Corrosion of metals by bio-oil obtained by vacuum pyrolysis of softwood bark residues. An X-ray photoelectron spectroscopy and Auger electron spectroscopy study

The corrosion of three metals (aluminum, copper, and austenitic steel (SS 316)) at 80 °C by bio-oil obtained by vacuum pyrolysis of softwood bark residue was studied using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiling. The bio-oil is very acidic (pH value of 3) and contains significant amounts of water and metal ions that are not present in significant concentrations in the feedstock material (e.g. Cu, Pb, and Fe). These metals were most probably leached from the pyrolysis reactor or peripheral installations. Aluminum and, to a much smaller degree, copper were corroded by bio-oil, whereas SS 316 was not affected. Oxide and/or hydroxide layers were formed on all three metals. However, in the case of aluminum and copper, these layers did not protect the underlying metal against further oxidation. For SS 316, after initial modification of the surface (e.g., leaching of Fe species), subsequent oxidation was prevented by the chromium oxide layer.