An electrochemical cell was specifically designed for testing in situ the quality of surface films formed on the cooling water side of condenser tubes in various situations of technical interest. Standard electrochemical techniques can be easily carried out even with the plant in operation, just requiring a half-reduction of the output. Rest potentials and polarization resistance results, together with cyclic voltammograms are presented. The results are in good agreement with those previously obtained by destructive sampling, mounting and using conventional electrochemical cells.