GB760327A - Improvements in the formation of phosphate coatings on wire or strip - Google Patents

Abstract

A method of forming a phosphate coating on a wire or strip comprises passing the wire or strip with a chemically clean surface continuously in an axial direction through an accelerated phosphate coating solution having a pointage greated than 30 and so correlating the speed of travel and the duration of immersion of the wire or strip in the solution with the degree of acceleration of the solution that initial formation of the coating takes place in the solution, that the wire or strip emerges from the solution carrying a uniform film of the solution by which the coating formation is completed and that the coating is crystalline and weighs not less than 250 mgms. per square foot. Wire or strip made from ferrous metals, zinc, cadmium and aluminium may be coated and the chemically clean surface is obtained by patenting or close-annealing, if necessary followed by pickling, e.g. in 10 per cent HCl, and passage through a grit or other mechanical scrubbing device. Preferably, the accelerator is a chlorate, the solutions used those disclosed in Specification 706,315, i.e. dihydrogen phosphate solutions accelerated with a chlorate or a mixture of a chlorate and a nitrate, the contents of phosphate (as P2O5), chlorate (as CIO3), and nitrate (as NO3), being such that (chlorate + nitrate)/ phosphate lies between 0.1 and 0.6, the metal of the solution is zinc, the pointage 60-80 and temperature of use 150 DEG -180 DEG F. The coating weight may be increased by the addition to the solution of 0.1-0.2 per cent of an -amino polycarboxylic acid or 0.02-0.08 per cent of a soluble copper salt. To assist the coating formation the wire or strip may be heated as it emerges from the solution, e.g. by a Bunsen burner flame or an induction coil; in this case, the solution pointage is preferably 50-70. If the wire is to be drawn a lubricant, e.g. drawing soap, may be applied directly to the phosphate coating or over a coating of borax or lime. If the wire is to be stored, corrosion thereof may be prevented by applying a protective material over the phosphate coating, this may be the lubricant in the subsequent drawing operation and is advantageously the drawing soap to which a corrosion inhibitor, e.g. sodium nitrite, chromate or benzoate, has been added. Among solution specified in the examples are those containing (a) zinc oxide, phosphonic acid and sodium nitrophenol sulphonate (b) zinc oxide, nitric acid, phosphoric acid and sodium nitrite and (c) sulzinc primary phosphate and nitroguanidine.