There are many severe corrosive environments that require the use of highly corrosion resistant materials, like zirconium. Richard C Sutherlin reports.
Zirconium has been used in the chemical processing industry for more than 40 years and is commonly used in the production of organic acids such as acetic acid and formic acid. It is also used in chemical processes utilizing mineral acids, such as nitric, sulphuric and hydrochloric acid. It also is extremely resistant to hydrogen peroxide and urea applications.
Zirconium's unique corrosion properties will allow its use in the full pH range from strong acids to strong caustics. These attributes of zirconium will allow this material to be used in a variety of severe environments that would destroy other materials of construction.
Zirconium for the chemical processing industry is available in three primary alloys, Zr700, Zr702 and Zr705. Two of these alloys (Zr702 and Zr705) are currently approved as a material of construction for use in the ASME Boiler and Pressure Vessel Code.
Unalloyed Zr700 zirconium has the lowest strength and is used primarily for explosion cladding to carbon steel and stainless steel plates. Zr702 alloy is used for pressure vessels, columns, tubing, piping, trays, internals, and other ancillary equipment. The Zr705 alloy, with 2-3 per cent niobium has higher strength than the other CPI alloys and is used primarily for fasteners, pumps, valves, etc.
Zirconium is highly resistant to nitric acid, hydrochloric acid and sulphuric acid. The iso-corrosion curves in Figs. 1 and 2, show the resistance of zirconium to these mineral acids.
In nitric acid, zirconium is resistant in the full range of acid concentrations to over 95 per cent and temperatures well above boiling.
Even in the area where zirconium shows susceptibility to stress corrosion cracking, proper design and stress relief heat treatments mitigate the susceptibility for this localised attack at the higher concentrations.
Applications for zirconium include cooler condensers, boiler feed water and tail gas heaters in the production of nitric acid for fertilisers.
In hydrochloric acid environments, zirconium has shown good corrosion resistance through the full concentration range and above boiling temperatures.
Zirconium is however susceptible to localise attack when ferric or cupric ions are present. Applications for zirconium include the production of azo dyes, fine chemicals, herbicides and pesticides.
In sulphuric acid environments, zirconium has excellent resistance through the full range of concentrations up to 70 per cent and temperatures well above boiling. Although the material is commonly used in the lower to middle concentrations for the heating of acid pickling solutions and in the production of acrylic monomers, it is also one of the few materials able to resist the 60-70 per cent acid above boiling in the production of isopropyl and secondary butyl alcohols.
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Richard C Sutherlin is PE Manager, Technical Services, ATI Wah Chang, Albany, OR, USA. www.ATImetals.com