Sulfuric acid is one of the most complicated corrosives for passive alloys to handle. It gradually changes from the highly reducing character of dilute solutions to the strongly oxidizing character of concentrated solutions. It becomes even more complicated when other variables, such as temperature, pressure and impurities, are involved. The usefulness of many alloys in sulfuric acid is restricted to some specific conditions, such as low concentration, high concentrations, or low temperatures. The corrosion of zirconium in sulfuric acid solutions is rather straightforward. It resists attack by sulfuric acid at all concentrations up to 70% and at temperatures to boiling and above. In 70 to 80% sulfuric acid, the resistance of zirconium depends strongly on temperature. In higher concentrations, the corrosion rate of zirconium increases rapidly with increasing concentration. Certainly, the performance of zirconium in sulfuric acid solutions is affected not just by concentration and temperature, but also by many other factors. When these factors are properly addressed, zirconium can perform well in many sulfuric acid solutions. This paper discusses metallurgical, environmental, mechanical, design, fabrication, operational and maintenance issues that affect the performance of zirconium in sulfuric acid solutions. Examples will be given of the applications of zirconium in sulfuric acid service.
Sulfuric acid is one of the most important industrial chemicals for use in a wide range of chemical processes. For example, the acid is used as a steel pickling solution, a dehydrating agent, an oxidizer, a catalyst, and a reagent in chemical syntheses. Sulfuric acid is very difficult for many metals and alloys to handle. The usefulness of most alloys is restricted to some specific conditions, such as low concentrations, high concentrations or low temperatures. Table 1 indicates concentration and temperature limits for zirconium and various other alloys. This difficulty can be attributed to the ever changing nature of the acid and the high affinity of most metals for sulfur and sulfur compounds. Unlike the other acids, which are either distinctively reducing like hydrochloric acid or oxidizing like nitric acid, sulfuric acid changes gradually from reducing to oxidizing with increasing concentration. Table 1 gives the limits for the reducing or oxidizing nature of sulfuric acid1.
Figure 1, Metals suitable for use in sulfuric acid.(available in full paper)
The Limits for the Reducing or Oxidizing Nature of Sulfuric Acid(available in full paper)
Zirconium is one of the few metals that resists attack by sulfuric acid at all concentrations to 70% and at temperatures to boiling and above. In 70 to 80% sulfuric acid, zirconium?s corrosion resistance depends strongly on temperature. In > 80% sulfuric acid, the corrosion rate of zirconium increases rapidly with increasing concentration. The suitable domain for zirconium falls well within the reducing limits for sulfuric acid as given in Table 1. Often, reducing acids are corrosive to common passive alloys but are compatible with zirconium. Nevertheless, there are many factors affecting the performance of zirconium in sulfuric acid2. This paper collects recently generated data to discuss metallurgical, environmental, mechanical, design, fabrication, operational and maintenance issues that affect the performance of zirconium in sulfuric acid solutions.
