Page 330 - Corrosion Engineering Principles and Practice

300 C h a p t e r 8 C o r r o s i o n b y W a t e r 301

Zircaloys are preferred because they do not “poison” the nuclear
reaction as much as do the stainless steels. The Zircaloys perform
well in the 285 to 315°C temperature of the primary water for the two
to three-year expected life of a nuclear core. They develop a shiny,
adherent black oxide film, which is protective and has excellent heat
transfer properties. However, after long exposure times (two to four
years), or shorter times at higher temperatures, for example, 40 to 120
days at 360 to 400°C, Zircaloys corrosion rate increases and a white,
relatively nonadherent insulating film develops.
Stainless steels, on the other hand, develop a relatively thick tarnish
film with a thin, powdery surface film. Both films have a nominal
composition of M O where M represents iron, nickel, or chromium. The
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corrosion rate of stainless steel is not greatly affected by temperature in
the range of 260 to 400°C, and it does not exhibit the marked effect of
temperature on corrosion rate that is characteristic of Zircaloys.
The piping, steam generator, and pressure vessel comprise much
of the total exposed area in the primary section of a pressurized water
reactor. The internal surface of the pressure vessel is clad with
austenitic stainless steel. The piping and steam-generator components
are primarily austenitic stainless steel or Alloy 600 (N06600). Carbon
steel has been used in a few systems.
The corrosion rates of austenitic stainless steels and Alloy 600 are
about the same, approximately 1.5 µm/y. The corrosion rate of carbon
steels is 5 to 10 times higher, to a maximum of about 13 µm/y. These
rates are acceptable from a structural standpoint. However, as much
as half of this oxide may not remain on the surface, but becomes a
radioactive “crud” in its passage through the reactor core. Its
subsequent deposition constitutes a personnel hazard.
The isotopes in the corrosion products which contribute most to
radioactivity are Co-60, Co-58, Fe-59, Mn-54, and Cr-51. These
isotopes are formed from the elements in stainless and nickel-base
alloys. Their half-lives* range from 27 days to more than five years.
Co-60 has a long half-life, and this is the reason for minimizing cobalt
content in nuclear-grade alloys.
It is generally agreed upon that pH in the range of 6 to 10, oxygen
up to 5 ppm, or irradiation have little or no effect on the corrosion
rate of stainless steels. However, these factors do affect the amount of
crud released into the water, the amount increasing with decreasing
pH, increasing oxygen content, and irradiation.
Since the primary water is pressurized, it does not boil. Traces of
chloride contamination or caustic from the water treatment may
therefore concentrate enough to cause SCC. The dissolved chloride
content is usually restricted to less than 0.1 ppm. Flow rates are of the
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order of 7 to 10 m s , and the temperature is about 260 to 290°C, with
* A half-life of a radioactive element is the period over which one-half of the initial
activity decays.