Liquid Metal Corrosion and Corrosion Protection
Forschungszentrum Karlsruhe, Institute of Pulsed Power and Microwave Technology (IHM)
Herrmann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen,
A key problem in development of heavy liquid metal cooled nuclear energy and transmutation reactors is the corrosion of structural and fuel cladding materials in contact with the liquid metal. Lead and lead bismuth, which are preferred as a heavy liquid metal coolant, attack unprotected steel surfaces by dissolution of the metallic components into the liquid metal.
During the last years the corrosion behaviour of austenitic and martensitic steels in liquid eutectic lead – bismuth alloy (LBE) was investigated in many laboratories around the world. It was found that oxide scales on the surface provide the best protection against dissolution attack. Therefore, oxygen is dissolved in the LBE to enable the steel to form an oxide scale on the surface and to support the self healing process in case of damage in the protective scale. However, at temperatures above 500°C austenitic steels suffer from severe dissolution attack, while martensitic steels form thick oxide scales which hinder heat transfer from the fuel pins and which may break off and eventually lead to a blocking of the coolant channel.
Above 500°C steels have to be protected by stable, thin oxide scales that have the potential of self healing in case of cracks and other defects in the scale. A well understood measure is alloying of stable oxide formers into the surface. Al has shown its ability to form such oxide scales. In the range of 4 – 10 wt% Al on the surface a stable thin alumina scale is formed by Al diffusion to the surface and selective oxidation. The alumina scale grows only very slowly and prevents migration of oxygen into the steel as well as migration of steel components onto the surface. A number of corrosion experiments showed the good protective behaviour of Al scales in LBE with 10-6 wt% oxygen up to 650°C and for exposure times up to 10000 h.
Alloying Al into the surface was done by diffusion processes and also by pulsed electron beam (GESA) melting of a thin layer on the surface on which Al was precipitated before. Another method to modify the surface properties is coating with an alloy that contains Al in the required concentration range.
This presentation gives an overview on investigations of the steel behaviour in HLM environment carried out to explore their suitability for systems with Pb/LBE coolants. Results of experiments with static and flowing LBE are discussed. The behaviour of steels examined and their respective application ranges are described. Part of the presentation deals with protective barrier development on the steel surface by alloying of Al and its effect on the corrosion resistance. Furthermore the influence is discussed of parameters like stresses in the cladding wall, different flow velocities of the LBE and changing temperatures and oxygen concentrations in LBE.