Pitting Corrosion Resistance Influencing Corrosion Fatigue Behavior of an Austenitic Stainless Steel in Chloride-Containing Environments

Strain-hardened austenitic stainless steels are commonly used as structural materials in drilling equipment because they meet the demanding requirements in terms of mechanical, magnetic, and chemical properties necessary for drilling technologies in subterranean energy resources exploration. Drilling operational conditions might become a challenge for the integrity of these materials due to the cyclic loading the drillstring is subjected to, in combination with the downhole temperature, and the corrosivity of the drilling fluid. In this research work, the relationship among the pitting corrosion resistance of one Mn-stabilized austenitic stainless steel and its corrosion fatigue behavior has been determined by means of electrochemical methods, advanced surface characterization, and corrosion fatigue testing in brines of near-neutral pH with different chloride contents at room temperature (RT) and 150°C. It has been determined that the corrosion fatigue behavior of the investigated CrMn stainless steel is strongly affected by its susceptibility to pitting corrosion. The synergistic effect between the corrosive environment and the mechanical load depends upon the applied stress amplitude and the pitting resistance of the material. The corrosion fatigue behavior of the austenitic stainless steel at RT was synergistically affected by the environmental and loading conditions at low stress amplitudes. In contrast, the large susceptibility to pitting of the material at 150°C has a significant detrimental effect on its corrosion fatigue behavior when subjected to high stress amplitudes. The observed damage mechanism at 150°C can be described as pitting-induced corrosion fatigue because pit propagation controlled the corrosion fatigue behavior of the CrMn stainless steel. The obtained experimental results have shown that the pitting resistance, assessed for instance by multiple electrochemical methods, could in cases where pitting susceptibility has a large influence on the environmentally sustained cracking mechanism, be used as an indicator of the expected corrosion fatigue behavior of the material. As demonstrated in this study, however, results from accelerated electrochemical testing solely might have a limited prediction capability of long-term corrosion behavior.