Overview
Corrosion inhibition involves the addition of a chemical to the environment to make the environment less aggressive or to protect the metal. Since the chemicals are added to the environment, this approach is often used for closed systems which are not open to the atmosphere. Examples are recirculated cooling water systems or acid pickling baths. Corrosion inhibitors are also added into organic coatings, and can be released to alter the local environment for a coating exposed under atmospheric conditions. One reference with many examples of corrosion inhibitors, the metals they are used for, and the mechanisms by which they act is "Organic Corrosion Inhibitors" by Y. I. Kuznetsov, Plenum Press, New York, 1996".
Types of Corrosion Inhibitors
Corrosion inhibitors can be categorized in different ways:
Film Formers or Environment Alteration
- Film forming inhibitors that act by forming a protective film on the surface by adsorbing on the surface and thereby block the environment.
- An example is the inhibition of copper corrosion by benzotriazole or tolyltriazole in which these molecules adsorb on the copper surface to protect the copper metal below the film.
- Film forming inhibitors that act by reacting with the metal surface to create a passive layer between the metal and the environment.
- An example is the inhibition of carbon steel in water by phosphonate type compounds in which the phosphonate acts by reaction with the metal and components in the water (such as calcium) to create a more impervious passive layer on the steel surface.
- Inhibitors that alter the environment
- Molecules that act by removing aggressive species from the environment by reacting with them. such as oxygen getters that remove dissolved oxygen
- Oxygen which can be an aggressive corrosion agent has been removed by this procedure. Examples of molecules that can react with oxygen are sodium sulfite and hydrazine. These agents are sometimes called oxygen getters.
- Buffer the environment to prevent changes in acidity (pH shifts)
- Borate is an example of a common buffer
- Provide oxidizing power to the environment to passivate the metal.
- This approach can be dangerous and should be used with caution. For example, too much oxidizing power could result in the corrosion potential of the metal being pushed into a region in which pitting or stress corrosion cracking could be more prone to initiate.
- Molecules that act by removing aggressive species from the environment by reacting with them. such as oxygen getters that remove dissolved oxygen
Anodic, Cathodic, or Mixed Inhibitors
These types of inhibitors act by interfering with parts of the corrosion reaction. They are classified in the following way:
- Anodic inhibitors primarily slows the reaction kinetics of the anodic reaction. The corrosion potential is usually shifted in the positive direction.
- Cathodic inhibitors primarily slows the reaction kinetics of the cathodic reaction. The corrosion potential is usually shifted in the negative direction.
- Mixed inhibitors tend to slow the reaction kinetics of both the anodic and cathodic reactions about equally. The corrosion potential may or may not shift.
Organic or Inorganic
- Organic inhibitors are commonly used in acid solutions and they usually act by inhibiting the cathodic reaction. A common application is acid pickling which is used to remove oxide scale from steel ingots. Acids chemically dissolve the oxide scales, and inhibitors are needed to inhibit the electrochemical dissolution of the metal after the scale is removed.
- Inorganic inhibitors are commonly used in neutral to alkaline environments and they usually act by forming a film and inhibiting the anodic reaction.
- chromate is a common inorganic inhibitor that is a film former (but it also inhibits the cathodic reaction)
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