Plasma Electrolytic Oxidation Of Aluminum Research Articles

Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions

In this paper, basic electrochemical processes (such as oxide film growth, anodic dissolution and oxygen liberation) on an aluminium anode in a model alkaline solution are considered under conditions of galvanostatic DC plasma electrolytic oxidation (PEO). The experiments performed include: (i) recording and analysis of the main electrical characteristics of the process; (ii) determination of the oxide layer thickness; (iii) anodic gas collection and composition analysis and (iv) electrolyte analysis to determine dissolved aluminium. Four different stages of the PEO process have been identified, characterised by various rate proportions of the partial anodic processes. Overall current efficiency of the oxide film formation has been estimated to be in the 10–30% range. The film growth rate decreases significantly with increasing electrolyte concentration from 0.5 to 2 g l −1 KOH, since the rate of anodic dissolution increases. Oxygen evolution is shown to be the main electrochemical process at the potentials corresponding to the plasma stages of the electrolysis. The overall rate of oxygen liberation at the anode exceeds the Faraday yield, which is probably due to the radiolytic effect of the plasma discharge on the adjacent electrolyte volume.

Discharge characterization in plasma electrolytic oxidation of aluminium

Digital video imaging of the plasma electrolytic oxidation (PEO) of aluminium has been performed, which allowed evaluation of both dimensional characteristics of individual microdischarges appearing at the oxide–electrolyte interface and their collective behaviour throughout the oxidation process. It has been shown that the microdischarge cross-sectional dimensions vary within the range 0.01–1.35 mm2. In the course of PEO processing, small localized events (<0.03 mm3) always dominate in the microdischarge spatial distribution and the relative proportion of medium-sized to very large microdischarges is gradually redistributed in favour of the latter. Temporal dependences have been found for the fraction of surface area instantaneously experiencing the discharge, as well as for the spatial and current densities of the microdischarge. Discharge mechanisms occurring during PEO are discussed and a model of microdischarge formation is suggested, assuming the possibility of free-electron generation and glow discharge ignition in the gaseous media developed at the oxide–electrolyte interface. First approximation evaluations of thermal processes in the oxide layer under the discharge conditions have been considered. The estimated ranges of the microdischarge current density (50–18 kA m−2) and duration (0.25–3.5 ms) sufficient for initiating phase transitions (e.g. γ–α transformation and melting) in the surface oxide layer are shown to be in good agreement with experimental data.

On the initial steps in the decomposition of energetic materials from excited electronic states

Decomposition studies of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX-C3H6N6O6, see Fig. 1) isolated in the gas phase and cooled in a supersonic expansion are reported for the excited electronic state near 225 nm. The RDX is handled safely and effectively through matrix-assisted laser desorption (MALD) of a thin film of RDX/R6G laser dye (1:1) adsorbed on an aluminum oxide coating on an aluminum drum. The aluminum oxide coating is generated by plasma electrolytic oxidation of aluminum. The combination of MALD and supersonic molecular beam techniques generates intact and cold RDX molecules isolated in the gas phase. Two basic conclusions are reached in this study. (1) Photodissociation of RDX at ≈225 nm generates NO as an initial product. (2) Nascent NO thus generated is vibrationally hot (Tvib∼1800 K) and rotationally cold (Trot∼20 K).