Point Defect Model Research Articles

Mechanistic Analysis of Anodic Oxidation of Gold in KOH (0.1 M) Solution Using the Point Defect Model

The potentiostatic, anodic formation of gold oxide at potentials of 0.55 to 0.80 V versus SHE in aqueous KOH (0.1 M) was studied using an impedance-based Point Defect Model (PDM). The film thickness and refractive indices at each formation potential were estimated using spectroscopic ellipsometry. The thickness of the oxide increases linearly with increasing applied voltage within this range. Mott-Schottky (MS) analysis showed that gold oxide formed in KOH (0.1 M) is an n-type semiconductor, and the dominant defect (Aui3+) density is calculated to be in the order of 1021–1022 (1/cm3). The steady-state current density of the oxide formation was independent of voltage, also in agreement with an n-type oxide. Reasonable agreement between PDM predictions and experimental observations of dominant defect density, steady-state current density, and thickness, demonstrates the value of the PDM in this system.

On corrosion and passivity of Inconel alloy 625 in HNO3 solution

Exploring the passivity and electrochemical behavior of Inconel alloy 625 in nitric acid solution could open up possibilities for potential applications, either in bulk form or as claddings. Hence, this research investigates corrosion behavior and passivity of Inconel alloy 625 in nitric acid solutions of varying concentrations (0.01–1.00 mol/L) using a range of analytical techniques, including potentiodynamic polarization testing, chronoamperometry measurement, Mott-Schottky (MS) analysis, X-ray diffraction (XRD) approaches, and point defect modeling (PDM). Results demonstrate that corrosion current density does not necessarily decrease with increasing electrolyte concentration (Please check it out). Additionally, the alloy exhibits higher passive film transpassive potentials in more concentrated solutions. Potentiostatic polarization tests reveal an increase in steady-state current densities attributed to the passive films formed in more concentrated solutions. Lower electrolyte concentrations lead to the formation of more intact bilayered passive films. In this context, the formation of passive films is elucidated by examining the generation and annihilation of point defects. The semiconducting behavior of passive layers is further examined through XRD patterns and electrochemical properties. According to PDM, the diffusivity of point defects within the passive films and their respective donor densities can be closely correlated.

Study on growth kinetics of passivation film of Ti-6Al-3Nb-2Zr-1Mo alloy Based on point defect model

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.

Unraveling the passive film characteristic and pitting mechanism of the 7xxx high-strength Al alloy with different microstructures: Experimental and FEM simulation

The pitting mechanism of 7 series high-strength aluminum (Al) alloys (7xxx) is investigated experimentally and numerically (finite element method, FEM) by comparing the characteristics of passive films and pitting kinetics of two types of 7xxx Al alloys. The point defect model (PDM) is used to evaluate the passive film properties in the two varieties of 7xxx Al alloy, and the FEM is employed to calculate the point defect diffusion process in detail. The results reveal that the two 7xxx Al alloys exhibit various microstructures, particularly the 7xxx-1, characterized by a robust (111) fiber texture and a lower proportion of low-angle grain boundaries than the 7xxx-2. The diffusivity D of point defects in 7xxx-1 (3.85 × 10−18–52.74 × 10−18 cm2 s−1) is lower than that in 7xxx-2 (5.47 × 10−18–66.61 × 10−18 cm2 s−1) and the diffusion duration of 7xxx-1 is longer than 7xxx-2, which is closely associated with the rupture of passive film. Additionally, FEM proved that the initial shapes of pits dictate the progression of pitting, manifesting that the shallow dish shape grows laterally while the deep hole shape grows longitudinally.

Understanding copper corrosion in bentonite-enriched environments: Insights from the point defect model

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 VSCE. 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.

