Digby D. Macdonald

The electronic structure of the passive film on tungsten

Abstract The capacitance of the passive film on tungsten in 1 M phosphoric acid solution has been measured as a function of frequency and voltage and Mott–Schottky analysis is used to derive dopant type and concentration data. The passive films were grown potentiodynamically to formation potentials within the range 3–10 V and were allowed to age until steady-state conditions were achieved. The capacitance was measured during a potential sweep from the formation voltage in the negative direction at a sufficiently high sweep rate that the thickness of the film remains constant. The data are interpreted in terms of a modified point defect model that takes into account the existence of an insulating outer layer on the defective WO3 barrier layer. The primary defect in the barrier layer was found to be the oxygen vacancy, which is concentrated in the region of the barrier layer adjacent to the metal, with the outer layer being postulated to form either by restructuring of the outer surface of the barrier layer or by hydrolysis of emerging cations or both. The data suggests that the space charge capacitance in the barrier layer is dominant at low formation voltages, but that at higher voltages the capacitance of the insulating outer layer makes an important contribution to the measured capacitance.

Corrosion in Supercritical Water Oxidation Systems: A Phenomenological Analysis

Supercritical water oxidation (SCWO) is now being actively developed as a means of destroying highly toxic organic waste (including physiological agents) and for reducing the volume of low level nuclear waste. Pronounced corrosion damage occurs in supercritical water oxidation reactors, and few materials are immune to attack. In this paper, the authors describe a phenomenological model for the corrosion process, and they discuss the effect of electrolyte dissociation and water density, as influenced by temperature and pressure, upon the kinetics of corrosion of metals and alloys in supercritical water. The corrosion process at near-critical temperatures is believed to involve acid attack, with the concentration of H{sup +} being a function of the dissociation constant of HCl, which is a major product of the oxidation of chlorinated organic waste, and of the density of the solution. The authors show that the competing effects of temperature on the heterogeneous rate constant and on the concentrations of H{sup +} and O{sub 2} leads to a pressure (and hence density)-dependent maximum in the corrosion rate in the vicinity of the critical temperature. This result is in general agreement with experimental data on corrosion in aqueous solutions at near-critical temperatures.

Determination of the fate of the current in the stress corrosion cracking of sensitized type 304SS in high temperature aqueous systems

Abstract The experimental work reported in this paper was designed to resolve the question concerning the fate of the current that is generated during crack growth in engineering alloys in power plant environments. Two limiting possibilities exist: the cathodic processes that consume the positive ion current generated at the crack tip reside within the confines of the crack; or they reside on the external surfaces. Experiments were conducted which made use of sensitized type 304 stainless steel (SS) compact specimens C(T) that had been modified to provide external surfaces that are isolated from the specimen, so that the current flowing between the crack and the external cathode could be readily monitored. This was done by coating the C(T) specimens with baked-on polytetrafluoroethylene so that only the crack (after fatigue precracking) was exposed to the environment. Cathodes were then mounted on the sides of the C(T) specimen and the current was monitored as the stress intensity and water chemistry were varied. The experiments reported here clearly demonstrate that current flows from the crack to the external cathode during crack propagation in sensitized type 304SS in pure water at the elevated temperatures typical of light water reactor heat transport systems. Thus, in spite of the high resistivity of the environment, the cathodic reaction is not restricted to the flanks of the crack or to the crack mouth, but occurs predominantly on the external surfaces, as postulated in the coupled environment fracture model. New insight has been gained into the physical mechanism of crack growth in high temperature aqueous environments.

The corrosion of carbon steel in oil-in-water emulsions under controlled hydrodynamic conditions

