J.L. Luo

A Mechanistic Study on Lead-Induced Passivity-Degradation of Nickel-Based Alloy

The mechanism of lead-induced passivity-degradation in a nickel-based alloy (UNS N06690) was explored in a simulated stream generator alkaline crevice chemistry. The pitting induction test indicated that the detrimental impact of lead contamination on pitting resistance was significantly enhanced by the presence of calcium ions. An X-ray photoelectron spectroscopy analysis revealed the incorporation of lead into the passive film. Calcium ions in the alkaline chemistry entered the passive films with the aid of the lead species but the ingress of calcium did not occur in the solution free of lead contamination. The incorporation of lead reduced chromium and iron content in passive films and hindered the dehydration processes during the passivation. Mott-Schottky and photoelectrochemical measurements showed that passive films on the nickel-based alloy are p-type semiconductors and the incorporation of lead may reduce the acceptors in the passive films. A further analysis suggested that the lead-induced passivity degradation of nickel-based alloys may be related to the increases in M-O and M-OH 2 bonds, the electronic structural changes of spinel oxides in the passive film, and reduced ion-selectivity due to the surface adsorption of the lead species.

A Theoretical Model on Electrochemical Response of Passivated Metals to Solid Particle Impingement

A theoretical model based on kinetic analysis has been developed for predicting the electrochemical response of passivated metals to solid particle impingement in corrosive slurries. The applicability of the new model was verified using experimental data from the literature. The model accurately predicts correlations between corrosion current density and erosion rate, and the effects of various hydrodynamic parameters on corrosion in flowing slurries, including flow velocity, concentration of solid particles, and impingement angle. The model confirms theoretical predictions that corrosion is controlled mainly by the rate of generation of active area due to particle impingement and repassivation kinetics of the active area. Consequently, the factors that promote mechanical erosion are shown to accelerate corrosion of passivated metals in flowing slurries through the effects of erosion-enhanced corrosion.

Condition for Lead-Induced Corrosion of Alloy 690 in an Alkaline Steam Generator Crevice Solution

Abstract The effects of dissolved lead on the anodic dissolution behavior of Alloy 690 (UNS N06690) were experimentally investigated in an alkaline solution that is used to simulate the environment in a nuclear steam generator crevice. Lead inoculation and scratch techniques were applied to exclude the inference of the impact of oxidization of the metallic lead deposit. It has been found that lead deposit can promote transient dissolution on the bare metal surface in the alkaline crevice solution before a fully passive state is achieved. The experimental measurements suggest that the promoted transient dissolution is likely to result from the selected dissolution of nickel. The presence of dissolved lead contamination may retard the passivation or repassivation of the alloy. The effect of lead contamination is very limited on the uniform anodic dissolution of the alloy at stable passive state in the alkaline crevice solution.

A Method to Measure the Proton Conductivity of LiNaSO4 under Fuel Cell Conditions

A method has been developed for measurement of proton conductivity of hydrogen-reducible ionic conductors under protonic fuel cell conditions in which the reducible electrolyte is sandwiched between layers of a nonreducible proton-conducting electrolyte in a disk. The proton conductivity of LiNaSO 4 under the fuel-cell condition was determined with Y-doped BaCeO 3 (BCY) as the nonreducible sandwich layers, using conventional methods under hydrogen at high temperature, under which conditions LiNaSO 4 would otherwise be chemically unstable. LiNaSO 4 and BCY had good chemical and mechanical compatibility in the sandwich disk. The major portion of the ionic conductivity of LiNaSO 4 at 600-680°C is due to Li + and Na + ion transport, and proton conductivity (7.01 X 10 -3 S cm -1 at 600°C to 1.23 X 10 -2 S cm -1 at 680°C) accounts for only 5-10% of total ion conductivity (1.25 X 10 -1 S cm -1 at 600°C to 1.35 X 10 -1 S cm -1 at 680°C). These data explain the poor performance of hydrocarbon conversion fuel cells using LiNaSO 4 as the electrolyte.

Abnormal photocurrent of hydrogen-containing passive films on X70 microalloyed steel

In situ potentiostatic and photoelectrochemical measurements were employed concurrently to investigate the effects of hydrogen on the properties of the passive films on X70 microalloyed steel. The photocurrent of the passive film increases with film formation time and reaches a steady value after a long time of passivation. After a steady-state current was attained, the hydrogen charged into the specimen resulted in an increase in the anodic current density of the film, which then resumed to the steady state after the hydrogen charging was cut off. Nevertheless, concurrent photoelectrochemical experiments revealed that the photocurrent of the film was also enhanced by hydrogen, but the photocurrent did not undergo recovery to the initial value before hydrogen charging, which is contrary to the result obtained for the anodic current after the cessation of hydrogen charging. The discovery of the irreversible effect of hydrogen on the photocurrent of the passive film is first reported by this team.

Detrimental effect of lead on the passivity of UNS N06690 alloy

The pitting induction times under the potentiostatic conditions indicate that the detrimental impact of lead on the passivity degradation is markedly enhanced by calcium ions present in alkaline crevice solutions. The mechanism is discussed based on the results of X-ray photoelectron spectroscopy (XPS) analyses, photoelectrochemical and Mott-Schottky measurements.