Electrochemical study of corrosion phenomena in zirconium alloys

Abstract

Shadow corrosion of zirconium alloy fuel cladding in BWR environments, the phenomenon in which accelerated corrosion is experienced when the cladding surface is in close proximity to other metals, has become a potential life-limiting issue for BWR fuel. Recent results from experimentation at MIT, Halden and Studvik suggest that a galvanic coupling drives the phenomenon between the cladding and the adjacent material. However, the actual processes involved are not understood. One key parameter that would help in the understanding of the phenonenon would be a measurement of the actual corrosion current between fuel cladding and adjacent materials in the actual in-reactor environment. The limitations placed on the burn-up of uranium oxide fuel corelates to the amount of corrosion seen through a directly measurable oxide thickness on the waterside of the zirconium alloy cladding. This oxide corrosion product directly correlates to distance from structure components, leading the effect commonly referred to as shadow corrosion. In recent experiments, Studvik determined that there are large ECP differences associated with Inconel and zirconium alloys that correlate to increased galvanic current density when the materials are coupled. In this thesis research, four electrode pairs wre used to measure galvanic cirrent densities in the irradiation environment:Pt-Pt, Zircalloy (Zr-2), Inconel (X-750)-Pt, and Zr-2-X-750. To determine the changes in the coolant water conductivity dus to the presence of radiolysis products, electrochemical potential mesurements of Pt-Pt coupled electrodes were analyzed. Finally, attempts to characterize the observed oxide behavior using measurement from Electrochemical Impedance Spectroscopy (EIS), also known as Alternating Current Impedance, were conducted. Through the measurements taken, analysis of the mechanisms potentially causing the shadow corrosion phenomenon was conducted. The results of the observations included: Measurement of increased conductivity of coolant water correlating to increases in reactor power, measurement of increased galvanic current measurements correlating to increases in reactor power.