Our previous studies have revealed that a "hybrid" structure of the amorphous and crystalline phases is formed in the outermost surface region of the austenitic stainless steel tubes of steam generators (SGs) as an undesired consequence of the industrial application of the AP-CITROX decontamination technology. The formation of this mobile oxide-layer increased the amount of the corrosion products in the primary circuit significantly (∼80 kg/block), resulting in magnetite deposition on fuel assemblies. As deposits blocked the cooling channels, the flow rate of water coolant through the reactor core decreased. Consequently, the power capacity of three nuclear reactor units had to be reduced, and full core fuel replacement became necessary. In the light of the above events, the present work gives a brief overview on the general corrosion state of the heat exchanger tubes of SGs, concerning the long-term effects of the AP-CITROX procedure on the chemical composition and structure of the protective oxide-layer. 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 project based on sampling as well as on ex-situ electrochemical and surface analytical measurements was elaborated. Within the frame of this project, comprehensive investigation of the general corrosion state and metallographic features of 36 stainless steel specimens, cut out from various locations of the four steam generators of the Paks NPP has been performed. Electrochemical (voltammetric) and surface analytical (SEM-EDX, CEMS, and XRD) results reveal that some beneficial changes in the corrosion properties, morphology and chemical composition of the inner surfaces of heat exchanger tubes can be observed in the time period of 2000-2006.