The most effective means for preventing SCC are proper design, reducing stress, removing
critical environmental contributors (for example, hydroxides, chlorides, and oxygen), and
avoiding stagnant areas and crevices in heat exchangers where chlorides and hydroxides might
become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are
subject to SCC in water containing chloride ions. Nickel based alloys are not affected by
chloride or hydroxide ions.
Two types of SCC are of major concern to a nuclear facility.
Chloride Stress Corrosion Cracking (Stainless Steels)
The three conditions that must be present for chloride stress corrosion to occur are as
Chloride ions are present in the environment
Dissolved oxygen is present in the environment
Metal is under tensile stress
Austenitic stainless steel is a non-magnetic alloy consisting of iron, chromium, and
nickel, with a low carbon content. This alloy is highly corrosion resistant and has
desirable mechanical properties. One type of corrosion which can attack austenitic
stainless steel is chloride stress corrosion. Chloride stress corrosion is a type of
Chloride stress corrosion involves selective attack of the metal along grain boundaries.
In the formation of the steel, a chromium-rich carbide precipitates at the grain boundaries
leaving these areas low in protective chromium, and thereby, susceptible to attack. It has
been found that this is closely associated with certain heat treatments resulting from
welding. This can be minimized considerably by proper annealing processes.
This form of corrosion is controlled by maintaining low chloride ion and oxygen content
in the environment and the use of low carbon steels. Environments containing dissolved
oxygen and chloride ions can readily be created in auxiliary water systems. Chloride ions
can enter these systems via leaks in condensers or at other locations where auxiliary
systems associated with the nuclear facility are cooled by unpurified cooling water.
Dissolved oxygen can readily enter these systems with feed and makeup water. Thus,
chloride stress corrosion cracking is of concern, and controls must be used to prevent