Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 137

1800

M5

1600

Zy4 RXA

1400

1200

200

1000

150

800

100

600

50

400

0

200 5 10 15

0

0 50 100

External + internal oxide thickness (

150

m)

4.52 Hydrogen pickup fraction (HPUF) of RXA Zircaloy-4 and M5
at 315 ° C (588K) in PWR water chemistry. (Source: Reprinted, with
permission, from Bossis et al . (2007), copyright ASTM International, 100
Barr Harbor Drive, West Conshohocken, PA 19428.)

be dissolved into the matrix, and SPPs of Nb will exist without apprecia-

ble dissolution. Again it appears that it is the existence of the SPPs which is
important.

β

Mechanism implications

Nodular corrosion has been shown to be very sensitive to the concentra-
tion of Fe and Ni in the zirconium matrix. For a recent complete review
see Franklin (2010). For unirradiated Zircaloy-2 annealing in the high alpha
temperature range (<800°C, 1073K) causes a small increase in solute matrix
concentration which is correlated to a sharp decrease in nodular corro-
sion, without a signifi cant change in size of the SPPs (Kruger et al ., 1992 )
and without a signifi cant change in the Fe/Cr ratio in the Zr(Fe,Cr) 2 SPP, but
with a signifi cant decrease in the Ni/Fe ratio in the Zr 2 (Fe,Ni) SPP (Cheng
et al ., 1987). The mechanistic interpretation of these phenomena, as gleaned
from Sections 4.2 and 4.3.1, is either that oxide conductivity is markedly
increased by the solutes, or that the Galvanic potential between the SPPs
and the matrix is decreased by increased solutes and chemistry changes in
the SPPs. The importance of Ni in decreasing nodular corrosion by high
alpha annealing is shown by the fact that such treatments for Zircaloy-4,
with no alloying Ni, unlike those for Zircaloy-2 do not result in any decrease
in nodular corrosion in the 520°C (793K) steam tests used in those studies.

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