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Electrochemical dissolution behavior of laser powder bed fusion (L-PBF) Ti–6Al–4V alloy is investigated in a 3.5 wt% NaCl solution through electrochemical impedance spectroscopy (EIS), potentiostatic polarization, and Mott–Schottky analysis. To investigate the effect of microstructure on the corrosion passivation film and corrosion resistance, the microstructure (martensitic α′/α + β phases and α grain boundaries (αGB)) of L-PBF Ti–6Al–4V alloys was modified via heat treatment. The microstructural results indicate that the heat treatment at 950 °C (HT-950) lead to a higher volume fraction of β phase and finer α-laths, compared with those under other heat treatment temperatures. Intriguingly, the corrosion resistance of the HT-950 sample is significantly higher than that of the others because the refinement of grains contributes to preventing corrosion. The stable passivation film is attributed to the decrease in donor density (oxygen vacancies) and vacancy diffusion coefficients with increasing the film formation potential. This work illustrates the mechanism of passivation film during the corrosion process of L-PBF Ti–6Al–4V alloy, and would be helpful to develop additively manufactured Ti–6Al–4V alloys with a better corrosion resistance. [ABSTRACT FROM AUTHOR]

A mixed potential model (MPM) comprising the Point Defect Model (PDM) and the Generalized Butler Volmer Equation (GBVE) was successfully developed to study the passivity of HDSS 2707. The results show that the dominant point defect within the passive film is the Min+, thereby rendering n-type semiconductor behavior of the barrier layer. The anodic current is independent of the film formation potential whereas it increases with increasing temperature and follows the Arrhenius law. The partial cathodic current of hydrogen reaction is affected by the applied voltage. The anodizing constant of the passive film is calculated to be 2.91 nm/V.

Passivity breakdown on HDSS 2707 has been studied and the electrochemical data are interpreted in terms of the Point Defect Model. Pitting parameters of HDSS 2707 are determined, including the polarizability at bl/s interface, the defect annihilation rate, and the defect diffusion coefficient. The breakdown potential is demonstrated to be linearly related to log [Br-], pH, and the square root of potential scan rate (v1/2), and follows a near-normal distribution. The critical vacancy concentration calculated from the PDM is consistent with that estimated crystallographically from the chromic barrier layer, and the dominant point defect is further confirmed by Mott-Schottky measurements.

In this paper, the passivation kinetics of AA2098-T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098-T851 in NaHCO3 solution in a CO2 atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler-Volmer (B-V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface.

Prediction of the accumulated pitting corrosion damage in aluminum-lithium (Al-Li) is of great importance due to the wide application of these alloys in the aerospace industry. The Point Defect Model (PDM) is arguably one of the most well-developed techniques for evaluating the electrochemical behavior of passive metals. In this paper, the passivity breakdown and pitting corrosion performance of AA 2098-T851 was investigated using the PDM with the potentiodynamic polarization (PDP) technique in NaCl solutions at different scan rates, Cl- concentrations and pH. Both the PDM predictions and experiments reveal linear relationships between the critical breakdown potential (Ec) of the alloy and various independent variables, such as aCl- and pH. Optimization of the PDM of the near-normally distributed Ec as measured in at least 20 replicate experiments under each set of conditions, allowing for the estimation of some of the critical parameters on barrier layer generation and dissolution, such as the critical areal concentration of condensed cation vacancies (ξ) at the metal/barrier layer interface and the mean diffusivity of the cation vacancy in the barrier layer (D). With these values obtained-using PDM optimization-in one set of conditions, the Ec distribution can be predicted for any other set of conditions (combinations of aCl-, pH and T). The PDM predictions and experimental observations in this work are in close agreement.

The study simulated copper corrosion in a bentonite/groundwater solution, analyzing the kinetics of metal passivation through potentiodynamic polarization, potentiostatic polarization, and electrochemical impedance spectroscopy experiments. Morphological characterization provided insights into corrosion development patterns, and the mechanisms by which bentonite affects copper corrosion were comprehensively elucidated based on the point defect model. The findings revealed that bentonite affects copper corrosion resistance through Donnan exclusion, cation adsorption, and hydrolysis salt effects. At lower potentials, bentonite aids in inhibiting anodic dissolution, reducing the polarization current by approximately 75 % under certain conditions. With increasing bentonite content, the induction time for stable pitting corrosion shortens significantly, down to 10 %, the current surge before breakdown intensifies, and the breakdown potential decreases. When the bentonite volume ratio exceeds 0.5/0.1, the breakdown potential of OFP-Cu/OF-Cu markedly decreases and stabilizes at 0.1/0 V SCE. The microscopic mechanism of pitting corrosion induced by bentonite hydrolysis salts was elucidated using the point defect model, calculating the polarizability of the passive layer/outer layer interface as α = 0.1. The reliability of the cation adsorption and adsorption-desorption mechanisms was confirmed through experimental data fitting, point defect model fitting, and theoretical analysis. [ABSTRACT FROM AUTHOR]

