Pyrolysis bio-oils are corrosive to low alloy steels, e.g., 2.25Cr-1Mo, 5Cr-1Mo, and 9Cr-1Mo grades. To identify the alloys with sufficient bio-oil compatibility, several commercial stainless steels were examined in bio-oil using electrochemical impedance spectroscopy to semi quantitatively assess their corrosion resistance. Low-Ash Low-Moisture (LALM) bio-oil, produced from a forest residue feedstock by the National Renewable Energy Laboratory in Golden, CO, was used as a test liquid for electrochemical impedance spectroscopy measurements. Three organic corrodents, formic acid, catechol, and lactobionic acid, were added into LALM bio-oil to produce test liquids with intentionally increased corrosivity. Corrosion reaction resistance, determined from the impedance data, was used to evaluate the corrosion compatibility of each stainless steel in LALM bio-oil and LALM bio-oil + organic corrodent(s). The results from corrosion reaction resistance indicated that the critical Cr content of stainless steels for corrosion resistance would be greater than 14 wt % if Ni and Mo contents are low but can be as low as 12–13 wt % with appreciable amounts of Ni and Mo.
Biomass-derived pyrolysis oils (bio-oils) are recognized as a renewable energy source that could aid in the reduction of fossil fuel use. Bio-oils exhibit higher corrosivity to common ferrous alloys because the oils contain organic acids and water.1–5 A series of corrosion studies were previously performed to determine the corrosion rates of ferrous alloys exposed in bio-oils for a quantitative evaluation of the material compatibility.6–9 The key information from these previous studies is that ferrous alloys with more Cr, Ni, and Mo are needed for compatibility with bio-oils.
Along with the compatibility studies that focused on the corrosion rates, a quick and semiquantitative method based on electrochemical impedance spectroscopy (EIS) was also introduced to assess corrosion susceptibility or resistance of different ferrous alloys in bio-oils and organic acid/constituent solutions.9–14 In this method, impedance spectra of a ferrous alloy sample immersed in a bio-oil are used for data fitting to determine the charge transfer resistance (R2) associated with corrosion kinetics. The higher a fitted resistance value becomes, the more resistant a ferrous alloy would be in the bio-oil used for the measurement.