Nickel alloys are highly resistant to corrosion,and, in most environments, nickel alloys outperform the most advanced stainless steels. One of the reasons is because nickel can be alloyed more heavily than iron. That is, large amounts of specific elements can be dissolved purposely into nickel to tailor an alloy for a particular environment.
In general, industrial environments can be divided into two broad categories: reducing and oxidizing. These terms refer to the range of electrode potential that the alloy experiences,and it is controlled by the cathodic reaction in the system.
Thus, a reducing condition is generally controlled by the discharge of hydrogen from a reducing acid, such as hydrochloric acid.An oxidizing environment has a potential that is higher than the potential for hydrogen discharge.
This potential may be established by cathodic reactions, such as reduction of dissolved oxygen
(O2) from the atmosphere, chlorine gas (Cl2), hydrogen peroxide (H2O2), chromates (CrO42),nitrates (NO3), and
metallic ions in solution, such as ferric (Fe3+) and cupric (Cu2+). Nickel alloys, the same as other alloys, may suffer two main types of corrosion: uniform corrosion and localized corrosion. Uniform corrosion may happen under reducing conditions in the active region of potentials and also under oxidizing conditions in the form of a slow, passive corrosion.
Localized corrosion, such as pitting and crevice corrosion, generally occurs under oxidizing conditions. Stress-corrosion
cracking (SCC) or environmentally assisted cracking (EAC) could occur at any electrochemical potential range. Three conditions are necessary for the occurrence of SCC: a susceptible material composition and microstructure, a specific aggressive environment, and the presence of tensile stresses.