Metal corrosion and the electrochemical series

Corrosion is fundamentally a redox reaction: a metal loses electrons (oxidises) to its surroundings, typically reacting with oxygen and moisture to form an oxide, hydroxide, or other compound — rust, in the case of iron, is hydrated iron oxide. This chemical (not merely physical) nature of corrosion is confirmed by the fact that different metals corrode at very different rates and in different ways, exactly tracking their position in the electrochemical (activity) series, a ranked list of metals ordered by how readily they lose electrons and oxidise. Metals high in the series (highly reactive, such as sodium and magnesium) oxidise very readily; metals low in the series (relatively unreactive, such as gold and platinum) resist oxidation strongly, which is exactly why they are found in nature as pure elements rather than only as ores.

When two different metals are placed in electrical contact in the presence of an electrolyte (such as moisture containing dissolved ions), a galvanic cell forms, and the electrochemical series predicts which metal corrodes. The more reactive metal (higher in the series) acts as the anode and is preferentially oxidised, losing electrons that flow to the less reactive metal (the cathode), which is thereby protected from oxidation — the electrons it would otherwise lose are instead supplied by the sacrificial metal. This is not a minor detail but a directly exploitable engineering principle: cathodic protection deliberately attaches a more reactive "sacrificial" metal, commonly zinc or magnesium, to a structure that needs protecting, such as a ship's steel hull, an underground pipeline, or a bridge support. The sacrificial metal corrodes in place of the structure, supplying electrons that keep the protected metal in its reduced, non-corroding state; the sacrificial anode is simply replaced periodically once it has been consumed, which is far cheaper and easier than repairing corroded structural steel.

Galvanising — coating steel with a thin layer of zinc — combines two protective mechanisms: the zinc coating is a physical barrier that keeps oxygen and moisture away from the steel, and even where the coating is scratched or damaged, the exposed zinc still acts as a sacrificial anode, continuing to protect the underlying steel electrochemically.