Primary Water Stress Corrosion Cracking (PWSCC) of Ni-base alloys, such as Alloy 600 and Alloy 690, still remains one of the main issues in Nuclear Power Systems (NPPs). The Internal Oxidation model has been proposed by Scott and Le Calvar to develop a mechanistic understanding of the early stages of PWSCC and more recent advanced characterizations have consolidated this model. Preferential intergranular Oxide (PIO) have been observed to penetrate along High Angle Grain Boundaries (HAGBs) that exhibits local Diffusion-Induced Grain Boundary Migration (DIGM). However, the role of these 'precursor events' (namely PIO and DIGM) in the early stages of PWSCC is still unclear and the role of GB migration on crack initiation has never been clarified. In this study, oxidation experiments were performed in a low pressure superheated H2-steam system that is capable of accelerating the oxidation kinetics of Ni-base Alloys. The capability of the H2-steam system to reproduce the correct Electrochemical Corrosion Potential (EcP) with respect to the Ni/NiO transition was assessed by in-situ electrochemical measurements with a Y2O3-ZrO2 solid state reference electrode. The environmental conditions reproducing the Ni/NiO transition were identified between 372 °C and 480 °C and confirmed the possibility to use the superheated H2-steam as surrogate environment to perform oxidation experiments in conditions that are relevant to PWR primary water. Solution annealed (SA) Alloy 600 samples with an OPS surface finishing were exposed to H2-steam at 480 °C at different oxidizing conditions to clarify the role of PIO/DIGM and local microchemical segregation during the early stages of PWSCC. Advanced electron microscopy techniques showed that the PIO was more susceptible to occur under reducing conditions, whereas DIGM occurred independently from the environment. These results suggest that DIGM is a not sufficient condition for the occurrence of PIO and subsequent SCC initiation. Furthermore, it was found that SCC initiation occurred in correspondence of the intergranular oxide that developed along a migrated grain boundary and displayed a local Al/Ti rich oxide. It was postulated that the local Al and Ti enrichment can create incoherent surfaces along the migrated GB and provide an accelerating factor for the inward oxygen diffusion. Evidence of the occurrence of DIGM and minor segregation was also found in Alloy 690 TT and SA after exposure to H2-steam at 480 °C under reducing conditions. However, the material underwent DIGM but not to PIO. It is probable that the high Cr content induced the formation of a dense and protective external Cr- rich oxide that inhibited the PIO despite providing the driving force for the occurrence of DIGM; therefore, these results show that also for the Alloy 690, the DIGM can be considered as a secondary (e.g. accelerating) and not sufficient phenomena for the occurrence of PIO. Therefore, the early stages of SCC of Ni base Alloys cannot be explained only as the result of one single dominant phenomenon but due to the synergistic interactions between Al/Ti oxide and DIGM which are accelerating factors for the development of PIO which fracture upon the effect of applied stress if chemical and environmental conditions can lead to the film instability.