Oxide microstructures of advanced Zr alloys corroded in 360 °C water loop

The corrosion behavior of Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr (HANA-4), Zr–1.1Nb–0.05Cu (HANA-6) and Zr–1.3Sn–0.2Fe–0.1Cr (Zry4) in weight percent was investigated in a 360 °C water loop containing 2.2 ppm Li and 650 ppm B. The corrosion rate of the HANA-4 and HANA-6 alloys was much lower than that of Zry4 and it increased with an increase of the final annealing temperature. Oxide microstructure observations revealed that all the oxides had a layer structure regardless of the alloy composition. The thickness of the fully developed layer was the highest in the 470 °C annealed HANA-6 which showed the best corrosion resistance among the alloys tested. The oxide consisted mainly of columnar grains in the interface region and equiaxed grains in the outer surface region. The lateral cracks were only observed in the regions where the equiaxed grains were dominant. The grain morphology was changed periodically from the interface to the outer surface, which was well correlated with the layer structure and the transition behavior in the corrosion kinetics. The precipitates incorporated into the oxide maintained the crystallite in the interface region but transformed to the amorphous state by an oxidation when far away from the interface. The precipitate was not fully oxidized even at distances very close to the outer surface of the oxide.