PEO protective coatings on inner surface of tubes

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO) or anode spark deposition (ASD), is a novel technique to produce hard ceramic coatings on metals, such as aluminum, titanium, magnesium (called valve metals) and their alloys. Up to now, many researchers focused on the PEO process for workpieces of regular shapes such as cube, cake or stick samples, etc. But no one paid attention to the irregular samples such as tubular materials. This research emphasized the PEO process of long tubes, especially the way to obtain uniform thickness of ceramic coatings on inner surface of tubes. Furthermore, the PEO kinetic behaviors of aluminum tubes were also investigated. The potential difference between the electrodes indicated a linear relationship with the coating thickness. A central accessory electrode was axially used to eliminate the shielding effect of electric field and was effective to obtain axially uniform coating on the inner surface of the tubes. The combination of hot-dipping aluminum and PEO process to obtain ceramic coating on 45# steel tubes was also performed in this work.