NPP Life Management_vs02

Figure 9: Simplified E-pH diagram for nickel and iron at 300°C showing the principal pH-potential combinations for PWR primary and secondary water, BWR Normal Water Chemistry (NWC) and BWR Hydrogen Water Chemistry (HWC) and the modes of stress corrosion cracking of Alloy 600 25

Hydrogen and NPP Life Management: Doel 3 and Tihange 2

There are a number of alternative reactions possible at these low potentials at 300°C, such as the oxidation of Fe to Fe 3 O 4 (see Figure 9), or possibly even – depending on pH – conversion of Cr to e.g. Cr 2 O 3 or some mixed Fe-Cr oxides (Figure 10). The corrosion potential (ECP), i.e. the “mixed potential”, may in this case be almost the same, but it would create much higher driving forces for the hydrogen evolution reaction (300 to 400 mV for iron oxidation, or up to 600 or 700 mV for the latter cases), with orders of magnitude higher hydrogen evolution rates.

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