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This work investigates the effects of hydrogen and stress on the electrochemical and passivation behaviour of 304 stainless steel in 0.5 M sulfuric acid with 2 ppm sodium fluoride. Three major results are revealed: hydrogen and stress significantly increase the critical current density (icrit) and passive current density (ipass); hydrogen has a more obvious impact on icrit than that on ipass; a high synergistic effect between hydrogen and stress on current densities is observed at the intermediate hydrogen charging current densities range.

Electrochemical CO2 reduction reaction (CO2RR) to value-added chemicals at low cost is highly attractive in mitigating the environmental issues caused by excessive greenhouse gases emission. In this work, Ag2O-derived Ag nanoparticles (OD-Ag NPs) with the optimized subsurface oxygen were obtained through steering the reducing current density (ire) upon in situ Galvano-statically reducing Ag2O NPs to metallic Ag, and it is found that the OD-Ag NPs thus obtained exhibit high catalytic activity and selectivity towards CO production. Specifically, the OD-Ag NPs derived from an intermediate ire of ‒1 mA•cm‒2 achieve the high Faradaic efficiencies (FECO) of over 90% in a wide potential window over 450 mV, whereas either smaller (‒0.1 mA•cm‒2) or larger (‒10 mA•cm‒2) ire reduces the overall CO2RR performance. This suggests that tuning the ire to change the subsurface oxygen of oxide-derived catalysts is beneficial to improve their selectivity. Theoretical DFT calculations and analysis rationalize the experimental observation, and reveal that the outstanding performance of the OD-Ag NPs benefits from the suitable concentration of subsurface oxygen which not only enhances the adsorption of intermediate *COOH, but also optimizes the free energy of the reaction pathways.

Yttria doped strontium titanate (Y0.07Sr0.89TiO3, YST) material is synthesized using a conventional combustion method as anode for solid oxide fuel cells (SOFCs) in sulfur- and hydrocarbon-containing fuels. The YST perovskite exhibits good stability and chemical compatibility with yttria stabilized zirconia (YSZ) electrolyte in 0.5% H2S-containing fuel at high temperatures. Moreover, the YST shows high electrical conductivity of 35 S cm−1 at 900 °C and the cell with this anode achieves a maximum power density of 200, 162 and 70 mW cm−2 when the anode is fed by 0.5% H2S-containing hydrogen, syngas and methane fuels, respectively. More importantly, the cell shows negligible degradation in the sulfur-, carbon- and hydrocarbon-containing fuels. Thus, YST holds the promise as anode material for SOFCs, especially in high concentration sulfur-, carbon- and hydrocarbon-containing fuels.

Proton-conducting solid oxide fuel cells have the potential ability to simultaneously convert ethane to electrical energy and value-added ethylene with high selectivity and the absence of CO2 emissions. However, their development is impeded by a lack of efficient and stable anode materials. Herein, in-situ exsolved FeNi nanoparticles on (La0.6Sr0.4)0.95Fe0.8Ni0.1Mo0.1O3-δ (LSFNM) perovskite oxide are realised by controlling A-site deficiency and Mo-doping. The introduction of high-valence molybdenum ions in (La0.6Sr0.4)0.95Fe0.8Ni0.2O3-δ (LSFN) not only stabilises its cubic perovskite structure under a reducing atmosphere, but also facilities the exsolution of FeNi nanoparticles from the parent perovskite matrix. The power density of the BaZr0.1Ce0.7Y0.2O3−δ (BZCY) electrolyte-supported single cells with a LSFNM/BZCY/LSCF-SDC configuration reaches 172mW•cm−2 at 750°C in C2H6; meanwhile, the ethylene yield also reaches approximately 40.5%. The presented results extend the functionality and applicability of proton conducting fuel cells for carrying out efficient hydrocarbon-related electrochemical conversion for producing power and co-production of value-added chemicals.

