Corrosion-resistant alloys
Nickel and nickel alloys have useful resistance to a wide variety of aqueous corrosive environments typically encountered in the chemical process industry and energy technology. Nickel by itself is a versatile corrosion resistant metal. More importantly, its metallurgical compatibility over a considerable composition range with a number of other metals as alloying elements has become the basis for many binary, ternary, and other complex nickel alloy systems, having very unique and specific corrosion resistant behaviour for handling the modern day corrosive environments of the chemical process industry.
Corrosion resistance depends on the chemical composition of the alloy, the alloy’s micro-structural features, the chemical nature of the environment, and the nature of the alloy / environment interface.
One of the main alloying element is chromium, which is necessary for a stable passive surface layer, upon which the corrosion resistance depends. After chromium, molybdenum is the most important alloying element by increasing the resistance to general corrosive attack in reducing media. Together with chromium it is of paramount importance for pitting and crevice corrosion resistance.
Zirconium owes its excellent corrosion resistance to a passivating oxide layer (ZrO2) which forms very quickly on the surface. This layer makes the material resistant to virtually all organic acids and alkaline solutions as well as to mineral acids.