GB200892A - Process for treating iron, steel and the like to produce a protective surface layer thereon - Google Patents

Abstract

200,892. Mai, A. April 19,1922. Coating.-Iron, iron alloys, and steel are treated to produce a protective layer thereon in an alkaline bath, e.g. caustic soda, containing an oxidizing agent and also slaked lime or other caustic alkaline earth. The oxidizing agents may be bichromates, permanganates, nitrates, nitrites, peroxides, chlorates &c. and also organic oxidizing agents, e.g. nitrocellulose, nitrophenol, nitrotoluol or organic peroxides. The bath is employed at temperatures of 120-130‹ C. or higher, such as 180-200‹ C., the treatment at higher temperatures and with more concentrated baths occupying less time than the treatment at lower temperatures. The oxidation and coloration of the metal surface takes place more rapidly if small quantities of decomposition products of the oxidizing agent are added to the bath at the beginning of the operation. The same result may be obtained by adding small quantities of an iron salt, e.g. ferric nitrate. Brighter colours are obtained if the metal is first provided with a preliminary protective coating, e.g. by immersion in dilute copper sulphate solution or by heating in air. The bath subsequently employed is preferably concentrated and employed at 180-200‹ C. On removal from the bath, the objects may be immersed in dilute acetic acid or other liquid to dissolve any adhering lime, and then washed in water, dried, and lacquered, varnished or oiled.