Chemistry
Rust & corrosion protection
Why bikes rust but alloy wheels don't, and how to protect metal cleverly.
What you need first
A bike that spends the winter outside often looks sad in spring: the chain and bolts are covered in brown rust. Rust eats away at cars, bridges and ships too, and experts estimate that corrosion costs a country around 3 percent of its yearly economic output. The good news: rust is chemistry, and once you understand the reaction, you can prevent it deliberately.
The rust recipe: three ingredients
Rust forms when iron reacts with oxygen and water. All three ingredients must be present at the same time: iron, water AND oxygen. If even one is missing, nothing happens. In dry desert air a nail barely rusts because the water is missing. In boiled, airtight sealed water it does not rust either, because the oxygen is missing. Only moist air delivers both at once, and salt on winter roads speeds the reaction up even more. The result is iron oxide, a porous brown layer that keeps crumbling away and exposing fresh metal.
Substances are identified by their properties: every substance has its own profile.
Protection means removing one ingredient
Every kind of corrosion protection relies on the same trick: it removes one ingredient from the rust recipe. Paint, grease or a plastic coating separate the iron from water and air. In galvanising, the iron gets a zinc coating that shields it, and even if the coating gets scratched, the zinc sacrifices itself first because it reacts more readily than iron. The sacrificial anode uses the same idea: on ship hulls and in hot water tanks, a block of zinc or magnesium is bolted on. These metals are less noble than steel, which means they react more readily. So the block dissolves instead of the steel, and every few years you swap it for a new one.
Why aluminium doesn't rust through
Aluminium actually reacts with oxygen even more readily than iron. Yet wheels and window frames do not fall apart. The reason: aluminium oxide forms a wafer-thin, dense layer that sits firmly on the metal and seals everything underneath. The metal protects itself. Iron oxide, on the other hand, is porous and flakes off, so the reaction keeps going. Whether a metal corrodes therefore depends not only on how readily it reacts, but also on what kind of oxide layer forms.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Which three ingredients does rust need?
How many of the three rust ingredients must be missing, at minimum, for no rust to form?
Why don't aluminium objects fall apart like rusting iron?
Aluminium does not fall apart like rusting iron because its oxide layer is … and seals the metal.
A garden gate needs 3 coats of anti-rust paint. One can covers one coat on 2 gates. How many cans do you need for 4 gates?
Match each protection method to how it works.
Where this leads