LIMITATIONS

While zirconium does exhibit superior corrosion resistance in most sulfuric acid environments, there are a few factors that limit zirconiumÕs effectiveness. The presence of small amounts of fluoride ion can dramatically increase the corrosion rate of reactive metals like zirconium. Figure 4 shows the impact.

FIGURE 4: EFFECT OF FLUORIDE IONS ON THE CORROSION OF Zr 702 IN BOILING SULFURIC ACID

Fluoride is not the only impurity that must be controlled. The presence of oxidizing impurities will increase the corrosion rate of zirconium in sulfuric acid as shown in Figure 5.

FIGURE 5: EFFECT OF IMPURITIES ON CORROSION OF ZIRCONIUM (5 MPY ISO-CORROSION CURVES)

Generally, the presence of non-oxidizing chlorides will not have an effect on corrosion rate of zirconium. Sulfate ions will inhibit pitting of zirconium when oxidizing halides are present. For example, if the ratio of sulfate to iron chloride is 100:1, the iron chloride will not cause pit corrosion. Figure 6 shows the affect of chloride ions in sulfuric acid on the corrosion rate of zirconium. It is excess of sulfate to chloride ion.

Five of Six 1 2 3 4 5 6