We developed a specific reliable apparatus to perform electrochemical measurements (i.e. current-potential curves) in an environment (pressure : 15MPa, temperature 300°C, ionic composition of the media) similar to those of steam generators of nuclear power plants. We demonstrated the important influence of nickel on the dissolution of alloys such as 316L, alloy 800 or alloy 625 in hot pressurized water. On the basis of a dissolution-passivation model fitted to the experimental results, we pointed out the beneficial role of phosphates and dissolved oxygen on the corrosion resistance of the alloys, even in presence of chlorides.