US2520475A - Process for the chemical oxidation of aluminum and alloys thereof - Google Patents

Description

Patented Aug. 29, 1950 PROCESS FOR THE CHEDIICAL OXIDATION OF ALUMINUIVI AND ALLOYS THEREOF Carlo Sonnino, Milan, Italy No Drawing. Application May 29,1947, Serial No. 751,455. In Switzerland October 6, 1944 Section 1, Public m 690, August a. 1946 Patent expires October 6, 1964 This invention relates to a process for the chemical oxidation of aluminum and its alloys whereby a colorless coating may be produced.

As already known, the most widely spread process for the chemical oxidation of aluminum and alloys thereof, called MBV process, produces a layer consisting of approximately 73% aluminum hydroxide and 25% chromium hydroxide, the latter adding a dark coloring to the layer. This coloring is a serious drawback for certain applications.

Various attempts have been made in order to eliminate this drawback of the MBV process and. to obtain colorless and transparent layers on aluminum and alloys thereof.

It was noticed that an addition of small quantities of sodium silicate to the solutions of sodium carbonate and sodium chromate used in the chemical oxidation speeded up the hydrolysis of the sodium aluminate formed during the oxidation and reduced considerably the quantity of chromium hydroxide present in the layer, making it of a much lighter color. This method is however very diificult, since even a slight excess of sodium silicate is sufficient to stop the reaction almost completely, giving a very poor and hardly protective oxide coating, and in silicon containing alloys producing no light colored layers.

It was therefore suggested to eliminate most of the chromium hydroxide from the layer by adding to the sodium carbonate and sodium chromate solutions some sodium fluoride, which reacts with chromium hydroxide according to the following reaction:

and therefore eliminates the chromium hydroxide and makes the oxide layer much more transparent.

It was however noticed that, although on aluminum and certain alloys thereof a very light colored layer is indeed obtained, the layer is nevertheless still rather grey on other alloys.

It was noticed that said coloring was due to the presence of graphitic silicon and hydroxides of metals present in the alloy. In particular, the silicon is only eliminated through the action of fluorine or of its acids, which are very active in the presence of oxidants, as for instance sodium chromate.

The inventor of the present process had already noticed the interesting properties of hydrofluosilicic acid (HzsiFs) and of its alkaline salts, which have a more regular and efficient 5 Claims. (01. 148-6.2)

2, action than sodium silicate upon the attack of aluminum and alloys thereof in alkaline solutions, while they reduce the violence of such attack and speed up the hydrolysis of sodium aluminate.

These properties are utilized in accordance with the present invention to make more regular the action of sodium carbonate and sodium chromate solutions for the chemical oxidation of aluminum and its alloys, and. at the same time to utilize the action of fluorine on the chromium hydroxide of the layer, so as to transform it into soluble chromium fluoride, and on the graphitic silicon so as to make it soluble as well in the form of fluoride compounds.

In practice, the addition of 0.1% HzSiFc to the salts for the chemical oxidation has proved sufficient, the sodium carbonate and sodium chromate mixture being 5:15, in order to obtain with commercial aluminum a perfectly colorless and transparent layer, which has however a remarkable thickness.

For silicon alloys, the hydrofluosilicic acid addition should be considerably increased, depending on the silicon percentage, and a further addition of an alkaline hydrofiuosilicate is also useful. The same is true for copper, manganese and magnesium alloys, where the best results are obtained both in color and thickness of the layer.

In practice, quantities varying from 0.1 to 10% by weight of hydrofluosilicic acid HzSiFs are added to a solution of sodium carbonate and sodium chromate in the ratio 5: 1.5, either alone or in addition to 0.1 to 10% of alkaline hydrofluosilicates. In certain cases an addition Of alkaline hydrofiuosilicates in percentages from 0.1 to 15% is suflicient. The solution thus prepared is heated to 95-100 C. and the articles of aluminum or alloys thereof that are to be oxidized are immersed in the solution for a period varying from 10 to minutes, depending on the desired thickness and on the nature of the alloy.

The articles are then washed and treated for 20 minutes with a boiling solution of sodium silicate in order to fill the pores of the layer.

The layers thus obtained are perfectly colorless and transparent, have a remarkably higher thickness and resistance against corrosion than those obtained through other processes.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

11 A process for the chemical oxidation of aluminum and its alloys which comprises treating the metal to be oxidized in a hot bath consisting of an aqueous solution consisting of a major proportion of sodium chromate and sodium carbonate and a minor proportion comprising at least 0.1% of a compound selected from the class consistinggof liydifofiuosilicic acid; alkaline hydro: fiuosilicatesand mixtures of hydrofluosilicic'aeid and alkali fiuosilicates.

2. Process according to claim 1 wherein the solution comprises from 0.15'130.10%*h'ydl.0fll'10.-

silicic acid.

3. Process according to claim 1 wherein the solution comprises from 0.1 to 15% alkaline-hydrofluosilicates.

4. Process according to claim;.1; whereinlthe solution comprises from 0.1 to 10% hydrofluosilicic acid and from 0.1 to 10 alkaline hydro fluosilicates.

5. Process according to claim 1, whereinthe metal is treated for a periodof from 5 to 60 min- 20 utes in a bath maintained at a temperature of from 95 to 100 C.

CARLO SONNINO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number: Name Date 1,723,067" Pacz Aug. 6, 1929 ,205,708., Zurbrugg June 25, 1940 2,247,580 Jauch July 1, 1941 2276353 Thompson Mar. 17, 1942 2,296,884 Thompson Sept. 29, 1942 2,312,855. Thompson Mar. 2, 1943 FOREIGN PATENTS Number: Country Date 415,636 Italy Oct. 28, 1946

Claims (1)

1. A PROCESS FOR THE CHEMICAL OXIDATION OF ALUMINUM AND ITS ALLOYS WHICH COMPRISES TREATING THE METAL TO BE OXIDIZED IN A HOT BATH CONSISTING OF AN AQUEOUS SOLUTION CONSISTING OF A MAJOR PROPORTION OF SODIUM CHROMATE AND SODIUM CARBONATE AND A MINOR PROPORTION COMPRISING AT LEAST 0.1% OF A COMPOUND SELECTED FROM THE CLASS CONSISTING OF HYDROFLUOSILICIC ACID, ALKALINE HYDROFLUOSILICATES AND MIXTURES OF HYDROFLUOSILICIC ACID AND ALKALI FLUOSILICATES.