Co-processing of bio-oils with conventional petroleum-based feedstocks is an attractive initial option to make use of renewable biomass as a fuel source while leveraging existing refinery infrastructures. However, bio-oils and their processing intermediates have high concentrations of organic oxygenates, which, among their other negative qualities, can result in increased corrosion issues. A range of stainless steel alloys (409, 410, 304L, 3161, 317L, and 201) was exposed at the base of the riser in a fluid catalytic cracking pilot plant while co-processing vacuum gas oil with pine-derived pyrolysis bio-oilg that had been catalytically hydrodeoxygeriated to similar to 2 to 28% oxygen. A catalyst temperature of 704 degrees C, a reaction exit temperature of 520 degrees C, and total co-processing run times of 57-75 h were studied. External oxide scaling 5-30 mu m thick and internal attack 1-5 mu m deep were observed in these short-duration exposures. The greatest extent of internal attack was observed for co-processing with the least stabilized bio-oil, and More so for types 409, 410, 3044 3161, 317L stainless steel than for type 201. The internal attack involved porous Cr-rich oxide formation, associated with local Ni-metal enrichment and S-rich nanoparticles, primarily containing Cr or Mn. Implications for alloy selection and corrosion are discussed. (C) 2017 Elsevier B.V. All rights reserved.