Kinetics of electrochemical dissolution of metals in porous media

Aerobic and anaerobic oxidation of ferrous ions in near-neutral solutions

Whilst the oxidation of Fe(II) in aerobic conditions has been studied thoroughly, an in-depth knowhow on the fate or stability of Fe(II) in solutions with near-neutral pH under anaerobic conditions is still lacking. Here, we experimentally investigated the kinetics of Fe(II) oxidation in solutions with pH ranging between ∼5 and 9, under aerobic (when solutions were in equilibrium with atmospheric oxygen) and anaerobic conditions (when the dissolved oxygen concentration was ∼10^-10 mol/L), by colorimetric means. Experimental results and thermodynamic considerations presented here, show that Fe(II) oxidation in anaerobic conditions is first-order w.r.t. [Fe(II)], and proceeds with set of parallel reactions involving different hydrolysed and non-hydrolysed Fe(II) and Fe(III) species, similar to that observed in aerobic conditions. However, in the absence of oxygen, the cathodic reaction accompanying the anodic oxidation of Fe(II), is the reduction of H₂O (l) releasing H₂ (g). Hydrolysed Fe(II) species oxidise much faster than Fe²⁺ and their concentrations increases with pH, leading to increased Fe(II) oxidation rates. Additionally, we also show the importance of the type of buffer used to study Fe(II) oxidation. Therefore, for the oxidation of Fe(II) in near-neutral solutions, the speciation of Fe(II) and Fe(III), the presence of other anions and the pH of the solution are critical parameters that must be considered. We anticipate that our results and hypothesis will find use in reactive-transport models simulating different processes occurring in anaerobic conditions such as corrosion of the steel in concrete structures, or in nuclear waste repositories.

Nanoscale electrochemical charge transfer kinetics investigated by electrochemical scanning microwave microscopy

We show how microwave microscopy can be used to probe local charge transfer reactions with unprecedented sensitivity, visualizing surface reactions with only a few hundred molecules involved. While microwaves are too fast under classical conditions to interact and sense electrochemical processes, this is different at the nanoscale, where our heterodyne microwave sensing method allows for highly sensitive local cyclic voltammetry (LCV) and local electrochemical impedance spectroscopy (LEIS). LCV and LEIS allow for precise measurement of the localized charge transfer kinetics, as illustrated in this study for a ferrocene self-assembled monolayer immersed in an electrolyte. The theoretical analysis presented here enables a consistent mapping of the faradaic kinetics and the parasitic contributions (nonfaradaic) to be spectrally resolved and subtracted. In particular, this methodology reveals an undistorted assessment of accessible redox site density of states associated with faradaic capacitance, fractional surface coverage and electron transfer kinetics at the nanoscale. The developed methodology opens a new perspective on comprehending electrochemical reactivity at the nanoscale.

Challenges in material recycling for postwar reconstruction

Besides the fact that concrete recycling allows to avoid landfills disposal and contributes to a closed-cycle economy, such option may be very much in demand in war struck regions such as Ukraine, which after the end of the war, are faced with the problem of rebuilding and reconstructing. Beyond this emergency, even in peacetime extensive parts of the building stock will sooner or later need to be replaced and concrete recycling is called to play an increasing role there. However, depending on the technology and degree to which aggregates are recycled, concrete may be characterized by poor workability, reduced mechanical properties, increased shrinkage and reduced durability. This deterioration in the properties of recycled concrete is usually attributed to the characteristics of the old cement mortar remaining on the surface of the recycled aggregates, which is best considered as an additional volume of hardened cement paste with fine aggregate and additional porosity. This article attempts to underline how such key concepts help frame the current state of knowledge about concrete recycling, understand the implications of existing regulations, in order to define pragmatic and efficient routes for broadening the use of concrete recycling in war struck regions, with specific examples regarding Ukraine.

