Investigation of Pitting Corrosion Mechanisms in Carbon Steel Pipelines Carrying Sour Crude Oil in the South South Zone

This study investigates the mechanisms and morphological characteristics of pitting corrosion in low alloy carbon steel pipelines transporting sour crude oil with elevated H2S content in the South South geopolitical zone of Nigeria. Pitting corrosion is a dominant failure mechanism in Nigerian onshore crude oil trunklines, causing leaks that contribute to oil spills and significant production losses, yet mechanistic studies of pit initiation and growth under actual Nigerian sour crude compositions are scarce. Failed pipe sections with documented pitting are obtained from two operating companies. Pit morphology, depth distribution, and growth geometry are characterised by optical microscopy, SEM, and three-dimensional profilometry. Corrosion products within pits are identified by XRD and Raman spectroscopy. Electrochemical pit initiation experiments are performed in synthetic sour crude brine using potentiostatic polarisation to isolate the roles of chloride, H2S, and CO2 partial pressure. Repassivation kinetics are determined from current transients following pit stabilisation. Findings identify H2S-facilitated dissolution of surface FeS films as the primary pit initiation trigger, producing wide, shallow pits under flowing conditions and narrow, deep pits in stagnant sections consistent with dead-leg corrosion morphology. The study provides mechanistic data supporting the selection of appropriate inhibitor types and inspection intervals for Nigerian sour service pipelines. Keywords: pitting corrosion, carbon steel pipeline, sour crude oil, H2S, South South Nigeria.