Zirconium Alloys
Zirconium alloys are solid solutions of zirconium or other metals. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.
Advantages of Zirconium Alloys
High melting point: Zirconium alloy has a high melting point, which can be used for processing and application in high temperature environment.
Corrosion resistance: Zirconium alloys have excellent corrosion resistance and can be used for a long time in harsh environments such as strong acid, strong alkali, high temperature and high pressure, so they are widely used in the fields of chemical industry, marine and nuclear industry.
Good biocompatibility: Zirconium alloy will not cause rejection when it comes into contact with biological tissues, and can be used in the manufacture of medical devices and artificial joints and other medical materials, with good biocompatibility.
Good mechanical properties: Zirconium alloy has excellent mechanical properties, including high strength, high hardness, high toughness and high wear resistance, etc., which can be used to manufacture high-quality mechanical parts and tools.
Low thermal neutron absorption cross-section: Zirconium alloy has a very low thermal neutron absorption cross-section, which can be used as core structural materials for nuclear reactors, such as fuel cladding, pressure tubes, stents and orifice tubes.
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Density of typical zirconium alloy is 6.6 g/cm3 (0.24 lb/in3).
Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:
ρ = m/V
The density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).
Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious that the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm3),
Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the atom’s total volume or less, but it contains all the positive charge and at least 99.95% of the atom’s total mass. Therefore it is determined by the mass number (number of protons and neutrons).
Atomic Number Density. The atomic number density (N; atoms/cm3), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm3) of the material. The atomic number density (N; atoms/cm3) of a pure material having an atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm3)
Crystal Structure. The density of a crystalline substance is significantly affected by its crystal structure. FCC structure, along with its hexagonal relative (hcp), has the most efficient packing factor (74%). Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium.
Zirconium alloys readily react with oxygen, forming a nanometer-thin passivation layer. The corrosion resistance of the alloys may degrade significantly when some impurities (e.G. More than 40 ppm of carbon or more than 300 ppm of nitrogen) are present. Corrosion resistance of zirconium alloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide. Nitride coatings might also be used.
Whereas there is no consensus on whether zirconium and zirconium alloy have the same oxidation rate, zircaloys do behave very similarly in this respect. Oxidation occurs at the same rate in air or in water and proceeds in ambient condition or in high vacuum. A sub-micrometer thin layer of zirconium dioxide is rapidly formed in the surface and stops the further diffusion of oxygen to the bulk and the subsequent oxidation. The dependence of oxidation rate r on temperature and pressure can be expressed as
R = 13.9·p1/6·exp(−1.47/kbt)
The oxidation rate r is here expressed in gram/(cm2·second); p is the pressure in atmosphere, that is the factor p1/6 = 1 at ambient pressure; the activation energy is 1.47 ev; kb is the boltzmann constant (8.617×10−5 ev/k) and t is the absolute temperature in kelvins.
Thus the oxidation rate r is 10−20 g per 1 m2 area per second at 0 °c, 6×10−8 g m−2 s−1 at 300 °c, 5.4 mg m−2 s−1 at 700 °c and 300 mg m−2 s−1 at 1000 °c. Whereas there is no clear threshold of oxidation, it becomes noticeable at macroscopic scales at temperatures of several hundred °c.
Applications of Zirconium
One of the major applications of Zirconium is as a corrosion-resistant material of construction for the chemical processing industry. Zirconium exhibits excellent resistance to corrosive attack in most organic and inorganic acids, salt solutions, strong alkalis, and some molten salts. In certain applications of Zirconium, the unique corrosion resistance properties can extend its useful life beyond that of the remainder of the plant. Consequently, maintenance costs are reduced and downtime is minimized. Some of the more important areas in the chemical processing industry where Zirconium is being used include Reboilers, Evaporators, Tanks, Packings, Trays, Reactor Vessels, Pumps, Valves and Piping.
Acetic Acid
Acetic acid is one of the basic components in a wide range of organic materials including acetate esters, acetic anhydride, terephthalic acid, aspirin and other pharmaceuticals. Zirconium is considered the most corrosion resistant material in virtually all acetic acid solutions.
Formic Acid
More corrosive than acetic acid, formic acid is used in the production of pharmaceuticals, dyes and artificial flavors. The leather, textile, rubber, and pulp and paper industries also use formic acid in their process.
Nitric Acid
Nitric acid is one of the most widely used acids in the chemical processing Industry. It is a key raw material in the production of ammonium nitrate for fertilizer, and is also utilized in a variety of manufacturing processes, including the production of industrial explosives, dyes, plastics, synthetic fibers, metal pickling and the recovery of uranium.
Sulfuric Acid
Sulfuric acid is undoubtedly the most important raw material in the chemical and pharmaceutical industry today. Few chemicals are manufactured without sulfuric acid being involved. It is a strong dibasic acid and can be a reducing acid, an oxidizing acid, and/or a dehydrating agent.
Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. There are various fuel failure root causes, that have been identified in the past. These causes were predominantly fabrication defects or fretting in the early days of pwr and bwr operations. One possible cause is also:
Internal hydriding. Inadvertent inclusion of hydrogen-containing materials inside a fuel rod can result in hydriding and thus embrittlement fuel cladding. Hydrogen sources were mainly residual moisture or organic contamination in fuel pellets/rods. This cause of failure has been practically eliminated through improved manufacturing.