• Investigated Ti-6Al-3Nb-2Zr-1Mo alloy's electrochemical behavior under varying pH and potentials. • Passivation film growth was analyzed using a point defect model and electrochemical testing, which provided the point defect density and oxygen vacancy diffusion coefficient. • Detailed Analysis of Passivation Films Using Localized Electrochemical Impedance Spectroscopy and Glow Discharge Optical Emission Spectrometry. This study aims to explore the passivation behavior and growth mechanism of the Ti-6Al-3Nb-2Zr-1Mo alloy in an oxygen-free environment, taking into account various film-forming potentials and solution pH levels in the process. The point defect model is employed to study the growth mechanism of the passivation film. A film-forming potential is selected within the stable passivation region for polarization, followed by conventional electrochemical tests, Localized Electrochemical Impedance Spectroscopy, X-ray photoelectron spectroscopy characterization, and Glow Discharge Optical Emission Spectrometry. Experimental findings indicate that the passivation film on the Ti-6Al-3Nb-2Zr-1Mo alloy surface demonstrates characteristics of an n -type semiconductor. Increasing the film-forming potential leads to a decrease in the donor point defect density. Meanwhile, elevating the pH of the solution decreases the oxygen vacancy diffusion coefficient from 7.8 × 1019 to 4.3 × 1019, thus improving the corrosion resistance of the passivation film. [ABSTRACT FROM AUTHOR]

The aim of this research was to study the growth kinetics of anodic oxide films on cobalt silicides in sulphuric acid solutions under potentiostatic conditions with various pretreatment of the electrode surface. For the study, we used low and high silicon silicides (Co2Si and CoSi2) in 0.05 and 0.5 M H2SO4. We obtained the chronoamperograms in the time interval t = 0.3–3000 s with the oxide formation potentials of Ef = 0.2, 0.5, and 1.0 V (SHE). It was determined that the kinetics of the growth of oxide layers on cobalt silicides in acidic solutions greatly depends on the method of the silicide surface pretreatment (mechanical polishing; cathodic pre-polarisation in a H2SO4 solution; exposure to H2SO4 solution at the open circuit potential; exposure in a 2 M KOH solution; and exposure to 2% HF solution). In most cases, at low t (up to 30–50 s), the oxide films grew due to the ion migration in the strong electric field generated in the film during anodic polarisation. In some cases (Co2Si silicide with higher cobalt content; pretreatment of Co2Si in alkaline solution, further enriching the silicide surface with cobalt; and the region of high values of t), the point defect model seemed to be executed.

The Point Defect Model was successfully developed to study the passivation of 316LN. Steel's passive film exhibits n-type semi-conductivity with interstitial cations as the dominant point defect. The transfer coefficients α i and rate constants k i are independent of the applied potential but vary with temperature. The electronic structure of the barrier layer (bl) is discussed as a degenerate defect semiconductor in which the bl comprises essentially the depletion zone with the electronic charge and the electron-hole charge being concentrated at the metal/bl and the bl/solution interfaces, respectively. The closest classical analog is postulated to be a tunnel diode. • The Point defect model (PDM) is successfully applied to studying the passivation of 316LN in synthetic seawater. • The dominant point defect within the passive film of 316LN is the interstitial cation. • The rate constants of defect reactions for 316LN are independent of the applied potential. • The near-normal distribution of breakdown potential of 316LN is consistent with the PDM calculation. [ABSTRACT FROM AUTHOR]

This research presents a comprehensive comparative analysis of the passivation kinetics of OFP-Cu and OF-Cu in simulated repository electrolyte. The study employs a range of techniques, including potentiodynamic polarization, multi-step potentiostatic polarization, electrochemical impedance spectroscopy, and Mott-Schottky analysis, to explore the intricacies of corrosion behavior. The Point Defect Model (PDM) is used to interpret the steady-state physicochemical processes. Utilizing the results of PDM optimization, defect distribution diagrams within the passivation film were constructed, shedding light on the pivotal transition from p-type to n-type semiconductor properties of the film. The study concludes with an interpretation of the experimental data, highlighting the enhancement of copper's corrosion resistance due to phosphorus doping, as elucidated by the Solute-Vacancy Interaction Model. [Display omitted] [ABSTRACT FROM AUTHOR]

