A simple device was designed with two mild steel electrodes placed face to face in the same phosphate solution and coupled through an external electrical circuit. A dialysis membrane retained hydrogenase from Ralstonia eutropha in contact with one electrode only. The simultaneous measurements of the electron flux in the electrical circuit and of nicotinamide adenine dinucleotide (NADH) production catalysed by hydrogenase proved that the enzyme induced the occurrence of cathodic and anodic micro-sites on the same electrode surface. A clear galvanic current was observed, which stopped after a few hours, because of the formation of a protective film of vivianite on the electrode that was in contact with hydrogenase. Hydrogenase in phosphate solution proved to be an effective trigger of mild steel corrosion. These results may be the basis of a new and easy-to-handle hydrogenase-catalysed phosphating process, which operates under mild conditions, avoids using toxic compounds, and is quite rapid. (C) 2004 Elsevier Ltd. All rights reserved.