The cause for enhanced corrosion of zirconium alloys by hydrides

The cause of the accelerated corrosion of zirconium alloys by hydrides, is studied by investigating the corrosion of three section planes of Zr-2.5Nb tubes with different texture: the longitudinal normal section (LS) plane, the transverse normal section (TS) plane, and the radial normal section (RS) plane. Corrosion tests were conducted on those section planes taken from the unhydrided and prehydrided Zr-2.5Nb tubes with Lip to 450 ppm, H in water at 350degreesC or in steam at 400degreesC. For Zr-2.5Nb tube with a strong circumferential texture, the deleterious effect of hydrides on enhanced corrosion was most striking an the LS specimen, while beneficial and little hydride effect on the corrosion was observed on the TS and RS specimens, respectively. However, for Zr-2.5Nb tube with a comparatively radial texture, the deleterious effect of hydrides on enhanced corrosion was observed on all the three section planes. The lattice broadening and the interplanar spacing in the zirconium matrix were measured by using X-rays on those section planes taken from Zr-2.5Nb tubes with a circumferential texture before and after charging with hydrogen. The precipitation of hydrides in the Zr-2.5Nb tube subjected the US plane to residual tensile stress expanding the zirconium lattice in the LS, and the TS plane to compressive stress, contracting it in the TS. Based on these results, the corrosion acceleration by hydrides is discussed by correlating the change in the zirconium lattice, distance or lattice distortion including residual stress and the corrosion on each plane before and after charging with hydrogen. This finding leads us to the conclusion that the major controlling factor to the corrosion of zirconium alloys is the lattice coherency between the metal and the oxide.