Electrochmical Corrosion Behavior of Alloy 600 and SA508Cl.3 in Solutions with Various Sodium-to-Chloride Ratios

The laboratory and field experience data indicate that both IGA and SCC of Alloy 600 nuclear steam generator tube material are minimized at a near neutral pH of the range between 6~8. The neutral pH can be achieved by controlling the molar ratio of the cation to anion impurities dissolved in the crevice to be near one. Tubesheet and tubes are galvanically contacted in the tube to tubesheet crevice region of a steam generator. Therefore, the corrosion behavior of tubesheet material should be considered in the application of the molar ratio control. In addition, since the molar ratio is a simple ratio of the cations to anions, the total concentration of the soluble impurities is not considered.
In this paper, the corrosion behaviors of tube and tubesheet material are examined by a electrochemical polarization method in solutions with various sodium-to-chloride ratios at room temperature. The effect of the impurity concentration at a constant sodium-to-chloride ratio is also evaluated. Both the corrosion potentials and corrosion rates of Alloy 600 and SA508Cl.3 were significantly decreased as the sodium-to-chloride ratio increased from 0.1 to 10. The corrosion potential and corrosion rate were not affected by a change of the impurity concentration from 0.011 M to 0.11 M at a constant sodium-to-chloride ratio. The corrosion rate of SA508Cl.3, acting as an anode, was increased by about 2~6 times by a galvanic coupling with Alloy 600.