TABLE 4: CORROSION OF TI AND ZR IN SEAWATER (SAMPLES EXPOSED FOR 275 DAYS AT 101C)

LIMITATIONS

Despite the superiority of its corrosion resistance in most chloride salts, zirconium will undergo localized corrosion in oxidizing solutions. The presence of oxidizers, such as ferric or cupric ions, will polarize the metal surface, allowing the breakdown of the passive oxide layer at preferred sites. This can lead to pitting, intergranular corrosion, and/or stress corrosion cracking of zirconium. There are several ways to improve the corrosion resistance of zirconium in oxidizing chloride salts. The zirconium metal can be pickled to give a well-conditioned surface, free from embedded particles such as silicon carbide and aluminum oxide. Since zirconium is much more susceptible to pitting in low pH solutions, maintaining pH in the neutral region can also have a significant impact on its corrosion resistance in solutions containing oxidizing ions (see Table 5). Adding corrosion inhibitors to the chloride salt solutions is another effective means of altering the environment to make it suitable for using zirconium; possible inhibitors include sulfate, nitrate and stannous ions. Finally, electrochemical protection can be used to prevent the breakdown of the passive zirconium oxide film, and thus maintain its favorable corrosion resistance.

TABLE 5: EFFECT OF PH CONTROL ON THE CORROSION OF ZIRCONIUM (NACL SOLUTIONS CONTAINING 500 PPM CU+2)

* = pitting

Four of Five 1 2 3 4 5