IronThe metal that corrodes. It also carries the electrons, anode to cathode.
Water drop (electrolyte)The electrolyte, which carries the ions. Dissolved salt makes iron rust faster.
Anode regionWhere the iron dissolves and a pit forms. Each atom leaves two electrons behind in the metal.
Cathode regionWhere oxygen takes up the electrons. It is the surface most open to the air.
What moves and forms 5
Electron flowElectrons move through the metal, anode to cathode.
Iron ionsIron atoms that have lost two electrons. They leave the metal and go into the water.
OxygenOxygen from the air dissolves in the water. At the cathode it takes up the electrons.
Hydroxide ionsFormed at the cathode from oxygen, water and electrons. They drift toward the iron ions.
RustForms where iron ions meet hydroxide ions and oxygen. It is a mix of hydrated iron oxides.
How this list was compiled: Every name and fact checked on 2026-10-01, and the layout under the drop on 2026-10-02, and kept only where two independent sources give it. (1) S. Lower, Chem1 virtual textbook, Electrochemical corrosion, https://www.chem1.com/acad/webtext/elchem/ec-7.html : oxidation and reduction happen at separate places on the metal; the electrons travel inside the metal, from anodic regions to cathodic ones; water is needed to carry the ions; iron goes to iron(II) ions at the anode; oxygen, water and electrons give hydroxide ions at the cathode; hydroxide ions react with the iron ions to form the hydrous iron oxides known as rust; places short of oxygen become anodic and pits form there; any surface open to oxygen can act as a cathode; two unlike metals in contact with moisture form a cell. (2) Wikipedia, text only: Rust, https://en.wikipedia.org/wiki/Rust (electrons pass from iron to oxygen; oxygen is reduced to hydroxide; iron is oxidized to iron(II); rust is hydrated iron(III) oxides and iron oxide-hydroxide; electrolytes such as salt speed it up); Corrosion, https://en.wikipedia.org/wiki/Corrosion (one spot on the iron behaves as an anode, the electrons move through the metal to another spot, which behaves as a cathode; the more active metal of a couple is the anode); Galvanic corrosion, https://en.wikipedia.org/wiki/Galvanic_corrosion. (3) Chemistry LibreTexts, the map of a general chemistry textbook, 20.8 Corrosion, https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/20%3A_Electrochemistry/20.08%3A_Corrosion : the cathode is the surface that gets more air, where oxygen is reduced; the anode is the less exposed spot, where iron is oxidized; rust forms where the iron ions meet oxygen, often away from the anode; both water and oxygen are needed. (4) U. R. Evans, An Introduction to Metallic Corrosion (1948), section Corrosion by Drops, full text at https://archive.org/details/cftri.1435introductiontome0000ulic_n5a1 : a drop of salt solution resting on iron attacks the metal at the center of the drop while the zone around the edge stays free of attack; a ring of rust appears between them, where the products of the two places meet; currents flow between an anode at the center and a cathode at the edge, where the oxygen is. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
When iron rusts, one spot on the metal acts as an anode and another as a cathode, with water joining them. This galvanic cell corrosion diagram shows that cell under a single drop.
Read the full guide
Follow the electrons
Under the middle of the drop, where little air reaches, iron atoms give up electrons and go into the water as iron ions. That spot is the anode, and a pit forms there. The electrons run through the metal to the edges of the drop, the cathode, where oxygen from the air takes them up and hydroxide ions form.
Where the rust forms
The iron ions and the hydroxide ions meet in the water and, with more oxygen, turn into rust. That is why rust often sits beside the pit and not in it.
Salt in the water makes it a better electrolyte, so iron rusts faster.
Rust needs both water and oxygen.
Questions and answers
What is a corrosion cell?
A small electric cell that forms on a metal as it corrodes. It has an anode, where the metal dissolves; a cathode, where oxygen takes up electrons; the metal that joins them; and an electrolyte, such as water, touching both.
Where are the anode and the cathode when iron rusts?
The anode is the spot least open to the air, here under the middle of the drop. The cathode is the surface most open to the air, here near the edge. Electrons travel between them through the iron.
What is galvanic corrosion?
Corrosion where two different metals touch in an electrolyte. The more active metal becomes the anode and corrodes faster, and the less active metal becomes the cathode. Zinc on steel works this way: the zinc corrodes first.
What is rust made of?
Hydrated iron oxides. It forms when iron ions from the anode react with hydroxide ions and oxygen in the water.