Elucidating Corrosion Performance of Weld in CO<sub>2</sub> Corrosion Environment

Abstract

Weld corrosion is suspected to contribute to localized corrosion failure of pipelines in CO2 environment. Some studies on the mitigation of the weld corrosion had been focused on weld metal micro-alloying producing cathodic weld metal but with mixed success result. The inconsistent result is due to lack of understanding on the mechanism of weld corrosion in this corrosive CO2 environment. This paper presents the study of the possibility of weld corrosion from the galvanic and self-corrosion effects of welded low carbon steel ASTM 106 B pipeline in pH 5-CO2 saturated environment at 25 °C and 60 °C under stagnant condition. The weld region samples were prepared by metallography method to identify the parent metal (PM), heat affected zone (HAZ) and weld metal (WM) microstructures. Galvanic current was measured for each weld region by zero resistance ammeters (ZRA) and self-corrosion rate was measured by electrochemical impedance spectroscopy (EIS). The total corrosion rate was determined from the summation of galvanic corrosion and self-corrosion for all weld regions. From the galvanic current test result, weld metal was found to be anodic for both 25 °C and 60 °C. Weld metal also experienced the highest self-corrosion rate in the range of 40-98% as compared to parent metal and HAZ region for both 25 °C and 60°C. The total corrosion rate was mainly contributed by the self-corrosion rates and the contribution from galvanic corrosion was considered insignificant. Possible preferential weld corrosion can occur on weld metal region due to differential self-corrosion rates of each weld regions.