Stainless Steel
Summary: Stainless steel is an alloy of iron with chromium, nickel, and carbon. It is hard, strong, and highly resistant to rusting and corrosion. Used for cutlery, kitchen sinks, surgical instruments, and chemical containers. Tags: igcse chemistry metals alloy Created: 2026-07-14 Last Updated: 2026-07-23
What Is Stainless Steel?
Stainless steel is an alloy (mixture) of:
- Iron (Fe) — main component (~70–75%)
- Chromium (Cr) — ~18% (provides corrosion resistance)
- Nickel (Ni) — ~8% (adds strength and lustre)
- Carbon (C) — <0.1% (adds hardness)
Chromium is the key element — it reacts with oxygen to form a thin, transparent, protective layer of chromium(III) oxide (Cr₂O₃) on the surface. This layer is:
- Self-healing: if scratched, more chromium reacts with oxygen to re-form the layer
- Impermeable: prevents oxygen and water from reaching the iron beneath
Properties
| Property | Description | Reason |
|---|---|---|
| Corrosion resistance | Does not rust | Chromium forms protective Cr₂O₃ layer |
| Hardness | Harder than pure iron | Different sized atoms disrupt the regular lattice |
| Strength | Strong and durable | Alloying elements prevent layers from sliding |
| Appearance | Shiny, attractive lustre | Nickel adds a polished finish |
| Hygienic | Easy to clean, non-porous | Smooth surface; used in medical and food settings |
Why Alloys Are Harder Than Pure Metals
See Alloy for the full explanation.
In pure iron, atoms are arranged in regular layers that can slide over each other when a force is applied → iron is relatively soft and malleable.
In stainless steel, the chromium, nickel, and carbon atoms have different sizes from iron atoms. These differently-sized atoms disrupt the regular layer arrangement, preventing the layers from sliding → the alloy is harder and stronger.
Uses of Stainless Steel
| Use | Property Exploited |
|---|---|
| Cutlery (knives, forks, spoons) | Hardness + resistance to rusting |
| Kitchen sinks | Corrosion resistance + hygienic surface |
| Surgical instruments | Can be sterilised; does not corrode |
| Chemical containers / pipes | Resists attack from chemicals |
| Architectural panels | Attractive appearance + weather resistance |
| Cookware (pots, pans) | Heat resistant + does not react with food |
Comparison: Stainless Steel vs Mild Steel vs Pure Iron
| Material | Rusts? | Hardness | Uses |
|---|---|---|---|
| Pure iron | Yes (readily) | Soft | Almost never used pure |
| Mild steel (~0.2% C) | Yes | Harder than iron | Construction, car bodies |
| Stainless steel (~18% Cr, 8% Ni) | No (highly resistant) | Hard and strong | Cutlery, medical, kitchen |
Key Facts
- Stainless steel = iron + chromium + nickel + carbon
- Chromium forms a protective Cr₂O₃ layer → corrosion resistance
- Different sized atoms in the alloy disrupt lattice layers → harder and stronger
- Does NOT rust — unlike pure iron or mild steel
- Used where hardness AND corrosion resistance are both needed
- More expensive than mild steel due to costly alloying elements