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Figure 14: Sieverts’ constant and diffusion coefficient of hydrogen in steel as a function of temperature; remark the very large data variations at lower temperatures.

According to general corrosion engineering literature and the current authors’ opinion, it should merely be seen that the existing “ hydrogen flakes” will act as an almost irreversible trap for diffusing hydrogen. The effect of the flaws on hydrogen diffusivity can be compared to the effect of a capacitor on electrical current flow (Figure 15). Like Ohm’s law is not applicable to the electrical current transfer across the capacitor, also Sieverts’ law only determines the overall driving force, but not the behavior and H-transfer across the flake. It also means that transient conditions, i.e. regarding temperature, could be very important (cf. Section 4.3). This is also recognized by Bosch and Vankeerberghen 42 when describing possible situations during reactor shutdown and outage. Recent studies on the effect of hydrogen in flake-bearing steels 56 have also shown the possible effects on hydrogen-induced delayed cracking and the effect of flakes on reduced impact toughness and increased fatigue crack growth rates .

Hydrogen and NPP Life Management: Doel 3 and Tihange 2

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