Effect of weld heat input on corrosion of dissimilar welded pipeline steels under simulated coating disbondment protected by cathodic protection

Abstract

Dissimilar welding is extensively used for oil and gas pipelines for new installations or repairs. Despite applying cathodic protection (CP) and coatings, corrosion and cracking can occur, usually on external surface of pipelines under coating disbondments close to weld lines. In this study, corrosion of dissimilar welded X-42 and grade B pipelines, under a simulated coating disbondment, is investigated. Welding was performed using shielded metal arc welding (SMAW) process. In order to simulate a coating disbondment, a corrosion cell containing 10 coupons was utilized. Coupons were exposed to a simulated soil solution purged with 5%CO2–95%N2 to maintain an anaerobic environment. CP potential of −870 mVSCE was applied to the open mouth of the disbondment. Results showed that by decreasing weld heat input from 0.74 to 0.61 kJ/mm, corrosion rate at the weld line increased. This was explained by formation of phases with higher corrosion rates (e.g. Widmanstatten and acicular ferrite). It was also observed that the corrosion rate at the weld line was higher than that at the heat affected zone (HAZ), followed by the base metals. The higher corrosion rate at the weld line and HAZ was explained by formation of acicular ferrite and secondary phases during the welding process. It was also observed that for similar heat inputs, the corrosion rate of dissimilar welded X-42 and grade B was higher than that of similar welded samples. This was possibly due to the minor galvanic effect between X-42 and the grade B pipeline steel.