The Corrosion Process of an API 5L X80 Welded Joint in a System with Different pH and H2S Concentration

The corrosion evaluation of steels in the oil and gas industry environments is a crucial issue because corrosion can cause economic and human losses. It is well known that H2S can be originated from different processes in the oil and gas industry, accelerating the corrosion process. The objective of this work was to evaluate the H2S corrosion resistance of an API 5L X80 steel and its welded joint obtained by submerged arc welding process (SAW). All tests were performed in an aerated 5% wt NaCl and thiosulphate aqueous solution. The H2S concentration, pH, weight loss, electrochemical tests, and microstructure were considered. The results obtained showed an increase of the corrosion rate values, with decreasing pH and increasing concentration of H2S generated by the thiosulphate. For the lowest H2S concentrations, the corrosion process was inhibited, due to the formation of a partially protective film on the samples' surfaces. The heat affected zone (HAZ) showed severe localized corrosion attack which was attributed to the microstructural characteristics of this region.