Top‐ and Bottom‐of‐the‐Line Corrosion (TLC and BLC): Effects of 30°C Bottom Liquid and 12°C Steel Temperature in H ₂ S and CO ₂ Environments

ABSTRACT Corrosion in oil and gas pipelines poses operational and safety challenges, especially where temperature gradients lead to different corrosion behaviors. Although mixed H₂S/CO₂ corrosion has been studied, few works isolate the effects of H₂S/N₂ and CO₂ on top‐of‐the‐line (TLC) and bottom‐of‐the‐line (BLC) corrosion. This study investigates API 5L X65 carbon steel under sweet (CO₂) and sour (H₂S/N₂) environments, focusing on steel position and environmental chemistry. Samples were exposed in situ for 7 days at 12°C steel and 30°C liquid temperatures. Analyses included weight loss (ASTM G1‐03), surface morphology/composition (SEM/EDX), and phase identification (XRD). Results show nearly identical TLC and BLC corrosion rates in the sweet environment, while BLC corrosion is much higher in the sour case. SEM/EDX and XRD reveal flaky oxide scales under CO₂ and dense, multilayered FeS scales under H₂S/N₂, with NaCl deposits contributing to localized BLC attack. Findings underscore the role of chemistry and steel position in corrosion mechanisms and provide insights for mitigation and pipeline design.