Patent GB-659927-A (GB)
Title: Improvements relating to the coating electrolytically of metal articles
IPC: C25D3/66
Filed: 1949-04-04
Abstract: 659,927. Electrolytic production of alloys. AMERICAN ELECTRO METAL CORPORATION. April 4, 1949, No. 9131. Convention date, April 5, 1948. [Class 41] In a method of coating a metal article, especially one consisting of or containing a highly refractory metal such as tantalum, tungsten, molybdenum, titanium or an alloy thereof by immersing it in a liquid electrolyte containing ions of the coating metal, the electrolyte containing the coating metal is heated to a temperature above its melting point and that of the coating metal but below the melting point of the article, an electric current of a minimum density of one-half ampere per square inch is passed to deposit molten coating metal, and the flow of current is continued to cause alloying of the molten coating metal with surface metal of the solid article to an approximate minimum depth of 0.003 inch. If only a portion of the coating metal is alloyed with the article, the remainder of the coating metal may, if of suitable metal, form its own corrosion resisting surface in the form of a refractory oxide. If the coating metal is completely alloyed, the alloy serves as a foundation for a layer which may form its own corrosion-resisting surface or may have a surface of such nature produced by baking a mixture of refractory oxides, for example, those of aluminium, zirconium, magnesium, and chromium, and vitreous frits, for example, metal silicates, on the outside of the layer. If two much unalloyed coating metal is pro. duced on the article it may be removed by passing an electric current in reverse direction this current being produced either from an external source, or by a self-exciting process. The coating metals may be aluminium, cerium, lanthenum, or thallium, all of which form a corrosion-resistant impervious layer. In the arrangement shown, a graphite crucible 10 is embedded in heat-insulating material 11 which is placed in a container 12 preferably of ceramic material. A hollow induction coil 13, through which cooling water is circulated, surrounds the container. The article 16 to be coated is connected by means of a clamp 18 and rod 19 to a terminal 29 leading to a source of current, the other terminal 22 being connected by a rod 21 to the crucible 10. The article is immersed in an electrolyte 24 containing compounds of the coating metal. In the case of aluminium coating, the electrolyte may contain a mixture of aluminium oxide and cryolite, cryolite and chiolite, or sodium potassium chloride and cryolite. The article 16 and the rod 21 may be connected together in a circuit containing a variable resistor 26, an ammeter 25 and a switch 30. In normal operation of the process the electrolyte is heated by means of the high-frequency current supplied to the coil 13 and current for coating the article 16 is supplied to the electrolyte by way of terminals 22, 29 and switch 23. Any surplus metal deposited may be removed by closing switch 30 and opening switch 23, whereby a selfexciting circuit through resistor 26 is completed and excess metal is re-dissolved in the electrolyte 24.