Materials for mid furnace environments of kraft recovery boilers
Kraft recovery boilers experience localized accelerated corrosion in mid furnace areas. Corrosion patterns may vary from one boiler to other depending upon the operation parameters and general design of the boiler. Local gaseous environments were characterized in areas with corrosion and equivalent areas without accelerated corrosion in a selected boiler over the period of two years. Results from this study indicated that in the areas with accelerated corrosion there was a large variation in the local gas composition at the waterwall surface. Reducing and sulfidizing gases were found in the areas with accelerated corrosion whereas the gas compositions were more oxidizing in the low corrosion areas. Observations during field tests have indicated that the local waterwall temperature may also vary in the mid furnace due to localized combustion and air flow patterns. Mid-furnace environments were simulated in laboratory to evaluate the effect of gas composition and fluctuations on corrosion kinetics of carbon steel tube material. Laboratory tests with SA-210 carbon steel samples with gas composition cycling between oxidizing and reducing/sulfidizing led to an unstable scale formation on the surface. Extent of accelerated corrosion was found to depend upon the extreme partial pressure values of sulfur and oxygen. Carbon steel samples were coated with chromized or aluminized diffusion coatings in the laboratory and were tested under sulfidizing conditions. Both type of diffusion coatings had superior performance compared to the carbon steel samples under similar conditions. Midfurnace corrosion can be controlled by making gaseous environments in the mid furnace more stable. However, more practical solution seems to be to use waterwall tubes with protective outer layer. This may be done by using composite tubes (stainless steel outer layer), thermal spray coatings of resistant alloys or diffusion barrier coatings like chromizing or aluminizing treatment.
The oxidation resistance of nitrided mild steel
Abstract The oxidation resistance of mild steel in a simulated boiler flue gas atmosphere has been shown in laboratory tests to be significantly enhanced by a surface nitriding treatment which forms a controllable fine-grained martensitic layer without precipitation of iron nitrides. A mechanism is suggested by which improved protection results from the refinement of oxide grain size and the condensation of cation vacancies in the nitrided layer instead of at the metal/oxide interface. A parallel is drawn with the development of improved oxidation resistance in superalloys by sub-surface dispersion of incoherent stable oxide particles.
“Social Media Misinformation”—An Epidemic within the COVID-19 Pandemic
"Social Media Misinformation"-An Epidemic within the COVID-19 Pandemic.