Effect of Sulfuric Acid, Oxygen, and Hydrogen in High Temperature Water on Stress Corrosion Cracking of Sensitized AISI 304 Stainless Steel

The influence of dissolved oxygen and hydrogen and dilute sulfuric acid in 289 C water on the stress corrosion cracking susceptibility of lightly and moderately sensitized AISI 304 stainless steel was determined in constant extension rate tensile (CERT) tests. The CERT parameters and the fracture surface morphologies were correlated with the concentrations of dissolved oxygen and sulfate, and the electrochemical potentials of platinum and AISI 304 stainless steel electrodes in simulated boiling water reactor (BWR) environments. A particularly high susceptibility to intergranular cracking was found for the lightly sensitized steel at oxygen concentrations between ~0.05 and 0.2 ppm under slightly acidic conditions (pH ~6.0 at 25 C), which may, in part, account for the pervasive nature of intergranular cracking in BWR piping systems. Scanning-transmission electron microscopy analyses revealed significant differences between samples in the lightly and the moderately sensitized conditions with respect to the width, but not the depth, of the chromium-depleted region at the grain boundaries. The addition of 0.5 ppm hydrogen to the water had only a slight mitigating effect on intergranular cracking in water containing oxygen and sulfuric acid at low concentrations; however, oxygen suppression to ≲0.05 ppm in the reactor coolant water by means of hydrogen additions to the feedwater would be beneficial, provided impurities are also maintained at low levels.