GB453704A - Process for the pickling of metals - Google Patents

Abstract

Iron or iron alloy is pickled to provide a blank surface suitable for subsequent enamelling, painting, or metallizing, by immersion in an oxidizing pickling bath heated to above 50 DEG C. which contains an acid and not more than 5 per cent of a salt the anion of which is different from that of the acid and the cation of which is more electropositive than iron, the initial concentration of the acid being so low that as the pickling proceeds it falls to a value at which hydrolysis occurs, the pickling taking place with precipitation of insoluble basic iron compounds. Fresh acid is added to the bath in such small quantities that hydrolysis is not prevented. A mixture of acids may be employed, the anion of the acid present in largest amount being different from that of the salt. If a mixture of salts is used the anion of the salt present in largest amount must be different from that of the acid and the cations of all the salts must be more electropositive than iron. Examples of pickling baths are (a) 1 per cent of nitric acid and 2 per cent of magnesium sulphate, (b) 1 per cent of hydrochloric acid and 2 per cent of sodium nitrate, and (c) 1 per cent of sulphuric acid and 2 per cent of potassium nitrate. If the iron is heavily rusted the rust may be first removed in a stronger acid solution, e.g. 2 per cent of sulphuric acid and 2 per cent of potassium nitrate, and the blank pickling then effected in a bath of 0,25 per cent sulphuric acid and 2 per cent potassium nitrate. British Specification 412,244 and U.S.A. Specification 1,824,932 are referred to.