Small amount of oxygen in production or injection systems can be responsible for the initiation of crevice corrosion of stainless steel. The current limit of 10 ppb dissolved oxygen (DO) recommended for the use of Corrosion Resistant Alloys (CRAs) in produced water system, has been challenged by several authors. The aim of this study is to report the use of electrochemical methods for the study of oxygen limits for stainless steels UNS(1) S31603 (Type 316L) and UNS S31803 (Type 22Cr Duplex) in produced water environments. Crevice corrosion potential (Ecrev) and repassivation potential (Erep) were obtained from cyclic polarization experiments performed in part 1 of this study. These two potentials were used as acceptance criteria during open circuit potential (OCP) measurements to determine the oxygen limits of Type 316L and Type 22Cr Duplex at given test conditions. The effect of the duration of exposure to oxygen as well as the effect of increase of oxygen content were discussed. Results obtained for Type 316L indicate that the current oxygen limit of 10 ppb is relevant for long-term and permanent exposure. A precise determination of oxygen limits for Type 22Cr Duplex could not be performed due to uncertainties related to the variations of Ecrev and Erep at low oxygen concentrations. A re-assessment of oxygen limits based on the data obtained in this study is planned based on revised acceptance criteria.
Crevice corrosion is a geometrical-dependent type of localized attack that occurs in occluded regions where a stagnant and corrosive electrolyte is in contact with the surface of a passive metal1,2. Crevices are present in all industrial designs and can lead to major failure since their detection is often challenging3,4. Main strategies for the prevention and mitigation of crevice corrosion include design awareness and adequate materials selection5.
The formulation of a universal mechanism of crevice corrosion is still a source of debate. Currently, three main mechanisms have been proposed: the acidification model (Oldfield and Sutton7,8), the IR drop model (Frankenthal and Pickering9) and the metastable pitting model10. All these mechanisms require a combination of factors for crevice corrosion to initiate1,6. Among them, the influence of pH, temperature and chloride concentration has been widely investigated in both produced water and seawater systems11-13. Another major factor affecting the development of crevice corrosion is the presence of DO in bulk solution. Indeed, oxygen influences the cathodic reaction of crevice corrosion: