Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 143

The early thoughts on the mechanism of shadow corrosion centred on it
being a form of galvanic corrosion. As such, the mechanism required a path
for electron transfer between the cathodic shadower (the material that
causes the shadow) and the anodic component (the Zircaloy or zirconium
alloy component on which the enhanced corrosion occurs) and a conduc-
tive path through the water separating the two parts. But since the shadow
phenomenon could not be reproduced in the laboratory, it was clear that
some sort of radiation effect was also required. Problems with the galvanic
hypothesis included lack of evidence, in some cases, of any electrical con-
nection between the shadower and component (although for commercial
reactor components an obscure path can always be suspected) and concern
that the zirconium oxide, which is always present on component surfaces,
was not conductive enough to allow the postulated conductive paths to
operate.
Another hypothesis arose when Chen and Adamson (1994) noted that the
range in water of beta particles from Mn-56 and Zr-97 (originating in the
shadower material) could explain the shape and size of observed shadows if
a beta-damage mode could be found. Lemaignan (1992) proposed that extra
radiolysis caused by the imposed local beta fl ux could result in accelerated
corrosion. However, additional calculations by Andersson et al . ( 2002 ) and
Shimada et al . (2002) indicated that the extra beta fl ux from the shadower
does not make a signifi cant change to the overall beta fl ux in the reactor, so
this hypothesis has been discounted. Also, as noted above, it has been shown
that the alloy Nitronic 32 does not cause shadows, even though the fl ux of
beta particles from activated Mn-56 from that alloy is much higher than for
the known shadowers Inconel and stainless steel.
The latest view of the mechanism is that it is indeed a form of irradiation-
assisted galvanic corrosion. Points which support this hypothesis include:

1 It is known that there is a corrosion potential difference between, for
instance, stainless steel or Inconel and Zircaloy in non-hydrogenated
water (BWR type) (Table 4.9). Also, this potential difference is enhanced
in-reactor (Lysell et al ., 2001) and by ultraviolet light outside the reactor
(Kim et al ., 2010 ).
2 The observed relationship between component separation distance and
shadow oxide thickness is roughly as expected (Adamson et al ., 2000 ;
Lysell et al ., 2001 ; Andersson et al ., 2002 ).
3 A true stainless steel/Zircaloy galvanic couple in-reactor produced thick
oxide and low hydrogen pickup in Zircaloy, similar to that observed for a
control blade handle/Zircaloy shadow (Adamson et al ., 2000 ; Lysell et al .,
2005 ).
4 A radiation enhancement of electrical conductivity of oxides has been
reported. Electrical conductivity of oxide fi lms on Zircaloy markedly

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