Corrosion of APS- and HVOF-Sprayed Coatings of the Al2O3-TiO2 System

In this paper results of corrosion studies on thermally sprayed coatings of the Al2O3-TiO2 system (Al2O3, Al2O3-3%TiO2, Al2O3-13%TiO2, Al2O3-40%TiO2, Al2TiO5 and TiOx) are presented. The coatings were deposited by APS and HVOF onto corrosion-resistant steel substrates in order to prevent the substrate influence on the corrosion results for the coatings. Coatings characterisation was performed on metallographically prepared cross-sections by optical microscopy and SEM as well as on coating surfaces before and after corrosion testing. It has been found that for true porosity determination by image analysis, SEM micrographs have to be used instead of optical micrographs. The phase compositions were studied by X-ray diffraction and were linked with the corrosion results. Corrosion experiments were performed in NaOH and H2SO4 (1 N solutions) at room temperature, 60 °C and 85 °C, as well as for hydrothermal conditions. In contrast to the expectations, APS-sprayed coatings were found to be more corrosion resistant than HVOF-sprayed coatings.