A hydrogen uptake micro-mechanism for Zr alloys

Past attempts to develop a micro-mechanistic model for the hydrogen uptake process during zirconium oxidation have concentrated on solid state diffusion through a barrier oxide. Experimental results have often been invalidated by artefacts. Recently it has become evident that the uptake may be via flaws in the oxide film which penetrate, at least briefly, up to the metal-oxide interface. An attempt to characterise and locate such flaws in oxide films on specimens from a number of batches of several zirconium alloys is reported here, using cathodic polarization to deposit small Cu balls on the active cathodic sites and scanning electron microscopy to identify and characterize these sites. In oxides on Zircaloy-2 samples, the cathodic sites were usually small cracks in the oxide, sometimes (but not always) at pits where intermetallic particles had been etched out during the initial surface preparation. The same cathodic sites could be decorated during successive cathodic polarizations, but even a short subsequent oxidation (1 day, 400 degrees C steam) repassivated all previous sites, and created new ones. In one batch of Zr-2.5%Nb alloy, that contained an array of large carbides, cracks in the oxide at the sites of these particles provided the cathodic sites. In the absence of such particles, far fewer cathodic sites were found in oxides on Zr-2.5%Nb alloy specimens than on Zircaloy-2 specimens. These few sites were always small cracks in the oxide film. Exposure to a low pressure hydrogen atmosphere created many more cracks and cathodic sites in the oxide on Zr-2.5%Nb. (C) 1999 Elsevier Science B.V. All rights reserved.