WEB Initial Stages of Stress Corrosion Cracking in High-Strength Austenitic Stainless Steel at Elevated Temperature monitored ElectrochemicallyWednesday (23.09.2020) 11:20 - 11:50 S: Structural Materials 1 Part of:
Austenitic CrMn-stainless steels have been developed to meet the requirements in terms of high strength in combination with high ductility and toughness necessary for oil and gas drilling applications and are the most commonly used structural materials in directional and logging-while drilling tools. Current applications involving streams with large chloride concentrations and elevated temperatures have pushed CrMn-stainless steels to their limits. Similar to other types of austenitic stainless steels, these materials become susceptible to pitting corrosion as well as to stress corrosion cracking (SCC) in this kind of environments. In fact, stress corrosion cracks in these materials are in most cases related to the occurrence of pitting. Therefore, it is important to understand how the pitting corrosion susceptibility of CrMn-stainless steels might influence the initiation and propagation of stress corrosion cracks at service conditions.
In this study a CrMn-stainless steel in strain-hardened condition was exposed to a 2.25 M Cl- buffer solution of pH 8.6 at 88°C with and without additional tensile loading at a constant load. The applied tensile stress, which was set in all cases below the corresponding yield strength of the material, was varied between several levels to evaluate the influence of (elastic) strain on localized corrosion as well as on propagation by SCC. Different techniques including electrochemical noise (ECN), strain-measurement and time-lapse macro photography were used to monitor the transition from pitting to SCC. Analysis of the experimental results reveals that elastic strain induces metastable pitting on the investigated CrMn-stainless steel. Once the tensile load was further increased to 90% of yield strength of the material at RT, few of the pits became stable and serve as a precursor for SCC. ECN monitoring was very successful for monitoring pit initiation and propagation. The electrochemical signals correlated well with the optical documentation of the exposed surface.
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