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Figure 15: Electrical analogue for hydrogen transfer and trapping in (micro-)voids in a steel wall.

4.3.

Potential Effects on Material Properties: Mechanical Aspects, HIC, Delayed Cracking or Other Phenomena

It is well known that the mechanical properties of high-strength ferritic steels can be severely degraded by hydrogen and therefore the hydrogen embrittlement phenomenon has great practical significance for this class of steels. On the other hand lower-strength steels (yield strength less than 690 MN/m 2 ) have generally been considered to be immune to hydrogen. However, the last two decades or so, a number of studies have demonstrated that such ferritic steels are also susceptible to hydrogen embrittlement 57,58,59 . At the lower end of the low-strength steels (< especially ca. 535 MPa yield strength), which will normally remain ductile after hydrogen absorption, and in the presence of thinner sections and very high hydrogen pressures, a real externally visibl e “blister” will form. Hydrogen blisters will typically nucleate at non-metallic inclusions; this occurs particularly when the non-metallic inclusions are distributed in layers or fibers arranged perpendicularly to the flux of the diffusing hydrogen (compare with the current failure case). In higher strength steels or embrittled steels (little plastic deformation) or in thick sections, a crack – rather than a blister – develops when the hydrogen gas precipitates at internal interfaces. This form of crack is normally called hydrogen-induced cracking (HIC) . Sometimes small hydrogen-induced cracks in adjacent parallel planes, as shown in Figure 7, link in the through-thickness direction and develop stepwise cracks .

Hydrogen and NPP Life Management: Doel 3 and Tihange 2

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