Abstract The effect of the oil content on the corrosion of AISI-SAE 1010 carbon steel in oil-in-water emulsions under controlled hydrodynamic conditions was studied by means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The systems that were studied included brine (0.2 wt% NaCl), surfactant solution (Dioctyl sodium sulfosuccinate, 1 wt%, +NaCl, 0.2 wt%), and oil-in-water emulsions, in which the aqueous phase was the surfactant solution and the oil phase was a mineral oil. Corrosion studies employed various controlled hydrodynamic systems, which included a rotating disk electrode (RDE) and a jet impingement electrochemical cell (JIEC), with the working electrode located in the stagnant region on the jet axis or on the wall jet region (displaced from the axis). This study found that the effect of the oil content on the electrochemical activity of carbon steel (as indicated by the current density in the active state) varies with the ‘internal phase relationship’, IPR. For emulsion with low IPR (oil contents up to 20 wt%), the electrochemical activity was slightly higher than that of the base surfactant solution. The electrochemical activity of emulsions with medium IPR (oil contents between 20 and 45 wt%) showed no major variation with oil content, while for emulsions with high IPR (oil contents between 45 and 70 wt%) the activity was diminished. The data are explained in terms of a model that postulates the formation of an ‘oily phase’ on the steel surface, the stability of which depends on the magnitude of the hydrodynamically induced shear stress at the interface. However, the coverage of the oil phase on the surface is postulated to depend on the normalized IPR, such that as the IPR increases the coverage also increases. Furthermore, the oil phase is envisioned to facilitate the cathodic partial process (reduction of oxygen) due to the enhanced solubility of oxygen compared with water, while inhibiting the dissolution of the metal (anodic partial process). Because the two partial processes are strongly coupled, the corrosion rate is predicted to pass through a maximum with increasing oil content of the emulsion, as observed.

Theoretical Prediction of the Scan Rate Dependencies of the Pitting Potential and the Probability Distribution in the Induction Time

The point defect model (PDM) has been used to derive the dependence of the pitting potential on the voltage scan rate. Relationships derived from the PDM predict that the observed pitting potential is a linear function of the square root of voltage scan rate at low scan rates, which agrees with experimental data reported in the literature. Furthermore, the critical concentration of condensed cation vacancies that give rise to passivity breakdown, as estimated from the sweep rate dependence of the pitting potential, is found to be in good agreement with that estimated from structural considerations. The PDM is also used to predict the probability distribution function in the induction time for pitting (t{sub ind}), that is, the survival probability, by assuming that the maximum diffusivity of the cation vacancy in a population of specimens is log-normally distributed. This calculated external distribution in the induction time is found to be in reasonable agreement with experimental induction time data for passivity breakdown on multiple specimens of single crystal (100) Ni and polycrystalline nickel buffered chloride solutions.

Modeling Water Chemistry, Electrochemical Corrosion Potential, and Crack Growth Rate in the Boiling Water Reactor Heat Transport Circuits—I: The DAMAGE-PREDICTOR Algorithm

A computer code with the capability of simultaneously estimating the concentrations of radiolysis species, the electrochemical corrosion potential, and the kinetics of growth of a reference crack in sensitized Type 304 stainless steel is developed for the heat transport circuits of boiling water reactors (BWRs). The primary objective of this code, DAMAGE-PREDICTOR, is to theoretically evaluate the effectiveness of hydrogen water chemistry (HWC) in the BWRs as a function of feedwater hydrogen concentration and reactor power level. The power level determines various important thermal-hydraulic parameters and the neutron and gamma energy deposition rate in the core and near-core regions. These input parameters are estimated using well-established algorithms, and the simulations are carried out for full-power conditions for two reactors that differ markedly in their responses to HWC. The DAMAGE-PREDICTOR code is found to successfully account for plant data from both reactors using a single set of model parameter values.

A photoelectrochemical impedance spectroscopic study of passive tungsten

Abstract In order to obtain some insight into the interaction between the electronic structure and ionic defects in passivating oxide films, photoelectrochemical impedance spectroscopy was employed to explore passive films formed on tungsten in phosphoric acid. The photoelectrochemical impedance could be measured at low frequencies and at photon energies of up to 5.5 eV using either square-wave-modulated irradiation or step-function excitation. Three different procedures were employed to demonstrate that the systems photocurrent response fulfilled the requirements (linearity, causality, stability and finity) necessary for impedance spectroscopy. Along with photoelectrochemical impedance spectra of passive tungsten in phosphoric acid, photocurrent transients, the electrochemical impedance and transient capacitance and reflectance responses were measured. Photocurrent transients show a fast exponential decay during the first 2 s and a linear increase for the following 10–20 s. Equivalent circuits could be derived to model this behavior. The properties of the first atomic layer of the oxide next to the solution seem to be responsible for the observed effects, which we attribute to the kinetics of electron–hole recombination via surface states: initially, surface recombination reduces the photocurrent and, subsequently, the number of surface states diminishes gradually by dissolution of the first atomic layer. This dissolution could actually be monitored by transient capacitance and transient reflectance measurements.

