Zirconium alloy

Because of good corrosion resistance in both acids and bases, zirconium alloys are widely used in chemical plants. Commercial zirconium, as used primarily for corrosion resistance in the chemical industry [4], contains up to 4.5% hafnium, which is difficult to separate because of the similar chemical properties of zirconium and hafnium. TTie presence or absence of hafnium has no effect on the corrosion resistance, which is controlled by a very stable oxide. At ambient temperature, this passive oxide is 2-5 nm thick [4]. The pure metal low in hafnium (0.02% max) has a low thermal neutron capture, making it useful for nuclear-power applications. 

The outstanding corrosion property of zirconium is its resistance to alkalies at all concentrations up to the boiling point, ft also resists fused sodium hydroxide. In this respect, it is distinguished from tantalum and, to a lesser extent, titanium, which are attacked by hot alkalies. Zirconium is resistant to hydrochloric and nitric acids at all concentrations and to 70% H2SO4 up to boiling temperatures. In HCI and similar media, the metal must be low in carbon ( 0.06%) for optimum resistance. In boiling 20% HCI, a transition or breakaway point is observed in the corrosion rate (see below) after a specihc time of exposure. The 

Critical pitting potentials of 0.38 V (S.H.E.) in IV NaCl and 0.45 V in 0.1 V NaCl [6] indicate that the metal is vulnerable to pitting in seawater. It undergoes intergranular S.C.C. in anhydrous methyl or ethyl alcohol containing HCl, but not when a small amount of water is added [7]. This behavior, similar to that of commercial titanium, suggests that stress may not be necessary and that the failure is perhaps better described as intergranular corrosion. 

To help improve the corrosion resistance of Zircaloy, several new zirconium alloys have been developed, such as Zirlo (Zr-1.0% Nb-1.0% Sn-0.1% Fe). Notwithstanding the progress so far, materials reliability does have a significant effect on the economics of nuclear power plants, and there is considerable incentive to develop a full understanding of the mechanisms of corrosion of zirconium alloys in reactors and to develop alloys that are resistant to both irradiation and corrosion in reactors [18]. 

The good resistance of zirconium to deaerated hot water and steam is of special importance in nuclear-power applications. The metal or its alloys can be exposed 

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R. K. McGeary, inM Symposium on Zirconium and Zirconium Alloys, American Society for Testing and Materials, Philadelphia, 1953, p. 168. 

By contrast, uranium fuels for lightwater reactors fall between these extremes. A typical pressurized water reactor (PWR) fuel element begins life at an enrichment of about 3.2% and is discharged at a bum-up of about 30 x 10 MW-d/t, at which time it contains about 0.8 wt % and about 1.0 wt % total plutonium. Boiling water reactor (BWR) fuel is lower in both initial enrichment and bum-up. The uranium in LWR fuel is present as oxide pellets, clad in zirconium alloy tubes about 4.6 m long. The tubes are assembled in arrays that are held in place by spacers and end-fittings. 

Table 6. Hafnium-Free Zirconium Alloys for Nuclear Service...

Hafnium-free zirconium alloys containing tin or niobium are used for tubing to hold uranium oxide fuel pellets inside water-cooled nuclear reactors. Zirconium —niobium alloys are used for pressure tubes and stmctural components in Canadian, the former USSR, and Germany reactor designs. 

Above 40 wt % hydrogen content at room temperature, zirconium hydride is brittle, ie, has no tensile ductiHty, and it becomes more friable with increasing hydrogen content. This behavior and the reversibiHty of the hydride reaction are utilized ki preparing zirconium alloy powders for powder metallurgy purposes by the hydride—dehydride process. The mechanical and physical properties of zirconium hydride, and thek variation with hydrogen content of the hydride, are reviewed in Reference 127. 

Zirconium and zirconium alloys. The possibihty of deterioration of zirconium and zirconium alloys above 315°C (600°F). 

Cladding. The Magnox reactors get their name from the magnesium-aluminium alloy used to clad the fuel elements, and stainless steels are used in other gas-cooled reactors. In water reactors zirconium alloys are the favoured cladding materials. 

Finally, the enriched uranium of converted back into UO,. The UO, is pressed into small fuel pellets and packaged in a metal tube (made of a zirconium alloy) for use in a nuclear reactor. 

The growth of nuclear engineering with its specialised demands for materials having a low neutron absorption coupled with adequate strength and corrosion resistance at elevated temperatures, has necessitated the production of zirconium in relatively large commercial quantities. This specific demand has resulted in development of specially purified zirconium, and certain zirconium alloys, for use in particular types of nuclear reactor. 

Table 5.20 Minimum mechanical properties of nuclear grade zirconium alloys...

Zirconium alloys have been much less thoroughly studied than titanium alloys. The main application of interest has been for nuclear reactor components where good corrosion resistance combined with a low neutron capture cross-section has been required. Corrosion fatigue crack growth in these alloys in high temperature (260-290°C) aqueous environments typical of... 

Test method for sandwich corrosion test Recommended practice for preparing, cleaning, and evaluating corrosion test specimens Practice for aqueous corrosion testing of samples of zirconium and zirconium alloys Test method for corrosion testing of products of zirconium, hafnium and their alloys in water at 633 K or in steam at 673 K [metric] Recommended practice for conventions applicable to electrochemical measurements in corrosion testing... 

Phillips, J. F. Dissolution of Oxide-Coated Zirconium and Zirconium Alloys, U.S. AEC Report BNWL-600, Pacific Northwest Laboratory, Richland, WA, 1968. 

Zirconium and zirconium alloys are used in the nuclear industry, because of their low neutron absorption cross-section and resistance to hot water at high pressures. 

Elektron A family of processes, operated by Magnesium Elektron, UK, for making magnesium, magnesium-zirconium alloys, and zirconium chemicals. In the 1920s and 30s, the names elektron and elektronmetall were used colloquially in Germany for magnesium metal. 

Hafnium-free zirconium alloys, 26 6351 Hafnium halides, 73 91-93 Hafnium hydride, 73 93 Hafnium hydroxide chloride heptahydrate, 13 92... 

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