Oxidation
Oxidation is generally described as the most commonly encountered form of high-temperature corrosion. However, the oxidation process itself is not always detrimental. In fact, most corrosion and heatresistant alloys rely on the formation of an oxide film to provide corrosion resistance. Chromium oxide (Cr2O3, chromia) is the most common of such films. In many industrial corrosion problems, oxidation does not occur in isolation; rather a combination of high-temperature corrosion mechanisms causes material degradation when contaminants (sulfur, chlorine, vanadium, etc.) are present in the atmosphere. Strictly speaking, the oxidation process is only applicable to uncontaminated air and clean combustion atmospheres.
Apart from chromium, alloying additions used to enhance oxidation resistance include aluminum, silicon, nickel, and some of the rare earth metals. For oxidation resistance above 1200 °C, alloys that rely on protective Al2O3 (alumina) scale formation are to be preferred over those forming chromia. Increasing the nickel content of the austenitic stainless steels up to about 30%, can have a strong beneficial synergistic effect with chromium.Carbon Steels and Low alloy Ferritic Steels are probably the most widely used engineering material. It is extensively used for high temperature applications in power plants, chemical and Petrochemical industries, oil refining and in many other industries. Boiler tubes in power plants, reactor vessels in process industries, heat treating fixtures and exhaust train piping are examples of a few components made out of mild steel.
The Oxidation behaviour of mild steels is very similar to that of iron. Carbon steels have negligible oxidation in air up to temperature of about 250-300 °C.Oxidation attack is less than 20 mg/cm2 up to 450 0C.Above this temperature, the oxidation rate increase rapidly, especially above 600 0C.High Strength Low Alloy (HSLA) steels which contain minor addition of alloying elements, such as Mn, Si ,Cr and Ni have a lower oxidation rate. Chromium –Molybdenum steels are considered to be a better choice for moderate temperature.