Chapter 16 · Oxidation and Reduction
16.8Corrosion: Undesirable Redox Reactions
9 min · two checks
Predict
Why does iron rust faster in salt water, and how does zinc protect it?
The idea
- Describe rusting as redox.
- Explain paint, galvanizing, and stainless alloys as defenses.
Iron rusts when it is oxidized by oxygen in the presence of water. A simplified picture: iron is oxidized to Fe²⁺ and then to iron(III) oxide hydrates, the flaky red-brown solid, while oxygen is reduced. The rust does not stick tightly, so fresh metal stays exposed. Salt water does not appear in the simplest equation, but dissolved ions let charges move, so the electrochemical cell on the surface works better and rusting speeds up.
Defenses interrupt the cell. Paint keeps out oxygen and water until it cracks. Galvanizing coats steel with zinc. Zinc is above iron in the activity series, so if the coating is scratched, zinc oxidizes preferentially and iron remains reduced. That is a sacrificial anode. Stainless steels alloy in chromium, which forms a tough oxide skin. Cathodic protection of pipelines uses the same sacrificial idea with attached blocks of a more active metal.
Keep these
- Rust is oxidation of iron by oxygen, with water present.
- Salt speeds it by conducting ions.
- Zinc protects steel by being oxidized instead.