•Corrosion resistance of zirconium alloys is improved by Ge addition.•Corrosion resistance is optimal when Ge contents is 0.05%.•Ge retards the evolution from columnar grains to equiaxed grains.•Ge delays the developing process of defects to form micro-cracks in oxide.•The sub-oxide layer identified as cubic-ZrO was detected at oxide/metal interface.The corrosion behavior and microstructures of the oxide film on Zr-1Nb-xGe alloys (x = 0, 0.05, 0.1 and 0.2, wt%) corroded in deionized water at 360 °C/18.6 MPa were investigated. Results showed that the Zr-1Nb-0.05Ge alloy possessed much better corrosion resistance than the Zr-1Nb alloy. The Ge dissolved in the α-Zr matrix can retard the microstructural transformation of ZrO2 from the columnar grains to equiaxed grains and delay the process of the defects developing into micro-cracks in the oxide. The amorphous phase produced around the [Zr–Nb–Fe–Cr–O] SPPs can improve the corrosion resistance by relaxing the compressive stress in the oxide films. The sub-oxide layer identified to be cubic-ZrO may improve the corrosion resistance after corrosion transition.

•A small amount of Ge and Cu addition can improve the corrosion resistance.•Corrosion resistance is optimal when Cu and Ge content is 0.05%.•Columnar grains to equiaxed grains evolution was retarded by Cu and Ge addition.•Coarse Zr2Cu and Zr3Ge will precipitate by adding more Cu and Ge.•Cracks can be easily formed around the coarse SPPs in oxide layers.The corrosion behavior of Zr-0.7Sn-1Nb-0.2Fe-xGe-xCu alloys (x = 0, 0.05, 0.1, 0.2, wt%) and the microstructures of the oxide film formed on the specimens were investigated in lithiated water at 360 °C by autoclave tests. Results show that the corrosion resistance of the Zr-0.7Sn-1Nb-0.2Fe alloy is remarkably improved with 0.05% Cu and 0.05% Ge addition, since a small amount of Cu and Ge can dissolve in α-Zr matrix and Cu- or Ge-containing Zr(Nb,Fe)2 second phase particles(SPPs) can delay the microstructural evolution of oxide film. But the coarse tet-Zr2Cu and tet-Zr3Ge SPPs, which will be formed by the further increase of Cu and Ge content, are disadvantageous to the corrosion resistance of the alloys.

•Corrosion resistance of zirconium alloys is improved by Ge addition.•Corrosion resistance is optimal when Ge content is 0.1 wt%.•There are Zr(Fe,Cr,Nb)2, Zr(Fe,Cr,Nb,Ge)2 and Zr3Ge SPPs in alloys.•Ge retards the evolution from columnar grains to equiaxed grains.•Ge delays the developing process of defects to form micro-cracks in oxide.The results of corrosion tests conducted in lithiated water with 0.01 M LiOH at 360 °C/18.6 MPa revealed that the corrosion resistance of the Zr–0.7Sn–0.35Nb–0.3Fe alloy in wt% was considerably improved by Ge addition of 0.05–0.2 wt%. To further understand the mechanism on the effect of Ge addition on the corrosion resistance of the Zr–0.7Sn–0.35Nb–0.3Fe alloy in wt%, the microstructures of alloys and oxides were investigated by transmission electron microscopy and scanning electron microscopy. For the Zr–0.7Sn–0.35Nb–0.3Fe–0.2Ge alloy, the outer layer of oxide mainly consisted of equiaxed grains, while the middle layer of oxide mainly consisted of many columnar grains in a direction perpendicular to the oxide/metal interface. The suitable amount of Ge can retard the evolution from the columnar grains to the equiaxed grains and delay the developing process of the defects to form the micro-cracks in oxide. The amorphous phase produced around the [Zr(Fe,Cr,Nb,Ge)2]O SPPs could relax the compressive stress, which could improve the corrosion resistance of zirconium alloy. The corrosion resistance could be degraded by the formation of cracks from the coarse [Zr3Ge]O SPPs towards the outer surface of oxide layers.

The corrosion resistance of a Zr–0.7Sn–1Nb–0.03Fe–0.2Ge (wt%) alloy was investigated by autoclave test in lithiated water with 0.01 M LiOH at 360 °C under a pressure of 18.6 MPa. The microstructure of the oxide film which formed was examined by TEM and SEM. The results revealed that there were a few micro-cracks and more ZrO2 columnar grains in the oxide film formed after exposure for 190 days. The oxidation of second-phase particles (SPPs) was slower than that of α-Zr matrix. The c-ZrO2 was observed around the [Zr–Nb–Fe–Cr–Ge]O SPPs. The amorphous phase produced around the [Zr–Nb–Fe–Cr–Ge]O SPPs could relax the stress in the oxide film. The addition of Ge can reduce micro-pores and micro-cracks formed in oxide film, and delay the microstructural evolution from columnar grains to equiaxed grains. Therefore, the addition of Ge can improve the corrosion resistance of the Zr–0.7Sn–1Nb–0.03Fe alloy.