US2352076A - Process for the protection of magnesium and magnesium base alloys against corrosion - Google Patents

Description

Patented June 20, 1944 PROCESS FOR THE PROTECTION OF MAG- NESIUM AND MAGNESIUM BASE ALLOYS AGAINST CORROSION Charles James Bushrod, Prestwich, England, as-

signor to Magnesium Elektron Limited, London,

England, a company of Great Britain No Drawing. Application April 5, 1943, Serial No.

. 481,952. In Great Britain April 2, 1942 3 Claims.

This invention relates to processes for protection of magnesium and magnesium base alloys against corrosion by treatment of the metal by a solution which reacts chemically with the metal to produce a protective film on the surface of the metal. Two such processes are widely used in Great Britain, viz. the 30 minute hot chromate bath and the cold acid chromate bath, described in paragraphs 3 (iii) and 3 (iv) of D'I'D911 and more fully described in the Handbook "Elektron Magnesium Alloys 1937 pages 91-95. The 30 minute bath whilst giving good surface finishes has the two disadvantages of lengthy period of treatment and necessity for maintaining the solution near to boiling point. The acid chromate treatment however involves a cold solution and a very short period of immersion but the solution is strongly acid and therefore removes a small but appreciable amount of the metal (of the order of 0.0005" total reduction in thickness)- during treatment and is therefore not so suitable for use on parts machined to fine tolerances.

Th process described in paragraph 3 (i) in D'I'D911 involves the use of electrical equipment and a period of treatment of 20 minutes and the process described under paragraph 3 (ii) involves ganese dioxide by treatment in solutions at normal atmospheric temperatures and containing sodium hypobromite, potassium permanganate, or other suitable oxidising agent.

The initial film can be produced by immersion in a solution at normal atmospheric temperature and having the following features:

(1) An acid reaction with a pH value of less than 5 and preferably less than 4.

(2) One or more sulphates, nitrates, or chlorides, or similar activating salts are in solution.

(3) The solution should contain ions'of one or more elements which form sparingly soluble hydroxides e. g., chromium, aluminium,-iron, or manganese, or alternatively, compounds'which can be reduced to produce such ions e. g., dichromates and permanganates.

(4) The solution should be well buii'ered so that the pH will vary only slowly with the area of metal treated to ensure a long life for the solution.

The treatment may be in accordance with the invention described in British Patent No. 510,353

a cold solution but requires a rather lengthy period of treatment, viz. minutes; and these processes form the subject of British Patents Nos. 493,935 and 510,353.

All four processes produce films on the metal which are somewhat porous and the degree of protection afforded to the metal and of the adherence of the film to the metal depends to some extent at least on the degree of porosity. The main object of the present invention is to produce protective films having a greater degree of con-.

. is used in its broader meaning viz. the addition to a compound of an electronegative atom or radicle. I

I have found that manganese compounds in the film are particularly suitable and for instance manganous hydroxide can 'be oxidised to manprovided that the solution produces a film containing a compound which can be oxidised.

I have found that a solution containing 20-30 grams per litre of potash alum and 20-80 grams of MnSO4.5I-I2O per litre of solution will produce an oxidisable film after an immersion of three minutes. In these particular experiments sheets of Elektron (registered trade-mark) AM503 alloy consisting of about 1.5% manganese balance magnesium were used. The sheets were subsequently 4 drained for about 10 seconds and then dipped whilst wet in an aqueous solution containing 5% NaBrO.

Another solution capable of producing an oxidisable film contains manganese sulphate, ammonium sulphate, and potassium dichromate e. g. 80, and 40 grams respectively per litre of water.

The oxidising solution should, of course, be incapable of attacking the metal to any serious extent. The oxidising eflect can be observed to some extent from the colour of the films which changes from light brown produced by the first solution to a deep chestnut colour after immersion in th second solution. If desired the effect of the oxidising solution may be accelerated by maintaining the solution above normal atmospheric temperature e. g. the potassium permanganate solution may be at C.

The formation of the film and/or its oxidation may be effected or facilitated if desired by anodic or cathodic action.

In addition to successful tests on magnesium base alloys containing substantially only manganese as an alloying constituent, I ha e also successfully treated alloys containing substantial proportions of aluminium e. g. 6 to 9%. I claim: 1. A process for the protection of magnesium and magnesium base alloys against corrosion, comprising first treating the metal with a solution, said solution containing principally manganous sulfate and being capable of forming a film of a manganous compound on the surface of the metal, then treating the metal with an aqueous solution containing principally sodium hypobromite to oxidize the manganous compound film.

2. A process for the protection of magnesium and magnesium base alloys against corrosion,

comprising first treating the metal with a solution, said solution containing principally man- 20 assaovs ganous sulfate and being capable of forming a film of manganous hydroxide on the surface of the metal, then treating the metal with an aqueous solution of an oxidizing salt capable of oxidizing the manganous hydroxide to manganese dioxide.

3. A process for the protection of magnesium and magnesium base alloys against corrosion, comprising first treating the metal with a solution, said solution containingprincipally manganous sulfateand being capable of forming a film of a manganous compound on the surface of the metal, then treating the metal with an aqueous solution containing principally sodium hypobromite to oxidize the manganous compound film, such first solution also having incorporated therein ammonium sulfate and potassium dichromate.

CHARLES JAMES BUSHROD.