Corrosion of Zr Alloys in Water and Steam

Abstract

The oxide, Zr0 2 , is formed by the reaction of Zr with water. Water molecules dissociate on the oxide surface, the oxygen formed diffuses through anion vacancies in the oxide to the metal-oxide boundary, where part of the oxygen dissolves in the metal and part reacts to form new oxide. As the oxide has a greater volume than the metal from which it was formed, strain is produced at the boundary. Hydrogen formed in the reaction partly diffused through the oxide into the metal.The process of oxidation of Zr and its alloys in water and steam passes through several stages with correspondingly different laws for the rate of oxide growth as a function of temperature and the corrosion exposure time. In the initial stage, especially at low temperatures, the oxide growth has a complex character, controlled by either a logarithmic or parabolic rate law. This period is a result of localized oxidation of the grain boundaries and of intermetallic species, preferential oxidation of some grains, etc.At temperatures above 300 °C, the oxidation rate is controlled by a cubic or parabolic law. Black, adhering and non-stoichiometric Zr0 2 is also formed. After a certain period of time, given by the oxidation conditions, a change occurs in the oxidation kinetics, appearing as a break on the kinetic curves, after which the oxidation kinetics obey a linear law. This is mostly accompanied by the formation of white or light grey (light brown) oxide, that adheres poorly to the metal and peels off.The increase in the mass of the Zr alloys sample with time can be expressed by the relationship ΔΜ = kt n (9.1)where Am is the mass increase per unit surface area, t is the oxidation time, k is the oxidation rate constant, and n is a dimensionless coefficient. The corrosion process is affected by a great many factors, the most important of which are: the chemical composition of the Zr alloy, both alloying elements and impurities in the metal, the metal structure determined by the technology of alloy working, the quality of the alloy surface, determined by the technology, the composition of the corrosive medium, the influence of radiation, especially the flux and the fluence of fast neutrons. 9.