Figure 2. Zirconium in H2SO4: Heat Treatment Relieves Weld Worries
Zirconium is vulnerable to attack at high sulfuric acid concentrations, as precipitates form within the weld. Heat treatment will improve resistance.
Figure 3. Zirconium in HCI: Vulnerable to High Concentrations
Zirconium is vulnerable at high HCl concentrations. For equipment being developed for HCl applications, care must be taken to ensure that the metal surface does not come in contact with iron or copper during fabrication. A chemical conditioning pre-treatment should be used to be on the safe side.
HCl: Hold the ionsZirconium is used in numerous processes that involve hydrochloric acid, and the metal's corrosion resistance approaches that of tantalum in pure HCl media. Applications include production of azo dyes and fine chemicals, geothermal applications to acidify salt solutions and in hydrochloric evaporators.
You'll note in Fig. 3, however, that the metal is susceptible to localized corrosion attack, including pitting, stress corrosion attack and intergranular attack in oxidizing chloride solutions. Zirconium should never be used where ferric or cupric ions are present.
In addition, care should be taken during the fabrication of zirconium vessels destined for hydrochloric acid service to ensure that no iron or copper is embedded on the metal surface. Consider chemical conditioning pre-treatment prior to placing zirconium equipment into hydrochloric acid service.
Chloride salt resistanceIn chloride salt solutions, most materials are susceptible to localized corrosion attack such as pitting, crevice corrosion or stress corrosion cracking. Zirconium, however, has good resistance to severe chloride environments, even at elevated temperatures. In addition to seawater, brackish and pure waters, zirconium is very resistant to a wide range of chloride salt solutions. Zirconium has limited use, however, in ferric or cupric chloride solutions and will undergo localized corrosion, including pitting or intergranular attack and stress corrosion cracking in these solutions. The presence of the ferric or cupric ions will cause the localized breakdown of the metal's passive oxide layer.
Excellent with organicsZirconium offers excellent resistance to organic media including formic acid, acetic acid, hydroxyacetic acid, lactic acid and urea. Process vessels for the production of acetic acid have been the single largest organics application for zirconium. Only a few conditions will cause zirconium to corrode in acetic acid. These include the presence of cupric ions, free chlorine and extremely low water content. Under highly stressed conditions, a minimum of 650 ppm water is needed to prevent stress corrosion cracking in acetic acid.
Zirconium also is highly resistant to attack by caustics, including sodium and potassium hydroxide, allowing design of equipment that simultaneously resists acidic and alkaline environments. In summary, zirconium can be a cost-effective, highly corrosion-resistant material of construction for chemical processing applications. It has been successfully applied in many severely corrosive environments. In many cases, the metal has not only helped reduce downtime, but lowered maintenance costs, increased process efficiency and eliminated corrosion products in the final product.
Richard Sutherlin is manager of technical services at Wah Chang. A Metallurgical Engineer and a registered Professional Engineer in the state of Oregon, he has more than 25 years experience in the field of corrosion, welding and fabrication of reactive and refractory metals.
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