The invention relates to a method for determining the corrosion risk of installations in natural waters due to biofilm formation, comprising: a) providing a device for measuring electrode potentials, which comprises at least one metal sample and at least one reference electrode, at a point in the immediate vicinity of the installation within the body of water of the natural water; b) measuring potential differences between the at least one metal sample and the at least one reference electrode over a sufficient period of time such that any potential drift of the sample caused by ennoblement due to biofilm formation can be determined; c) comparing the measured potential difference with one or more reference values for the metal of the metal sample; and d) estimating if or when the potential difference, because of biofilm formation, has reached or can reach a critical value that brings about corrosion of said metal at said point of the natural water.