Corrosion and its Prevention

Corrosion is the gradual, unwanted reaction of a metal with substances in its environment — moisture, oxygen, acidic gases, or dissolved salts — that converts the free metal into a compound (an oxide, sulphide, or carbonate) on its surface. Chemically it is an oxidation: the metal atom loses electrons and its oxidation state rises, exactly as in the simple oxidation reactions met earlier in "Chemical Reactions and Equations." Different metals corrode into different products and at very different rates. Iron rusts into a reddish-brown flaky solid; copper develops a dull green coating (basic copper carbonate) when exposed to moist air over time; silver tarnishes to a black sulphide when it reacts with trace sulphur compounds in air. Some metals, like aluminium, form a thin, tightly adhering oxide layer that actually protects the metal underneath from further attack — but plain iron's rust is loose and flaky, so it keeps exposing fresh metal and the damage keeps deepening.

Rusting of iron, the syllabus's central case, is the reaction of iron with oxygen and water together to give hydrated iron(III) oxide:

where: $\text{Fe}(s)$ is solid iron, $\text{O}_2(g)$ is atmospheric oxygen, $\text{H}_2\text{O}(l)$ is liquid water (moisture), and $\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}(s)$ is rust — iron(III) oxide bonded with a variable number of water molecules, which is why rust is written with an "$x$" rather than a fixed integer. The single most heavily examined fact here is that both oxygen and water must be present together for rusting to proceed at a noticeable rate; neither alone is enough. Iron kept in perfectly dry air stays bright for a long time, because there is no water to take part in the reaction. Iron submerged in freshly boiled, oxygen-free water also resists rusting, because there is no dissolved oxygen available. Only when a metal surface is simultaneously wet and exposed to air does the reaction proceed steadily, which is exactly why rusting is so much worse in humid coastal climates and during the rainy season than in dry desert air.

Prevention of corrosion always works on the same underlying logic: interrupt the metal's contact with air and/or moisture, or make its surface itself more resistant to attack.

• Painting — a continuous coat of paint physically blocks air and moisture from reaching the metal surface; it is cheap and widely used on gates, railings, bridges and vehicle bodies.
• Oiling or greasing — a thin film of oil or grease repels water and is easy to reapply, so it suits machine parts and tools that are handled or serviced often.
• Galvanising — coating iron or steel with a thin layer of zinc. This protects in two ways: as a physical barrier while intact, and — its real advantage — even after the coating is scratched, because zinc is more reactive than iron and corrodes preferentially, sacrificing itself to keep the exposed iron safe.
• Alloying — mixing iron with other elements to change its properties. The standard example is stainless steel (iron alloyed with chromium and nickel), which resists rusting far better than plain iron because chromium forms a thin, stable, self-repairing oxide layer on the surface.
• Chrome plating — depositing a thin layer of chromium metal onto a surface by electroplating, giving a shiny, decorative, and corrosion-resistant finish, common on taps, bumpers and cycle handlebars.

Every prevention method traces back to the same chemistry as the rusting equation itself: remove either reactant, oxygen or water, from contact with the metal surface, and the reaction that consumes iron simply cannot proceed. A barrier method (painting, oiling, chrome plating, the intact zinc layer of galvanising) works purely mechanically — it physically keeps the two reactants away from the metal, with no chemistry of its own. This is also exactly why any break, scratch, or crack in a painted or chrome-plated surface is dangerous: once air and moisture reach the bare metal underneath through that gap, rusting resumes there, often hidden from view, and can spread beneath the coating. Galvanising is the interesting exception, because it keeps working even after it is damaged. Zinc is chemically more reactive than iron — it loses electrons more readily. When a scratch exposes both metals to air and moisture at once, the zinc corrodes in preference to the iron, effectively taking the "hit" so the iron beneath is spared, at least until the sacrificial zinc is used up. This is why galvanised iron sheets used for roofing can survive years of scratches and abrasion far better than simply painted iron of the same age. Alloying works differently again — it is not a barrier at all but a change to the metal itself. Adding chromium to iron (to make stainless steel) causes a very thin, tough, and continuously self-healing layer of chromium oxide to form on the surface the moment it is exposed to air. Unlike rust, this oxide layer clings tightly and does not flake off, so it seals the surface against further attack rather than exposing fresh metal the way loose rust does.

Question: Three identical iron nails are treated as follows before being left in a humid room for two weeks: Nail A is left bare and untreated. Nail B is coated with a continuous, unbroken layer of grease. Nail C is galvanised, but a single deep scratch is deliberately made through the zinc coating down to the bare iron. Predict what happens to each nail, and explain your reasoning using the requirement for rusting.

Step 1 — Recall the requirement. Rusting of iron needs both atmospheric oxygen and water/moisture in contact with the bare metal surface at the same time; a humid room supplies plenty of both.

Step 2 — Analyse Nail A. The bare nail has its metal surface directly exposed to the humid air, so both oxygen and water reach it freely. Nail A rusts, developing the familiar reddish-brown flaky $\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$ coating.

Step 3 — Analyse Nail B. The unbroken grease layer forms a continuous physical barrier that keeps both air and moisture away from the iron surface underneath. With no reactant reaching the metal, Nail B stays essentially rust-free as long as the grease film remains intact.

Step 4 — Analyse Nail C. Over most of its surface the intact zinc coating keeps the iron protected, exactly like Nail B's grease. But at the scratch, both bare zinc and bare iron are exposed to humid air together. Because zinc is more reactive than iron, the zinc at the scratch corrodes preferentially, while the iron immediately beneath the scratch stays largely protected — this is the sacrificial action that distinguishes galvanising from a simple barrier coating.

Conclusion: Nail A rusts visibly and steadily; Nail B stays rust-free while the grease film is unbroken; Nail C shows corrosion of the zinc at the scratch but the underlying iron itself remains protected there, unlike a scratched painted nail, which would rust at the exposed spot with nothing to stop it. This is the key distinguishing behaviour of galvanising versus painting or oiling at a break in the coating.

• Saying iron rusts because of "air" or because of "water" alone — the correct statement always names both together as the joint requirement.
• Treating "corrosion" and "rusting" as interchangeable — rusting is specifically iron's corrosion product; other metals corrode into different compounds (copper's green coating, silver's black tarnish), and "rust" should never be used for those.
• Describing galvanising as "just a coating like paint" — this misses that zinc protects the iron even after the coating is scratched, because zinc corrodes preferentially, whereas a scratched paint layer offers the exposed spot no protection at all.
• Reaching for anode/cathode electrochemical mechanism to explain rusting at this level — that detailed mechanism is Class 12 material; Class 10 answers should state the air-and-water requirement and the prevention methods, not the electrode reactions.

Ask the tutor: Want to work through why a scratched galvanised sheet still resists rust for years while a scratched painted one starts rusting almost immediately?