Abstract 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.

The H2S corrosion resistance of API 5L X80 steel and its welded joint (WJ) were evaluated using the weight loss method in a 5%wt brine solution and different corrosive environments based on a sodium thiosulphate solution (10–3 and 10–2 mol/l). The weight loss method, scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD) were applied to measure the effects of different H2S concentrations and pH (3 and 5) on the corrosion process and formation of corrosion product films. The results showed that the obtained corrosion rate and corrosion product films for both API 5L X80 steel and its submerged arc welding (SAW) WJ depend on the pH, H2S concentration and metal surface microstructural characteristics. The corrosion product film consists of two layers with different morphologies. The heat affected zone (HAZ) shows severe localized corrosion attack relative to the base metal (BM) and weld metal (WM), which is attributed to the microstructural characteristics of this region.

Longitudinal welded joints samples of X70 and X80 API grade steel pipes were exposed to a deaerated 1% NaCl solution containing CO2, at temperatures in the range of 25 ºC to 80 ºC for 48 hours. Analogous samples were exposed to the adjacent vapour phase. The layer of corrosion products which formed on the base metal and welded regions of the samples was investigated using mass loss determinations, scanning electron microscopy and X-ray diffraction. The corrosion products formed in tests at 60 ºC and were composed of FeCO3. The FeCO3 which had initially formed at 80 ºC was observed to have subsequently decomposed, resulting in a layer of Fe2O3.