Coatings 2022, 12, 1935. https://doi.org/10.3390/coatings12121935 www.mdpi.com/journal/coatings Article Electrochemical Behavior of Nickel Aluminide Coatings Produced by CAFSY Method in Aqueous NaCl Solution Amalia Marinou 1,2, *, Angeliki G. Lekatou 1,3 , Galina Xanthopoulou 2 and George Vekinis 2 1 Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece 2 Institute of Nanoscience and Nanotechnology, National Centre of Scientific Research “Demokritos”, 15310 Athens, Greece 3 Institute of Materials Science and Computing, University Research Center of Ioannina (URCI), 45110 Ioannina, Greece * Correspondence: a.marinou@inn.demokritos.gr Abstract: Combustion‐assisted flame spraying (CAFSY) is a novel method that allows in‐flight synthesis of alloys during flame spraying. The in‐flight synthesis of alloys by the CAFSY method during flame spraying combines two different methods: the self‐propagating high‐temperature synthesis (SHS) and flame spraying (FS). The present work studies the corrosion performance (by cyclic polarization and chronoamperometry in aerated 3.5 wt.% NaCl) of NiAl coatings fabricated by the CAFSY technique in relation to main process parameters (composition of the initial feed‐ stock, spraying distance, substrate temperature, postdeposition heat treatment) and their effect on the microstructure and porosity of the coatings. Most of the coatings exhibited limited susceptibil‐ ity to localized corrosion. In all cases, the steel substrate remained intact despite corrosion. Inter‐ connected porosity was the main parameter accelerating uniform corrosion. Localized corrosion had the form of pitting and/or crevice corrosion in the coating that propagated dissolving Al and Al‐rich nickel aluminides along coating defects. Substrate preheating and postdeposition heat treatment negatively affected the corrosion resistance. A short spraying distance (1.5 inches) in‐ creased the corrosion resistance of the coatings. Keywords: combustion‐assisted flame spraying (CAFSY); in‐flight synthesis; flame spraying; SHS; intermetallic phases; coatings; corrosion resistance; cyclic polarization; chronoamperometry 1. Introduction The intermetallic compounds (IC) of NiAl and Ni3Al are important for the industry, owing to their outstanding properties as protective coatings [1,2]. Because of their high melting points, these intermetallic compounds are used in high‐temperature applica‐ tions, such as heat treatment furnaces, gas turbines, aircraft connectors, automotive tur‐ bochargers, pistons and valves, tools, and permanent molds [3]. The application of nick‐ el aluminide coatings on metals and alloys has a beneficial effect on the high‐temperature performance of boilers and turbines that operate at high temperatures [4]. The intermetallic compounds of the Ni‐Al system are known for their high‐temperature mechanical strength and can improve the resistance to oxidation and corrosion by forming a protective outer alumina film [5]. It has also been reported that the two‐phase material Ni3Al + NiAl exhibits a synergistic beneficial effect on the prop‐ erties of these alloys and has been used in aerospace engines [6]. In addition, intermetal‐ lic compounds in the Ni‐Al system, as well as the Ti‐Al system, are considered strong candidates as new alternative structural systems for high‐temperature applications [7– 10]. The in‐flight synthesis of alloys by the CAFSY method during flame spraying com‐ bines two different methods: the self‐propagating high‐temperature synthesis (SHS) and Citation: Marinou, A.; Lekatou, A.G.; Xanthopoulou, G.; Vekinis, G. Electrochemical Behavior of Nickel Aluminide Coatings Produced by CAFSY Method in Aqueous NaCl Solution. Coatings 2022, 12, 1935. https:// doi.org/10.3390/coatings12121935 Academic Editor: Lech Pawlowski Received: 31 October 2022 Accepted: 5 December 2022 Published: 8 December 2022 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and insti‐ tutional affiliations. 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 (http://creativecommons.org/licenses /by/4.0/).