To alleviate the cavitation damage of metallic engineering components in hydrodynamic systems operating in marine environments, a NbN nanoceramic coating was synthesized on to a Ti-6Al-4V substrate via a double cathode glow discharge technique. The microstructure of the coating consisted of a ~13 μm thick deposition layer of a hexagonal δ′-NbN phase and a diffusion layer ~2 μm in thickness composed of face-centered cubic (fcc) B1-NaCl–structured (Ti,Nb)N. The NbN coating not only exhibited higher values of H/E and H2/E than those measured from NbN coatings deposited by other techniques, but also possessed good adhesion to the substrate. The cavitation erosion resistance of the NbN coating in a 3.5 wt% NaCl solution was investigated using an ultrasonic cavitation-induced apparatus combined with a range of electrochemical test methods. Potentiodynamic polarization measurements demonstrated that the NbN coated specimens demonstrated both a higher corrosion potential (Ecorr) and lower corrosion current density (icorr) than the uncoated substrate. Mott-Schottky analysis, combined with the point defect model (PDM), revealed that, for a given cavitation time, the donor density (ND) of the passive film on the NbN coating was reduced by 1 ~ 2 orders of magnitude relative to the uncoated Ti-6Al-4V, and the diffusivity of the point defects (D0) in the passive film grown on the NbN coating was nearly one order of magnitude lower than that on the uncoated substrate. In order to better understand the experimental observations obtained from Mott-Schottky analysis and double-charge layer capacitance measurements, first-principles density-functional theory was employed to calculate the energy of vacancy formation and the adsorption energy for chloride ions for the passive films present on both the NbN coating and bare Ti-6Al-4V.

This research aimed to apply different models on electrochemical impedance spectroscopy to study the influence of grain refinement and straining obtained from surface mechanical attrition treatment on the passivation of carbon steel in an alkaline solution at open circuit and different passivating potentials. Based on the power law model, the graphical presentation of capacitances at high frequencies and the SIMPLEX method at low frequencies were used to evaluate the electrochemical properties of the interface. The point defect model was used to estimate point defects diffusivity of the passive film in the passivating potential range. Results showed the oxide layer formed on a more strained and grain refined surface was thinner but less porous. [ABSTRACT FROM AUTHOR]

Pitting corrosion is a significant concern for the broader application of stainless steel in modern industries in which metal and metal alloy are detached preferentially from susceptible parts on the surface, resulting in the creation of holes in passivated alloys that are exposed to an aqueous, neutral electrolyte containing corrosive species. Exposure of SS to brines leads to the localized loss of surface passivity and the onset of isolated pitting, which render the equipment or piping unfit for service. In the present study, the passive layer behavior and the pitting corrosion of the modified martensitic stainless steel (MMSS) were evaluated in a saturated CO2 environment (pH~5) with different NaCl concentrations and temperatures, using various electrochemical techniques. It was found that by increasing the temperature up to 60 °C, the corrosion resistance of the MMSS increased; however, the corrosion rate dramatically increased at 80 °C, indicating the destruction of the oxide layer. According to the point defect model (PDM) results, the calculated values of polarizability (α), metal cation diffusivity (D), and the rate of annihilation of cation vacancies (jm), reveal a strong dependence on the solution temperature.

Transuranic metals such as plutonium usually need to be protected in inert gases to inhibit oxidation and produce aerosols. In the paper, the oxidation reaction process of plutonium was depicted based on the point defect model. The aerosol generation rate was approximated to the oxidation rate of plutonium when the oxidation layer keeps constant. The results show that the rate is strongly dependent on the vacancy diffusivity and reaction rate constant of the vacancy generation and consumption. The aerosol generation rate at the temperature of 298 K is in the order of 108 particles/cm2/s. [ABSTRACT FROM AUTHOR]

TiO 2 film structure has a significant impact on its electrochemical performance. Improving the film structure has become an important means to promote the electrochemical performance of TiO 2. Growth of high quality TiO 2 film is extremely important for corrosion resistance or optoelectronics applications; however, it is considerable challenging using conventional methods. Herein, we report on a novel control method based on the defect engineering. The results show that the defect concentration is controlled by adjusting the mechanical activation time with tunable film thickness from 155 nm to 998 nm. Moreover, the pure titanium with high activity anodized display the best high crystallinity and corrosion resistance probably due to the Gibbs free energy educed by mechanical activation, which increased the defect concentration and reaction rate constant, and promoted the transition of unstable Ti3+ and Ti2+ to stable Ti4+. Our findings provide an effective method for the fabrication of TiO 2 films, which can pave the way to further provide theoretical support for surface interfacial reactions. [ABSTRACT FROM AUTHOR]

The passivation mechanism of the film formed on the alloy Ti-6Al-4V was evaluated in Hank's solution to infer the properties of this alloy as an implant material. Alloy passivation was found from electrochemical measurements and X-ray photoelectron spectroscopy (XPS) to strongly depend on the oxidation of Ti and Al, microstructural changes associated with the Al and V, and the formation of metallic hydroxides and oxyhydroxides that disrupt the TiO2 matrix. Experimental impedance diagrams were fitted using the point defect model (PDM, transfer function) to describe the passive character of the alloy. According to this analysis, the transport of oxygen and hydroxide defects across the film on the alloy surface determines the adsorption of oxygen from water dissociation and/or phosphate and the precipitation of calcium phosphate. Therefore, osseointegration of the alloy Ti-6Al-4V occurs across the entire surface and strongly depends on the defects present in the film, Al incorporation, the penetration of hydroxide ions (hydration), and oxygen adsorption. [ABSTRACT FROM AUTHOR]

