Effects of Trench Generated by Ni on SSC Initiation Limits for Low-Alloy Carbon Steels

Abstract In the oil and gas industry, pipelines are exposed to wet H2S environments (sour environments) that cause sulfide stress cracking (SSC) in low-alloy carbon steels in some cases. Recently, a judgment flowchart was proposed to determine the SSC susceptibility based on the corrosion pit morphologies or stress groove geometries observed on the steel surface in the early stages of SSC. However, it remains unclear whether the crack-like corrosion morphologies, known as trenches or fissures, form in low-alloy carbon steels containing Ni cause failure or not. In this study, the effects of trenches on SSC initiation limits in Ni-containing steels were investigated. Constant-load tests, hydrogen permeation tests, double cantilever beam (DCB) tests, and finite element analysis (FEA) were conducted using TS 800 MPa grade steels with and without 1% Ni. Trenches were observed in the Ni-containing steel following the constant-load tests. The features of the trenches were mimicked and the stress intensity factors (K) at the trench tips were evaluated using FEA. Hydrogen absorption increased and the fracture toughness (KISSC) decreased with increasing in H2S partial pressure. In this study, SSC initiation was evaluated by comparing the K and KISSC values of low-alloy carbon steel, and the SSC judgement flowchart for Ni-containing steel was discussed.