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Influence of H2S on the localised corrosion of 316L stainless steel Part 2 - Electrochemical testing

Hesketh, J; Dickinson, E J F; Turnbull, A; Hinds, G (2020) Influence of H2S on the localised corrosion of 316L stainless steel Part 2 - Electrochemical testing. NPL Report. National Physical Laboratory. MAT 94

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This report describes the second part of a two-part study to elucidate the mechanism by which H2S leads to pitting of stainless-steel in environments representative of upstream oil and gas applications. In this part, a series of electrochemical tests was performed to measure the impact of H2S on the anodic and cathodic reaction kinetics of 316L stainless steel. The electrochemical testing included cyclic polarisation, cathodic polarisation, and lead-in-pencil pit measurements.

The passivity of the film appeared to be reduced with increasing H2S but the trend depended on the pH/buffering combination and was not fully consistent. The repassivation potentials derived from anodic polarisation tests with different partial pressures of H2S in simulated oilfield environments did not reflect the pitting behaviour observed in coupon tests, suggesting that the methodology used for the cyclic polarisation tests was over-severe.

Cathodic polarisation tests in 0.1 bar H2S supported other published work indicating that the role of H2S in solution was that of a weak buffer rather than being itself cathodically reduced. However, this deduction was not consistent with the repeatable data obtained at 0.01 bar, where a clearly discerned increase in cathodic current density near the open circuit potential (OCP) attributable to H2S was apparent, together with a shift in OCP to less negative potentials. For this case, pre-exposure appeared also to suggest a catalytic effect of the modified passive film on the current density.

Item Type: Report/Guide (NPL Report)
NPL Report No.: MAT 94
Keywords: stainless steel, pitting corrosion, oil and gas
Subjects: Advanced Materials > Corrosion
Divisions: Electromagnetic & Electrochemical Technologies
Publisher: National Physical Laboratory
Identification number/DOI: 10.47120/npl.MAT94
Last Modified: 08 Jan 2021 15:43
URI: http://eprintspublications.npl.co.uk/id/eprint/9006

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