In one line
Galvanising coats iron with a more active metal (zinc), giving an anodic coating that protects even when scratched, while tinning coats iron with a more noble metal (tin), giving a cathodic coating that protects only while unbroken.
Explained simply
A metallic coating is a metal "jacket" on the base metal. If the jacket metal is more active than iron (zinc), it acts as a bodyguard: even if scratched, zinc corrodes instead of iron. If the jacket metal is less active than iron (tin), it is only a barrier: a scratch exposes iron as a tiny anode next to a big tin cathode, and the iron corrodes faster than if it were bare.
How it works
Galvanising (hot dipping in zinc)
- Clean the iron/steel surface; pickle in dilute H2SO4 (about 60–90 °C) to remove rust and scale; wash and dry.
- Dip in molten zinc at about 425–450 °C; the surface is covered with a flux of NH4Cl to prevent oxidation of the molten metal.
- Pass through rollers to remove excess zinc and give uniform thickness; anneal and cool slowly.
Tinning (hot dipping in tin)
- Clean and pickle the iron sheet in dilute acid.
- Pass through a flux of molten ZnCl2, then through molten tin.
- Pass through rollers immersed in palm oil to remove excess tin and prevent oxidation of the coating.
Key formula
No formula. Principle: coating metal above iron in the galvanic series → anodic coating; below iron → cathodic coating.
| Galvanising | Tinning |
|---|---|
| Zn coating on Fe; anodic coating | Sn coating on Fe; cathodic coating |
| Protects iron sacrificially even if coating is broken | Protects only as a barrier; a break causes intense local corrosion of iron |
| Zn dissolves in acids and forms toxic compounds; not used for food containers | Sn is non-toxic; used for food cans and containers |
| Uses: roofing sheets, wires, pipes, buckets, nuts and bolts | Uses: food cans, utensils, copper wires (before insulation) |
Other methods of applying metallic coatings include electroplating, metal cladding (e.g., Alclad: pure Al on duralumin), metal spraying and cementation (e.g., sherardizing with Zn dust).
Quick example
A galvanised roofing sheet with a scratch still does not rust at the scratch for a long time; a scratched tin can rusts quickly at the scratch.
Common mistakes
- Saying tinning is better because tin is less reactive. It is better only for non-toxicity, not for scratch protection.
- Using galvanised vessels for acidic foods like pickles or curd.
- Calling galvanising "electroplating with zinc"; standard galvanising is hot dipping.
How MU asks it
"Distinguish between galvanising and tinning" (3–5 marks) or "Explain anodic and cathodic coatings with examples" (5 marks), sometimes with a process diagram. For full marks: definition of anodic/cathodic coating, process steps with temperatures and flux, a comparison table of at least 4 points, and uses.