Zirconium is one of a very few materials of construction for processes involving high concentrations of sulfuric acid at temperatures above the atmospheric boiling point. In this severe environment only zirconium or tantalum can be used with any reliability. Most corrosion data for zirconium has been developed using welded wrought material as test coupons. The majority of this information is related to the corrosion resistance of Zr 702 with very little information available for Zr 705. In addition, almost no information is available as to the corrosion resistance of the cast forms of either alloy. This paper presents laboratory data comparing the corrosion resistance of Zr 702 and Zr 705 in the welded and wrought forms and in the cast form in sulfuric acid. A newly expanded iso-corrosion diagram is presented for both wrought and cast Zr 702 and Zr 705. Zr 705 is a zirconium alloy containing 2.5% Niobium. Because of the possibility of hydrogen absorption by the Niobium phase during the corrosion process, hydrogen absorption data is also presented for Zr 705. This hydrogen absorption data is compared with Zr 702 tested under the same conditions.
Frequently users consider Zr 702 and Zr 705 as interchangeable in regards to corrosion. In some cases this is true, but not for sulfuric acid applications. For many applications in the chemical processing industry (CPI) involving sulfuric acid, zirconium is the material of choice. This is especially true in concentrated sulfuric acid and when the operating temperature is above the atmospheric boiling point. Although Zr 702 is the most corrosion resistant of the zirconium alloys for this application, Zr 705 can be used in the form of cast pump parts and for fasteners of various types. There are, however, many differences between the two alloys. Zr 702 is generally more corrosion resistant than Zr 705 but Zr 705 has considerably higher strength and is more formable than Zr 702. Both alloys can be produced into large castings by the rammed graphite casting method .
Cast zirconium has similar metallographic structure to that found in the welded wrought material. In both cases, the corrosion resistance of the cast material is somewhat lower than the wrought material especially at higher concentrations and temperatures in sulfuric acid. In this paper, all corrosion and mechanical data, including Table 4, used Zircadyne® zirconium.(1)
(Table in full paper)
Zr 702C and Zr 705C cast zirconium is normally produced according to ASTM B752 “Standard Specification for Castings, Zirconium-Base, Corrosion Resistant, for General Application”. Zirconium sheet and plate is normally produced according to ASTM B751 “Standard Specification for Zirconium and Zirconium Alloy Strip, Sheet, and Plate”. Table 1 gives the chemical requirements as specified in these ASTM standards.
Table 2 gives the physical and mechanical differences between these two zirconium alloys 2. Notice that the Zr 705 is actually a two phase alloy as opposed to Zr 702 which, for practical purposes, has a single phase.