ENVIRONMENT ASSISTED CRACKING OF 316L STAINLESS STEEL IN ACETIC ACID ENVIRONMENTS

摘要

The stress corrosion cracking susceptibility of a 316L stainless steel in concentrated acetic acid solutions at 90 deg C has been investigated. The solutions tested were designed to model a chemical process environment and contained 70 percent to 90 percent acetic acid with 1500 ppm Br and 200 ppm Na~+ as the base chemistry. Of key interest was the impact of CI ions, in the concentration range 200-3000 ppm, representing uncontrolled excursions in sys tem chemistry. Proof ring and slow strain rate tests were conducted in the acetic acid solutions with and without chloride and one proof ring test was conducted with cyclical incursions of chloride ions. In the timescale of the proof ring tests, fracture of the specimens did not occur for any of the test conditions but sectioning of the specimen revealed intergranular cracks in the 90 percent acetic acid solutions containing 1500 ppm chloride but not 400 ppm. The crack growth rate, estimated from proof ring specimens exposed for varying times, decreased with crack depth and was of the order of 2.8X10~7 mm s~(-1) (24 ~tm/day) for the deepest crack. The slow strain rate testing was conducted mainly at a strain rate of 4X10~(-6) s~(-1) in order to limit the extent of section loss due to corrosion. In 90 percent acetic acid solution, a lower reduction in area compared to air coupled with fractographic examination and with observation of cracks in the sectioned specimens confirmed intergranular stress corrosion cracking at 400 ppm chloride but not at 200 ppm. No intergranular cracking was observed in 80 percent or 70 percent acetic acid solutions with chloride concentrations up to 3000 ppm. When the specimens associated with the latter environments were examined in section, features were observed which appear best explained as very blunt transgranular cracks or corrosion trenches. On the basis of the crack growth results, short-term incursions of chloride may not have a major impact, particularly as there appears to be no crack growth when normal chemistry is restored. However, this conclusion must be tempered with caution in view of the simplified solutions used for testing and the possible role of cold work in service.