Stress Corrosion Cracking of Carbon Steel in Ethanol

Ethanol (C2H5OH) is used as a gasoline additive, serving both as an octane enhancer and an oxygenate to promote complete combustion and reduce harmful emissions. The specifications and procedures for handling fuel ethanol have been described in a document published by the Renewable Fuels Association (RFA).1 The RFA-recommended standard for fuel ethanol is governed by ASTM D 4806.2 The ethanol quality specification under ASTM D 4806 is given in Table 1. ASTM D 4806 also restricts the type of denaturant to be natural gasoline, gasoline components, or unleaded gasoline. The purpose of the denaturant is to render the ethanol undrinkable. For this study, unleaded gasoline was used as the denaturant. Furthermore, ASTM D 4806 cites California and Federal ethanol requirements that limit the ethanol sulfur content to 10 ppm, benzene (C6H6) to 10 vol%, olefins (CnH2n) to 0.5 vol%, and aromatics to 1.7 vol%. Somewhat similar fuel ethanol standards exist in other countries with either more or less restricted water contents.3 It must be noted that the pHe is a measured value using the specific procedure described in ASTM D 6423.2 The value measured using this procedure is time-dependent and does not correspond to equilibrium or even a steady-state hydrogen activity. The RFA fuel ethanol guidelines1 further recommend the addition of one of a number of corrosion inhibitors. Among the nonaqueous environments, the stress corrosion cracking (SCC) of steel in anhydrous ammonia (NH3) and methanol (CH3OH) are well known. The