Inhibitors

Inhibitors are chemicals that react with the surface of a material decreasing the material’s corrosion rate, or interact with the operating environment to reduce its corrosivity. Inhibitors may be introduced into the environment in which the material is operating as solutions or dispersions to form a protective film. For instance, they can be injected into a completely aqueous recirculating system (e.g. automobile radiators) to reduce the corrosion rate in that system. They may also be used as additives in coating products, such as surface treatments, primers, sealants, hard coatings, and corrosion preventive compounds (CPCs). Furthermore, some inhibitors can be added to water that is used to wash a vehicle, system or component.

Corrosion inhibitors interact with the metal, slowing the corrosion process by:

• shifting the corrosion potential of the metal’s surface toward either the cathodic or anodic end
• preventing permeation of ions into the metal
• increasing the electrical resistance of the surface

The corrosion potential of a metal is shifted toward the anodic end by inhibiting the cathodic process. This is accomplished by using chemicals that inhibit the corrosion reactions taking place at the cathodic site of the corrosion cell, for example, blocking the hydrogen ions at the metal’s surface from combining to form hydrogen gas. Likewise, the corrosion potential of a metal is shifted toward the cathodic end by inhibiting the anodic process. This is accomplished by using chemicals that inhibit the corrosion reactions taking place at the anodic site of the corrosion cell for example, by keeping the metal from dissociating into ions.

Preventing the permeation of ions into the metal is accomplished by forming a protective film or layer on the metal surface. Inhibitors can form a protective barrier film, which effectively isolates the metal from the corrosive environment, or they can induce the formation of precipitates that block the corrosive agents from accessing the metal. Inhibitors can also increase the electrical resistance of the metal by passivating the surface.

Inhibitors are usually grouped into five different categories: passivating, cathodic, organic, precipitation, and vapor phase. Each of these groups is discussed separately in the following sections.