The effect of boiling bubbles on the passivation degradation of Alloy 800 in thiosulphate-containing aqueous electrolyte was experimentally investigated using corrosion potential, anodic polarization, combined with Secondary Ion Mass Spectrometry (SIMS) and X-ray Photoelectron Spectroscopy (XPS). The results showed that the boiling bubbles decreased the pitting potential by enhancing the mass transport. The specimen immersed in boiling electrolyte have a thicker anodic film and a greater amount of sulfur incorporation into the passive layer compared with the experiments at 99 °C. Moreover, a higher ratio of low valance sulfur species was detected on the specimen in boiling test electrolyte, indicating that the boiling bubbles reduced the corrosion resistance of Alloy 800.

The solid oxide electrolysis cell (SOEC) has attracted increased attention in recent years due to its capability to reduce CO2 emissions in a highly efficient and environmentally sustainable fashion. Previous work in our group has fabricated an A-site Ce doped La0.7Sr0.3Cr0.5Fe0.5O3-δ (LSCrF) with gadolinium doped ceria (GDC) as the cathode material in SOEC by the conventional method. This composite cathode presents a satisfying electrochemical performance and good stability due to the presence of excessive oxygen vacancies and strong CO2 adsorption ability. However, its electrochemical catalytic activity is still limited by the catalyst specific area. Hence, the optimization of electrode microstructure is considered as a promising way to further improve the SOEC performance by increasing the active reaction area. In this study, highly active (La0.65Sr0.3Ce0.05)0.9(Cr0.5Fe0.5)0.85Ni0.15O3-δ (Ni-LSCeCrF)/GDC nanostructured cathode was successfully fabricated by incorporating infiltration and in situ exsolution processes. The optimized microstructure contains fine and uniformly distributed perovskite particles on GDC backbone with nano-socketed Ni-Fe alloy nanoparticles. The Ni-LSCeCrF/GDC cathode shows significantly improved electrochemical performance, CO production rate, and Faraday efficiency for CO2 reduction reaction. Furthermore, the density functional theory calculations show that Ni doping could reduce the segregation energy of Fe, revealing a new strategy of multiple elements doping to form active alloy by in situ exsolution.

In this work, LSBT–LDC (La 0.4 Sr 0.5 Ba 0.1 TiO 3 –La 0.4 Ce 0.6 O 1.8 ) was successfully prepared by impregnating LSBT and LDC solutions on porous YSZ matrix. The prepared anode material exhibited higher catalytic performance and better resistance against carbon deposition than LSBT anode material in 5000 ppm H 2 S containing methane. Electrochemical impedance spectra (EIS) characterizations revealed that the addition of LDC significantly reduces the activation polarization resistance. Energy-dispersive X-ray spectroscopy (EDX) results indicated that the distribution of impregnated oxides in the matrix was uniform. The results of X-ray photoelectron spectroscopy (XPS) and temperature-programed oxidation (TPO) indicated that LSBT–LDC anode could enhance the tolerance against carbon deposition. The gas chromatography (GC) analysis proved that the modification of LDC could effectively promote the activation of methane and contribute to remove carbon deposition formed on the catalyst simultaneously.

This paper reports thiosulfate-enhanced effect of chloride ions on the passivity degradation of Alloy 800 and discusses how thiosulfate influences the semiconductor properties as well as breakdown of the passive films. Experimental results reveal that in concentrations of 10−6 to 0.075 mol/L, thiosulfate lowers the breakdown potential of Alloy 800 by 100–400 mV in solutions containing 0.6 mol/L chloride ions. This degradation is further verified by polarization resistance and noise resistance obtained from electrochemical impedance spectroscopy (EIS) and micro-nanoscale electrochemical noise (EN) data, respectively. Mott–Schottky analysis is employed to explore the corrosion mechanism and the results show that the passive film exhibits as p-type semiconductivity in a thiosulfate-only solution and n-type semiconductivity in a chloride-only solution, and the addition of thiosulfate into chloride solutions increases the concentration of oxygen vacancies in the passive film at the potential close to the pitting potential. Thiosulfate cannot be easily reduced on well-passivated surfaces in chloride-free solutions but can be readily reduced to sulfide in chloride solutions at active sites when the passive film breaks down – the reduced sulfur retards the healing process of repassivation, leading to an increase in oxygen vacancies.

