In metallurgy, scaling (also called high-temperature oxidation) is the growth of a layer of oxide on a metal surface as a result of reaction with a hot oxidizing atmosphere. The oxide layer is itself called the scale. Scaling is one form of high-temperature corrosion, alongside sulfidation and carburization, in which the metal reacts instead with sulfur- or carbon-bearing atmospheres. A particular case of scaling is mill scale, which forms on hot-rolled steel. Scaling differs from the fouling sense of "scale", in which mineral solids precipitate onto a surface from a fluid. For example, minerals like CaCO3 precipitate onto the inside of a metal pipe that carries hard water, which can then be chipped off into scale-shaped chunks. In metallurgical scaling, the material that builds up comes from oxidizing the metal itself, which is progressively consumed. Scaling may remain intact and thicken slowly, which would protect the underlying material from further oxidation. Scaling may crack, fall off (spall), or thicken quickly, which would disrupt the underlying material. Material with good protective scaling property may have worse performance in other properties, such as mechanical properties (strength, creep resistance), fabricability (formability, weldability), cost, etc. Material engineers balance these criteria when designing material meant for high temperature applications.
Kinetics Three kinetic regimes are commonly observed. When solid-state diffusion through the scale is rate-limiting, scale thickness x {\displaystyle x} grows as x 2 = k t {\displaystyle x^{2}=kt} , the parabolic rate law derived by Wagner in 1933. According to Wagner's theory, oxidation rate is controlled by partial ionic and electronic conductivities of oxides and their dependence on the chemical potential of the metal or oxygen in the oxide. Notably, this is the same law as that of a random walk. Wagner's theory is based on lattice diffusion, whereas the transport properties of slow-growing protective oxides are largely determined by their grain boundaries and possibly, microporosity, so a fully quantitative description of real protective scales remains empirical. When a surface reaction or diffusion through the gas phase controls the rate, oxidation becomes linear in time at a constant rate. The scale thickness grows as x = k l t {\displaystyle x=k_{l}t} . For very thin films, roughly 2 to 4 nm, at low temperatures, oxidation follows a logarithmic law. Two forms are observed. The direct logarithmic law is x = k log log ( t + t 0 ) + A {\displaystyle x=k_{\log }\,\log(t+t_{0})+A} . The inverse logarithmic law is x = ( B − k i l log t ) − 1 {\displaystyle x=(B-k_{il}\log t)^{-1}} . The inverse form was explained by Cabrera and Mott. Chemisorbed oxygen sets up an electric field across the thin film. The field accelerates ion migration through the oxide. As the film thickens, the field weakens, and the rate falls. A material may have more than one regime. Niobium in air at about 1000 °C, for example, starts parabolic and transitions to linear at long times.
Protective scaling Scaling may be protective, or destructive. In protective scaling, the scaling layer grows slowly, and does not spall, crack, or flake off. In destructive scaling, the opposite is true. Whether a material has protective scaling is largely empirical, but several factors have been identified.
Growth stresses
The growing scale rarely forms without internal stress. Birks, Meier & Pettit (2006) identify seven mechanisms that generate growth stresses.
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