Modeling of Thermal Cycling on Initiation of Stress Corrosion Cracking

Abstract This paper develops a theoretical model to investigate the impact of thermal cycling on the initiation of stress corrosion cracking (SCC). Prior to commissioning, coatings may be disbonded in places due to poor material selection or 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 governed 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 arrives at the metal surface directly 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 value, surface potential, and tensile stress are within certain bounds, stress corrosion cracking may take place longitudinally or circumferentially depending on stress orientation. Types of SCC such as high-pH SCC or near-neutral pH SCC can be characterized. The findings should be useful for stakeholders in proactively developing preventive and mitigative measures for effective SCC threat management.