Volume 5, Issue 5 (May 2008)
Corrosion and Oxide Properties of HANA Alloys
The corrosion behavior of HANA alloys has been investigated at 360°C in a pure water autoclave as well as in a pressurized water reactor (PWR)-simulating loop. The grain morphology and the incorporated precipitates of the oxide were characterized to elucidate the corrosion mechanism of HANA alloys. HANA alloys showed a much lower corrosion rate when compared to Zircaloy-4 in the investigated corrosion environments with HANA-6 (Zr-1.1Nb-0.05Cu), revealing a lower corrosion rate than HANA-4 (Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr). The corrosion rate was increased with an increase of the final annealing temperature of the alloys and the dissolved oxygen content in the corrosion environment. The oxide grain morphology of the HANA alloys was changed periodically from equiaxed grains to columnar ones during its growth to form a layered structure of the oxide. The change of the oxide grain morphology correlated well with the transition of the corrosion kinetics. It was revealed that the fully developed layer was thicker and the columnar grain size was larger in the protective oxide with a lower corrosion rate. The precipitates of HANA alloys were more finely distributed and oxidized more slowly in the oxide when compared to those of Zircaloy-4. Such oxidation characteristics of the precipitates along with the reduction of the Sn content were considered to be more favorable for growing an oxide to a greater thickness before transition and increasing the columnar grain size of the oxide, thereby improving the corrosion resistance of HANA alloys.