Citation: Senderowski, C.; Rejmer, W.; Bilko, P. Effect of Low Chloride and Sulfate Concentrations on Corrosion Behavior of Aluminum and Zinc Arc Thermal Sprayed Coatings. Coatings 2022, 12, 653. https://doi.org/10.3390/ coatings12050653 Academic Editor: Lech Pawlowski Received: 7 April 2022 Accepted: 6 May 2022 Published: 10 May 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 Effect of Low Chloride and Sulfate Concentrations on Corrosion Behavior of Aluminum and Zinc Arc Thermal Sprayed Coatings Cezary Senderowski 1, * , Wojciech Rejmer 1 and Piotr Bilko 2, * 1 Department of Materials and Machines Technology, Faculty of Technical Sciences, University of Warmia and Mazury in Olsztyn, 10-719 Olsztyn, Poland; wojciech.rejmer@uwm.edu.pl 2 Department of Mechanics and Constructions, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, 10-719 Olsztyn, Poland * Correspondence: cezary.senderowski@uwm.edu.pl (C.S.); piotr.bilko@uwm.edu.pl (P.B.) Abstract: The aim of this study was to determine the suitability of arc sprayed zinc and aluminum coatings as materials for protective coatings of different heating systems. The most aggressive chemical agents occurring in heating water are chloride and sulfate anions. Both ions are responsible for the corrosion of metals due to their high electronegativity and standard electrochemical potential. Water in heating systems should not contain more than 150 mg/L anions, including no more than 50 mg/L of chlorides and 100 mg/L of sulfates. To determine the corrosion resistance of three types of zinc and aluminum coatings, open circuit potential and linear polarization resistance (LPR) tests were conducted in eight alkaline solutions with different sulfate and chloride contents. The SEM/EDS structural properties of sprayed coatings at specific arc process parameters were investigated. Zinc coatings exhibit the most stable corrosion potentials in varying environments but have higher corrosion current density. Aluminum coatings exhibit much higher potential values in a chloride environment than in any other. A chloride environment also causes the lowest corrosion rates for aluminum-coated samples. A small addition of aluminum to the zinc coating (15 wt.%) does not appear to affect the stability of the corrosion potential but does result in a reduction in corrosion rates in chloride solutions. Keywords: arc spraying; corrosion; heating installations; alkaline solutions; chlorides; sulfides 1. Introduction One of the most important parameters of materials used for the transmission of heating water is their resistance to corrosion. According to PN-93 C-04607 standard, water for central heating applications should be characterized by a low concentration of anions (<150 mg/L), particularly sulfates and chlorides [1]. Additionally, the heating medium should be alkaline to further inhibit corrosion processes. Alkaline solutions have different effects on the corrosion behavior of various coatings. Especially, aluminum, zinc, and their alloys are commonly used as thermal sprayed coating materials on steel substrate immersed in water. The sacrificial corrosion protection of these coatings materials, in combination with their relatively low corrosion rates, make them suitable for such harsh environments [2–4]. Zinc coatings show no significant change in corrosion resistance in acidic and alkaline solutions (pH = 4–10), whereas the formation of zinc hydroxide and crystalline zinc oxide in alkaline solutions has been reported [5]. A similar situation occurs on the aluminum surface, where the concentration of hydroxide anions is needed to precipitate Al 3+ ions to form protective coatings. This effect was reported when aluminum alloys were tested at pH = 3–10 [6]. Chlorides and sulfides also have significant effects on the corrosion behavior of zinc and aluminum materials. It was found that high sulfate concentrations (1–4%) in low alkaline solutions (pH about 8) lead to a decrease in corrosion rate with increasing concentration [7]. It Coatings 2022, 12, 653. https://doi.org/10.3390/coatings12050653 https://www.mdpi.com/journal/coatings