Corrosion and Rusting: The Electrochemistry Behind It, and How It Is Stopped

Rusting is a cell, not a simple reaction

The usual mistake is to treat rusting as iron combining directly with oxygen, in the way magnesium burns. It is not. Dry iron in dry oxygen barely corrodes at all, and iron in oxygen-free water barely corrodes either. Both water and oxygen are required, and that requirement is the clue that an electrochemical cell is involved.

On a wet iron surface, small differences — impurities, strain from bending, differences in oxygen concentration — make some regions slightly more prone to oxidation than others. Those regions become anodes, the rest become cathodes, and the water film containing dissolved ions acts as the electrolyte. The metal itself is the wire connecting them.

The two half reactions

At the anodic region, iron dissolves:

Those electrons travel through the metal to a cathodic region, where dissolved oxygen is reduced:

The overall cell potential is large and positive, so the process is thermodynamically spontaneous. The Fe²⁺ then migrates and is oxidised further by oxygen to Fe³⁺, which precipitates as hydrated iron(III) oxide.

Why rust does not protect the metal underneath

Aluminium is thermodynamically more reactive than iron, yet aluminium objects survive outdoors for decades. The difference is the nature of the oxide layer. Aluminium oxide is dense, adherent and closely matched in volume to the metal it replaces, so it seals the surface and stops further attack.

Rust is porous and flaky, occupies a much larger volume than the iron it came from, and therefore spalls off and exposes fresh metal. Corrosion continues until the object is consumed. This comparison is a frequent short-answer question and the expected answer is about the physical nature of the oxide layer, not about reactivity.

Conditions that make it faster

Prevention: four strategies, three mechanisms

Barrier protection

Paint, grease and oil keep water and oxygen off the surface. It is simple and cheap, and it fails completely the moment the coating is scratched, because the tiny exposed area becomes a concentrated anode.

Galvanisation

Coating iron with zinc does two jobs at once, and the second is the important one. Zinc is a barrier, but zinc is also more reactive than iron, so if the coating is scratched the zinc still corrodes preferentially and the exposed iron is protected. Tin-plating behaves in the opposite way: tin is less reactive than iron, so a scratch in tin plate makes the iron the anode and accelerates the corrosion.

Cathodic protection

Attaching blocks of magnesium or zinc to a buried pipeline or a ship hull makes the iron the cathode of a cell in which the attached metal is the anode. The sacrificial block corrodes and is replaced periodically. Nothing needs to coat the iron for this to work, which is why it is used where a coating cannot be maintained.

Alloying and passivation

Stainless steel contains chromium, which forms a thin, dense, adherent chromium oxide layer of the aluminium type. The surface passivates rather than flaking, and the protection reforms if it is scratched.

Frequently asked questions

Why are both air and water needed for rusting?

Water provides the electrolyte that carries ions between the anodic and cathodic regions, and oxygen is the species reduced at the cathode. Remove either one and the cell cannot operate, which is why iron does not rust in dry air or in boiled, oxygen-free water.

Why does salt water corrode iron so much faster?

Dissolved salt increases the conductivity of the water film. The internal resistance of the corrosion cell drops, more current flows, and the metal dissolves faster. The salt is not consumed and does not change the chemistry, only the rate.

Why does a scratch in tin plate cause worse corrosion than a scratch in galvanised iron?

Tin is less reactive than iron, so at a scratch iron becomes the anode and corrodes rapidly. Zinc is more reactive, so zinc remains the anode and continues to protect the exposed iron. The relative position in the reactivity series decides which metal is sacrificed.

What is a sacrificial anode?

A block of a more reactive metal, usually magnesium or zinc, connected to the structure being protected. It oxidises in place of the iron, keeping the iron cathodic. It is consumed over time and replaced during maintenance.

Why is aluminium more resistant than iron despite being more reactive?

Aluminium forms a thin, dense, strongly adherent oxide that seals the surface. Rust is porous and flakes away, exposing fresh iron. The difference is in the physical quality of the oxide layer, not in the thermodynamics of oxidation.