Corrosion of Other Metals

Introduction

Iron is not the only metal that reacts with its surroundings. Silver cutlery turns dark, copper roofs turn green and bronze statues develop coloured crusts. All of these are examples of corrosion : the slow chemical attack on a metal surface. Unlike rusting, however, corrosion of some metals can actually protect them. Understanding why some layers crumble while others seal the metal explains why aluminium aircraft and copper roofs last so long.

Core explanation

Corrosion versus rusting. Corrosion is the general term for a metal reacting with substances around it, such as oxygen, water, carbon dioxide or sulfur compounds. Rusting is the name for the corrosion of iron and steel only.

How reactivity affects corrosion. More reactive metals tend to corrode more readily. Gold and platinum are so unreactive that they stay shiny for thousands of years, which is why ancient gold jewellery is found in perfect condition. However, reactivity is not the whole story, because the nature of the corrosion layer matters too.

Tarnishing of silver. Silver does not react with oxygen in the air at room temperature, but it reacts with tiny amounts of sulfur compounds, such as hydrogen sulfide from pollution, eggs or rubber. A thin black layer of silver sulfide forms. This dull layer is called tarnish . It can be removed by polishing, which rubs away a little silver each time.

Copper and patina. Copper slowly reacts with oxygen, water and carbon dioxide in the air. It first darkens to brown, then over many years a green layer called patina forms. This is mainly basic copper carbonate, often with copper sulfates or chlorides in polluted or coastal air. Unlike rust, patina sticks firmly to the copper and slows further corrosion, so copper roofs can last for centuries.

Aluminium: reactive but resistant. Aluminium is above iron in the reactivity series, so we might expect it to corrode badly. In fact, it is very resistant. As soon as fresh aluminium meets air, it forms a very thin, tough layer of aluminium oxide , Al₂O₃. This layer is tightly bonded to the metal and is not porous, so oxygen and water cannot pass through. If it is scratched, a new oxide layer forms almost instantly. This is why aluminium is used for window frames, drinks cans and aircraft.

Comparing corrosion layers.

Metal Corrosion product Layer behaviour --- --- --- Iron Hydrated iron(III) oxide (rust) Porous and flaky; corrosion continues Aluminium Aluminium oxide Thin, tight; protects the metal Copper Patina (basic copper carbonate) Adheres well; slows corrosion Silver Silver sulfide (tarnish) Thin dark film; mainly cosmetic Gold None No reaction

Anodising. The protective oxide on aluminium can be deliberately thickened by an electrical process called anodising. The thicker layer is harder and can absorb dyes, giving coloured, durable finishes on items such as phone cases and bicycle parts.

Step-by-step reasoning

To predict how a metal will behave outdoors:

1. Consider its reactivity: very unreactive metals such as gold barely corrode. 2. Identify what it reacts with: oxygen, water, carbon dioxide or sulfur compounds. 3. Decide whether the corrosion layer is tight and non-porous or loose and flaky. 4. A tight layer protects; a flaky one allows corrosion to continue.

Visual explanation

Picture two cross-sections. On the left, iron with a thick, cracked orange layer and water seeping through cracks to the metal. On the right, aluminium with an extremely thin, continuous grey oxide film, with water droplets and oxygen molecules sitting on top unable to get through.

Real-world analogy

A protective oxide layer is like a scab on a cut. It forms quickly, seals the wound and stops further damage. Rust is more like a soggy plaster that peels away and lets dirt reach the skin, so the wound never heals.

Real-world example

The Statue of Liberty in New York is made of thin copper sheets over an iron frame. When it was unveiled in 1886 it was a shiny reddish-brown. Over about three decades it turned green as a patina formed. The patina has protected the copper ever since, and officials chose not to remove it.

Why?

Why is aluminium more corrosion-resistant than iron even though it is more reactive? Its oxide layer is thin, strong and firmly attached with no gaps, so it seals off the metal. Rust is porous and flakes away, constantly exposing fresh iron.

Common misconception

"More reactive metals always corrode faster." Aluminium is more reactive than iron but resists corrosion far better, because its oxide layer forms a protective barrier. The properties of the corrosion layer matter as much as reactivity.

Worked example

Question: A silver necklace stored in a drawer beside rubber bands turns black. Explain what has happened and whether this is a physical or chemical change.

Reasoning: Rubber can release sulfur compounds, which react with silver.

Answer: The silver has reacted with sulfur compounds to form black silver sulfide (tarnish); this is a chemical change because a new substance has formed.

Quick check

1. What is the green layer that forms on old copper roofs called? Answer: Patina.

Exam focus

Be clear that rusting is a special case of corrosion that applies only to iron. The aluminium "paradox" is a favourite exam question: explain its resistance using the thin, non-porous, strongly bonded oxide layer. Link silver tarnishing to sulfur compounds, not oxygen.

Advanced insight

Stainless steel resists corrosion because it contains at least about 10.5% chromium. The chromium forms a very thin, invisible chromium(III) oxide layer that heals itself when scratched, just like the oxide on aluminium. This property, called passivation , is used in cutlery, surgical tools and chemical plant.

Summary

Corrosion is the chemical attack of a metal by its surroundings; rusting applies only to iron. Silver tarnishes by forming black silver sulfide. Copper develops a green patina that slows further corrosion. Aluminium, despite being reactive, is protected by a thin, tight oxide layer that reforms when scratched. Whether corrosion continues depends on whether the layer formed is porous or protective.

Practice questions

1. What is the difference between corrosion and rusting? Answer: Corrosion is the chemical attack on any metal by its surroundings; rusting is the corrosion of iron and steel only. 2. Name the substance that forms when silver tarnishes and state what causes it. Answer: Silver sulfide, caused by reaction with sulfur compounds such as hydrogen sulfide in the air. 3. Explain why aluminium drinks cans do not corrode away quickly. Answer: Aluminium forms a thin, tough, non-porous oxide layer that stops oxygen and water reaching the metal below. 4. Why is patina described as protective but rust is not? Answer: Patina sticks firmly to the copper and slows further corrosion, while rust is porous and flakes off, exposing fresh iron.