GB1088287A - Brightening of aluminium and alloys thereof - Google Patents

Abstract

A composition for brightening Al and Al alloys from which free nitric acid is absent comprises phosphoric acid, cupric nitrate and water. Al or Al alloy articles are brightened in a bath containing ortho, pyro or other condensed phosphoric acid, 0.5-20% by weight cupric nitrate, up to 50% H2SO4 up to 5% NH4 ions and less than 25% water, at 80-125 DEG C. for 0.5-10 minutes. The cupric nitrate may be formed in the bath by mixing cupric sulphate and NH4, Al or an alkali metal nitrate. The NH4 ions may be added as ammonia, NH4 nitrate, sulphate or phosphate. 5% of each of HCl, boric acid, ferrous sulphate, titanium or vanadium compounds, oxalic or tartaric acid and carboxymethylcellulose as a wetting agent or non-ionic surface active agents may be present. The process may be accompanied by electro-polishing in which the article is the anode, the bath the electrolyte and a potential of 10-20v applied. During use the bath may be replenished by adding cupric or ammonium nitrate or HNO3, topping up with the original solution and adding water to maintain the specific gravity within defined limits. After brightening the article is washed in hot water, in a desmudge solution of HNO3 and water, chromic and sulphuric or phosphoric acids or a mixture of an acid bisulphate and Na dichromate and is finally rinsed in cold water and hot demineralized water before drying. The brightened surface may be anodized or coated with a transparent lacquer.