SEM examination of the oxide-metal interface formed during the aqueous corrosion of a Zr-2.5 wt% Nb alloy

Abstract

The oxide-metal interface formed during the aqueous corrosion of a Zr-2.5 wt% Nb alloy has been studied by SEM, using a specimen preparation method which also has potential application to other materials containing an oxide-metal interface. The oxidation of cold-worked Zr-2.5 wt% Nb pressure tubing in pressurized lithiated water proceeds first along grain boundaries at which there is β-zirconium or its decomposition products, and then continues on the α-zirconium grains. Oxides at the oxide-metal interface formed in an aqueous environment were mainly extended along grain boundaries, and were characterized by long filaments which consisted of many fine zirconia grains. This oxidation behaviour is attributed to short-circuit diffusion at the grain boundaries which is caused by the nature of the crystallite boundaries of the oxide, the flaws arising from the oxidation of the grain boundary phases (β-Zr, its decomposition products as well as impurities), and cracking of the oxide due to phase transformations in ZrO₂. When different plane sections of the tubing are corroded, there are different corrosion rates, with the sections containing a higher area fraction of the grain boundaries with β-zirconium exhibiting higher corrosion rates. The formation of long ‘fingers’ of oxide made up of longer filaments of oxide, together with the higher corrosion rates in the post-transition specimens, result in the destruction of the barrier oxide layer and an increase in the oxide rate.