Influence of thermal processing and microstructure on the corrosion behaviour of Zircaloy-4 tubing

To minimize the corrosion, and hence the fuel cycle costs, of Zr-4 cladding, optimization of thermal processing and microstructure of the tubing is a necessity. This investigation was carried out in order to correlate the effect of thermal processing and microstructure of Zr-4 tubing with the 400 deg. C steam corrosion behaviour in a refreshed autoclave. To achieve this a number of different intermediate annealing treatments were performed on various specimens. It was found that shorter-lower temperature heat treatments at the intermediate annealing stage resulted in poor corrosion resistance when compared with longer-higher temperature treatments. A critical heat treatment existed which had to be exceeded to obtain good corrosion behaviour. For the given composition there was a saturation value of the corrosion behaviour, i.e. a minimum weight gain which was always obtained. The corrosion behaviour was modelled to the annealing parameter A=t exp (-Q/RT), which can be interpreted as a normalized annealing time. The factor governing the corrosion appeared to be the matrix solute content rather than the intermetallic particle size. The activation energy determined from the model, Q=63,000 cal/mol, did not correspond to any simple self-solute diffusion process. (author)