Electrochemical Techniques for Determining Localized Corrosion of Iron and Ni-base Corrosion Resistant Alloys – A Comparison of ASTM Methodologies

Abstract Subsurface components in the Oil and Gas industry can undergo localized corrosion, pitting or crevice corrosion, due to myriad of factors where oxygen contamination can result in some of the most aggressive corrosion of typically corrosion-resistant alloys. Examples of oxygen corrosion issues in subsurface components include contaminated completion or packer brines or exposure in injection streams, e.g. seawater injection. In this paper, we have compared ASTM approaches viz. ASTM G192 (Tsujikawa-Hisamatsu Electrochemical Method), ASTM G150 and Chronoamperometric technique (CREVCORR) for determining the localized corrosion propensity of Iron and Ni-base alloys. All three methodologies were used to determine Critical Crevice Temperature (CCT) of precipitation hardened Ni-base Alloy 955 (UNS N09955), Cr-Ni-N Fe-base Alloy P750 (DIN 1.4675), Super Austenitic Fe-base alloy high nitrogen stainless steel UNS S31010 and 25Cr Super Duplex stainless steel (UNS S39274) in brines at various chloride concentrations. For 25Cr Super Duplex, CCT were determined based upon ASTM G150 and ASTM G192 standards. Comparative tests were also performed to determine repassivation potential (Erp) using ASTM G61 methodology (Cyclic potentiodynamic polarization method) and certain advantages of using ASTM G192 method over the former has been elucidated. Long-term OCP of these alloys were also evaluated to determine CCT based on their repassivation capabilities (Erp – OCP) under the same environmental conditions. Each of the alloys tested under ASTM G192 had higher CCT when compared to that determined by ASTM G150 or chronoamperometric techniques.