US2231373A - Coating of articles of aluminum or aluminum alloys - Google Patents

Description

Patented Feb. 11, 1 941 PATENT. OFFiCE COATING OF ARTICLES OF ALUMINIlM R ALUMINUM ALLOYS i Max Schenk, Basel, switzerlaniassignor to the firm Ematal Electrochemical Corporation, New York, N. Y., a corporation of New York -No,Drawlng. Application November 24, 1937,Se-

rial No.- 176,319. In Switzerland January 8,

This, invention relates to the manufacture of coated articlesof aluminum or aluminum alloys.

It contemplates the provision of a very hard and corrosion-resistant, durable coating on articles of 5 aluminum or aluminum alloys, and it also contemplates the production of articles of aluminum or aluminum alloys surfaced witha strongly adherent film having an opaque and enamel-like appearance This applicationis a continua'tionin part of my copending application ,Serial No. 58.223.

The invention comprises a process of electroplating articles of aluminum or aluminum alloys in an aqueous bath containing salts of titanium in solution, and it also comprises an article of aluminum or aluminum alloy surfaced with "a film containing aluminum oxide and titanium dioxide, the latter being present in an amount between 0.2 and 5%.

. The term salts of titanium as used herein and in the appended claims, is to be.'understood as comprising the salts of the titanium metal, such as the titanium cation (Ti++++) ,ltitanyl ion (Tio and even salts of a more complex form, such as TiO(KCzO4)z.2HzO in which titanium is present in the cation in contradistinction to and tothe exclusion of the titanates which in fact are salts of titanic acid.

I have found thatv a very hard and very resistant protective layer of opaque, enamel-like appearance can be obtained on articles of aluminum or aluminum alloys by treating them electrolytically in an aqueous bath containing salts of titanium in solution. The so produced coating securely adheres to the surfaces to which it is'applied, and is very'resistant to corrosion. Such coating differs essentially, in its appearance, that is to say, in its opacity hitherto un known and its density of pores, from the oxide film obtainable y the conventional aluminum oxidation process. I I In electro-plating articles according to my 1 invention; either' 'alternatingcurrent drdire'ct current; may be "used. If a direct "current is ap- 45 plied, the'art-icle to be coated must be the anode, When alternating 'current is used the article'is, of course, anodic during apportion ofthebycle. The cathode may consist ofany "suitable 'material such as lead," carbon, graphite,'magnetite. The layers produced by the treatment with alternating current are .less ahard and-more, flexible thanthose produced with the ,aid oi direct current. In other respects I havenot found essential differences. The aqueous 55 bath used in my process comto maintain the bath on a pH-value between 1 Claims. (Cl. 204-35) prises at least one kind of the salts of titanium in solution. Said salts of titanium may belong to those vmentioned hereinbefore, or they may ever, prefer to prepare the bath with a content. 10

of salts of titanium in the neighborhood of 2 to 5 per cent in weight. Contrary to the salts or titanium as defined hereinbefore, the titanates are not useful as components of said baths. Although it appears theoretically possible to pre- 15 pare a bath containing titanium cation by adding an acid to an aqueous solution of a titanate, and thus transforming the titanate into titanium cation,,the desired result cannot be obtained in this manner for the following reason: Titanatcs 20 can be present only in alkaline solutions. AsI have found by experiments confirmed bythe standard work Handworterbuch der Naturwissenschaften", 2nd edition, volume V, page 1004, only the alkali titanates are soluble in water, 20 the maximum quantity of such tltanates in an aqueous solution being approximately at the rate of l per 1000. Such quantity, however, is much too small to produce the opaque layers according to my invention. i 30 Mineral acid-components as well as organic acid-components of the trivalent or 'tetravalent titanium are suited as anions. It is necessary and 3.5. .In order to comply with this prerequi- 30 site, quantities of inorganic and/or organic acids, and/or their salts havingxallra'li, ammonium or organic bases may be added. to the bath. Among-the inorganic acids, Lioundto'ibe par ticularly useful arethe phosphoric, chromicyar- 40 senic and boric acids, and among the; organic acids: ,dicarboxylic acids, such as oxalic, succinic and Hmalonic acids: hydroxy-carboxylic acids,

