Oxidation behaviour of CVD and electroplated Al-base coatings for EUROFER97

Abstract

The reduced activation ferritic/martensitic (RAF/M) EUROFER97 is the primary candidate for coolant-facing alloys of the European DEMO reactor. However, EUROFER97 will face harsh environment, and it might experience enhanced degradation due to the synergistic exposure to flowing high temperature water coolant, tritium embrittlement and ionising radiation. A possible solution is to use self-passivating coatings that can interact with the environment to provide corrosion and tritium permeation protection during in service operation. In this study, EUROFER97 was firstly Al-base coated using either a chemical vapour deposition (CVD at 700 °C or 750 °C) or electroplating process (room temperature); and then oxidised in a 2-step process using the same tempering (980 °C) and normalisation (760 °C) temperatures used for EUROFER97. These temperatures were used to assess their suitability for obtaining a stable and protective Al-base oxide without altering the microstructure of EUROFER97. Advanced microscopy characterizations revealed the formation of a ≈0.6 µm α-Al₂O₃ layer over an intermetallic Al-Fe-rich interdiffusion microstructure of ≈80 µm depth. However, the interdiffusion layer was highly decorated with voids that might act as stress concentrators during in-plant service, thus being detrimental to the material performance. This study compares different Al-base coating techniques and provides a preliminary insight on the selection of oxidation temperatures for EUROFER97, finding that the current 2-step oxidation process needs further optimisation before being industrialised.