Zirconium alloys, environment-alloy

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... 

Tantalum-hafnium and tantalum-zirconium alloys are less suitable for aggressive acid environments. Compared with tantalum-tungsten alloys, tantalum rhenium alloys are superior in corrosion resistance and to hydrogen embrittlement, but the major disadvantage is the high cost of rhenium. 

The environments, along with the cracking modes of zirconium and titanium, are given in Table 4.88. It is obvious from the table that zirconium alloys are susceptible to stress-corrosion cracking in a variety of environments. It is necessary to subject the weld to heat treatment in order to lower the stress in the weld. The most serious problem encountered in the nuclear applications is delayed hydride cracking in addition to stress-corrosion cracking, particularly in Zr-2.5% Nb alloy. 

Environment Aluminum alloys Carbon steels Copper alloys Nickel alloys Stainless steels Austenitic Duplex Martensitic Titanium Zirconium alloys alloys... 

Metallic materials with the exception of noble metals are also thermodynamically not stable in the acidic environment under the PEFC operating conditions and therefore subject to corrosion. Nevertheless, many different metals such as stainless steels, aluminum, aluminum composites, copper, nickel and nickel alloys, titanium alloys and even highly corrosion resistant materials used in chemical industry such as tantalum, hafnium, niobium or zirconium have been investigated with respect to applicability in PEFC with respect to corrosion resistance [68—71]. 

J. Busby, Irradiation effects on corrosion of zirconium alloys, in ASM Handbook, Vol. 13C, Corrosion Environments and Industries, ASM International, Materials Park, OH, 2006, pp. 406-408. See also C. Lemaignan, Corrosion of zirconium alloy components in light water reactors, in ASM Handbook, Vol. 13C, Corrosion Environments and Industries, ASM International, Materials Park, OH 2006, pp. 415-420. 

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. 

Exposure to fast neutron fluxes increases the corrosion rate of zirconium-based alloys. Even in the highly oxidizing environment of a BWR coolant, however, the corrosion rate of the principal zirconium alloys is low enough that cladding corrosion is not a limiting factor on the life of the fuel element. 

Most of these problems arise from the need to minimize parasitic neutron absorption In the pressure tubes. As well as limiting directly the choice of alloy systems of interest in this application, it also makes It desirable that the material shows a high strength and Is able to tolerate the chemical. Irradiation and thermal environments without the need for large corrosion allowances, shielding or Insulation. It Is such considerations which have led to the use of zirconium alloys for pressure tubes in water reactors. 

The in-plle and out-of-plle aqueous corrosion and hydrogen pick-up of zirconium alloys used as reactor pressure tube materials Is discussed particularly in relation to differing behaviour under oxidizing (neutral) and comparatively reducing environments (ammonia). The materials selection and chemical aspects of the moderator circuit are outlined. 

Titanium and zirconium alloys are used in process equipment subjected to severe environment. In the ASME Code, VUI-1, unalloyed titanium is listed for grades 1,2, and 3, and alloyed titanium is listed for grade 7. Two zirconium grades also given in the Code are unalloyed alloy 702 and alloyed alloy 705. 

Environment Alnminmn aHo Carbon steels C(qq er alloys Nickel alloys Austenitic Stainless Steds Duplex Martensitic Titanium alloys Zirconium alloys... 

In the chemical process industry molybdenum has found use as washers and bolts to patch glass-lined vessels used in sulphuric acid and acid environments where nascent hydrogen is produced. Molybdenum thermocouples and valves have also been used in sulphuric acid applications, and molybdenum alloys have been used as reactor linings in plant used for the production of n-butyl chloride by reactions involving hydrochloric and sulphuric acids at temperatures in excess of 170°C. Miscellaneous applications where molybdenum has been used include the liquid phase Zircex hydrochlorination process, the Van Arkel Iodide process for zirconium production and the Metal Hydrides process for the production of super-pure thorium from thorium iodide. 

It should be noted that swarf from a zirconium-titanium alloy containing approximately 50% by weight of each element is prone to pyrophoricity in air. It has also been reported that when zirconium is welded to titanium, the welded zone is much more sensitive to corrosion than either of the parent metals. If, therefore, it is proposed to use my construction in which zirconium is welded to titanium, caution should be observed in the machining of welds, and the corrosion behaviour of the weld should be checked by prior testing in the environment with which the construction will be employed. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

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