Laboratory experiments have been carried out in synthetic pore water solutions and in mortar slabs with and without chloride additions. Field studies were performed at a 840 m long, 15 year old bridge at the coast of Northern Norway. The corrosion film on steel in concrete is changed by polarisation. The impressed current also induces changes of the pore water in contact with the metal. This leads to a complicated behaviour under polarisation and a large pseudo-capacitance. Potentiodynamic scans could not be used to determine the linear polarisation resistance of the cut reinforcement bars in the 15 year old bridge due to the large pseudo-capacitance of steel in concrete, giving a charging current that overshadowed the Faraday current. Polarisation resistance obtained by potentiostatic exposures varied more than a decade depending upon the polarisation time. Nevertheless, this technique discriminates between sites with high and low corrosion rates.