A nail, when exposed to the elements for any extended length of time, undergos some familiar changes. The silvery sheen of a new nail gives way to reddish-brown spots, which then spread to cover the entire nail. The sharp outline softens, covered in rough scale and eaten away with tiny pits. Eventually, the rust reaches the core, until you can break the nail between your fingers. Finally, the nail crumbles entirely, leaving only a powdery stain. The cause of all this is a chemical reaction between the iron in the nail and oxygen dissolved in the water it encounters.
The formation of rust depends on two chemical reactions. The first is known as anodic dissolution, which takes place when the iron in the nail is exposed to water. The water reacts with the iron by stealing two electrons from the iron, leaving it positively charged. Any oxygen dissolved in the water then interacts with the positively charged iron in a second chemical reaction, bonding with it to create ferrous oxide. Ferrous oxide is the reddish substance most commonly referred to as rust.
Causes of Rust
Since one of the chemical reactions that causes rust requires the presence of water and the second reaction requires oxygen, rust can only form when both water and oxygen can reach the iron molecules in the nail. Unfortunately, both water and oxygen are readily available in the atmosphere, so even unprotected nails in a desert environment will succumb to rust, although iron exposed to high humidity or seawater will rust much more quickly. Steel rusts as well as iron because it is an alloy chiefly composed of iron.
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Scaling is the ferrous oxide which remains attached to the nail. Since the ferrous oxide is a bulkier molecule than the original iron, it takes up more space, which distorts the shape of the nail as it rusts. This also accounts for the fact that when a whole barrel of nails rusts, they meld together into a cohesive mass. The ferrous oxide from one nail is bonding with the ferrous oxide of its neighbors, welding them together. Scaling is what causes rusty hinges to stick and squeak and rusty chains to creak.
Corrosion is the most destructive aspect of rust. Since the ferrous oxide is less durable than the original iron, it easily pits and flakes away. Worse, unlike the oxides of copper, ferrous oxide does not provide any sort of protective patina. A rusty nail can rust to the core without the outer coating of rust providing any protection. When too much of the original iron has been converted to fragile ferrous oxide, it will lose structural integrity and crumble to dust. Given enough time, water and oxygen, even large chunks of iron machinery will literally rust away to nothing.