In this work, a flow-through electrochemical–ICP-AES platform for operando monitoring of pitting corrosion on the CoCrFeMnNi high-entropy alloy is introduced. This setup combines localized chloride injection, potentiostatic control, and online, element-resolved dissolution analysis, thereby addressing a long-standing gap in mechanistic studies of early pit initiation and repassivation. Experiments in 0.5 M H2SO4 with Cl- injection enabled the continuous transfer of dissolved species from the electrode surface to the ICP-AES detector, achieving sub-ppb sensitivity and allowing quantification of Co, Cr, Fe, Mn, and Ni dissolution rates during the pitting process. The results reveal four characteristic stages, namely, incubation, initiation, propagation, and repassivation, with subtle but systematic differences between alloying elements. Co and Fe contribute slightly more during initiation, while Cr plays a dominant role during repassivation, reflecting its critical involvement in passive film regeneration. Charge analysis demonstrates that repassivation consumes a quantity of charge far greater than expected for compact passive films, pointing instead to a slow, iterative re-formation and partial dissolution of hydrated oxides. This methodology provides new mechanistic insight into the dynamic sequence of film breakdown, localized dissolution, and film repair in multicomponent alloys, and establishes a versatile framework for studying localized corrosion processes with element-specific resolution.

High-entropy alloys (HEAs), such as the equiatomic CoCrFeMnNi "Cantor" alloy, are being considered more and more as model materials for understanding corrosion in chemically complex solid solutions where passivity emerges from multicomponent oxide films. While the Cantor alloy often exhibits strong passivation, chloride-induced pitting is a significant form of degradation in acidic environments relevant to energy, chemical processing, and marine applications. A key limitation of current mechanistic descriptions of early pit initiation and repassivation is the absence of time-resolved, element-specific chemical information. Classical electrochemical signals integrate all contributions and cannot distinguish the roles of individual alloying elements in film breakdown, selective dissolution, and passive film repair [1, 2]. Here, we present an integrated, flow-through, electrochemical-ICP-AES (AESEC)-type platform designed to monitor element-resolved dissolution in situ during a controlled pitting event on CoCrFeMnNi. This approach involves ( i ) localized chloride delivery via a microcapillary positioned above the surface, ( ii ) potentiostatic control to trigger passivity breakdown, and ( iii ) online ICP-AES detection of Co, Cr, Fe, Mn, and Ni in the cell effluent with sub-ppb sensitivity. During passivation, all elements exhibit similar dissolution behavior at open circuit. This is followed by a sharp decrease under cathodic polarization and transient dissolution peaks when transitioning anodically into the passive domain. Chromium dissolves at a markedly lower rate than the other elements during passive film formation, which is consistent with Cr enrichment in the protective film. Once chloride injection begins, the synchronized electrochemical and dissolution-derived currents reveal the pitting sequence with electrical and chemical specificity. Four regimes are identified: (1) an incubation period with a current close to the baseline, indicating an intact passive film, (2) an initiation/nucleation stage, marked by a sharp increase in current and elemental dissolution, (3) a propagation stage, during which the current stabilizes, consistent with quasi-steady pit growth, dominated by depth advance, and (4) a repassivation stage, which follows the interruption of chloride injection and involves a gradual return of the current to the baseline over the course of several minutes. A central outcome is the nearly quantitative agreement between the charge from the electrochemical transient and the integrated dissolution charge, which supports the reliability of the oxidation state assumptions used to convert the elemental fluxes into equivalent currents. Post-mortem microscopy confirms that under the imposed hydrodynamic conditions, the intended single pit evolved into a small cluster of adjacent pits with a total projected area of approximately 1.6×10⁻³ cm² and comparable depths across cavities. This platform provides an unprecedented operando, element-specific view of the full pitting lifecycle in a multicomponent alloy. Beyond HEAs, this coupled methodology establishes a versatile framework that quantitatively links electrochemical transients to dissolution pathways. It can also selectively probe film breakdown/repair and generate high-resolution datasets suitable for mechanistic modeling and data-driven corrosion prediction. [1] Y. Hou, B. Dou, C. Xie, F. Sun, S. Rioual, B. Lescop, K. Ogle, A. Miche, O. Gharbi, M. Turmine, V. Vivier, On the corrosion resistance of the CoCrFeMnNi high entropy alloys in chloride-containing sulfuric acid solutions, Appl. Surf. Sci., 681 (2025). DOI: 10.1016/j.apsusc.2024.161487. [2] Y. Hou, O. Gharbi, K. Ogle, F. Sun, M. Turmine, V. Vivier, Generation of pits on CoCrFeMnNi high entropy alloy: an electrochemical impedance study of a single event, Electrochim. Acta, 526 (2025). DOI: 10.1016/j.electacta.2025.146193.

