Corrosion fatigue and repassivation investigations of CrNiMo austenitic stainless steels in chloride environment

Austenitic stainless steels of the CrNiMo type for the oil and gas industry have to withstand extreme conditions such as mechanical stress in corrosive environments combined with raised temperatures. The susceptibility to corrosion is effectively decreased by the alloying elements. CrNiMo alloys show a high corrosion resistance due to their ability to form a stable passive layer. The passive layer can be damaged by slip steps, whereupon this activated area is susceptible to dissolution. Therefore localized corrosion has to be prevented by quick repassivation.
This article picks two steel-grades: first, the austenitic steel CrNiMo18-14-3 and, secondly, the so-called “superaustenite” CrNiMoN28-30-3. Characterization of their performance under exacerbated corrosive testing conditions was done with three different tests. These are the cyclic load test under inert and aggressive conditions, measurement of repassivation kinetics, and electrochemical characterization by cyclic polarization curves.
Both austenitic steels show a high resistance to corrosion fatigue under testing conditions. They also repassivate very well even at high CaCl2 contents and raised temperatures, which affirms the conclusions drawn by the results of the electrochemical test.