Initial Stages Of Stress Corrosion Cracking In Austenitic Stainless Steels

Abstract

Abstract Initial stages of stress corrosion cracking in Types 304 and 304L stainless steel were investigated with optical and electron microscopy techniques. Studies of U-bend samples and electron-transmission quality thin foils revealed that cracks initiated preferentially on the {111} family of planes and that formation of corrosion products accompanied both crack nucleation and propagation. U-bend samples formed from metal-lographically polished strips were studied after exposures in 42 percent MgCl2 solutions at 154 C (309.2 F). Tiny corrosion pits were observed initially, then extension of corrosion formed linear traces of corrosion product on the sample surfaces. These traces lay parallel to the {ill} traces on grain surfaces and cracking was seen later in the corroded regions. Samples then had many small cracks which often formed Widmanstatten-like patterns in several grains. All the small cracks were in corroded regions and were on the {111} family of planes. Continued exposure caused the small cracks to propagate, interconnect and form characteristic trans-granular branching crack patterns. Electron transmission studies of thin foils exposed to dilute chloride solutions revealed a similar sequence of crack initiation and propagation. Preferential pitting formed linear arrays along {111} traces on the foil surfaces. Also, preferential attack occurred at stacking faults and dislocation arrays, producing crack-like voids in the foil. Extent of preferential attack increased with increasing exposure time and temperature and corrosion products were observed in regions of attack. Foils exposed under similar conditions to deionized water and solutions of NaI did not contain the attacked regions, demonstrating importance of chloride ion in the stress corrosion process. These results support the electrochemical mechanism of crack propagation by localized anodic dissolution and show that crystallographic defects provide the sites for such attack.