Semiconductive Tendency of the Passive Film Formed on Super Austenitic Stainless Steel SR-50A in Acidic or Alkaline Chloride Solutions

Stainless steel is widely used in various industrial fields due to its excellent corrosion resistance and mechanical properties. The key to this corrosion resistance is the thin passive film that naturally forms on the metal surface. Passive films are characterized by oxide film theory and adsorption theory, each uniquely explaining the structure and mechanism of the protective film on the metal surface. Research on the semiconductive properties of passive films on stainless steel offers diverse viewpoints, classifying theories into the point defect model and the bipolar fixed charge-induced passivity. Specific changes in passive film attributes that lead to degradation, however, are not fully understood. In this study, we analyzed the inner and outer layers of the passive film on super austenitic stainless steel SR-50A under various conditions in acidic and alkaline chloride environments. The interpretations of these results were based on the point defect model and the bipolar model for the passivation mechanism, and correlations between p-type and n-type semiconductor properties and passivation behavior were examined. The surface of the stainless steel forms a passive film comprising two layers with p-type and n-type semiconductive properties, independent of the pH of the solutions. The corrosion resistance increases as the p-type and n-type semiconductive tendencies become more balanced, consequently enhancing the properties of the passive film.

Scalable nano-crystallization approach over multiple passivation behavior for Fe-based amorphous coating under chloride exposure

Structure-aware complicated Fe-based amorphous coating induced by scalable nano-crystallization approach proved influential in this work. The integration of experimental and mathematical findings uncovered that the dual nanocrystalline phases, interacting within the amorphous matrix of the coating subjected to annealing at 893 K, demonstrated the most superior electrochemical performance against chloride ion attack. At this singularity temperature, compositional intricacies were strengthened by structural relaxation, a consequence of the compatible collaboration of chemical constituents. The categorization of the vacancy-triggered point defects model (VR-PDM) indicated that the involvement and subsequent collapse of oxygen vacancies played a pivotal role in governing the multifaceted passivation behavior. A circulation of film spatial extension evaluated by d′-parameter in assistance with diffusion coefficients D of oxygen vacancies provided the direct evidence that reliable nano-crystallization strategy can reach a critical safety point against corrosion degradation. The total adaptive contribution of passivity to corrosion attack can be beneficial from the precise segmentation of surficial positions superimposed by target metallic atoms in competition with Cl- ions.

Ab Initio Materials Modeling of Point Defects in a High-κ Metal Gate Stack of Scaled CMOS Devices: Variability Versus Engineering the Effective Work Function

Ab Initio Materials Modeling of Point Defects in a High-κ Metal Gate Stack of Scaled CMOS Devices: Variability Versus Engineering the Effective Work Function

Application of the Point Defect Model to the Oscillatory Anodic Oxidation of Illuminated n-Type Silicon in the Presence of Fluoride Ions Using Electrochemical Impedance Spectroscopy

This work aims to provide insight into the oscillations occurring during the anodic electrooxidation of Si in fluoride-containing electrolytes using electrochemical impedance spectroscopy (EIS). The EIS measurements were conducted within less than a tenth of the oscillation periods allowing changes in the electrical properties of the silicon/oxide/electrolyte interfaces to be monitored during an oscillatory cycle. Application of the power law model to the experimental data revealed a significant change in resistivity at the oxide/semiconductor interface while the properties at the oxide/electrolyte interface remained constant and the oxide layer varied only by about 1 nm around an average value of about 4.9 nm. The application of the point defect model to the semiconductor/oxide/F−-containing electrolyte interface suggests that the oscillations are linked to the time delay between the production of oxygen vacancies at the Si/oxide interface and their consumption at the oxide/electrolyte interface.

Study on the passivation properties of austenitic stainless steel 316LN based on the point defect model

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 αi and rate constants ki 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.

Influence of Grain Size and Film Formation Potential on the Diffusivity of Point Defects in the Passive Film of Pure Aluminum in NaCl Solution

The influence of grain size on the corrosion behavior of pure aluminum and the defect density and diffusion coefficient of surface passive films were investigated using electron backscatter diffraction (EBSD) and electrochemical testing techniques, based on the point defect model (PDM). Samples with three different grain sizes (23 ± 11, 134 ± 52, and 462 ± 203 μm) were obtained by annealing at different temperatures and times. The polarization curves and electrochemical impedance spectroscopy results for the pure aluminum in the 3.5% NaCl solution showed that with decreasing grain size, the corrosion current (icorr) decreased monotonously, giving rise to a noble corrosion potential and a large polarization resistance. The Motte–Schottky results showed that the passive films that formed on pure aluminum with fine grains of 23 ± 11 μm had a low density (3.82 × 1020 cm−3) of point defects, such as oxygen vacancies and/or metal interstitials, and a small diffusion coefficient (1.94 × 10−17 cm2/s). The influence of grain size on corrosion resistance was discussed. This work demonstrated that grain refinement could be an effective approach to achieving high corrosion resistance of passive metals.

