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The effects of pH on sulfur-induced passivity degradation of Alloy 800 in simulated crevice chemistries were evaluated using electrochemical impedance spectroscopy (EIS), secondary ion mass spectroscopy (SIMS), and Mott-Schottky analysis methods. Experimental results reveal that the detrimental effect of impurities containing sulfur at the reduced or intermediate oxidation level (S x ) on the passivity of Alloy 800 depends significantly on the solution pH. In neutral crevice (NC) chemistry containing chloride ions, the S x obstructs the healing process during repassivation, retards the dehydration reaction and also incorporates H, -OH and S into the passive film. However, the detrimental effect of S x is insignificant in an alkaline crevice (AKC) solution due to a change in surface adsorption and surface charge. Impurities containing sulfur at the reduced or intermediate oxidation level would cause Alloy 800 to lose its passivity and become active in an acidic crevice (AC) chemistry. Experimental evidence also indicates that the solution pH alters the semiconductor type of the surface film -from n-type in NC solution to p-type in AKC solution.

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Semiconductive properties of the passive films formed on Alloy 800 in 0.1 M chloride solutions containing various concentrations of thiosulfate are investigated using Mott-Schottky analysis. The results indicate that when the concentration of the thiosulfate ions increases, the semiconductivity is converted from the n-type to the p-type. This conversion is postulated to be due to the incorporation of sulfur into the passive film, which changes the vacancy type. The breakdown behavior of the passive films is correlated with the semiconductivity of the film in this system.

Heat Exchanger Technology Branch, Atomic Energy of Canada Limited, Chalk River Laboratories, Chalk River, ONKOJ IJO, Canada D egradation of the thermal performance of steam generators (SCs) is a serious problem in nuclear power stations throughout the world (Lovett and Dow, 1991). In pressurized-heavy-water reactors (PWHRs), the reduced thermal performance of the SCs is manifested by an increase of the primary coolant reactor inlet header temperature (RIHT). In pressurized-light-water reactors (PWRs), which operate with fixed primary coolant temperature, the loss of thermal performance is manifested by a reduction of the steam pressure.Degradation mechanisms that may contribute to the loss of SC thermal performance include:fouling of the boiler tube inner surfaces (primary-side fouling); fouling of the boiler tube outer surfaces (secondary-side fouling); divider and thermal plate leakage that causes the coolant to bypass either the SC or the integral preheater; and fouling of the steam separators. The relative contribution of these various degradation mechanisms to the overall loss of thermal performance is still under investigation. Soulard et al.(1 990) examined the relative contributions of tube bundle fouling, divider plate leakage, and thermal plate leakage to the increase in RIHT at the Point Lepreau Nuclear Generating Station, and concluded that tube fouling contributes to a significant fraction of the loss of thermal performance. Corrosion products deposit on both the inner (primary) and outer (secondary) surfaces of the boiler tubes. Thus, a complete understanding of the reasons for the loss of thermal performance and the development of strategies to mitigate this loss requires a knowledge of the thermal resistance of tube deposits under primary-and secondary-side heat-transfer conditions. We present here the results of measurements of the thermal resistance of primary-side and secondary-side boiler tube deposits performed under single-phase forced convection and flow-boiling conditions, respectively.The results are discussed in terms of the physical properties of the deposit and the mode of heat transfer.A schematic of the loop used for the measurements of deposit resistance under primary-and secondary-side heat-transfer conditions has been reported elsewhere (Turner and Codin, 1994). The loop is a waterrecirculating facility that can operate at temperatures and pressures up to 320°C and 14 MPa, respectively. The loop water chemistry is controlled in the loop makeup tank. High-purity, deoxygenated water was used throughout the measurements. Residual hydrazine was maintained at a Measurements were made of the thermal resistance of porous deposits of various thicknesses under both single-phase forced convection and flow-boiling conditions. Both synthetic deposits and deposits on tubes removed from operating steam generators were used in this investigation. The thermal resistance was modeled as the sum of two components: one associated with conduction through the porous deposit and a second associated with...