oxidescale

Oxidescale is a term used to describe the thin film of metal oxide that forms on the surface of metals and alloys when they are exposed to oxygen, particularly at elevated temperatures or in oxidizing environments. The phrase is often used interchangeably with oxide scale, though specific fields may prefer one form or the other.

The formation of oxidescale involves diffusion processes at the metal-oxide interface. As metal atoms diffuse outward and oxygen diffuses inward, a layered structure can develop. Inner sublayers tend to adhere firmly to the substrate, while outer portions may be more porous or cracked. The exact composition of the scales depends on the base metal or alloy and the operating environment; for example, iron forms magnetite (Fe3O4) and hematite (Fe2O3) scales, chromium-rich alloys develop chromia (Cr2O3) scales, and aluminum alloys form alumina (Al2O3) scales, which are often highly protective.

Oxidescale can be either protective or non-protective. Protective scales are dense, adherent, and slow-growing, significantly reducing further oxidation. Non-protective scales tend to spall or crack under thermal cycling, exposing fresh metal to the environment and accelerating degradation. The protective quality of oxidescale depends on factors such as alloy composition (notably chromium and aluminum content), temperature, oxygen partial pressure, and mechanical stresses.

Assessing oxidescale involves weight-change measurements, microscopy, X-ray diffraction, and cross-sectional analysis to determine thickness, adherence, and phase composition. Understanding its properties is crucial for high-temperature materials engineering, corrosion resistance, and the design of alloys for oxidative environments. Related concepts include oxide layers, passivation, scale spallation, and high-temperature oxidation.