US2095519A - Method for producing galvanic coatings on aluminum or aluminum alloys - Google Patents

Description

40 tageous for the purpose according to the invensible to polish the same. It is, however, possible to produce onthe metals Patented 0ct.12, 1 937 i 2,095,519 METHOD FOR PRODUCING GALVANIQ COAT- (PATENT OFFICE INGS ON ALUIWINUM OR ALUMINUM AL- LOYS Johannes Fischer, Berlin-Siemensstadt, Germany, assignor to Siemens 8; Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany No Drawing. Application June 29, 1935, Serial No. 29,027. -In Germany May 7, 1934 7 Claims.

My invention relates to a method for producing galvanic coatings on aluminum or aluminum alloys. 1 1' l It is well known in the art to prepare aluminum or aluminum alloys by means of an anodic preliminary etching for the subsequent application of galvanic layers. According to a known method the etching process is eiiected in sulphuric acid or in solutions of sulphates, hydro? ,10. chloric acid being also in some cases mixed therewith. This method of pretreating aluminum has proved to be too rough, insofar as the aluminum is attacked to such an extent by the strong mineral acids that the metallic coating subsequently applied thereto, on the one hand, adheres as a matter of fact firmly'to the aluminum "but, on the other hand, it is 'so uneven that it is imposto be treated metallic coatings capable of being polished according to the processes described in my prior U. S. patents, Nos. 1,947,981 and 1,968,483. These processes involve the following three steps: Artificially preoxidizing the starting material; partially removing the artificial oxide layer applied thereto and simultaneously applying thereto ametallic intermediate flash-coating and finally galvanizing the flash coating.

My present invention has for its object to provide a method whereby my previous methods are simplified by eliminating the second step. This may be accomplished according to the invention by coating aluminum or aluminum alloys with an oxide-containing layer by means of" an, anodic pretreatment in an electrolyte free of S04 ions. 1 have found, as a' result of the research work carried out on a great number of acid electrolytes free of S04 ions, substantially on so-called weak acids, that hydrofluoric and hydriodic. acid as well as phosphoric acid are particularly advam tion. With the last-named acid the effect is only attained at a temperature above 50 0. Also mixtures of the first-named ,acids as well as various organic acids may be employed.

50. by are the same as those according to the method hitherto employed comprising three steps and at the same time the further advantage is obtained in that a complete step is dispensed, with.

The electrolyte may be composed of a solution containing aphosphoric acid having, 'for in and the temperature thereof playing also an important part.- Thus, for instance, 1 per cent Lsolution of hydrofluoric acid at room temperature and a current action lasting threeminutes result in a good preparation of the metal to be treated whereasthe corresponding current action lasting seven minutes produces a surface to'which the coat firmly adheres but which is so rough that it cannot be polished. With the above-described comparatively slightly ionized acids the capability of the surfaces of being polished decreases, as a rule, with increasing concentration, working time and heat. The reverse is the case as to the acid concentration with strong mineral acids which cause a passivity of the metal to be treated.

The electrolyte may also contain a mixture of hydrofluoric acid and phosphoric acid, 1 per cent phosphoric acid with 5 per cent hydrofluoric acid being preferably employed; .However, such mixtures have a slight disadvantage, insofar as they entail a relatively high current density of about 800'amperes per square meter, whereas for the first step of the novel method the current density isto-be chosen, as a rule, lower but "preferably not too low. In generaLcurrent densities above 200 amperes per square meter have proved to be suitable. The increase of the temperature for preventing the formation of blocking or insulating layers required when using phosphoric acid may be chosen at will as far as the purpose of the present method is concerned, since the formation of the blocking layersis prevented the surer, the higher the temperature is. Since it, however, presents dimculties to maintain higher temperatures constant during the electrolysis preferably temperatures lying between 50 and 70 C. are chosen; for instance a temadvantageous.

A mixture of per cent hydrofluoric acid with 5 per cent phosphoric acid has also proved to be suitable according to the invention.

Also the very good stray capacity of the electrolytes employed is advantageous in the various forms of the novel method.

Unexpectedly according to the invention a faultless galvanization of the aluminum or aluminum alloys is attained also in such cases in which the object to be treated has soldered or welded joints. Even such objects may be properly galvanized without a particular thermal subsequent treatment being necessary.

I claim as my invention:

1. In the plating of aluminum and aluminum alloy articles with metals without the use of a 3. The process of claim, 1 wherein said bath contains hydriodic acid.

2,095,519 perature of 60 C. has proved to be particularly 4. In the plating of aluminum and aluminum alloy articles with metals without the use of a flash-coating step, the process which comprises making such an article an anode in an oxide coating bath consisting substantially of an aqueous phosphoric acid solution of a concentration ranging from about 5 to 20 per cent, with a current density of about 200 to 800 amperes per square meter, at a temperature of about 50 to 70 C. and for a time required to produce a substantial oxide-containing coating on said article which is capable oi receiving 'a firmly adherent, polishable metal plating, and then metal plating said oxide-coated article without intermediate chemical treatment.

5. In the metal plating of aluminum and aluminum alloy articles without the use of a flashcoating step, the two-step process which comprises the steps of anodically pretreating such an article at a temperature above 50 C. in an electrolytic bath consisting substantially of a dilute aqueous solution of phosphoric acid, under conditions of concentration, temperature, time and current density producing a substantial oxide coating which is capable of receiving a firmly adherent, polishable metal coating, and finally metal plating said article.

6. The process oi! claim 5 wherein said article has soldered or welded joints.

7. The process of claim 5 wherein the concentration of phosphoric acid in the oxide coating bath ranges from about 5 to 20 per cent by weight. JOHANNES FISCHER.