Corrosion and Cracking of Carbon Steel in Fuel Grade Ethanol—Supporting Electrolyte and Susceptible Potential Regime

Carbon steel, which is commonly used to fabricate storage tanks and pipelines, is susceptible to stress corrosion cracking (SCC) in a fuel grade ethanol (FGE) environment. Dissolved oxygen and corrosion potential have been identified as the critical factors contributing to ethanol SCC. The investigation of the electrochemical response of carbon steel in FGE is essential to develop a mechanistic understanding of SCC to mitigate the SCC risk, including the influences of different FGE sources and additives. The implementation of electrochemical measurements in FGE is limited by its high electrical resistivity. Adding non-complexing supporting electrolyte is considered to be the most practical method to carry out any measurements at controlled potential, including slow strain rate (SSR) tests. To find an ideal supporting electrolyte in ethanol solution, a series of cyclic potentiodynamic polarization and electrochemical impedance spectrum tests were performed on carbon steel specimens in simulated fuel grade ethanol (SFGE) with a variety of salt candidates. Carbon steel micro-electrodes were used to verify the effects of the supporting electrolyte. A supporting electrolyte with no influence on the system electrochemical response is preferred. All of the salts investigated have some effect on the electrochemical response of carbon steel in SFGE. However, tetrabutylammonium tetrafluoroborate (TBA-TFB) has the least effects particularly in a deaerated environment. Deaerated conditions are of interest so that the effects of high potential on SCC susceptibility can be determined separate from other possible effects of oxygen. Cracking was reproduced at high applied potential without oxygen in the deaerated SFGE + TBA-TFB, indicating that the role of oxygen in ethanol SCC might be simple anodic polarization and a potential regime for SCC susceptibility of carbon steel in SFGE could be determined.