Effects of elastic deformation on the anodic dissolution of X70 steel have been investigated in 0.5 mol dm −3 H 2 SO 4 solution. In the active region, the elastic stress exerts no obvious influence on the active dissolution rate of the specimen. However, in the oscillatory region, it leads to a marked increase in the frequency of current oscillations, while the mode remains essentially the same. In the passive region, it induces oscillations at the initial stage, but shows no significant effect on the passive current when the potential is controlled at the more positive stage. Two explanations are postulated: the elastic deformation degrades the stability and increases the defects of the passive film; it increases the surface energy (Δ G S ), causing the effective potential ( E V ) to move negative.

A cathode-supported SOFC consisting of LSM (La 0.8 Sr 0.2 MnO 3− δ ) cathode supporter, LSM–Sm 0.2 Ce 0.8 O 2− δ (SDC) cathode functional layer (CFL), yttria stabilized zirconia (YSZ)/SDC bi-layered electrolyte and Ni-YSZ anode layer was fabricated by a slurry spin coating technique. The influence of the porosity in both the CFL and cathode supporter on the electrochemical properties of the cells has been investigated. It was found that properly controlling the porosity in the CFL would improve the performance of the cells using O 2 in the cathode side (O 2 -cells), with a maximum power density (MPD) value achieving as high as 0.58 W cm −2 at 850 °C. However, this improvement is not so evident for the cells using air in the cathode side (air-cells). When increasing the porosity in the cathode-supporter, a significant increase of the power density for the air cells due to the decreasing R conc,c (cathode concentration polarization to the cell resistance) can be ascertained. In terms of our analysis on various electrochemical parameters, the R act (activation polarization to the cell resistance) is assumed to be mainly responsible for the impedance arcs measured under the OCV condition, with a negligible R conc,c value being able to be detected in our impedances. In this case, a significant decreasing size of the impedance arcs due to the increasing porosity in the cathode supporter would correspond to a decrease of the R act values, which was proved to be induced by the decreasing R conc,c .

The effects of Ca 2+ and Mg 2+ ions on the stress corrosion cracking (SCC) susceptibility of UNS N08800 are investigated using constant extension rate tensile (CERT) tests at 300 °C in simulated crevice chemistries. The presence of lead contamination in the crevice chemistries increases significantly the SCC susceptibility of the alloy. The lead-assisted SCC (PbSCC) susceptibility is reduced markedly by the addition of Ca 2+ and Mg 2+ ions into the solution and this mitigating effect is enhanced by increasing the total concentration of Ca 2+ + Mg 2+ . The CERT test results are consistent with the types of fracture surfaces shown by Scanning Electron Microscopy (SEM). There is a reasonable correlation between the SCC susceptibility and the donor densities in the anodic films in accord with the role of lead-induced passivity degradation in PbSCC.

This paper studies the hydrodynamic effects of erosive slurry on corrosion of passive target (type 304 stainless steel). The transient current response to disruption of passive film is investigated using the single particle impingement technique. The transient current density over the damaged surface is approximately independent of the hydrodynamics of fluid when the flowing velocity is in range of 5–10 m/s. It is characterized by a sharp rise caused by disruption of passive film and a slow decay due to repassivation. Therefore, the difference in transient current response is a result of different damaged surface area produced by solid particle impingement. After the kinetic parameters of repassivation are determined, the hydrodynamic effects on average corrosion rates of the passive target are quantitatively predictable with aid of the physical model developed by Lu and Luo.

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler–Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.

