Corrosion inhibitors, also known as corrosion retardants, are substances added to corrosive media to interfere with the electrochemical corrosion process and prevent or reduce the corrosion rate of metals. They are chemical substances or mixtures of multiple chemical compounds that, when present in the environment (medium) at an appropriate concentration and form, maintain the original physical and mechanical properties of metallic materials and significantly reduce, even halt, their corrosion rate. Their anti-corrosion function is achieved by forming a protective film on the metal surface.

一、Classification of Corrosion Inhibitors

Corrosion inhibitors have a wide variety of types and complex inhibition mechanisms. When properly applied, they adsorb onto the surface of components and hinder corrosion of the metal by water and oxygen in the air. The objective is to bring the corrosion rate under control via passivation. The mechanism relies on redox reactions to generate a dense oxide film or compound layer on the metal surface, forming a protective film and improving the corrosion resistance of components during service.

1. Oxide-film Corrosion Inhibitors

Also known as passivation-film corrosion inhibitors. They oxidize the metal surface to form a dense, corrosion-resistant passive film for corrosion protection. For instance, chromates can form a γ‑Fe₂O₃ oxide film on carbon steel in solution. This film adheres tightly and firmly to the metal surface, shifts the corrosion potential of the metal, and reduces the rate of corrosion reactions through passivation. Oxide-film inhibitors deliver excellent anti-corrosion performance. However, if the dosage is insufficient to fully passivate the anodic sites, corrosion will concentrate on incompletely passivated areas, leading to dangerous pitting corrosion. Therefore, relatively high dosages are generally required for this type of inhibitor.

Oxide-film inhibitors are consumed during film formation. A high initial concentration is needed to build the film, and the dosage can be reduced once the film is formed; subsequent dosing only serves to repair damaged oxide films. Chloride ions, high temperatures and high flow velocities can damage the oxide film, so the concentration should be increased accordingly in such service conditions.

2. Precipitation-film Corrosion Inhibitors

They react with certain ions in water or metal ions dissolved by corrosion, precipitating on the metal surface to form a layer of poorly soluble precipitates or surface complexes, which stops further metal corrosion. This anti-corrosion film is not chemically bonded directly to the metal substrate and is porous, often showing poor adhesion to the metal. Consequently, their inhibition performance is generally inferior to that of oxide-film inhibitors.

Polyphosphates widely used in water treatment, such as sodium hexametaphosphate and sodium tripolyphosphate, belong to precipitation-film corrosion inhibitors.

3. Adsorption-film Corrosion Inhibitors

These are organic compounds. Their inhibition effect arises from their molecular structure, which contains hydrophilic groups capable of adsorbing onto the metal surface and hydrophobic groups that shield the metal surface. The hydrophilic groups adsorb onto the metal surface in an oriented arrangement, while the hydrophobic groups block the diffusion of water and dissolved oxygen toward the metal, achieving corrosion inhibition.

Satisfactory adsorption films and good anti-corrosion performance are obtained when the metal surface is active and clean. If the metal surface is covered with corrosion products or scale deposits, conditions are unfavorable for forming the adsorption protective film. For this reason, wetting agents may be added together with this type of inhibitor to help the inhibitor penetrate rust-covered metal surfaces and boost inhibition efficiency.

二、Function and Mechanism

1. Adsorption Film Theory

It mainly explains the inhibition mechanism of organic corrosion inhibitors. When a metal is immersed in the medium, the corrosion inhibitor undergoes physical or chemical adsorption with the charges on the metal surface.

2. Phase Film Formation Theory

Corrosion inhibitor molecules react chemically at the interface between the metal surface and corrosive media, generating oxides or sparingly soluble substances that precipitate on the metal surface to form a protective film. This prevents direct contact between the metal and corrosive media, so as to protect the metal from corrosion.

3. Electrochemical Theory

Galvanic cell reactions take place on the metal surface. Corrosion inhibitors can increase the overpotential of anodic or cathodic reactions, suppress these reactions and reduce the metal corrosion rate.

三、Principles for Selecting Corrosion Inhibitors

1. Selection of corrosion inhibitors varies with corrosive media

Passivation-type (anodic) and precipitation-type inhibitors, mostly inorganic substances, are commonly used for neutral aqueous media. Adsorption-type inhibitors, which are mostly organic compounds, are applied in acidic media.

2. Selection of corrosion inhibitors varies with substrate metals

Single-component inhibitors deliver lower inhibition efficiency compared with blended inhibitors. For this reason, blended inhibitors are widely preferred over single-component ones in modern applications.