Impact of heat treatment on surface chemistry of Al-coated Eurofer for application as anti-corrosion and T-permeation barriers in a flowing Pb–15.7Li environment

Highlights: ► Electro-chemical Al deposition is industrially relevant for barrier formation. ► Al scales have to be converted into protective layers by heat treatments. ► Morphology of scales depend on deposition parameters. ► Solid state diffusion step at 640 °C avoids critical Al melting and activates steel surface. ► Al2O3 protective scales detected. - Abstract: The compatibility testing of Eurofer steel in flowing Pb–15.7Li has shown that corrosion attack is a serious concern at least under view of precipitate formation from dissolved steel components in cooler system sections, leading to a high risk of tube blockages. Additionally, the T-permeation from the liquid breeder through the steel structure into the He-cooling system is an important safety issue. Both topics may be reduced by the application of barriers. Hot dip aluminization (HDA) showed that Al scales exhibit such ability but claimed also the development of improved coating technologies. Thus, two electro-chemically-based deposition processes, which exhibit industrial relevance and adjustable layer thickness of deposited Al, were developed, tested and characterized. Both are working with water-free electrolytes of toluene or ionic liquid (IL) base near room temperature (RT). The successfully deposited homogeneous layers need a heat treatment to establish the required protective/functional properties. During this stage, the Al reacts with the steel and forms an Al-enriched zone with a thin protective Al2O3 surface scale. However, the topology is rough and pores are visible. Thus, for optimization of surface structure and scale quality, investigations concerning the required heat treatment were performed including effects coming from the applied coating technology.