GB714078A - Improvements in or relating to processes for electrolytic polishing of metals - Google Patents

Abstract

714,078. Electrolytic surface treatment of aluminium. EVER READY CO. (GREAT BRITAIN), Ltd. Aug. 27, 1952, [Sept. 24, 1951.] No. 2334.4/51. Class 4I. Articles of aluminium or an aluminium-rich alloy are polished by immersion in a heated electrolyte comprising phosphoric acid, sulphuric acid, a phosphoric acid ester of a polyhydric alcohol or a salt thereof, and water; after a period, during which etching is effected, the article is connected as anode to direct current voltage. Suitable proportions for the electrolyte are 45-80 per cent by weight of phosphoric acid, preferably orthophosphoric, 5-20 per cent of sulphuric acid, and 1-5 per cent of the phosphoric acid ester or salt. The latter constituent may be glycerophosphoric acid or its sodium or potassium salt. The period before passage of current may be 3o-6o seconds, the bath temperature 70‹-90‹C. and the anode current density 100-180 amps per sq. ft. The anode support may be reciprocated to agitate the electrolyte. The cathodes may be of lead or graphite and are preferably of area substantially greater than the projected area of the article under treatment. The article after electrolytic treatment may be washed in water containing a wetting agent, and a thin surface film present thereon removed by immersion in a solution of chromic and phosphoric, or chromic and sulphuric, acids or of salts of these acids. The articles may then be anodized in a chromic acid or sulphuric acid bath to produce an opaque or transparent coating and the coating dyed and/or sealed. An impact-extruded or deep-drawn article e.g. an electric torch case may be subjected to a light mechanical polishing before the electrolyte polishing.