Corrosion by external currents (electrocorrosion) of metallic constructions has to be regarded as a strongest factor of aggressiveness in conductive aggressive environments. Corrosion by stray currents of underground and underwater metal constructions was comprehensively and profoundly studied long ago and it is considered in this book only in the form of a review. The primary attention is focused on corrosion by external anodic (mainly) and cathodic currents of metal constructions in environments of high aggressiveness typical for electrochemical plants of chemical and non-ferrous metallurgical brunches of industry, where penetration of the external currents (leakage currents) from the electrolytic baths to the metal constructions is unavoidable.Structural metals selected for the media of a high aggressiveness are initially in a passive state that provides their corrosion resistance at the absence of the attack by external currents. However, under the attack by leakage current the metals are brought out of the boundaries of their passive field and undergo intensive corrosion damage.A new approach to the problem of electrocorrosion protection of the passive structural metals which is considered in this book consists in keeping the metals attacked by external currents in the boundaries of their passive field. The systems, developed in accordance with this approach, are based on modification of existing and on elaboration of new methods and means of electrocorrosion protection. These systems take into account corrosion and electrochemical characteristics of the aggressive media (redox potential, conductivity etc.) and of the passive metal (corrosion and activation potentials, current density in a passive state etc) as well as sizes and distribution character of the external currents.Analysis of leakage current distribution in electrochemical plants, character of their influence, methods of estimation of corrosion stability of metallic structures in the field of external currents and a lot of concrete examples of successful introducing of the developed corrosion protection systems in many operating plants of chemical and non ferrous metallurgical branches of industry are given.