What is Corrosion?

Chemical reactions may be found everywhere around us, from our bodies’ food metabolism to how the light we receive from the sun is produced through chemical reactions. It’s crucial to understand physical and chemical changes before starting with chemical reactions. The best example of physical and chemical change is a burning candle.

Take a candle and put it on the table. We can see how the candle turns to wax as time goes on. The candle will go out if you cover it with a jar. The burning of the candle is a chemical change, whereas the conversion of the candle to wax is a physical change in the demonstration. A physical change primarily results in a change in the state of the substance, whereas a chemical change primarily results in the formation of a new substance in which energy is either released or absorbed. As a result, we can deduce that chemical changes are followed by physical modifications.

What is Corrosion?

One of the most typical phenomena we see in our daily lives is corrosion. You’ve probably seen that over time, some iron objects become covered in an orange or reddish-brown coloured layer. This layer is formed as a result of a chemical reaction known as rusting, which is a type of corrosion.

Corrosion is the process by which refined metals are transformed into more stable compounds such as metal oxides, metal sulphides, and metal hydroxides. The development of iron oxides occurs as a result of the action of air moisture and oxygen on iron. Corrosion is commonly regarded as a bad phenomenon since it compromises the metal’s good characteristics. 

Iron, for example, is recognised for its tensile strength and stiffness (especially alloyed with a few other elements). Rusting, on the other hand, causes iron items to become brittle, flaky, and structurally unsound. Corrosion is an electrochemical process because it usually involves redox interactions between the metal and certain atmospheric agents including water, oxygen, and Sulphur dioxide, among others.

Do All Metals Corrode?

Metals with a greater reactivity series, such as iron and zinc, corrode quickly, whereas metals with a lower reactivity series, such as gold, platinum, and palladium, do not corrode. The reason for this is because corrosion requires the oxidation of metals. The tendency to oxidise decreases as we progress down the reactivity series (oxidation potentials is very low). Interestingly, although being reactive, aluminium does not corrode like other metals. This is due to the fact that aluminium is already covered with an oxide layer. It is protected from further corrosion by this layer of aluminium oxide.

Factors Affecting Corrosion:

• Metals are exposed to gases such as CO₂, SO₂, and SO₃ in the air.
• Metals are exposed to moisture, particularly saltwater (which increases the rate of corrosion).
• Impurities such as salt are present (e.g. NaCl).
• Temperature: As the temperature rises, so does the rate of corrosion.
• The nature of the first oxide layer that forms: some oxides, such as Al₂O₃, generate an insoluble protective coating that can prevent further corrosion. Rust, for example, crumbles readily and exposes the rest of the metal.
• Presence of acid in the atmosphere: acids have the ability to speed up the corrosion process.

Types of Corrosion

The following are the types of corrosion types:

1. Crevice Corrosion: A limited kind of corrosion known as crevice corrosion can occur whenever there is a difference in ionic concentration between any two local locations of a metal. Gaskets, the underside of washers, and bolt heads are all places where crevice corrosion can occur. Crevice corrosion occurs in all grades of aluminium alloys and stainless steel, for example.
2. Stress Corrosion Cracking: Corrosion Due to Stress SCC refers to the breaking of metal as a result of the corrosive environment and the tensile stress exerted on it. It happens a lot when the weather is hot. In a chloride solution, stress corrosion cracking of austenitic stainless steel is an example.
3. Intergranular Corrosion: The presence of contaminants in the grain boundaries that separate the grain generated during the solidification of the metal alloy causes intergranular corrosion. Depletion or enrichment of the alloy at these grain boundaries can also cause it. IGC, for example, has an impact on aluminium-base alloys.
4. Galvanic Corrosion: Galvanic corrosion can occur when an electric contact develops between two metals that are electrochemically different and are in an electrolytic environment. It describes the breakdown of one of these metals at a joint or junction. The degradation that occurs when copper comes into contact with steel in a saltwater environment is a good illustration of this form of corrosion. When aluminium and carbon steel are linked and submerged in seawater, the aluminium corrodes faster while the steel is protected.
5. Pitting Corrosion: Pitting Corrosion is unpredictably unpredictable, making it difficult to detect. It is regarded as one of the most hazardous forms of corrosion. It starts at a single location and progresses to the production of a corrosion cell encircled by the regular metallic surface. Once established, the ‘Pit’ continues to develop and can take on a variety of shapes. The pit progressively eats away at metal from the surface in a vertical direction, eventually leading to structural failure if not addressed. Consider a droplet of water on a steel surface; pitting will begin near the water droplet’s centre (anodic site).
6. Uniform Corrosion: This is the most prevalent type of corrosion, in which the environment attacks the metal’s surface. The degree of the rusting can be seen clearly. This sort of corrosion has a minimal impact on the material’s performance. A piece of zinc or steel immersed in diluted sulphuric acid would normally dissolve at a constant rate throughout its whole 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 combines with oxygen in the air to produce copper (I) oxide, which is a reddish-brown substance.

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

Silver combines with Sulphur in the air to form silver sulphide (Ag₂S), which is a dark substance. Exposed silver reacts with H₂S in the environment, which is present due to some industrial processes, to generate Ag₂S.

