Use of Pourbaix diagrams to infer local pitting conditions
Technical Report
·
OSTI ID:7145843
Corrosion products identified on Alloy 600 tubes pulled from the Millstone Unit 2 nuclear steam generator were compared to phase fields of appropriate Pourbaix diagrams. The diagrams were constructed using new thermodynamic data. They were used to analyze local corrosive conditions and corrosion processes that occur in the steam generators. Indications are that the pH within the pits and wastage areas ranges from 1 to 4 - the higher pH occurs at the tube surface and in pits near the tubesheet; the lower pH occurs in pits away from the tubesheet. The wastage band near the tubesheet is attributed to active dissolution of Alloy 600, which occurs at low potentials. Pitting occurs in areas where reduction of copper oxides raises the Alloy 600 surface potential into the passive range. Local solute concentrations of 0.001 to 0.1 molar are consistent with the postulated pH. This work corroborates the postulated pitting mechanism and suggests a mechanism of formation for the observed wastage bands. It also indicates the need to minimize accumulation of copper and iron oxides and of acid-forming salts in the steam generators.
- Research Organization:
- Babcock and Wilcox Co., Alliance, OH (USA). Research and Development Div.
- OSTI ID:
- 7145843
- Report Number(s):
- EPRI-NP-4831; ON: TI87920091
- Country of Publication:
- United States
- Language:
- English
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21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
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36 MATERIALS SCIENCE
360105 -- Metals & Alloys-- Corrosion & Erosion
ALLOYS
BOILERS
CHEMICAL REACTIONS
CHEMISTRY
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36 MATERIALS SCIENCE
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ALLOYS
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CHEMICAL REACTIONS
CHEMISTRY
CHROMIUM ALLOYS
CHROMIUM STEELS
COPPER
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ENRICHED URANIUM REACTORS
INCONEL 600
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IRON
IRON ALLOYS
IRON BASE ALLOYS
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NICKEL BASE ALLOYS
NIOBIUM ALLOYS
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TRANSITION ELEMENTS
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VAPOR GENERATORS
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WATER MODERATED REACTORS