Preferential Weld Corrosion During Inhibitor Qualification for Sour Service Available to Purchase

Production pipelines which are made of carbon steel typically rely on corrosion inhibitors (CI) for corrosion control, and generally the inhibitors are qualified by laboratory testing to mitigate the expected forms of CO₂ and/or H₂S corrosion. In qualification tests for sour service conditions, inhibitors usually pass the general corrosion criteria, but frequently fail the pitting criteria. Preferential Weld Corrosion (PWC) is a well-known concern for sweet systems, but there is relatively limited published information on PWC in sour conditions and qualification tests are often completed using only parent material. This work reports testing of several commercial corrosion inhibitors in sour service conditions, using both parent material and welded samples. Stirred autoclave tests with weight loss and Linear Polarization Resistance (LPR) measurements, and PWC tests with measurement of galvanic current have been carried out. In some cases, over 50% additional inhibitor dosage was required to mitigate pitting corrosion to standard acceptance criteria. For some inhibitor and environment combinations, the pitting occurred preferentially in the weld or heat affected zone, but this was not always the case. This paper adds significant new insights to the published data on the possibility of PWC in sour service conditions. The findings from this study is expected to help to enable more robust procedures for corrosion inhibitor qualification.