Corrosion

Corrosion is the spontaneous oxidation of metal in the presence of an oxidizing agent, usually oxygen. Protective corrosion, like copper oxidizing to form blue-green basic copper carbonate, creates a stable layer shielding underlying metal. Detrimental corrosion, such as iron rusting, produces unstable metal-oxides that flake off, continuously exposing fresh metal to oxidation and structural damage.

Rusting forms a galvanic cell at iron's surface. Surface defects act as anodic regions where iron oxidizes to iron(II) at −0.44 volts. Electrons migrate to cathodic regions where oxygen reduces to water at +1.23 volts. Iron(II) ions then migrate via surface moisture and oxidize to iron(III) oxide hydrate, commonly called rust, which continuously flakes away.

Rust formation accelerates with increased moisture, acids, and electrolytes. These factors enhance charge flow and increase hydrogen ion concentration. Atmospheric carbon dioxide reacting with water forms carbonic acid, supplying hydrogen ions that drive the electrochemical oxidation process faster, leading to rapid rust development and metal degradation.

Stainless steel is an alloy of iron containing small amounts of chromium. Chromium collects near the surface and oxidizes preferentially, forming an unreactive oxide layer that protects the underlying iron from further oxidation. This protective chromium oxide layer remains intact and stable, unlike rust, preventing continuous metal degradation.

Galvanization coats iron with a layer of more readily oxidized metal, typically zinc. When intact, the zinc layer prevents air and water from contacting underlying iron. If the zinc layer is breached, the iron remains protected because zinc is more easily oxidized, sacrificing itself to prevent iron corrosion through cathodic protection.

Cathodic protection converts the metal being protected into a cathode by connecting it to a more easily oxidized metal, called a sacrificial anode, such as zinc or magnesium. The anode corrodes and gets consumed protecting the cathode metal. This principle is widely used in water heaters, storage tanks, and underground pipes to prevent corrosion of valuable infrastructure.

Iron(II) oxidizes to iron(III) oxide hydrate, commonly known as rust, whose stoichiometry varies with water exposure. The hydrate formula contains a variable x representing different water content ratios. Greater moisture exposure produces different rust compositions, affecting the layer's properties and the rate at which it flakes off to expose fresh iron.