High-Temperature Corrosion Behavior of Superheater Materials at 600 °C: Insights from Laboratory and Field Exposures

Combustion of biomass and waste results in release of corrosive species, such as alkali chlorides and water vapor, which accelerate the corrosion of superheaters in the boiler. To improve our understanding of alkali-induced corrosion, long-term corrosion investigations are needed. This study utilizes a systematic approach based on long-term corrosion studies (up to 8000 h) in a well-controlled laboratory environment to understand the corrosion behavior and protectiveness of oxide scales formed on a FeCr alloy (marginal chromia former) and three overlay weld coating systems (lean FeCrAl, FeCrAl, and Ni-based alloy) in a KCl-rich environment at 600 °C. The results show that all the alloys undergo fast breakaway corrosion and form duplex-layered scales consisting of outward- and inward-growing oxide scales. The marginal chromia former exhibits parabolic oxidation kinetics and forms Fe-rich oxides in the outer scale and mixed Fe- and Cr-oxides in the inner layer. These oxide microstructures are compared to the scales formed on probe-exposed samples in boilers, and similar microstructures are typically found after exposures. The FeCrAl coatings form Fe-rich oxides in the outer layer and Fe-, Cr-, and Al-oxides in the inner layer. All alloy systems except the Ni-based coating show corrosion rates in the boiler in good agreement with the laboratory test prediction. The Ni-based coating exhibits the slowest oxidation kinetics in the laboratory, forming thin oxide scales with Ni-rich oxides in the outer layer and Cr-rich oxides in the inner layers, while this is not the case in the waste-fired boiler.