A model is developed for determination of the efficiency of employment of protector protection of an equipment in dependence on different physical-chemical parameters of a medium. A series of model calculations is carried out for the system of the protector (zinc and aluminum based alloy) - steel (St3) in two media; chemically treated water and wash water of ionite filters. Data on the effect of the medium electric conductivity on the protection zone radius are obtained. All calculations are based on the kinetic and thermodynamic parameters of electrode processes in media under investigations.