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.
Alloy degradation under oxidizing-sulfidizing conditions at elevated temperatures
Exposure of high-temperature alloys to a low-PO2 oxidizing-sulfidizing environment above about 550 °C is often characterized by an initial period of protective oxidation followed by more rapid corrosion. This paper will provide a brief overview of alloy corrosion in oxidizing-sulfidizing atmospheres, with specific consideration given to the thermodynamic and kinetic aspects of the process, resulting modes of degradation, and alloy selection. The concept of a "critical microstructure" in the subsurface of the alloy as a prerequisite to initiating protective scale breakdown will also be discussed. The paper will conclude with a specific example of alloy development for the filtration of hot oxidizing-sulfidizing gases.
High-temperature sulfidation behavior of Ni-Nb alloys
AbstractThe sulfidation properties of Ni-Nb alloys containing additions of niobium up to 40 wt.% have been studied at
$$P_{S_2 } = 0.01$$
 atm over the temperature range 550-700 °C. The sulfidation reactions followed the parabolic rate law; the sulfidation rates decreased with increasing amounts of niobium. An Arrhenius plot of the rate constants gave activation energies of 25.0+3.5 kcal/ mole. The scales formed on Ni-Nb alloys were multilayered, generally consisting of an outer layer of nickel sulfide ( NiS1+x and Ni3S2) and an inner complex layer of NiNb3S6 plus NbS2. The position of the original metal surface was notedy platinum-wire marker experiments to be the interface between the inner andouter layers. The location of the marker indicates that the outer layer, generally greater in thickness than the inner layer, grew by outward diffusion of the nickel cations, and the inner layer formed probably by the inward diffusion of sulfur. Neither preferential sulfidation nor internal sulfidation was observed. The development of the scale structures from the transient stage to steady state was also studied.