GB473285A - Improvements in or relating to the coating of surfaces of iron or steel - Google Patents

Abstract

A cyclic process for phosphatizing iron or steel comprises spraying upon the metal with sufficient force to ensure thorough and uniform welting a dilute solution containing a phosphate or phosphates not readily oxidizable by spraying through air and a small amount of free acid, draining the solution from the surface, removing the sludge produced by the oxidation of ferrous phosphate and re-spraying the solution. Ferrous phosphate is formed during the coating reaction and is oxidized when the solution is reflected from the surface in a state of subdivision. The non-oxidizable phosphates may be one or more of the dihydrogen phosphates of manganese, zinc, cadmium, and calcium, or a mixture of one or more of these phosphates with sodium phosphate, and the spraying pressure is preferably about 20 pounds per square inch. Accelerating agents, e.g. nitrates or copper compounds, may be added to the solution, the acid ratio of which is preferably as high as 10 to 1. The preferred spraying temperature is 65-75 DEG C. To avoid sludge due to the hydrolysis of the non-oxidizable phosphates, a small amount of nitric acid is preferably added to the liquid used for replenishing the solution before re-spraying. For example a coating solution consisting of 350 lb. of sodium nitrate, 110 lb. of zinc dihydrogen phosphate, about 1 lb. of copper carbonate, and 2000 gallons of water may be replenished by addition of a solution containing 7,5 parts by weight of zinc dihydrogen phosphate, 1 part of 100 per cent nitric acid, and 0,4-0,5 per cent of copper carbonate, the nitric acid reacting with normal zinc phosphate formed by hydrolysis to produce zinc nitrate. Specifications 390,834, 397,179, 473,326, and 473,974, are referred to.