What Is a Corrosion Inhibitor and How It Prevents Metal Corrosion
Corrosion destroys various metals such as iron, steel, and copper. Various techniques and substances are used to inhibit or prevent the process of corrosion. One of the processes or techniques is Corrosion inhibitors to prevent or reduce the rate of corrosion.
Chromate, molybdate, and nitrate are the most commonly used corrosion inhibitors. Corrosion inhibitor spray is also used to inhibit the rusting process.
WD-40 is one corrosion inhibitor spray. There are various manufacturers of corrosion inhibitors.
What is Corrosion?
Corrosion is the process of destruction of a material as a result of its interaction with its surroundings.
Examples of Corrosion
Formation of rust on the iron surface which is also an electrochemical process
The blue-green coating on the surface of copper metal
Tarnishing of silver
What are Corrosion Inhibitors?
Corrosion inhibitors are chemical substances that when added to the environment reduce the rate of corrosion and are hence used to protect the substance from corrosion.
Chemicals that are used to prevent and slow down the process of corrosion are generally liquid or gases.
Corrosion Inhibitors for Water - when corrosion inhibitors are mixed with water, their effect may depend on the quality or properties of water such as viscosity, temperature, etc.
Types of Corrosion Inhibitors
Corrosion inhibitors are generally of three types depending on the type of method used by them.
Anodic Inhibitor - In this type of corrosion inhibitor. It helps in the formation of a protective oxide layer on a metal surface. This reaction causes a considerable anodic move transforming the metallic exterior into a passivation area. This passivation helps in decreasing the corrosion of the metal. Examples of anodic corrosion inhibitors - Chromates, nitrates, and molybdates can be used as anodic inhibitors.
Cathodic Inhibitor - This type of inhibitor is used for decreasing the cathodic reaction. They also work to hasten the cathodic metal region to confine the dispersion to the metal surface of the eroded metal. Examples of cathodic corrosion inhibitors -
Sulfite and bisulfite can be used as cathodic inhibitors Which react with the oxygen to make sulfates.
Redox reaction catalysts by nickel are also another example of a cathodic reaction.
Mixed Inhibitors - Mixed corrosion inhibitors also form a thin protective layer on the surface of the metal. They worked to lessen the process of anodic as well as cathodic reactions. This is done through the formation of a precipitate on the surface of a metal.
Silicates and phosphates can be used as mixed inhibitors. These chemicals react with water to stop the process of corrosion and are also used as water softeners to stop the rusting of water.
Examples of mixed corrosion inhibitors - are silicates and phosphates.
Mechanisms of Corrosion Inhibitors
Cathodic Toxic Substances: These are utilized by smothering the procedures of cathodic decrease to adjust the reaction at the anode. The vulnerability of the metal to hydrogen-initiated breaking can be inclined to cathodic inhibition because the metal can retain hydrogen during cathodic charging or aqueous corrosion. In low-pH solutions, some decreased hydrogen diffuses as atomic hydrogen into the metal as opposed to the gas formation. This occurs during electroplating or pickling of the metal.
Oxygen Scavengers: These are synthetic chemicals that react with dissolved oxygen for a reduction in corrosion. The best examples are Sulfite and bi-sulfite ions that lead to the formation of sulfates while reacting with oxygen. Before any bringing down of oxygen dissolved in mud is finished by a scavenger, the air is expelled from the mud through mechanical foaming and degassing.
Cathodic Precipitates: These incorporate zinc, calcium and magnesium. They are accelerated on the metal surface to shape into a defensive layer. Since the task of an inhibitor is to diminish the anodic procedure rate, the possible corrosion change after an inhibitor has been included demonstrates a hindrance in the procedure. Positive displacement of the corrosion potential demonstrates an obstacle of the anodic method. The negative displacement of the potential shows the impediment of the cathodic process.
Inhibitors
An inhibitor is a chemical compound or mixture of chemicals that, when given at extremely low concentrations to a corrosive environment, effectively prevents or lowers corrosion while causing no substantial interaction with the environment's components. Inhibitors are useful in closed environmental systems with excellent circulation because they ensure an appropriate and regulated concentration of inhibitors. These conditions can be reached in a variety of applications, including cooling water recirculation systems, oil refining, oil production and acid pickling of steel components. Antifreeze for vehicle radiators is one of the most well-known applications for inhibitors. Inhibitors can be organic or inorganic chemicals, and they are often dissolved in aqueous solutions. They reduce corrosion by acting as a barrier, by forming an adsorbed layer or by delaying the anodic, cathodic, or both corrosion processes.
Factors to Consider While Selecting an Inhibitor
Cost of the inhibitor.
The toxicity of the inhibitor can cause ill effects on human beings and other living spices.
The availability of the inhibitor determines its selection.
Inhibitors should be environment friendly.
Corrosion Classification
Corrosion has been divided into the following methods:
Low-temperature corrosion and high-temperature corrosion (or)
Electrochemical corrosion and chemical corrosion (or)
Wet and Dry corrosion.
