Zirconium is well known and widely used in sulfuric acid containing processes where it is used in heat exchangers, tanks, piping and other ancillary equipment. A considerable body of corrosion data has been developed describing the use of zirconium for use in sulfuric acid environments. Virtually all of this information, however, has been developed using zirconium in wrought form such as sheet and plate. Very little information is available as to the corrosion characteristics of cast zirconium when used in applications involving sulfuric acid in high concentrations and under high temperatures. In addition to wrought products, zirconium pumps and valves are commonly specified in a cast product form of zirconium. This paper will provide information related to the production using the rammed graphite method along. New corrosion data will be given for Zr 702C. A new iso-corrosion diagram is presented comparing wrought Zr 702 vs. cast Zr 702 in sulfuric acid.
Design engineers are faced with multiple considerations when it comes to component design and fabrication for the chemical processing industry (CPI), including ease of assembly, structural integrity, corrosion resistance, long term maintenance costs, and overall affordability. These considerations often conflict as design engineers begin to weigh the costs and benefits of each option. In the quest to achieve the best cost vs. benefit ratio, older technologies can get overlooked in favor of newer technologies. A prime example of an older technology that can meet today's needs is zirconium rammed graphite castings for CPI applications.1 Cast zirconium has similar metallographic structure that is found in a weld in wrought material. Along with this structure the corrosion characteristics and mechanical properties are similar to the welded structure in wrought material. The corrosion resistance is, however, somewhat lower than the wrought material at higher concentrations and temperatures of sulfuric acid. Zircadyne is a trademark or a registered trademark of ATI Properties, Inc., an Allegheny Technologies Company. Zirconium cast material is normally produced according to ASTM B752 “Standard Specification for Castings, Zirconium-Base, Corrosion Resistant, for General Application”. Table 1 gives the chemical requirements as specified in this ASTM standard. Exceptions to this standard are possible, however, with agreement between the purchaser and the producer. MECHANICAL PROPERTIES Unlike aluminum and steel alloys which tend to lose structural integrity and strength when cast, zirconium tends to maintain structural integrity and strength that is comparable to wrought zirconium products. Table 2 illustrates this point by comparing tensile and the yield minimums and other physical characteristics for wrought Zircadyne® 702 and cast zirconium. As shown in Table 2, the minimum elongation is reduced somewhat, although the yield strength actually increase as a result of casting. CORROSION Zirconium is rather straightforward in its corrosion resistance in sulfuric acid as indicated in Figure 2. There are many factors affecting the performance of the alloys in actual service. The factors include impurities in the alloys and the environments, equipment design and fabrication, operating conditions and maintenance among others.
