Aluminium is listed among the most useful construction materials for many branches of industry. This is owed to its light weight and decent mechanical properties, which can be enhanced further with alloying and mechanical pretreatments. It is especially useful for aerospace and automotive industries, where low density is sought after because of fuel savings. This aspect is more crucial than ever due to the advent of commercially available electric vehicles. Despite many advantages of aluminium it is relatively susceptible to corrosion in many basic and acidic conditions as well as under atmospheric conditions. Therefore, it is imperative to protect the metal from corrosion, typically by means of coatings.

The standard procedure for preparing aluminium surface for aerospace industry requires the use of Cr(VI). However, current regulations in the European Union restrict the use of hexavalent chromium leading to accelerated research focused on replacing this technology. Aluminium surfaces can be passivated by the electrochemical means, e.g. by anodizing. The conventional anodizing comprises the anodic polarization of a metal workpiece in an electrolyte solution resulting in the formation of oxide layers on top of the underlying substrate. There exist an extension of this process utilizing the voltages of formation higher than the dielectric breakdown voltage. Such treatment is often called plasma electrolytic oxidation (PEO), microarc oxidation (MAO) or microplasma oxidation (MPO). The formation of coatings is a consequence of combined electrochemical oxidation of the metal workpiece and the thermal processing in the plasma formed in the strong electric field across the oxide. The PEO coatings are typically much thicker than the conventionally anodized oxide layers and their properties rely heavily on the processing regime. DC PEO treatment produces thick coatings with considerable porosity, while the AC or pulsed bipolar current modes lead to the formation of much denser oxide layers.

In the present study, the authors explored the possibility of application of AC PEO for the formation of oxide coatings on aluminium, which will be the base for corrosion inhibitors loading. The resulting coatings will be better equipped to withstand corrosion even in the event of coating damage. The study comprised the determination of processing parameters (e.g. duty ratio, voltage, time, waveform) necessary for attaining soft sparking regime of aluminium, which give rise to dense coatings and is easier to control. The resulting coatings were characterized by SEM and XRD, while their corrosion resistance was determined by EIS.

This work was supported by the National Science Centre, Poland, under research Project No.: 2016/23/N/ST5/02022.