Impedance Studies on Stress Corrosion Cracking Behavior of Steel Pipeline in NS4 Solution under SSRT Test Condition

Abstract

Stress corrosion cracking behavior of API 5L X65 steel in a NS4 solution with different pH was investigated by electrochemical impedance under slow strain rate tensile test. NS4 solution was selected as corrosive electrolyte under disbonded pipeline coatings. According to the analysis of tensile test, steel was susceptible to SCC in a NS4 solution. The reduction area ratio in the air was measured to be 70.71%, which was more significant than the reduction obtained in a NS4 solution with different pH. EIS in different strain time in acidic solution showed perfect semicircles, which indicated high dissolution rate and some hydrogen penetration effect in this environment. EIS in neutral and basic NS4 solution presented effective constant phase elements in the transmission line model for the porous electrode, which indicated SCC behavior of steel in these solutions. Phase angle analysis indicated that cracks grow considerably after 12 h. Besides, SEM illustrated a ductile type of fracture in air and brittle type of fracture in all NS4 solution.