In situ marine exposure study on corrosion behaviors of five alloys in coastal waters of western Pacific Ocean

The aim of this study is to investigate the effects of seawater retention inside subsea oil and gas pipelines on the corrosion of several corrosion resistant alloys commonly used in seawater. Pitting corrosion, crevice corrosion, and electrochemical test samples were immersed in coastal waters at the depth of about 170 m in South China Sea (western Pacific Ocean). Assessments of the pitting and crevice corrosion for different alloys were carried out through electrochemical measurements and surface characterizations. The results showed that, the order of the pitting corrosion resistance of five alloys in marine environment was: 825 Ni-based alloy similar to 2507 super duplex stainless steel > 2205 duplex stainless steel similar to 317L similar to 316L, while the crevice corrosion resistance was: 825 Ni-based alloy similar to 2507 super duplex stainless steel > 2205 duplex stainless steel > 317L > 316L. The pitting initiations of 2205 duplex stainless steel, 2507 super duplex stainless steel, and 825 Ni-based alloy were later than those of 316L and 317L stainless steels. After the exposure for 592 d, the passivation for 316L, 317L and 2205 stainless steels were completely broken down while 2507 super duplex stainless steel and 825 Ni-based alloy still had a wide and stable passive zone. In crevice corrosion samples, the most severe corrosion was observed in the crevice edge area. A boat shaped corrosion pit was found in the inner coverage area far from the edge of crevice, which was probably related to the distribution of microorganisms in this area. (C) 2020 The Author(s). Published by Elsevier B.V.