such as glycollic, lactic, and gluconic acids; and hydroxy-polycarboxylic acids, such as tartaric and citric acids. The anions of the above acids and theircorrespondingsalts are all oxygen containing" and promotethe formation of hard, adherent, opaque and enamel-like films which are integral-with the base Qandcontai'n titanium dioxide. -Excluded fromi-the baths arev'the acids of the halogens. and their "salts, and the sulfuric and, sulfurous, and the nitric and nitrous acids and their salts. The suitable acids are added and mixed to the baths in such quantities as necessary to adjust the required pH value.

As the acids are consumed by and by owing to the metali going into solution, they may be replaced during the treatment in adequate quantities.

The addition of the'aforementioned salts of said acids helps to increase the conductivity of the bath particularly if alkali salts of said acids are used for this purpose, or to keep the pH value constant if such salts as borates, phosphates, chromates, citrates, lactates, tartrates, etc. are added. Titanates cannot be used in this connection because of their instability in the limits of the pH-value between 1 to 3.5.

Other substances may be added to act as protective colloids counteracting the tendency of the aluminum-oxide film'to redissolve which film is produced on the surface of the treated article and contains the titanium dioxide embedded therein. Substances useful for this purpose are watersoluble polyhydric alcohols, such as glycol, glycerine, polyvinyl alcohols, mannite, and watersoluble carbohydrates, such as glucose, cane sugar, dimethyl cellulose, dextrine.

The temperature of the bath during the treatment varies according to the acidity, the concentration of the components and the working voltage and may amount to -95 degrees centigrade; it may be made uniform and kept constant by means of agitators and cooling devices in the bath. The temperature e. g. with mineralacid baths should not amount to substantially more than 55 C., whereas the bath in the case of weak organic-acid electrolytes works best at temperatures of 65-95 C. I I

Before the articles are treated in a bath containing salts of titanium, they are to be freed from grease in a suitable manner, for instance by means of kerosene, benzine or spirit. Inasmuch as the brightness and the fineness of structure of the non-coated article influences the appearance of the finished coating, desired effects may be obtained by a preceding treatment. Such preceding treatment may be a mechanical, a chemical or an electrochemical one. The articles may be roughened by means of sand-blasting or brushing, or they may be highly polished. For

chemically corroding the article surfaces an alkaline mordant such as a solution of caustic soda or basic phosphates, may be applied, or a strongly mineral acid mordant, such as nitric acid. The well known hydrofluoric acid mordant is well suited for the purpose. It is also possible either electrolytically or by another suitable process, toprovide the article with a securely adhering aluminum oxide film before it is treated in the bath containing salts of titanium.

In preparing said bath, I heat the water to a temperature of about 50 C. and add, while stirring first the salts of titanium, then the acid, thereafter the neutral salt, and finally the indiiferent admixtures. The articles to be coated may be attached to electrodes of aluminum or aluminum alloys and immersed into the bath liquid. Electrolysis is then efiected with either alternating or direct current of a density between 0.1 and 8 amperes per square decimeter, at temperatures as stated hereinbefore.

Examples 1. The bath is made up of'a solution of 50 kg.

of titanyl potassium oxalate, TiO(KCzO4) 2.2H2O,

kg. of citric acid, kg. of glucose and 6 kg. of phosphoric'acid in 1000 kg. of distilled water. Electrolysis is efiected with alternating current of 110 volts. at a temperature of 75 C., with current .glycerine in 180 kg. of distilled water.

'such drying the coating produced densities of 5- 6 amperes per square decimeter until 2.5 ampere-hours per square decimeter have been used.

