Corrosion kinetics under high pressure of steam of pure zirconium and zirconium alloys followed by in situ thermogravimetry.
Résumé
A new experimental thermogravimetric device has been installed to study in situ the corrosion behaviour of zirconium alloys under high pressure of steam. Corrosion tests up to 5 MPa of steam pressure have been performed on two materials, pure zirconium and Zircaloy-4 (Zy4), at around 415 degrees C. The rate-limiting step assumption was experimentally verified on Zy4. Unlike pure zirconium, its oxidation rate is not dependent on steam pressure. The experimental result obtained on this material is consistent with an oxygen vacancy diffusion rate-limiting step. For pure zirconium, the kinetic law is nearly linear during the corrosion process, which leads to propose an interface reaction rate-limiting step. Moreover, according to the isotope exchange experiments, the oxygen diffusion in the oxide formed on pure zirconium under high pressure of steam is very fast compared to that in the oxide of Zy4, which supports the thermogravimetric results. Finally, the impact of the SPPs on the corrosion resistance is briefly discussed in the last part of this paper from photoelectrochemical results.