Patent GB-745452-A (GB)
Title: Method of diffusing tin in metal articles
IPC: C23C10/24
Filed: 1953-06-16
Abstract: Tin-containing alloy layers are formed on the surfaces of articles of iron, copper and alloys thereof by immersion of the articles in an anhydrous fused salt bath (B-bath) consisting of one or more anhydrous chlorides of alkali metals, alkaline earth metals, ammonium, zinc and magnesium until the articles acquire approximately the temperature of the bath and then in a molten chloride salt bath (T-bath), containing stannous chloride.  The B-bath may comprise Na Cl 5-10 per cent, K Cl 5-10 per cent, Zn Cl2 80-90 per cent; Na Cl 10-30 per cent, K Cl 10-30 per cent, Zn Cl2 35-75 per cent, NH4 Cl 0-5 per cent; Na Cl 5-30 per cent, K Cl 5-30 per cent, Zn Cl2 10-90 per cent, NH4 Cl 0-20 per cent, Mg Cl2 0-10 per cent; Na Cl 30-50 per cent, K Cl 30-50 per cent, Zn Cl2 0-40 per cent, Mg Cl2 0-10 per cent.  Prior to treatment in this bath, however, the articles may be subjected to oxidising baths in the form of aqueous oxidising acids or salts or oxidising fused salt baths; the latter may comprise Na NO3 40-60 per cent, KNO3 40-60 per cent; Na NO3 20-40 per cent, KNO3 20-40 per cent, Na Cl 10-30 per cent; or Na NO3 10-50 per cent, KNO3 10-50 per cent, Na Cl 10-20 per cent, K Cl 1-5 per cent, Mn O2 1-5 per cent.  Compositions of T-baths given comprise K Cl. Sn Cl2 5-30 per cent, K Cl 5-20 per cent, Na Cl 5-20 per cent, Zn Cl2 50-80 per cent; K Cl.Sn Cl2 1-30 per cent, K Cl 2-30 per cent, Na Cl 2-30 per cent, Zn Cl2 40-90 per cent, NH4 Cl 1-10 per cent; K Cl.Sn Cl2 5-40 per cent, K Cl 5-10 per cent, Na Cl 5-10 per cent, Zn Cl2 50-80 per cent, NH4 Cl 1-5 per cent, Mg Cl2 1-10 per cent; or K Cl.Sn Cl2 5-30 per cent, K Cl 0-10 per cent, Na Cl 0-10 per cent, Zn Cl2 50-80 per cent, NH4 Cl 0-5 per cent, Mg Cl2 0-10 per cent.  The articles may be conveyed from B-bath to T-bath under a hot bonnet or through a heated medium to avoid a drop of temperature.  The T-bath may be regenerated, if contaminated with iron, by adding tin or zinc powder or sponge and separating the solid Fe Sn or Fe Zn crystals; if contaminated with copper, iron may be added when metallic copper deposits thereon.  Two T-baths may be used, the first containing 1-4 per cent tin, the second 6-25 per cent. After removal from the T-bath, the article is cleaned in water which may contain trisodium phosphate: it may then be subjected to a N-bath which may comprise a zincate, titanate, chlorate, vanadate, tungstate, molybdate or aluminate with or without agaragar or a silicate followed by phosphoric or chromic acid or both: butyl titanate, polymerised butyl titanate or butyl vanadate or the corresponding ethyl compounds may be used. Alternatively, hot solutions of nitrate, chromates or phosphates, e.g. zinc dihydrogen phosphate may be used which may be followed by drying in an oxygen-containing atmosphere up to 130 DEG C.; they may then be immersed in a cold solution of polyvinyl acetate or polyvinyl chloride and dried at 90-170 DEG C. or in oils, fats or resins, or they may be painted or lacquered.  The articles or baths may be vibrated at sonic or supersonic speed during any of these treatments.