Even though nickel based Alloy 600 (Ni 75 wt%, Cr 15 wt% and Fe 10 wt%) shows superior corrosion and mechanical properties, numerous corrosion problems, caused by a long time exposure of more than 30 years under a high temperature and high pressure water chemical environment have occurred. Especially, a lead induced stress corrosion cracking (SCC) has been a very important issue. A SCC is deeply related to a nano-sized oxide properties, formed on an Alloy 600 surface, because a crack initiates and propagates through a breakdown and modification of a thin surface oxide formed naturally on Alloy 600 in an aqueous solution. Therefore an investigation of an oxide properties would provide key information to elucidate the mechanism of a SCC and to establish a countermeasure. In the present work, a surface oxide film formed on Alloy 600 in an aqueous solution at 315 degrees C without/with a lead oxide and nickel boride as a SCC accelerator and an inhibitor, respectively, was analyzed by using a transmission electron microscopy, equipped with an energy dispersive X-ray spectroscopy and an X-ray photoelectron spectroscopy, equipped with an ion sputter for a depth profiling. In both the ammonia solutions without/with NiB, a duplex oxide layer was formed, i.e., a porous outer oxide mainly composed of NiO and Ni(OH)2 and a relatively dense inner Cr2O3 layer. Lead was incorporated into the oxide layer leading to a Cr depletion in the oxide layer and a passivity degradation. The passivity of the surface oxide was increased when NiB was added into a solution with PbO, which was in accordance with the increase of the SCC resistance. Passivity degradation by lead and the inhibitive mechanism of nickel boride were explained by a defect chemistry in an oxide semiconductor and an adhesion preference as a function of the wetting angle, respectively.