GB673391A - Improvements in or relating to the production of protective coatings upon aluminum and aluminum alloys - Google Patents

Abstract

673,391. Anodic treatment of aluminium base metal. JUNGHANS, S. Feb. 13, 1950, No. 3672/50. Class 41. A process for the production of an oxidic coating on an aluminium base metal comprises anodizing the metal on an electrolyte consisting of a substantially water-free fused salt bath, which is such that upon electrolysis oxygen is liberated at the anode, the bath being maintained at a temperature between 140‹ C. and the melting point of the metal. The salt used is preferably an alkali-metal or ammonium nitrate or nitrite. Potassium dichromate is also suitable. Decomposition of the nitrates may be accelerated by the addition to the bath of catalysts, for example copper or iron compounds or lead oxides. Copper oxide and copper nitrate are mentioned as examples. Copper and iron compounds may be obtained by using copper and iron electrodes. To ensure the production of light transparent coatings a local potential which normally persists for a short time after current has been switched off, should be reduced or destroyed. This may be done by using an electrolysing current which is interrupted or which is an alternating current or a pulsating current. A suitable current is provided by superposing a direct current of N-2 volts upon an alternating current of N volts. Anodizing treatment may be preceded by etching for a short time in a caustic soda solution of about 5 to 15 per cent concentration, and may be followed by boiling in a 2 to 5 per cent waterglass solution.