For 9–12Cr ferritic–martensitic steels in supercritical water, the dependencies of the thicknesses of three oxide layers (diffusion, inner, and outer layers) on each of seven principal independent variables were separately investigated using three supervised artificial neural networks (ANN) and fuzzy curve analyses. The latter were employed to evaluate the relative significances of independent variables, indicating that on the whole, temperature and exposure time are the most important variables, while the oxide dispersion strengthening (ODS) comes in the first place for the thickness of the diffusion layer. A thicker diffusion layer occurs easily at intermediate temperature approximately 600 °C and/or on ODS steels, due to higher growth rate of the barrier layer than that of the outer layer. The periodic growth of the diffusion layer, including the "shrinking"/"thickening" stages, was revealed by ANN prediction, which may be the basic cause of periodic pore-assembled layers within the inner layer. Finally, the physicochemical basis of classical point defect model was extended to describe the growth of oxide scales for explaining the ANN predictions at the atomic level. The growth of the inner layer into the metal is attributed to the inward transport of oxide ions (actually via outward transport of oxygen vacancies), while the outward transport of cations through the inner layer via a cation vacancy mechanism or as interstitials results in the thickening of the outer layer. The periodic variation in oxygen potentials at the diffusion/inner layer interface is responsible for the periodic growth of the diffusion layer. The iron caves at the inner–diffusion interface left behind by the generation reaction of cation interstitials may be the intrinsic sources of pores within the inner layer. [ABSTRACT FROM AUTHOR]

• Electric-field strength in the passive film on tungsten is explored using photo-electrochemical techniques. • A theoretical expression for the photo-stimulated growth of the film has been derived. • Super band gap light suppresses the electric field in the barrier layer. • Super band gap light stimulates anodic oxide film growth. • The electric field strength in the steady state passive film is independent of the applied potential and film thickness. The nature of the electric-field strength in the passive film on tungsten is explored using photo-electrochemical techniques. A theoretical expression for the photo-stimulated growth of the film has been derived. Rotating ring-disk electrode (RRDE) experiments indicate that photo-corrosion of a tungsten electrode is negligible under super-band gap light illumination. XPS results show that tungsten is in the maximum possible oxidation state (VI) in the film and hence no higher oxidation state is available. The photo-stimulated transient film growth of tungsten in pH 8.5 ± 0.1 boric-borax buffer was recorded as a function of film formation potential (1 V SCE , 2 V SCE , 3 V SCE , 4 V SCE , 5 V SCE , 6 V SCE) and light intensities (50, 200, 1000 mW/cm2). Steady state passive film thickness measurements of the passive film on tungsten indicate that super band gap, UV light suppresses the electric field in the barrier layer, and hence stimulates anodic oxide film growth. The data obtained in this study demonstrate that the electric field strength in the steady state is independent of the applied potential and film thickness, as postulated in the Point Defect Model. [ABSTRACT FROM AUTHOR]

• Interactions of Cl with the α-Fe 2 O 3 (0001) surface are studied. • The Cl adsorption is stronger on the lower charged Fe sites. • The subsurface Cl is less stable than adsorbed Cl. • Cl lowers the surface Fe vacancy formation energy. • Fe vacancy is more stable in the bulk while O vacancy is more stable on the surface. This study provides molecular insights into the initial stages of Cl-induced depassivation of iron passive film by studying the interactions of Cl with pristine and vacancy surfaces of α-Fe 2 O 3 (0001). Subsurface Cl is found to be less stable than adsorbed Cl which contradicts the ion exchange model that postulates Cl ingress into the passive film. Comparison of vacancy formation energy shows that Cl can enhance the surface Fe vacancy formation, which is consistent with the point defect model. Additionally, the energy difference of vacancies in different oxide layers supports the diffusion direction proposed by the point defect model. [ABSTRACT FROM AUTHOR]

Abstract Upon performing electrochemical measurements, XPS and TEM characterizations, the influence of sulfide on the passive behavior of Super 13Cr martensitic stainless steel was deciphered. Mechanism interpretation is performed by optimizing the Point Defect Model based upon the impedance data. It is revealed that the passive film display n-type semiconductivity, and the donor density dropped with increasing concentration of sulfide ion. Sulfide ion can induce a current peak at −250 mV SCE (E peak). The addition of Na 2 S generates a continuous outer layer which is comprised of Cr(OH) 3 , FeS 2 and FeS at potential lower than E peak , and Cr(OH) 3 at potential higher than E peak. This result leads to the thinner barrier layer and lower charge transfer resistance, implying a weaker protection of the substrate by the passive film. Highlights • The passive behavior of Super 13Cr martensitic stainless steel in sulfide-containing environment was studied. • The influence of sulfide ion on the charactersitic of the passive film was characterized. • Sulfide ion can thin the barrier layer and result in reduced protection effect of passive film, while the outer layer was thickened with the addition of Na 2 S. [ABSTRACT FROM AUTHOR]

