Plasma Electrolytic Oxidation (PEO)

The project of PEO in our laboratory was put up, launched and maintained by Eng. A. Krasnopolski and Eng. A. Kossenko. My role is much more modest.

 

Plasma electrolytic oxidation (PEO) is a technology allowing obtaining hard wear- and corrosion-resistant coatings in the form of thick and highly-adhesive oxide layer on aluminum, magnesium and other metal surfaces. Although the processes in plasma processing occur in electrolyte solutions under the action of electric charge and therefore can be classified as electrochemical, they are significantly different than "conventional" electrochemical processes, in which relatively small currents and voltages cause relatively tranquil chemical reactions.

For a conventional electrochemical (for instance, electroplating) processing, anodic reactions can sometimes cause the passivation of the electrode surface due to formation of impenetrable to reactants (or products). For aluminum or magnesium (or some other metals) such passivation occurs when the coating is only <10 nm thick. Such thickness cannot provide the metal any reliable protection against abrasion or corrosion. What distinguishes the PEO from a regular electroplating, is the continuation of the anodic process. As the surface has been passivated by non-conductive oxide coating, the potential difference between the metal and the electrolyte keeps increasing. It increases until it has become too high for the dielectric coating and a microscopic plasma discharge breaks the coating. This breakdown results in the formation of a bit thicker coating, which will be broken again in the course of the next cycle, under a bit higher potential difference. The cycles continue until desired coating thickness has been achieved (normally, 20-50 μm).

In practice, PEO is realized in a working station combining a High Voltage AC Supply and a cooled electrolyte bath:

An item to be processes is loaded in the bath and AC pulses cause micro-arc plasma discharges:

The obtained oxide layer consists of two sub-layers: the outer brittle porosive sub-layer and the inner functional very hard and much less porosive sub-layer. The outer brittle sub-layer can be then removed by polishing and the inner harder sub-layer can be finished to smooth marble surface.