Electrochemical impedance spectroscopy (EIS) was used to assess the corrosion behavior of three steels, 2.25Cr-1Mo, 5Cr-1Mo and 9Cr-1Mo (UNS K21590, S50200 and K90941) in bio-oils and catechol, an organic constituent commonly found in bio-oils. Two types of biomass pyrolysis oils produced from selected forest residue feedstocks, classified as high-ash and high-moisture (HAHM) and low-ash and low-moisture (LALM) types and 10 wt.% catechol, were used to test the alloys. According to previous results, 10 wt.% catechol in a water and methanol mixture was corrosive to 2.25Cr-1Mo but not to 9Cr-1Mo, highlighting that increasing Cr content in the steels resulted in improved corrosion resistance. 5Cr-1Mo, a steel with an intermediate Cr content, was tested in catechol solutions to better estimate the critical value of Cr for corrosion resistance under these conditions. Also, the three alloys were tested in both HAHM and LALM bio-oils to assess corrosion susceptibility. For EIS measurements, a customized electrochemical setup was employed to minimize the ohmic path through the bio-oils. The corrosion susceptibility was evaluated by determining the resistance at the alloy-corrosive medium interface.
Biomass-derived pyrolysis oil, a renewable energy source, has corrosivity originating from its water and organic acid contents.1-5 For example, a plain carbon steel and 2.25Cr-1Mo steel suffered from severe pitting attack, causing some pits deeper than 500 µm, after 1000 h exposure to a pine sawdust bio-oil at 50°C.6 To avoid corrosion attack by bio-oils, the use of stainless steels can be a practical, but expensive solution.
It was previously reported that mixed hardwood bio-oil exposure resulted in relatively small and essentially no corrosion mass loss for types 409 and 316L stainless steels (UNS S40900 and UNS S31603), respectively, as opposed to the greater mass losses measured in a plain carbon and 2.25Cr-1Mo steel.7 Lower or no mass loss of stainless steels was also observed in a similar study that used a different bio-oil.8