Citation: Niaz, A.; Al Fuhaid, A.F.; Faraz, M.I. Understanding Corrosion Degradation Processes of a Multi- Component CoNiCrAlY-Coating System. Coatings 2022, 12, 1396. https://doi.org/10.3390/ coatings12101396 Academic Editor: Changheui Jang Received: 27 August 2022 Accepted: 21 September 2022 Published: 25 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Article Understanding Corrosion Degradation Processes of a Multi-Component CoNiCrAlY-Coating System Akbar Niaz 1, * , Abdulrahman Fahad Al Fuhaid 1,2 and Muhammad Iftikhar Faraz 3 1 Materials Engineering Department, College of Engineering, King Faisal University, Al Hofuf 31982, Saudi Arabia 2 Civil Engineering Department, College of Engineering, King Faisal University, Al Hofuf 31982, Saudi Arabia 3 Mechanical Engineering Department, College of Engineering, King Faisal University, Al Hofuf 31982, Saudi Arabia * Correspondence: abutt@kfu.edu.sa; Tel.: +966-13-5899725 Abstract: 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 μm area was carried out by microelectrode in an electrolyte at a distance of 5 μm 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 μm height to 2 μm 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. Keywords: CoNiCrAlY corrosion; multicomponent system; SECM study; oxide characterization; charging current in polarization curves 1. Introduction To maintain thermal insulation and integrity at very high temperatures, thermal barrier coatings are made from multiple materials [1–4]. This multiple-component system provides unique insulation properties but also causes corrosion degradation problems. If these corrosion problems get worse with time, they severely hamper the integrity of coatings [5,6]. Coatings for high-temperature applications consist of a heat-insulating top ceramic coat and an intermediate bond coat of MCrAlY. The bond coat improves the adherence with the substrate on one side, while improving the bond with the insulating top coat on the other side [7,8]. A number of experimental sequences are reported to study coating microstructure, composition, and corrosion degradation properties. Among the reported work, researchers used electron microscopy, X-ray diffraction, phases/composition analysis, and electrochemical testing techniques [8–11]. A detailed standalone electrochemical aspect of the corrosion degradation is missing. In this study, we will use traditionally reported electrochemical testing techniques alongside advanced scanning electrochemical microscopy to study the corrosion process in the closest vicinity of the coating surface. The air-grown and voltage-assisted passive layer is studied by X-ray photoelectron spectroscopy to find the types of oxides formed with the severity of the environment. Although different aspects of coatings have been studied by researchers in the past, only the most relevant work will be presented here. Coatings 2022, 12, 1396. https://doi.org/10.3390/coatings12101396 https://www.mdpi.com/journal/coatings