GB625065A - Improvements in and relating to the depositing of protective coatings or films on metal surfaces - Google Patents

Abstract

A composition which in aqueous solution will deposit an amorphous metal-phosphate coating on metals-particularly iron, steel, zinc, copper, silver, magnesium and their alloys-comprises essentially (a) at least one water-soluble salt of a coating metal selected from Ca, Zn, Co, Mg, Cd, Fe, Al, Ba, Sr, Mn, Cr, Ni; (b) a molecularly dehydrated alkali-metal or ammonium phosphate glass; (c) a material which functions in the solution as an oxidizing agent, e.g. a nitrate, nitrite, sulphite, permanganate, chromate, dichromate, perborate, ferricyanide, hypochlorite, together with if necessary a pH adjusting material to ensure a pH value of the solution between 3 and 8; the proportions of the components (a) and (b) being such that when the composition is dissolved in water the ratio of (b) to the metal ion of (a) is from 0.05 : 1 to 12 : 1, whilst the amount of (c) is such as to give a concentration of at least 10 p.p.m. In a modification, the component (a) comprises salts of both Ca and Al, and the components (and pH adjusting material if used) are chosen to give a pH between 2 and 8. Preferably the phosphate glass has a molecular ratio of alkali metal oxide (M2O) to P2O5 of from 0.4 : 1 to 1.7 : 1, and typical compositions are by weight, 49.2 per cent sodium metaphosphate glass, 34.7 per cent of 78 per cent calcium chloride, 6.6 per cent sodium nitrate, 9.5 per cent acid sodium sulphate; 54.62 per cent sodium metaphosphate glass, 38.6 per cent of 78 per cent calcium chloride, 0.7 per cent sodium bichromate, 6.1 per cent acid sodium sulphate, the M2O : P2O5 ratio in the phosphate glass of these examples being 1.1 : 1. The compositions are dissolved in water sufficient to ensure the presence in solution of 100-10,000 p.p.m. of the metal coating ion of component (a), and at least 200 p.p.m. of the phosphate glass (b)-their sum being from 300 to 50,000 p.p.m. and the ratio of phosphate glass to metal coating ion being preferably between 2 : 1 and 12 : 1. The solution is applied by dipping, spraying, rolling, &c. and preferably at temperatures between 15 DEG C. and 50 DEG C., and the examples given show the use of solutions containing 1.4-2.5 per cent by weight of the composition. The composition ratio of the components (a) and (b) is said to remain constant down to a depletion of 35 per cent of the original value of the solution; and the solution may be restored to its original effectiveness by the addition of more of the composition. The phosphating treatment may be succeeded by a rinse with an adjusted solution of chromic acid or a salt thereof, the solution having a concentration of CrO3 between 0.05 and 0.5 per cent and pH bewteen 3.9 and 5.5. The coating of the process is said to form a good basis for a subsequent organic finish, e.g. of paint, varnish, lacquer or of enamel, and the use of a Ni salt in component (a) is said to be particularly effective when phosphating iron to which a subsequent vitreous enamel is to be applied.