Abstract: A method for determining the corrosion risk caused by biofilm formation for installations in natural bodies of water includes a) providing a device for measuring electrode potentials, comprising at least one metal sample and at least one reference electrode at a location close to the installation within the natural body of water b) continuously or intermittently measuring potential differences between said at least one metal sample and said at least one reference electrode over a sufficiently long period of time to allow for identifying any changes of potential of the sample resulting from ennoblement caused by biofilm formation, c) comparing the measured potential difference to one or more reference values for the respective metal of the metal sample, and d) estimating if and when the potential difference resulting from biofilm formation attains or may attain a critical value which would result in the corrosion of the respective metal at the respective location of the natural body of water.