Internal oxidation is a process whereby oxygen diffuses into a metal and reacts with certain elements in the metal or alloy (such as chromium, aluminum, or silicon), creating oxide particles in the interior of the metal instead of just on the surface. This occurs predominately in metals or alloys that are used in high-temperature oxidizing environments and are typically described as alloys.

Key Features:

• Formation of oxides occurs below the surface: The formation of oxides is often occurring inside the metal matrix rather than on the surface.
• The reactive element is the target: The reactive elements in the metal (e.g., chromium in steel) will react with oxygen, which is the desired reaction.
• Mechanical property changes happen over time: Although mechanical property changes may occur over time, some of those initial changes are unrecognizable until late in service, when changes in microstructure are noticeable.

The microstructural phenomenon of internal oxidation is often experienced by components exposed to elevated temperatures, such as turbine blades, exhaust systems, or heat exchangers. This is generally an issue where metals or alloys are in oxygen-rich atmospheres. Internal oxidation is more of a concern with creep-resistant alloys, as modifications to microstructural properties can lead to irreversible changes and instability to the microstructure.