Sealing of oxide coatings for service at high temperatures

Sealing is one of the most common post-treatment procedures for thermal spray coatings, directed mostly to prevention of attack of corrosive media to the substrate. Organic sealants are dominating but are limited to service at room or slightly elevated temperature. Technical solutions for sealers allowing coating service up to 800 °C are seldom, and have been focussed on aluminum orthophosphate in the past. Possible changes in the coating microstructure at temperatures up to 800 °C are neglected sofar. In this study we studied two new commercial silicate-based sealers in comparison with aluminum orthophosphate. The penetration and curing of these sealers into selected oxide coatings (alumina, alumina-titania, spinel, zirconia and chromia) sprayed by APS was studied, including determination of the wetting behavior of the sealers to the coatings. Alumina free-standing deposits sprayed by water stabilized plasma were studied as well. The changes of the coating microstructures after treatment at 800 °C were taken into consideration. Extensive microstructural studies were carried out. The success of the sealing operation was estimated using electrochemical tests.