| 摘要 |
Protective coatings are formed on metals by depositing first a high lustre nickel coating then an intermediate layer of metal from a bath of pH above 6 containing in suspension a solid non-acid resistant silicate obtained by reaction of lime and/ or magnesia with diatomaceous earth and having in suspension in water an alkaline reaction, and finally chromium. Typically the bright nickel is electrodeposited from a Watts type bath of pH4 containing also quinaldinium ethiodide, p-toluenesulphonamide and sodium lauryl sulphate at 55 DEG C. and the chromium is electrodeposited from a chromic acid-sulphuric acid bath at 40 DEG C., while the intermediate coating may be electrodeposited or formed by electroless plating and may be of Ni, Co, or Ag. For such electrodeposited nickel layers, the baths may be of the Watts type and include brighteners such as saccharin and quinaldinium ethiodide, for electrodeposited cobalt the basic bath may be of similar composition and include saccharin, and for electrodeposited silver the bath may be a cyanide bath containing as brightener an equimolecular mixture of selinium diethyl dithiocarbomate and tetraethyl-thiuramdisulphide, while for electroless nickel deposition the bath comprises nickel chloride, sodium hypophosphite and sodium citrate. The silicates included in the intermediate layer baths comprise in addition to SiO2 considerable amounts of other oxides e.g. a combined CaO + MgO content of 18 to 36%, 3 to 9% Al2O3 and from 12 to 23% total of Na2O, K2O and Fe2O3. For comparison purposes, the intermediate layer is formed from baths including, instead of the silicates, magnesium hydroxide, calcium hydroxide, alumina, barium sulphate, kaolin, diatomaceous earths, or the silicates used in the invention after washing with acid, e.g. HCl, and the initial bright nickel layer is omitted. |
