Stability of the Soft Sparking State in the Plasma Electrolytic Oxidation Process

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.