**1. Introduction**

One of the most widespread kinds of steel is low-carbon or mild steel, not only for structural purposes but also for several applications in industry. Therefore, its usage and operation range from machinery parts, building materials, and domestic appliances to cutting tools, conveying tubes, cables, and magnets. Due to its low concentration in carbon (up to 0.29 wt %), mild steel exhibits extreme durability, grea<sup>t</sup> affordability, and significant mechanical, thermal, and magnetic properties [1–3]. Due to mild steel's increasing utilization, there has been grea<sup>t</sup> concern for the susceptibility of such steels caused by environmental corrosive factors, such as the humidity, acidity, or salinity of the atmosphere [4–6]. Thus, lots of studies have been focusing on low-alloy (weathering) steel corrosion and how to encounter it in a functional way [7–9].

In general, metallic corrosion is associated with any chemical alteration of the metal that stems from interaction with its environment [10]. Although intense research studies have taken place over all these decades in order to face corrosion, it still remains in the foreground since it is a continuous and inevitable process [11]. The main classification of corrosion types emphasized in the literature is between uniform and localized corrosion [12]. Pitting corrosion is a type of localized corrosion in which the metal is corroded in depth via the formation of indistinguishable pits. In the case of water and usually oil-conveying pipes, the corrosion effect often triggers uniform surface degradation internally, in parallel with localized corrosion underneath and oxide film formation [13,14]. Taking into account that pitting may cause the initiation of stress corrosion cracking [15], it is considered as the most detrimental corrosion effect. While uniform corrosion occurs during the subjection of mild steel

in various environmental conditions [8,16], localized corrosion, such as pitting and scaling, takes place in the presence of heavy metal ions [17,18] or in locally high pH values [19]. Localized corrosion phenomena may thin locally the inner wall in a pipe, resulting in the creation of areas susceptible to cracking [20].

The protection of metallic materials from the corrosion e ffect could be achieved by intervening either in the alloy structure or in the alloy environment. The intervention in the alloy structure can be performed by changing the alloying ratio, by creating/applying metallic or organic coatings/films on the surface, by decreasing mechanical operation tensions, by the use of anodic or cathodic protection, and by adding corrosion inhibitors in the corroding solution [21]. In the literature, several studies indicate corrosion inhibition as an e ffective protection method [22,23]. A conventional classification of corrosion inhibitors is according to their inhibiting action. Therefore, there are adsorption inhibitors that undergo chemisorption in the metallic surface, film-forming inhibitors, which are divided into passivation inhibitors (oxidizing or non-oxidizing) and precipitation inhibitors (deposition of three-dimensional film). However, the most common discrimination is between anodic, cathodic, and mixed-type inhibitors, considering which half-reaction they suppress during corrosion phenomena. Most organic inhibitors are of mixed-type and act as chemisorptive inhibitors. Phosphates act as cathodic inhibitors, whereas benzoates and azelates represent non-oxidizing film-forming inhibitors [24].

Concerning inorganic inhibitors, ideally the central atom of the inhibitor tends to form a complex entering the metallic lattice, without the need of additional energy and with the necessary stability [25]. Refaey et al. compared the inhibition ability of phosphate, chromate, molybdate, and nitrite as potential inhibitors for mild steel, in 0.1 M NaCl near neutral solutions. They concluded that phosphate demonstrated higher inhibition e fficiency compared to the other inhibitors. Furthermore, phosphate deposited a strongly adherent layer, consisting of γ-Fe2O3 and FePO4·2H2O, from the solution on the metal surface [26]. Regarding organic corrosion inhibitors, a perusal of the literature reveals a variety of organic compounds that have been suggested for encountering pitting corrosion on mild steel. An outstanding investigation was carried out by Marczewska-Boczkowska and Kosmulski using steel samples in aqueous solutions and indicating certain derivatives of azoles and thiazoles. They reported that imidazoles, benzothiazoles, and mercaptobenzothiazoles behaved as significantly efficient corrosion inhibitors against pitting. They claimed that these organic substances are capable of forming self-assembled monolayers, which improve inhibition and confer a protective layer of grea<sup>t</sup> stability, due to the spontaneous self-assembly process [27]. Moreover, Wang et al. synthesized and utilized an organic chemical compound, 4-salicylideneamino-3-phenyl-5-mercapto-1,2,4-triazole, as a corrosion inhibitor for mild steel, in electrolytic solution of 1 M HCl solution in several temperatures, which was based on the enhanced corrosion inhibition of the combined N and S elements in heterocyclic organic compounds [26]. They also pointed out that the presence of both elements functions more efficiently instead of the use of substances that contain only nitrogen or sulfur separately. This outcome was attributed to the combination of phenyl, mercapto, and azomethine reactive groups. Taking into account these claims, 2-mercaptobenzothiazole (MBT) was rendered as one of the most preferable mixed-type organic inhibitors for mild steel in aqueous solutions [28].

