GB510353A - Improvements in or relating to the protection of magnesium and magnesium alloys against corrosion - Google Patents

Abstract

510,353. Electrolytic surface treatment of magnesium. SUTTON, H., and LE BROCQ, L. F. Dec. 23, 1937, No. 35575. Drawings to Specification. [Class 41] [Also in Group II] A protecting coating is formed on magnesium and magnesium alloys by immersion, with or without the application of electric current, in a cold aqueous solution containing chromic acid ions and one or more activator salts, the pH of the solution being within the range necessary for the production of an adherent protective film in the cold in a period not exceeding two hours. The constituents of the solution and their relative concentrations are selected so that the solution has a hydrogen ion concentration curve which has a portion of such small gradient within the range above referred to that, within this range, 12 grammes of potassium hydroxide can be added to 1 litre of the solution without causing an increase of more than 2 in the pH of the solution. A solution of this type can be used for several treatments without too great a drop in the reaction velocity ; when necessary it can be revived by adding acid, e.g., chromic or sulphuric acid. The preferred activator salts which will give a solution of the required type are aluminium salts, particularly aluminium sulphate, nitrate, and aluminium alums ; chrome alums and iron alums may also be used. The chromic acid ions may be added as chromates or dichromates of the alkali metals, ammonium, magnesium, aluminium, or zinc. Permanganates may be added to the solution to improve the coating and, when aluminium salts are used, chlorides may be added to keep the alumina formed in colloidal suspension. The pH range necessary for producing the coatings in the cold in less than two hours is usually about 2-5 ; with some activator salts or when electric current is used it may be higher. When using electric current the magnesium may be made either the anode or cathode with direct current, or alternating current may be used. With certain combinations of constituents the desired pH may be obtained with concentrated or saturated solutions, e.g., a saturated solution of potassium dichromate and potassium or ammonium aluminium alum may be used, the salts being kept in the solution in perforated containers. Specifications 331,853, [Class 82 (ii)], and 506,836, [Group II], are referred to.