Understanding Corrosion Degradation Processes of a Multi-Component CoNiCrAlY-Coating System

The thermal insulation and integrity of the thermal barrier coating is hampered by the formation of mixed oxide at intermediate bond coat. The existing reported work correlates growth of mixed oxide to the microstructural and phase changes. The track mostly used to study these changes is scanning electron microscopy, X-ray diffraction, and electrochemical testing. Oxide growth is principally an electrochemical process; hence a thirst exists to study this aspect by using advanced electrochemical techniques. In this study scanning electrochemical microscopy is used to reveal the electrochemical activity in the closest vicinity of the surface. A raster scan of 500 mu m area was carried out by microelectrode in an electrolyte at a distance of 5 mu m above the surface to record the current profile. The activity at the surface was confirmed by current distance curves. The tip of the microelectrode was approached from 60 mu m height to 2 mu m above the surface. The current-distance curves for the coating without heat-treatment show an active surface while the heat-treated one show non active surface. The average coating electrochemical response was further studied by polarization curves impedance spectroscopy. The X-ray photoelectron spectroscopy results show that oxidation and formation of the mixed oxide increase with polarization.