The self-assembling e ffect [29] and the chemisorption ability in conjunction with the aromatic nature of the inhibitor, which confers increased stability [25], lead to the need to investigate how to optimize inhibition by the use of MBT. Furthermore, its e fficiency in near neutral aqueous solutions was taken into account. A common utilization of MBT was in coolant mixtures as a corrosion inhibitor for low-carbon steel pipes [30]. It exhibited improved inhibition e fficiency when mixed with di fferent organic inhibitors, denoting the advanced synergistic inhibition e ffect through smart combinations. Gunasekaran et al. evaluated the synergistic corrosion inhibition of several phosphonic acids substituting metal ions for azoles, provided that phosphorous inhibition ability could be exploited in chemically di fferent systems [31]. A conventional use of phosphate anions in corrosion inhibitors is in cooling systems that maintain cavitation problems and intense corrosion phenomena [32]. Calmon et al. patented certain mixtures against pitting and the galvanic corrosion of copper and iron surface, consisting of sodium phosphates (Na2HPO4) or polyphosphates (Na2P4O13) synergistically with sodium mercaptobenzothiazole in aqueous solutions emphasizing the immediate connection between inhibition e fficiency and inhibitor concentration [33].

The aim of this study is to investigate the susceptibility of mild steel to corrosion in industrial hot/cooling systems after its exposure in 3.5 wt % NaCl as well as in the water of the Athens city supply system. The main goal of the presented experiments is to identify which inhibitors perform best at restricting the aforementioned corrosion process. The importance of these experiments is directly linked to the current urgen<sup>t</sup> need for industrial mild steel pipeline corrosion protection.

In this regard, the protection e ffectiveness of the complexes, oxides, or salts that are created on the mild steel surface, as well as the morphological conversions that occur on the surface of the metal alloy panels following exposure in the presence or absence of MBT and Na2HPO4, were evaluated by three families of techniques: electrochemical, microscopy, and spectroscopy. The electrochemical characterization demonstrated that MBT and Na2HPO4 can be considered as corrosion inhibitors of mild steel as they reduced the corresponding anodic and/or cathodic corroding reactions. Moreover, the synergistic e ffect of the corrosion inhibition behavior of MBT and Na2HPO4 in a molar ratio 1:1 at several concentrations was studied. The analysis revealed that the admixtures performed e ffectively with inhibition e fficiency above 90%. The presence of both aforementioned inhibitors into the corrosive environment exhibited the highest impedance modulus (|Z|) and polarization resistance ( *R*p) value as the exposure time elapsed.

Regarding the X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM) characterization, it was disclosed that the exposure of mild steel to a corrosive environment in the presence or absence of inhibitors resulted in the creation of several oxide, hydroxide, and hydroxide–phosphate compounds on the steel surface. According to the aforementioned characterization techniques, the corrosion protection mechanisms of steel can be ascribed to the protective films created onto the metal surface because of the inhibitors' presence, which prevent chlorides' insertion. The performed experiments shed light on the corrosion mechanisms of mild steel in industrial hot/cooling systems.

#### **2. Materials and Methods**

#### *2.1. Reagents and Solutions*

All the compounds and reagents were of analytical reagen<sup>t</sup> grade. Sodium phosphate dibasic dihydrate (Na2HPO4·2H2O, Sigma-Aldrich) and 2-mercaptobenzothiazole (MBT, Sigma-Aldrich) were used without further purification. Hot-rolled black (non-galvanized) mild steel panels were manufactured by TMK-ARTROM S.A. and accompanied with all certifications required (ASTM A568/A568M-09 [34]). Electrochemical measurements were performed on mild steel grade API 5L X42 conveying a pipeline with the following chemical composition (wt %): C: 0.15, Mn: 0.56, S: 0.002, P: 0.12, Si: 0.21, Ni: 0.07, Cr: 0.04, Mo: 0.01, Cu: 0.22, Al: 0.020, N: 0.009, V+Ti+Nb: 0.004, and Fe as remainder.

#### *2.2. Preparation of Substrates*

The mild steel panels were abraded with SiC paper up to 5 μm grain size (P4000) and then cleaned in agreemen<sup>t</sup> with ASTM D6386-99 (reapproved 2005) [35] prior to being used in the conducted experiments. This procedure is necessary because the exposed steel interacts with the environment to form several iron oxides such as α-FeO(OH), γ-FeO(OH), β-FeO(OH), and Fe3O4 [36]. According to the aforementioned standard, the cleaning procedure includes the degreasing of steel panels with their subjection in a mixture of acetone and ethanol (50:50) of purity 96% *v*/*v* for about 20 min, and then their exposure to NaOH solution of pH 11 for 5 min at 60 ◦C. Finally, the panels are rinsed with distilled water and dried in a desiccator in order to avoid the formation of new corrosion products, as uncoated black mild steel is susceptible to corrosion by humidity, as analyzed above.