Electrochemical insight into the passivity and corrosion of 316 L stainless steel fabricated through wire arc additive manufacturing

This study explores the microstructure of AISI 316L stainless steel fabricated by wire arc additive manufacturing (WAAM) and establishes correlations with its passivity and corrosion characteristics in a 0.9 wt% NaCl solution, while making comparisons with the wrought alloy counterpart. The WAAM-fabricated alloy demonstrates a favorable chemical composition in comparison to the wrought alloy, exhibiting a heterogeneous microstructure comprised of residual delta ferrite within the austenitic matrix. The pitting vulnerability of the WAAM-printed alloy is observed to be lower than that of the wrought counterpart, a phenomenon further investigated through passive film behavior analysis. Mott-Schottky analysis and the point defect model (PDM) revealed the development of a less defective passive layer on the WAAM-fabricated alloy, enhancing overall passivity and electrochemical response, attributed to the interplay between microstructural features and chemical composition.

Dynamics of early-stage oxide formation on a Ni-Cr-Mo alloy

Corrosion results in large costs and environmental impact but can be controlled by thin oxide films that passivate the metal surfaces and hinder further oxidation or dissolution in an aqueous environment. The structure, chemistry, and thickness of these oxide films play a significant role in determining their anti-corrosion properties and the early-stage oxidation dynamics affect the properties of the developed oxide. Here, we use in situ X-ray Photoelectron Spectroscopy (XPS) to study the early-stage oxidation of a Ni-Cr-Mo alloy at room temperature and up to 400 °C. Cr and Mo begin to oxidize immediately after exposure to O2, and Cr3+, Mo4+, and Mo6+ oxides are formed. In contrast, Ni does not contribute significantly to the oxide film. A self-limiting oxide thickness, which did not depend on temperature below 400 °C, is observed. This is attributed to the consumption of available Cr and Mo near the surface, which results in an enrichment of metallic Ni under the oxide. The self-limited oxide thickness is 6–8 Å, which corresponds to 3–4 atomic layers of cations in the oxide. At 400 °C, sublimation of Mo6+ oxide is observed, resulting in the formation of an almost pure layer of Cr2O3 on the alloy surface. Lastly, a mechanism is presented that explains the formation of the bi-layer oxide structure observed for Ni-Cr-Mo alloys, which involves the enhanced migration of hexavalent Mo ions in the electric field, which drives mass transport during oxidation according to both the Cabrera Mott model and the Point Defect Model.

Theoretical explanation of passivation behavior in OFP-Cu and OF-Cu

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.

Estimation of Some Parameters in the Point Defect Model (PDM) for the Passivity of Metals

In this paper, we justify that the potential drop at the barrier layer/solution (bl/s) interface of the barrier layer of the passive film on a metal, φf/s, is a linear function of the applied voltage V and pH in the case of sufficiently thin barrier layer. Furthermore, we show that a relationship exists between parameters α (the dependence of φf/s on V) and β (the dependence of φf/s on pH), that is, those parameters are not independent, in the general case. It is also shown that some dependence of parameters α and β on temperature can exist. However, this dependence in the practical region (0 °C–100 °C) is weak and, in many cases, it is possible to neglect the dependence of parameters α and β on temperature. Finally, we show that it is possible to estimate, theoretically, the parameter β (and accordingly parameter α) by using models for specific systems. For the system considered in the current report (carbon steel in concrete pore water), the estimated value of parameter β is confirmed experimentally. These findings are discussed in terms of the electronic and point defect structures of the barrier layer.

Corrosion resistance of electroplated coatings based on chromium trivalent-baths

Cr-C coatings were obtained from two different Cr(III)-baths based on sulphate and chloride salts, respectively. A tailored characterization was carried out by scanning electron microscopy, several spectroscopies and Mott-Schottky approaches (including Point Defect Model) to find out the impact in the corrosion performance of the morphology, composition, and semiconducting properties of the native oxide layers. The polarisation curves using 0.01 M NaCl have shown better corrosion resistance capabilities for the chloride-based Cr(III) coatings which is in agreement with the lower crystal size and the less defective native oxide layer (using a borate buffer electrolyte).