Abstract Understanding the kinetics of passive oxide formation and breakdown has been an ongoing problem for corrosion scientists for several decades. Here, we present a model for the formation of a passive oxide film on a metal that is an extension of the Point Defect Model (PDM) by Macdonald and coworkers. We replace the potential description of the PDM with a boundary value problem and ensure that Gauss's law is satisfied at the oxide-solution interface by considering the presence of the Helmholtz layer in solution. Our model predicts the observed linear variation of the steady-state film thickness with anode potential, and an increasing film thickness with increasing pH. We perform a linear stability analysis of the model and show that, depending on the parameters, the passive film may be unstable to morphological perturbations of the film interfaces, leading to nonplanar films and potentially the formation of a pit in the oxide. This also implies that one-dimensional models, which assume planar interfaces, can be inapplicable in broad classes of corrosion processes. The analysis shows that a morphological instability exists if the oxide dissolution mechanism is such that an increasing Helmholtz layer potential drop leads to an increasing dissolution rate. The instability behavior is consistent with the literature on breakdown of passivity in the presence of chloride ions. The theory provides insights on the initiation of passivity breakdown leading to pitting corrosion and the role that interfacial energy plays in determining stability. [ABSTRACT FROM AUTHOR]

The addition of small amounts of tungsten (W) on the passivity breakdown of nickel (Ni) in borate buffer solutions as a function of chloride activity, pH, and voltage sweep rate has been evaluated and the data are interpreted in terms of the point defect model (PDM). The critical pitting potential of Ni–W alloys was found to become more positive (correspondingly a higher resistance to pitting) upon increasing the W content up to 3 at.% but decreases with further additions. The initial increase in the breakdown voltage with increasing [W] is attributed to changes in the diffusivity of the cation vacancy, the electric field strength, the annihilation rate of cation vacancies (Jm), changes in the standard Gibbs energy of adsorption of Cl− into surface oxygen vacancies, and in the standard Gibbs energy of generation of Schottky pairs, as predicted by the SVIM. At high [W] (4–5 at.%), it is postulated that all of the mobile cation vacancies are complexed with immobile WNi4⋅ [W(VI) cation substituted onto the Ni(II) vacancy of the cation sublattice of the barrier layer], so that further additions of W are predicted to have little positive effect in improving the pitting resistance of the alloy. Instead, W additions above 3% are postulated to increase the population density of breakdown sites on the surface via the formation of intermetallic precipitates, which has the effect of displacing the distribution in the breakdown voltage in the negative direction. The addition of small amounts of tungsten (W) on the passivity breakdown of nickel (Ni) in borate buffer solutions as a function of chloride activity, pH, and voltage sweep rate has been evaluated and the data are interpreted in terms of the point defect model (PDM). The critical pitting potential of Ni–W alloys was found to become more positive upon increasing the W content up to 3 at.% but decreases with further additions. [ABSTRACT FROM AUTHOR]

Passive films were prepared at different film formation potential (E ff) on a Ni16Cr13Co4Mo alloy in NaCl-Na 2 SO 4 eutectic at 700 ℃. The electrochemical property of the passive film was measured and the diffusivity (D 0) of the point defects in the passive film was calculated based on point defect model (PDM), which was around 10−14 cm2/s. TEM and XPS proved that passive film was composed of outer Na 2 CrO 4 -Cr 2 O 3 mixture, middle Cr 2 O 3 -TiO 2 mixture, and inner amorphous Al 2 O 3. With increased E ff , the passive film became compact and had better crystallinity. The corrosion mechanism of the alloy under potentiostatic condition was discussed. • Passive films were potentiostatically prepared on a Ni16Cr13Co4Mo alloy in NaCl-Na 2 SO 4 eutectic. • The diffusivity (D 0) of point defects in the passive film was around 10−14 cm2/s. • Passive films were composed of outer Na 2 CrO 4 -Cr 2 O 3 , middle Cr 2 O 3 -TiO 2 , and inner amorphous Al 2 O 3. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) technology realized the surgical personalized devices. However, LPBF-produced titanium alloys face several limitations, such as obvious metastable pits. This work presented a promising strategy to enhance the corrosion resistance of LPBF-produced Ti6Al4V alloy by the Cu alloying. Compared to the Ti6Al4V alloy, Ti6Al4V-Cu alloy possessed a more protective passive film with stable oxides due to its finer grains. Furthermore, Ti6Al4V-Cu alloy demonstrated superior metastable pitting corrosion resistance in a 3.5 wt% NaCl solution, because Cu substituents effectively consumed the cationic vacancies in the passive film, thus inhibited the nucleation of the metastable pits. [Display omitted] • LPBF-produced Ti6Al4V-Cu alloy had finer grains to promote passive film formation. • Nucleation of metastable pits was inhibited in the passive film of Ti6Al4V-Cu alloy. • Cu addition enhanced metastable pitting corrosion resistance of LPBF-produced Ti6Al4V-Cu alloy. [ABSTRACT FROM AUTHOR]

