Physical and Nuclear Properties
Zirconium is a transition metal with a density of about 6.49 g/cm3, a melting point of about 1852 degrees C and a boiling point of about 4377 degrees C. It is silver-grey, ductile at room temperature and forms a dense, self-healing oxide film that is the basis of its corrosion resistance. Its thermal expansion is low and its heat transfer efficiency is high, which is why it appears in heat exchangers rather than only in laboratory apparatus.
The property that separates zirconium from other corrosion-resistant metals is its very low absorption cross-section for thermal neutrons, roughly 0.18 barn per atom. Zirconium is therefore almost transparent to neutrons, while the hafnium that occurs with it in nature absorbs them strongly, which is why nuclear grades are specified on a hafnium limit rather than only on zirconium content.
Grades and UNS Designations
Industrial zirconium is ordered in four widely used grades, each with a UNS number that appears on the mill certificate. The grade is chosen for the corrosion duty and for the strength required, not for purity alone.
| Grade | UNS number | Distinguishing feature | Common product forms |
|---|---|---|---|
| Zr702 | R60702 | Purest standard grade; Zr + Hf 99.2 % min with Hf 4.5 % max | Plate, sheet, strip, tube, bar |
| Zr704 | R60704 | Tin-bearing grade, about 1.2-1.7 % Sn, for higher strength | Plate, sheet, tube |
| Zr705 | R60705 | Niobium-bearing grade, 2.0-3.0 % Nb, the strongest of the four | Plate, tube, bar |
| Zr706 | R60706 | Alternative niobium-bearing grade with a different alloy balance | Plate, sheet, tube |
Product forms are covered by separate material specifications: ASTM B551 for plate, sheet and strip, ASTM B523 for seamless and welded tube, and ASTM B550 for bar and wire. A purchase order that names the grade but not the product specification leaves tolerances and test frequency undefined, so both should appear on the order.
Corrosion Behaviour
Zirconium resists hydrochloric acid, nitric acid, most organic acids, chlorides and alkaline media, and it performs well in urea and hydrogen peroxide service. Its limits are just as important as its strengths. It is attacked by hydrofluoric acid and by fluoride-containing solutions, and it is unsuitable in concentrated sulphuric acid, so a heat exchanger specified for a mixed acid stream has to be checked against the fluoride content before the grade is released.
Corrosion resistance depends on the oxide film staying intact. Air or oxygen must therefore be present in many aqueous services, and equipment that runs fully deaerated for long periods can behave differently from the same equipment in an aerated loop.
Fabrication and Mechanical Behaviour
Zirconium is readily hot and cold formed and can be welded under inert gas shielding with matching filler. It has moderate strength and high ductility in the annealed condition, so forming is not usually the limiting step in fabrication; cleanliness is. Iron contamination from steel tools, work benches or grinding wheels causes hydriding and premature failure, so a dedicated area and dedicated tooling are normal practice. A light acid pickling or mechanical cleaning after forming removes embedded iron before service.
Applications
The largest uses are chemical process equipment such as heat exchangers, columns, reactor liners, pumps and valves, especially where hydrochloric acid or chloride-bearing streams make stainless steel impractical. Nuclear applications use hafnium-controlled zirconium alloys for fuel cladding and core internals, and smaller volumes go into medical and specialty applications where biocompatibility and corrosion resistance are both required.
Frequently Asked Questions
Q: What is the difference between Zr702 and Zr705?
A: Zr702 (UNS R60702) is the unalloyed grade with the best general corrosion resistance. Zr705 (UNS R60705) contains 2.0-3.0 % niobium, which raises strength but slightly reduces corrosion resistance in some media.
Q: Why is hafnium limited in zirconium?
A: Hafnium absorbs thermal neutrons strongly. In nuclear service a low hafnium content is required, and even in chemical service the Hf limit is part of the grade definition, for example Hf 4.5 % max in R60702.
Q: Can zirconium be used with hydrofluoric acid?
A: No. Fluoride ions break down the protective oxide film, and zirconium corrodes rapidly in hydrofluoric acid and in fluoride-contaminated streams.
Q: How is zirconium tube ordered?
A: By grade and UNS number, product specification (for example ASTM B523 for tube), outside diameter, wall thickness, temper and test requirements, with the hafnium and oxygen limits stated on the order.
Q: What causes early failure in zirconium equipment?
A: Iron contamination and hydriding are the usual causes. Embedded iron from carbon steel tools or benches creates sites where hydrogen is absorbed and the metal cracks.
Q: Is zirconium magnetic?
A: No. It is a non-magnetic transition metal, which is one practical way to separate it from iron contamination during fabrication checks.