Sacrificial Protection and Galvanising

Introduction

Paint and grease protect iron only while they are intact. But what if the protection could keep working even after a scratch? Sacrificial protection does exactly this. By attaching a metal that is more reactive than iron, we make that metal corrode instead. It "sacrifices" itself to save the iron. The most common sacrificial metal is zinc, used in galvanised steel all around us, from buckets to motorway crash barriers.

Core explanation

Reactivity is the key. Metals differ in how readily they react and form compounds. Part of the reactivity series runs: magnesium > aluminium > zinc > iron > tin > copper. A metal higher in the series loses electrons and forms compounds more readily than one below it.

How sacrificial protection works. When a more reactive metal such as zinc is in electrical contact with iron and both are in contact with water, the zinc is oxidised in preference to the iron. The zinc atoms give up electrons more readily, and those electrons flow into the iron, which stops iron atoms being oxidised. The zinc slowly corrodes away; the iron stays intact. As long as some zinc remains in contact, the iron is protected.

Galvanising. Galvanised steel has a coating of zinc, usually applied by dipping the cleaned steel into molten zinc. The coating protects in two ways:

1. Barrier: the zinc layer, and the dull zinc compounds that form on its surface, keep water and oxygen away from the iron. 2. Sacrificial: if the coating is scratched and some steel is exposed, the surrounding zinc still corrodes in preference to the iron, so the exposed steel does not rust.

This double action makes galvanising far more reliable than paint or tin on its own.

Comparison with tin plating. Tin is less reactive than iron. It works only as a barrier. When a tin coating is broken, the iron corrodes in preference to the tin, and rusting is actually faster. Zinc is more reactive than iron, so a broken zinc coating still protects.

Coating metal Reactivity compared with iron Protection when scratched --- --- --- Zinc More reactive Yes — zinc corrodes instead Magnesium Much more reactive Yes — used as sacrificial blocks Tin Less reactive No — iron corrodes faster

Sacrificial blocks. Large structures cannot always be coated completely, so blocks of zinc or magnesium are bolted or welded to them. Ships' hulls, underground pipelines, oil rigs and domestic hot-water tanks all use these sacrificial anodes . The blocks corrode steadily and are replaced during maintenance.

Step-by-step reasoning

To decide whether a metal can protect iron sacrificially:

1. Find the metal and iron in the reactivity series. 2. If the metal is above iron, it will be oxidised first and can protect iron. 3. If it is below iron, it cannot; iron would corrode instead. 4. Check that the metals are in electrical contact and share the same water film.

Visual explanation

Imagine a steel ship's hull under water with grey zinc blocks fixed near the stern. Arrows show electrons flowing from each zinc block into the steel. Over time the blocks shrink and become pitted, while the steel hull beside them remains smooth and unrusted.

Real-world analogy

A sacrificial metal is like a bodyguard who steps in front of the person being protected and takes the blows instead. The bodyguard gets worn out, but the person stays unharmed — as long as the bodyguard is still there.

Real-world example

Motorway crash barriers, lamp posts, farm gates and steel roofing sheets are usually galvanised. Their dull, speckled grey surface, sometimes showing a crystal pattern called "spangle", is zinc. Galvanised structures can last many decades outdoors with little maintenance, even after scrapes from vehicles.

Why?

Why does zinc protect iron even through a scratch? Zinc is more reactive, so where zinc and exposed iron share water, the zinc gives up electrons more readily. It is oxidised instead of the iron, so the exposed iron is not attacked until the nearby zinc is used up.

Common misconception

"Galvanising works just like paint." Paint is only a barrier, so a scratch lets rust start. Zinc acts as a barrier and as a sacrificial metal, so it keeps protecting the steel even when damaged.

Worked example

Question: An engineer can fix blocks of copper, magnesium or tin to a steel pipeline to stop it corroding. Which should she choose and why?

Reasoning: Only a metal more reactive than iron will corrode in its place. Magnesium is above iron; copper and tin are below it.

Answer: Magnesium, because it is more reactive than iron and will be oxidised instead of the steel.

Quick check

1. What is galvanising? Answer: Coating iron or steel with a layer of zinc.

Exam focus

Explain sacrificial protection using reactivity: the more reactive metal is oxidised (loses electrons) instead of iron. Be ready to compare galvanising with tin plating when both are scratched, and to explain why sacrificial blocks need replacing regularly.

Advanced insight

An alternative to sacrificial anodes is impressed-current cathodic protection , where a power supply pushes electrons into the steel structure through an inert anode. It is used on long pipelines and large offshore platforms, because it can be adjusted and does not need blocks to be replaced.

Summary

Sacrificial protection uses a metal more reactive than iron, usually zinc or magnesium, which is oxidised instead of the iron. Galvanising coats steel with zinc, giving both barrier and sacrificial protection, so it keeps working when scratched. Tin, being less reactive, protects only as a barrier. Ships, pipelines and hot-water tanks use replaceable sacrificial blocks.

Practice questions

1. Explain why zinc can be used for sacrificial protection of iron. Answer: Zinc is more reactive than iron, so it is oxidised in preference, and the iron is protected. 2. Give two ways in which galvanising protects steel. Answer: The zinc coating acts as a barrier to water and oxygen, and it corrodes sacrificially instead of the iron if scratched. 3. Why must the magnesium blocks on a ship's hull be replaced from time to time? Answer: They corrode away as they protect the hull, and once gone they no longer protect the steel. 4. A galvanised bucket and a tin-plated bucket are both scratched. Which one rusts at the scratch? Explain. Answer: The tin-plated bucket, because tin is less reactive than iron, so the iron corrodes; zinc on the galvanised bucket corrodes instead of the iron.