The present work investigates the corrosion resistance of as-cast FeCoCrNiMn high-entropy alloy in borate buffer solution. Compared with 304 and 316L stainless steels steel, as-cast FeCoCrNiMn high-entropy alloy exhibited excellent passivation ability. This should be attributed to homogenization of alloying elements. Highly corrosion-resistant elements in as-cast FeCoCrNiMn alloys, for example, Cr, Co and Ni, could promote the formation of protective passive film in borate buffer solution. Moreover, combined effect of high mixing entropy and sluggish diffusion also could improve the corrosion resistance. The formation mechanism of the passive films on the surface of as-cast FeCoCrNiMn alloys satisfied the point defect model.

Abstract In this work, the passivation behavior of Nb in acid fluoride-containing solutions with pH of 4, 5 and 6 was investigated. Both electrochemical measurements and surface characterization were systematically used to explore the pH effect on the electrochemical stability of Nb in aggressive oral cavity environment. The point defect model (PDM) and surface charge approach (SCA) were used as theoretical basis to analyze the passive film growth and dissolution kinetics. As the pH value decreases, the passive film becomes more reactive and suffers more severe dissolution, leading to higher point defect density across the passive film and more pronounced surface charge behavior at the film/solution interface. Furthermore, the surface becomes rougher and the passive film is thinner with less amount of suboxide as the solution is more acid. Since high electrochemical stability of the passive films is desirable for Nb implants, the results from this work shed insight into the use of fluoride-containing prophylactic substances. Highlights • pH dependent passivation behavior of Nb in acid F−‒containing is examined. • The PDM and SCA are used to analyze the film growth and dissolution kinetics. • The pH effect is quantitatively characterized with kinetics parameters. • The decrease of pH promotes the dissolution reactivity of the passive film. • The decrease of pH leads to a rougher and thinner passive film with less suboxide. [ABSTRACT FROM AUTHOR]

In this paper, the passivation kinetics of AA2098−T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098−T851 in NaHCO3 solution in a CO2 atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler−Volmer (B−V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface.

Prediction of the accumulated pitting corrosion damage in aluminum-lithium (Al-Li) is of great importance due to the wide application of these alloys in the aerospace industry. The Point Defect Model (PDM) is arguably one of the most well-developed techniques for evaluating the electrochemical behavior of passive metals. In this paper, the passivity breakdown and pitting corrosion performance of AA 2098-T851 was investigated using the PDM with the potentiodynamic polarization (PDP) technique in NaCl solutions at different scan rates, Cl− concentrations and pH. Both the PDM predictions and experiments reveal linear relationships between the critical breakdown potential (Ec) of the alloy and various independent variables, such as a C l − and pH. Optimization of the PDM of the near-normally distributed Ec as measured in at least 20 replicate experiments under each set of conditions, allowing for the estimation of some of the critical parameters on barrier layer generation and dissolution, such as the critical areal concentration of condensed cation vacancies (ξ) at the metal/barrier layer interface and the mean diffusivity of the cation vacancy in the barrier layer (D). With these values obtained—using PDM optimization—in one set of conditions, the Ec distribution can be predicted for any other set of conditions (combinations of a Cl − , pH and T). The PDM predictions and experimental observations in this work are in close agreement.

In the present research, the passive and electrochemical responses of pure copper, heavily deformed by equal channel angular pressing (ECAP), in a phosphate buffer solution (pH = 10.69) were investigated. For this purpose, various electrochemical tests such as potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and Mott–Schottky (M–S) analysis were carried out. Also, the influences of ECAP on distribution of the plastic strain and maximum principal stress were analyzed using a three-dimensional (3D) simulation by finite element methods (FEM). According to the field emission scanning electron microscope (FESEM) micrograph, by increasing the ECAP passes, finer microstructure was obtained. Considering all the electrochemical tests results, it can be concluded that the passive and electrochemical responses of pure copper are improved under influence of ECAP process, mainly due to the formation of thicker and less defective passive film. [ABSTRACT FROM AUTHOR]

The corrosion resistance of passive film is investigated by in situ scanning electrochemical microscopy (SECM), and is also diagnosed by modified point defect model (PDM). Thus the constant rate related to defect reactions present in the passive film, can be obtained. SECM results indicate that both compressed and tensile stress decreases the stability of passive film, leading to an increase in surface reactivity and dissolution rate-which is possibly due to the decreased binding force between atoms caused by the stress and therefore make them more easy to dissolve. PDM results indicate that the reason of the decreased stability is an increase in the rate constant of defect reactions present in the passive layer, including transportation rate of oxygen and cation dissolution rate. [ABSTRACT FROM AUTHOR]

