of corrosion of metal surfaces. Many inorganic complexes, ions, and salts were successfully used against the corrosion of metal surfaces in different corrosive environments [83–85]. Inorganic corrosion inhibitors prevent the corrosion via reaction of anodic and cathodic parts of the system. On the other hand, organic corrosion inhibitors prevent the corrosion process adsorbing on metal surfaces. The most widely used inorganic corrosion inhibitors are the salts of zinc, copper, nickel, arsenic, and additional metals. It should be noted that arsenic compounds are widely considered compared to others. The mentioned arsenic compounds scrape at the cathode cell of metal surfaces when they are mixed in the corrosive medium. It is important to note that the plating decreases the percentage of hydrogen ion interchange. The reason of this situation is the formation of iron sulfide. The reaction between iron sulfide and acid is known as a dynamical process. In the literature, some advantages and disadvantages regarding the using of inorganic corrosion inhibitors are reported. The advantages of them are that they can be used for a long time at high temperatures. Additionally, compared to organic corrosion inhibitors, they are cheaper. As disadvantage, it can be noted that they lose speedily their abilities to connect in the acid solutions that are stronger than 17% hydrochloric acid [86]. Inorganic corrosion inhibitors are classified as anodic and cathodic inhibitors.
4.2.1.8 Anodic Inhibitors
Anodic inhibitors are also known as passivation inhibitors. They cause a reducing anodic reaction. Namely, they support the metal surfaces blocking the anode reaction. In addition, they form a film adsorbed on metal surface. Usually, these inhibitors form the mentioned cohesive and insoluble film reacting with corrosion product initially formed. The corrosion inhibitors and the corrosion potentials of the metals studied affect the anodic reaction [87]. As a result of the reaction with the metal ions (Mn+) on anode of corrosion inhibitors, insoluble and impermeable metallic ions hydroxide films occur. If concentrations of inhibitor molecules reach to sufficient height, the cathodic current density becomes higher than the critical anodic current density. Consequently, the metal is passivated. In anodic inhibitors, it is quite important that concentrations of inhibitor molecules should be high in the solution considered. If concentration of inhibitor is low, the film formed cannot cover the entire metal surface. This situation causes a localized corrosion [2]. Nitrates, molybdates, sodium chromates, phosphates, hydroxides, and silicates are the examples of anodic corrosion inhibitors.
4.2.1.9 Cathodic Inhibitors
In the course of corrosion process, the cathodic corrosion inhibitors prevent the occurrence of the cathodic reaction of the metal surfaces. These mentioned inhibitors having some metal ions form insoluble compounds that precipitate in cathodic sites. Here, a compact and adherent film restricting the diffusion of reducible species in these areas settles down on metal surface. The oxygen diffusion and electrons conductive in these areas provide that these inhibitors have a high cathodic inhibition. Magnesium, zinc, and nickel ions can be given as example for cathodic inhibitors because they form the insoluble hydroxides as (Mg(OH)2, Zn(OH)2, Ni(OH)2 reacting with the hydroxide ions of water. The formed insoluble hydroxides are deposited on the cathodic sites of the metal surfaces to protect them. As other examples of cathodic inhibitors, the oxides and salts of antimony, arsenic, and bismuth, which are deposited on the cathode region in acid solutions, can be presented. It is well‐known that these inhibitors minimize the release of hydrogen ions [88]. In the current literature, many studies regarding the performances against the corrosion of metal surfaces of inorganic corrosion inhibitors are available [84,89–91].
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