For superalloys operating at high temperatures and exposed to corrosive environments, oxidation behavior is a concern. Oxidation involves chemical reactions of the alloying elements with oxygen to form new oxide phases, generally at the alloy surface. If unmitigated, oxidation can degrade the alloy over time in a variety of ways, including:
* sequential surface oxidation, cracking, and spalling, eroding the alloy over time
* surface embrittlement through the introduction of oxide phases, promoting crack formation and fatigue failure
* depletion of key alloying elements, affecting mechanical properties and possibly compromising performance
Selective oxidation is the primary strategy used to limit these deleterious processes. The ratio of alloying elements promotes formation of a specific oxide phase that then acts as a barrier to further oxidation. Most commonly, aluminum and chromium are used in this role, because they form relatively thin and continuous oxide layers of alumina (AlO) and chromia (CrO), respectively. They offer low oxygen diffusivities, effectively halting further oxidation beneath this layer. In the ideal case, oxidation proceeds through two stages. First, transient oxidation involves the conversion of various elements, especially the majority elements (e.g. nickel or cobalt). Transient oxidation proceeds until the selective oxidation of the sacrificial element forms a complete barrier layer.
The protective effect of selective oxidation can be undermined. The continuity of the oxide layer can be compromised by mechanical disruption due to stress or may be disrupted as a result of oxidation kinetics (e.g. if oxygen diffuses too quickly). If the layer is not continuous, its effectiveness as a diffusion barrier to oxygen is compromised. The stability of the oxide layer is strongly influenced by the presence of other minority elements. For example, the addition of boron, silicon, and yttrium to superalloys promotes oxide layer adhesion, reducing spalling and maintaining continuity.
Oxidation is the most basic form of chemical degradation superalloys may experience. More complex corrosion processes are common when operating environments include salts and sulfur compounds, or under chemical conditions that change dramatically over time. These issues are also often addressed through comparable coatings.