2. The bath is made up of a solution of 6 kg. titanyl potassium oxalate, TiO(KC2O4)2.2H2O (TlO(C2O4)K2.2H2O), 2 kg. borax, .5 kg. boric acid, 1 kg. citric acid, .5 kg. oxalic acid and 2 kg.

chemicals being free from chlorine and nitrate.) Electrolysis is effected with direct current of 120 volts at a temperature of 68 C., with current densities of 6 amperes per square decimeter until 3.5 ampere hours per square decimeter have been used.

The electrolytic treatment being completed the articles are removed from the bath and first suitably washed in water or diluted ammonia or a diluted solution of soda or a. diluted solution of, caustic soda and then dried. The drying takes place preferably at a raised temperature of for instance 100 to 150 C. depending upon the size and quality of the surface for a period of a quarter to three quarters of an hour. By means of in the bath becomes harder.

An article of aluminum or aluminum alloy treated according to the new process has a coating which comprises essentially aluminum oxide and titanium dioxide, the latter being chemically or physically retained by the former so much so that a separation of the two is impossible without destruction. Both the aluminum oxide and may be present partly or entirely in a hydrated form. The content of in the aluminum oxide film varies between .2 and 5%. Depending upon the kind of acid used in the bath, the oxides of t1- tanium and aluminum may contain, embedded thereininsoluble alkaline salts, such as alkaline phosphates or borates which, however, are without considerable influence. The structure of the film is microcrystalline and has very fine pores in contradistinction to protective films produced on aluminum by other methods. coating according to the invention shows a resistance to corrosion, in many instances considerably greater than a pure aluminum oxide film, and protects-the metal of the treated article excellently against chemical attacks. The coating is opaque and enamel-like in appearance. The thickness of the coating may amount up to half a millimeter. However, coatings of .02 millimeter thickness are very opaque so that the articles provided with such films no longer show a metallic effect. This feature renders the process very economical. The appearance of the protective coating produced according to the new process depends upon the conditions of the surface of the article prior to the application of the electrolytic bath, upon the composition of the latter, and upon other circumstances of the treatment. The opacity increases with the thickness of the coating. Phosphoric, arsenic, chromic, and boric acids act to make the coating more opaque while large quantities of tartaric or citric acids cause the formation of less opaque coatings. A low pH value of the bath produces transparent films with coarser. pores, and permits the production of thicker coatings than'a higher pH value. Dull films may be obtained in two ways: either by creating a thin coating on a surface already roughened and corroded with a mordant, or by oxidizing a brightly polished surface over a long period of'time with the aid (All This protective according to the new process may be subjected to an additional treatment intended to affect the film pores which are able to receive additional substances. Such treatment preferably takes place after the washing and prior to the final drying described hereinbefore. It may have the purpose either of drying the film, or of making it water repellent, or of sealing the still open pores.

It is also possible to apply one of said additional treatments after the other to the same film.

The opaque film may be dyed with organic dyes in an aqueous bath, or in a dyers bath containing an organic solvent, or by means of any suitable application of or coating with dyed oils, metal soaps, or varnishes. For producing pure white effects the film may be subjected to a treatment with a salt having titanium as one of its components. In the last-mentioned additional treatment, a wider group of salts is available for selection than in the preparation of the aforementioned electrolytic bath in which the opaque film is produced. Said wider group comprises all soluble salts which are suited to form insoluble titanium compositions by hydrolysis under the application of heat or with the aid of chemical agents. To this group belong not only the salts of titanium as defined hereinbefore, but also the titanates and such salts as for instance the sulphates, chlorides and nitrates of titanium. The additional treatment consists in-immersing the articles with its opaque coating in a cold or warm solution of a salt of the said group, thereafter impregnating with precipitating substances, and finally washing.

Another treatment for making the original opaque layer whiter, consists in subjecting the layer or coating film to the action of suitable oxidizing chemicals in solution. Such chemicals are for instance persulfate, potassium permanganate, bichromate, chlorate, concentrated nitric acid, acid solution of hydrogen peroxide and perborate.

In order to make the opaque film water repellent, it may be treated with either a fatty acid, or an alkali metal salt of a fatty acid or a strong base, or a sulfonated oil to form insoluble salts or soaps of the titanium and aluminum of the film. Qr, the coating film may be covered with a suitable varnish or burnt-in resin.

