In our days, the nuclear energy becomes more and more important. The efficient operation of a Nuclear Power Plant (NPP) supposes the assurance of the performances established by design for all entire life of the NPP key components. Steam Generator (SG) is one of the key components for a NPP because this equipment assures the separation boundary between the primary and secondary circuit and its unavailability suppose the NPP shutdown. The synergetic action of the high pressures and temperatures, constraints (stresses, vibrations) and the chemical parameters of the cooling agents make the steam generator susceptible for more types of degradations. Because of the unavailability of the measures for monitoring and mitigation of these degradations the performances of the steam generators decrease and determine direct and indirect losses. The replacement of the steam generator is very expensive and very difficult. For these reasons it becomes necessary the assurance of the steam generator performances for the entire life established by design, 30 years with the possibility of extension to 40 or 60 years. The optimization of the steam generator operation, by implementation of the management complex system for the monitoring of the operation processes, periodical inspections, and preventive maintenance determine economies by order of hundreds of millions dollars, for entire life of a nuclear power plant. In this scope it is very important to intensify of the applicative research in the purpose of establishing the newest solutions, methods, mechanisms in order to characterize the specific processes for the operation of the steam generator. The principal objective of the research consists in the establishing of the fundamental knowledge, theories, methods and models necessary for qualitative and quantitative characterization of steam generator degradation processes. The complementary research activity should be oriented towards the management of ageing and, implicitly, towards the preservation of the steam generator structural integrity. The principal objective of the work presented in this chapter consists in the characterization of specific processes and mechanisms referring to steam generator tubing degradation. The specific objectives are the following: the establishment of the main corrosive degradation mechanisms which contribute at steam generator tubing material (Incolloy-800) failures; knowledge of the phenomena which appear in steam generator because of the material/environment interaction; elucidation of corrosion product release, transport and