Machine learning modeling of predictive external corrosion rates of spent nuclear fuel carbon steel canister in soil

Soil corrosion is always a critical concern to corrosion engineering because of the economic influence of soil infrastructures as has been and has recently been the focus of spent nuclear fuel canisters. Besides corrosion protection, the corrosion prediction of the canister is also important. Advanced knowledge of the corrosion rate of spent nuclear fuel canister material in a particular environment can be extremely helpful in choosing the best protection method. Applying machine learning (ML) to corrosion rate prediction solves all the challenges because of the number of variables affecting soil corrosion. In this study, several algorithms of ML, including series individual, boosting, bagging artificial neural network (ANN), series individual, boosting, bagging Chi-squared automatic interaction detection (CHAID) tree decision, linear regression (LR) and an ensemble learning (EL) merge the best option that collects from 3 algorithm methods above. From the performance of each model to find the model with the highest accuracy is the ensemble stacking method. Mean absolute error performance matrices are shown in Fig. 15. Besides applying ML, the significance of the input variables was also determined through sensitivity analysis using the feature importance criterion, and the carbon steel corrosion rate is the most sensitive to temperature and chloride.

Nanoscale Colocalized Electrochemical and Structural Mapping of Metal Dissolution Reaction

Understanding the structure-activity relationship in electrochemical metal dissolution reactions is fundamentally important, from designing higher density batteries to mitigating corrosions. The kinetics of metal dissolution reaction is highly dependent on surface structures, including grain boundaries and local defects. However, directly probing the electrochemical activity at these sites is difficult because the conventional bulk electrochemistry measures an averaged kinetics, obscuring the structure-activity correlation. Herein, we report the colocalized mapping of an electrochemical metal dissolution reaction using Ag as a model system. The local dissolution kinetics is voltammetrically mapped via scanning electrochemical cell microscopy (SECCM), which is correlated with local structures obtained via colocalized electron backscattering diffraction (EBSD). Individual pits of ∼200 nm are formed, and their geometries suggest dissolution is fastest in the direction parallel to the {111} planes. Enhanced dissolution kinetics is observed at the high-angle grain boundaries but not at twin boundaries, which are attributed to the different binding energy of Ag atoms. Furthermore, the faster local dissolution correlates with the geometrically necessary dislocation density. The work demonstrates the importance of nanoscale local electrochemical mapping and colocalized microscopic measurement in obtaining the structure-activity relationship for electrochemical reactions at complex interfaces.

Should the term 'metallic iron' appear in the title of a research paper?

Over the past three decades, groundwater remediation using permeable reactive barriers (PRBs) has proven to be effective. The majority of installed PRBs uses metallic iron (Fe(0)) as a reactive material. However, the success of implemented Fe(0) PRBs is yet to be rationalized as Fe(0) is a generator of iron oxides (contaminant scavengers) and secondary reducing agents (e.g. Fe(II), Fe₃O₄, H₂, green rust), This communication demonstrates that Fe(0) is not an environmental reducing agent. Therefore, more science-based investigations are needed to optimize the operation of Fe(0) PRBs. In particular, Fe(0) PRBs and Fe(0)-based water filters should be regarded as particular cases of "metal corrosion in porous media". A key feature of such systems is that the extent of Fe⁰ corrosion temporally depends on the residual porosity (capillarity). Thus, the functionality of any Fe⁰ PRB should be monitored in a way that the time-dependent variation of the kinetic of iron corrosion is discussed.

Macrocell Corrosion of Steel in Concrete under Carbonation, Internal Chloride Admixing and Accelerated Chloride Penetration Conditions

Steel corrosion has become the main reason for the deterioration of reinforced concrete structures. Due to the heterogeneity of concrete and the spatial variation of environmental conditions, macrocell corrosion is often formed by localized corrosion, which is more detrimental if the anode is supported by large numbers of cathodes. The macrocell corrosion caused by concrete carbonation has been seldom studied. Furthermore, the influence of geometrical conditions on cathode-controlled corrosion in the chloride environment needs to be further clarified. In this work, the macrocell corrosion of steel embedded in concrete specimens exposed to accelerated carbonation, chloride contamination, and chloride penetration is studied using a modified ASTM G109 method. Concrete specimens with various binder types, geometrical parameters (i.e., concrete cover thickness and the diameter of embedded steel), and boundary conditions were tested. A simplified mathematical model for the prediction of the steel corrosion rate was developed considering two-dimensional oxygen diffusion. The results showed that, at the same level of anodic potential drops, the corrosion current rate in chloride-induced corrosion is higher than that of carbonation-induced corrosion. Chloride contamination is less detrimental to concrete incorporated with slag and pulverized fly ash than it is to pure ordinary Portland cement (OPC), likely due to enhanced chloride binding capacity. The results also indicated that the model considering two-dimensional diffusion can accurately predict the cathodic reaction process on corroded steel bars, which provides a theoretical basis for considering the correction coefficient of steel bar position in the establishment of a steel bar corrosion rate model.

