Behavior of a barrier layer of corrosion films on zirconium alloys

Abstract

Capacitances of oxide films obtained on a EZ-1 alloy during the corrosion tests in water-vapor environment at 300, 350, and 400°C are measured. In order to estimate the thickness of a barrier layer, the barrier potential at a constant anodic current (dielectric puncture potential) is measured. It is shown that the barrier layer cannot be treated as a homogeneous environment. At the sites of intermetallide inclusions in the oxide film, the thickness of the dielectrics is locally decreased. In the first approximation, the heterogeneity of the oxide film can be taken into account by inserting two parallel RC subcircuits in the equivalent scheme. One subcircuit (C ₁, R ₁) describes the electrophysical properties of the capacitance whose insulator thickness corresponds to the total thickness of the oxide film. The other subcircuit (C ₂, R ₂) describes the electrophysical properties of the nonporous part of the oxide film between the intermetallide particles and the outer surface. Then, the results of measurements can be written as follows: C _exp = θC ₂ + (1 − θ)C ₁, where θ is the surface part of the oxide film whose dielectric properties are changed due to the inclusion of intermetallide particles. Assuming that the mean spatial size of intermetallide particles falls in the range of 200–400 nm, one can estimate the mean concentration of the particles on the metal surface in agreement with the metallographically determined concentration of the second-phase particles (approximately 10⁶–10⁷ cm⁻²). The obtained results indicate the substantial heterogeneity of the barrier layer structure, which may cause local corrosion and premature failure of zirconium items.