Zirconium and alloys of it, have found increased use in sulfuric acid service in media where many other engineering materials have fallen short.

The economics of using zirconium are primarily the result of its good corrosion resistance. The corrosion resistance of the common grades of zirconium in aqueous sulfuric acid media is illustrated by the isocorrosion diagrams in figures 1 through 4.

Fig 1 Zirconium in H2S04, constructed from data in literature generated by L.B. Golden, et al, W.E. Kuhn, and C.R. Bishop
Fig 2 Zirconium grade ASTM-UNS R60702 (unalloyed), from recent TWCA tests
Fig 3 Zirconium grade ASTM-UNS R60704 (Zr-Sn-Fe-Cr), from recent TWCA tests
Fig 4 Zirconium grade ASTM-UNS R60705 (Zr-Nb), from recent TWCA tests

Three of these diagrams, as indicated in the captions, were drawn from data generated in TWCA's corrosion research laboratory, and the fourth was drawn from data in published literature. As the diagrams show zirconium is highly corrosion resistant even at concentrations as high as 75% H2SO4 and at temperatures well exceeding the boiling point.

Zirconium's corrosion resistance in H2SO4 is due to the presence of a dense, stable ZrO2 film that forms naturally on the metal surface prior to exposure to the acid. This adherent oxide film protects the base metal from chemical attack, mechanical wear and abrasion.

Zr702, Zr704 and Zr705 are all equally resistant to sulfuric acid until high concentrations and temperatures are reached. At high concentrations and temperatures Zr702 is preferable due to its slightly better corrosion resistance.

In severe applications where acid concentrations and temperatures require improved weld corrosion resistance, the weld areas are heat treated with a full recrystallization anneal. This treatment imparts a corrosion resistance to the welds and HAZ that is very near that of the parent material. Zirconium weld areas in the as-welded condition are less resistant to H2SO4 than the parent material as shown by the dashed lines on the diagrams. This is generally not a problem in many H2SO4 applications since the acid concentrations or temperatures are generally below this level, allowing the use of welded zirconium structures in the as-welded condition.

Care should be taken to avoid the presence of fluoride ions in the H2SO4 environment. The presence of only a few ppm of fluoride ions in the H2SO4 can result in severe corrosion of zirconium. Care also should be taken to avoid concentration or temperature excursions into regions where zirconium begins to corrode. Zirconium that has begun to corrode in H2SO4 can be repaired by totally removing the corrosion product to allow a new oxide layer to form in the air. Repair welding may be desirable in the areas that have thinned excessively.

The oxidizing cupric and ferric ions can increase the corrosion rate of zirconium in H2SO4 if they are present in sufficient amounts. Current testing programs in Wah Chang's corrosion research laboratory are defining the acceptable limits for these ions. These results will be published in a subsequent newsletter.

Zirconium is available in the same mill product forms as other common structural metals such as steel, copper and nickel-base alloys. Both ASTM Grades R60702 (702) and R60705 (705) are ASME Boiler and Pressure Vessel Code qualified structural materials. Zirconium shell and tube heat exchangers have been available for over 10 years. Heat exchangers with lined, solid and clad zirconium tube sheets and vessels are all available from a long list of experienced and qualified fabricators.