scaleforming

Scaleforming refers to the formation and growth of oxide scales on metal surfaces when they are exposed to high-temperature, oxidizing environments. The scales arise from the diffusion of metal cations to the surface and oxygen anions inward, producing oxide films that can be protective or detrimental depending on their properties and adherence.

In many alloys, especially iron- and steel-based ones, oxide layers develop in a sequence: wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3). Some alloys are designed to form protective scales, such as chromium oxide (Cr2O3) or aluminum oxide (Al2O3), which slow further oxidation. Scale growth is generally diffusion-controlled and often follows a parabolic rate law at elevated temperatures. Thermal cycling and mechanical stresses can cause scales to spall, exposing fresh metal to rapid oxidation and potential degradation of surface integrity.

Factors influencing scale formation include temperature, exposure time, oxygen partial pressure, water vapor, and the chemistry of the alloy. Additions of chromium, aluminum, silicon, and other elements can promote protective, adherent scales, while impurities or coarse microstructures may lead to non-protective, porous scales.

Mitigation strategies aim to control scale formation or enhance adhesion. These include selecting alloys with protective oxide-forming elements, applying coatings (such as aluminide or chromia-forming coatings or ceramic thermal barriers), modifying the service atmosphere, and controlling temperature and cooling rates to reduce thermal stresses. In industry, understanding scale formation is important for planning maintenance, predicting component life, and designing protective surface treatments.

See also oxidation, oxide scale, diffusion, thermal barrier coatings.