Influence of aqueous aging on surface properties of plasma sprayed oxide coatings

Abstract

The influence of aging in mild aqueous conditions (pH 4, 7 and 9) on surface properties of plasma sprayed oxide was studied using electrophoretic mobility studies and measuring concentrations of dissolved species from exposure liquids. In addition, required acid/base additions to maintain constant pH, redox potentials suspension conductivities were measured. The experiment time was two weeks. The plasma sprayed materials were based on Al2O3, TiO2 and Cr2O3. Materials based on Al2O3 dissolved easily at pH 4 due to presence of metastable γ-Al2O3 phase. In addition there was clear change in surface charging properties (zeta potential) of Al2O3 surfaces so that the estimated IEP value drifted from >9 at the beginning of aging and dropped down to 8.5-8.7 after 2 weeks of treatment. Plasma sprayed TiO2 did not dissolve under the experiment conditions. Even thought the surface charging (zeta potential) changed during the exposure, the estimated IEP remained close to the values reported for pure TiO2 materials. Plasma sprayed Cr2O3 based materials were also insoluble at the studied pH values. On the other hand, the estimated IEP values deviated radically from the reported PZC values of similar materials. © 2007 Elsevier Inc. All rights reserved.