Corrosion Inhibitor: Definition, Types, Application and Mechanisms

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What is Corrosion?

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The surface of most metals gets corroded when they are exposed to air, water, acids etc. for a long period of time, this process is called Corrosion

Corrosion of metal is a greater threat than estimation. Each year tons and tons of iron (Fe) gets corroded, and a large sum of money has to be spent on it in order to retrieve the material. Small measures such as painting the surfaces, oiling, greasing etc. can be done to prevent corrosion.

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What are Corrosion Inhibitors?

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Corrosion Inhibitor is a chemical substance that is added to the environment in small concentrations to effectively reduce the rate of corrosion of the materials (especially metals) exposed in the same environment. Corrosion Inhibitor is usually denoted as CI.

• It acts as the first line defence against corrosion.
• These products tend to form a protective barrier on metal surfaces and break the chemical reaction, preventing rust to occur.

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Different Kinds of Corrosion Inhibitors

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The substance which is used to inhibit the corrosion rate are called corrosion inhibitors. Fundamentally there are three types of corrosion inhibitors namely Anodic inhibitors, Cathodic inhibitors, and Mixed inhibitors.

• Anodic Inhibitors: Anodic inhibitors also known as passivators are chemical substances that form a protective layer of oxide on the metal surface resulting in resistance to corrosion. These inhibitors alter the anodic reactions in a chemical cell, forcing the metal into passivate region. Anodic inhibitors are mostly considered unsafe due to their chemical properties.

Examples of anodic inhibitors might include:

1. Nitrite
2. Orthophosphate
3. Chromate
4. Molybdate

• Cathodic Inhibitors: Cathodic inhibitors either slow down the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface resistivity and limit the distribution of reducible species to these areas.

There are three different mechanisms by which cathodic inhibitors can provide inhibition:

1. Cathodic poisons: Arsenic and antimony are some of the cathodic inhibitors that work by making it difficult for hydrogen to reduce or recombine.
2. Cathodic precipitates: Ions such as calcium, zinc or magnesium are the cathodic inhibitors that form a protective layer on the metal by precipitating as oxides.
3. Oxygen scavenger: Oxygen scavengers by preventing the cathodic depolarization caused by oxygen help to inhibit corrosion. The most commonly used oxygen scavenger at ambient temperature(70-80^oF) is sodium sulphite (Na₂SO₃).

• Mixed Inhibitors: These are the types of corrosion inhibitors that tend to form a firm film on the surface of the metal. Both cationic and anionic reactions can be reduced by the help of mixed inhibitors as this process utilizes the precipitate formed on the surface of the metal. Silicates and Phosphates are used as water conditioners to prevent corrosion.

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Applications of Corrosion Inhibitors

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Following are some of the applications of Corrosion Inhibitors:

• Corrosion inhibitors are used for different purposes such as in petrochemical plants, oil and gas wells, reinforcing bars in concrete, and the mining industry etc.
• Corrosion inhibitors (CI) are used to prevent rusting of fuel pipes and make sure that no corrosion occurs.
• Water transmission and distribution systems uses corrosion inhibitors like phosphates, amines volatiles (cyclohexylamine, morphine) along with pH adjusters and alkalinity controls towards an efficient protection. 
• Thermoelectric uses Ammonia, Cyclohexylamine, alkanol and Morpholine as inhibitors in boilers in various processes.
• The corrosion inhibitors are added in the hydrochloric acid used for the solubilization of limescale to prevent the attack on pipes.

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Mechanisms of Corrosion Inhibitors

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Following are some of the mechanisms of Corrosion Inhibitors:

• A protective thin film with inhibitor effect or by combination between inhibitor ions and metallic surface is formed; by the chemical adsorption (chemisorption) of the inhibitor on the surface of the metal.
• The reaction of the inhibitor with a potential corrosive component present in aqueous media and results as a complex.
• By oxide protection of the base metal, the inhibitor leads a formation of film.

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Points to Remember

Following are some important points:

• The silicates and the phosphates are included in the list of most common Corrosion Inhibitors.
• Passivation is a process for prevention from corrosion inhibitors as it is a light coating of a material such as metal oxide which creates a layer of protection on the metal as a barrier against corrosion.
• Volatile Corrosion Inhibitors (VCI) is also a type of corrosion inhibitors which are transported in a closed environment to the corrosion site. It is due toa process of volatilization from a source.
• However, we would get 4 types of corrosion inhibitors, namely, Anodic inhibitors, Cathodic inhibitors, Mixed inhibitors, and Volatile Corrosion Inhibitors (VCI).
• urotropine, thiourea, o-xylene thiourea, ruthenium, industrial xylene thiourea are some examples of corrosion inhibitors.