Relaxation kinetics of PEO coated Al electrode: Insight into the role of negative current

Plasma Electrolytic Oxidation (PEO) is an advanced coating process based on high-voltage anodising. Notwithstanding the anodic nature of PEO process, it is known that negative polarisation leads to synergetic effects in oxide formation efficiency and characteristics of resulting coatings. In this work, we used dynamic anodic voltammograms derived from polarisation signal combining working and diagnostic segments to evaluate in real time the effects of negative polarisation on the formation of PEO on coating on Al in the bipolar regime with frequency of 50 Hz and negative to positive charge ratio of 1.3. It was found that the hysteresis between ascending and descending branches of the voltammogram can be both caused by prior cathodic polarisation and spontaneously generated under unpolarised conditions. This indicated the existence of a quasi-equilibrium in the chemical state of the coating material, which could be perturbed by the external bipolar polarisation. The characteristic relaxation time for this system was found to be 40-370 ms. The quasi-equilibrium was attributed to a reversible hydration/dehydration reaction taking place in the active zone of anodic alumina layer (degree of hydration: 10-40%). Coating response analysis via kinetic hydration model allowed both explanation to be provided to a number of previous experimental observations and practical recommendations to be made for the design of efficient electrical regimes for intelligent PEO processes. The latter includes recommendations on avoiding long pauses during negative to positive switching.