Zirconium alloys are solid solutions of

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...

or other

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...

s, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption

cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

of thermal neutrons, high hardness,

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

and

corrosion resistance Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engin ...

. One of the main uses of zirconium alloys is in nuclear technology, as

cladding Cladding is an outer layer of material covering another. It may refer to the following: * Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...

of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95

weight percent In chemistry, the mass fraction of a substance within a mixture is the ratio w_i (alternatively denoted Y_i) of the mass m_i of that substance to the total mass m_\text of the mixture. Expressed as a formula, the mass fraction is: : w_i = \frac ...

zirconium and less than 2% of tin,

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has si ...

, iron,

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

, nickel and other metals, which are added to improve mechanical properties and corrosion resistance. The water cooling of reactor zirconium alloys elevates requirement for their resistance to oxidation-related

nodular corrosion Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A simi ...

. Furthermore, oxidative reaction of zirconium with water releases hydrogen gas, which partly diffuses into the alloy and forms zirconium hydrides. The hydrides are less dense and are weaker mechanically than the alloy; their formation results in blistering and cracking of the cladding – a phenomenon known as

hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...

.Delayed hydride cracking in zirconium alloys in pressure tube nuclear reactors
Final report of a coordinated research project 1998–2002, IAEA, October 2004

Production and properties

Commercial non-nuclear grade zirconium typically contains 1–5% of

hafnium Hafnium is a chemical element with the Symbol (chemistry), symbol Hf and atomic number 72. A lustre (mineralogy), lustrous, silvery gray, tetravalence, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirco ...

, whose neutron absorption cross-section is 600 times that of zirconium. Hafnium must therefore be almost entirely removed (reduced to < 0.02% of the alloy) for reactor applications. Nuclear-grade zirconium alloys contain more than 95% Zr, and therefore most of their properties are similar to those of pure

. The absorption cross section for thermal neutrons is 0.18

barn A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G. N ...

for zirconium, which is much lower than that for such common metals as iron (2.4 barn) and nickel (4.5 barn). The composition and the main applications of common reactor-grade alloys are summarized below. These alloys contain less than 0.3% of iron and chromium and 0.1–0.14% oxygen. ^*ZIRLO stands for zirconium low oxidation.

Microstructure

At temperatures below 1100 K, zirconium alloys belong to the

hexagonal crystal family In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...

(HCP). Its microstructure, revealed by chemical attack, shows needle-like grains typical of a

Widmanstätten pattern Widmanstätten patterns, also known as Thomson structures, are figures of long nickel–iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons c ...

. Upon annealing below the phase transition temperature (α-Zr to β-Zr) the grains are equiaxed with sizes varying from 3 to 5 μm.

Development

Zircaloy 1 was developed after Zirconium was selected by Admiral H.G. Rickover as the structural material for high flux zone reactor components and cladding for fuel pellet tube bundles in prototype submarine reactors in the late 1940s. The choice was owing to a combination of strength, low neutron cross section and corrosion resistance. Zircaloy-2 was inadvertently developed, by melting Zircaloy-1 in a crucible previously used for stainless steel. Newer alloys are Ni-free, including Zircaloy-4, ZIRLO and M5 (with 1%

Oxidation of zirconium alloy

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 Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...

. Nitride coatings might also be used. Whereas there is no consensus on whether zirconium and zirconium alloy have the same oxidation rate, Zircaloys 2 and 4 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·P^1/6·exp(−1.47/k_BT) The oxidation rate R is here expressed in gram/(cm²·second); P is the pressure in

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

, that is the factor P^1/6 = 1 at ambient pressure; the

activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...

is 1.47 eV; k_B is the

Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...

(8.617 eV/K) and T is the

absolute temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics. Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic wo ...

in kelvins. Thus the oxidation rate R is 10⁻²⁰ g per 1 m² area per second at 0 °C, 6 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.

