What is corrosion?
Corrosion is the electrochemical destruction of a metal as it reacts with its environment. Learn the science: anodes, cathodes, electrolytes, and why metals return to ore.
The destruction of metal by its environment
Corrosion is the gradual degradation of a material — most often a metal — caused by chemical or electrochemical reaction with its environment. For iron and steel, the familiar result is rust: the metal combining with oxygen and water to become iron oxide, the same compound it was smelted from.
It helps to think of corrosion as metallurgy in reverse. A blast furnace spends enormous energy to pull iron out of its ore. Corrosion is that energy being released again, slowly, as the refined metal reacts its way back toward the low-energy oxide it came from. This is why corrosion is sometimes called *the slow fire* — it is oxidation, the same chemistry as burning, only thousands of times slower.
Four things every corrosion reaction needs
Aqueous corrosion is electrochemical: it works like a tiny short-circuited battery on the metal surface. Remove any one of these four and corrosion stops.
Anode
The site where metal dissolves and gives up electrons — for iron, Fe → Fe²⁺ + 2e⁻. This is where metal is actually lost.
Cathode
The site where those electrons are consumed, usually by oxygen and water forming hydroxide ions. No metal is lost here.
Electrolyte
A conductive solution — rainwater, seawater, soil moisture, condensation — that carries ions between anode and cathode.
Metallic path
An electrical connection (the metal itself) so electrons can flow from anode to cathode and complete the circuit.
At the anode, iron dissolves: Fe → Fe²⁺ + 2e⁻. At the cathode, oxygen is reduced: O₂ + 2H₂O + 4e⁻ → 4OH⁻. The ions meet and react to iron hydroxides, which oxidise further into hydrated iron(III) oxide — Fe₂O₃·xH₂O — the flaky brown solid we call rust.
Why metals want to corrode
Most engineering metals are thermodynamically unstable in air and water. Their oxides sit at a lower energy state, so there is always a driving force pushing the metal to react. The galvanic series ranks metals by how readily they do: noble metals like gold and platinum resist; active metals like magnesium and zinc corrode eagerly.
That ranking is not just academic — it predicts which metal corrodes when two are joined. The more active one becomes the anode and protects the other, the principle behind both galvanic corrosion (a failure) and cathodic protection (a defence).
Corrosion takes many forms
Rusting steel is only the most visible case. Corrosion also attacks copper (forming green verdigris), aluminium (a thin self-protecting oxide), and even reinforced concrete, where corroding rebar cracks the structure from within. It can be uniform across a surface, or concentrate into pits and cracks that pierce a wall while most of the metal still looks pristine.
Understanding which form you face is the first step to stopping it. From there, the engineer's toolbox — coatings, cathodic protection, inhibitors and smart material choices — can slow corrosion to a crawl. Explore corrosion protection, or find the conferences where specialists share the latest methods at Corrosion Congress.
Corrosion, answered
Is corrosion the same as rust?
Rust is one kind of corrosion — specifically the corrosion of iron and steel into iron oxide. All rust is corrosion, but not all corrosion is rust: copper, aluminium and other metals corrode without ever forming rust.
Is corrosion always bad?
Usually, but not always. On aluminium and stainless steel a thin oxide layer forms and then protects the metal underneath — a controlled corrosion that works in our favour. The green patina on copper roofs behaves the same way.
How fast does corrosion happen?
Anywhere from microns to millimetres per year, depending entirely on the metal and the environment. Dry indoor air is nearly harmless; warm salt spray can be aggressive. The ISO 9223 corrosivity categories put numbers to this for atmospheric corrosion.