Evaluation of the Influence of the Combination of pH, Chloride, and Sulfate on the Corrosion Behavior of Pipeline Steel in Soil Using Response Surface Methodology

External damage to buried pipelines is mainly caused by corrosive components in soil solution. The reality that numerous agents are present in the corrosive environment simultaneously makes it troublesome to study. To solve that issue, this study aims to determine the influence of the combination of pH, chloride, and sulfate by using a statistical method according to the design of experiment (DOE). Response surface methodology (RSM) using the Box–Behnken design (BBD) was selected and applied to the design matrix for those three factors. The input corrosion current density was evaluated by electrochemical tests under variable conditions given in the design matrix. The output of this method is an equation that calculates the corrosion current density as a function of pH, chloride, and sulfate concentration. The level of influence of each factor on the corrosion current density was investigated and response surface plots, contour plots of each factor were created in this study.

Attoampere Nanoelectrochemistry

Electrochemical microscopy techniques have extended the understanding of surface chemistry to the micrometer and even sub-micrometer level. However, fundamental questions related to charge transport at the solid-electrolyte interface, such as catalytic reactions or operation of individual ion channels, require improved spatial resolutions down to the nanoscale. A prerequisite for single-molecule electrochemical sensitivity is the reliable detection of a few electrons per second, that is, currents in the atto-Ampere (10^-18 A) range, 1000 times below today's electrochemical microscopes. This work reports local cyclic voltammetry (CV) measurements at the solid-liquid interface on ferrocene self-assembled monolayer (SAM) with sub-atto-Ampere sensitivity and simultaneous spatial resolution < 80 nm. Such sensitivity is obtained through measurements of the charging of the local faradaic interface capacitance at GHz frequencies. Nanometer-scale details of different molecular organizations with a 19% packing density difference are resolved, with an extremely small dispersion of the molecular electrical properties. This is predicted previously based on weak electrostatic interactions between neighboring redox molecules in a SAM configuration. These results open new perspectives for nano-electrochemistry like the study of quantum mechanical resonance in complex molecules and a wide range of applications from electrochemical catalysis to biophysics.

Formation of three-dimensional bicontinuous structures via molten salt dealloying studied in real-time by in situ synchrotron X-ray nano-tomography

Three-dimensional bicontinuous porous materials formed by dealloying contribute significantly to various applications including catalysis, sensor development and energy storage. This work studies a method of molten salt dealloying via real-time in situ synchrotron three-dimensional X-ray nano-tomography. Quantification of morphological parameters determined that long-range diffusion is the rate-determining step for the dealloying process. The subsequent coarsening rate was primarily surface diffusion controlled, with Rayleigh instability leading to ligament pinch-off and creating isolated bubbles in ligaments, while bulk diffusion leads to a slight densification. Chemical environments characterized by X-ray absorption near edge structure spectroscopic imaging show that molten salt dealloying prevents surface oxidation of the metal. In this work, gaining a fundamental mechanistic understanding of the molten salt dealloying process in forming porous structures provides a nontoxic, tunable dealloying technique and has important implications for molten salt corrosion processes, which is one of the major challenges in molten salt reactors and concentrated solar power plants.

The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe0/MnO2/sand systems

Metallic iron (Fe0) has shown outstanding performances for water decontamination and its efficiency has been improved by the presence of sand (Fe0/sand) and manganese oxide (Fe0/MnOx). In this study, a ternary Fe0/MnOx/sand system is characterized for its discoloration efficiency of methylene blue (MB) in quiescent batch studies for 7, 18, 25 and 47 days. The objective was to understand the fundamental mechanisms of water treatment in Fe0/H2O systems using MB as an operational tracer of reactivity. The premise was that, in the short term, both MnO2 and sand delay MB discoloration by avoiding the availability of free iron corrosion products (FeCPs). Results clearly demonstrate no monotonous increase in MB discoloration with increasing contact time. As a rule, the extent of MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the vessels (test-tubes). The presence of MnOx and sand enabled the long-term generation of iron hydroxides for MB discoloration by adsorption and co-precipitation. Results clearly reveal the complexity of the Fe0/MnOx/sand system, while establishing that both MnOx and sand improve the efficiency of Fe0/H2O systems in the long-term. This study establishes the mechanisms of the promotion of water decontamination by amending Fe0-based systems with reactive MnOx.

