FILM BREAKDOWN OF CARBON STEEL IN BIO-ETHANOL DURING STRESS CORROSION CRACKING

Fuel-grade ethanol (FGE), a major form of bio-ethanol, has been widely used with gasoline as a blend-fuel in the United States and other countries. One critical concern in using the existing gas-pipelines to transport the fuel-grade ethanol or blended fuel is the potential stress corrosion cracking (SCC) susceptibility of carbon steel pipelines in these environments. In this contribution, we report a systematic investigation into the mechanism of stress corrosion cracking in simulated fuel-grade ethanol environment under aerated condition. Slow strain rate test (SSRT) method, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), open circuit potential (OCP), and potentiostatic current monitoring were used to understand the passivity breakdown and crack growth. It was found, for the first time in FGE environment, that the growth of surface film as well as its breakdown presents a close relationship to the SCC initiation and growth. Anodic dissolution at the crack tip dominates the crack propagation. Onset of film breakdown takes place closely after the yield point of carbon steel. Further propagation of cracks was controlled by a competition among the film breakdown, anodic-dissolution, and repassivation. By stopping the anodic dissolution, SCC in FGE can be mitigated through a cathodic polarization.