The effect of sulfate on the passivity of oxygen free phosphorus (OFP) copper in simulated, anaerobic geological saline solutions was investigated by DC polarization and electrochemical impedance spectroscopy (EIS). The dominant point defect within the passive film is the cation vacancy as identified by Point Defect Model (PDM) coupled EIS optimization (PEO) and Mott-Schottky analysis. Kinetic parameters for the generation/annihilation of point defects were extracted by PEO. Sulfate is found to induce passivity breakdown via facilitating the ejection of cation vacancies and to catalyze the dissolution of the barrier layer leading to a thinning of the layer and a larger passive dissolution current density. [ABSTRACT FROM AUTHOR]

Corrosion has become a decisive factor hindering supercritical water technologies' long-cycle, safe implementation. Electrochemical experiments performed in supercritical aqueous systems can capture essential, real-time information about the micro/nanoscale processes of corrosion and their kinetics. As a starting point, this paper first summarizes the critical chemical properties of supercritical aqueous systems and analyzes their role in determining the duality of corrosion mechanisms (whether electrochemical or chemical micro-processes predominate). Next, it reviews the available electrochemical techniques such as pH sensors, electrochemical noise measurements, and related applications for quantitatively elaborating the corrosion issues in supercritical aqueous systems. With thick and heavy colors, a series of mechanistic models, such as the mixed potential model and the coupled environment models were introduced from the fundamental theories to the related applications, which have been applied widely in ambient and subcritical aqueous systems and exhibit great values and potential to be employed in high-density supercritical water environments. The passivity, depassivation, and passivity breakdown of metals and alloys in terms of the Point Defect Model (PDM) were also successfully implemented and discussed. The newly built SCW_PDM describes the film growth kinetics at the atomic level of metals and alloys in supercritical water and is being developed to quantitatively analyze and resolve a series of corrosion issues such as the passivity breakdown and the stress corrosion cracking that occur in supercritical aqueous systems. This review is helpful by promoting the development of the electrochemical corrosion theory in supercritical aqueous systems and by identifying corrosion prevention and control methods for the safe implementation of relevant supercritical water technologies. [Display omitted] • Electrochemical techniques for corrosion testing/monitoring in SCW environments. • Mechanistic models for describing electrochemical corrosion. • Point defect models for the passivity, depassivation, and passivity breakdown. • Film growth kinetics at the atomic level of alloys in SCW. [ABSTRACT FROM AUTHOR]

The pitting behavior of copper in aqueous chloride-containing environments was investigated by the doping of the P-element and the data was interpreted using a point defect model (PDM). Cyclic polarization tests investigated the passivation breakdown behavior of copper and phosphorus-doped copper, with phosphorus-doped copper exhibiting a higher pitting potential. And the cumulative distribution of breakdown potentials for both copper and phosphorus-doped copper is close to normal, demonstrating the accuracy of the PDM predictions. The scanning electron microscope observed the microscopic morphology of the passivated film surface and the XPS analyzed the composition of the passivated film. • Both copper and phosphorus-doped copper exhibited significant pitting behavior in deep geological disposal environments. • Phosphorus-doped copper exhibited higher pitting potential and better corrosion resistance under different environments (pH). • Under the same conditions, the passivation films on the surface of phosphorus-doped copper were denser, with a higher content of Cu 2 O. • The pitting corrosion behavior of copper reduced by phosphorus doping was explained according to the Point Defect Model. • The phosphorus doping has been shown to reduce copper pitting by calculating the flux of cation vacancies in copper passivation films. [ABSTRACT FROM AUTHOR]

Point Defect Model (PDM) has been successfully employed for aqueous solutions to describe the rate of surface corrosion of structural materials. In this work, the adaptation of PDM for the case of lead–bismuth coolant is developed. The necessary modifications in the set of electrochemical reactions and in the model of electrostatic potential distribution in the oxide film are made. Parameters of the proposed model, PDM_PB, are optimized using the experimental data for T91 grade steel. The obtained adaptation of PDM shows applicability to the case of lead and lead–bismuth coolants which is of critical importance for the next-generation fast-neutron reactors. • A Point Defect Model for steel oxidation by a heavy liquid metal coolant. • A set of interfacial reactions specific to the case of a liquid metal coolant. • Model of electrostatic potential distribution in the oxide layer. • Optimization of PDM parameters for T91 grade steel in contact with LBE. • Model verification via temperature dependence of the oxidation kinetics. [ABSTRACT FROM AUTHOR]

