GB963129A - Improvements in or relating to the formation of chemical conversion coatings upon metal surfaces - Google Patents

Abstract

Iron and steel surfaces are phosphated with aqueous acidic alkali metal (to include ammonium mono-hydrogen- and ammonium di-hydrogen-) phosphate solutions containing nitrite ion (NO2-) and a heterocyclic compound, and when the heterocyclic compound concentration ranges 0.05-1.0 gm./litre then the nitrite ion concentration = 0.05-1.0/molecular weight of heterocyclic compound X 46 gms./litre and when the heterocyclic compound exceeds 1.0 gm./litre the nitrite ion concentration ranges from 0.05/molecular weight of heterocyclic compound X 46 to 10.0 gms./litre. The pH=4.0-5.8, preferably 4.5-5.7. The temperature is at least 120 DEG F. (49 DEG C.), the operating range being 120-150 DEG F. (49-66 DEG C.) and preferably 130-145 DEG F. (57-63 DEG C.). The heterocyclic compounds comprise melamine, 3-amino-1,2,4-triazole, imidazole, succinimide, 2-oxazolidone. Coating varies between 30 seconds and 5 minutes, preferably 1-3 minutes. Concentration of the nitrite ion varies directly with respect to concentration of the heterocyclic compound up to 1.0 gm./litre of the latter as prescribed by the above formula. For heterocyclic compound contents in excess of 1.0 gm./litre the nitrite ion concentration may rise to a miximum of 10 gms./litre. 30 gms./litre or more of heterocyclic compound may be used as determined only by solubility considerations but in practice quantities are limited to 10.0 gms./litre. The nitrite ion may be introduced as a salt of alkali or alkaline earth metals, preferably NaNO2. Although at least 0.05 gm./litre of heterocyclic compound must be incorporated in the solution, it is preferred to use 0.1 gm./litre of the heterocyclic compound as the practical minimum concentration due to drag-out losses. A wetting agent may be incorporated.