Biojalostamon vetykäsittelyreaktoreiden jännityskorroosiotutkimus

The use of renewable feedstocks in biofuel production introduces corrosion challenges that differ from those encountered in conventional oil refining, leading to new material selection requirements. This master’s thesis examined corrosion conditions in hydrotreating reactors and the susceptibility of five corrosion‑resistant metallic alloys (316L, 904L, Super Duplex 2507, Sanicro 28 and Sanicro 35) to stress corrosion cracking (SCC) in renewable diesel production. The study combined a literature review, electrochemical corrosion evaluation, and the design of long‑term in‑situ SCC testing to assess SCC initiation susceptibility. A key contribution of this work is the design and implementation of in-situ SCC testing under hydrotreating conditions, which provides a level of industrial relevance and environmental realism that laboratory tests cannot fully replicate. In-situ testing captures realistic fluctuations in temperature, pressure, feedstock impurities and shutdown-startup transients, which are the factors known to influence localized corrosion and SCC initiation. The literature review showed that renewable feedstocks contain high levels of free fatty acids, impurities and water, increasing the risk of localized corrosion and SCC. Chloride‑induced SCC, polythionic acid SCC, high‑temperature hydrogen attack and fatty acid corrosion were identified as key degradation mechanisms in hydrotreating. Material comparisons indicated that super-austenitic and nickel-based alloys offer superior resistance to localized corrosion, while duplex stainless steels provide a balance between strength and corrosion resistance. In the experimental part of the study, U‑bend test specimens were designed and installed at two different locations in the hydrotreating reactors for SCC exposure testing. The actual results will only become available after the next scheduled shutdown, the combined use of electrochemical screening and in‑situ testing provides a mechanistically grounded basis for evaluating long‑term material performance and SCC initiation susceptibility in hydrotreating reactors.