Corrosion and Its Prevention: Definition, Types, Causes, Methods, and Applications in Electrochemistry

Corrosion is the gradual deterioration of pure materials, especially metals, when they react with environmental elements such as moisture, air, or acids. It is a redox (reduction-oxidation) process that results in the conversion of metals into more stable compounds like oxides, sulphides, or hydroxides, leading to the loss of the metal’s strength and other physical properties.

For example, when iron is exposed to moisture and oxygen in the air, it forms iron oxide, commonly known as rust. This process weakens the metal, making it brittle, flaky, and structurally unreliable. Although metals like iron are valued for their strength and rigidity (especially when alloyed with other elements), corrosion severely diminishes these qualities.

Since it involves redox reactions between the metal and atmospheric agents like water, oxygen, or sulfur dioxide, corrosion is considered an electrochemical process. It is generally viewed as a harmful phenomenon due to the damage it causes to metal structures and components.

Factors Affecting Corrosion:

• Exposure to atmospheric gases: Metals can corrode when exposed to gases like carbon dioxide (CO₂), sulfur dioxide (SO₂), and sulfur trioxide (SO₃) present in the air.

• Moisture, especially saltwater: The presence of moisture, particularly saline water, significantly accelerates the rate of corrosion.

• Presence of impurities: Impurities such as salts (e.g., sodium chloride, NaCl) on metal surfaces can enhance corrosion.

• Temperature effects: Higher temperatures tend to increase the rate of corrosion.

• Type of oxide layer formed: Some metals form a stable, insoluble oxide layer (e.g., aluminum forming Al₂O₃) that protects the surface and slows further corrosion.

• The presence of acidic compounds in the atmosphere can greatly accelerate the corrosion process.

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Types of Corrosion

The following are the types of corrosion types:

1. Crevice Corrosion:

Crevice corrosion is a localized form of corrosion that occurs in confined spaces where there is a difference in ionic concentration between adjacent areas on a metal surface. These crevices create stagnant conditions that promote corrosion. Common sites include gaps under gaskets, washers, bolt heads, and other shielded areas. This type of corrosion can affect all grades of stainless steel and aluminium alloys.

2. Stress Corrosion Cracking (SCC):

Stress corrosion cracking is a type of corrosion that leads to the sudden failure of metals due to the combined influence of a corrosive environment and sustained tensile stress. It is more likely to occur at elevated temperatures. A common example is the cracking of austenitic stainless steel when exposed to chloride-containing solutions.

3. Intergranular Corrosion (IGC):

Intergranular corrosion occurs along the grain boundaries of a metal, often due to the presence of impurities or the depletion/enrichment of certain elements at these boundaries during solidification. This localized attack weakens the metal without visibly affecting the surface. Aluminium-base alloys, among others, are particularly susceptible to IGC.

4. Galvanic Corrosion:

Galvanic corrosion occurs when two electrochemically dissimilar metals come into electrical contact in the presence of an electrolyte. One metal (the more active one) corrodes faster, while the other is protected. This type of corrosion typically occurs at the junction between the two metals. For example, when copper and steel are in contact in saltwater, steel tends to corrode. Similarly, when aluminium is coupled with carbon steel in seawater, the aluminium corrodes more rapidly while the steel remains protected.

5. Pitting Corrosion:

Pitting corrosion is a highly localized and dangerous form of corrosion that is difficult to detect due to its unpredictable nature. It typically begins at a single point on the metal surface where the protective layer is compromised, forming a small anodic site surrounded by the unaffected metal acting as the cathode. This leads to the creation of a corrosion cell. Once a pit is initiated, it continues to grow—often vertically into the metal—potentially leading to serious structural damage over time. For example, a droplet of water on a steel surface can initiate pitting at its center, where the oxygen concentration is lowest.

6. Uniform Corrosion:

Uniform corrosion is the most common and easily recognizable type of corrosion, where the entire surface of a metal is attacked evenly by the surrounding environment. The extent of rusting is usually visible and predictable. Because the material degrades uniformly, it often has a relatively minor impact on overall structural performance—especially when monitored and managed. For instance, when a piece of zinc or steel is placed in diluted sulfuric acid, it tends to corrode at a consistent rate across its entire surface.

