The efficiency of sodium-diethyl-dithiocarbamate (NaDDTC) as corrosion inhibitor for CuNi10Fe alloy was studied in natural seawater by means of immersion tests, polarization measurements, electrochemical impedance spectroscopy (EIS) and Auger electron spectroscopy (AES). After immersion of the alloy for 24 h in various concentrations of NaDDTC, layers were formed which act as a physical barrier to the corrosion attack. The polarization curves obtained with electrodes in seawater indicate retardation of both the anodic and cathodic processes. From EIS measurements equivalent circuits, which illustrate the behaviour of the reaction product film, were proposed and individual circuit elements at the open circuit potential and anodic polarization were defined. The results obtained for inhibitor efficiency from weight losses, corrosion currents and from impedance measurements are in fair agreement. The inhibition film does not lose efficiency even at anodic polarization, although the AES measurements indicate a change in the organic molecule structure. The hypothesis proposed is that a possible reconstruction of the organic molecule takes place, with release of CS2.