Metals such as aluminum naturally form a
passivating oxide layer which provides moderate protection against corrosion. The layer is strongly
adherent to the metal surface, and it will regrow quickly if scratched off. In
conventional anodizing, this layer of oxide is grown on the surface of the metal by the application of electrical
potential, while the part is immersed in an acidic
electrolyte. In plasma electrolytic oxidation, higher
potentials are applied. For example, in the plasma electrolytic oxidation of aluminum, at least 200 V must be applied. This locally exceeds the
dielectric breakdown potential of the growing oxide film, and
discharges occur. These discharges result in localized plasma reactions, with conditions of high temperature and pressure which modify the growing oxide. Processes include melting, melt-flow, re-solidification,
sintering and densification of the growing oxide. One of the most significant effects, is that the oxide is partially converted from
amorphous alumina into crystalline forms such as
corundum (α-Al2O3) which is much harder. As a result, mechanical properties such as
wear resistance and
toughness are enhanced.
Equipment used The part to be coated is immersed in a bath of
electrolyte which usually consists of a dilute
alkaline solution such as KOH. It is electrically connected, so as to become one of the
electrodes in the
electrochemical cell, with the other "counter-electrode" typically being made from an inert material such as
stainless steel, and often consisting of the wall of the bath itself. Potentials of over 200 V are applied between these two electrodes. These may be continuous or pulsed
direct current (DC) (in which case the part is simply an
anode in DC operation), or alternating pulses (
alternating current or "pulsed bi-polar" operation) where the stainless steel counter electrode might just be
earthed. ==Coating properties==