The stress corrosion cracking of austenitic stainless steels in H 3BO 3 + NaCl solutions between 100°C and 200°C

Abstract

The stress corrosion cracking of 316L austenitic stainless steel in H 3BO 3 + NaCl aqueous media between 100 and 200°C has been studied. The results obtained from slow strain rate and constant load tests, the polarization curves, the rapid tensile straining tests and the fractographic investigation of the fracture surface by scanning electron microscopy, permit the examination of the nature of mechanisms at work. The experimental crack propagation velocities and those calculated from Faraday's law have been compared. The analysis of the electrochemical behaviour in the absence of stress has revealed the potential ranges of maximum stress corrosion cracking susceptibility. Etch-pitting and stereographic observations are employed in order to determine the cracking crystallography. The transgranular fracture surfaces show crystallographic morphology and though the cracking is cleavage-like and is discontinuous in some cases, the cracks propagate mainly under the control of localized anodic dissolution. This process seems to be the limiting factor of the phenomenon whatever the proposed mechanisms may be. In fact, the crack propagation velocity seems to be clearly consistent with a film rupture-dissolution-repassivation process, although it proves insufficient to quantify the phenomenon. The influence of the various environment parameters (temperature, concentration of chemical species, oxygen) which have been studied, has confirmed the importance of the localized anodic dissolution on the cracks propagation.