A 1898 repair to the Parthenon did more harm than 2,500 years
Ancient Greek builders fastened the Parthenon's marble blocks with iron clamps sealed in molten lead, and the joints held for some 2,500 years. Restorers in 1898 used bare iron clamps instead. Rust made the iron swell and crack the marble, and within a century those repairs were close to collapse.
Rust is a mix of iron oxides and hydroxides that forms when iron meets oxygen with water present. Pure water alone or dry oxygen barely affects it. Unlike the tight oxide skins on aluminium, copper or tin, rust is flaky, crumbly and lets air and water through, so the metal beneath keeps corroding until, given enough time, a whole iron object can turn to rust. Those oxides also take up more room than the original metal, and the swelling can exert enormous force.
Chemically it is an electrochemical process: iron gives up electrons and oxygen accepts them. Water seeps into microscopic pits and cracks, and dissolved salts speed everything up, which is why seawater and road salt are so hard on ships and cars. Acidity accelerates it too. With little oxygen, black magnetite tends to form; underwater concrete pillars with chloride but no oxygen can grow green rust. A lab indicator shows the process starting as blue and pink patches.
Keeping iron intact is a major industry. Stainless steel protects itself with a film of chromium oxide, and weathering steels like Cor-Ten grow a rust layer that clings and slows further damage. Galvanising coats steel with zinc, which corrodes first to spare the steel; a typical 85-micrometre coating wears away at about 1 micrometre a year, giving roughly 85 years. Buried pipes and ship hulls get sacrificial anodes of zinc, aluminium or magnesium, or an applied electric current.
Simpler barriers work as well. Paint, lacquer and wax seal the surface, and ships and cars have waxy slushing oil injected into hollow sections. Concrete's alkalinity guards embedded steel, though expanding rust can still split it from inside. Mothballed equipment gets coated in grease or Cosmoline, and silica gel packets keep sea cargo dry.
Source: Rust
