GB604325A - Improvements relating to the production of phosphate coatings ferrous surfaces - Google Patents

Abstract

A metal phosphate coating solution, or a replenishing solution therefor, is produced by dissolving a phosphate of the metal more basic than its primary orthophosphate in aqueous orthophosphoric acid in quantity less than that required to convert all the phosphate into primary orthophosphate; zinc, cadmium, manganese, calcium and iron phosphate coating solutions are referred to. An initial solution may be made in the tank in which it is to be used, or separately. The metal phosphate employed may be the tertiary, secondary, meta-, or pyrophosphate or mixtures thereof, and may be the sludge from the coating bath. The reaction may take place in the coating tank, provided it is not close to the work, the acid being added continuously or intermittently. If ferrous iron present is oxidized and lost as ferric phosphate, metal must be added to compensate for the loss. Primary ferrous phosphate may be added if a ferrous surface is being coated with iron phosphate, or manganese primary phosphate may be added if manganese phosphate is being used to coat a ferrous surface, some of the manganese entering the coating in place of the iron lost by oxidation. When zinc phosphate is applied to ferrous surfaces, the zinc used up in the process must be replaced, and the iron dissolved should be removed by oxidation, precipitating as ferric phosphate. An oxidizing accelerator may be used, and the free phosphoric acid produced should be neutralized by adding a tertiary or other salt more basic than the primary. The attack on the surface of the work may be increased by the addition of mineral acid salts of the coating metal, which may be prepared by dissolving the basic phosphate in say sulphuric acid. In solutions containing such a salt and used for coating ferrous surfaces, the ferrous iron in part of the solution is oxidized by air or other agent to ferric orthophosphate, which is removed, and the mineral acid liberated is reacted with phosphate more basic than the primary, but less than the tertiary; the solution removed, now containing primary phosphate so formed, is returned to the main body. The oxidation may however take place in the main body. The solutions may contain accelerators, e.g. nitrates or chlorates formed by adding the corresponding acids. When replenishment is effected by a solution of primary orthophosphate p formed as above, a "basic phosphate" chamber is provided, either by a partition in the processing tank or as a separate small tank, in which the sludge is treated with phosphoric acid. In the dipping process, the displacement of the solution by the articles dipped serves to transfer the sludge to the "basic phosphate" chamber; alternatively, circulation may be caused by convection currents.ALSO:A metal phosphate coating solution, or a replenishing solution therefor, is produced by dissolving a phosphate of the metal more basic than its primary orthophosphate in aqueous phosphoric acid in quantity less than that required to convert all the phosphate into primary orthophosphate. Zinc, cadmium, manganese, calcium and iron phosphate coating solutions are referred to. An initial solution may be made in the tank in which it is to be used, or separately. The metal phosphate employed may be the secondary, tertiary, meta-, or pyrophosphate or mixtures thereof, and may be the sludge from the coating bath. The reaction may take place in the coating tank, provided it is not close to the work, the acid being added continuosly or intermittently. If ferrous iron present is oxydized and lost as ferric phosphate, metal must be added to compensate for the loss. Primary ferrous phosphate may be added if a ferrous surface is being coated with iron phosphate, or manganese primary phosphate may be added if manganese phosphate is being used to coat a ferrous surface, some of the manganese entering the coating in place of the iron lost by oxidation. When zinc phosphate is applied to a ferrous surface, the zinc used up in the process must be replaced, and the iron dissolved should be removed by oxidation, precipitating as ferric phosphate. An oxidizing accelerator may be used, and the free phosphoric acid produced should be neutralized by adding a tertiary or other salt more basic than the primary. The attack on the surface of the work may be increased by the addition of mineral acid salts of the coating metal, which may be prepared by dissolving the basic phosphate in say sulphuric acid. In solutions containing such a salt, and used for coating ferrous surfaces, the ferrous iron in part of the solution is oxidized by air or other agent to ferric orthophosphate, which is removed, and the mineral acid liberated is reacted with phosphate more basic than the primary, but less than the tertiary; the solution removed, now containing primary phosphate so formed, is returned to the main body. The oxidation may however take place in the main body. The solutions may contain accelerators, e.g. nitrates or chlorates formed by adding the corresponding acids. When replenishment is effected by a solution of primary phosphate formed as above, a "basic phosphate" chamber is provided, either by a partition in the processing tank, or as a separate small tank, in which the sludge is treated with phosphoric acid. In the dipping process, the displacement of the solution by the articles dipped serves to transfer the sludge to the "basic phosphate" chamber; alternatively, circulation may be caused by convection currents.