Patent GB-464982-A (GB)
Title: Process of cleaning and coating metal surfaces
IPC: C23G1/32
Filed: 1935-10-26
Abstract: Grease, oil, scale, sand, and existing surface coatings such as paint, enamel, and zinc, tin, or cadmium platings are removed from metal articles, particularly iron, steel and other ferrous alloys, copper, and cuprous alloys, by subjecting them to the action of a fused mixture containing at least two members of the group consisting of alkali metal nitrates, carbonates, and hydroxides, and which may also contain oxygen-carrying agents such as manganese dioxide and copper sulphate.  The treatment is usually effected by immersion and when removing metal coatings the metal is precipitated, mostly in combined form, as a sludge which may be drawn off periodically. Specific examples of molten baths are (a) sodium nitrate and potassium nitrate, with or without sodium hydroxide, (b) potassium carbonate and sodium hydroxide, (c) potassium nitrate and sodium hydroxide.  To any of the specified mixtures 0,5--10 per cent of manganese dioxide or copper sulphate may be added.  The proportions and temperatures appropriate to various circumstances are given.  The Specification as open to inspection under Sect. 91 refers to the treatment of aluminium and its alloys and includes treatment of the articles in molten baths containing only one alkali metal salt which may be a nitrate, carbonate, hydroxide, or phosphate, and also baths containing a mixture of salts of which one is a phosphate.  This subject-matter does not appear in the Specification as accepted.ALSO:A tough, strongly adherent rust and corrosion resisting coating is formed on metal articles by subjecting them to the action of a fused mixture containing at least two members of the group consisting of alkali metal nitrates, carbonates, and hydroxides, and which may also contain oxygen-carrying agents such as manganese dioxide and copper sulphate, then washing them in an aqueous liquid, and finally subjecting them to the action of an aqueous solution containing one or more substances which will colour the surface either by dyeing or by reaction with the surface, such as hematoxylin, iron sulphate, water soluble nigrosine, and tannic acid. The process is particularly applicable to iron, steel and other ferrous alloys, copper, and cuprous alloys.  The treatment with the fused mixture, which is usually effected by immersion, removes grease, oil, scale, sand, and existing surface coatings such as paint, vitreous and non-vitreous enamel, and zinc, tin, or cadmium platings and also forms an adherent coating on the metal surface. When removing metal coatings the metal is precipitated, mostly in combined form but in some cases in the molten state, as a sludge which may be drawn off periodically.  If vitreous enamel is being removed, deterioration of the bath may be avoided by introducing steam to precipitate the siliceous sludge.  Specific examples of molten baths are (a) sodium nitrate and potassium nitrate with or without sodium hydroxide, (b) potassium carbonate and sodium hydroxide, (c) potassium nitrate and sodium hydroxide.  To any of these mixtures 0,5-10 per cent of manganese dioxide or copper sulphate may be added.  The proportions and temperatures appropriate to various circumstances are given.  After the treatment with the molten mixture the articles are washed with an aqueous liquid, which may be water, or an aqueous solution containing hydrochloric acid, sulphuric acid, oxalic acid, or iron sulphate.  When the neutralizing solution is used the articles may be washed with water both before and after immersion in the neutralizing bath.  The final colouring bath, which acts also to seal the pores of the coating, comprises a solution of a colouring agent such as iron sulphate, hematoxylin, nigrosine, or tannic acid.  Hematoxylin may be used in the form of logwood chips &c. The solution is preferably used at a temperature of from 100 to 200 DEG  F., and after treatment the articles are washed. They may subsequently be given any desired surface coating, e.g. oil, paint, lacquer, enamel, or metal such as a zinc, tin, or cadmium plating.  The treatment of metal plates, castings, bolts, nuts, and washers is referred to.  The Specification as open to inspection under Sect. 91 refers to the treatment of aluminium and its alloys and includes treatment of the articles in molten baths containing only one alkali metal salt which may be a nitrate, carbonate, hydroxide, or phosphate, or containing mixtures of such salts of which one is a phosphate, and also describes the electrolytic treatment of the articles in the molten bath, the articles being made either cathode or anode or both cathode and anode.  The electrolytic treatment is particularly applicable when the baths are used for removing vitreous enamel as it causes precipitation of the sludge. It also states that the molten baths when used at 600-900 DEG  F. may be employed to effect annealing of the article and that the baths may in certain cases be heated to 1200-1500 DEG  F. to remove internal stresses in the metal.  The use of the colouring solution is stated to be optional.  This subject-matter does not appear in the Specification as accepted.