Investigation of the passive film of nanocrystalline 304 stainless steel in 3.5 wt% NaCl solution under hydrostatic pressure

In this paper, the effect of hydrostatic pressure on the passive film of coarse crystalline 304 stainless steel (304 SS) and sputtered nanocrystalline stainless steel (304 NC) in 3.5 wt% NaCl solution has been investigated using in-situ electrochemical measurement techniques. The results indicate that nanosizing promotes the growth of the passive film. Hydrostatic pressure changes the nucleation mechanism of the passive film, decreases the carrier density and oxygen vacancy diffusivity, and decreases the content of oxides and hydroxides within the film, significantly deteriorating the corrosion resistance of the passive film. The point defect model (PDM) was introduced to elaborate the microscopic process of the surface passivation of stainless steel under hydrostatic pressure.

Characterization Using Point Defect Theory of the Microstructure Effect on Passivity Stability in Austenitic Stainless Steel

The influence of the microstructure on the resistance to pitting potential in austenitic stainless steel UNS S31603 is evaluated to explain the effect of the distribution of features such as carbides. Different microstructures were obtained by processing, via surface laser melting (SLM) and sensitized at 600°C, 700°C, and 800°C. The test solution used for electrochemical testing included a buffer pH 8 brine at room conditions. The characterization of the passive conditions is done by using potentiodynamic, potentiostatic, and Mott Schottky techniques. The results show that the UNS S31603 samples produced by SLM have higher passive layer stability. The correlation with the microstructural features attributes this to a lack of inclusions and carbides characteristic of the SLM process. The analysis of the experimental results using the point defect model description of the passive layer behavior indicates that the stability of the passive layer is a priori inversely proportional to both the metal cation and anion vacancy diffusivities. Experiments reveal the close dependence and explain the properties of the passive layer with respect to a point defect model.

Computer Simulation of Pitting Corrosion in Galvanostatic Conditions

Pitting corrosion of stainless steels has been the subject of substantial research over many years. The overall mechanism can be separated into nucleation and propagation stages, and the development of a reliable predictive model requires a robust treatment for both these processes. Over the years, several purely stochastic models have been developed, including those by Shibata and Takeyama [1], Williams et al. [2], Baroux [3] and Wu et al. [4]. Alternatively, Macdonald and co-workers [e.g., 5] have focused on a deterministic approach, based on the point defect model of passivity breakdown. Newman, Laycock and co-workers developed a deterministic model for the propagation of individual corrosion pits [6-8], which was then combined with a stochastic model of pit nucleation to enable simulation of pitting potential measurements [9]. Li, Scully and Frankel later published a series of papers based on a similar approach [e.g., 10-11].The majority of the prior modelling work has focused on potential-controlled conditions, where the pitting outcomes are determined mainly by the pit propagation element; for example, a limiting lower bound distribution of the pitting potential can be calculated without any consideration of pit nucleation processes [9]. However, real corrosion does not occur under potential control; rather, there is a limited supply of cathodic current that must be shared between all simultaneously propagating pits [12,13]. This situation is closer to that of experiments under galvanostatic control [14]. Krouse et al [15] described simulations that included possible interactions between multiple simultaneously propagating pits under galvanostatic conditions, supporting earlier suggestions that pits compete for the available current, and that ‘champion pits’ will ultimately use all available resources (see, e.g., Figure 1).In more recent work [16], we have further developed the earlier propagation model [6-9] to include the interactions and possible mergers between two simultaneously propagating pits. Here we expand on the work of Krouse et al [15] to carry out simulations of galvanostatic experiments that now incorporate the possibility of mergers between propagating pits. References Shibata, T. Takeyama, Corrosion 33 (1997) 243.E. Williams, C. Westcott, M. Fleischmann, J. Electrochem. Soc. 132 (1985) 1796.Baroux, Corros. Sci. 28 (1988) 969.Wu, J.R. Scully, J.L. Hudson, A.S. Mikhailov, J. Electrochem. Soc. 144 (1997) 1614.Engelhardt, D.D. Macdonald, Corrosion 54 (1998) 469.Ernst, N.J. Laycock, M.H. Moayed, R.C. Newman, Corros. Sci. 39 (1997) 1133.J. Laycock, S.P. White, J.S. Noh, P.T. Wilson, R.C. Newman, J. Electrochem. Soc. 145 (1998) 1101.J. Laycock, S.P. White, J. Electrochem. Soc. 148 (2001) B264.J. Laycock, J.S. Noh, S.P. White and D.P. Krouse, Corros. Sci, 47, 3140 (2005).S. Frankel, T. Li, and J. R. Scully, Journal of the Electrochemical Society, 164, C180 (2017).Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165, C484 (2018).Y. Chen, F. Cui and R.G. Kelly, J. Electrochem. Soc., 155, C360-C368 (2008).Y. Chen and R.G. Kelly, J. Electrochem. Soc., 157, C69 (2010).I. Suleiman and R. C. Newman, Corros. Sci., 36, 1657 (1994).Krouse, P. McGavin and N. Laycock, in Proceedings of Corrosion & Prevention 2008, Paper # 97, ACA, Wellington, 16-19 November (2008).A Nguyen, R.C. Newman and N.J. Laycock, J. Electrochem. Soc., 169, 081503 (2022) Figure 1

