Figure 86 is a cross-sectional view of the place where a steel wall joins a copper floor. The diagram resembles the capital letter L, with the steel forming the upright, and the copper the horizontal part. The space above and to the right of the L is filled with an oxygen-bearing electrolyte. This region contains a number of small circles with plus signs inside them, and others with minus signs in them. The ones with the plus signs are labelled 'metal ions'; the ones with the minuses 'oxygen ions'. A large blue arrow coming from the top right of the diagram and labelled 'oxygen' indicates an abundant supply of this element from elsewhere. A red arrow leading from within the steel plate into the electrolyte zone indicates the movement of iron ions into the electrolyte. A white arrow starting from within the steel and ending in the copper, without passing through the electrolyte, indicates the transfer of electrons from one metal to the other. This is the local current that flows in any corrosion process. A mauve arrow from the copper into the electrolyte shows the destination of these electrons.

 5.4 Batteries, chemistry and corrosion