Corrosion Risk and Repassivation of Duplex Stainless Steel UNS S82551 in Treated Seawater Injection Service

Abstract

In seawater injection wells, the available well tubing materials are generally low alloy steel, glass-reinforced epoxy-lined low alloy steel or corrosion-resistant alloys such as super duplex stainless steel. However, in treated seawater the corrosion risk can be controlled and lower-grade alloys (low alloy steel) can be considered. But for long well lifetime designs (20 y plus), then low alloy steel tubing can be challenged. In this respect, recent efforts have focused attention on better dissolved oxygen control which permits the investigation and on the possible use of more cost-effective materials such as the duplex stainless steels UNS S82551 and UNS S82541 (the latter is a higher strength version, but same PRENw). Full-scale testing of tubes joined together with a proprietary premium threaded connection (pipe-coupling premium connection [PCPC] couplings) was performed in controlled seawater loops simulating service conditions at 30°C. The flow rate and dissolved oxygen were controlled at 5\xa0m/s and <20\xa0ppb, respectively. Weekly dissolved oxygen excursions corresponding to 24\xa0h at 100\xa0ppb followed by 1 h at 300\xa0ppb were performed during the 5 months exposure. Corrosion results of UNS S82551/S82541 tubing were compared to UNS S31803 and UNS S39274. In parallel, laboratory exposures of creviced coupons for parametric study were performed in dissolved oxygen-controlled cells, allowing the measurement of electrochemical potentials as function of dissolved oxygen content and the related corrosion resistance. The results showed that dissolved oxygen content should be properly controlled below critical values to avoid crevice corrosion of the lesser alloyed duplex stainless steels. The ability of UNS S82551/S82541 to recover or repassivate after prolonged exposures to high dissolved oxygen concentrations (DOC) was also determined with both the use of full-sized PCPC test cells and electrochemical testing involving a remote crevice assembly. The repassivation potential was investigated after different active crevice corrosion durations. The results of the study allowed to precisely define the limits of use of UNS S82551/S82541 in treated seawater, i.e., the critical DOC conditions for corrosion initiation and for repassivation of UNS S82551/S82541. For all tested conditions, the UNS S82551/S82541 showed a rather good ability to repassivation when normal service conditions (i.e., low dissolved oxygen) are recovered.