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 High Temperature Corrosion of Metals in Sulfidizing-Oxidizing Environments: A Critical Review
Resultats experimentaux obtenus sur plusieurs metaux (Ti,Cr,Mn,Fe,Co,Ni,Cu) en atmosphere gazeuse mixte contenant du soufre et de l'oxygene. Aspects theoriques: thermodynamiques, mecanismes des reactions, transport dans la souche. Interpretation des donnees experimentales. Influence de la preoxydation sur la reaction ulterieure en atmosphere mixte. Conclusions
Influence of filler addition, storage medium and evaluation time on biaxial flexure strength and modulus of adhesive systems
Abstract Objective. This study evaluated the effects of filler addition, storage medium and time on biaxial flexural strength and flexural modulus of six adhesive systems. Materials and methods. The adhesives were either unfilled resins: Single Bond, Prime&Bond 2.1 and One-Step; or filled resins: Single Bond Plus, Prime&Bond NT and One-Step Plus. Resin discs of each product (0.5 mm thick × 6.0 mm diameter) were prepared using silicon molds (n = 10). The discs were stored dry, in water, mineral oil or ethanol for 1 week or 3 months before biaxial flexural testing. Data were statistically analyzed by 3-way ANOVA and Tukey's post-hoc test (α = 0.05). The specimens were tested in a universal testing machine at 1.27 mm/min until failure occurred. Results. In general, the storage in ethanol led to significantly lower flexural strength and flexural modulus values than the other storage conditions for all adhesives in both storage periods. Filler addition increased flexural strength and flexural modulus for Prime&Bond NT, when it was stored dry or in water for 1 week. For Single Bond Plus, the filler addition resulted in higher flexural modulus, when it was stored for 1 week in oil and in ethanol. No significant differences in flexural modulus were observed between One-Step and One-Step Plus in any storage medium. Conclusions. Results suggested that filler addition does not necessarily have to increase the flexural strength and flexural modulus. Ethanol and oil storages tended to yield opposite effects.