Role of the Elements
A nickel alloy has been defined by the relevant ASTM Committee as “a material that conforms to a specification that requires, by weight percent, more nickel in the alloy than any other element with the possible exception of iron”. With this definition is it any wonder that sometimes there is confusion as to which specific grades may be considered nickel alloys.
In addition to having its own inherent corrosion resistance a feature of nickel is that it has a high capacity for alloying. This means that a wide range of other elements may be added to enhance corrosion resistance, improve strength at elevated temperatures and alter physical properties. The converse of this is that the metallurgy of nickel alloys may become quite complex and needs to be understood in order to avoid costly problems.
The role of chromium in nickel alloys is primarily the same as that in stainless steels: it enhances the formation of passive surface films in the presence of oxygen and these passive films impede the corrosion process. Iron also affects passivity as does silicon. Thus alloys containing these elements will display a corrosion resistance in oxidising media. On the other hand molybdenum and copper enhance corrosion resistance of nickel in reducing media. Nickel alloys containing both chromium and molybdenum will thus be versatile alloys for a broad spectrum of conditions although there may be some conditions in which other alloys may be the optimum. A useful way of understanding this is commonly known as the “Y” of corrosion.