Effect of carbides on the hydriding and oxidation behavior of a Zr-2.5Nb alloy

The surface carbide layer, interior chain-like carbide arrangements and the carbide/matrix interfaces present in an initial Zr-2.5Nb alloy were studied in hydriding and oxidation experiments. Hydriding performed by cathodic charge in 0.1 M H ₂SO ₄ solution showed that neither the inner carbide chains nor the corresponding interfaces offered short circuit paths for hydrogen diffusion, while the surface carbide layer acted as a protective layer. The oxidation tests performed in autoclaves during 24 h in deaerated steam at 400°C and 105 bar showed weight gains higher than one order of magnitude with respect to the samples free of carbides. The surface carbides promoted the accelerated oxidation of the surrounding matrix and were stripped during the experiments. Evidence was found of the oxidation of the inner carbides, probably due to the transformation strain energy produced by the former oxidation of the surrounding matrix. Moreover, the carbide/ matrix interfaces acted as short circuit paths for oxygen diffusion.