Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 138

Uniform corrosion appears not to be so sensitive to solute content, as
even low fl uence irradiation results in large changes in the chemistry of
the SPPs and signifi cant increases in solute matrix concentration without
a signifi cant change in oxide thickness. The experiments described above
clearly indicate, however, that disappearance of the SPPs is correlated to
oxide thickness increase.
The HPUF at high fl uence appears to be correlated to both SPP disap-
pearance and solute concentrations in the matrix. HPUF increases earlier
in fl uence compared to oxide thickness, indicating a connection to solutes,
but there does not appear to be a clear correlation to Fe concentration in
the matrix. When the SPPs do disappear (dissolved by irradiation effects,
dispersing Fe and Ni in the matrix and leaving a local concentration of Cr)
however, the HPUF sharply increases.
The role of Ni in HPUF issues is emphasized by the observations that
HPUF in Zircaloy-4 does not increase at the high fl uences reported thus
far. A mechanism has been proposed (Garzarolli et al ., 2011b ) whereby
Ni distribution in the growing Zircaloy-2 metal/oxide interface acts as an
easy path for hydrogen ingress to the metal. This hypothesis and the data of
Ishimoto et al . (2006) suggest a high value of the Fe/Ni ratio in Zircaloy-2
is desirable.

4.5.2 Shadow corrosion

Enhanced corrosion of zirconium alloys may occur when the corroding
surface is close to, or in contact with, certain other metallic components.
The shape of the component is often reproduced in the shape of an area of
enhanced corrosion, suggestive of a shadow cast by the component on the
zirconium alloy surface. The term 'shadow corrosion' is therefore often used
to describe the phenomenon. Observations of shadow corrosion on water
reactor components have been noted for many years. In 1974, Johnson et al .
(1974), reported enhanced corrosion in Zircaloy coupons located near, but
not touching, small pieces of platinum in the advanced test reactor (ATR).
Also, Trowse et al . (1977) reported enhanced fuel rod corrosion beneath
steel spacers in steam generating heavy water reactors (SGHWRs).
Most commonly observed are the control blade shadows on BWR chan-
nel surfaces adjacent to the control blade handles, such as shown in Fig.
4.53. In this case the stainless steel control blade handle is imaged as a black
shadow on a light background, but the reverse is sometimes also observed.
It was shown (Chen & Adamson, 1994) that the handle image is faithfully
reproduced on the channel surface, but is larger than the actual handle,
shown schematically in Fig. 4.54. Hot cell examination of a similar channel
shows that oxide thickness within the shadow area can be much higher than
outside the shadow (Adamson et al ., 2000 ).

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