Stress Corrosion Cracking Behavior and Surface Oxidation Properties of Alloy 600 and 690 in Secondary Environment with Impurities

The maintenance activity of SG may be hindered and its life may be shortened. Alloy 600 (Ni-15Cr-9Fe) has been used until now as a material for SG tubes due to its slow corrosion rate and excellent resistance to stress corrosion cracking in primary and secondary cooling water environments of pressurized water reactors(PWR). However, issues such as pitting corrosion, stress corrosion cracking, and intergranular attack (IGA) have occurred over long-term use. To solve this problem, Alloy 690 whose Cr content is higher by about twice than that of Alloy 600 is used. The increase in Cr content of Alloy 690 is believed to inhibit Cr depletion of crystal grain boundaries and accelerate the formation of chromium oxides, which are problematic in the precipitation of carbides, in turn, increasing corrosion resistance. The secondary system of a nuclear power plant may have a small number of impurities (S, Pb, and Cl etc.) that are introduced during the initial construction, maintenance, and normal operation. The concentrations of these impurities are very low. The movement of cooling water, however, is limited. Therefore, impurities can be concentrated in the gap between the top of tubesheet and the tube where sludge accumulates. The environment is chemically complicated and has a wide range of pH values from acidic to alkaline. In this study, we aimed to analyze the characteristics of the film after corrosion experiment using existing heat transfer tube material in an aqueous solution containing Pb, Cl,and S to identify mechanisms of stress corrosion cracking. The SCC of Alloy 690 showed a large susceptibility in the Pb-added environment and was not affected by other impurities (Cl, S).