An Investigation of the Mechanism of IGA/SCC of Alloy 500 in Corrosion Accelerating Heated Crevice Environments. Technical progress report Page: 4 of 9
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in some plants; however, other plants using the same measures have
continued to experience IGA/SCC.
The present strategy for mitigating IGA/SCC is based on the
assumption that crack initiation and propagation rates depend on
pH and the electrochemical potential (ECP). Laboratory data, using
static autoclaves, show that IGA/SCC crack growth rates reach a
minimum at pH's between 5 and 9 under electrochemically reducing
conditions. Some plants are injecting Na and Cl ions into the
feedwater to adjust the crevice pH. There are several uncertainties
in this approach. Since measurements of crevice chemistry and
electrochemical potential (ECP) cannot be made in an operating
steam generator, estimates are made using computer codes based on
hypothesized processes believed to occur in crevices. Moreover,
laboratory IGA/SCC data were obtained in static autoclaves using
simulated crevice solutions. The IGA/SCC mechanism may be
different under heat flux conditions, during which steam is being
generated. Crevice chemistries are complex and pH may not be the
important factor or the only important factor.
The objective of this program is to develop an understanding
of the corrosion accelerating mechanisms, particularly IGA/SCC, in
steam generator crevices. The important variables will be
identified, including the relationship between bulk water chemistry
and corrosion accelerating chemistries in a crevice. The approach
will use an instrumented heated crevice, which is a replica of a PWR
steam generator tube/TSP crevice. With the system operating at
simulated steam generator thermal conditions, measurements can
be made of the chemical, electrochemical, and thermal conditions in
the crevice. Damage to the tube due to IGA/SCC and other
corrosion processes will be monitored using electrochemical noise.
Task 1: Modification of Heated Crevice for SCC and Electrochemical
1. Task Status.
The design of the system has been completed and construction is
underway on schedule. Figure 1 is a drawing showing a cross
sectional view of the heated crevice and shows only the portion of
the steam generator tube, which is inside the autoclave. The heated
crevice is constructed so that the autoclave, the ring, simulating the
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Lumsden, Jesse. An Investigation of the Mechanism of IGA/SCC of Alloy 500 in Corrosion Accelerating Heated Crevice Environments. Technical progress report, report, March 1, 2000; United States. (digital.library.unt.edu/ark:/67531/metadc717027/m1/4/: accessed November 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.