Effect of applied stress on chloride induced external stress corrosion cracking of type 304 stainless steel in air atmosphere

Austenitic stainless steels (SS) are widely used in various components of chemical plants, nuclear power plants, etc, because of the superior mechanical property and general corrosion resistance. However, it is also well known that austenitic stainless steels are susceptible to localized corrosion in the environments containing chloride ions, and several equipment in the plants built in coastal area has been suffering from chloride induced external stress corrosion cracking (ESCC). Hence, for the establishment of the countermeasures it is very important to clarify the factors governing ESCC process from the view points of stress, material and environmental conditions. The purpose of this study is to investigate the effect of applied stress on ESCC of type 304 stainless steel. ESCC tests were conducted on type 304 SS specimens, which were fabricated from a cold rolled plate, by a uniaxial constant load method using springs. After loading, droplets of synthetic sea water were put on the gage section of specimen and dried, and then the specimens were placed in a chamber with a constant temperature of 353 K and a relative humidity of 35%. The test specimens after the test were observed by a scanning electron microscope to measure the crack length and depth. No clear difference was found in the maximum values of the average crack propagation rate (crack depth divided by test time) among the applied stress conditions. In addition, most of ESCC were initiated from the bottom or periphery of pits under the low applied stress condition (0.5 sigma(0.2)).