ADVANTAGES OF Zr OVER OTHER MATERIALS

Because of its passivating power, nitric acid is not considered to be a difficult acid for passive metals to handle. Nevertheless, it does become highly corrosive to most metals at elevated temperatures, very high or very low concentrations, or when impurities are present. Zirconium is considerably more suitable than most passive metals for handling HNO3 under these conditions, and its performance is even better than the noble metals in some cases (Fig. 2).

FIGURE 2

Stainless steel has long been used in nitric acid plants, and it does exhibit good corrosion resistance over a wide range of concentrations and temperatures in nitric acid. There are still considerable problems with stainless steel, particularly in pressurized HNO3 at elevated temperatures. As shown in Figure 3 and Table 4, stainless steel suffers significant corrosion as nitric acid concentration increase. The presence of chromium in certain products resulting from the corrosion of stainless steel has also become a serious concern, as this poses an environmental hazard. Data for specifically selected competitor alloys is listed in Table 5.

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