Characterisation of Fe–20Cr–6Al–Y model alloy in supercritical water

Abstract

MCrAlY (M = Fe, Ni or Co) alloys belong to a class of coating materials that provide corrosion and oxidation resistance to Fe or Ni based substrate alloys by forming a dense alumina layer on the surface. In order to assess its potential as a corrosion resistant coating on components in supercritical water cooled reactor core, a Fe–22Cr–6Al–0·6Y model alloy was tested in supercritical water (SCW) (500°C and 25 MPa) for over 6000 h. The long-term corrosion behaviour of samples with various surface preparation means was evaluated by measuring weight change per unit surface area and examining surface microstructure and oxide formation using scanning electron microscopy and X-ray diffraction. The results showed that surface preparation alone led to changes in weight gain as great as seven times. Finely polished surface allowed for more oxidation to take place in SCW, hence more weight change due to Fe₃O₄ (and spinel) formation. Simple grinding with abrasive paper yielded the least and the most stable weight change while grit blasting had resulted in the greatest weight gain, probably due to the increased surface area and more oxygen inward diffusion. Comparing to other alloys tested under similar condition, the ground Fe–22Cr–6Al–0·6Y had the lowest weight change. Although not detected, the formation of Al₂O₃ or an Al/Y modified Cr₂O₃ superficial layer was likely the reason for such low weight change.

Keywords:

• Oxidation
• Supercritical water
• FeCrAlY
• Surface preparation

Acknowledgement

Funding to the Canada Gen-IV National Program was provided by Natural Resources Canada through the Office of Energy Research and Development, Atomic Energy of Canada Limited, and Natural Sciences and Engineering Research Council of Canada.