GB869218A - Improvements relating to the picking of ferrous surfaces - Google Patents

Abstract

Ferrous metal surfaces are pickled using an aqueous phosphoric acid solution containing ferrous and ferric ions in a ratio between 11 : 1 and 1 : 7 the ferrous ion concentration being controlled by removing some of them by treatment with an ion exchange resin, so as to maintain the concentration of ferric ions between 20% and 85% of the saturation concentration. The pickling solution may contain 15-45% phosphoric acid, and the desired ferric ion concentration may be maintained by continuously recirculating a portion of the solution through a bed of said resin. The solution may contain inhibitors such as dibutylthiourea, di-isopropylthiourea, or sodium iodide. Pickled surfaces may be given a flash coating of nickel, cobalt, or a nickel cobalt alloy prior to vitreous enamelling. The nickel or cobalt coating may be formed from an aqueous solution of a nickel or cobalt salt which also contains controlled quantities of hypophosphite radicals, and a nickel cobalt coating by treating the surface with a bath containing nickel sulphate, cobalt sulphate, boric acid, citric acid, dextrose, sodium hydroxide, chromic oxide, glutamic acid, and sodium bisulphite. In using this solution the bath is maintained at a pH of 4.9-5.1 and the pickled surface is contacted with the solution for 15/20 minutes. The coated surface may be heated in an oxidizing atmosphere at 1100-1900 DEG F. for 1-10 minutes. Instead of using a nickel or cobalt coating a phosphate coating may be formed in the conventional manner and heated in air at 1100-1900 DEG F. for 3-15 minutes, to convert it to an oxide-phosphate coating prior to vitreous enamelling.