Electrochemical method to evaluate the connected porosity in ceramic coatings

The electrochemical behaviour of plasma-sprayed SS41 steels with the Al₂O₃-TiO₂ system, Cr₂O₃ and ZrO₂ stabilized by Y₂O₃ ceramics was studied using a potentiostat by measuring their natural potential and anodic polarization curves in 3.5% NaCl aqueous solution. The natural potential of ceramic-sprayed SS41 steels is the same as that of the substrate. The anodic polarization behaviour of ceramic-sprayed SS41 steels is also dominated by the substrate through connected porosity. However, the polarization current density of ceramic-sprayed SS41 steel is lower than that of the substrate because the current density is controlled greatly by the amount of porosity in the coating. The connected porosity of ceramic coatings was obtained quantitatively from the polarization current density ratio between the ceramic-sprayed specimen and the substrate at the corresponding polarization potential. The strong relation between the connected porosity and the porosity of cross-sections of the ceramic coatings, shown using micrographs, was recognized. By measuring electrochemically the connected porosity in coatings obtained under various spraying conditions, it would be possible to select the optimum spraying conditions to spray coatings which have less porosity.