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<p>In oil and gas production systems, marginally sour environments containing low concentrations of H<sub>2</sub>S in CO<sub>2</sub>-rich conditions present a significant corrosion risk to carbon steel pipelines, where protective scale formation is often inconsistent and poorly understood. This study investigates the corrosion behavior of API 5L X65 carbon steel at 50 °C over a 7-day exposure period, under incremental H<sub>2</sub>S concentrations from 0, 30, 60, and 100&#xa0;ppm. Corrosion rates and mechanisms were evaluated using weight loss, solution analyses, SEM and EDX, and surface profilometry. In the CO<sub>2</sub>-only system, corrosion was predominantly uniform with the highest uniform corrosion rate (UCR) of 3.36&#xa0;mm/yr recorded, with a relatively low pitting rate of 1.50&#xa0;mm/yr. At 30&#xa0;ppm H<sub>2</sub>S, a steep decrease in UCR was observed at 0.46&#xa0;mm/yr, coupled with a steep increase in pitting rate, which peaked at 8.44&#xa0;mm/yr, marking a shift toward localized corrosion. At 60&#xa0;ppm H<sub>2</sub>S, UCR remained low at 0.45&#xa0;mm/yr, and the pitting rate decreased to 1.72&#xa0;mm/yr. And finally, at 100&#xa0;ppm H<sub>2</sub>S, both UCR and pitting rate further declined to 0.40&#xa0;mm/yr and 0.48&#xa0;mm/yr, respectively, consistent with the formation of a protective corrosion product film. These findings highlight the shift in corrosion behavior and severity with increasing H<sub>2</sub>S, whereby a transition from predominantly uniform to highly localized and finally passivating behavior occurs through the formation of a protective corrosion product layer.</p>

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Transitional Corrosion Behavior of API 5L X65 Steel under Marginally Sour Environment

  • 期刊论文

  • 发表日期：

    2026年1月14日

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  • Liyana Yasmin Azhar,
  • Dinna Geraldine Ramlan,
  • Nur Anis Asyiqin Shahrizal,
  • Najmiddin Yaakob

摘要

In oil and gas production systems, marginally sour environments containing low concentrations of H2S in CO2-rich conditions present a significant corrosion risk to carbon steel pipelines, where protective scale formation is often inconsistent and poorly understood. This study investigates the corrosion behavior of API 5L X65 carbon steel at 50 °C over a 7-day exposure period, under incremental H2S concentrations from 0, 30, 60, and 100 ppm. Corrosion rates and mechanisms were evaluated using weight loss, solution analyses, SEM and EDX, and surface profilometry. In the CO2-only system, corrosion was predominantly uniform with the highest uniform corrosion rate (UCR) of 3.36 mm/yr recorded, with a relatively low pitting rate of 1.50 mm/yr. At 30 ppm H2S, a steep decrease in UCR was observed at 0.46 mm/yr, coupled with a steep increase in pitting rate, which peaked at 8.44 mm/yr, marking a shift toward localized corrosion. At 60 ppm H2S, UCR remained low at 0.45 mm/yr, and the pitting rate decreased to 1.72 mm/yr. And finally, at 100 ppm H2S, both UCR and pitting rate further declined to 0.40 mm/yr and 0.48 mm/yr, respectively, consistent with the formation of a protective corrosion product film. These findings highlight the shift in corrosion behavior and severity with increasing H2S, whereby a transition from predominantly uniform to highly localized and finally passivating behavior occurs through the formation of a protective corrosion product layer.