Hydrothermal liquefaction (HTL) is an important thermochemical technology which uses hot
pressurized water to convert wet biomass or biowaste feedstocks into biocrude oils and other marketable bio-chemicals. The presence of hot pressurized water, aggressive catalyst, and organic products can lead to serious corrosion damage and even stress corrosion cracking risk on the HTL reactors. Up to now, very limited information is available about the corrosion of HTL reactor alloys under HTL processes. In this study, the corrosion of a candidate constructional steel (UNS S31000) was investigated under the batch-mode HTL conversion of different biomass feedstocks, including bamboo (a typical lignocellulosic biomass) and black liquor (a common industrial biowaste). The bio-oil produced from black liquor had higher contents of organic acids and phenols compared to that converted from bamboo. The corrosion rate of the steel in the HTL of black liquor was about twenty-five times higher than that in the HTL of bamboo. The corrosion layer formed in the HTL of black liquor is spalling.