Corrosion inhibitors and their types

Inhibitors

Today, due to the understanding of the metal-electrolyte system, it has become common to prevent corrosion with the help of corrosion inhibitors. Inhibitors are chemicals that, when added in small amounts to a corrosive environment, greatly reduce the rate of corrosion. The composition and types of inhibitors are very diverse. One of the most important methods of classifying these materials is based on their mechanism of action and composition. Knowledge of the mechanism of corrosion inhibition is very essential in the design and selection of inhibitors.

Classification of Inhibitors

The classification of inhibitors has been presented in various ways, none of which is completely satisfactory. Because in these classifications, chemicals often do not find a fixed specific place. The various categories of inhibitors are briefly presented below:

1. By their application range

•      Acids
•      Neutral solutions
•      Colors
•      Gas phase

2. By their effect on electrochemical reactions

•      Anodic
•      Cathodic
•      Mixed

3. By their reaction mechanism

•      Adsorption
•      Emulsification
•      Layer deposition
•      Oxygen removal

A large number of inhibitors with different compositions are available. Most of these materials have been found and modified through experimental tests, and many of them are marketed under trade names and their chemical composition is kept secret. For this reason, their corrosion protection process is not completely known. Therefore, inhibitors can be classified according to mechanism and composition. The most important classifications of inhibitors are:

A) Safe and dangerous inhibitors:

Each inhibitor has a specific effective concentration, which in practice should be used in slightly higher amounts. So that if less than the critical concentration (minimum) is used, the corrosion rate may increase. Therefore, safe inhibitors are those that, if their concentration is less than the critical concentration, only uniform corrosion occurs and the corrosion rate is less than if the inhibitor has not been added. Dangerous inhibitors are those that, at concentrations less than the critical concentration, cause severe damage and pitting corrosion, the rate of which is much higher than if the inhibitor has not been added.

B) Anodic inhibitors:

The mechanism of action of these inhibitors is associated with an increase in anodic polarization. Anodic inhibitors will be effective if they are added to the system in sufficient quantities. If their concentration is less than the critical concentration, it will reduce the effective surface area, and ultimately lead to the intensification of localized corrosion. In fact, the mechanism of action of anodic inhibitors is the formation of a protective layer that is only formed at relatively noble potentials, and its formation reduces the rate of anodic dissolution. Anodic inhibitors are dangerous in cracks and corners and stagnant points or points where particles in the environment easily accumulate. Some anodic inhibitors include: sodium carbonate, sodium silicate and sodium chromate.

c) Cathodic inhibitors:

Since the cathodic reaction in neutral solutions is mainly oxygen reduction, therefore, by reducing the transfer of oxygen to the metal surface, the severity of corrosion can be greatly reduced. In general, any chemical that interferes with the cathodic reduction reaction is classified as a cathodic corrosion inhibitor. These inhibitors are salts of metals such as zinc, magnesium, manganese or nickel that form precipitated hydroxides on the metal surface, especially in the cathodic regions. The hydroxyl ion (OH-), which is formed as a result of oxygen reduction in the cathodic regions, increases the alkalinity of the environment and ultimately an insoluble hydroxide is formed from the combination of the metal ion and OH-. Cathodic corrosion inhibitors are considered safe inhibitors because their concentration will not cause corrosion to intensify. These inhibitors cannot corrode the metal surface like oxidant corrosion inhibitors. However, they greatly reduce the corrosion rate. In addition, the presence of aggressive ions such as chloride and sulfate has little effect on the efficiency of these types of inhibitors. Cathodic inhibitors are generally less efficient than anodic and mixed inhibitors, and the layer formed on the metal is often thick and visible. While the layer formed by anodic corrosion inhibitors will often be very thin.

d) Oxidant and non-oxidant inhibitors:

This classification is based on their ability to corrode the metal. In general, non-oxidant inhibitors require the presence of oxygen in the solution to create and maintain the corroding oxide layer, while oxidant inhibitors do not require oxygen in the solution.

e) Organic and inorganic inhibitors:

This classification is based on the type of compounds and components of the inhibitor. Organic substances that cover the metal through an oily surface layer will protect it. These inhibitors are usually used in acids, although some of them are also effective in neutral or alkaline solutions.

Note: The inhibitory effect of many organic substances is very similar to that of inorganic inhibitors. (such as sodium carboxylates)

Note:Chemical inhibitory compounds are usually dangerous to the environment and have destructive effects on living organisms. Today, scientists' efforts are focused on using natural inhibitor compounds. Some plant extracts have been studied as metal corrosion inhibitors. Extracts of prickly pear, which is from the cactus family, are used as anti-corrosive compounds. Its anti-corrosion effect is due to the formation of a protective layer on the metal surface. Plant tannins are natural and non-toxic compounds that are obtained cheaply. They act especially to inhibit the corrosion of iron and stainless steel.

Factors effective in inhibition

The most important parameters effective in the corrosion control rate of an inhibitor can be briefly mentioned as follows:

• Nature of the metal surface
• Chemical composition of the environment
• Inhibitor concentration
• Environmental pH
• Temperature
• Effects of microorganisms
• Sediment formation
• Liquid movement speed and aeration