Nucleation of Stress Corrosion Cracking in High-Strength Austenitic CrMn-Stainless Steel exposed to NaCl Solution at Elevated Temperature

摘要

Strain-hardened austenitic CrMn-stainless steels are the most commonly used structural materialsin oil and gas directional and logging-while drilling technology. In addition to their non-magneticproperties, these materials have been designed to meet the requirements in terms of high strengthin combination with high ductility and toughness, necessary for drilling applications. With thecontinued increase in operations with demanding service conditions, mainly characterized by hightemperatures and corrosive environments containing large chloride concentrations, CrMn-stainlesssteels have been pushed to their limits. Similar to other types of austenitic stainless steels, thesematerials become prone to pitting corrosion as well as to environmental assisted cracking (EAC) inthis kind of environments. Pitting corrosion is indeed one of the most common causes forscrapping drilling tools. In addition, environmentally assisted cracks in these materials are in mostcases related to the occurrence of pitting. However, different from pitting, EAC might lead to acatastrophic failure of the drillstring. It is, therefore, important to understand how the pittingcorrosion susceptibility of CrMn-stainless steels used in drilling technology might affect theinitiation and propagation of environmentally assisted cracks at service conditions.In this study a CrMn-stainless steel in strain-hardened condition was exposed to a 2.25 M Clbuffersolution of pH 8.6 at 88℃ with and without additional tensile loading at a constant load. Theapplied tensile stress, which was set in all cases below the corresponding yield strength of thematerial, was varied between several levels to evaluate the influence of (elastic) strain on localizedcorrosion as well as on propagation by stress corrosion cracking (SCC). Four simultaneous in-situmonitoring techniques including electrochemical noise (ECN), acoustic emission (AE), strainmeasurementand time-lapse macro photography were applied during the exposure.Analysis of the experimental results reveals that elastic strain induces metastable pitting on theinvestigated CrMn-stainless steel. Once the tensile load was further increased to 90% of yieldstrength of the material at RT, few of the pits became stable and serve as a precursor for SCC.Cracks propagate from the surface into the bulk material and leave a rather small footprint on thesurface. Secondary cracks were found only in few instances. ECN monitoring was verysuccessful for monitoring pit initiation and propagation. ECN also delivered information regardingthe propagation of stress corrosion cracks, but only in cases where there were none or just fewsmall pits that did not develop into stress corrosion cracks. AE was found to be less sensitive topitting and nucleation stages of SCC, the crack growth activity could be detected only when thecrack was already fully propagating into the austenitic microstructure of the material.