Influence of yttrium additions on the oxide scale adhesion to an iron--chromium--aluminum alloy

The oxidation behavior of an Fe--27% Cr--4% Al alloy and similar alloys containing 0.023 % and 0.28% Y in 1 atm oxygen at 12000C has been examined. The oxide formed on the yttrium-free alloy develops a highly convoluted configuration, apparently resulting from lateral growth of the oxide. The latter leads to oxide detachment from the alloy at temperature and extensive spalling during cooling. It is postulated that lateral growth results from the formation of oxide within the existing oxide layer by reaction between oxygen diffusing inward down the oxide grain boundaries and aluminum diffusing outward through the bulk oxide. Additions of yttrium to the alloy apparently prevent the formation of oxide within the oxide layer, the oxide-forming reaction occurring as the alloy-oxide interface. Thus lateral growth is prevented and spalling during cooling does not occur. Secondary advantages conferred by the addition of 0.28% Y to the alloy are the prevention of void formation at the alloy-oxide interface, the avoidance of alloy grain growth during oxidation, and the creation of an oxide ''keying'' or ''pegging'' effect