Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 144

Table 4.9 Corrosion potential differences (mV) between Zircaloy-4 and
Inconel in BWR and PWR environments

Environment Material coupling

Inconel-Zry-4 ground Inconel-Zry-4 pickled

PWR type ෥60 ෥75

BWR type ෥420 ෥640

Source: A.N.T. International (2011) and Garzarolli et al . (2001b).

increased during electron irradiation (Howla et al ., 1999 ), gamma irra-
diation (Kang et al ., 1994) and in-reactor (Shannon, 1962). Also, the con-
ductivities of various ceramics, including Al 2 O 3 were reported to increase
dramatically under proton or x-ray irradiation (Hobbs et al ., 1994 ). So
perhaps a way to allow closing of the galvanic circuit is indicated. Note
that Al 2 O 3 was used as insulation in the MIT experiment (Andersson
et al ., 2002 ) and ZrO 2 as insulation in others (Chen & Adamson, 1994;
Shimada et al ., 2002). Some distinction must be made between surface
conduction and bulk conduction in thick ceramics, but little is known.
5 Shadows were not formed when the components were confi rmed to be
electrically insulated (Lysell et al ., 2005 ).

Points which bring doubt to the irradiation-assisted galvanic mechanism
hypothesis include:

6 It is not certain that an electrically-conducting path truly exists between
the shadower and component.
7 Conventional galvanic reactions are inhibited when the cathode is small
and the anode is large, as is often the case with observed shadows.
8 The MIT experiments (Andersson et al ., 2002) appear to produce shad-
ows in a region of very low radiation.

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It is also quite possible that several different mechanisms contribute to
shadow corrosion depending on specifi c conditions.
It is obvious that the mechanism of shadow corrosion is not precisely
defi ned, although the number of observations which help to provide a work-
ing hypothesis are many. It is clear that enhanced corrosion can and will
occur in cases where Zircaloy and a variety of other metals or alloys are in
close contact. Potential problems include loss of strength and integrity due
to wall thinning and oxide spallation. In most cases, the enhanced corro-
sion does not cause serious operational problems. The oxide thicknesses are
moderate, hydrogen absorption is not increased by straightforward shadows
and spalling usually does not occur. The severe problem of enhanced spacer