Galvanising & Sacrificial Protection
Section: 9. Metals | Syllabus: Cambridge AS Level Physics 9702
What is Galvanising? Galvanising The process of coating iron or steel with a layer of zinc to protect it from rusting. Galvanising is one of the most effective methods of preventing iron from rusting.
It works in two ways: Barrier protection: Zinc forms a physical barrier that prevents oxygen and water from reaching the iron Sacrificial protection: If the zinc coating is scratched or damaged, the zinc corrodes instead of the iron Why Zinc?
Zinc is used because it is more reactive than iron in the reactivity series. This means zinc will lose electrons (be oxidized) more easily than iron, so zinc corrodes preferentially to protect the iron.
The Galvanising Process Method 1: Hot-Dip Galvanising Clean the iron or steel to remove dirt, grease, and rust Heat zinc to about 450°C until it melts Dip the iron/steel object into the molten zinc bath Remove and allow to cool A layer of zinc bonds to the surface of the iron Diagram Process diagram showing hot-dip galvanising in 4 steps.
Step 1: Steel object being cleaned (show brush/cleaning). Step 2: Molten zinc bath at 450°C in a large tank (show orange glow indicating heat). Step 3: Steel object being dipped into molten zinc (show object partially submerged).
Step 4: Finished galvanized product with silvery-gray zinc coating. Label each step and include arrows showing progression. Add cross-section view showing layers: iron core (gray) covered by zinc layer (silver).
Method 2: Electroplating with Zinc Uses electrolysis to deposit a thin layer of zinc onto the iron Iron object acts as the cathode (negative electrode) Zinc electrode acts as the anode (positive electrode) Zinc ions (Zn²⁺) from the solution are deposited onto the iron Common Galvanised Products: Steel buckets, dustbins, and watering cans Corrugated iron roofing sheets Steel fences and gates Nails and screws Car body panels (before painting) Steel pylons and structural supports How Does Galvanising Work?
1. Barrier Protection (When Coating Is Intact) When the zinc coating is undamaged: Zinc forms a continuous layer over the iron surface Oxygen and water cannot reach the iron beneath The iron is physically protected from the environment Zinc itself forms a thin layer of zinc oxide (ZnO) which is self-sealing and protects the zinc from further corrosion Zinc oxide formation: 2Zn(s) + O₂(g) → 2ZnO(s) This thin oxide layer protects the zinc underneath 2.
Sacrificial Protection (When Coating Is Scratched) This is where galvanising becomes superior to other coating methods like tin plating: Key Concept If the galvanised coating is scratched and the iron is exposed, the zinc corrodes instead of the iron .
The zinc "sacrifices" itself to protect the iron. Why Does Zinc Corrode Instead of Iron? This is explained by the reactivity series: Zinc is more reactive than iron (higher in the reactivity series) More reactive metals lose electrons more easily In a redox reaction, zinc is preferentially oxidized (loses electrons) instead of iron The iron remains protected as long as there is zinc present Interactive Diagram Split diagram comparing scratched galvanized steel vs scratched tin-plated steel.
Left side: Galvanized steel with scratch exposing iron - show zinc corroding at the scratch (labeled "Zn → Zn²⁺ + 2e⁻") while iron remains protected (labeled "Fe protected"). Right side: Tin-plated steel with scratch - show iron rusting at the scratch (labeled "Fe → Fe²⁺ + 2e⁻, rust forms") while tin remains intact.
Include reactivity series reference showing Zn above Fe, and Sn below Fe. Annotation: "Zinc is MORE reactive - sacrifices itself" vs "Tin is LESS reactive - iron rusts". Sacrificial Protection Explained Sacrificial Protection A method of protecting a metal from corrosion by attaching a more reactive metal to it.
The more reactive metal corrodes instead (is "sacrificed") to protect the less reactive metal. The Science Behind It: When zinc and iron are in contact in the presence of water and oxygen: Zinc is oxidized (loses electrons): Zn(s) → Zn²⁺(aq) + 2e⁻ Zinc corrodes Iron is NOT oxidized: The electrons released by zinc prevent iron from being oxidized Iron remains protected Reactivity Series Link Reactivity Series (partial): Magnesium > Zinc > Iron > Tin > Copper Rule: The more reactive metal (zinc) will be oxidized preferentially.
This protects the less reactive metal (iron) from corrosion. Comparing Galvanising with Tin Plating Property Galvanising (Zinc coating) Tin Plating (Tin coating) Metal used Zinc (Zn) Tin (Sn) Reactivity compared to iron More reactive than iron Less reactive than iron Position in reactivity series Zn > Fe Fe > Sn When coating is intact Barrier protection - iron protected Barrier protection - iron protected When coating is scratched Sacrificial protection - zinc corrodes, iron protected No sacrificial protection - iron rusts at the scratch Durability Very durable - protects even when damaged Only works if coating is intact Uses Outdoor structures, roofing, buckets, fences Food cans (…
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