| 摘要 |
In electroplating on iron or steel, etching is prevented in the non-plating low current density areas of the cathode by placing adjacent said areas an auxiliary electrode which is maintained more cathodic than these areas of the cathode. The plating metal may be copper, chromium, nickel, tin, or zinc deposited from acid baths. The auxiliary electrode may be of carbon or any metal not appreciably affected by the plating bath e.g. Pb, Cr, Ag, Ru or the low hydrogen overvoltage metals set out below and may be of steel, especially if treated to resist corrosion and plating. Parts of the auxiliary electrode may be protected with masking tape and other parts by being placed in intimate electrical contact, e.g. by chemical or electrolytic plating, with a metal having a hydrogen overvoltage less than 0.33 volt measured at a current density of 0.1 amp/dm2 (see The Corrosion Handbook by H. Uhlig, John Wiley 1948) such as Pt, Pd, Rh, Ir, Au and Ni. As described the interior of a steel pipe is protected from etching when the outside is plated with chromium from a chromic acid bath including sulphuric acid and potassium silicofluoride by means of a more cathodic steel rod placed inside the pipe and masked with tape except adjacent the pipe ends where it is chemically coated with platinum from a chloroplatinic acid solution.
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