Diffusion Coatings for Corrosion Protection of Nickel-Plated Steels for Biomass Combustion Plants

摘要

In the course of energy transition the use of existing power plants for co-firing of biomass plays a substantial role in terms of base load supply. While co-firing reduces the CO_2 footprint of the plant, it also leads to higher amounts of corrosive species such as chlorine and sulphur within the system Consequently, higher corrosion rates and material losses are observed in the firing chamber components. Especially superheater tubes are subjected to increased corrosive attack and are prone to early replacement. Thus, co-firing is commonly limited to temperatures below 500°C and small biomass-to-coal ratios (10%). This study deals with the evaluation of innovative diffusion coatings applied on selected steels commonly used as superheater tube material in order to protect the metal surfaces against high temperature corrosion. For this purpose martensitic steel of type X20 was first coated with a thin nickel layer. This interlayer is supposed to serve as nickel reservoir for the subsequent diffusion process of potentially protective elements like Al, Cr and Si. For comparison, austenitic steel DMV 310 N and pure X20 without interlayer were coated by this diffusion process, as well. The morphology and the chemical composition were analysed as a function of nickel plating parameters. Characterization of the manufactured intermetallic layers was conducted by means of optical microscope, EPMA' and XRD analysis.