Why does iron rust continue to promote further corrosion?

Iron rust continues to promote further corrosion because it forms a porous, nonprotective layer on the metal's surface. Unlike metals such as aluminum or copper, which develop compact, adherent oxide films that act as a barrier to limit further attack, the rust layer on iron is permeable.1

This porosity allows moisture and oxygen to penetrate the surface and reach the underlying metal, where the electrochemical process of corrosion continues. Because the rust does not seal the surface, the iron remains exposed to the environment, facilitating ongoing oxidation and the gradual degradation of the material. To prevent this, various methods such as protective coating, galvanization, or cathodic protection are used to create a barrier or interrupt the electrochemical reactions that drive the rusting process.21