Plasma Electrolytic Oxidation (PEO) Anodize

Plasma Electrolytic Oxidation (PEO) or Micro Arc Oxidation (MAO), applies cyclic anodic (+) and cathodic (-) high-voltages to the part while immersed in a high-pH (basic) electrolyte solution (Figure 1). Dissolution of the electrolyte and electrolysis of the water produces charged ions that migrate to the anodic part, resulting in the formation of crystalline oxides (e.g., alumina (Al2O3) or zirconia (ZrO2)) on the part surface. While the process is ideal for Al, Ti, and Mg alloys, due to some conversion of the underlying material surface and subsequent build-up of the substrate oxide, it has also recently been demonstrated on steels with successful results. It should be noted that for the steel substrates the oxide coating is an add-on and not an integral part of the Al, Ti, or Mg alloy surface.

The resultant crystalline oxide surface is 3-5 times harder than traditional hard anodize and consequently offers greater resistance to sand-abrasion and erosion. NanoCoatings, Inc. researchers have also added nanoparticles to the electrolyte solution which can modify certain properties, such as increased toughness of the oxide layer. Addition of solid-lubricating materials, e.g., graphite or MoS2, to the electrolyte will become incorporated into the oxide surface and result in low-friction surfaces. Furthermore, the PEO surface produces a pore-free sealed near-surface layer, which resists corrosion; hence the PEO surface does not require a final sealing treatment as required for traditional anodize. As a result, the corrosion resistance of PEO surfaces is very high.

Finally, since the process is an immersion, conversion process, complex shaped parts can be treated on both inside and outside surfaces. This enables the protective coating of tubing, pipe, and other complex shapes that would be hard to process using line-of-sight spray-deposition methods.

Figure 1 Experimental Plasma Electrolytic Oxidation Process Set-up