Modeling Water Chemistry, Electrochemical Corrosion Potential, and Crack Growth Rate in the Boiling Water Reactor Heat Transport Circuits—II: Simulation of Operating Reactors

The DAMAGE-PREDICTOR computer code, which has the capability of simultaneously estimating the concentrations of radiolysis species, the electrochemical corrosion potential (ECP), and the crack growth rate (CGR) of a reference crack in sensitized Type 304 stainless steel, is used to evaluate the responses of the Dresden-2 and Duane Arnold boiling water reactors (BWRs) to hydrogen water chemistry (HWC). The HWC simulations for these two BWRs are carried out for feedwater hydrogen concentrations ([H 2 ]FW) ranging from 0.0 to 2.0 parts per million (ppm). Results such as species concentrations (H 2 , O 2 , H 2 O 2 , etc.), ECP, and CGR are predicted for various components in the heat transport circuits (HTCs) of the two reactors. It is found that while 1.3 ppm of feedwater hydrogen is needed to protect part of the lower downcomer, the recirculation system, and the lower plenum in Dresden-2 from intergranular stress corrosion cracking, only 0.3 ppm is needed to achieve the same goal in Duane Arnold. However, it is also found that the ECP in many regions (core channel, core bypass, upper plenum, downcomer, etc.) in the HTCs cannot be lowered to below the critical corrosion potential of -0.23 V SHE for sensitized Type 304 stainless steels, even when [H 2 ] FW is as high as 2.0 ppm.

Effect of variable intensity ultraviolet radiation on passivity breakdown of AISI Type 304 stainless steel

The effect of ultraviolet illumination on passivity breakdown of a commercial grade (CG) of AISI Type 304 stainless steel and of a high purity heat (HPH) of the same material in neutral 0.5 M NaCl solution and in 0.025 M NaCl + 0.15 M H3BO3/0.007 M Na2B4O7, pH = 7.5, solution was experimentally studied. Passivity breakdown on the CG, as indicated by the measured breakdown potential and the intensity of the noise observed in the current under potentiostatic conditions, was inhibited at low illumination intensities but was enhanced at high illumination intensities. The low intensity photo-inhibition of passivity breakdown (PIPB) is consistent with previous observations on Fe, Ni, Cu/Ni alloys, and stainless steels, and has been attributed to quenching of the electric field and a subsequent modification of the vacancy structure in the barrier layer. These processes inhibit the formation of a critical cation vacancy condensate at the metal/barrier layer interface. The breakdown sites are envisioned to be regions of inherently high cation vacancy flux, and to include the points of intersection of the barrier layer with inclusions (e.g., MnS) and second phase particles (e.g., Cr2O3). Thus, at low intensities (power densities less than 100 mW/cm2, λ=325 nm), cation vacancy condensation/cap dissolution at the periphery of the inclusion is considered to be the fundamental breakdown event, even though, MnS dissolution eventually occurs, as an acidic crevice develops adjacent to the inclusion. On the other hand, under high intensity illumination conditions (power density > 100 mW/cm2), enhancement of passivity breakdown is attributed to photoactivation and dissolution of the MnS inclusions, which occur in parallel with vacancy condensation. The HPH exhibited breakdown voltages that were about 0.5 V more positive than that for the commercial grade of AISI Type 304 SS, under identical experimental conditions. The breakdown voltage, Vb was shifted in the positive direction, upon irradiation with super band-gap light at a high power density in 0.5 M NaCl, but no change in Vb was noted when irradiated in 1.0 M NaCl.

Development of Dissolved Hydrogen Sensors Based on Yttria‐Stabilized Zirconia Solid Electrolyte with Noble Metal Electrodes

A hydrogen sensor has been developed for in situ measurements of the concentration of hydrogen in aqueous solutions at elevated temperatures. The sensor was based on an electrochemical cell employing an yttria-stabilized zirconia (9% Y{sub 2}O{sub 3}) solid electrolyte, a Ag-O{sub 2} (in air) reference electrode, and a noble metal working electrode (Pt or Pd) covered with a polytetrafluorene-ethyl membrane. The response of the sensor to hydrogen has been examined in gas mixtures and aqueous solutions at temperatures as high as 300 C, and for hydrogen concentrations ranging from 0.001 to 100% or 7.61 {times} 10{sup {minus}6} to 7.61 {times} 10{sup {minus}4} mol/kg, respectively. The sensor displayed rapid responses to changes in concentration of hydrogen in both gas mixtures and aqueous solutions. Linear relationships with Nernstian slopes (2.303RT/2F) were observed between the measured potential of the sensor and the logarithm of the hydrogen concentration. Oxygen in solution had little effect on the potential of the sensor when the concentration of oxygen was lower than that of hydrogen, while there was a significant change in the potential with oxygen concentration in gas mixtures. The potential of the sensor was not affected by the pH of the solution.