Plasma Electrolytic Oxide (PEO) Coatings are hard, dense, wear-resistant, and well-adhered oxide coatings for metals such as aluminium and magnesium. The process by which they are grown may also be referred to as micro-arc oxidation (MAO) or spark discharge anodizing. Essentially, it involves the modification of a conventional anodically grown oxide film by the application of an electric field greater than the dielectric breakdown field for the oxide.  Discharges occur, and the resulting plasma-chemical reactions contribute to the growth of the coating. More significantly, local conditions of heat and pressure sinter and anneal the coating. Rapid cooling also modifies the oxide, resulting in a complex mixture of amorphous material and nanocrytalline phases.

The process may be applied to any valve metal such as aluminium, magnesium or titanium, and to a wide range of their alloys. Coating properties depend on the substrate alloy, but also on the electrolyte used and on the many parameters of the electrical system. On aluminum, dense alumina coatings (approximately 3% porosity) up to 130 microns thick can be formed, but more porous coatings up to 600 microns in thickness can also be formed by using different electrolytes. Typical alumina coatings consist of a relatively dense polycrystalline layer of alpha-alumina, with a softer, more porous layer of gamma-alumina formed on top.

The process is currently being developed and marketed in the UK by Keronite Ltd. (owners of the registered trade mark Keronite) who are based on the Granta Science Park, near Cambridge.