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Zirconium- Its Applications and Anticorrosion Behaviour

Segun is a registered chemical engineer from the Federal University of Rio de Janeiro (UFRJ). He is also a high school chemistry teacher.

Brief History

Zirconium (Zr) is a metallic element that is extracted from a mineral called zircon, from which its name is derived. It was first identified in Sri Lanka in 1789 by the German chemist Martin Heinrich Klaproth. This mineral contains principally zirconium orthosilicate (ZrSiO4), it was then converted to its oxide.

The metal was later isolated in 1824 in its impure form by the Swedish chemist Jons Jacob Berzelius. Despite this, it remained unknown until the late 1940s, when it became a significant engineering material for nuclear energy applications due to its high transparency to neutrons.

The major countries that produce zircon are Australia, Brazil, China, Russia, the United States, India and South Africa, responsible for over two thirds of the world´s production.


Chemical Industry Application

Due to its high resistance in both acidic and basic media, zirconium alloys are widely used in the chemical industry. An alloy is a mixture containing a metal and another chemical element that may be another metal or a non metal.

Owning to its high corrosion resistance, Zr has widespread use in the fabrication of pumps, pipes, fittings, valves and heat exchangers. It is also used as an alloying agent in the production of some magnesium alloys and as an additive in the manufacture of certain steels as a hardening component. It is also used in surgical instruments.

It is also used in the fabrication of superconductive magnets, as a refractory and an opacifier.


Nuclear Industry Applications

Zirconium is used in the constructon of nuclear reactors.They are used in the cladding, structural components and pressure tubes of the reactors.

At ambient temperature, the passive oxide is about 2-5 nm (10-9 m) thick. The pure metal, low in hafnium (Hf) (0.02% maximum) has a low thermal neutron capture, making it useful in nuclear power applications which helps it in the cladding (coating) of fuel rods thereby facilitating the journey of neutrons. A neutron capture is when an atomic nucleus and one or more neutrons collide to form a heavier nucleus.

Zirconium alloys used in nuclear reactors usually contain iron, chromium and oxygen. The other major components used are tin (Sn) and/or niobium (Nb) which are used in the construction of nuclear reactors like Boiling Water Reactor (BWR), and Pressurized Water Reactor (PWR), usually having the major function of generating electric power in many countries including Russia, Japan, Canada and Argentina.



Table Showing the Resistance Behaviour to Corrosion of Zirconium Towards Some Substances or Reagents.

Zirconium is resistant to the followingZirconium is not resistant to the following

Alkalis, like NaOH and KOH at all temperatures up to boiling point, including fused caustic ones upto the boiling point

Oxidizing metal chlorides, for example, iiron (III) chloride and copper (II) chloride.

Hydrochloric acid (HCl) at all concentrations up to boiling point. Embrittlement of the metal and higher corrosion rates occur above boiling temperatures under pressure.

Hydrofluoric acid (HF) and fluosilicic (H2SiF6) acid.

Nitric acid (HNO3), all concentrations up to boiling point, including red fuming acid. Stress Corrosion Cracking (S.C.C.) may occur under slow strain rate conditions.

Wet chlorine.

Sulfuric acid (H2SO4), below 70% concentration to boiling.

Oxygen, nitrogen and hydrogen at elevated temperatures.

Phosphoric acid (H3PO4), below 55% concentration, boiling point.

Aqua Regia (mixture of 3 parts of HCl acid and 1 part of HNO3)..

Formic, acetic, lactic, and citric acids. (Acids found in some ants, vinegar, milk and citrus fruits like orange and lemon respectively).

Trichloroacetic or oxalic acids, at boiling.

 

Boiling calcium chloride above 55% concentration.

 

Carbon tetrachloride, at 200°C explodes.

Chemical Properties of Zirconium

Zirconium is an active metal in the Electromotive (Em.f.) series, normally exhibiting a very stable passivity.

Zirconium is passive over a very wide range of electric potential and pH.

It reacts readily with oxygen, nitrogen and hydrogen at temperatures above 800°C.

It is a by-product of the mining and processing of titanium minerais, ilmenite and rutile, as well as tin mining.

Physical Properties of Zirconium

Zirconium is a strong, maleable, ductile and lustrous grey-white, transitional metal that resembles hafnium, and, to a lesser extent, titanium.

It is solid at room temperature, with melting and boiling points of 1852°C and 4377°C respectively.

It possesses a density of 6.45 g cm-3, slightly lower than iron (7.88 g cm-3), and greater than water (1.0 g cm-3).

It is regarded as a non toxic chemical element with the symbol Zr, atomic number 40, and an atomic mass of 91.2. It occupies the group 4B (or 4) of the fifth period of the periodic table of chemical elements.


The Temelín Nuclear Power Plant, South Bohemia, Czech Republic, which went into full operation in 2003, using two Russian-designed pressurized-water reactors. © Josef Mohyla/iStock.com

The Temelín Nuclear Power Plant, South Bohemia, Czech Republic, which went into full operation in 2003, using two Russian-designed pressurized-water reactors. © Josef Mohyla/iStock.com

Zirconium Alloys

Although, zirconium alloys do have a good resistance to Stress Corrosion Cracking (S.C.C.), they are vulnerable in many environments. S.C.C. is the growth of crack formation in a corrosive environment. Unalloyed zirconium is known to be resistant to S.C.C. due to its low yield strength, whereas the higher strength alloys are more susceptible. Commercial non-nuclear grade zirconium contains 1-5% hafnium, whose neutron absorption section is about 600 times that of zirconium. For this reason, hafnium must be removed as much as possible (to below 0,02%) from the zirconium. Although this does not affect the corrosion resistance of the zirconium, it does restricts its applications in the nuclear industry. Nuclear grade zirconium alloys contain more than 95% zirconium, therefore, its properties are similar to those of pure zirconium

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