Functional composite electrochemical coating Ni-Co-Al2O3 -an alternative to chromium plating

Abstract

The physico-mechanical properties of composite electrochemical coatings (CEC) nickel-cobalt-aluminum oxide were investigated depending on the electrolyte parameters and electrolysis conditions. The previously developed low-concentration chloride electrolyte for nickel plating was used as an electrolyte to replace environmentally hazardous chromium plating electrolytes containing hexavalent chromium, which is prohibited by the laws of many countries. The wear resistance of the obtained CEC was determined on a three-ball friction machine. This made it possible to establish that the wear resistance of the CEC exceeds the wear of chrome coatings in dry friction mode by 2-2,5 times and is comparable to chromium in the friction mode with lubrication. In the dry friction mode, the higher wear resistance of the nickel-cobalt-alumina coating is explained only by the higher hardness of the latter. Probably, upon destruction of the coating, the particles of the alloying addition act as a solid lubricant, which causes an increase in the resistance of the CEC during wear. The microhardness of the CEC was determined using a PMT-3 microhardness tester and amounted to 6-25 GPa. The microhardness value was influenced by the concentration and properties of the dispersed phase, as well as the electrolysis conditions - the temperature and pH of the electrolyte, and the cathode current density. “Corrodcote” test was used in the study of corrosion. According to its data, the corrosion resistance of CEC is 2-3 times higher than the corrosion resistance of chromium deposits. The results obtained make it possible to recommend the developed functional CEC of nickel-cobalt-alumina instead of chromium coatings as corrosion- and wear-resistant.