Effect of microstructural features on caustic stress corrosion cracking of steam generator tubing materials

Effects of microstructural features on stress corrosion cracking (SCC) of steam generator tubing materials have been studied in 40% NaOH at 315 .deg. C at potential of +200mV above corrosion potential using C-ring specimen and reverse U bend specimen. The tubing materials used were commercial Alloy 600, Alloy 690 and laboratory alloys, Ni-χCr-10Fe. SCC resistance of Ni base alloys increased as the Ni base alloys have following microstructural features : high bulk Cr content, high fraction of intergranular carbide, large grain size, serrated grain boundary, slightly Cr depleted grain boundary rather than sensitized grain boundary. Role of impurities such as P, S, B in SCC seems to be very unclear at present