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FIGURE 6: EFFECT OF CHLORIDE IONS ON CORROSION
OF ZIRCONIUM
Finally, the amount of tin present
in zirconium is also a factor in sulfuric acid applications.
While tin is not usually a problem in most environments, it
should be controlled when zirconium is to be used in sulfuric
acid. The ASTM specification for Zr 702 allows tin at a level
of 500 - 5000 ppm. When the zirconium is to be used in concentrations
of sulfuric acid above 40%, the amount of the tin in the ingot
should be 1500 - 2500 ppm; Wah Chang makes a special alloy,
Zircadyne 702-S, for this purpose.
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 steam or hot air at 240° C
for 20 minutes or 120° C for 3 days through the equipment to
make sure all the zirconium in the oxide film is reacted before
opening to air.
SUMMARY / CORROSION LAB AND OTHER WAH CHANG RESOURCES
As demonstrated above, zirconium can
be the best alternative for material selection in many sulfuric
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 sulfuric acid environments,
the best way to determine zirconium's 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 sulfuric acid applications.
For further information or any questions regarding the use
of zirconium in sulfuric acid applications, please contact
Technical Services at Wah Chang, phone 541-917-6777 fax 541-967-6987.
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