Improving the corrosion protection of copper‐nickel‐chromium plating

Introduction There is no doubt that the development of duplex nickel was a major step forward in enhancing the corrosion resistance of nickel/chromium deposits. Various multilayer nickel deposit systems followed, but it became increasingly obvious that any further corrosion protection could only be achieved by improved chromium deposits. Chromium, due to the thin, passive oxide film formed on its surface, under normal circumstances is very resistant to most atmospheric corrosive conditions. However, it is an acknowledged fact that conventional chromium cannot be deposited free from random discontinuities, thus creating relatively active galvanic corrosion cells in the presence of electrolytes such as salt solutions (conditions arising from de‐icing and snow melting operations), sulphurous acid (produced from sulphur dioxide in industrial areas), etc. Since nickel is anodic to chromium, these cells are quite active, and with a small area of nickel exposed the effect is that of having a large cathode area in a cell with a small anode. With the galvanic current thus concentrated at a few sites, the corrosion of the nickel is rapid and penetration to the base metal quickly follows.