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What is rust?

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Rust is a pesky foe. Removing it can be quite difficult and doing it wrong can cause permanent damage. Knowing what rust is and finding a technique is recommended for your rust problem is the only way to remove it properly.

When iron compounds corrode due to the presence of oxygen and water, rust is formed. Rust is a mixture of iron oxides and hydroxides.
“Rusting” is a common term for corrosion, and usually corrosion of steel.

Iron is found naturally in the ore haematite as iron oxide, and metallic iron tends to return to a similar state when exposed to air, (hydrogen, oxygen, nitrogen, etc.) and water. This corrosion is due to the oxidation reaction when iron metal returns to an energetically favourable state.

Energy is given off when rust forms. The process of rusting can be summarized as three basic stages: The formation of iron(II) ions from the metal; the formation of hydroxide ions; and their reaction together, with the addition of oxygen, to create rust.

Iron is the main component of steel and the corrosion of steel is observed more frequently, since iron is rarely used without alloying in the present day.

When steel contacts water, an electrochemical process starts. On the surface of the metal, iron is oxidized to iron(II). The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen. They reduce the oxygen and water to hydroxide ions. The hydroxide ions react with the iron(II) ions and more dissolved oxygen to form iron oxide. The hydration is variable, however in its most general form.

Rust is hydrated iron(III) oxide. Corrosion tends to progress faster in seawater than fresh water due to higher concentration of sodium chloride ions, making the solution more conductive. Rusting is also accelerated in the presence of acids, but inhibited by alkalis. Rust can often be removed through electrolysis, however the base metal object can not be restored through this method.

Hydrated iron oxide is permeable to air and water, meaning that the metal continues to corrode after rust has formed. The iron mass eventually converts entirely to rust, and disintegrates. Corrosion of aluminium is different from steel or iron, in that aluminium oxide formed on the surface of aluminium metal forms a protective, corrosion resistant coating, a process known as passivation. Stainless steel similarly resists rusting by forming a passivation layer of chromium(III) oxide. This is also true of magnesium, copper and zinc.

Galvanization consists of coating metal with a thin layer of another such metal. Typically, zinc is applied by either hot-dip galvanizing or electroplating. Zinc is traditionally used because it is cheap, easy to refine and adheres well to steel. Zinc also provides cathodic protection to metal that itself is unplated, but close enough that any water touching bare iron is also in contact with some zinc. The zinc layer acts as a galvanic anode rusting in preference. More modern coatings add aluminium to the coating as zinc-alume, aluminium will migrate to cover scratches and thus provide protection for longer. These rely on the aluminium and zinc oxides protecting the once-scratched surface rather than oxiding as a sacrificial anode.

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