About 20 nm precursor powders for BaCe0.85Y0.15O3−δ (BCY) were synthesized by combustion method. The nanopowder had about 100 times larger specific volume than sintered BCY. A bi-layered proton conducting membrane having a thick porous BCY substrate and an integrally supported dense BCY thin film were co-fabricated facilely by pressing two layers comprising the precursor powder and its mixture with starch, followed by co-sintering at high temperature. Pt was impregnated into the porous BCY layer matrix as anode catalyst for dehydrogenation of ethane to ethylene. A hydrocarbon solid oxide fuel cell with the BCY thin film electrolyte and Pt electrodes demonstrated high selectivity (90.5%) to ethylene at 36.7% ethane conversion with co-generation of 216 mW cm−2 electrical energy output at 700 °C. The ethane conversion and ethylene selectivity increased with current density.

The effect of hydrodynamics of flowing slurry on anodic dissolution rate of passive metals was quantitatively evaluated using a theoretical model recently developed by the authors. The enhanced anodic dissolution over a passive metal in flowing slurry is dominated by the passive film breakdown caused by the impingement of solid particles and the decay of local current density over the impacted area due to repassivation. In the present study, the anodic current densities of 304 stainless steel and carbon steels were measured in flowing slurries under potentiostatic control condition. The difference in the repassivation modes indicates different repassivation mechanisms that depend on electrode material and corrosive medium. The parameters of repassivation kinetics experimentally determined enable estimation of the average anodic current density on the electrode surface in flowing slurry using the theoretical model. The theoretical predictions are in good agreement with the experimental data.

The fracture ductility of anodic films was measured for Alloy 800 at 300 °C in a neutral crevice solution with and without lead contamination. The film fracture ductility decreased, generally, with rising potential and increasing lead impurity concentration. The lead contamination altered the compositions of anodic films significantly and the effects of lead depended heavily on the passivation potentials. Anodic films containing more M OH and M OH2 bonds displayed lower film rupture ductility. The film rupture ductility correlated well with the stress corrosion cracking (SCC) resistance. It was found that the sensitivity of SCC resistance to lead contamination increased with an increase in the impact of lead contamination on the film rupture ductility. It is concluded that the SCC of Alloy 800 is likely related to film rupture at the crack tip and that the high SCC susceptibility in lead-contaminated environments may be related to the reduced rupture ductility of anodic films.

Yttrium-doped barium zirconate (BZY) perovskite structured materials are the first protonic electrolytes proven to be stable for use at intermediate temperatures (450–700 °C) in hydrogen containing 5000 ppm H 2 S. Structures of materials were characterized and stability was confirmed using XRD, SEM and EDX methods. While BZY powders and membranes are stable proton conductors in H 2 S-containing atmosphere, in contrast Y-doped barium cerate has poor chemical stability. The resistance of BZY, measured by impedance spectroscopy, is strongly dependent on temperature and atmosphere. Total conductivity of BZY increased with temperature in both hydrogen and dilute H 2 S atmospheres.

The erosion-enhanced corrosion of carbon steel A1045 was investigated by electrochemical methods in three borate buffer solutions with different concentrations, and the resistance of material to erosion-enhanced corrosion was discussed from the passive film growth kinetics. In polarization tests, the carbon steel exhibited comparable passive behavior in the three solutions. However, the dependence of passive current density on sand concentration became more significant with a decrease in borate concentration. The scratching simulation tests showed that after the passive film was damaged the peak current and decay constant were higher in the dilute solution, indicating a slower film growth rate. Based on point defect model (PDM), the key parameters for passive film growth are the diffusivity and density of the defects within the film. Mott-Schottky analysis in conjunction with PDM was performed and the results indicated that the diffusivities of defects were comparable in the films while the donor density increased with solution concentration. The higher donor density could result in higher diffusion flux and thus higher film growth rate, which could explain the better erosion-enhanced corrosion resistance in the concentrated solution.

