GB717015A - Improved process for the electrolytic oxidizing of aluminium or aluminium alloys - Google Patents

Abstract

717,015. Electrolytic surface oxidation of aluminium and its alloys. FINSTERWALDER ISOLATIONS GES. Aug. 22, 1952, [Aug. 22, 1951.] No. 21178/52. Class 41. [Also in Group XXIII] An oxide coating is produced on aluminium or an alloy thereof by electrolytic treatment in an aqueous bath containing an alkali metal salt of an organic acid, e.g. citrate or tartrate, and a weak inorganic acid, preferably boric acid, employing, after a short introductory period at lower voltages, a voltageexceeding 250 such that continual vigorous sparking occurs on the surface of the objects treated. The concentration of the inorganic acid should exceed that of the alkali metal salt; the latter concentration should not exceed 5 per cent by weight. The applied voltage may be increased continuously or by steps from an initial value of 100 volts to a maximum of 450-550 volts. The current may be supplied by a transformer through a variable resistance, ohmic or inductive, taking up 10-25 per cent of the supply voltage. Direct current may alternatively be used. The following values are preferably maintained, current density 0.7-0.6 amps per sq. dcm., pH value 5.5-7.5, specific conductivity of electrolyte 0.01 ohms<SP>-1</SP>: cms<SP>-1</SP>, bath temperature 60-70‹C. Preferred baths contain not less than 2.5 per cent by weight of sodium citrate and 8-16 per cent by weight of boric acid. A small part of the boric acid content may be replaced by phosphoric acid or arsenic acid. White hard coatings of o.2 mm. thickness or more may be obtained. They may be impregnated or coloured after formation, or colouring matter may be included in the electrolyte bath. The treatment may be applied to unhardened or hardened aluminium alloys containing alloying materials, preferably magnesium and silicon, not exceeding 2.5 per cent by weight; grinding tools, such as discs, may be of such alloys coated according to the invention (see Group XXIII). Aluminium foil coated according to the invention may be used for abrasion.