Materials related problems in Kraft recovery boilersare generally associated with corrosion, and the highestcorrosion rates are usually observed in the lower furnace.Presently, there are two main classes of tubing materials used inthe lower furnace: (1) plain carbon steel and (2) compositetubing consisting of an inner tube of plain carbon steel and anouter tube of Type 304L stainless steel. While the plain carbonsteel tubing is relatively inexpensive, it experiences severecorrosion in the sulfidizing environment of the lower furnace.The composite tubing generally performs well; however, theinitial cost of the material is high and corrosion problems havebeen observed around the air ports of some recovery boilers. Thiseffort will pursue the development of a new class of alloys withcorrosion resistance that is equal or superior to 304L, but witha reduced cost due to significantly less chromium and nickel inthe alloy. The corrosion resistance of this alloy will resultfrom the synergistic use of chromium and silicon. Phase I willfocus on the corrosion properties of the alloy, bothexperimentally and theoretically. Iron-based alloys prepared inthe laboratory and containing different amounts of chromium plussilicon will be tested in gas atmospheres that simulate the lowerfurnaces. A control sample of 304L stainless steel will alsoundergo corrosion testing. The results will allow thedetermination of the necessary chromium and silicon contentsrequired for this new alloy to show similar performance to 304L.A thermodynamic and kinetic model for the corrosion of this alloywill also be developed. The experimental and theoretical studywill lay the foundation for Phase II, where issues such as thedevelopment of high temperature mechanical properties,thermomechanical processing, and weldability will be addressed. Anticipated Results/Potential Com.m.ercial Applications as described by the awardee: The development of a new class of lowcost iron-based ferritic alloys that exhibit excellent corrosionresistance in the severe environments experienced in Kraftrecovery boilers will be achieved. The result of this developmentwill be lower capital costs due to lower initial material costs,lower maintenance costs because of fewer tube replacements, andlower production costs because of prolonged operation cycles.This alloy should also have applications in other industries(e.g. petrochemical) where severe corrosion problems areexperienced.
