How Do Corrosion Inhibitors Prevent Metal Damage?
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 Inhibitors: Types, Examples, and Applications
1. What is a corrosion inhibitor?
A corrosion inhibitor is a chemical substance that, when added in a small concentration to an environment (like water or a coating), significantly slows down the rate of corrosion of a metal or alloy. It functions by interfering with the electrochemical reactions that cause corrosion, typically by forming a protective, passive film on the metal's surface.
2. What are the main types of corrosion inhibitors?
Corrosion inhibitors are primarily classified based on their mechanism of action and chemical nature. The main types are:
Anodic Inhibitors: These form a protective passive film over the anodic (oxidation) sites of the metal. Examples include chromates, nitrites, and molybdates. They are highly effective but must be used in sufficient concentration.
Cathodic Inhibitors: These slow down the cathodic (reduction) reaction. They either precipitate as a film on cathodic sites (e.g., zinc salts) or poison the reaction itself (e.g., compounds of arsenic and antimony).
Mixed Inhibitors: These reduce the rate of both anodic and cathodic reactions, often by adsorbing over the entire metal surface. Many organic inhibitors, like amines, fall into this category.
Vapour Phase Inhibitors (VPI): These are volatile compounds used to protect metal components in enclosed spaces. They vaporise and then condense on the metal surface to form a protective film.
3. What are some common examples of corrosion inhibitors used in industries?
Different industries use specific inhibitors tailored to their unique environments. For example:
In cooling water systems, phosphonates and molybdates are used to protect pipes and equipment.
For protecting steel reinforcement in concrete, calcium nitrite is a common and effective inhibitor.
In the oil and gas industry, long-chain organic molecules like amines and imidazolines are used to protect pipelines from internal corrosion.
For automotive engine coolants, sodium nitrite and silicates are added to prevent corrosion of the engine block and radiator.
4. What are the primary methods used to control or prevent corrosion?
While corrosion inhibitors are a key method, several strategies are used to control corrosion, often in combination:
Protective Coatings: Applying a barrier like paint, plastic, or a metallic coating (e.g., galvanising steel with zinc) to isolate the metal.
Cathodic Protection: Making the metal structure the cathode of an electrochemical cell, either by connecting it to a more reactive metal (sacrificial anode) or by using an external power source (impressed current).
Alloying: Creating corrosion-resistant alloys, such as stainless steel (iron, chromium, nickel), which forms its own passive layer.
Using Corrosion Inhibitors: Adding specific chemicals to the environment to slow the corrosion reactions.
Design Modification: Designing structures to avoid trapping moisture and ensure proper drainage.
5. How do anodic and cathodic inhibitors differ in their mechanism of action?
Anodic and cathodic inhibitors protect metals in fundamentally different ways. Anodic inhibitors, like nitrites, work by reacting with the metal at the anode (where oxidation occurs) to form an insoluble passive film. This film acts as a barrier, directly stopping the metal from dissolving. In contrast, cathodic inhibitors target the cathode (where reduction occurs). They either precipitate on cathodic sites to physically block the reaction (like zinc ions forming zinc hydroxide) or they directly poison the reduction reaction (like arsenic compounds slowing hydrogen evolution). The key difference is their target: anodic inhibitors block metal loss, while cathodic inhibitors block the reacting species.
6. Why is it crucial to use the correct concentration of a corrosion inhibitor?
Using the correct concentration of a corrosion inhibitor is critical for both safety and effectiveness. If the concentration is too low, especially for an anodic inhibitor, it can be more dangerous than using no inhibitor at all. An insufficient amount may only cover parts of the metal surface, leaving small anodic areas exposed. The entire corrosion current is then focused on these tiny spots, leading to severe, localised pitting corrosion, which can cause rapid and unexpected material failure. An excessive amount is uneconomical and can lead to environmental issues or interfere with other system processes.
7. What is the importance of adsorption in how organic corrosion inhibitors work?
Adsorption is the primary mechanism for most organic corrosion inhibitors. These molecules are designed with a polar 'head' containing heteroatoms (like Nitrogen, Sulphur, or Oxygen) and a non-polar hydrocarbon 'tail'. The polar head has a strong affinity for the metal surface and physically or chemically adsorbs onto it. This process forms a tightly packed, thin film across the surface. This adsorbed layer acts as a physical barrier, effectively blocking corrosive species in the environment from reaching the metal and preventing the electrochemical reactions of corrosion.
8. Can a single corrosion inhibitor work effectively in any environment?
No, corrosion inhibitors are highly specific to the metal-environment system they are intended for. An inhibitor that works perfectly for steel in an acidic solution might be completely ineffective or even accelerate corrosion for aluminum in an alkaline solution. The effectiveness of an inhibitor depends on multiple factors, including the type of metal, the pH of the environment, temperature, fluid velocity, and the presence of other ions like chlorides. Therefore, selecting the right inhibitor always requires a careful analysis of the specific corrosive conditions.
9. What are 'green' corrosion inhibitors and why are they becoming more important?
'Green' corrosion inhibitors are substances derived from natural, renewable sources, such as plant extracts, that can slow down corrosion. These extracts contain compounds like alkaloids, tannins, and amino acids. They are gaining importance because they offer an alternative to traditional, highly toxic inhibitors (like chromates). The primary advantages of green inhibitors are:
Low Toxicity and Biodegradability: They are environmentally benign and pose minimal risk to health.
Sustainability: They are sourced from renewable plant materials.
Cost-Effectiveness: They can often be produced cheaply from abundant natural resources or agricultural byproducts.