2Ag + H₂S → Ag₂S+ H⁺₂

• Corrosion of Iron (Rusting)

When iron comes into touch with air or water, rusting occurs, which is the most typical occurrence. The reaction resembles that of a normal electrochemical cell. Metal iron loses electrons and is converted to Fe²⁺ in this process (this could be considered as the anode position). The electrons that are lost will travel to the opposite side and interact with H+ ions. H+ ions are emitted in the atmosphere by either H₂O or H₂CO₃ (this could be considered as the cathode position).

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: It’s an electrolysis-based method that coats a metal (I) with a thin layer of another metal (II). The new metal covering protects the metal (I) from corrosion in this way. Metal (I) (metal to be plated) is used as the anode and metal (I) (metal to be plated) is used as the cathode in this procedure. Metal ‘I’ is connected to the negative terminal, while metal ‘II’ is connected to the positive terminal. When electricity is applied to these two electrodes, oxidation occurs in the anode, resulting in the dissolution of metal II ions in the electrolyte. At the cathode, these dissolved metal II ions are reduced, resulting in a coating on metal I. Copper, Nickel, Gold, Silver, Zinc, and other metals are often used as anodes.
2. Cathodic Protection: The base metal is connected to a sacrificial metal that corrodes instead of the base metal in this procedure. This sacrificial metal (which is more reactive than the base metal) will release electrons and become oxidised as a result. The ions produced as a result of this process participate in corrosion reactions, preserving the base metal.
3. Galvanization: This procedure includes applying a thin layer of zinc to iron. In most cases, this is accomplished by dipping iron in molten zinc. As a result, the zinc layer protects the iron against corrosion.
4. Painting and Greasing: Applying a layer of paint or grease to the metal can keep it from coming into contact with the outside world, preventing corrosion.
5. Using Corrosion Inhibitor: Corrosion inhibitors are substances that, when introduced into a corrosion environment, reduce the rate of corrosion.
6. Make use of the Right Material: Corrosion can also be avoided by selecting the proper material. Aluminium and stainless steel, for example, are extremely corrosion resistant.

Sample Problems

Question 1: What do you understand by Corrosion?

Answer:

Corrosion is the process by which refined metals are transformed into more stable compounds such metal oxides, metal sulphides, and metal hydroxides. The development of iron oxides occurs as a result of the action of air moisture and oxygen on iron. Corrosion is commonly regarded as a bad phenomenon since it compromises the metal’s good characteristics. Iron, for example, is recognised for its tensile strength and stiffness (especially alloyed with a few other elements).

Rusting, on the other hand, causes iron items to become brittle, flaky, and structurally unsound. Corrosion is an electrochemical process because it usually involves redox interactions between the metal and certain atmospheric agents including water, oxygen, and Sulphur dioxide, among others.

Question 2: What are the factors that affect corrosion?

Answer:

The factors that affect corrosion are as follow:

• Metals are exposed to gases such as CO2, SO2, and SO3 in the air.
• Metals exposed to moisture, particularly salt water (which increases the rate of corrosion).
• Impurities such as salt are present (eg. NaCl).
• As the temperature rises, so does the rate of corrosion.
• The nature of the first oxide layer that forms: some oxides, such as Al2O3, generate an insoluble protective coating that can prevent further corrosion. Rust, for example, crumbles readily and exposes the rest of the metal.
• Presence of acid in the atmosphere: acids have the ability to speed up the corrosion process.

Question 3: What will happen when copper metal is exposed to the environment?

Answer:

When copper metal is exposed to the environment, it combines with oxygen in the air to produce copper (I) oxide, which is a reddish-brown substance.

2Cu + 1/2 O₂ → Cu₂O

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

Cu₂O+ 1/2O₂ → 2CuO

Question 4: What will happen when iron comes into touch with air or water?

Answer:

When iron comes into touch with air or water, rusting occurs, which is the most typical occurrence. The reaction resembles that of a normal electrochemical cell.

Metal iron loses electrons and is converted to Fe²⁺ in this process (this could be considered as the anode position). The electrons that are lost will travel to the opposite side and interact with H+ ions. H+ ions are emitted in the atmosphere by either H₂O or H₂CO₃ (this could be considered as the cathode position).

H₂O ⇌ H⁺ + OH^–

H₂CO³ ⇌ 2H⁺ + CO₃²

Question 5: What will happen when Silver combines with Sulphur in the air?

Answer

Silver combines with Sulphur in the air to form silver sulphide (Ag₂S), which is a dark substance. Exposed silver reacts with H₂S in the environment, which is present due to some industrial processes, to generate Ag₂S.

2Ag + H₂S → Ag₂S+ H⁺₂

Question 6:  How do you prevent metals from getting corrode?

Answer:

Corrosion prevention is critical in order to avoid significant losses. Metals make up the majority of the structures we employ. Bridges, autos, machines, and home items such as window grills, doors, and railway lines are all examples. Electroplating, galvanization, painting and lubrication, and the use of corrosion inhibitors are just a few of the popular methods for preventing corrosion.