Review and Hypothesis for the Stress Corrosion Mechanism of Carbon Steel in Alcohols

The stress corrosion cracking (SCC) of carbon steel in oxygenated ethanol (C₂H₅OH) or methanol (CH₃OH) is reviewed. This kind of cracking is almost certainly not due to hydrogen embrittlement, since a marked reduction in maximum load is seen in slow strain rate testing. The load-extension behavior resembles that seen in environments where an “anodic” cracking mechanism—not necessarily slip-dissolution—is believed to operate. The crack path can be either intergranular or cleavage-like transgranular, depending on the details of the environment. The transgranular fracture mode is particularly significant mechanistically, since it suggests some commonality with other peculiar SCC environments such as anhydrous ammonia (NH₃) and carbon monoxide-carbon dioxide-water (CO-CO₂-H₂O). Alcohols themselves are not known to have any particular aggressive effect on iron, so a reasonable hypothesis is that the alcohol is being oxidized in the cracks to something that causes SCC. Literature on direct-alcohol fuel-cell anodes, alcohol sensors, and free-radical oxidation of alcohols is cited, and critical experiments are suggested. It is proposed that acetic acid (CH₃COOH) or formic acid (HCOOH), and carbon monoxide (CO), in varying proportions, are responsible for SCC in alcoholic environments.