GB877313A - Improvements in or relating to materials and processes for producing chemical coatings on the surfaces of corrosion-resistant metals and alloys - Google Patents

Abstract

An aqueous acid solution for chemically coating corrosion resistant metals and alloys such as nickel base alloys and stainless steels, consists of the oxalate anion and cation, the borate anion and cation, and a sulphur bearing compound which yields sulphur dioxide and sulphur upon decomposition in acid solution. Oxalic acid and boric acid are the preferred sources of the oxalate and borate ions but the sodium, potassium and ammonium salts may also be used, in which case a mineral acid such as sulphuric, hydrochloric, nitric or phosphoric acid is added to reduce the pH of the solution which should be between 0.5 and 2.5. The sulphur compounds used are the water soluble thiosulphates, sulphites, thionates, and hydrosulphites in an amount sufficient to give a titration limit of 2 to 15 mls. of 0.05 N iodine solution for a 10 mm. sample of the solution. The solution contains 0.1 to 4 parts by weight of boric acid per part of oxalic acid which is present from 2 to 500 gms per litre, and it may be prepared or replenished with a dry powdered mixture of boron trioxide and commercial grade oxalic acid which may also contain the sulphur compound, the dry mixture having the following composition by weight, 5% to 47.5% boron trioxide, 10% to 70% oxalic acid and 5% to 80% sulphur compound. The solution is maintained at 120 DEG F. to 180 DEG F. during the coating process and the surface to be coated may be activated prior to coating.