During the formation of the oxide layer by plasma electrolytic oxidation (PEO), the electrochemical processes are accompanied by the plasma micro-discharges, occurring uniformly over the coated electrode. Application of an alternating current with strictly controlled electrical parameters can affect the character of the discharges, and consequently the properties of the obtained layers. When the cathodic current density exceeds anodic, at some point, a sudden change in the appearance of micro-discharges and a decrease in the intensity of the acoustic emission can be observed - in literature this effect is called “soft sparking”. In the present work, the evolution of the electrical properties of the layers at various stages of their formation has been characterized, using electrochemical impedance spectroscopy. The study showed a significant decrease in charge transfer resistance and increase in capacitance of the oxide layer after reaching the ”soft sparking”. This indicates a significant change in the structure of the oxide layer, in the barrier and main part, which is additionally confirmed by measuring the breakdown voltage.

Soft sparking is a phenomenon observed during plasma electrolytic oxidation (PEO) performed under alternating current (AC) conditions. It is directly associated with the presence of cathodic polarization during the oxidation process, contributing to enhanced functional coating properties. However, the role of cathodic current in oxide-layer formation remains ambiguous. This study presents findings suggesting that soft sparking is a dynamic equilibrium state occurring within a certain stability window, primarily governed by the anodic to cathodic charge ratio in AC cycles. By analyzing soft-sparking behavior under varying cathodic-to-anodic charge ratios, frequency, cathodic pulse duty cycles, and alkalinity of electrolytes, the proposal of a mechanism underlying this process is presented. The authors suggest that the soft-sparking state may be linked to the formation of active sites during cathodic polarization and their subsequent suppression during anodic polarization. This occurs due to oxidation and deposition of Al(OH)₃, facilitated by localized OH⁻ ion accumulation in these regions. Additionally, the restricted diffusion of water molecules toward the substrate may play a crucial role in sustaining the soft-sparking state.

During different aluminum smelting processes occur direct contact of liquid metal and carbon materials, which are the main constituent for the lining of the cells, furnaces, crucibles and ladles, etc. As a result, processes of aluminium carbide formation at the interfacial area and its subsequent dissolution occurs. Those are recognized as one of the most important mechanisms causing surface wear and decrease lifetime of the equipment, especially in aluminium electrolysis. Present work is aimed at deeper study of the initial steps of Al₄C₃ formation at the aluminium/ carbon interface. Three types of carbonaceous materials: amorphous, semigraphitic and graphitized, in the presence and absence of cryolite melts, were examined. As it is very difficult to study layer of Al₄C₃ in situ, two indirect experimental techniques were used to investigate aluminium carbide formation: measurements of the potential and the electrical resistance. It was concluded that the process of early formation of aluminium carbide depends on many processes associated with the presence of electrolyte (intercalation, penetration and dissolution) as well as the structure of carbon materials - especially the presence of the disordered phase.