GB412244A - Process for the surface coating of iron and steel - Google Patents

Abstract

A process for surface coating iron or steel consists in first cleaning or pickling, for example with sulphuric or hydrochloric acid, treating with a reagent which during the treatment contains a ferrous salt, an acid, and an oxidizing agent, such a reagent being nitric acid, so that a ferric salt is produced in situ, said treatment being continued until the planar structure of the metal grains is exhibited, cleaning the surface, and finally coating with vitreous enamel or other paint-like coating. The pickled sheets may, for example, be submerged for 40 secs. in 8 per cent nitric acid, scrubbed, neutralized with hot 2 per cent trisodium phosphate, and dried by hot air. The temperature and concentration of the acid, and time of immersion may be varied. Solutions containing ferrous salts together with ferric salts, chlorates, nitrates, nitric acid, chromates, or oxygen, which is bubbled therethrough, may be employed. A sufficient concentration of oxidizing agent and of an acid should be present to maintain a concentration of ferric salt of 5 per cent or more. A solution containing 2-3 per cent or less of sodium chlorate and 4 per cent of sulphuric acid is efficient at about 160 DEG F. Other reagent baths contain about 20 per cent of ferric chloride, or 2 per cent or 5 per cent of nitric acid. The black scum left by the etching may be removed by treatment with a dilute solution of sodium nitrate, e.g. 2-4 per cent, and sulphuric acid, e.g. 4-5 per cent, or of nitric acid, prior to the scrubbing. The surface is thus rendered passive to oxidation. The treated surface can be white enamelled using two coatings only and scaling is eliminated. Reference has been directed by the Comptroller to Specifications 16451/88, [Class 82], 25393/13, 236,241, [both in Class 82 (ii)], and 375,599.