Delayed hydride cracking (dhc). Delayed hydride cracking is time-dependent crack initiation and propagation through fracture of hydrides that can form ahead of the crack tip. This type of failure can be initiated by long cracks at the outer surface of the cladding, which can propagate in an axial/radial direction. This failure mechanism may potentially limit high burnup operation.
Extraction and Refining
Extracting zircon
The sand and gravel that contain zircon mixed with silicate, ilmenite, and rutile are typically collected from coastal waters by a floating dredge, a large steam shovel fitted on a floating barge. After the shovel has scooped up the gravel and sand, they are purified by means of spiral concentrators, which separate on the basis of density. The ilmenite and rutile are then removed by magnetic and electrostatic separators. The purest concentrates of zircon are shipped to end-product manufacturers to be used in metal production, while less pure concentrations are used for refractories.
Refining zircon
End-product manufacturers of zircon further refine the nearly pure zircon into zirconium by using a reducing agent (usually chlorine) to purify the metal and then sintering (heating) it until it becomes sufficiently ductile—workable—for industrial use. For small-scale laboratory use, zirconium metal may be produced by means of a chemical reaction in which chloride is used to reduce the zircon.
The less-pure zircon is made into zirconia, an oxide of zirconium, by fusing the zircon with coke, iron borings, and lime until the silica is reduced to silicon that alloys with the iron. The zirconia is then stabilized by heating it to about 3,095 degrees Fahrenheit (1,700 degrees Celsius), with additions of lime and magnesia totalling about five percent.
Refining baddeleyite
Baddeleyite contains relatively high, pure concentrations of zirconium oxide that can be used without filtering or cleansing. The only refining process used on baddeleyite involves grinding the gravel or sand to a powder and sizing the powder with different sized sieves. All zirconium oxide that comes from baddeleyite is used for refractories and, increasingly, advanced ceramics.
A Guide to Machining Zirconium Alloys
Milling:
Both vertical face and horizontal slab milling give good results. Wherever possible zirconium should be climb milled to penetrate the work at the maximum approach angle and depth of cut while emerging through the work hardened area. The faces and edges of milling cutters should be kpt very sharp. A set of herringbone cutters will permit positive axial rake angles to be effective at both sides of a recess. Optimum surface finish and tool life are obtained when the tool is ground with a positive 12° to 15° radial rake along with cutting corner. A high spiral flute should also be used. The work should be flooded or sprayed with a coolant to completely wash away all chips from the tool. The penetration can range from .005 to .010 inch per tooth at 150 to 250 sfpm. The work absorbs about 10 percent of the cutting energy with sharp cutters. Hafnium requires only about 75 percent of the horsepower required for sae 1020 cr steel.
Grinding:
The grinding methods used for zirconium involve standard grinding machine equipment. The grinding characteristics of zirconium are similar to those of other metals, and both wheel and belt grinding can be used. The use of straight grinding oil or oil coolant produces a better finish and higher yields; these substances also prevent ignition of dry grinding swarf. Conventional grinding speeds and feeds can be used. Both silicon carbide and aluminum oxide can be used as abrasives, but silicon carbide generally gives better results.
Wheel grinding:
Zirconium produces a white stream of sparks. Conventional speeds and feeds are satisfactory and silicon carbide generally gives better results than aluminum oxide. At light in feeds and slow wheel speeds, higher grinding ratios are produced. At heavier in feeds and slow wheel speeds, lower grinding ratios are produced. The finishes produced are in relation to the grinding ratios. Higher grinding ratios, which mean less wheel breakdown, produce finer finishes. The effect of grinding fluid on hafnium is the same as for other metals. Straight grinding oils produce higher grinding ratios than water miscible fluids at all in feeds.
Belt grinding:
Belt speed and contact wheel selection are two primary considerations when grinding zirconium. Recommended belt speeds are 2,000 to 3,000 sfpm at low grinding pressures with 50 grit and coarser material, and 2,500 to 3,500 sfpm with 60 grit and finer belts with similar working pressure. At high grinding pressures, 2,500 to 3,500 sfpm are recommended with 50 grit and coarser and 3,000 to 4,000 sfpm with 60 grit and finer.
Contact wheels should be relatively hard and aggressive. Soluble oil coolants alone, or mixed with water and applied in a flood are recommended. Resin abrasive cloth may be used with oil and rubber contact wheels on general polishing operations. Resin industrial cloth type 3 or type 6 are recommended for use with oil in grinding operations where high grinding pressures are used. Similarly, waterproof cloth silicon carbide for light work and aluminum oxide for heavy work may be effectively employed with soluble oil and water coolants.
Welding:
Zirconium has better weldability than some more common construction materials, provided that the proper procedure is followed. Proper shielding from air with inert gasses such as argon or helium is very important when welding these metals. Because of the reactivity of zirconium to most gases at welding temperatures, welding without proper shielding will allow the absorption of oxygen, hydrogen and nitrogen from the atmosphere and thus embrittle the weld. Zirconium is most commonly welded by the gas tungsten arc welding (gtaw) technique. Other welding methods used for this material include; gas metal arc welding (gmaw), plasma arc welding, electron beam welding and resistance welding.
The Nuclear Grade Zirconium Alloy Market plays a crucial role in the global nuclear industry, primarily due to its unique properties that make it ideal for use in nuclear reactors. Zirconium alloys, such as Zircaloy, are renowned for their exceptional corrosion resistance, mechanical strength, and low neutron absorption cross-section, which are essential qualities for nuclear reactor components. These alloys are primarily used in fuel cladding, where they provide a barrier between the nuclear fuel and the coolant, ensuring the integrity and safety of the reactor core.