Stress and stress gradient are two necessary
requirements for DHC to occur, and, fortunately,
they are controllable. High stresses without
gradients will not induce DHC. Hydrogen
cannot move when high stresses are uniformly
distributed in the structure. High-stress gradients
are needed to move hydrogen but cannot be
created without high stresses. High stresses are
also required in fracturing hydrides.
It is estimated that hydrogen can hardly move
from stress gradients generated by stressing up
to 240 MPa (35 ksi). The common ASME
requirement for unfired pressure vessels is to set
the maximum allowable stresses at one-fourth
the tensile strength at the operating temperature.
As indicated in Table 7, the maximum allowable
design stresses for zirconium alloys 702
and 705 are much lower than 200 MPa (29 ksi).
Consequently, DHC should not occur when
ASME requirements are met.