Crystallization and degradation of zirconium oxide in various pH solutions

A series of modeling experiments was conducted to understand crystallization and degradation behavior of zirconium oxide occurring during corrosion of zirconium alloys. The crystallization of zirconia from amorphous Zr(OH)(4) was investigated through its hydrothermal treatment in various metal hydroxide solutions with different pH at 250 degrees C for 6 h in mini-autoclaves. The degradation behavior of the tetragonal zirconia stabilized by 3 mol% Y2O3 was investigated at low temperatures of 90-200 degrees C in water and LiOH solution with 3.5 and 350 ppm Li, respectively. It is demonstrated that pH is a main factor governing the crystallization of zirconia, leading to 35% tetragonal and 65% monoclinic zirconia at pH 7-10 and 100% monoclinic zirconia at pH > 12. Therefore, the change of the corrosion resistance of zirconium alloys with pH at the beginning of corrosion is suggested to be attributed to the crystallization behavior of the zirconia varying with pH. On the other hand, the tetragonal zirconia degraded due to the tetragonal-monoclinic transformation, which was accelerated only in LiOH solution, not in LiNO3 or KOH. On the basis of this finding, the accelerated corrosion of zirconium alloys in LiOH solutions is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.