The oxidation behavior of an Fe-27%Cr-4%Al alloy and similar alloys containing 0.023% and 0.82% Y in 1 atm oxygen at 1200°C 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 0.82% 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.

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

Some recent developments in the understanding of the oxidation of alloys at elevated temperatures are reviewed, with special reference to binary and ternary alloys upon which many commercial materials are based. Following an initial classification of alloy systems, certain basic principles and their limitations are considered, including factors determining whether an alloy displays surface scaling only, internal oxidation only, or both phenomena; the mode of distribution of cations in an oxide solid solution growing on an alloy; and doping and the Wagner-Hauffe rules. Previous alloy oxidation theory has concerned itself principally with steady-state oxidation but, in practice, behavior of the oxidizing sample in approaching the steady state, both upon initial exposure to the environment and after scale spalling induced isothermally or during thermal cycling, is often as important. Such behavior is illustrated in terms of scale establishment and with respect to compositional changes in the subjacent alloy, the importance of certain alloy and oxide properties being emphasized. After further brief consideration of steady-state scaling, the causes and consequences of breakaway oxidation are presented. The role of major and minor concentrations of third elements on scaling behavior is discussed. The paper concludes with brief reference to situations not considered in detail and with aspects requiring further study.

High-temperature oxidation of alloys

Reactive element additions may promote the formation of protective oxide scales on high temperature alloys, and improve long term oxidation resistance by limiting the rate of scale growth and inhibiting scale failure processes. Various modifications to transport properties and mechanical behaviour have been proposed to explain the observed benefits. These mechanisms are reviewed with reference to available experimental evidence. Key steps in enhancing oxidation resistance are probably the provision of reactive element oxide particles for incorporation into a scale and consequent segregation of reactive element ions to high diffusivity pathways and interfaces. Such behaviour is consistent with known variations in protection resulting from changes in reactive element type, concentration, and initial distribution.MST/1055

Role of reactive elements in alloy protection

Oxidation tests in a CO2-based atmosphere at 1123 K have been conducted on a series of 20Cr-25Ni stainless steels containing a dispersion of TiN particles and with silicon contents ranging from 0.05 to 2.35 wt.%. In all cases, a chromia surface film whose thickness increased parabolically with time was formed. The parabolic rate constant was a minimum at an intermediate alloy silicon content when an interlayer of silica was formed between the outer oxide and the metal. The significance of this duplex structure on the processes determining the oxidation rate and the extent of oxide spalling is discussed. An additional feature of the attack was the formation of subsurface voidage whose depth again increased parabolically with time. The appropriate rate constant also showed a minimum at an intermediate silicon level and the relationships between this form of attack and the development of the surface oxides are highlighted.

M.G. Hobby

Papers

The identification of thin healing layers at the base of oxide scales on FeCr base alloys