Specialized Corrosion Summary (Cont.)
Stress corrosion cracking (SCC) is a great concern due to the hazard that it can
readily crack metal of appreciable thickness. The crack can lead to a serious
failure of the component, or system, and all the attendant results such as:
Loss of coolant
Loss of pressure
SCC is prevented in reactor system by:
Reducing stress levels
Avoiding stagnant areas and crevices in heat exchangers, where chlorides
and hydroxides might become concentrated.
Chloride stress corrosion
The three conditions that must be present for chloride stress corrosion to occur
Chloride ions present
Dissolved oxygen present
Metal under tensile stress
Chloride stress corrosion is controlled by maintaining low chloride ion and
oxygen content in the environment and the use of low carbon steels.
Caustic stress corrosion
Caustic stress corrosion is similar to chloride stress corrosion. The metals
involved are mild steels (steel with low carbon and low alloy content). The
metals are exposed to concentrated caustic environments and under tensile stress.
The presence of oxygen is not required for cracking to initiate. Concentration of
greater than 10,000 ppm is required for cracking to initiate. The level may be
higher for different caustic environments.