The effect of electrolyte concentration on the microdischarge behaviour during plasma electrolytic oxidation (PEO) on aluminium and titanium*

The production of oxide-ceramic coatings on light metals aluminium, magnesium and titanium can be achieved by plasma electrolytic oxidation (PEO). High corrosion resistance and a good adhesion of the coating to the substrate are the main advantages of this method. During the process, anodic dielectric breakdowns in form of short-living microdischarges are generated in a conductive liquid.

To analyse the effect of the electrolyte concentration on these microdischarges, the active anode surface is reduced to the tip of a wire with a diameter of 1 mm. The electrolyte consists of varying concentrations of potassium hydroxide (1 - 4 g/l) in distilled water.

Fast optical measurements with a high-speed camera are carried out for a better understanding of the development and evolution of single microdischarges and the accompanying gas evolution.

Optical emission spectroscopy allows the determination of electron densities with Stark broadening of H­­­_α and scanning the treated wire tips with an electron microscope (SEM) enables to investigate the morphology of oxide layer. The measurements are performed in galvanostatic DC Mode with a current density of 1.27A/cm².