The weight loss of carbon steel in flowing 0.1 M Na 2 SO 4 solution was investigated under galvanostatic condition. It was found that in corrosive solution even without solid particles, the mass loss rate of sample experimentally measured with the traditional weight loss method was higher than that calculated with Faraday's law as the flowing velocity of solution was high enough. The extra mass loss is regarded as non-Faraday material loss because it cannot be predicted with Faraday's law. According to the experimental observations, the non-Faraday material loss increases with flow velocity but decreases with surface hardness of the target, indicating a role of mechanical force. However, the non-Faraday material loss increases with the anodic current density and is inhibited under the cathodic protected condition, suggesting it is not solely caused by a mechanical force. Upon comparison with the experimental results in flowing slurry, it was found that the dependence of the non-Faraday wastage on anodic current density follows analogous trend with the corrosion-enhanced erosion observed in flowing slurries. It implies that the non-Faraday material loss may result from certain interaction between electrochemical and mechanical processes, i.e. corrosion-induced erosion.

The interaction of mechanical and electrochemical factors in the erosion–corrosion process of carbon steel A1045 was investigated in Na2SO4 solution/silica sand slurry with a rotating cylinder electrode system. In line with the mechanism associated with material wastage, the synergism due to the interaction of erosion and corrosion was divided into corrosion-enhanced erosion and erosion-enhanced corrosion. The electrochemical experimental results indicated that the carbon steel showed active dissolution behavior, and the corrosion process was not controlled by the mass transfer when the rotating velocity is high enough. Scratching electrode tests and tensile tests were conducted to simulate the effect of sand impingement on corrosion process. Results showed that the impingement of the solid did not affect the corrosion behavior significantly in the present system. For the effect of corrosion on erosion, a half-logarithm relationship was found between weight loss due to corrosion-enhanced erosion and the applied anodic current density. According to a chemo-mechanical model, this effect can be attributed to the hardness degradation in metal surface with corrosion current.

This paper investigated the effect of chloride ion perturbation on the electrodissolution behavior of carbon steel in 5.0 M H 3 PO 4 solution. The results indicated that the oscillatory electrodissolution changes significantly when the electrode/electrolyte interface was disturbed by injecting small quality of chloride ion-containing solution. The mode of current oscillations changed from the mono-periodic to the nonperiodic behavior with higher frequencies but lower amplitudes. After a transient response disappeared, the permanent influence in the current oscillations was observed. Potential distributions on the surface of the electrode were monitored in situ by using scanning reference electrode technology during the electrodissolution process. The potential maps show clearly that localized corrosion was induced after the chloride perturbation. The enforced solution homogenization by bubbling nitrogen at the electrode/electrolyte interface after a few seconds of the chloride perturbation could not eliminate such influence, indicating that chloride ions may enter the passive film and resulted in degradation in chemical stability of the film because of permanent damage. Such a degradation of the passive film only appears when the transient concentration gradient of the chloride ions on the surface is sufficiently high. The long duration of impact on electrodissolution caused by chloride ion perturbation suggests that the passive film does not totally dissolve during the active period in each oscillatory cycle.

The effect of microstructural changes in 304 austenitic stainless steel induced by the processes of gas tungsten arc welding (GTAW) and laser-beam welding (LBW) on the pitting and stress corrosion cracking (SCC) behaviors was investigated. According to the in situ observations with scanning reference electrode technique (SRET) and the breakdown potentials of the test material with various microstructures, the GTAW process made the weld metal (WM) and heat-affected zone (HAZ) more sensitive to pitting corrosion than base metal (BM), but the LBW process improved the pitting resistance of the WM. In the initiation stage of SCC, the cracks in the BM and HAZ propagated in a transgranular mode. Then, the crack growth mechanism changed gradually into a mixed transgranular + intergranular mode. The cracks in the WM were likely to propagate along the dendritic boundaries. The crack initiation rate, crack initiation lifetime and crack propagation rate indicated that the high-to-low order of SCC resistance is almost the same as that for pitting resistance. High heat-input (and low cooling rate) was likely to induce the segregation of alloying elements and formation of Cr-depleted zones, resulting in the degradation in the corrosion resistance.

