Zirconium Corrosion Resistance in Nitric and Phosphoric Acid Service

Why Zirconium Is Chosen for Aggressive Chemical Service

Zirconium is a reactive metal whose corrosion resistance comes from a thin, tenacious and self-repairing oxide film. Once that film has formed in air or in an oxidising medium, the metal withstands many process streams that destroy carbon steel and even stainless steel. Unalloyed industrial zirconium is supplied as strip, sheet and plate to ASTM B551/B551M, as bar and wire to ASTM B550/B550M, and as seamless or welded tube to ASTM B523/B523M, so it can be specified as a fully controlled material of construction for pressure equipment, heat exchangers, columns, piping and pumps in chemical plants.

Corrosion Behaviour in Nitric Acid

Zirconium has good corrosion resistance in 95% nitric acid below 200 °C. The passive film stays stable as long as the acid remains oxidising and retains a minimum water content. In dry red fuming nitric acid the situation is different: as with titanium, rapid oxidation and even explosive reaction can be triggered when the water content and the oxygen to nitrogen oxide balance fall outside the safe window. The established countermeasure is to keep a defined balance of water vapour and oxygen in the vapour space above the liquid rather than relying on the metal alone.

Uniform corrosion rates in nitric acid are low, yet zirconium is susceptible to stress corrosion cracking. The risk has to be assessed across the whole range of nitric acid concentrations and temperatures of interest, in concentrated as well as dilute media, and it is controlled mainly through metallurgy and fabrication discipline: fully annealed material, low residual stress and crack-free welds.

The practical consequences for equipment design are straightforward. Vapour spaces should be evaluated as carefully as the liquid, since condensate chemistry can differ sharply from bulk acid chemistry, and areas where liquid can evaporate and concentrate deserve particular attention.

Corrosion Behaviour in Phosphoric Acid

Zirconium is corrosion resistant in boiling phosphoric acid at concentrations below 60%. At 60% H3PO4 the corrosion rate increases sharply as temperature rises, so the usable window narrows quickly above that concentration. To judge whether zirconium suits a given phosphoric acid duty, the actual composition of the medium must be determined first, because it is normally the minor constituents rather than the acid itself that decide the outcome.

Fluoride (F-) is markedly unfavourable to the corrosion resistance of zirconium and has to be tightly limited, while P2O5 present in the process liquor acts as an effective corrosion inhibitor. Wet-process phosphoric acid plants therefore compare the full liquor analysis, the temperature profile and the expected fluoride level before committing to zirconium for heat exchangers, reboilers, agitators or piping.

Service Envelope at a Glance

Medium and condition	Zirconium behaviour	Design note
95% nitric acid, below 200 °C	Good corrosion resistance	Oxidising, water-bearing acid keeps the passive film stable
Dry red fuming nitric acid	Risk of rapid oxidation	Maintain a balanced water vapour and oxygen level in the vapour space
Nitric acid, concentrated or dilute	Low uniform rate, cracking possible	Fully anneal, avoid residual tensile stress, qualify welding
Phosphoric acid below 60%, boiling	Corrosion resistant	Confirm liquor composition before selection
Phosphoric acid at 60% and rising temperature	Corrosion rate rises sharply	Set a conservative maximum operating temperature
Phosphoric acid containing fluoride	Attack accelerated by F-	Limit fluoride; P2O5 content acts as an inhibitor

Fabrication and Design Practice

Fully anneal formed and welded components to remove residual tensile stress and reduce cracking risk.

Weld under inert gas shielding with qualified procedures, and keep heat input and interpass temperature under control.

Exclude fluoride-bearing cleaning agents, pickling residues and marking inks from the surface of finished equipment.

Design for complete drainage and avoid crevices and stagnant zones where aggressive species can concentrate.

Account for hydrogen uptake and hydride formation when equipment operates in wet, hydrogen-charged or cathodically protected service.

Check thermal expansion, low thermal conductivity and galling tendencies when selecting tube-to-tubesheet joints and fasteners.

Typical Applications

Zirconium equipment is used for nitric acid coolers and condensers, urea and ammonium nitrate plant equipment, phosphoric acid heat exchangers and reboilers, organic chemical reactors, piping and valves handling hot oxidising acids, and hydrochloric and sulfuric acid service where the concentration and temperature rule out cheaper alternatives. The same combination of corrosion resistance and heat transfer performance also supports use in specialty instruments, chemical pumps and laboratory apparatus.

Frequently Asked Questions

Q: Is zirconium resistant to nitric acid at all concentrations?
It resists 95% nitric acid below 200 °C well, but dry red fuming nitric acid requires vapour-phase control, and stress corrosion cracking must be considered across concentrated and dilute duties alike.

Q: Why is stress corrosion cracking a concern in nitric acid?
Because the uniform corrosion rate is low but the alloy can crack under tensile stress in HNO3 service, so residual stress and weld quality, not only corrosion allowances, govern equipment life.

Q: How does fluoride affect zirconium in phosphoric acid?
Fluoride strongly accelerates attack and must be limited, while the P2O5 naturally present in process acid acts as an inhibitor and improves the outlook.

Q: At what phosphoric acid concentration does zirconium stop being suitable?
It is corrosion resistant in boiling acid below 60%; at 60% the corrosion rate rises sharply with temperature, so higher concentrations need a case-by-case evaluation of the real liquor.

Q: Which standards cover industrial zirconium mill products?
ASTM B551/B551M covers strip, sheet and plate, ASTM B550/B550M covers bar and wire, and ASTM B523/B523M covers seamless and welded tube for general corrosion-resistant service.

Q: What is the single most useful step before ordering zirconium equipment?
Establish the true composition of the process medium, including fluoride, water and inhibitor levels, at every temperature the equipment will see, including shutdown and cleaning conditions.