Theoretical Framework for Stress Corrosion Cracking Susceptibility in High Strength Pipeline Steels Operating in Nigerian Sour Environments

This study develops a theoretical framework for predicting stress corrosion cracking (SCC) susceptibility in high strength API 5L X70 and X80 pipeline steels operating in sour (H2S-containing) environments representative of onshore and offshore Nigerian gas gathering systems. SCC is a critical integrity threat for high pressure gas pipelines, and the combined effects of high H2S partial pressure, cathodic protection, and near-neutral pH groundwater conditions prevalent in the Niger Delta create a multi-mechanism SCC environment requiring an integrated theoretical treatment. The framework synthesises hydrogen-assisted cracking (HAC) theory, linear elastic and elastic-plastic fracture mechanics, and electrochemical dissolution mechanisms into a unified susceptibility parameter applicable to the Nigerian environment. A hydrogen permeation and trapping model is coupled with a decohesion-based fracture criterion to define critical conditions for crack initiation as a function of applied stress intensity, hydrogen fugacity, and microstructural trap density. The framework is validated through an experimental campaign involving slow strain rate testing, modified double cantilever beam SCC growth rate measurements, and hydrogen permeation cell experiments on X70 and X80 steel specimens in NACE TM0177 Solution A at varied H2S partial pressures. Findings define quantitative susceptibility boundaries for the two steel grades and provide a mechanistic basis for selecting maximum allowable operating stress levels in sour service Nigerian pipelines. Keywords: stress corrosion cracking, hydrogen embrittlement, pipeline steel, sour service, fracture mechanics.