Zirconium and its Alloys

Zirconium alloys are chiefly used as the cladding (structural) metal in nuclear reactors. The alloys are also used as grid spacers, guide tubes, pressure tubes, calandria tubes and other minor components. 

The low cross-section for absorption of neutrons and high-temperature (330-350°C) aqueous corrosion resistance as well as its good mechanical properties promote the use of zirconium alloys in the nuclear reactors. In the development of zirconium alloys care must be taken that the added minor elements do not posses high cross-sections for the absorption of neutrons and contribute to greater corrosion resistance and improved mechanical properties. The good corrosion resistance of the alloys in acids and bases favors the use of zirconium alloys in chemical plants. 

The zirconium alloys with the ASTM specifications are given in Table 4.85. The composition of nuclear-grade alloys is given in Table 4.86. The composition of other alloys developed for nuclear applications is given in Table 4.87. 

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A problem of obtaining zirconium with lowest possible contents of hafnium comes from construction requirements when using zirconium and its alloys in building nuclear reactors. The construction material must have good mechanical properties and must be resistant to corrosion in contact with heat carriers. Since reactor power is proportional to the quantity of neutrons, their absorption into construction materials should be as small as possible. Zirconium and its alloys are unique materials that satisfy these requirements. However, hafnium has approximately the same chemical characteristics as zirconium but it absorbs neutrons strongly. 

Brhacek, L., Kalandra, C. Determination of hydrogen, oxygen and nitrogen in zirconium and its alloys. Chem. Prumysl 18, 417 (1968). 

Table 4.89 Corrosion data for zirconium and its alloys in HCIa...

The curcumin method (in either the rosocyanin or rubrocurcumin version) has been applied for determining traee amounts of boron in biologieal materials [10], soils and plants [17], waters [51], silicon [52], chlorosilanes [20], uranium [1,53], zirconium and its alloys [53,54], nickel [55,56], copper alloys [56], cast iron and steel [12,57-59], beryllium and magnesium [53], and phosphates [2]. This method was also used for determining boric acid admixtures (about 0.05%) in powdered boron [11]. Some synthetic compounds having the structure similar to that of curcumin, were used in determining boron in water [60]. 

The method involving the Mo-V-P acid has been used in determinations of phosphorus in biological tissues [127], plant material [128], fruits [129], fish products [130], foodstuffs [131], phosphate minerals [132], cast iron and steel [133,134], niobium, zirconium and its alloys, titanium and tungsten, aluminium, copper, and white metal [135], nickel alloys [134,135], metallurgy products [136], molybdenum concentrates [137], silicon tetrachloride [7], cement [138], and lubricants[139]. The flow injection technique has been applied for determining phosphate in minerals [140] and in plant materials [141]. 

Brown, M.L. and Walton, G.N., Polarisation of zirconium and its alloys in high temperature water. J. Nucl. Materials, 66, 44 (1977). 

B. Cox, Oxidation of zirconium and its alloys. Advances in Corrosion Science and Technology, Vol. 5, M. Fontana and R. Staehle, editors, Plenum Press, New York, 1976, p. 173. 

Zirconium and its alloys are nontoxic and compatible with bodily fluids and thus are used in making surgical implants and prosthetic devices. The high oxygen affinity of the divided metal powder allow zirconium to be used as a getter in vacuum tubes or in photoflash bulbs or explosive primers. On the other hand, it is used as an alloying agent in steel. Other applications include the manufacture of rayon spinnerets and lamp filaments. 

Zirconium and its alloys are susceptible to stress corrosion cracking (SCC) in such environments as Fe - or Cu -containing chloride solution, CH3OH -H hahdes, concentrated HNO3, halogen vapors, and liquid mercury or cesium [4,5]. Common test methods, e.g., U-bend, C-ring, split ring, direct tension, double cantilever, and slow strain rate tension, have been used to determine zirconium s susceptibility to SCC. 

Specimens of zirconium and its alloys can be covered with a layer of pyrophoric film when they are tested under certain specific conditions, e.g., a stagnant solution of 77.5 % sulfuric acid and 200 ppm ferric ion at 80 C for ten days [7]. Treating in hot air or steam can eliminate this pyrophoric tendency. This takes 20 to 30 min at 250 C. It should be noted that most corroded specimens do not have a pyrophoric layer. The formation of pyrophoric films is possible, but not guaranteed when both the following conditions exist ... 

Yau, T. L., Andrews, J. A., Henson, H. R., and Holmes, D. R., Practice for Conducting Corrosion Coupon Tests on Zirconium and Its Alloys, Corrosion Testing and Evaluation Silver Anniversary Volume, ASTM STP 1000, ASTM International, West Conshohocken, PA, 1990, pp. 303-311. 

Zirconium and its alloys can be classified into two major categories nuclear and nonnuclear. They all have low alloy contents. They are based on the a structure with dilute additions of solid solution strengthening and a-stabilizing elements like oxygen and tin. However, in niobium-containing alloys, there is the presence of some niobium-rich P particles. 

Zirconium ores contain a few percent of its sister element, hafnium. Hafnium has chemical and metallurgical properties similar to those of zirconium, although their nuclear properties are markedly different. Flafnium is a neutron absorber but zirconium is not. As a result, there are nuclear and nonnudear grades of zirconium and zirconium alloys. Some commerdally available grades of zirconium and its alloys are listed in Table 22.2. 

