Stress Corrosion Cracking and Pitting Corrosion of Carbon Steel in Ethanolic Environments

Several studies have recently claimed that the Stress Corrosion Cracking (SCC) of carbon steel in ethanolic environments occurs by an anodic dissolution mechanism, but the exact mechanism is not well understood. In this research project, potentiostatic slow strain rate testing was conducted on SCC test specimens exposed to ethanolic environments prepared from pure dehydrated ethanol. Cracks of various types—intergranular and transgranular open cracks, and sharp closed transgranular cracks—were found by altering several testing parameters. The presence of chloride was found to be essential for crack initiation. SEM examination indicated that an “anodic” cracking mechanism, not necessarily slip-dissolution, was most likely operating at high elongations. Sharp closed transgranular cracks, with a maximum depth of 4 µm, were detected at elongations below 3% in ethanol solutions containing 2.5 mM LiCl. A focused ion beam was used to extract the tip of such a transgranular crack tip for analytical transmission electron microscopy using electron energy loss spectroscopy, which confirmed that the crack was in a ferrite grain. The sharp closed transgranular cracks seem to ally with the cracks observed in CO-CO2-H2O and anhydrous ammonia environments, which are proposed to grow by unique cleavage mechanisms. The possibility of embrittlement by carbon interstitials produced by ethanol electro-oxidation within the crack is discussed.The similarities of ethanol SCC and CO-CO2-H2O SCC prompted the investigation of the possibility of ethanol electrochemical oxidation into CO on ferrite (Fe) and cementite (Fe3C) surfaces. Density functional theory (DFT) computations on (110) surfaces reveal that the catalytic activity of Fe and Fe3C through the α dehydrogenation pathway can significantly reduce the energy barrier of electro-oxidation of ethanol and production of CO to 0.575 and 0.480 eV vs SHE, respectively. These first principles calculations indicate that at the anodic potentials applied during potentiostatic slow strain rate testing (SSRT), ethanol electro-oxidation to CO is thermodynamically viable on carbon steel, giving further credit to the involvement of cleavage type SCC of carbon steel in ethanolic environments.Ph.D