NPP Life Management_vs02

Figure 10: E-pH diagram for chromium at 300°C 26 .

The likelihood of this is illustrated by a series of experimental electrochemical polarization curves from our own laboratory. 27,28 Slow scan potentiodynamic polarization tests on stainless steel type 304, as well as on the alloying elements Fe and Ni, in a range of high-temperature aqueous solutions at 300°C show that the polarization curve (E-i) for the hydrogen reaction is mostly very flat; i.e. the reaction is very little polarized, with low cathodic Tafel constants. This means that, the corrosion reactions are almost fully under so- called ‘anodic control’ and that, whatever the anodic reaction is, there will be little shift in the corrosion potential (ECP) and the ECP will in any case be close to the hydrogen evolution line. It is the type of anodic reaction and its polarization behavior that will almost totally determine the corrosion and the hydrogen evolution rate. This hydrogen evolution rate can vary by orders of magnitude, depending on the anodic reaction. It should be mentioned that such additional or alternative oxidation reactions for the Ni/NiO couple might also explain the variation in actual potential measurement for stainless steel under PWR conditions (+/- 300 mV, not taking into account potential excursions during transient RCS conditions).

Hydrogen and NPP Life Management: Doel 3 and Tihange 2

So, there is no concluding experimental proof or evidence that the hydrogen evolution driving force would indeed be small.

15

Although a native “hydrogen blistering” phenomenon in the 20 cm thick-walled reactor pressure vessel is not to be expected, a contribution of hydrogen and hydrogen pressures to phenomena