INTRODUCTION

Nitric acid is one of the most widely used acids in the Chemical Processing Industry. It is a key raw material in the production of ammonium nitrate for fertilizer, and is also utilized in a variety of manufacturing processes, including the production of industrial explosives, dyes, plastics, synthetic fibers, metal pickling and the recovery of uranium. As the demands on nitric acid process equipment have increased, material selection for that equipment has become crucial. Suitable structural materials need to be cost-effective, reliable, durable, efficient, and non-contaminating to the product and the environment. For over 20 years, zirconium has proven to be the best solution for many nitric acid applications (Table 1).

Most nitric acid is produced by the oxidation of ammonia with air over platinum catalysts. The resulting nitric acid is further oxidized into nitrogen oxide and then absorbed into water to form HNO3. This process produces acid of up to 70% concentration, with higher concentration acid produced by distilling the dilute acid with a dehydrating agent. Stainless steel has long been used in nitric acid applications; however, it has developed certain serious problems over the years and is subject to several limitations. The superior corrosion resistance of zirconium can overcome some of these limitations, making it an ideal replacement material in many specific nitric acid environments.

TABLE 1: MAJOR NITRIC ACID PROCESSES SUITABLE FOR ZIRCONIUM USE

CORROSION DATA

The excellent corrosion resistance of zirconium in nitric acid has been recognized for more than 30 years. As shown in the isocorrosion diagram (Fig. 1), the corrosion rate of zirconium is conservatively stated as less than 5 mpy (mils per year) at concentrations up to 98% HNO3 and temperatures below the boiling point, as well as 70% HNO3 and up to 250¡C.

One of Seven 1 2 3 4 5 6 7