Effect of Ethanol Chemistry on Stress Corrosion Cracking of Carbon Steel in Fuel-Grade Ethanol

Abstract

A systematic study was carried out to understand the effect of ethanol chemistry (chloride, water, pHe, and oxygen level) as well as other controlling parameters on the stress corrosion cracking (SCC) behavior of X-65 carbon steel in simulated fuel-grade ethanol (SFGE). The SCC susceptibility of carbon steel was evaluated using the slow strain rate testing (SSRT) method. Chlorides strongly affect the SCC initiation and growth, and a higher concentration of chloride leads to a higher crack density and velocity. The addition of water in the ethanol influences the surface passivation in SFGE. SCC-to-pitting corrosion transition was observed above 2.5% water concentration in SFGE. pHe was also found to be a critical factor influencing the SCC susceptibility with alkaline SFGE inhibiting SCC initiation in carbon steel. Strain rate affected the SCC behavior of carbon steel, where slower strain rate caused a larger crack length and a higher crack density, but a lower crack velocity. Hard inclusions (alumina and silicate) in X-65 steel acted as early crack initiation sites due to the higher local plastic deformation that occurred near the inclusions. By adjusting the ethanol chemistry, SCC susceptibility of a commercial fuel-grade ethanol can be mitigated.