Corrosion of pipeline steel weld metals containing alloying elements in CO2/H2S environment

Abstract

The corrosion of weld metals with Cr, Ni, Cu or Mo in gas field produced water with saturated CO2 and H2S was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), optical profiler, and electrochemical tests. The addition of alloying elements led to a decrease in polygonal ferrite and pearlite, whereas an increase in acicular ferrite in weld metal. Morphological and compositional analysis manifested that corrosion scale on weld metals primarily consisted of FeCO3 and iron sulfides, and Cr(OH)3 also existed on Cr-containing weld metals. With increased alloying elements, especially chromium, localized corrosion resistance was improved. This improvement was ascribed to the presence of alloying elements in substrate and Cr-rich products in scale, which can inhibit the oxidation of iron sulfides, thereby improving the structure of corrosion scale. Electrochemical tests showed that the anodic dissolution reaction of weld metal with higher Cr, Ni, Cu or Mo was inhibited in more extent, and the increase in total reaction impedance depended on weld metal surface active state. The preliminary corrosion mechanism of weld metal in CO2/H2S environment was proposed.