Evaluating the water chemistry in the primary circuit and the effect of chemical decontamination of the heat exchanger tubes performed by the AP-CITROX procedure at Paks NPP (Hungary), a project dealing with the comprehensive investigation of the general corrosion state of the steam generators (SGs) has been initiated. Owing to the fact that there is no investigation method available for the in-situ monitoring of the inner surfaces of heat exchanger tubes, a research program based on sampling as well as on ex-situ electrochemical and surface analytical measurements were developed and elaborated. In the time period of 2000-2008 - within the frame of the above project - 45 stainless steel specimens, cut out from various locations of the steam generators of the Paks NPP were investigated. Besides to the corrosion characteristics (corrosion rate, thickness and chemical composition of the protective oxide-layer) surface properties (morphology, chemical and phase compositions) of the passive layer formed on the inner surface of above heat exchanger tubes were studied, too. The passivity of the inside surface of the stainless steel specimens was measured by voltammetry, the morphology, chemical and phase compositions of the oxide layer formed on the surface were analyzed by SEM-EDX, XRD and CEMS methods. The great number of experimental results allowed us to develop an electronic database which involves the results of the above corrosion experiments, and also some special characteristics of the tubes (e.g. location in the SGs, surface pretreatment by decontamination, if any, etc). Evaluating the main relations among these parameters may contribute to the identification of important processes affecting the corrosion state of steam generators, and highly decisive concerning a life time prolongation project of VVER-type nuclear reactors. In the present work we provide a brief overview on these experiments, some characteristic results, the database developed, as well as some novel findings.