Effect of Temperature and Ionic Impurities at Very Low Concentrations on Stress Corrosion Cracking of AISI 304 Stainless Steel

The relative effect of 13 anionic species, in conjunction with hydrogen and sodium cations, on the stress corrosion cracking (SCC) behavior of lightly sensitized AISI(1) 304 stainless steel (SS) was investigated in constant extension rate tests (CERTs) at 289 C in water with 0.2 ppm dissolved oxygen at total conductivity values of ≤1 μS/cm. The results show that the sulfur species, either in acid or sodium form, produce the highest degree of intergranular stress corrosion cracking (IGSCC) relative to the other anions. The effect of temperature on SCC behavior was investigated in CERTs over the range of 110 to 320 C in high-purity water and in water containing 0.1 and 1.0 ppm sulfate as H2SO4 at a dissolved oxygen concentration of 0.2 ppm. The CERT parameters were correlated with the electrochemical potential of platinum and AISI 304 SS electrodes in the high-temperature environments. Maximum IGSCC occurred at temperatures between ~200 and 250 C in high-purity water, and the addition of sulfate increased the average crack growth rates at all temperatures >150 C and broadened the temperature range over which maximum susceptibility occurred. A distinct transition in the crack morphology from intergranular to transgranular and ultimately to a ductile failure mode was observed as the temperature increased from ~270 to 320 C in high-purity water. This transition was at tributed to a decrease in the open circuit corrosion potential of steel <~0 mV[standard hydrogen electrode (SHE)] at the higher temperature. A large decrease in the crack growth rates of fracture-mechanics-type specimens of the steel was also found when the temperature was increased from 289 to 320 C in high-purity water with 0.2 ppm dissolved oxygen.