Mechanistic Approach to Corrosion of Zirconium by Water - A First Principle Study

Abstract: Zirconium and zirconium oxides are of major technological importance. There are broad applications for these materials, from fuel cell electrolyte to semiconductors and in hip-implants. In nuclear power plants, zirconium alloys are widely used as cladding material to contain the fission fuel in the reactor cores. A limiting factor for fuel longevity is the corrosion properties of the zirconium alloys. The main corrodent in the reactor core is water. The oxidation process of zirconium alloys with water should ideally be accompanied by molecular hydrogen release into the surrounding, but a significant amount of hydrogen is absorbed into the alloy. This process is called hydrogen pick-up, and is along with the oxidation rate decisive to the durability of the cladding. Mechanisms controlling hydrogen pick-up are to a large extent unknown. In this study density functional theory, DFT, is used to gain insights into the mechanism for water induced corrosion of zirconium. The purpose is to build understanding by deconstructing the corrosion phenomenon into computationally accessible and at the same time experimentally relevant quantum chemical modules. Anode and cathode reactions of the system are explored and a charge dependent oxygen vacancy transport through zirconia is identified. A detailed mechanism for electro-catalytic hydrogen evolution is articulated. It comprises formation of a transition metal associated hydride ion that recombines with a proton to form molecular hydrogen. The concentration dependence of the anode potential on absorbed oxygen in the alloy is examined along with the impact of co-absorption of oxygen and hydrogen in the α-Zr matrix. Two channels are taken to jointly constitute the oxidation process: one according to classical oxidation theory involving hydrogen evolution and the second reflected by inwards transport of protons causing hydrogen pick-up. Wagner theory and Tedmon kinetics are modified to include effects of oxide scale charging by augmenting the activation energy for diffusion of charged oxygen vacancies to also include the actual charging upon formation. Hydrogen assisted build-up of nano-porosity is also addressed.