On the Use of NaOH Solution to Simulate the Crevice Conditions of a Nuclear Steam Generator.

The corrosion behavior and integrity of steam generator (SG) tube materials have frequently been tested in solutions containing sodium hydroxide (NaOH), assuming that NaOH is a typical contaminant concentrated in the crevices of SGs in a pressurized water reactor. The purpose of this study was to investigate the adequacy of using concentrated NaOH solutions to simulate the crevice environments of SGs. The dissolution behavior of magnetite deposit flakes formed in an operating SG was tested in a 0.4 wt.% NaOH solution at 300 °C, and the thermodynamic stability of magnetite was investigated using the potential-pH diagram for an iron-water system. The magnetite deposits were rapidly dissolved in the test solution, which was supported by the fact that magnetite is thermodynamically unstable under the test condition to dissolve to dihypoferrite ions (HFeO2-). These results indicate that research data obtained from concentrated NaOH solutions are not appropriate to apply to the crevice environments of SGs.

Stress Corrosion Cracking Behavior of Alloy 600 Coupled to Magnetite under High-Temperature Caustic Conditions.

This study aims to investigate and explain the magnetite-accelerated stress corrosion cracking phenomenon of Alloy 600 under caustic conditions, based on the electrochemical behavior. After the SCC test that lasted for 300 h, no cracks were observed in any of the magnetite-free specimens, whereas cracks with a depth of 150 to 280 µm were generated in all the magnetite-deposited specimens. Furthermore, the electrochemical behavior of magnetite and Alloy 600 demonstrated that Alloy 600 behaved as an anode in the coupling system with magnetite. In this coupling system, the electrochemical potential of Alloy 600 can be shifted into the range potentially susceptible to stress corrosion cracking.

Galvanic Corrosion of SA106 Gr.B Coupled with Magnetite in Alkaline Solution at Various Temperatures.

The effect of temperature on the galvanic corrosion behavior of SA106 Gr.B carbon-manganese steel was studied in an alkaline aqueous solution at various temperatures (30, 60, and 90 °C) via electrochemical corrosion tests. At all temperatures studied, carbon-manganese steel acted as the anode of the galvanic cell composed of carbon-manganese steel and magnetite because the corrosion potential of carbon-manganese steel was significantly lower than that of magnetite. The corrosion current density of carbon-manganese steel significantly increased due to the galvanic effect irrespective of temperature used in this study. With the increase in temperature, the extent of the galvanic effect on the corrosion current density of carbon-manganese steel and reductive dissolution of magnetite gradually increased. When the area ratio of magnetite to carbon-manganese steel increased, the corrosion rate of the carbon-manganese steel in contact with magnetite further increased.

Effects of Hydrogen Contents on Oxidation Behavior of Alloy 690TT and Associated Boron Accumulation within Oxides in High-Temperature Water.

The aim of this work is to characterize the oxide layer structure of Alloy 690TT in high-temperature water with different dissolved hydrogen (DH) contents by using an X-ray photoelectron spectroscopy. Under the low DH contents (0.4494-0.8988 mg/kg), the oxide layers were composed of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Ni, an intermediate layer of hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. Outermost NiO coexists with small amount of Cr2O3 layer, while in the inner oxide only Cr2O3 remains. The oxide layers at medium and high DH contents (3.1458- 8.9880 mg/kg) consisted of an outermost layer of Ni(OH)2 and Cr(OH)3 enriched in Cr, an intermediate layer of metallic Ni, hydroxides and oxides enriched in Cr, and an inner Cr2O3 layer. In addition, boron compounds containing B3+ ions were accumulated in the thick and porous NiO layer formed at low DH contents, whereas the accumulation of boron compounds did not occur in the thin and dense polyhedral oxide layer formed at medium and high DH contents.

Simulating Porous Magnetite Layer Deposited on Alloy 690TT Steam Generator Tubes.

In nuclear power plants, the main corrosion product that is deposited on the outside of steam generator tubes is porous magnetite. The objective of this study was to simulate porous magnetite that is deposited on thermally treated (TT) Alloy 690 steam generator tubes. A magnetite layer was electrodeposited on an Alloy 690TT substrate in an Fe(III)-triethanolamine solution. After electrodeposition, the dense magnetite layer was immersed to simulate porous magnetite deposits in alkaline solution for 50 days at room temperature. The dense morphology of the magnetite layer was changed to a porous structure by reductive dissolution reaction. The simulated porous magnetite layer was compared with flakes of steam generator tubes, which were collected from the secondary water system of a real nuclear power plant during sludge lancing. Possible nuclear research applications using simulated porous magnetite specimens are also proposed.

Optimization of polymeric dispersant concentration for the dispersion-stability of magnetite nanoparticles in water solution.

Fouling of various Fe oxide particulates on heat transfer tubes in the coolant of the secondary system of a nuclear power plant has been known to reduce the heat transfer performance and degrade the integrity of system components. Thus, in order to mitigate such a fouling problem, an addition of polymeric dispersant has been proposed to remove the oxide partculates. In this paper, experimental studies was conducted for evaluating the effect of polymeric dispersants (PAA: Polyacrylic acid, PMA: Polymethacrylic acid, PAAMA: Polymaleic acid-co-acrylic acid) on the dispersion stability of magnetite nanoparticles (MNPs, Fe3O4) for the reduction of fouling and corrosion of carbon steel by the settling test, the transmittance, zeta-potential, and particle size measurements, and the electrochemical corrosion tests. It was observed that the critical concentration for maximizing the dispersionstability of MNPs was in the range of concentration ratio (dispersant/MNPs) of 0.1 to 0.01 and the dispersion-stability of MNPs was not improved when the dispersant concentration is above this critical value. This non-linearity above a critical dispersant concentration may be explained by the agglomerations between MNPs. While there is no significant increase of corrosion rate with an addition of up to 10 ppm PAA, the addition of 100 ppm PAA increases the growth rate of oxide layer rapidly and deteriorates the formation of protective oxide on carbon steel. It is thus reasonably stated that the optimization of polymeric dispersants variables and its impacts on the corrosion of structural materials is necessary for the best application at plants.