Corrosion of Steel Rebars in Anoxic Environments. Part I: Electrochemical Measurements

The number of reinforced concrete structures subject to anoxic conditions such as offshore platforms and geological storage facilities is growing steadily. This study explored the behaviour of embedded steel reinforcement corrosion under anoxic conditions in the presence of different chloride concentrations. Corrosion rate values were obtained by three electrochemical techniques: Linear polarization resistance, electrochemical impedance spectroscopy, and chronopotenciometry. The corrosion rate ceiling observed was 0.98 µA/cm², irrespective of the chloride content in the concrete. By means of an Evans diagram, it was possible to estimate the value of the cathodic Tafel constant (b_c) to be 180 mV dec⁻¹, and the current limit yielded an i_lim value of 0.98 µA/cm². On the other hand, the corrosion potential would lie most likely in the −900 mV_Ag/AgCl to −1000 mV_Ag/AgCl range, whilst the bounds for the most probable corrosion rate were 0.61 µA/cm² to 0.22 µA/cm². The experiments conducted revealed clear evidence of corrosion-induced pitting that will be assessed in subsequent research.

Characterizing the impact of MnO2 addition on the efficiency of Fe0/H2O systems

The role of manganese dioxide (MnO₂) in the process of water treatment using metallic iron (Fe⁰/H₂O) was investigated in quiescent batch experiments for t ≤ 60 d. MnO₂ was used as an agent to control the availability of solid iron corrosion products (FeCPs) while methylene blue (MB) was an indicator of reactivity. The investigated systems were: (1) Fe⁰, (2) MnO₂, (3) sand, (4) Fe⁰/sand, (5) Fe⁰/MnO₂, and (6) Fe⁰/sand/MnO₂. The experiments were performed in test tubes each containing 22.0 mL of MB (10 mg L⁻¹) and the solid aggregates. The initial pH value was 8.2. Each system was characterized for the final concentration of H⁺, Fe, and MB. Results show no detectable level of dissolved iron after 47 days. Final pH values varied from 7.4 to 9.8. The MB discoloration efficiency varies from 40 to 80% as the MnO₂ loading increases from 2.3 to 45 g L⁻¹. MB discoloration is only quantitative when the operational fixation capacity of MnO₂ for Fe²⁺ was exhausted. This corresponds to the event where adsorption and co-precipitation with FeCPs is intensive. Adsorption and co-precipitation are thus the fundamental mechanisms of decontamination in Fe⁰/H₂O systems. Hybrid Fe⁰/MnO₂ systems are potential candidates for the design of more sustainable Fe⁰ filters.

Calibrating the Impressed Anodic Current Density for Accelerated Galvanostatic Testing to Simulate the Long-Term Corrosion Behavior of Buried Pipeline

Various studies have been conducted to better understand the long-term corrosion mechanism for steels in a soil environment. Here, electrochemical acceleration methods present the most efficient way to simulate long-term corrosion. Among the various methods, galvanostatic testing allows for accelerating the surface corrosion reactions through controlling the impressed anodic current density. However, a large deviation from the equilibrium state can induce different corrosion mechanisms to those in actual service. Therefore, applying a suitable anodic current density is important for shortening the test times and maintaining the stable dissolution of steel. In this paper, to calibrate the anodic current density, galvanostatic tests were performed at four different levels of anodic current density and time to accelerate a one-year corrosion reaction of pipeline steel. To validate the appropriate anodic current density, analysis of the potential vs. time curves, thermodynamic analysis, and analysis of the specimen’s cross-sections and products were conducted using a validation algorithm. The results indicated that 0.96 mA/cm² was the optimal impressed anodic current density in terms of a suitable polarized potential, uniform corrosion, and a valid corrosion product among the evaluated conditions.