Specialized Corrosion Summary (Cont.)

Corrosion DOE-HDBK-1015/1-93 SPECIALIZED CORROSION Rev. 0 CH-02 Page 37 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: Contamination Loss of coolant Loss of pressure SCC is prevented in reactor system by: Proper design 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 are: 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.