Thermodynamic and Kinetic Aspects of the Corrosion of High-Temperature Alloys in High-Temperature Gas-Cooled Reactor Helium

Abstract

In corrosion tests with iron- and nickel-based alloys in simulated cooling gases of the primary circuit of high-temperature gas-cooled reactors (HTGR helium), different effects have been found. The materials may be carburized or decarburized, depending on gas composition, gas supply rate, and test temperature. The surface scales may be composed of oxides and spinels, of mixed oxide/carbide layers, or of carbides, and internal oxidation may become significant. The basic corrosion mechanism could not be explained by the simple use of thermodynamics, but a significant step forward is possible if the kinetics of the different oxidation and carburization reactions are taken into account. The classical stability diagram for chromium, the most important alloying element in these alloys, can then be used for the prediction of the corrosion effects and the corrosion products.

Besides the usual description of reaction rates, the kinetics must include the changes in the oxidizing and carburizing potentials at the metallic surface caused by surface scale formation.

The influence of some additional alloying elements present in commercial high-temperature alloys can be estimated by comparing their stability with the stability of chromium.