(Corrosion Division Morris Cohen Graduate Student Award) Evolution of Passivity and Passivity-Breakdown for Cr and Cr-Containing Alloys

Among several corrosion prevention strategies for metallic alloys, alloying with so-called ‘passivating’ elements remains the most effective approach. For example, chromium (Cr) remains the key functional alloying element for Fe-based stainless steels, Ni-based Inconel® alloys and Co-based Stellite® alloys, which dominate the family of corrosion resistant alloys (CRAs) used in a wide range of applications. In addition to conventional alloys, recent exploration in the materials community has led to development of an emerging class of alloys, so-called high entropy alloys (known more broadly as multi-principal element alloys (MPEAs)), which contain multiple alloying elements in ‘principal’ fractions – and of which the corrosion performance has not been widely studied to date. It is however challenging to ascertain the key controlling factors responsible for the passivity in MPEAs using conventional electrochemical techniques due to the combination of low sensitivity and the lack of element resolved information. Moreover, the physical and chemical properties of the nano-metric thin passive film upon CRAs are challenging to evaluate using conventional microscopy and spectroscopy. Consequently, several fundamental aspects of passivity of Cr-containing CRAs remain open questions to be elucidated.The work herein sought to investigate the passivity and dissolution of Cr and Cr-containing alloys, including stainless steel and several MPEAs; primarily using an in-situ electrochemical method called atomic-spectroelectrochemistry (ASEC). ASEC uses a modified flow electrochemical cell coupled with an atomic spectrometer, revealing the real-time and element-resolved dissolution rate during electrochemical testing. In addition to ASEC analysis, the project herein employed a range of electrochemical testing, electron microscopy, X-ray photoelectron spectroscopy, and thermodynamic calculations to explore Cr-based passivity.Significance between composition and growth kinetics on the passivity of Cr and Cr-containing alloys was established. It was found that the passive films upon Cr and Cr-containing alloys (including MPEAs) obey a direct-logarithmic growth law, consistent with the assumptions of the point defect model. In addition, the results unveiled a complex interdependence of oxidation rate of material and dissolution rate of the passive film during anodic polarization, found to be an important parameter to evaluate barrier properties of passive films. The influence of microstructural features including grain boundaries and chemical heterogeneities on Cr-based passivity was explored for stainless steel 316L; whereas the role of alloying elements and electrolytic on the evolution of compositional morphology and thickness of the Cr2O3-based passive film was studied for MPEAs exhibiting single-phase microstructures. The origin of passive film breakdown and role of potential and chloride ions on the breakdown mechanism were elucidated for stainless steel 316L; whereas transpassive dissolution and origin of secondary passivation were studied for equiatomic CoCrFeNi and AlTiVCr MPEAs.Such insights advance the knowledge of underlying mechanisms and associated rate controlling factors of passivity in Cr and Cr-containing alloys, aiding towards an ‘oxide-by-design’ paradigm, to guide design of new CRAs.