| 摘要 |
In electroplating a shaped article of a relatively high hydrogen overvoltage metal having because of differences in geometry or accessibility one or more regions of non-plating relatively low current density, etching is prevented in the low current density regions by coating them or placing them in intimate electrical contact with a relatively low hydrogen overvoltage metal having an overvoltage of less than 0.33 volt at a current density of 0.1 amp/dm2 as defined in (The Corrosion Handbook by H. Uhlig, John Wiley 1948). The low hydrogen overvoltage metal may be applied to the low current density areas (a) from an aqueous solution containing the metal in ionic form by spraying, brushing, contacting, dipping, wicking, electroless or electrolytic plating, or by applying a paste containing such a solution or (b) from an organic solvent solution of compounds of the metal or a foramions sheet of or coated with such a metal may be pressed against the low current density areas. Specified low hydrogen overvoltage metals are platinum, palladium, rhodium, iridium, gold, black nickel and electroless nickel and the process is described as applied to electroplating ferrous articles with chromium or nickel. In a specific example the inside of a steel pipe after vapour degreasing, anodic cleaning in alkali and acid dip brushed over with an aqueous solution containing 4 g/1 palladium chloride and 20 g/1 sodium chloride and acidified to pH 1.5 with hydrochloric acid. The pipe was then anodically cleaned in a 200 g/1 chromic acid bath at a current density of 14.4 amps/dm2 of external surface and was then cathodically plated in an aqueous chromic acid bath at approx. 58 DEG C. containing sulphuric acid and potassium silicofluoride using lead alloy anodes containing 4% tin with a cathode current density of 49 amps/dm2. The inside of the pipe was neither plated nor etched. Examples of other low hydrogen overvoltage metal solutions are aqueous solutions containing 2.5 g/1 chloroplatinic acid or 4 g/1 hydrated chloroauric acid. Pastes are also described comprising 1 to 2 g/1 rhodium chloride or 2.5 to 4 g/1 chloroiridic acid thickened with silica gel, methyl cellulose, polyvinyl alcohol, gelatine, or agar-agar.
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