Microcell corrosion is when active corrosion and the cathodic half-cell reaction that it produces take place at different points along the same piece of metal. Corrosion is, of course, an electrochemical reaction that produces voltage across an anode and a cathode. On the surface of a contiguous piece of metal, it is possible to have multiple corrosion cells, where pairs of adjacent anode-cathode electrons cover a large area of a metal's surface, leading to relatively uniform corrosion.

Microcell corrosion is generally caused by concrete carbonation or high levels of chloride. This short video from CorrConnect shows how this process works at a molecular level.