Top‐of‐line corrosion in marginal sour environment: protectiveness of iron (II) sulphide in wet gas pipeline

Pipeline integrity remains challenged by limited understanding of top‐of‐line corrosion (TLC) in marginal sour environments, where mitigation strategies cannot be directly inferred from sweet or highly sour systems. Hence, this study evaluates the corrosion behaviour of API 5 L X65 carbon steel exposed to 30 ppm and 80 ppm H 2 S for 72 hours using a 2‐litre top‐of‐line corrosion setup. The uniform corrosion rate is determined using the weight loss method (ASTM G1‐03), while the pitting rate is assessed following ASTM G46‐21. The measured uniform corrosion rate values are 0.51 mm/yr at 30 ppm and 0.22 mm/yr at 80 ppm, both slightly lower than the control condition (0.62 mm/yr). The average pitting rates increase significantly to 1.66 mm/yr at 30 ppm and 0.51 mm/yr at 80 ppm, compared with 0.45 mm/yr for the control, indicating possible localized corrosion in both conditions. Surface morphology analysis shows that the corrosion product layer becomes more compact and adherent with increasing H 2 S concentration, while elemental composition analysis detects sulphur on the corroded surfaces, suggesting the formation of iron (II) sulphide corrosion products. These findings demonstrate that H 2 S concentration critically governs iron (II) sulphide protectiveness and corrosion performance in marginal sour top‐of‐line corrosion environments.