Effects of Mn and Microalloying Composition on Corrosion and Hydrogen-Induced Cracking of API 5L X65 Steels

The corrosion and hydrogen-induced cracking (HIC) resistance as well as hydrogen permeation behavior of two API 5L X65 steels with different amounts of Mn and microalloying elements were compared. The corrosion behavior of both steels, evaluated using electrochemical impedance spectroscopy and polarization curves in solution A of NACE TM0284-16 standard with and without H2S saturation (sour medium), showed no relevant differences in each medium, which can be ascribed to their similar microstructures; however, the corrosion resistance of both steels was lower in the sour medium. The investigation of resistance to hydrogen-induced failures disclosed better performance for the low Mn steel. This was confirmed using a harsher HIC test performed in an HCl acidified sour medium and was ascribed to the presence of Nb carbides nanoprecipitates, as revealed by the scanning transmission electron microsccopy analysis.