Corrosion Examples and Reactions

Here are some common examples of corrosion, which are typically encountered in metals.

Copper Corrosion

When copper metal is exposed to the environment, it reacts with oxygen in the air to form copper(I) oxide (Cu₂O), a reddish-brown compound.

2Cu + 1/2 O₂ → Cu₂O

Cu₂O is oxidised further to generate CuO, which is black in colour.

Cu₂O+ 1/2O₂ → 2CuO

CuO interacts with CO₂, SO₃, and H₂O in the environment to produce Cu₂(OH)₂ (Malachite), a blue mineral, and Cu₄SO₄(OH)₆ (Brochantite), a green mineral. The colour of the copper plating on the Statue of Liberty, which has turned blue-green, is a good example of this.

Silver Tarnishing

When exposed to air, silver reacts with sulfur—primarily in the form of hydrogen sulfide (H₂S), which is released from certain industrial processes—to form silver sulfide (Ag₂S), a dark-colored compound that tarnishes the metal’s surface.

2Ag + H₂S → Ag₂S+ H⁺₂

Corrosion of Iron (Rusting)

When iron comes into contact with air or water, rusting occurs—a common and well-known example of corrosion. This process resembles the operation of a typical electrochemical cell.

At the anode, iron (Fe) loses electrons and is oxidized to ferrous ions (Fe²⁺):

Fe → Fe²⁺ + 2e⁻

These electrons then travel through the metal to another region—the cathode—where they reduce hydrogen ions (H⁺). These H⁺ ions originate from water (H₂O) or carbonic acid (H₂CO₃), which forms when carbon dioxide (CO₂) dissolves in water:

2H⁺ + 2e⁻ → H₂ (gas)

This movement of electrons and the redox reactions between iron and the environment initiate the rusting process, ultimately resulting in the formation of iron oxides (rust).

H₂O ⇌ H⁺ + OH^–

H₂CO₃⇌ 2H⁺ + CO₃²

Prevention from Corrosion

Corrosion can be prevented in a number of ways. We’ll go over a few of the more popular ones below.

1. Electroplating:

Electroplating is an electrolysis-based proces used to coat a metal object (metal I) with a thin layer of another metal (metal II) to protect it from corrosion and improve its appearance.

In this process:

The metal to be plated (metal I) is made the cathode (connected to the negative terminal).

The coating metal (metal II) is used as the anode (connected to the positive terminal).

2. Cathodic Protection:

Cathodic protection is a method used to prevent corrosion by connecting the base metal (the metal to be protected) to a sacrificial metal that is more reactive. The sacrificial metal acts as the anode, undergoing oxidation and releasing electrons:

Sacrificial Metal → Metalⁿ⁺ + ne⁻

These electrons flow to the base metal, making it the cathode, which prevents it from corroding. The ions from the sacrificial metal take part in corrosion reactions, effectively sacrificing themselves to protect the base metal.

Commonly used sacrificial metals include zinc, magnesium, and aluminium.

3. Galvanization:

Galvanization is the process of coating iron or steel with a thin layer of zinc to protect it from corrosion. This is typically done by dipping the iron object into molten zinc. The zinc layer acts as a physical barrier and, being more reactive than iron, also serves as a sacrificial metal—corroding in place of the underlying iron if the coating is damaged.

4. Painting and Greasing:

Applying a protective layer of paint, grease, or oil to metal surfaces helps prevent corrosion by isolating the metal from air, moisture, and other corrosive agents. This barrier blocks the environmental elements that typically initiate the corrosion process. It’s a simple and cost-effective method commonly used for machinery, vehicles, and outdoor structures.

5. Use of Corrosion Inhibitors:

Corrosion inhibitors are chemicals that, when added to a corrosive environment, slow down or prevent the corrosion process. They work by forming a protective film on the metal surface or by altering the chemical environment to make it less aggressive. Corrosion inhibitors are commonly used in cooling systems, pipelines, and boilers.

6. Use of Appropriate Materials:

One effective way to prevent corrosion is by selecting materials that are inherently resistant to it. For example, aluminum and stainless steel form protective oxide layers on their surfaces, which shield them from further environmental damage. Using such corrosion-resistant materials in design and construction significantly reduces the risk of corrosion over time.

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