Types of Corrosion Inhibitors and Their Mechanisms of Action

Any substance added to a corrosive medium that can interfere with the electrochemical action of corrosion, preventing or reducing the rate of metal corrosion, is known as a corrosion inhibitor. Corrosion inhibitors produced by a corrosion inhibitor factory are chemical substances or mixtures of chemical substances that exist in the environment (medium) at an appropriate concentration and form, maintaining the original physical and mechanical properties of metal materials and significantly reducing the corrosion rate of metal materials to zero. Their function is to prevent corrosion by forming a protective film on the metal surface.

Classification of Corrosion Inhibitors

Currently, there are a wide variety of corrosion inhibitors produced by a corrosion inhibitor factory, with complex mechanisms. Correct use of corrosion inhibitors allows them to adsorb onto the surface of parts, obstructing the corrosion of the metal surface by water and oxygen in the air. The goal is to passivate the corrosion rate within a controllable range. The principle involves forming a dense oxide film or compound on the metal surface through redox reactions, creating a protective film, and enhancing the corrosion resistance of parts during use.

Oxide Film Type Corrosion Inhibitors

Oxide film type corrosion inhibitors are also known as passivation film type corrosion inhibitors. They can oxidize the metal surface to form a dense corrosion-resistant passivation film that prevents corrosion. For example, chromates in solution can create a layer of r-Fe₂O₃ metal oxide on the surface of carbon steel, which tightly and firmly adheres to the metal surface, changing the corrosion potential of the metal and reducing the speed of the corrosion reaction through passivation. The anti-corrosion effect of oxide film type corrosion inhibitors is very good, but if the amount added is not enough to completely passivate the anode, corrosion will concentrate on the parts that are not fully passivated, causing dangerous pitting corrosion. Therefore, a larger amount of such corrosion inhibitors is often used.

Precipitation Film Type Corrosion Inhibitors

Precipitation film type corrosion inhibitors can combine with certain ions in water or with metal ions from corrosion to precipitate on the metal surface, forming a layer of insoluble precipitate or surface complex, thereby preventing further corrosion of the metal. Because this anti-corrosion film does not directly combine with the metal surface, it is porous and often shows poor adhesion to the metal. Therefore, from the perspective of anti-corrosion effectiveness, these corrosion inhibitors are slightly inferior to oxide film type corrosion inhibitors.

Adsorption Film Type Corrosion Inhibitors

Adsorption film type corrosion inhibitors are all organic compounds. They can inhibit corrosion because their molecular structure contains hydrophilic groups that can adsorb onto the metal surface and hydrophobic groups that shield the metal surface. The hydrophilic groups adsorb directionally onto the metal surface, while the hydrophobic groups hinder the diffusion of water and dissolved oxygen to the metal, thereby achieving corrosion inhibition.

Mechanism of Action of Corrosion Inhibitors

Adsorption Film Theory

Mainly concerning the mechanism of action of organic corrosion inhibitors, when metal is immersed in a medium, the corrosion inhibitor will undergo physical or chemical adsorption with the metal surface charge.

Phase Formation Film Theory

Corrosion inhibitor molecules undergo chemical reactions between the metal surface and the corrosive medium to form oxides or insoluble substances that precipitate on the metal surface, forming a protective film that prevents direct contact between the metal and the corrosive medium, hence protecting the metal from corrosion.

Electrochemical Theory

A galvanic cell reaction occurs on the metal surface, and the corrosion inhibitor can increase the overpotential of the anodic or cathodic reaction, inhibiting the reaction and reducing the rate of metal corrosion.