In cathodic protection the efficiency of cathodic protection is driven by various environmental parameters. The main ones are: the geometry of the protected structure the electrolyte resistivity and of course the steel polarization curve. The aim of this study is to assess notably the influence of those parameters on the current distribution and on the potential attenuation. In particular an attempt to solve the apparently controversial influence of the resistivity will be proposed. In order to achieve these results the cases of storage tanks bottom are studied by 3D modeling for two configurations. The first one concern the tank bottom for which a membrane is installed few tens of cm underneath with the anodic grid system located in the confined space. The second one is the usual design where no membrane is installed below the tank and where some anodes are located at the periphery of the tank