## 1 Introduction
Furthermore, phosphate deposited a strongly adherent layer, consisting of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) and FePO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O, 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 [PERSON] and [PERSON] using steel samples in aqueous solutions and indicating certain derivatives of azoles and thiazoles. They reported that imidazoles, benzothazoles, 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 great stability, due to the spontaneous self-assembly process [27]. Moreover, [PERSON] 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 effect [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 efficiency 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 efficiency when mixed with different organic inhibitors, denoting the advanced synergistic inhibition effect through smart combinations. [PERSON] 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 different systems [31]. A conventional use of phosphate anions in corrosion inhibitors is in cooling systems that maintain cavitation problems and intense corrosion phenomena [32]. [PERSON] et al. patented certain mixtures against pitting and the galvanic corrosion of copper and iron surface, consisting of sodium phosphates (Na\({}_{2}\)HPO\({}_{4}\)) or polyphosphates (Na\({}_{2}\)P\({}_{4}\)O\({}_{13}\)) synergistically with sodium mercaptobenzothiazazole in aqueous solutions emphasizing the immediate connection between inhibition efficiency 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 urgent need for industrial mild steel pipeline corrosion protection. In this regard, the protection effectiveness 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 Na\({}_{2}\)HPO\({}_{4}\), were evaluated by three families of techniques: electrochemical, microscopy, and spectroscopy.