealing". Further, there is a minor difference in the pH of the solution used in this initial "clear sealing" operation. These differences are considered to point out the fact that the initial "clear sealing" used with the invention need only be what can be regarded as a partial sealing.

The bath used in this initial sealing can easily be created utilizing a conventional soluble tungstate salt such as a potassium, sodium, lithium or ammonium salt. It is also possible to utilize potassium, sodium or ammonium metatungstates or a combination of any of these salts. The chromate used may be in the form of chromium trioxide (which of course forms chromic acid in solution) or a conventional chromate compound such as a sodium, potassium, lithium or ammonium chromate or dichromate or any "mixture jbf these. This initial sealing solution snould consist essentially of these salts in the sense thiat it should contain onl a mixture of one or more tungstates and one or more chromates and if reasonably necessary, a suitable pH adjusting compound such as sodium hydroxide or the like containing ions such as alkali metals ions and hydroxide ions which will not significantly enter into the sealing action obtained, and, if desired 'one or more conventional wetting agents used in accordance with conventional practice so as to facilitate solution penetration into an anodized surface provided that such a wetting agent is of a known type not effecting the sealing action obtained.

The proportions of the tungstate (s) or chromate (s) used in the bath in this initial partial sealing are primarily dictated by practical considerations. If amounts of either of these two types of ions below those indicated are present in the sealing solution a partially sealed anodized film having less than a desired degree of corrosion resistance will be achieved. On the other hand, if greater amounts than are indicated are used, there tends to be an uneconomic or unnecessary use of material.

A pH within the range indicated is preferred during this initial sealing since a lower pH than 4.5 may result in some acidic-type attack: on the aluminum treated and/or on the oxide film present on this aluminum. With higher pH's than are indicated there is also the possibility of alkaline attack on Ithe aluminum and the oxide. When solution temperatures below the 130°F. figure are used it appears that a desired partial type seal action is not effectively obtained. If a maximum temperature of the sealing solution is used which corresponds to the boiling point of this solution the vaporization of water may tend to interfere with the formation of a uniform, at least partially sealed coating.

Although as indicated the "clear sealing" step carried out in accordance with this invention may be performed utilizing various broad ranges of ingredients and conditions as, indicated, it is preferred to carry out the invention utilizing the more specific ranges and conditions indicated in the preceding discussion as to prior, known "clear seal" process of sealing utilizing a combined tung-state-chromate bath. This is because it is considered that it is advisable to seal an anodized coating as completely as possible in the initial sealing step so as to avoid any chance of such a coating being completely sealed because of failure of any type of the secondary sealing operation performed in accordance with thes invention. In essence, however, this is a matter of choice.

This initial sealing step or partial sealing step in accordance with this . invention can be carried out in accordance with conventional practice by either dipping an article to be sealed in a bath as! specified or by spraying such a bath on such an article. The time of contact between this initial "clear sealing" solution will, of course, effect the completeness of the sealing accomplished during the treatment with it. It: is considered that effective sealing can be accomplished with a contact period of at least one minute. Preferably, however, the contact time between an article being partially sealed in the solution should be longer in order to make sure ithat there is adequate, reasonably complete partial sealing duriing this step; It is considered that if a time eriod in excess of about twent minutes is used that nothing is gained as a result of the use of this loncj a period. In general, the hotter the temperature of the sealing solution used the lower the time period recjuired.

An anodized film or coating at least partially sealed with a tungstate-chromate bath as indicated in the preceding is further sealed in accordance with this invention with a solution containing from about 0.1 to about 10% by weight of a soluble nickel or cobalt salt. In general, the lower the concentration of such a bath, the less effective it is in providing adequate stain resistance while if a solution is used having a higher concentration than indicated, significant benefit in sealing is observed.

It is considered significant that the nickel or cobalt salt used with the present invention should be a salt of a weak acid such as the salt of either of these metals and an acid selected from the group consisting of boric acid, citric acid, formic acid and acetic acid. Salts of these metals with' other acids than are indicated are considered to normally impart to a sealed anodized film as herein described a degree of color rendering an anodized, sealed film as herein indicated unacceptable for certain applications where a substantially clear, colorless film is required. Mixtures! of various salts as indicated can be employed with the present" invention.