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential Ep of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO3 solution. Pre-charged hydrogen does not have a significant effect on the pitting potential Ep and open circuit potential Ecorr of the steel in 0.5 M NaHCO3 solution. However, a synergistic effect of hydrogen and Cl− on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO3 solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed.

1-Octadecanethiol (C18SH) monolayers were self-assembled on the fresh and active copper surface pretreated by nitric acid etching method. The surface properties of the alkanethiol-modified copper electrode in halide-containing solutions were characterized systematically by using several electrochemical methods, including polarization curves, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN). The results show that C18SH self-assembled monolayers (SAMs) onto copper provide a flexible method that can protect the underlying copper against corrosion. With the immersion time of SAMs-coated copper electrode in NaCl and HCl corrosive solutions increasing, a slow loss of corrosion protection ability of SAMs indicates dynamic processes occurring at the electrode/solution interface and in the monolayers, such as expansion of the defects and transport of corrosive ions through defects of SAMs. Electrochemical noise (EN) is employed to detect the alkanethiol-modified copper surfaces immersed in HCl solution. This observation suggests the pitting process associate with dynamic processes in the 1-octadecanethiol layer.

Mott-Schottky analyses and photoelectrochemical measurements were used to explore the effects of film formation potentials, time, and chloride ions on the electronic properties of the passive film on X70 pipeline steel in 0.5 M NaHCO 3 . Mott-Schottky analyses showed that with increasing film formation potentials, the capacitance and donor density of the passive film decrease, and the flat band potential and thickness of the space charge layer increases. The addition of chloride ions increases the capacitance and donor density of the film and results in a more negative flat band potential and a thinner space charge layer. Photoelectrochemical measurements imply that photo-generated carriers with a low mobility exist in the passive film and the photocurrents of the film increase when the film formation potential becomes more positive. The bandgap energy, E g , of the passive film decreases with increasing film formation potentials. The extension of film formation time increases the photocurrents and leads to a decrease in the bandgap energy of the passive film. The effect of film formation time on the photocurrents of the film formed at 0.6 V SCE was less remarkable than that formed at 0.2 V SCE . The existence of chloride ions in 0.5 M NaHCO 3 decreases the photocurrents and increases the bandgap energy of the passive film.

Hydrogen-induced pitting active sites on iron were investigated by electrochemical noise (EN) and scanning reference electrode techniques (SERT). The significant increase in the number of current and potential fluctuations in EN measurements indicates that hydrogen increases the number of active sites for formation of metastable pits, which are precursors of stable pits. The potential distributions over the surfaces of hydrogen charged and uncharged iron polarized at 0.07 V (vs. SCE) for different polarization times also show clearly that hydrogen significantly increases the number of pitting active sites on the surface of charged iron. It was also observed that to reach equal numbers of pitting active sites, a higher applied anodic potential is required on uncharged iron than on hydrogen-charged iron. The increase in the number of pitting active sites due to hydrogen was discussed in terms of the effect of hydrogen on the nature of the passive film and the dissolution rate of iron.

The effects of hydrogen on the stability of passive films on iron were investigated by electrochemical methods: open circuit potential decay, cathodic galvanostatic reduction, electrochemical impedance spectroscopy, and breakdown potential measurements. The results show that hydrogen decreases the final static open circuit potential, the cathodic charge for reduction and the charge transfer resistance of the passive film, indicating that hydrogen decreases the stability of the passive film. The charge transfer resistance of the passive film formed on the charged specimen does not change with increasing the film formation potentials, suggesting that increasing film formation potentials under hydrogen charging conditions cannot improve the stability of the passive film. Hydrogen decreases the breakdown potential of the passive film, especially at lower chloride ion concentrations, confirming that hydrogen promotes the susceptibility of the passive film on iron to pitting corrosion. The reasons why hydrogen decreases the stability of the passive film were discussed.