Oxidation of zirconium by steam

One disadvantage of metallic zirconium is that in the case of a loss-of-coolant accident in a nuclear reactor. Zirconium cladding rapidly reacts with water steam above . Oxidation of zirconium by water is accompanied by release of hydrogen gas. This oxidation is accelerated at high temperatures, e.g. inside a reactor core if the fuel assemblies are no longer completely covered by liquid water and insufficiently cooled. Metallic zirconium is then oxidized by the protons of water to form hydrogen gas according to the following redox reaction: : Zr + 2 H₂O → ZrO₂ + 2 H₂ Zirconium cladding in the presence of D₂O deuterium oxide frequently used as the moderator and coolant in next gen pressurized heavy water reactors that

CANDU The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...

designed nuclear reactors use would express the same oxidation on exposure to deuterium oxide steam as follows: : Zr + 2 D₂O → ZrO₂ + 2 D₂ This exothermic reaction, although only occurring at high temperature, is similar to that of alkali metals (such as sodium or potassium) with water. It also closely resembles the anaerobic oxidation of iron by water (reaction used at high temperature by

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (

Three Mile Island Nuclear Generating Station Three Mile Island Nuclear Generating Station (commonly abbreviated as TMI) is a closed nuclear power plant on Three Mile Island in Londonderry Township, Dauphin County, Pennsylvania on Lake Frederic, a reservoir in the Susquehanna River just ...

in 1979 that did not damage the containment building. This same reaction occurred in

boiling water reactor A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nuc ...

s 1, 2 and 3 of the Fukushima Daiichi Nuclear Power Plant (Japan) after reactor cooling was interrupted by related earthquake and tsunami events during the disaster of March 11, 2011, leading to the Fukushima Daiichi nuclear disaster. Hydrogen gas was vented into the reactor maintenance halls and the resulting explosive mixture of hydrogen with air oxygen detonated. The explosions severely damaged external buildings and at least one containment building. The reaction also occurred during the

Chernobyl Accident The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the No. 4 nuclear reactor, reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian Soviet Socialist Republic, Ukrainia ...

, when the steam from the reactor began to escape. Many water cooled reactor containment buildings have

catalyst Catalysis () is the process of increasing the reaction rate, rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the ...

-based passive autocatalytic recombiner units installed to rapidly convert hydrogen and oxygen into water at room temperature before the explosive limit is reached.

Formation of hydrides and hydrogen embrittlement

Zirconium Hydride in the microstructure of Zircaloy-4

In the above oxidation scenario, 5–20% of the released hydrogen diffuses into the zirconium alloy cladding forming zirconium hydrides.DOE-HDBK-1017/2-93, January 1993
DOE Fundamentals Handbook, Material Science, Volume 2 of 2, U.S. Department of Energy, January 2003, pp. 12, 24. The hydrogen production process also mechanically weakens the rods cladding because the hydrides have lower ductility and density than zirconium or its alloys, and thus

blister A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, usually caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled w ...

s and cracks form upon hydrogen accumulation. This process is also known as

. It has been reported that the concentration of hydrogen within hydrides is also dependent on the nucleation site of the precipitates. In case of loss-of-coolant accident ( LOCA) in a damaged nuclear reactor, hydrogen embrittlement accelerates the degradation of the zirconium alloy cladding of the fuel rods exposed to high temperature steam.Nuclear Fuel Behaviour in Loss-of-coolant Accident (LOCA) Conditions. State-of-the-art Report. OECD 2009, NEA No. 6846. https://www.oecd-nea.org/nsd/reports/2009/nea6846_LOCA.pdf

Applications

Zirconium alloys are corrosion resistant and

biocompatible Biocompatibility is related to the behavior of biomaterials in various contexts. The term refers to the ability of a material to perform with an appropriate host response in a specific situation. The ambiguity of the term reflects the ongoing de ...

, and therefore can be used for body implants. In one particular application, a Zr-2.5Nb alloy is formed into a knee or hip implant and then oxidized to produce a hard ceramic surface for use in bearing against a polyethylene component. This oxidized zirconium alloy material provides the beneficial surface properties of a ceramic (reduced friction and increased abrasion resistance), while retaining the beneficial bulk properties of the underlying metal (manufacturability, fracture toughness, and ductility), providing a good solution for these medical implant applications. Reduction of zirconium demand in Russia due to nuclear demilitarization after the end of the cold war resulted in the exotic production of household zirconium items such as the vodka shot glass shown in the picture.