Comparative Study on Corrosion of X65 Pipeline Steel Welded Joint in Simulated Shallow and Deep Sea Environment

The short-term corrosion behavior of the welded joint of X65 pipeline steel was comparatively studied in artificial laboratory environments, which aims to simulate the real environment of shallow and deep sea respectively, by using mass loss test, potentiodynamic polarization curve measurement and corrosion morphological observation by scanning electron microscopy (SEM). Results show that the corrosion rate of the welded joint in simulated shallow sea water was faster than that in the simulated deep sea environment. In the simulated shallow sea environment the weld joint exhibited mainly pitting corrosion; however in the simulated deep sea environment, relatively serious corrosion occurred near the fusion-line (FL) while non obvious corrosion could be observed on the welded zone. The corrosion potential of heat affected zone (HAZ) was lower than that of the welded zone and matrix of X65 pipeline steel both in simulated deep and shallow sea environments, which caused galvanic effect in welded zone and accelerated the corrosion rate of HAZ. Thereby the formed corrosion product film on HAZ was thicker with a larger charge-transfer resistance Rt.