The effects of hydrogen and applied tensile stress on the breakdown of passive films on Type 304 stainless steel have been investigated in chloride-containing solutions. Hydrogen was cathodically introduced into specimens by applying a constant current density. The current responses to the application of a passivation potential of 0.3 V versus a saturated calomel electrode (SCE) and additions of various concentrations of chloride ions were recorded. Hydrogen greatly decreases the critical chloride concentration for the breakdown of passive films, which indicates that hydrogen promotes the breakdown of passive films. Hydrogen is also found to hinder the repassivation process. With an increase in applied tensile stress, the critical chloride concentration for the breakdown of a passive film decreases. The critical chloride concentrations are always significantly lower for charged specimens than for uncharged specimens at the applied tensile stress range and the effect of stress on lowering the critical chloride ions is more significant for charged specimens than uncharged specimens.

The effects of the presence of hydrogen in type 304 stainless steel and chloride ions in a solution on the anodic current density and breakdown potential were investigated. It is found that both hydrogen and chloride ions increase the anodic current density of the stainless steel in the passive range. The effect is not simply arithmetically additive; instead there is a synergistic effect of hydrogen and chloride ions on the anodic current density. Hydrogen also has an enhanced effect of chloride ions on a decrease in breakdown potential. The critical chloride concentration for film breakdown decreases with hydrogen-charging current density and the critical hydrogen-charging current density for the film breakdown decreases with chloride ion concentration. The passive films on both uncharged and charged specimens exhibit n-type semiconductivity. Hydrogen increases the donor concentration while chloride ions have no such effect. The effects of hydrogen and chloride ions on the passivity are explained with an electronic model.

The self-discharge mechanism during storage in open-circuit states of a Ni–MH battery using a Mg2Ni-type hydrogen storage alloy anode was investigated by electrochemical impedance spectroscopy (EIS) and X-ray diffraction (XRD). The loss of discharge capacity for this battery can be ascribed to two causes: (i) desorption of hydrogen from the Mg1.95Y0.05Ni0.92Al0.08 hydride anode; and (ii) anode surface degradation resulting from oxidation of the magnesium alloy in the electrolyte. At the higher open-circuit voltages (OCV), the former was mainly responsible for a high self-discharge rate, while the latter might dominate the loss of capacity at the lower OCV. XRD results confirmed that Mg(OH)2 formed on the magnesium alloy anode after storage in an open-circuit condition for 20 days.

The noise spectra under different transient shapes were calculated and the relationship between the spectral parameters (roll-off slope and roll-off frequency) in the frequency domain and the transient variables in the time domain were analyzed. The influence of corrosion types on noise spectra was considered. It is shown that a ‘white’ noise always appears in the low-frequency range of the noise spectrum and the roll-off slope of the spectrum in the high-frequency range depends on the transient shape, not the corrosion type. During pitting, the roll-off frequency of the noise spectrum is directly related to the time constant of the exponential function contained in the noise transient in the time domain and reflects the repassivation or growth rate of the metastable pits. For the triangular- and delta-shaped transients generally generated during passivity or general corrosion, the roll-off frequency is related to the time interval of the current transient during the current rise or drop stage. Although spectral analyses can extract useful information from the noise transients, the spectra cannot display the difference of any two transients that are symmetrical in shape in the time domain.

