Industrial Chemistry: Corrosion, Water, and Materials
Water Hardness and Softening Methods
Temporary Hardness: Calcium bicarbonate – Ca(HCO₃)₂, Magnesium bicarbonate – Mg(HCO₃)₂.
Permanent Hardness: CaSO₄, MgSO₄, CaCl₂, MgCl₂.
Softening Agents: Lime [Ca(OH)₂] and soda ash [Na₂CO₃] are added to hard water. Alternatively, hard water is passed through a bed of sodium zeolite (Na₂Z).
Types of Ion Exchange Resins
- Cation Exchange Resin (RH): Removes Ca²⁺, Mg²⁺, Na⁺
- Anion Exchange Resin (R′OH): Removes Cl⁻, SO₄²⁻, NO₃⁻
Mechanism of Dry Corrosion
Step-by-Step Mechanism
- Metal atoms at the surface react with dry gas.
- A compound layer (oxide, sulphide, or halide) forms on the metal surface.
- Further corrosion depends on the nature of this layer.
Nature of Oxide Film
The corrosion rate depends on whether the oxide film is:
(a) Protective Oxide Film
- ✔ Thin, stable, non-porous, and adherent.
- 🔹 Prevents further corrosion by blocking oxygen diffusion.
- Examples: Aluminium (Al₂O₃), Chromium (Cr₂O₃), Zinc (ZnO).
(b) Non-Protective Oxide Film
- ✖ Porous, unstable, and non-adherent.
- 🔹 Allows oxygen to reach metal continuously, leading to rapid corrosion.
- Examples: Na, K, Ca.
(c) Volatile Oxide Film
- Oxide evaporates at high temperatures.
- Fresh metal surface is exposed repeatedly.
- Example: Mo → MoO₃ (volatile).
Types of Dry Corrosion
1. Oxidation Corrosion
Reaction of metal with oxygen at elevated temperatures.
Reaction: 2M + O₂ → 2MO
Example (Iron): 2Fe + O₂ → 2FeO
2. Corrosion by Other Gases
Occurs due to gases like H₂S, SO₂, and Cl₂.
Example: Fe + H₂S → FeS + H₂
3. Liquid Metal Corrosion
One solid metal dissolves in molten liquid metal. This is common in nuclear reactors.
Example: Steel in molten sodium.
Factors Affecting Dry Corrosion
- Nature of metal
- Nature of oxide film
- Temperature
- Reactivity of gas
Characteristics of Dry Corrosion
- ✔ No electrolyte required
- ✔ Occurs at high temperatures
- ✔ Slow at room temperature
- ✔ Purely chemical reaction
Mechanism of Wet Corrosion
The metal surface becomes heterogeneous. Some regions act as an anode, while others act as a cathode, forming a corrosion cell.
Anodic Reaction (Oxidation)
Metal dissolves at the anode: Fe → Fe²⁺ + 2e⁻. 🔹 The anode always corrodes.
Cathodic Reaction (Reduction)
1. Hydrogen Evolution Mechanism (Acidic Medium)
Occurs in acidic solutions (H₂SO₄, HCl).
Reactions:
- Fe → Fe²⁺ + 2e⁻ (Anode)
- 2H⁺ + 2e⁻ → H₂↑ (Cathode)
2. Oxygen Absorption Mechanism (Neutral / Alkaline Medium)
Occurs in natural water and moist air.
Reactions:
- Fe → Fe²⁺ + 2e⁻
- O₂ + 2H₂O + 4e⁻ → 4OH⁻
Rust Formation:
- Fe²⁺ + 2OH⁻ → Fe(OH)₂
- Fe(OH)₂ + O₂ → Fe(OH)₃
- Fe(OH)₃ → Fe₂O₃·xH₂O (Rust)
Types of Wet Corrosion
1. Galvanic Corrosion
Two dissimilar metals in electrical contact in an electrolyte. The more active metal becomes the anode.
Example: Zn–Cu couple → Zinc corrodes.
2. Concentration Cell Corrosion
Occurs due to differences in oxygen or electrolyte concentration.
Examples: Corrosion under water droplets or in crevices.
3. Pitting Corrosion
Localized corrosion producing deep pits. This is very dangerous and sudden.
Example: Stainless steel in chloride solution.
4. Stress Corrosion
Combined effect of tensile stress and a corrosive environment.
Composite Materials
A. Based on Matrix Type
1. Polymer Matrix Composites (PMC)
- Matrix: Thermoplastics or thermosets (epoxy, polyester)
- Reinforcement: Glass, carbon, aramid fibres
- Examples: Glass Fibre Reinforced Plastic (GFRP), Carbon Fibre Reinforced Polymer (CFRP)
- Properties: Lightweight, high strength-to-weight ratio, corrosion resistant
2. Metal Matrix Composites (MMC)
- Matrix: Aluminium, magnesium, titanium
- Reinforcement: SiC, Al₂O₃, carbon fibres
- Examples: Al–SiC composite
- Properties: High temperature resistance, better wear resistance, high strength
3. Ceramic Matrix Composites (CMC)
- Matrix: Ceramic (SiC, Al₂O₃)
- Reinforcement: Ceramic fibres
- Examples: Carbon–carbon composites
- Properties: Very high temperature resistance, low thermal expansion, brittle but strong
B. Based on Reinforcement Type
1. Fibre Reinforced Composites
- Continuous or discontinuous fibres (Glass, carbon, Kevlar)
2. Particulate Composites
- Reinforced with fine particles. Example: Concrete
3. Laminated Composites
- Layers of different materials bonded together. Example: Plywood
Applications of Composite Materials
- Aerospace (aircraft bodies, wings)
- Automobiles (body panels, drive shafts)
- Construction (bridges, panels)
- Sports equipment (bats, helmets)
- Marine industry (boats)
- Medical implants
9991
Calorimetry and Fuel Analysis
A) Bomb Calorimeter
(Used for solid and liquid fuels)
Principle
A known mass of fuel is burnt in excess oxygen inside a sealed steel bomb, and the heat released is absorbed by a known mass of water. The temperature rise of water is used to calculate the calorific value.
Construction
- Strong steel bomb and oxygen inlet valve
- Ignition electrodes and fuse wire
- Platinum crucible and water jacket
- Thermometer and stirrer
Procedure
- A known mass of fuel is placed in the crucible.
- The bomb is filled with oxygen at 25–30 atm pressure.
- The bomb is immersed in water and the fuel is ignited electrically.
- The rise in temperature of water is noted.
Corrections Applied
- Fuse wire correction
- Acid correction (H₂SO₄, HNO₃ formation)
- Cotton thread correction
Advantages and Limitations
- Advantages: Highly accurate; suitable for coal, coke, and petroleum fuels.
- Limitations: Not suitable for gaseous fuels; expensive apparatus.
B) Boy’s Gas Calorimeter
(Used for gaseous fuels)
Principle
A known volume of gas is burnt, and the heat produced is absorbed by flowing water. The temperature rise of water is measured to calculate the calorific value.
Construction
- Gas burner and copper combustion chamber
- Water inlet and outlet, thermometers, and gas meter
Procedure
- Gas is burnt at a steady rate.
- Water flows continuously around the chamber.
- The temperature rise of water and the volume of gas burnt are recorded.
Uses
- Natural gas, coal gas, and LPG.