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of these components is attributed to the low stress under which they operate.
Intergranular attack in these components could lead to premature IGSCC ss was the case for
piping. Repairs oE these internal components would be extremely costly and difficult.
Therefore, it is advisable for the utillty/process vendor to thoroughly review and list
all materials (metals and non-metals) that will be wetted by the decontamination solution
and that will remain in service. This list should be used to make a preliminary
engineering Judgment of the effect of the decontamination solution on corrosion, both
during the decontamination operation and in subsequent service, for different commercially
available decontamination processes.
There are seven families of materials that should be characterized by relevant corrosion
data for decontamination evaluation. These are:
I. Austenftic Stainless Steels (Type 304, 316, 347, etc.)
2. Nickel Base Alloys (Alloy 600, 690, X-750, etc.)
3. Chromium Iron Alloys (Type 410, 420, 422, etc.)
4. Low Alloy Steels (all except SAS08-B)
5. SA508-B Low Alloy Steel)
6. Carbon Steels (SA333-B, SAI06-B, etc.)
7. Non-Metallic Materials
4.3 Fabrication history
The primary motivation for the materials review described in Section 4.2 is to minimize
the potential for intergranular stress corrosion cracking (IGSCC). Depending on the flow
path selected, it is possible for the decontamination solution to contact stainless steel
welds within the reactor pressure vessel, especially in the shroud-to-vessel annulus
region.
In general, the long term IGSCC performance of welded stainless steel internal
components has been excellent, principally because of the low sustained applied loads.
This contrasts with the situation in piping, where high stresses are present. Since the
total tensile stress must be over the yield stress to inlt~ate IGSCC in weld sensitized
Type 304 and 316 stainless steel, it is highly likely that the low stress internal weld
HAZs will not experience crack initiation over the plant lifetime. If a decontamination
solution was sufficiently aggressive to initiate intergranular attack (IGA), it might be
possible for subsequent intergranular crack propagation to occur during service. Thus, the
identification of weld locations in the decontamination flow path is critical. These are
the areas most susceptible to cracking due to the presence of a sensitized microstructure
in the weld heat-affected-zone (localized chromium depletion as evidenced by the presence
of chromium carbides at the austenitlc grain boundaries) and high tensile stress including
the weld residual stress.
Another material condition which should be carefully identified is the presence of cold
work. Cold work is the result of any mechanical process (such as cutting, sawing,
grinding, machining, shearing, drilling, boring, broaching, honing, tube expansion,
turning, hammering and bending), which results in plastic deformation at a temperature and
time interval such that the strain hardening is not relieved. Cold working not only
increases the gross chemical reactivity of a metal end thus leads to a general decrease in
the corrosion resistance of the metal, but also increases the susceptlbility of annealed
and sensitized stainless steel to stress corrosion cracking. Cold work also results in
stress corrosion cracking at applied tensile stress levels below the cold worked material's
yield stress.
4.4 Stress considerations
There are primarily four sources of stress: fabrication stresses, primary, secondary
and cyclic stresses. Fabrication stresses consist of stresses introduced during fit-up and
assembly in the shop or in the field, those introduced by machining or forming operations,
such as surface grinding or cold straightening, and by other operations such as welding.
For example, grinding can Introduce surface tensile stresses near to the yield point.