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 (CrO4
2),
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.