The introduction of plasma electrolytic oxidation (PEO) as a surface finishing technique has enabled a range of hard, dense oxide coatings to be produced on aluminium, magnesium, titanium and other lightweight alloy substrates. As with all surface coating technologies, successful development of PEO coatings requires adequate attention to substrate pretreatment together with careful control of electrolyte conditions and process variables. The principles and applications of the PEO coating process are considered, including the fundamentals of oxide deposition, the technology involved and the typical characteristics of the coatings. Industrial applications are considered together with their coating requirements. Plasma electrolytic oxidation coating is a specialised but well developed process. Suitable control of electrolyte and process conditions can realise a novel range of coatings having technologically attractive physical and chemical properties. The development of PEO technology over the last decade has provided coatings having controlled appearance, hardness, corrosion resistance and other tribological properties across an extending range of industrial sectors. Continuing developments are concisely reviewed and the PEO process is illustrated by the characterisation of anodised coatings on an AZ91 magnesium alloy surface.

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The development of anodic films at a constant current density of 10 mA cm−2 has been studied for Mg-W alloys, containing 0.4 and 1.0 atom % W, in 3 M ammonium hydroxide/0.05 M ammonium phosphate electrolyte at 293 K. The structure, morphology, and composition of the films were determined by X-ray diffraction, scanning electron microscopy, atomic force microscopy, glow discharge optical emission spectroscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. During anodizing to about 50 V, a relatively smooth film develops. Cross-sectional atomic force microscopy suggests the film may be finely porous. With an increase in voltage, the film transforms gradually to a coarse, porous morphology due to dielectric breakdown. The transformation coincides with a reduction in slope of the voltage-time response. Significant regions of both types of film morphology coexist during anodizing to 150 V. With further anodizing, a porous film, developed in the presence of sparking above about 270 V, eventually covers all of the macroscopic surface. Magnesium, phosphorus, oxygen, hydrogen, and nitrogen species are present throughout the film thickness at the resolution of the measurements. However, the films are initially free of tungsten species due to enrichment of tungsten in the alloy. The O:Mg atomic ratio is in the range 1.7 to 2.1, reducing with an increase in voltage for films formed up to 220 V, consistent with films composed primarily of hydroxide or oxyhydroxide. The P:O atomic ratio increases with an increase in voltage, starting at about 0.03 for voltages below 50 V and reaching 0.29 at 330 V. The films form at a reduced efficiency, typically about 40-50%. © 2001 The Electrochemical Society. All rights reserved.

The variations of the structure and the activity of a sputtered Co-C-N sample for the oxygen reduction reaction ͑ORR͒ with heat-treatment temperature are studied. The sample begins to show activity for ORR in acidic electrolyte at a critical heattreatment temperature ͑T c ͒ of 725°C, at which both substantial nitrogen release from the originally amorphous films and the transformation to a heterogeneous structure ͑␤-Co + N-containing graphitic carbon͒ occurs. The sample shows activity in alkaline electrolyte when heated above 500°C, the activity increases substantially near T c and the highest activity occurs at 850°C. This suggests that active sites, apparently created below T c , which are active below T c , are blocked in acidic electrolyte. When N loss, graphitization and formation of ␤-Co occurs at T c , sites apparently become available in acidic electrolyte. The variation of the transformation temperature, T c , with x and y in sputtered Co x C 1−x−y N y combinatorial libraries was mapped using electron microprobe and X-ray diffraction studies. T c decreases strongly as the Co content increases since Co is a graphitization catalyst and T c increases weakly as the N content increases.

Variations with heat-treatment temperature of the structure and the electrocatalytic activity of sputtered transition-metal (TM)–C–N ( TM=V , Cr, Mn, Co, and Ni) films for the oxygen reduction reaction (ORR) in acid and alkaline electrolytes were studied. The films prepared were all originally amorphous. At a critical heat-treatment temperature ( Tc , different for each composition) substantial nitrogen release occurred and the films transformed to a heterogeneous N-containing carbon structure with either normalCr3normalC2 , Co, Ni, normalV8normalC7 , or normalMn7normalC3 , depending on the TM used. In acid electrolyte, heat-treated normalCrxnormalC1−x−ynormalNy , normalCoxnormalC1−x−ynormalNy , and normalNixnormalC1−x−ynormalNy films began to show oxygen reduction activity at Tc , while normalVxnormalC1−x−ynormalNy and normalMnxnormalC1−x−ynormalNy films showed little or no activity for ORR in acid electrolyte for all temperatures studied. However, all of the heat-treated TMxnormalC1−x−ynormalNy ( TM=V , Cr, Mn, Co, and Ni) films showed oxygen reduction activity in alkaline electrolyte even when heated to temperatures below Tc . Because all the TMxnormalC1−x−ynormalNy films heated above Tc contained N-containing carbon, the activity of heat-treated sputtered normalC1−xnormalNx and C films, as well as graphite powder, were studied for comparison. Their activities were much lower in acid electrolyte than those of the normalCrxnormalC1−x−ynormalNy , normalCoxnormalC1−x−ynormalNy , and normalNixnormalC1−x−ynormalNy films heat-treated at the same temperatures. normalC1−xnormalNx films, C films, and graphite powder all showed activity in alkaline electrolyte but were less active than some heated normalCoxnormalC1−x−ynormalNy and normalNixnormalC1−x−ynormalNy films. The choice of TM and the heat-treatment temperature played important roles in determining the activity of the sputtered TMxnormalC1−x−ynormalNy films for ORR in acid and alkaline electrolytes. The corrosion stability of the heat-treated TMxnormalC1−x−ynormalNy libraries was studied as well. Heat-treated normalCrxnormalC1−x−ynormalNy libraries showed good passivation against corrosion.