Effects of Co, Al and Mo on the long term passivation of 9Cr-3W (wt.%) ferritic steels in air and wet air at 650 °C

9Cr-3W (wt.%) ferritic steels with different amounts of Co, Al and Mo added were oxidised in air and wet air at 650 °C for up to 9638 h. Steels with only 1.90 wt.% Al added was not passivated in air and wet air. They were passivated in air but not in wet air when 3 wt.% Cr was added either alone or with 1.83 wt.% Al. Long term passivation was achieved in both air and wet air when 2.22 wt.% Al, 3 wt.% Co and 2.2 wt.% Mo were added; without Mo, passivation in wet air was maintained only temporarily.