Long-Term Corrosion Behavior of Cr Diffusion Coatings on Ferritic–Martensitic Superheater Tube Material X20CrMoV12‑1 under Conditions Mimicking Biomass (Co-)firing

• Published in: Energy & fuels 2020-09, Vol.34 (9), p.10989-11002
• Main Authors: Meißner, Tobias M, Grégoire, Benjamin, Montero, Xabier, Miller, Eva, Maier, Jörg, Galetz, Mathias C
• Format: Article
• Language: English
• Subjects: Biofuels and Biomass
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.0c01474

Co-firing of biomass is currently attracting more attention because it is a major step toward reducing CO2 emissions from power generation and can be directly realized within existing plants. Despite its many benefits, considerable challenges in terms of corrosion prevention and durability of the plant components arise because of highly increased amounts of chlorine and alkali species inside the steam generator. Ferritic–martensitic superheater tube steels are particularly challenged and subjected to rapid degradation with the pursuit of achieving higher biomass-to-coal firing ratios. In order to improve the corrosion behavior of such structural materials in environments relevant for biomass (co-)­firing, the present paper suggests enrichment of Ni (against chlorine-induced attack) and Cr (against sulfur-induced attack) in the surfaces of the metallic tubes. For this purpose, a Cr and a combined Ni + Cr diffusion coating were manufactured on ferritic–martensitic X20CrMoV12-1 steel and investigated in environments simulating pure coal, co-firing, as well as pure biomass firing (straw). Exposure tests were conducted at 650 °C for up to 1900 h in SO2- and/or HCl-containing atmospheres with specimens embedded in real power plant combustion ashes. Pure biomass firing clearly accelerated the corrosion attack compared to partial substitution of coal and pure coal firing. However, the Ni + Cr coating performed very well and increased the corrosion resistance of the ferritic–martensitic substrate. As far as degradation mechanisms are concerned, the first stage of the attack turned out to be dominated by chlorine followed by a shift toward sulfur-induced corrosion.