Iron combining with oxygen
Why Does Metal Go Rusty?
Rust is iron slowly joining with oxygen and water to make a new, crumbly substance.
-
Step 1 of 5
Rust is a new substance.
It is not a coating on the iron — it is what the iron has become. The metal is consumed as the rust forms.
-
Step 2 of 5
It needs both oxygen and water.
Iron in dry air barely rusts, and iron in oxygen-free water barely rusts. Both must be present for the reaction to run.
-
Step 3 of 5
Salt speeds it up dramatically.
Dissolved salt lets electric charge move through the water far more easily, and the reaction depends on that movement.
-
Step 4 of 5
Rust flakes off and exposes fresh metal.
Rust takes up more space than the iron it came from, so it cracks and falls away — uncovering new iron to attack.
-
Step 5 of 5
Protection means keeping the two apart.
Paint, oil, plastic coating or a zinc layer all block oxygen and water from reaching the iron. Break the coating and rusting resumes at once.
Stainless steel contains chromium, which forms an invisible sealed oxide layer instead of a flaky one.
The short version
Rust is iron chemically combining with oxygen and water, and it flakes away to expose fresh metal underneath.
Try it yourself
Leave three nails: one dry, one in water, one in salt water. Compare after a few days.
Topics