LIMITATIONS

Despite the superiority of its corrosion resistance in HNO3, zirconium is not suited for all nitric acid environments. As mentioned previously, stress corrosion cracking becomes a concern at HNO3 concentrations above 70% or at 70% HNO3 and elevated temperatures. This can be overcome with proper equipment design and heat treatment for stress relief.

Zirconium may also suffer pitting in the vapor phase of nitric acid and chloride mixtures. Chlorides could be oxidized to produce chlorine gas, which may induce pitting on zirconium. Giving zirconium a good surface finish, using methods such as pickling, can prevent this problem.

Finally, the presence of fluoride ions in a nitric acid solution can significantly increase the corrosion rate of zirconium. The fluoride ions tend to form undissociated hydrofluoric acid in HNO3, with the corrosion rate of zirconium being directly proportional to the HF concentration. Adding an inhibitor to convert fluoride ions into non-corrosive complex ions can control this problem. Several compounds, including zirconium sponge and zirconium nitrate, can be used as inhibitors.

SAFETY

There is a special safety concern when using zirconium. Reactive metals like zirconium can develop pyrophoric films. Normally zirconium corrodes uniformly and all the zirconium is converted to zirconium oxide. If corrosion rates are low, <5 mpy, there is time to react all the zirconium uniformly. For very high corrosion rates, >200 mpy, the reaction rate is so high that all zirconium is also reacted.

At certain conditions, it is possible that the corrosion rate will attack grain boundaries and continue attacking the boundaries, trapping small pieces of Zr grains in the oxide and not completing the oxidation. Under these conditions, the oxide film may be pyrophoric. To passify the zirconium, the trapped zirconium pieces need to be completely oxidized before opening the equipment to air. This is achieved by passing hot air or steam through the equipment to make sure all the zirconium in the oxide film is reacted before opening to air. At 250¡C, air must flow for 20 minutes or steam for 30 minutes; at lower temperatures, several days of treatment are required.

SUMMARY / CORROSION LAB SERVICES AND OTHER WAH CHANG RESOURCES

As demonstrated above, zirconium can be the best alternative for material selection in many nitric acid applications. Longer equipment life, reduced maintenance downtime, and higher purity product streams are all possible with the proper application of zirconium, making it the most cost-effective option when compared with other alloys. Although zirconium has proven its outstanding corrosion resistance performance in a wide variety of HNO3 environments, the best way to determine zirconiumÕs suitability for a particular environment is to perform a corrosion test. Zirconium corrosion test kits are available from Wah Chang for use in on-line process equipment. These tests can show how zirconium will hold up under actual process conditions. Wah Chang also has a fully capable corrosion laboratory for complete testing and detailed analysis for specific nitric acid applications.

For further information or any questions regarding the use of zirconium in nitric acid applications, please contact the Technical Services Division at Wah Chang, phone 541-917-6777 fax 541-967-6987.

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