is adopted in aerospace and other critical applications and wherever the cathodic protection is not cost‐effective.
1.5.5 Corrosion Inhibitors
The most prevalent, economic, and effective measure against corrosion of metallic surfaces in aggressive media in closed systems is inhibitors [25–30]. Corrosion inhibitors are added in small concentration to effectively reduce the corrosion rate of a metal exposed to corrosive solution. It is similar to a retarding catalyst, which reduces the corrosion rate by increasing or decreasing the reaction of anode and/or cathode, decreasing the reactants’ diffusion rate to the metallic surface and decreasing the metallic surface’s electrical resistance. Without disrupting the setup, inhibitors can be added in situ. Adsorption theory best explains their functioning in a corrosion cell. Adsorption refers to the adhesion of a chemical species to the superficial single monolayer of the metal without bonding with the bulk of metal. The type of the corrosion medium, the magnitude of the charge at the metal/solution interface, the nature of metal, and the cathodic reaction decide the inhibitor. Three types of environments use inhibitors, namely, recirculating cooling water systems in the pH range of 5–9, primary and secondary production of crude oil and pickling acid solution for the removal of dust and mill scale during the production and fabrication of metals parts, or also for the post‐service cleaning. Based on their composition or mechanism of protection, inhibitors are classified as follows.
1.6 Adsorption Type Corrosion Inhibitors
As the name suggests, these organic compounds adsorb on the metal to suppress dissolution and the reduction reaction. Adsorption inhibitors affect both anodic and cathodic process equally or disproportionally.
1 Vapor‐phase corrosion inhibitorsVapor‐phase corrosion inhibitors also called volatile corrosion inhibitors. The vapor pressure of these compounds at 20–25°C is usually between 0.1 and 1.0 mm Hg. When kept in the vicinity of the metal to be protected, they sublime and condense in enclosed spaces [31]. For example, phenylthiourea and cyclohexylamine chromate are used for protecting brass. Dicyclohexylamine nitrite protects both ferrous and nonferrous metals/alloys.
2 Inorganic inhibitorsSome metal ions like Pb2+, Mn2+, and Cd2+ deposit on the iron surface in acidic environment [32]. Even Br− and I− inhibit corrosion in strongly acidic solutions [33]. As2O3 and Sb2O3are also corrosion inhibitors in acidic media. These substances form a metal oxide layer and increase the hydrogen over‐voltage to reduce the corrosion rate.
3 Organic inhibitorsIt includes large number of organic substances containing N, S, or O atoms in the molecule. Organic inhibitors possess a functional group as the reaction center for the adsorption process. They have heteroatoms like N, S, and O in their structures. The molecular structures majorly influence the extent of inhibition of corrosion.
1.6.1 Anodic Inhibitors
Those substances that reduce the anodic area by acting on the anodic sites and polarizing the anodic reaction are called anodic inhibitors [34]. Anodic inhibitors cause displacements in corrosion potential in positive direction, suppress corrosion current, and reduces corrosion rate. If an anodic inhibitor is not present at a concentration level sufficient to block off all the anodic sites, localized attack such as pitting corrosion can become a serious problem due to the oxidizing nature of the inhibitor, which raises the metal potential and encourages the anodic reaction. Anodic inhibitors are classified as unsafe because they may cause localized corrosion. Examples of anodic inhibitors include orthophosphate, chromate, nitrite, ferricyanide, and silicates.
1.6.2 Cathodic Inhibitors
There are substances that may lessen the cathodic area by polarizing the cathodic reactions on cathodic area [35]. Cathodic inhibitors transfer the corrosion potential in negative direction to retard cathodic reaction and suppress the corrosion rate. Cathodic area is reduced by precipitating the insoluble species on cathodic sites. Cathodic inhibitors do not cause localized corrosion hence safe. They are cathodic poisons/hydrogen‐evolution poisons like arsenic and antimony ions. Scavengers like sodium sulfite and hydrazine are filming inhibitors (cathodic precipitates).
1.6.3 Mixed Inhibitors
The formulation contains more than one inhibitor in this case. These inhibitors interfere with anodic, as well as cathodic reactions. These inhibitors are used for multi‐metallic substrates and when combined and optimized cathodic/anodic effect is required. The halide ions enhance the action of organic inhibitor in acid solutions.
1.6.4 Green Corrosion Inhibitors
As the chromates/arsenates were restricted, corrosion prevention with ecological green compounds (hazard‐free inhibitor formulations) in most oil field applications were designed to effectively meet safety standards and also protect the targeted metal substrates in their service life. In general, most of these efficient corrosion inhibitors are organic compounds containing hetero atoms such as S, N, O, P, and multiple bonds or aromatic rings. The number of lone pairs of electrons and loosely bound π‐electrons in these functional groups are determining factors for their activity. These biocompatible substances might be of plants and animal origin. In the past two decades, the researchers sought after the “green” or “eco‐friendly” corrosion inhibitors to use cheap, effective compounds at low or “zero” effect on nature. The ever‐increasing publications on green corrosion inhibition indicate the interest in exploring the new inhibitors to control the corrosion of various metals.
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