Corrosion in pressurized water reactors (PWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 8

the ruptured pipe downstream of the orifi ce. The bottom of the pipe on the
inside was also covered with a thick surface fi lm. These fi ndings are charac-
teristic of FAC.
Intergranular SCC (IGSCC) and intergranular attack (IGA) are the
most serious degradation processes affecting SG tubes, on the secondary
side. This degradation commonly occurs in crevice regions at tube support
plate and tube sheet locations or under sludge piles, 8 , 9 although intergranu-
lar SCC has also been observed in the free span of the tubes. The presence
of lead in the secondary circuit was supposed to enhance IGSCC 10 , 11 : Pb
ions would infl uence passivity of the Alloy 600 surface, being incorporated
into the Alloy 600 specimen surface and enhancing electronic conductance.
Lead may cover a signifi cant fraction of the Alloy and shift equilibria for
the Ni oxide formation. IGA was observed at the secondary side, at the roll
transition zone underneath crud deposits of SG tubes from McGuire Unit
1, for example. A wide variety of elements were present in the crud deposits,
including Fe, Ni, Cr, Al, Si, Mg, Cu, Ti, Mn, Ca, K, and S. The copper was pre-
sent in the deposit as metallic copper. The presence of metallic Cu indicates
that the electrochemical potential was below the Cu/Cu oxide equilibrium.
The SG unit had operated initially with Ni-Cu moisture separator reheaters.
On the primary side, IGSCC occurs at locations of high stress, typically at
regions where substantial plastic strain has occurred within the tube, during
the SG manufacturing process and from in-service straining. Thus, IGSCC
has been observed at the apex and at the transition from bent to straight
portions of small radius U-bends. 12 Examinations of SG tubes revealed the
presence of axial cracks mainly at regions of transitions from expanded
to non-expanded portions of the tube/tubesheet joint and circumferential
cracks at the end of the transition. 13
More recently, primary water SCC was found (2004) at the surface of
the warm side of the divider plate of the SG #171 at Chinon Unit B4, 14 , 15
exposed to the primary environment at 325°C. Cracks initiated in a area
which had been subject to grinding, on the hot side of the partition stub
made of Alloy 600, close to the welds (Alloy 182), where a signifi cant cold
work was present and where a limited intergranular precipitation was
observed. Examination showed intergranular and intragranular precipi-
tates in the materials. The divider plate exhibited large non-recrystallized
grains close to the surface. Cross-sections indicated the presence of IGSCC
perpendicular to the surface. The maximal crack depth was 1.2 mm (<4%
of the total thickness). After neutron diffraction examinations, the mean
plastic strain present in the Alloy 600 stub was estimated as 5.3%, while
the maximal strain in the heat affected zone reached 11%. In addition, the
deformation was higher at the surface than in the bulk of the material.
R&D studies 16 - 18 on representative hot rolled Alloy 600 have shown that
no signifi cant crack growth is expected as soon as the deformation is lower

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