Water vapor effects on the cyclic oxidation resistance of alumina forming alloys

Abstract

Water vapor is present in most environments in which alloys are used at elevated temperatures and there are papers in the literature that show water vapor usually has adverse effects on the oxidation resistance of alloys. However, the exact effect of water vapor is dependent on the particular alloy under consideration. This paper is concerned with the oxidation of alloys that rely upon the development of α-Al2O3 scales for oxidation resistance. This paper describes two major deleterious effects of water vapor on the oxidation of such alloys. One effect involves increased spalling of α-Al2O3 which will be shown to be less significant in the case of alloys with extremely adherent α-Al2O3 scales. It is proposed that water vapor causes the α-Al2O3-alloy interfacial toughness to be decreased, however this effect is not sufficient to cause spalling of extremely adherent α-Al2O3 scales. Another effect of water vapor is that it causes more transient oxides to be formed during the selective oxidation of aluminum in alloys. This condition becomes more severe at lower temperatures. Possible mechanisms by which water vapor affects the selective oxidation of aluminum in alloys will be described.