Page 31 - Curriculum Visions Dynamic Book

How iron becomes pitted
If a wetted iron surface is exposed either by being uncoated or because the paint on the surface has been chipped, oxygen atoms are able to enter the water through its surface skin. In this water one of the world’s tiniest batteries forms. The water is oxygen-rich and the iron forms an electrode, one terminal of a battery. The oxygen-poor region, in the scratch and farther from the air, forms the other electrode. The water forms the electrolyte. A minute electric current now flows, and iron is carried in solution to the oxygen-rich water, where it is oxidised and deposited.
Thus, iron is removed from one part of the metal and deposited as an oxide or hydroxide nearby. This explains why rusty material is often both pitted and lumpy.
corrosion: the slow decay of a substance resulting from contact with gases and liquids in the environment. The term is often applied to metals. Rust is the corrosion of iron.
electrode: a conductor that forms one terminal of a cell.
electrolyte: a solution that conducts electricity.
 These rusting chains show two forms
of rust. The light brown rust patches are recently formed iron hydroxide or Fe(OH)3. In contrast, the darker brown rust patches are the final solid state of ferric oxide (Fe2O3).
31
31