This work aims to characterize the influence of temperature and chloride solution concentration on the corrosion behavior of a newly developed high-entropy alloy system (Al 2 Cr 5 Cu 5 Fe 53 Ni 35) in a simulated marine environment. A direct correlation was found between temperature and chloride concentration with localized corrosion resistance. The Point Defect Model approach is employed to analyze the influence of the temperature and chloride concentration upon the properties of the passive film formation over the alloy surface in seawater solutions. It turns out that the present system alloy exhibits a better local corrosion resistance than conventional martensitic stainless steel UNS S40300 when temperature is below 60 ∘C in similar electrolytic environment. • A new high entropy alloy system is proposed for marine applications with promising anti-corrosive characteristics. • Electrochemical experiments show the localized corrosion resistance of the alloy is undermined by higher concentration of chloride in the solution. • Likewise, the resistance to local pitting formation also decreases with increasing temperature. • The stability of the passive film is strongly affected by the above factors. [ABSTRACT FROM AUTHOR]

The thermodynamic character and dynamic behavior of vacancies in a passivated copper surface were investigated by calculations and experiments, and the data were interpreted in terms of the Point Defect Model (PDM). Both the experimental and computational results show that the principal vacancy in the passive film on copper formed anodically in 0.1 M NaOH solution is the copper vacancy and the diffusion coefficient of the copper vacancy is of the order of 10 − 17 cm 2 /s. [ABSTRACT FROM AUTHOR]

The electronic properties of the passive film formed on pure chromium in an acidic sulfate medium at 30 °C was studied by using the multi-frequency Mott–Schottky approach and the point defect model. Conversely to common Mott–Schottky experiments, the relevance of the multi-frequency Mott–Schottky analysis relies on the determination of the space charge capacitance from a large frequency range impedance diagram. To analyze the Constant-Phase-Element behavior commonly observed for the impedance diagrams which characterize the passive film, the power-law model was used and allowed to determine the space charge capacitance. The consistency of the results showed that it was relevant to combine the power-law model to the predictive point defect model. The aim of this study is to extract the semi-conductive parameters of a passive film from impedance measurements using a sensitive analytical method. [ABSTRACT FROM AUTHOR]

This paper describes an approach for thermodynamically consistent modeling of perovskite redox cycles for thermochemical energy storage and chemical-looping combustion applications. Prior modeling approaches to perovskite redox processes do not provide exact closure of the energy balance or thermodynamic consistency for calculating enthalpies and exergies of multiphase solid–gas flows needed in system-level process analysis. The approach documented here implements solid species thermodynamic functions derived from the enthalpies and entropies of reactions including oxidation/reduction and additional point defect reactions. The approach is fundamentally different than the typical approach of using partial molar properties to perform process flow modeling. Coupling process flow modeling to point defect reactions captures complex trends observed for oxygen non-stoichiometry and varying specific heat capacities during reduction and oxidation, maintains thermodynamic consistency between the solid and gas phase species, and thereby enables modeling of flow conservation equations for both the solid and gas phases. The thermodynamic model is fit to reported measurements for a highly reducible perovskite, strontium-doped calcium manganite (Ca 0.6 Sr 0.4 MnO 3− δ ), and then demonstrated through equilibrium thermodynamic calculations in process energy and exergy balances. Energetic and exergetic analyses for Ca 0.6 Sr 0.4 MnO 3− δ redox cycles are presented for thermochemical energy storage and chemical-looping combustion systems where component exergy destructions are calculated. The model predicts roundtrip thermochemical energy storage efficiencies with Ca 0.6 Sr 0.4 MnO 3− δ as high as 89% and 63% by first and second law analyses, respectively. The chemical-looping combustion of methane using Ca 0.6 Sr 0.4 MnO 3− δ indicates first and second law efficiencies up to 90% and 66%, respectively. The modeling approach is used to explore trends in performance with operating conditions for both redox cycles thereby enabling new insight regarding design trade-offs for these emerging energy storage and conversion cycles. [ABSTRACT FROM AUTHOR]

Diagnostic criteria for growth of the anodic oxide film on platinum in KOH are reported. In this work, the analytical analysis of oxide film growth demonstrated that the electric field in the passive film formed anodically on platinum in KOH is constant, independent of the applied voltage and barrier layer thickness. This criterion intrinsically distinguishes the Point Defect Model (PDM) from the High Field Model (HFM). Unequivocally, the PDM provides a superior theoretical framework than does the HFM for interpreting oxide film growth on platinum. Importantly, we argue that the diagnostic criteria also apply to metal interstitial conduction, which is the mechanism proposed in the HFM for formation of the PtO oxide film on platinum, but with film growth occurring at the film/solution interface and with a thickness-dependent electric field, rather than being restricted to oxygen vacancy conductors alone, as originally derived for the PDM. Thus, the ability of the diagnostic criteria to differentiate between the HFM and the PDM, in this case, is a direct assessment of the dependence of the electric field strength on the applied voltage and barrier layer thickness, with the experimental results coming down unequivocally on the side of the PDM. [ABSTRACT FROM AUTHOR]