A primary bacterial film on a metal surface immersed in natural waters can create electrochemical conditions at the base of the film that are radically different from those in the bulk water. The concentrations of dissolved oxygen and other chemical species; as well as pH at the metal surface are usually affected. These changes influence electrochemical reactions taking place on the metal during corrosion. On the other hand, the electrochemistry of corrosion can also change the pH and ionic concentrations at the metal surface, thus influencing biofilm formation. This paper will examine what types of changes take place under marine biofilms, the mechanisms by which these changes influence several types of corrosion on a variety of metals, and the implications of that influence to both corrosion rates and corrosion control.