The sealing of the film pores may be accomplished by immersing the article with its opaque coating into an aqueous solution containing a wetting substance at a temperature of 70 to 100 C. for a period of from 20 to 100 minutes. 1 have found that diisopropylnaphthalene-sulfonic acid and also laurylsulfonate are suitable substances for this purpose.

It is to be understood that the invention is not limited to the specific details herein described but may be carried out in other ways without departure from its spirit.

I claim:

1. A process of producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing titanium cations and containing anions, said titanium cations being selected from the group consisting of those containing trivalent and tetravalent titanium, said anions consisting of those which are oxygen containing and will promote said articles as the anode.

the formation of hard, adherent, opaque and enamel-like films on said articles, and passing electric current throughsaid articles as the anode.

2. A process of producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises immersin said articles in an aqueous acid bath having a pH value of betweenabout 1 and 3.5 containing titanium cations and containing anions, said titanium cations being selected from the group consisting of those containing trivalent and tetravalent titanium, said anions consisting of those which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles and comprising anions of the aliphatic carboxylic acids. and passing electric current through said articles as the anode.

3. A process of producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing titanium cations and containing anions, said titanium cations being selected from the group consisting of those containing trivalent and tetravalent titanium, said anions consisting of those which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles and comprising those selected from the group consisting of the anions of phosphoric, chromic, arsenic and boric acids, and passing electric current through said articles as the anode.

4. A process of producingintegral coatings containing titanium dioxide on articles of alumi num or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing alkali titanyl cations and containing anions,

said anions consisting of those which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles, and passing electric current through said articles as the anode.

5. A process of producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing potassium titanyl cations and containing anions, said anions consisting of those which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles, and passing electric current through said articles as the anode.

6. A process of producing integral coatings 7. A process of producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing potassium titanyl cations and containing anions,

said anions consisting of those which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles and comprising a mixture of oxalate, citrate and phosphate anions, and passing electric current through said articles as the anode.

8. A process oi producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing potassium titanyl cations and containing anions,

said anions consisting of those which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles and comprising a mixture of oxalate. citrate and borate anions, andpassing electric current through said articles as the anode. I 9. A process according to claim 1 in which the bath temperature is maintained at a temperature of between 10 and 95 (I. during passage of the electric current. 10. A process of producing integral coatings containing titanium dioxide on articles of aluminum or its alloys, which comprises first forming on said articles a film of aluminum oxide, then immersing said articles in an aqueous acid bath having a pH value of between about 1-and 8.5

containing titanium cations and containing anions, said titanium cations being selected from the group consisting of those containing trivalent and tetravalent titanium, said anions consisting of those which are'oxygen containing and will promote the formation of hard, adherent. opaque and enamel-like films on said articles, and passaasns'rs ing electric current through said articles as the anode..

11. A process of producing integral coatings containing titanium dioxide on articles 01 aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath having a pH value of between about 1 and 3.5 containing titanium cations and containing anions, said titanium cations being selected from the group consisting of those containing trivalent and tetravalent titanium, said anions consisting of those which are oxygen containing and will promote thetormation 01' hard, adherent, opaque and enamel-like films on said articles, passing electric current through said articles as the anode, impregnating the coatings obtained thereby with a salt having titanium as one of its components, and converting said salt to an insoluble white titanium compound to increase the content of white in the coating.

12. An article of aluminum or its alloys having an opaque enamel-like coating containing aluminum oxide and titanium dioxide, said coating being integral with the base and having fine pores and a micro-crystalline structure, and having been produced by the process of claim 1.

13. An article of aluminum or its alloys having an opaque enamel-like coating containing aluminum oxide and titanium dioxide, the titanium dioxide being present in the coating in .the amount ofbetween 0.2 and 5 percent thereof, said coating being integral with the base and having fine pores and a microcrystalline structure. and having been produced by the process of claim 1.

. MAX SCHENK.