Dealloying as a precursor to stress corrosion cracking of Ni-Fe-Cr alloys in boiling caustic solutions

Laboratory experiments have shown that Alloy 800 steam generator tubing material and some other Ni-Fe-Cr alloys are susceptible to dealloying and SCC in hot caustic environments. This study examines the effects of alloy composition and time of exposure on dealloying and SCC susceptibility of Ni-Fe-Cr-(Mo) alloys in boiling caustic solutions. Electrochemical measurements indicate that increasing Ni content increases the resistance to dealloying. Electron microscopy analysis suggested that extending the exposure time could change the geometry of the dealloyed layer. The role of a dealloyed layer as a precursor to SCC of Alloy 800 in boiling caustic solutions was studied by exposure of U-Bend specimens and post-exposure in-SEM tensile testing. Results indicate that the ability of a dealloyed layer to transmit a microcrack into the substrate material, and initiate SCC, depends on the dealloyed layer's geometry and adherence to the substrate material. (author)