Low-Temperature Oxidation

The normal surface state for metallic materials is an oxide skin since a bare surface would be a highly unstable chemical situation in the oxidizing air atmosphere of our earth. Most metals form at ambient and even at low temperatures thin adherent oxide scales which protect the bulk of the metal from further attack by reactive gas molecules. The growth of such tarnishing layers is a complex process. The reacting elements, metal atoms and oxygen molecules, are separated by the oxide skin formed as reaction product. This is an ionic compound and, therefore, the more mobile atom of the two reactants has to be transferred first to a charged defect in the crystal lattice before it can migrate to the reaction front. An electric field can exist across the oxide layer and introduce additional effects that make understanding of the reaction mechanism more difficult.