A simplified predictive model of thermal cycling on initiation of stress corrosion cracking

Abstract

This paper develops a simplified predictive model to investigate the role of thermal cycling in the evolution of corrosive environment for the initiation of stress corrosion cracking (SCC) on the external surface of steel pipelines. Prior to commissioning, coatings may be disbonded in places due to improper material selection or poor application. Corrosive gases (i.e., O₂, CO₂) and water vapor diffuse into coating disbondment areas from the soil side. The diffused amount of each species is obtained by Fick’s First Law. The accumulation of condensed water vapor depends on the solubility change of water vapor with respect to thermal cycling frequency and magnitude. The concentrations of [H⁺], [HCO₃⁻], and [CO₃²⁻] are dependent on CO₂ partial pressure. The metal surface potential is predicted using a resistance circuit model. When an open coating holiday occurs, the cathodic protection (CP) current flows directly to the metal surface through the exposed electrolyte. The local surface potentials inside the crevice are simulated using a parametric equation. The pH value at an open holiday obeys Faraday’s Second Law of Electrolysis. Local tensile stress may come from hoop stress from design, residual stress from manufacturing and fabrication, thermal stress from operation, geotechnical stress from soil movement, and mechanical stress from deformation. When the pH, surface potential, and tensile stress are within specific ranges, SCC can occur either longitudinally or circumferentially, depending on stress orientation. Types of SCC, such as high-pH and near-neutral pH SCC, are characterized. The model was validated qualitatively by comparing the predicted results with field measurements from published industry literature. The findings should help stakeholders develop proactive preventive and mitigative measures for effective SCC management.