The influence of additives on the stability behavior of electrolyte, discharges and PEO films characteristics

The instability of the potassium fluorozirconate electrolyte during plasma electrolytic oxidation process was investigated. Phosphoric acid, sodium dihydrogen phosphate and disodium hydrogen phosphate were applied as additives. Ceramic films were prepared on magnesium alloy in different electrolytes under constant voltage. The effect of additives on the stability of the electrolyte, composition, structure, cross-section, corrosion properties of the films, dissolution of the substrate, the transmittance of electrolytes and the composition of plasma active species were investigated. It was found that the additives of phosphoric acid and sodium dihydrogen phosphate could restrain the hydrolysis of electrolytes and thus improve the stability of the electrolyte effectively. Sodium dihydrogen phosphate is one of the ideal additives to improve the stability of the electrolyte. Moreover, it was observed seldom depositions deposit in the electrolyte and exhibited uniform discharge. The films were mainly composed of monoclinic and tetragonal ZrO2 and MgF2 phases in three layers. The element distribution of F and Mg decreased and that of Zr increased from the inner layer to the outer layer. The corrosion properties depended on the microstructure of the films were improved by adding additives into the electrolyte. The electrolyte components and stability have little influence on the plasma species' composition of micro-discharges. (C) 2010 Elsevier B.V. All rights reserved.