High-temperature materials such as MCrAlY, aluminide, silicon-base environmental barrier, and thermal barrier coatings, as well as nickel- and cobalt-based superalloys, ceramic matrix composites, and ceramics are used in various corrosive environments. Materials that may be resistant to one environment may be susceptible in other environments. The interaction of the protective oxide of the given material changes as a function of a number of factors. The increasing desire to raise temperatures in different high-temperature applications requires a thorough understanding of those factors influencing passivity in order to develop and implement alternative, more corrosion-resistant materials.
This paper will examine some of the factors that influence the interaction of high-temperature materials with several corrosive species and offers observations and recommendations improve the service life of these materials.
High temperature applications of metals and alloys demand materials that have a variety of properties such as strength, toughness, creep resistance, fatigue resistance, as well as resistance to degradation by its interaction with the environment. All potential metallic materials are unstable in these high temperatures environments without the presence of a protective layer on the component surface. High temperature alloys derive their resistance to degradation by forming and maintaining a continuous protective oxide surface scale layer that is slow-growing, very stable, and adherent [1]. Most commercial alloys designed for high-temperature service rely on the formation of a chromia (Cr2O3), an alumina (Al2O3) scale, or possibly a silica scale (SiO2). Ceramics may depend on the formation of SiO2.
The performance of alloy protective oxides and high temperature coatings is dependent largely on the temperature, the environment, alloy substrate structure and composition, and the structure/composition of the oxide or coating. Oxides are seldom stoichiometric; they have either excess metal (n-type) or deficient metal (p-type) compositions. Metal-deficient oxides such as Cr2O3, Al2O3, or NiO grow as cations and electrons move outwardly through the scale toward the oxide/gas surface while cation vacancies and electron holes move inwardly toward the metal. Consequently, as the scale thickens, the flow of cation vacancies accumulate and condense to form voids at the metal/metal oxide interface [2,3] or condense in the metal immediately below the scale/metal interface to cause void formation [4-6]. Generally, n-type oxides require either interstitial cation or oxygen anion vacancy migration which leads to oxide growth occurring at the oxide/gas surface and the metal/oxide interface, respectively [7]. Protective oxide scales will eventually breakdown: thermal cycling tends to accelerate scale spallation and lead to less protective scale formation [8].
The nature of the protective oxide is dependent upon many variables, not all of which have been identified or understood [9]. It has been stated that there is a need to develop improved physics-based models linking the thermochemical and thermomechanical behavior of thermal barrier coatings to their combustion environments [10]. Bornstein acknowledged that there needs to be an increased understanding of the coatings manufacturing methods, coating structure, corrosion mechanisms, and coating failure mechanisms [10]. The use of thermodynamically-based modeling and development of multi-component phase diagrams will help in predicting possible harmful phases in the development of new high-tempera