When this second sealing solution has a pH below about 4.5 it is considered that there is a tendency for the acidity of this solution to attack the aluminum being treated with it and/or the treated dxi'de film on the aluminum. When the pH of this secondary treatment solution is relatively high such as at a pH of 8 or above, it is considered that there is normally a problem of maintaining a desired, effective concentration of the salt used. This is because nickel and cobalt salts as employed tend to precipitate out at relatively high pH's. Indeed, with certain salts there can be solubility problems at even pH's below this pH of 8.

Because it is considered that an acidic pH tends to promote the formation of the type of complex believed to be created by practicing the present invention, it is normally pre-ferred that this secondary treatment solution employed have a pH of no greater than 7.

The importance of pH in this secondary, final sealing is considered to make it desirable to buffer the solution used in accordance with conventional practice so that this solution will have a constant pH or a substantially constant pH within the range of from about 5.5 to about 6.5. This ultimate treatment solution can be conveniently buffered to within this pH range through the use of one or more of the enumerated acids and/or through the use of salts of such acids which do not effect the sealing action achieved. Thus, acids such as boric acid, citric acid, formic acid, acetic acid and salts of such, acids such as the sodium, lithium and ammonium salts of these acids can conveniently be utilized for buffering purposes. ,The amount of such a buffering reagent should of course, be determined on the basis of · conventional considerations.

In order to avoid the possibility of secondary ingredients interfering with this "secondary sealing step, it is preferred that the solution iised consist essentially of only one or more of the nicKel or cobalt salts indicated or consist essentially of one or more of such salts and one or more buffering reagents as described. Thus, the treatment solution used should be free from metallic ions other than alkali metal ions and from acidic ions other than those, of the acids specifically enumerated. If desired the solution used may contain one or more conventional wetting agents used in accordance with conventional practice so as to facilitate solution penetration provided that such a wetting agent is of a known type not affecting the sealing action obtained. Suitable wetting agents may be anionic, cationic, or nonionic and may be conveniently used in amounts up to about 3% by weight of the solution.

Substantially the same considerations as specified in the preceding regarding the temperature of the "clear sealing" solution govern the temperature of the solution during this final sealing step. In general, if temperatures below about 150 °F. are used, the sealing accomplished tends to be inadequate. If a temperature at or above the boiling point off the solution is employed there is a possiblity that a uniform sealing action will not be achieved due to the vaporization of water.

In general, the. higher the temperature required the less the time with which the bath need be in contact with the film or coating; being processed. Contact periods of from one to twenty minutes are normally considered satisfactory. If the contact time is less than this there is a danger that there may be inadequate sealing. On the other hand, if periods greater than about twenty minutes are used no significant benefits are achieved. In general, the higher the less the time required.

This secondary sealing step may also be carried out by either dipping or immersing an at least partially "clear sealed" article as indicated in a bath as specified in the preceding or by spraying such solution on to an at least partially sealed oxide film or coating as indicated so that there is contact between the film and coating and the solution for a time period and at a temperature as specified.

The fact that this secondary sealing achieves an unexpected type reaction and/or series of reactions is considered to be illustrated by the colorless character of a final sealed film produced. As indicated in the preceding this is important for commercial reasons. This unexpected-ness is indicated by the fact that nickel and cobalt chro-mates and tungstates normally possess a distinct color.

Thus, for example, nickel chromates are of a brown color and nickel tungstates are of a greenish type color. The absence of these colors in films sealed with nickel salts as indicated indicates that these compounds are not formed as a result of a final sealing step as herein described.

The following examples are given as an aid to understanding the present invention. It is understood that these examples are given for this purpose only. They are not to be considered as limiting the invention in any respect.

EXAMPLE 1 A test panel of 7075-T6 aluminum material anodized in a sulfuric acid electrolyte in accordance with conventional practice can be partially sealed by immersing it in an a ueous solution containin 5% b wei ht sodium tun state form, the preferred method of practicing the invention. The present invention, of course, is of an intangible character and is not directly indicated by the accompanying drawing. The features or concepts of this invention are considered to be fully defined by the scope of the appended claims. form, the preferred method of practicing the invention. The present invention, of course, is of an intangible character and is not directly indicated by the accompanying drawing. The features or concepts of this invention are considered to be fully defined by the scope of the appended claims . i ANODIZE AN ALUMINUM OBJECT TO PRODUCE AN ANODIZED COATING "CLEAR SEAL" THE ANODIZED COATING BY CONTACTING IT WITH AN AQUEOUS SOLUTION CONTAINING FROM ABOUT 0.1% TO ABOUT 5% BY WEIGHT CHROMIC OXIDE AND FROM ABOUT 0.5% TO ABOUT 10% BY WEIGHT OF SOLUBLE TUNGSTATE SALT HAVING A pH OF FROM ABOUT 4.5 TO ABOUT 8.0 AT A TEMPERATURE OF FROM ABOUT 130 °F. TO BELOW THE BOILING POINT OF THE SOLUTION.

