Effect of Field Bending and Girth Weld on the Initiation and Early Growth of Near-Neutral pH Circumferential Stress Corrosion Cracking (NNpHCSCC) in Thin-walled Pipeline

Abstract

Pipeline failure because of near-neutral pH stress corrosion cracking (NNpHSCC) has been a concern in Canada for over 30 years. Previous studies focused on axial NNpHSCC, which has happened more frequently than circumferential NNPHSCC (C-NNPHSCC). This study focuses on the significance of residual stress caused by the combination of girth weld and field bending on initiation and early-stage growth of C-NNPHSCC. To examine the effect of those factors, corrosion samples with and without girth welds were manually bent inwardly and outwardly to apply tensile or compressive bending residual stress on the outer surface of the pipeline steel. Based on the axial welding residual stress distribution induced by the girth weld from previous studies, the distances between the bend's center and the girth weld's center were selected from 0 mm to 30 mm to investigate the welding residual stress distribution. The distribution and depth profiles of corrosion pits and microcracks on those samples revealed that a higher density of microcracks and corrosion pits was observed on the samples with girth welds than on those without girth welds. Furthermore, it was found that the higher frequencies of deep pits and cracks (> 80 µm) were observed as the distance between the center of bend and weld centerline decreased. The interaction between welding residual stress and bending residual stress increased or decreased the significance of C-NNpHSCC, depending on the applied bending residual stress type. The mill scale on the metal substrate significantly impeded or enhanced the localized corrosion in the NNpH environment, significantly affecting the morphologies of pits and cracks on the samples. The future study requires the removal of the mill scale to rule out the effect of galvanic corrosion.