During most heat-treatment processes, a steel surface reacts with any gases present, even in a “vacuum” environment. In most applications, we try to avoid the formation of an oxide either on the surface or beneath the surface (internal oxidation) since it may negatively influence the mechanical properties and service life of the component. In some cases, however, the formation of an oxide coating on the surface of the material is highly beneficial. For example, we steam oxidize powder-metal parts to both seal their porous surfaces (to prevent corrosion) and improve lubricity in service.  

Formally, oxidation occurs when the metal in questions loses one or more of its electrons so that the atoms of the metal go from the neutral state to one of a positively charge ion, resulting in the formation of a metal oxide (in the case of iron, rust). There are three basic oxides of iron:

1.) FeO (wüstite)
2.) Fe₃O₄ (magnetite)
3.) Fe₂O₃ (hematite)  

Thin oxide layers (under approximately 3000 Å) are referred to as films or film layers. Thicker oxide layers (above 3000 Å) are considered scales (note: 1.0 Å = 10^-10 m). An oxide film does not form if the partial pressure of oxygen is higher than the oxide’s dissociation pressure for a given metal.  

The oxide type, or phase, that forms as well as the oxide thickness is a function of temperature and oxygen concentration (Fig. 1). The scale formed may be categorized as either protective scale or non-protective scale.

• Protective scale prevents access of oxygen to the metal surface due to non-porous continuous structure of the oxide layer.
• Non-protective scale has loose porous structure, providing free access of oxygen to the underlying metal.

The type of scale may be determined by the Pilling-Bedworth rule: The scale is protective (adherent and nonporous) if the volume of the oxide is not less than the volume of metal, from which the oxide was formed; or the scale is non-protective (porous) if the volume of the oxide is less than the volume of metal from which the oxide was formed.  

Oxides with volume much greater (twice and more) than the volume of metal from which the oxide was formed cause developing compressive stresses. These stresses may lead to cracking and spalling of the scale, which results in faster penetration of oxygen to the metal surface and possible in-service issues.  

It is interesting to note that these oxides form in successive layers, first wüstite (nearest the metal surface) then magnetite and finally hematite (in contact with the atmosphere). At tempering temperatures below 566ºC (1050ºF), only magnetite and hematite form, but all three can may form at higher temperatures.