Wet corrosion happens when a metal comes into touch with an electrolytic conducting liquid or when two different metals or alloys are submerged or partially immersed in the electrolytic conducting solutions. This is always connected with low temperatures. Dry corrosion occurs mostly as a result of the direct chemical action of air gasses and vapours in the environment. This is usually linked with a high temperature.
Effective Method of Using Corrosion Inhibitor
One of the most effective ways to fight corrosion is to utilize corrosion inhibitors. Three elements must be addressed for them to be used effectively:
Identification of corrosion problems
Anodic inhibition (polarization of the anode to be increased)
Cathodic inhibition (polarisation of the cathode to be increased)
Resistance inhibition (increase in circuit's electrical resistance simultaneously with creating a thin or thick layer on the metal's surface)
Diffusion restriction (preventing the diffusion of depolarizers)
Conclusion
Corrosion inhibitors are chemicals used to inhibit or slow down the process of corrosion.
There are three main types of corrosion inhibitors- anodic, cathodic, and mixed. Corrosion inhibitors are used to decrease the process of rusting. Corrosion inhibitor chemicals are synthesized by various methods. These are also available in spray form.
FAQs on Corrosion Inhibitor in Chemistry
1. What is a corrosion inhibitor in chemistry?
A corrosion inhibitor is a chemical substance that reduces or prevents the corrosion rate of metals when added in small amounts to a corrosive environment. It works by interfering with the electrochemical reactions involved in corrosion.
- Forms a protective film on the metal surface
- Reduces metal oxidation (loss of electrons)
- Slows down anodic, cathodic, or both reactions
2. How do corrosion inhibitors work?
Corrosion inhibitors work by forming a protective barrier or altering the electrochemical reactions that cause metal oxidation. They act through one or more mechanisms:
- Adsorption: Molecules adsorb onto the metal surface, forming a thin protective film.
- Anodic inhibition: Slows the metal oxidation reaction (e.g., Fe → Fe2+ + 2e-).
- Cathodic inhibition: Slows the reduction reaction, such as O2 + 2H2O + 4e- → 4OH-.
- Mixed inhibition: Affects both anodic and cathodic reactions.
3. What are the main types of corrosion inhibitors?
The main types of corrosion inhibitors are anodic inhibitors, cathodic inhibitors, and mixed inhibitors.
- Anodic inhibitors: Reduce metal dissolution; examples include chromates (CrO42-) and nitrites (NO2-).
- Cathodic inhibitors: Slow reduction reactions; examples include zinc salts and phosphates.
- Mixed inhibitors: Affect both anodic and cathodic processes; many organic inhibitors fall in this category.
4. What is an example of a corrosion reaction of iron?
An example of iron corrosion is the rusting reaction, where iron reacts with oxygen and water to form hydrated iron(III) oxide.
- Anodic reaction: Fe(s) → Fe2+(aq) + 2e-
- Cathodic reaction: O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
- Overall simplified reaction: 4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s)
5. What is the difference between anodic and cathodic corrosion inhibitors?
Anodic inhibitors slow down metal oxidation, while cathodic inhibitors slow down the reduction reaction in a corrosion cell.
- Anodic inhibitors: Reduce metal dissolution at the anode (e.g., Fe → Fe2+ + 2e-).
- Cathodic inhibitors: Reduce reactions such as oxygen reduction (O2 + 2H2O + 4e- → 4OH-).
- Anodic inhibitors must be used in adequate concentration to avoid localized corrosion.
6. What are organic corrosion inhibitors?
Organic corrosion inhibitors are carbon-containing compounds that protect metals by adsorbing onto their surfaces and forming a protective film.
- Often contain heteroatoms like N, O, or S
- Examples include amines, imidazolines, and thiourea derivatives
- Commonly used in acidic environments such as HCl pickling solutions
7. Why are corrosion inhibitors used in cooling water systems?
Corrosion inhibitors are used in cooling water systems to prevent metal degradation caused by dissolved oxygen, salts, and varying pH.
- Protect steel pipes and heat exchangers
- Reduce formation of rust and scale
- Extend equipment lifespan and reduce maintenance costs
8. What factors affect the efficiency of a corrosion inhibitor?
The efficiency of a corrosion inhibitor depends on concentration, temperature, pH, and the nature of the metal and corrosive medium.
- Concentration: Too low may be ineffective; too high may cause side effects.
- Temperature: Higher temperatures may reduce adsorption.
- pH: Influences inhibitor stability and ionization.
- Type of metal: Steel, aluminum, and copper respond differently.
9. What is the role of adsorption in corrosion inhibition?
Adsorption plays a key role in corrosion inhibition by allowing inhibitor molecules to attach to the metal surface and block active corrosion sites.
- Forms a thin, protective molecular layer
- Prevents contact between metal and corrosive agents
- May involve physical (physisorption) or chemical bonding (chemisorption)
10. Can you give an example of a common inorganic corrosion inhibitor?
A common inorganic corrosion inhibitor is sodium nitrite (NaNO2), which acts as an anodic inhibitor for steel.
- Promotes formation of a passive iron oxide layer
- Reduces the reaction Fe → Fe2+ + 2e-
- Widely used in closed cooling and boiler systems
