Relaxation Kinetics of Plasma Electrolytic Oxidation Coated Al Electrode: Insight into the Role of Negative Current

Abstract

Plasma electrolytic oxidation (PEO) is an advanced coating process based on high-voltage anodizing. Notwithstanding the anodic nature of the PEO process, it is known that negative polarization leads to synergetic effects in oxide formation efficiency and characteristics of resulting coatings. In this work, we used dynamic anodic voltammograms derived from polarization signal, combining working and diagnostic segments to evaluate in real time the effects of negative polarization on the formation of PEO on the coating on Al in the bipolar regime with a frequency of 50 Hz and a 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 polarization and spontaneously generated under unpolarized 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 polarization. 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 explanations 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.

A Systematic Study of the Role of Cathodic Polarization and New Findings on the Soft Sparking Phenomenon from Plasma Electrolytic Oxidation of an Al-Cu-Li Alloy

“…18 Although the "soft" regime has been long known, 25,26 it has not been possible so far to explain properly why the conditions concerned lead to those changes, and in general the current understanding of the process is still far from complete, 27 and the interest in this topic has been intensified in recent years. [28][29][30][31][32] Previously, the drop of the anodic potential is considered to be an indicator that PEO has stepped into the soft sparking regime (Researchers paid less attention to the cathodic behavior during soft sparking since the cathodic potential is lower in absolute value and its change is not so obvious during PEO. This phenomenon is called the current rectification effect since valve metals exhibit asymmetric response to the direction of external polarization 30,33 ).…”

Influences of Growth Species and Inclusions on the Current–Voltage Behavior of Plasma Electrolytic Oxidation: A Review

“…A more sophisticated diagnosis method was proposed recently by Rogov, Matthews, Yerokhin [33,134]. Its effectiveness was also demonstrated on the platform of soft sparking transition.…”

In-Liquid Plasma-Mediated Manganese Oxide Electrocatalysts for Quasi-Industrial Water Oxidation and Selective Dehydrogenation

“…27,28 Such conditions are indeed ideal to form a robust catalyst/substrate interface in the nanostructured anode that can overcome the persistent stability problem during intensive electrocatalytic oxidation (V < 2.0). Although PEO technique has been widely used for thin metal coatings, 29,30 its utilization to fabricate nanostructured anodes for electrocatalysis is relatively unexplored.…”

“…Under high applied voltages, the PEO generates a harsh environment at the MEI (with a local temperature of 10,000 K and a pressure inside discharge channels of about 10 2 –10 3 MPa). , Such conditions are indeed ideal to form a robust catalyst/substrate interface in the nanostructured anode that can overcome the persistent stability problem during intensive electrocatalytic oxidation ( V < 2.0). Although PEO technique has been widely used for thin metal coatings, , its utilization to fabricate nanostructured anodes for electrocatalysis is relatively unexplored.…”