metals Article Galvanic Sensor for Detecting Corrosion during Acid Cleaning of Magnetite in Steam Boilers Abdullah M. Al-Mayouf * and Matar N. Al-Shalwi   Citation: Al-Mayouf, A.M.; Al-Shalwi, M.N. Galvanic Sensor for Detecting Corrosion during Acid Cleaning of Magnetite in Steam Boilers. Metals 2021, 11, 343. https:// doi.org/10.3390/met11020343 Academic Editor: Belén Díaz Fernández Received: 6 January 2021 Accepted: 16 February 2021 Published: 18 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- iations. Copyright: © 2021 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/). Electrochemical Sciences Research Chair, (ESRC), Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia; malshalwi@ksu.edu.sa * Correspondence: amayouf@ksu.edu.sa; Tel.: +966-11-467-5959; Fax: +966-11-467-5992 Abstract: A simple galvanic sensor was developed to determine the onset of iron corrosion under conditions similar to those used for removing the iron oxide (magnetite) layer from steam boilers with a hy-drochloric acid solution. Since the potentials of magnetite and iron are different, the initiation of the dissolution of the magnetite layer without corroding the base metal can be determined by moni-toring the potential of a sensor made of magnetite layers of different thicknesses that can be placed directly on the iron surface. Moreover, the time at which the addition of corrosion inhibitors is required to protect the base metal can be speciﬁed using this sensor. It has been shown that the time required for the iron base metal to start was dependent on the magnetite layer thickness in the sensor. With this sensor, the onset of the steel corrosion during the cleaning can be detected in re-al-time which makes the oxide removal process less harmful to the base metal. Keywords: sensor; steam boiler; acid cleaning; magnetite; oxide 1. Introduction Steam, generated from boilers of different sizes and types, is used in many industrial plants for process applications and power generation. Industrial boilers are considered critical parts of industrial facilities, as they are directly linked to production to make the business proﬁtable. Steam boilers are made of carbon steel and the inner side of the boiler tubes are in contact with water. Thus, the formation of solid deposits on that side of the tubes is a progressive and inevitable process, even when the water and steam chemistry is carefully controlled, leading to reduced boiler efﬁciency and increased fuel consumption. In particular, a 2.8 mm thick deposit can increase fuel consumption by 16% [1]. Moreover, steam at a certain temperature can react with iron, forming iron oxide and particularly magnetite (Fe 3 O 4 ) when the environment inside the boiler is reductive. Thick magnetite layers may also lead to deposit corrosion, reduce the water circulation inside the boiler, and overheat the tubes. Therefore, the inner surface of the boiler tubes must be cleaned and kept free of unwanted deposits to ensure that the temperature of the tube wall will not cause damage to the boiler. Chemical cleaning is usually preferred compared to other cleaning methods (e.g., mechanical) to ensure that all deposits are removed from the boiler’s internal surfaces, while an inhibited acid solution is commonly used depending on the composition and thickness of the deposits. When the deposit is magnetite, hydrochloric acid (HCl) is the most commonly applied solvent [2]. Previous studies have shown that HCl is more effective than other inorganic and organic acids in dissolving scale deposits collected from the tubes of an industrial water tube boiler at a speciﬁed time [3]. In a typical procedure, the boiler is ﬁlled with 5–10% HCl at around 65 ◦ C in a static condition. As HCl is very corrosive, it takes about 500 min to remove the deposits [4]; the cleaning process can also cause corrosion unless carefully controlled [5]. Moreover, iron has shown a high corrosion rate during the removal of magnetite sludge with a solution containing 20% ethylenediamine tetraacetic acid (EDTA) at 120 ◦ C[6]. Metals 2021, 11, 343. https://doi.org/10.3390/met11020343 https://www.mdpi.com/journal/metals