The passivity and pitting behavior of A516-70 carbon steel in chromate solutions were studied using electrochemical measurements. The anodic Tafel slopes in the active region show that carbon steel dissolution involves two mechanisms in this range: formation and further oxidation of a pre-passive film of Fe(OH)2. The first current peak at −0.228 V (Ag | AgCl) in cyclic voltammograms is caused by the oxidation of the pre-passive film and the formation of a stable passive film of Cr3++Fe3+. The second peak at 0.612 V is ascribed to the oxidation of Cr3+ in passive film to Cr6+. The charge-transfer step at the electrode/solution interface controls the film formation and dissolution; the role of diffusion is negligible. Chromate ions play a prominent role in the formation of passive film, but hardly affect the stability of the passive state. More chromate ions in solution enhance the dissolution of Cr3+ at the second peak potential. Upon addition of chloride ions metastable pits are initiated, as indicated by a typical current transient: a quick current rise followed by a slow recovery. A maximum exists in the potential dependence of the pit initiation rate. Metastable pit growth is controlled by the ohmic potential drop mainly across the cover over the pits. Increasing potential is beneficial to the repassivation of metastable pits, as indicated by the decreasing average repassivation time. A pit stabilization criterion, the ratio of peak pit current to pit radius, must exceed 6×10−2 A cm−1 during pit growth to avoid repassivation in the present system.

Nanocrystalline Mg2Ni-type hydrogen storage alloys were synthesized using the powder metallurgical technique and mechanical milling with nickel powder under an argon atmosphere. Changes in phase structures with mechanical treatment were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM) with selected-area electron diffraction (SED). As expected, the phase structure of the synthesized material changes with milling time, from crystallite to nanocrystallite and amorphous. The electrochemical behavior of these materials with various phase structures in alkaline media, such as electrochemical capacity, high rate dischargeability and cycle life, were investigated and compared. The Mg2Ni type alloy electrode with nanocrystalline structure showed superior discharge characteristics compared with those with crystallite and amorphous structure. It was also found that the electrochemical hydriding/dehydriding behavior of Mg2Ni-type hydrogen storage alloys has been further improved by a new approach of particle inlay developed in this study. The analysis of the electrochemical impedance spectroscopy (EIS) reveals that for the nanocrystalline Mg2Ni type alloy electrode, both the charge transfer resistance of the hydrogen redox reaction on the surface and the diffusion resistance of the hydrogen atom in the bulk alloy were greatly reduced by mechanical treatment with nickel addition. The rate-determining step of the electrode reaction changed with the electrode nanocrystalline structure.

The semiconductive properties, electronic structure and pitting susceptibility of passive films formed on A516-70 carbon steel in chromate solutions with and without chloride ions are studied by capacitance measurements, Mott–Schottky analysis and the electrochemical noise technique. The passive film shows features of an amorphous or highly disordered n-type semiconductor. The donor concentration is in the range of 1026–1027 m−3 and decreases with the film formation potential (prepassivation potential). The thickness of the space charge layer ranges from 1 to 6 A. The flat band potential increases with an increase in the film formation potential. The nonlinearity of Mott–Schottky plots, depending upon the film formation potential, indicates the existence of two donor levels in the space charge layer. When the film formation potential is below −50 mV, the two levels are revealed by two slopes in the Mott–Schottky plots. For a film formed at a prepassivation potential of −50 mV, only the shallow donor level can be ionized due to increase in the stability of the passive film. The onset potential and the transitional potential of metastable pitting, determined by electrochemical noise measurements, coincide with the flat band potential of the film formed at the corrosion potential and the critical potential at which both deep and shallow donor levels are ionized. It is suggested that the divalent iron cation vacancies generated by the ionization of the deep donor level are mainly responsible for pitting corrosion.

Effects of metal oxide additions on the electrochemical hydriding and dehydriding behavior of Mg2Ni-type hydrogen storage alloy in 6 M KOH aqueous solution were investigated. The electrode characteristics of mechanically alloyed composites of Mg1.9Y0.1Ni0.9Al0.1–5 wt% MO (MO=Ag2O, Fe2O3, MoO3, RuO2 and V2O5) were examined such as discharge capacity, high-rate dischargability and cycle life. The discharge capacity and high-rate dischargability were greatly increased by the modification with the oxide additions, but the cycle life decreased. The electrochemical performances were characterized using both dc polarization and ac impedance analysis techniques. The hydrogen diffusivity in the alloys was estimated by an electrochemical method.