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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]

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]

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]

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.

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 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 plastic deformation of Ti can be divided into three stages. • The diffusivity (D 0) of the point defect increases with larger plastic strain. • The relationship between D 0 and the stress corrosion cracking of Ti is discussed. The influence of plastic deformation on the diffusivity (D 0) of the point defect in the passive film on Ti in 3.5% NaCl solution was investigated. Electrochemcial measurements including chronoamperometry measurement, electrochemical impedance spectroscopy and Mott-Schottky plots were performed, and the passive films were characterized by the Auger electron spectroscopy. The values of D 0 were calculated based on the point defect model. The results shown that the passive film exhibited the n-type semiconductive property at different conditions. With the increased plastic deformation, the donor density and D 0 increased, and the passive film became more defective. The relationship between D 0 and stress corrosion cracking of Ti was discussed. [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. [ABSTRACT FROM AUTHOR]

The passive corrosion properties of a series of Ni–Cr–Mo alloys were investigated. The alloys studied were C22, C2000, C276, C4 and 625. Potentiostatic experiments at potentials within the passive range were obtained as a function of temperature (25–85 °C) for each alloy. Each specimen was subsequently analyzed by time-of-flight secondary ion mass spectrometry (TOF SIMS) and X-ray photoelectron spectroscopy (XPS). Results indicated that much lower passive dissolution currents, and a much slower attainment of steady-state conditions, were observed on those alloys with >20 wt.% Cr content. These alloys also consistently showed only a minor temperature dependence of the passive current. The surface analyses showed that the high-Cr alloys were able to build thicker oxides with a layered structure consisting of an inner Cr–Ni oxide layer and an outer Mo/Cr oxide. By contrast, such a high-Cr content inner layer and structured elemental distribution were not achieved with alloys having a lower bulk Cr content.For low potentials (200 mV), when Cr dissolution can only occur as Cr(III), additional alloying elements, specifically Mo and W, exert little influence on passive current densities. At 500 mV, a potential at which Cr(VI) release appears to start, the presence of Mo, and especially W, in the outer regions of the oxide suppresses passive dissolution. This may be due to the low dissolution rate of the W-containing surface oxide layer, but some influence of W on the defect density within the oxide cannot be ruled out. The observed temperature dependence for high-Cr alloys (C22, C2000) reflects the lack of steady state and cannot be interpreted as an accurate activation energy. [Copyright &y& Elsevier]

• The chloride threshold for rebar in concrete pore solution has been calculated using the Point Defect Model and Mixed Potential Model. • Expressions have been derived for interconverting different definitions of CT. • CT are scattered over wide ranges, arising from inherent error in the breakdown voltage and the ECP. • The accuracy of CT is limited by fundamental theory- and determined by the standard deviation of cation vacancy diffusivity in the population of available breakdown sites. • The PDM/MPM theory yields CT values that are in good agreement with those reported experimentally. A theory is developed for calculating the chloride threshold (CT) from the Point Defect Model (PDM). CT is defined as the ratio [Cl−/[OH− at which the passivity breakdown voltage (E b) equals the corrosion potential, ECP. CT is sensitive to the T , ECP, pH, and E b , because all four appear in the exponent X in CT = 10X. Small uncertainties in these parameters translate into a large uncertainty in CT, emphasizing the need for accurate T , ECP, pH, and E b data. We demonstrate the theory by successfully calculating CT using data for selected systems. [ABSTRACT FROM AUTHOR]