Corrosion and Pollution

Corrosion

Definition and Causes: Corrosion is the gradual destruction of metals by chemical reactions with substances in their environment, especially oxygen and water. It weakens metals and causes damage to structures and machinery.

Rusting of Iron: Rusting is a specific type of corrosion that affects iron and its alloys (like steel). It occurs when iron reacts with oxygen and water to form hydrated iron(III) oxide, commonly known as rust.

The chemical reaction for rusting can be summarised as:

Iron reacts with oxygen and water:

The iron(III) hydroxide then dehydrates to form rust (iron(III) oxide):

Conditions for Rusting: Both water and oxygen must be present for rusting to occur. Without either, rusting slows or stops.

For example, iron left in dry air (oxygen but no water) rusts very slowly, and iron submerged in pure water with no oxygen rusts very little.

Prevention Methods: Although detailed prevention methods are covered in other topics, common ways include:

• Keeping iron dry to remove water
• Applying coatings like paint or oil to block oxygen and water
• Using sacrificial protection (e.g., zinc coating) to prevent rusting

Sources of Atmospheric Pollution

Combustion of Fossil Fuels: Burning fossil fuels such as coal, oil, and natural gas releases pollutants into the atmosphere. These fuels contain carbon and other elements that produce harmful gases when burned.

For example, power stations burning coal release smoke containing sulfur compounds and nitrogen oxides.

Industrial Emissions: Factories and industrial plants emit gases and particles from chemical processes and fuel combustion. These emissions include sulfur dioxide (SO₂), nitrogen oxides (NOₓ), and other pollutants.

Vehicle Exhausts: Cars, buses, and lorries burn petrol or diesel, producing exhaust gases such as carbon monoxide (CO), nitrogen oxides, and unburned hydrocarbons. Traffic congestion increases pollution levels in urban areas.

Natural Sources: Some atmospheric pollution comes from natural events like volcanic eruptions and wildfires. Volcanoes release sulfur dioxide and ash, while wildfires produce smoke and particulates.

These natural sources contribute to atmospheric pollution but are less controllable than human-made sources.

Effects of Atmospheric Pollution

Health Impacts: Air pollution can cause respiratory problems such as asthma, bronchitis, and other lung diseases. Pollutants irritate the airways and reduce lung function, especially in children and the elderly.

Environmental Damage: Pollutants like sulfur dioxide and nitrogen oxides can dissolve in rainwater, forming acid rain. Acid rain harms plants, soils, and aquatic life by lowering pH levels in ecosystems.

Corrosion of Buildings and Metals: Acid rain and other pollutants accelerate corrosion of metal structures and damage buildings made of limestone or marble. This leads to costly repairs and loss of cultural heritage.

For example, statues and old buildings in cities with high pollution often show signs of surface erosion and damage.

Pollutants and Their Impact on Corrosion

Sulfur Dioxide and Acid Rain: Sulfur dioxide (SO₂) is released mainly from burning fossil fuels containing sulfur. It reacts with water in the atmosphere to form sulfurous acid and eventually sulfuric acid, causing acid rain.

Acid rain increases the rate of corrosion on metals and damages buildings and statues made from materials like limestone.

Nitrogen Oxides: Nitrogen oxides (NOₓ) are produced from high-temperature combustion in vehicles and power stations. They also contribute to acid rain and can increase corrosion rates.

Particulates Enhancing Corrosion: Particulates (tiny solid particles) in polluted air can settle on metal surfaces, trapping moisture and pollutants. This creates a more corrosive environment, speeding up corrosion.

For example, soot and dust from industrial areas can make corrosion worse on exposed metal structures.

Learning Example: Calculating the Mass of Rust Formed

If 56 g of iron reacts completely with oxygen and water to form rust (iron(III) oxide, $Fe_2O_3$), what mass of rust is formed?

Step 1: Write the balanced equation for rust formation (simplified):

Step 2: Calculate moles of iron:

Molar mass of Fe = 56 g/mol

Step 3: From the equation, 4 moles Fe produce 2 moles $Fe_2O_3$.

So, 1 mole Fe produces $\frac{2}{4} = 0.5$ moles $Fe_2O_3$.

Step 4: Calculate mass of 0.5 moles $Fe_2O_3$:

Molar mass of $Fe_2O_3 = (2 \times 56) + (3 \times 16) = 112 + 48 = 160 \text{ g/mol}$

So, 56 g of iron produces 80 g of rust.