Nickel alloy UNS(1) N10276 is commonly used for pipes in the sulfur scrubbers of coal-fired power plants. The pipes in these sulfur scrubbers are exposed to both gaseous and liquid sulfuric acid at moderate temperatures (<3000F), necessitating the use of corrosion-resistant alloys. The biggest factor that controls the corrosion behavior of nickel alloy UNS N10276 is the microstructure. There are two primary options for changing the microstructure of cast alloys: chemical composition and heat treatment. This project focused on the effect of both chemistry and heat treatment on microstructure and corrosion resistance. Samples with three chemistries (low, mid, high) within the alloy’s specification were produced at the Virginia Tech foundry. Chromium, molybdenum, and tungsten were varied due to their known ability to influence the corrosion resistance of this alloy. Heat treatments at various temperatures, holding times, and cooling rates were evaluated. Samples were corroded in the vapor produced by a 20 wt% sulfuric acid solution in water at 930C.
The low and mid alloy chemistries exhibited weight losses, whereas the high alloy chemistry exhibited weight gains. Overall, none of the samples exhibited high corrosion rates (all were <0.114 mm/y). In terms of chemistry, none of the corrosion rates were statistically different from one another except for the mid and high alloy with an air cooled heat treatment. In terms of heat treatment, for the mid alloy chemistry, the corrosion rates from all three heat treatments were statistically different from that of the non-heat treated (as-cast). For the low and high alloy, none of the heat treatments caused corrosion rates to be statistically different from one another including the non-heat treated.
The heat treating temperature altered the shape of the intermetallic particles (µ and/or P phases) and the heat treating time affected both the shape and distribution of these intermetallic phases. The cooling rate, however, did not seem to affect them (this may have been a result of the small test specimen size). In addition, heat treating eliminated segregation that occurred during solidification.
Overall, varying the chemistry within the specification range had no statistically significant effect on corrosion resistance of cast nickel alloy UNS N10276 in the environment tested.