Potentiostatic slow strain rate testing was conducted on stress corrosion cracking (SCC) test specimens exposed to ethanolic environments prepared from pure dehydrated ethanol. The mechanism of SCC in such environments is not well‐understood. 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. A scanning electron microscope 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 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.

Striking mechanical and morphological similarities of the stress corrosion cracking (SCC) of carbon steel in ethanolic media with those governed by a cleavage-like mechanism in CO-CO2 aqueous solutions, prompted the investigation of the possibility of ethanol electrochemical oxidation into CO on ferrite (Fe) and cementite (Fe3C) surfaces. Density functional theory 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, respectively. These first principle calculations indicate that at the anodic potentials applied during potentiostatic slow strain rate testing, ethanol electrooxidation to CO is thermodynamically viable on carbon steel, giving further credit to the involvement of cleavage type SCC of carbon steel in ethanolic environments.

Stress Corrosion Cracking (SCC) of carbon steel in fuel-grade ethanol is typically intergranular in service, but it is nearly always transgranular, in short-term laboratory experiments. While this might be considered an inconvenience, it provides an important clue about the SCC mechanism. The important point is that the transgranular cracking occurs under much milder mechanical conditions than any known form of hydrogen embrittlement in ordinary carbon steel. For this and other reasons, hydrogen is not considered to be the cause of this form of SCC. Transgranular SCC of carbon steel occurs in a very particular set of environments. Anhydrous liquid ammonia, anhydrous ammonia-methanol and CO-CO2-H2O are the key systems to be considered, apart from alcohols. High-temperature water containing oxygen also causes transgranular SCC, but is a less severe environment, probably because a magnetite film imposes a compressive stress that partially opposes any embrittlement effect. In our view, the important factor connecting these examples is embrittlement by interstitials. Just as hydrogen can embrittle iron, so can nitrogen or carbon (or oxygen). The main distinction is the diffusivity of the interstitial in iron. Ammonia can be oxidized to N, and CO can be reduced to C (or, in the language of surface chemistry, this could be dissociative adsorption). Still, at ambient temperature, those interstitials can only diffuse a few nm in relevant times of seconds. But we know little about the effect of such near-surface embrittlement on the propagation of a crack, and research on such surface effects is very important. Striking mechanical and morphological similarities of the SCC of carbon steel in ethanolic media with those governed by a cleavage-like mechanism in CO-CO2 aqueous solutions prompted the investigation of the possibility of ethanol electrochemical oxidation into CO on ferrite (Fe) and cementite (Fe3C) surfaces. Density functional theory computations on (110) surfaces revealed 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, respectively. These first principle calculations indicate that at the anodic potentials applied during potentiostatic slow strain rate testing, 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.

This work is aimed at improving the understanding of the localized corrosion of carbon steel in ethanolic solutions. The role of ethanol dehydration, chloride, and oxygen level in the pitting behaviour of carbon steel in ethanolic environments in the presence of supporting electrolytes was investigated. Open Circuit Potential measurement, Cyclic Potentiodynamic Polarization and Potentiostatic testing were conducted on specimens exposed to ethanolic environments prepared from pure dehydrated ethanol to study the pitting behaviour of carbon steel. Corrosion and passivation potentials significantly reduce due to the change in the cathodic reaction and the decrease in passivation kinetics under de-aerated conditions. SEM and EDX examination indicated that no pitting corrosion is observed without chlorides, and chloride significantly destabilizes the surface film resulting in decreases of both corrosion potential and passivation potential. A decrease in the dissolved oxygen in the solution reduces but does not eliminate the pitting susceptibility. Iron oxide is identified as the significant corrosion product at different water and oxygen content. Therefore, ethanol aeration can be a proper method to increase pitting corrosion resistance in ethanolic solutions.