51316-7623-Scaling investigation of API 5L X65 steel in a ‘sweet’ followed by ‘sour’ environment

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Product Number: 51316-7623-SG

ISBN: 7623 2016 CP

Author: Shiladitya Paul

Publication Date: 2016

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This paper reports the corrosion behaviour of X65 steel in a mixed CO2/H2S immersion environment after being exposed to CO2 immersion environment. The objective was to identify and analyse the scales formed and understand the effect of scale formation on the performance of X65 steel when a transition from pure sweet to mixed sweet/sour conditions occur. Test coupons were prepared from API 5L X65 steel pipe and immersed in a de-aerated 3.5% NaCl solution continuously purged with pure 1barg CO2. After 20 days the exposure to CO2 conditions was interrupted and a gas mixture containing 1%H2S in CO2 was purged in the vessel for 28 days. The test was carried out at ambient conditions. During the test free corrosion potential was monitored and polarisation resistance was measured at regular intervals. At the same time NACE TM0177B test specimens were also prepared and subjected to four-point bend loading in the same environmental conditions to evaluate sulphide stress cracking (SSC) resistance. Upon completion of the tests the corrosion rate of the coupons was calculated from weight loss measurements and compared to the one estimated by Faraday’s law. Finally metallographic examination and SEM-EDX analysis was carried out on the surface and cross-sections of the test specimens.