Physical and Chemical Changes

Rusting (chemical change):

Iron + oxygen + water → iron oxide (rust) 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Fe₂O₃.nH₂O (hydrated iron oxide = rust)

Conditions required: Both oxygen AND water are needed — iron does NOT rust in:

• Dry air (no water)
• Boiled/degassed water sealed from air (no dissolved oxygen)

Prevention of rusting:

1. Painting: Barrier between iron and air/water (used for bridges, ships — frequent repainting needed)
2. Oiling/greasing: Blocks air and water contact
3. Galvanisation: Coating iron/steel with zinc — zinc corrodes preferentially protecting iron underneath; used for roofing sheets, pipes, buckets
4. Electroplating: Depositing a protective metal layer using electricity (chrome on car parts, tin on food cans, gold on jewellery)
5. Alloying: Mixing iron with other metals — stainless steel (iron + chromium + nickel) does NOT rust
6. Cathodic protection: Connecting iron structure to a more reactive metal (zinc, magnesium) which corrodes instead of iron; used for underground pipelines and ship hulls

Economic cost of corrosion:

• Globally: ~3–4% of GDP lost annually to corrosion damage
• India: Infrastructure projects (bridges, railways, coastal buildings) face significant corrosion costs
• Coastal areas (high humidity + salt water) have accelerated corrosion rates

Galvanisation vs Electroplating:

Crystallisation: Process of obtaining pure crystals of a substance from its solution by slow evaporation or cooling.

• Salt crystals from seawater (solar evaporation in salt pans)
• Sugar crystals from sugar solution
• Application: Purification of substances; growing large pure crystals for electronics (silicon wafers, quartz crystals)

Burning (combustion): Fuel + oxygen → CO₂ + H₂O + energy (heat and light)

• Complete combustion → CO₂ + H₂O (clean burning)
• Incomplete combustion → CO (carbon monoxide, toxic) + soot (particulates) — indoor air pollution from chulhas

Burning vs rusting: Both are oxidation reactions (combination with oxygen), but:

• Burning: Fast; produces heat and light; flame
• Rusting: Slow; no visible heat/light

Cooking (chemical change):

• Proteins are denatured (unfolded) by heat → texture changes permanently
• Sugars caramelise
• Starches gelatinise
• Maillard reaction (between amino acids and sugars) → browning, new flavours
• Cannot be reversed — cooked food cannot become raw

Neutralisation: Acid + Base → Salt + Water HCl + NaOH → NaCl + H₂O (Chemical change — new substance NaCl formed)

The Chromium Passive Layer — Self-Healing Corrosion Protection:

When stainless steel (minimum 10.5% chromium by weight) is exposed to oxygen in air or water, chromium atoms at the surface instantly react with oxygen to form an ultra-thin layer of chromium oxide (Cr₂O₃) — called the passive layer or passivation film.

• Thickness: Just 1–5 nanometres (nm) — too thin to see, but extremely effective as a barrier
• Self-healing: If the surface is scratched or cut, chromium atoms from the underlying steel migrate to the damaged area and reform the Cr₂O₃ film automatically within hours — as long as oxygen is present
• Why iron rusts but stainless steel doesn't: Iron oxide (rust, Fe₂O₃) is porous and flaky — it falls off, exposing fresh iron to further attack. Chromium oxide is dense, adherent, and stable — it does not flake, forming a permanent protective shield.

Composition of common stainless steels:

• 304 grade (most common): 18% Cr + 8% Ni — used in kitchenware, sinks, surgical instruments
• 316 grade (marine grade): 16% Cr + 10% Ni + 2% Mo (molybdenum) — used in coastal infrastructure, marine equipment, food processing; the molybdenum further strengthens the passive layer against chloride (salt) attack

India's stainless steel sector:

• India produces ~3.7 million tonnes of stainless steel per year (FY 2024-25, ISSDA/IBEF)
• Domestic consumption: ~4.8 million tonnes/year, growing at 7–8% annually
• Jindal Stainless (Hisar, Haryana): India's largest stainless steel producer (integrated plant)
• Uses: Railways (Metro rail coaches, Vande Bharat interiors), kitchenware, chemical plant equipment, defence and nuclear industry components