Environmental Engineering Reference
In-Depth Information
There is another type of localized corrosion in Zircaloys: the enhanced
in-reactor corrosion when Zircaloy is placed close to a noble metal (under
BWR conditions it is stainless steel or a nickel alloy), and where Zircaloy
'mimics' the noble metal corrosion. This is termed as 'shadow corrosion'. 55
The oxide thickness is unusually large and often appears to be particularly
dense and uncracked. This localized corrosion is a special case of crevice
corrosion and is predominantly seen in BWR components, although there
is no direct electrical contact between Zircaloy and the material producing
the shadow effect. The oxidation of H 2 O 2 to HO 2 +H + at the noble metal
surface is balanced by the regeneration of H 2 O 2 on the ZrO 2 surface and
the coupling between the two metals (Zircaloy and nickel) is maintained
by the ionic transport under a concentration gradient. The driving force is
the potential difference between the two metals and radiolysis of water is
required to sustain this reaction. 55 Shadow corrosion is invariably noted in
BWRs and not in PWRs where the coolant is high in hydrogen concentra-
tion, which in turn reduces or eliminates galvanic potentials between dis-
similar alloy components.
Environmentally assisted cracking is another manifestation of corrosion-
related problems and is very often encountered in the steam and feed water
piping as well as in condensate systems, RPV feed water nozzles and the
secondary circuit of LWRs. This process is accelerated by stress (i.e. SCC)
and neutron fl ux (i.e. IASCC). A typical fracture surface of IASCC is shown
in Fig. 1.30 . 56 Attempts are being made to reduce IASCC. Figure 1.31 shows
the effect of hydrogen injection into the BWR environment on IASCC
of 304 SS. The mechanism of crack growth mitigation by hydrogen injec-
tion could be explained by analyzing the corrosion potential of the system.
The presence of molecules like H 2 O 2 and O 2 increases the free corrosion
potential which falls into the cracking range and hence the crack velocity is
enhanced following the slip dissolution model and Faraday's law. Whereas,
when hydrogen is introduced into the environment it helps the recombi-
nation of species and thus reduces the corrosion potential far below the
cracking range. 57
Austenitic stainless steels (e.g. blade sheathing in BWR) at high tem-
perature and in a neutron-rich environment (>0.7 dpa), further infl uenced
by higher oxygen levels in the water (BWR environment), exhibit IASCC.
Other steels and nickel-base alloys also undergo IASCC at lower stress
levels. Another aspect is that IASCC occurs in almost all materials and is
known to occur in components at low stress levels. It is an expensive process
to detect and repair the affected component. SCC is a major issue of PWR
components like steam generator tubes, RPV penetrations, pressurizer noz-
zles, etc. While SCC can be controlled by modifying the water chemistry
and the composition of the alloy, that is by replacing components with those
resistant to SCC (e.g. Alloy 690, 52, 152), IASCC is more complex. Though
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