FURTHER SEAL THE PARTIALLY SEALED ANODIZED OBJECT BY CONTACTING IT WITH AN AQUEOUS SOLUTION CONTAINING FROM > ABOUT 0.1% TO ABOUT 10% BY WEIGHT OF A SALT OF NICKEL OR COBALT AND A1 WEAK ACID AND, IF DESIRED, A BUFFERING COMPOUND HAVING A pH OF FROM ABOUT 4.5 TO ABOUT 8.0 AT A TEMPERATURE OF FROM ABOUT 150 °F. TO BELOW THE BOILING POINT OF THE SOLUTION. (43118/2 US Patent o.3,616,298 does disclose a two-step process wherein an anodized aluminum surface is first treated with nickel acetate and then followed by sodium dichromate treatment. The present invention distinguishes from this state of the art because it produces a corrosion-resistant anodized surface that is clear (uncdlored) and which is resistant to staining by extraneous substances.

This is accomplished in the present invention by the two-step process using a tungstate-chromate solution in the first step and a nickel or cobalt salt solution in the second step. The two-step process utilized in US Patent No.3,616,298 fails to accomplish the object of the present invention in that the treatment by the sodium dichromate solution leaves a yellow or brownish color on the anodized surface. Thus, the two-step process of this US Patent cannot produce a clear (uncolored) surface.

Claims (11)

1. In a process of sealing an anodized aluminum coating in which said coating is contacted with a first solution containing tungstate and chromate ions so as to partially seal said anodized coating, the improvement which comprises: contacting the anodized coating which has been contacted with said first solution after it has been removed from said first solution with a further solution containing from about 0.1% to about 10% by weight of a soluble salt of a metal selected from the group consisting of nickel and cobalt and of a weak acid at a temperature of from about 150eF. to immediately below the boiling point of said solution for a period sufficient to further seal said anodized coating in order to render said anodized coating substantially stain resistant.

2. A process &s claimed ih claim 1, wherein said further solution consists essentially of water and said salt and said weak acid, is an acid selected from a group consisting of boric, citric, formic and acetic acids.

3. A process as claimed in claim 1, wherein: said further solution consists essentially of water, said salt and a buffering compound? said weak acid is an acid selected from a group consisting of boric, citric, formic and acetic acids; i¾nd said buffering compound is a compound selected from the group consisting of boric acid, citric acid, formic acid and acetic acid and ammonium and alkali metal salts of such acids.

4. A iprocess as claimed in claim 1, 2 or 3, wherein said further solution contacts said ano¬ dized coating for a period of from about 1 to about 20 minutes and has a pH of from about 4.5 to about 8.

5. A process as claimed in claim 4, wherein the pH of said further solution is from about 5.5. to about 6.5.

6. A process as claimed in claim 1, 2, 3, 4 or 5, wherein: said first solution consists essen¬ tially of water and from about 0.5 to about 10% by weigh of a soluble tungstate salt, from about 0.1 to about 5% by weight chromic oxide present as chromic acid or as a soluble chromate salt, has a pH of from about 4.5 to about 8 and is in contact with: said anodized coating for a period of from about 1 to about 20 minutes at a temperature of from about 130°F. to immediately below the boiling point of said solution. r

7. A process as claimed in any of the pre¬ ceding claims wherein said first solution includes a wetting agent serving to facilitate penetration of said first solution into said anodized coating. 4

8. A process as claimed in any of the preceding claims 1 to 6, wherein said further solution includes a wetting agent serving to facilitate penetration of said further solution into said anodized coating.

9. A process as claimed in any of the pre-ceding elaims 1 to 6, wherein both of said first and said further solutions include a wetting agent serving to facilitate penetration of said solutions into said anodized coating:

10. An anodized, sealed aluminum oxide coating produced by the process of any of the preceding claims.

11. A process for sealing an anodized aluminum coating substantially as herein described with reference to the accompanying examples. AGENTS FOR APPLICANTS