The corrosion resistance of zirconium and its alloys in steam is of special interest to nuclear power applications. They can be exposed for prolonged period without pronounced attack at temperatures up to 425°C. In 360 C steam, up to 350 ppm chloride and iodide ions, 100 ppm fluoride ions, and 10,000 ppm sulfate ions are acceptable for zirconium in general applications, but not in nuclear power applications. 

The development of water-cooled nuclear power reactors brought about the use of zirconium and its alloys for uranium fuel cladding and for structural components. As a result of these developments in the nuclear industry, the cost of zirconium and its alloys decreased considerably and became competitive... 

Zirconium and its alloys also find applications in other nuclear reactor systems, such as gas-cooled or organic coolant-cooled reactors. 

The process equipment must be made of the most corrosion-resistant materials, such as zirconium and its alloys. Zr 702 and Zr 705 are often used to construct process equipment, such as reactors, columns, heat exchangers, pumps, valves, piping systems, trays, and packing. In recent years, zirconium has been replacing stainless alloys after their failures in acetic acid service. ° Zirconium could be the most cost-effective structural material when all issues, such as process efficiency, product yield and quality, safety, maintenance and replacement costs, toxicity, and environmental protection are considered. 

Zirconium and its alloys have been identified to offer the best prospects from a cost standpoint as materials for an HI decomposer in hydrogen production. They resist attack by HI media (gas or liquid) from room temperature to 300°C. Most stainless alloys have adequate corrosion resistance only at low temperatures. 

Zirconium appears to be nontoxic and biocompatible. It has very low corrosion rates in many media. It is ideal for making equipment used in food processing, in the manufacture of fine chemicals, and in pharmaceutical preparations. Zirconium and its alloys are suitable for certain implant applications as well. 

As a result, many nuclear and industrial applications have been developed for zirconium and its alloys. These applications include fuel cladding and pressure tubes for nuclear reactors, process equipment for the CPI, superconducting materials, battery alloys, hydrogen storage alloys, ordnance applications, implant materials, and consumer goods. 

The manufacturing industry is emphasizing quality, efficiency, and environmental compatibility. Zirconium is well positioned to meet these needs. Interest in zirconium and its chemicals is on the rise. However, there is still a persistent perception that zirconium is exotic and costly. Actually, zirconium is plentiful. In the earth s crust, zirconium is more abimdant than many common elements, such as nickel, copper, chromium, zinc, lead, and cobalt. The prices of zirconium and its alloys have been relatively stable for many years. They are very competitive with other high-performance materials. Life cycles costs of zirconium equipment can be particularly attractive. There is much room for zirconium to grow in the coming years. 

J.E. LeSurf. 1969. The corrosion behavior of 2.5Nb zirconium alloys, in Symposium on applications, related phenomena in zirconium and its alloys, ASTM STP 458, Philadelphia, PA American Society for Testing and Materials. 

R.F. Koemg. 1953. Corrosion of Zirconium and its Alloys in Liquid Metals, Report No. KAPL-982, prepared for the U.S. Atomic Energy Commission by the General Electric Company. 

Ingots of zirconium and its alloys are most commonly 40 to 760 mm in diameter and weigh 1100 to 4500 kg. Wrought products are available in a variety of forms and sizes, such as sheet and strip, plate, foil, bar... 

Although zirconium and its alloys are costly compared with other common corrosion-resistant materials, their extremely low corrosion rates, resulting in long service life and reduced maintenance and downtime cost, make zirconium and its alloys quite cost effective. Table 8.45, which compares costs between S31600 stainless steel and various corrosion-resistant metals and alloys, shows that although R69702 is more costly than stainless steel. Inconel, and titanium alloys, it costs roughly the same as or less than some of the Hastelloys and considerably less than tantalum. 

Zirconium shows excellent corrosion resistance to hydrochloric acid and is superior to any other engineering metal for this application, with a corrosion rate of less than 0.125 mm y" at all concentrations and temperatures well in excess of the boiling point. Aeration does not affect its corrosion resistance, but the presence of oxidizing impurities such as cupric or ferric chlorides in relatively small amoimts will decrease it. Therefore, either these ions should be avoided, or suitable electrochemical protection should be provided. Zirconium also shows excellent corrosion resistance to nitric acid in all concentrations up to 90% and temperatures up to 200°C, with only platinum being equal to it for this service. Welded zirconium and its alloys retain this high corrosion resistance. In concentrated nitric acid, zirconium may exhibit SCC at nitric acid concentrations above 70%, if under high tensile stress. ... 

This method may be applied to zirconium and its alloys in the same way as described under 1.1.1.3. for aluminium. 

In general the concentrations of carbon and nitrogen in zirconium and its alloys are much higher than the boron concentration and the boron concentration of "real" samples is low. Therefore only the B(d,n) C, B(p,a) Be and B(d,an) Be reactions are of practical use for reasons of selectivity and sensitivity. 

Elwell and Wood (21) obtain more controllable combustion conditions by filling the combustion tube first with argon at the used temperature and furtheron forcing it gradually out with oxygen. An advantage is that larger samples can be burnt. Titanium and its alloys need a lead flux (3 g Pb for samples of 3 to 4 g), whereas zirconium and its alloys need no flux at all. The reproducibility of the method is 100 ig/g for carbon concentrations of 1000 /xg/g in titanium (39). The method is useful for concentrations above 500 /xg/g. 

The sample must be finely divided, distributed evenly in a thin layer over the bottom of the combustion boat and covered with the fluxing agent. For the analysis of zirconium and its alloys, it is advised to use 2 g bismuth chips and 1 to 2 g granulated crude iron (< 50 /ig/g carbon) as flux. 

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