Corrosion resistance of electrodeposited zinc-chromium alloy coatings

Recently, there have been some notable developments in electroplating technology which have caused much interest. The major area of development has been with zinc alloy coatings for improved corrosion resistance. Chromate conversion coatings are used widely for further protection of zinc electrodeposits. This involves the highly toxic, carcinogenic and environmentally-undesirable chromium (VI) solutions. It was therefore decided to investigate the electrodeposition of zinc-chromium alloys, in which the conversion coatings can be generated from within the deposit itself. Good quality zinc-chromium alloy coatings containing 0–10% chromium have been electrodeposited in the laboratory and characterised with respect to composition and structure. Corrosion behaviour has been studied using salt spray and electrochemical methods. Results obtained have been compared to those found for a selected range of commercially-produced deposits. The optimum range for maximum corrosion resistance was found to be 4–6% chromium. Conversion-coating treatments have also been investigated and found to improve corrosion resistance. The advantage of the new process is that the conversion coatings can be formed on these alloys by immersion in a simple oxidant solution, such as permanganate, thus eliminating the highly toxic chromium (VI) solution.