Corrosion characterization of new low manganese microalloyed X65 steels in sour and NON-SOUR synthetic seawater

API 5L X65 microalloyed steels are extensively used for oil and gas transportation. In many cases, reserves of such resources contain H2S, constituting what is called a sour medium, at which hydrogen-induced failures are critical. Even though essential for improving mechanical resistance, the addition of high contents of manganese (Mn) may increase the tendency for segregation and the precipitation of MnS inclusions, increasing the susceptibility to hydrogen-induced failures. In this study, electrochemical techniques were employed to compare the corrosion resistance in the B solution of ASTM D1141−98 (synthetic seawater) of three API 5L X65 microalloyed steels with low Mn content, designed for severe sour service applications. Experiments were performed in naturally aerated (pH 8.0), deaerated (pH 8.0 and 5.0), and H2S-saturated (sour pH 5.0) media. Although the steels exhibit very similar chemical compositions and mechanical properties, it was possible to rank their corrosion resistance. Microstructural characterization allowed associating corrosion susceptibility with the presence of pearlite grains in a ferritic matrix. The results also demonstrated the superior aggressiveness of the sour medium, and that the precipitation of a poorly adherent layer of Mackinawite slightly increases the corrosion resistance with immersion time.