Abstract Quantitatively resolving how electrochemical charge is distributed between dissolution and surface film formation remains a major challenge in interfacial electrochemistry because dissolved and surface-bound species are rarely quantified simultaneously under operando conditions. Here, we introduce a coupled atomic emission spectroelectrochemistry-quartz crystal microbalance (AESEC-QCM) platform that enables time-resolved and quantitatively constrained analysis of electrochemical reactions by simultaneously measuring electron transfer, elemental dissolution, and interfacial mass evolution. Using Cu as a well-established model electrochemical system, we demonstrate quantitative agreement between faradaic charge, dissolved Cu flux, and QCM-derived mass variations during electrodeposition and pulsed anodic dissolution. Residence time distribution analysis further separates intrinsic interfacial kinetics from hydrodynamic dispersion within the flow cell, enabling accurate interpretation of transient dissolution responses. The operando methodology provides quantitative insights into the partitioning of anodic charge during Cu oxidation in synthetic tap water, where dissolution and surface film formation occur concurrently. Combined AESEC-QCM analysis reveals that Cu oxidation proceeds through concurrent dissolution as Cu2+ and formation of a Cu2O surface layer. The oxide composition and quantity are independently validated through subsequent chemical dissolution in citrate buffer, establishing complete closure of the mass-charge balance across electrochemical and chemical transformation steps. The results demonstrate that the coupled AESEC-QCM approach enables direct and quantitative separation of dissolved and surface-bound reaction pathways with nanomole-level sensitivity. More broadly, the methodology provides a general operando platform for investigating complex electrochemical systems involving concurrent dissolution and surface transformation, including multicomponent alloys, conversion coatings, and electrocatalytic materials.

Precise control of oxygen interstitials content enhances the mechanical strength of metastable beta-phase titanium alloys while preserving satisfactory ductility. Here, we have demonstrated that this concept can be exploited also to increase corrosion resistance of metastable beta-phase Ti-12Mo-5Hf alloy in the simulated inflammatory conditions (PBS+H2O2). Results of atomic emission spectroelectrochemistry have shown that introducing excessive oxygen into the alloy matrix inhibits titanium dissolution at cathodic bias, which can be relevant to the regions adjacent to the fretting area. Ion-release measurements combined with scanning electron microscopy and transmission electron microscopy revealed that interstitial oxygen suppresses elemental dissolution, surface degradation and oxide film growth in case of 7 days of immersion in PBS+H2O2. Overall results indicate that increasing interstitial oxygen content in Ti-12Mo-5Hf alloy matrix potentially inhibits its long-term degradation for the post-inflammatory period after implantation.

The effect of 5 at% Mo, Cu, Ti, V, Al, or Mn on the polarization and spontaneous passivation of (CoCrFeNi)(95)X-5 was investigated in ae rated 2 M H2SO4. As compared to CoCrFeNi, enhanced spontaneous passivation was observed for Mo>Cu>Ti approximate to V. The CoCrFeNi alone, or with added Al, or Mn did not undergo spontaneous passivation. The addition of Mn increased the critical dissolution rate. Mo and Cu functioned by enriching on the surface during the active-passive transition, thereby inhibiting dissolution, as oxide and in metallic form respectively. The behavior of the alloys was compared with that of the pure elements to gain mechanistic insight.

A novel operando methodology is presented that couples atomic emission spectroelectrochemistry (AESEC) with an optical oxygen sensing method (respirometry) to simultaneously monitor the kinetics of metal dissolution and oxygen evolution in real time during electrochemical experiments. To demonstrate the methodology, an investigation of Ni dissolution and the oxygen evolution reaction (OER) in the transpassive domain of nickel in 0.5 M H2SO4 is presented. Prior to secondary passivation, Ni dissolution was essentially faradaic with minimal OER. At higher potentials secondary passivation occurred with a sharp drop in the current and the dissolution rate. Beyond this point, the dissolution rate is constant or slightly decreased, while the OER increased systematically with potential. This may be interpreted in terms of a secondary passive film which blocks dissolution but is catalytic for OER.

Controlling both the resorption rate and the formation of reactive oxygen species (ROS) of biodegradable iron (Fe) remains a central challenge for the fabrication of bioresorbable cardiovascular stents. Here, we introduce an innovative nanoengineered surface coating strategy to simultaneously accelerate Fe corrosion and suppress ROS generation without altering the bulk Fe materials. Aryl-diazonium salt chemistry (4-cyanobenzene diazonium tetrafluoroborate, DCN) was used to create robust polyaryl interphases that can immobilize gold nanoparticles (Au NPs) on Fe, establishing nanoscale microgalvanic and catalytic sites. Electrochemical analysis reveals that the coating increases the overall Fe corrosion rate while biasing the cathodic oxygen reduction reaction toward the four-electron pathway, thereby reducing peroxide/OH• formation. This mechanism is supported by the remarkably reduced OH• content detected by the terephthalate-probe assay. Corrosion metrics show a pronounced, controllable rate enhancement relative to bare Fe, and the postcorrosion exposure interfacial spectroscopy/microscopy verify the persistence of Au NPs, attributed to atomic Fe-FeO-Au interactions and anchoring by the DCN-derived polyaryl layer. Endothelial cell culture indicates favorable adhesion and viability, supporting cytocompatibility of the modified surface. This surface-chemistry-driven mechanism establishes a general interfacial principle for rate and pathway control of Fe biodegradation, offering a concise route to safer, faster resorbing Fe-based stents.

Reactive oxygen species (ROS), produced by immune cells during inflammatory reaction, are known to promote corrosion of standard biomedical materials such as CP-Ti and Ti-6Al-4V. Electrochemical corrosion in the ROS environment can be further accelerated in the vicinity of fretting regions, where titanium can be polarized towards negative potentials. This study considers both of these aspects and presents corrosion analysis under complex inflammatory conditions for Ti-Mo and Ti-Mo-Zr alloys, which offer exceptional strain-hardening behavior and ductility. Combining electrochemical impedance spectroscopy (EIS) and atomic emission spectroelectrochemistry (AESEC) allowed us to understand the origin of ROS-induced corrosion. At free corrosion conditions, Zr was found to suppress oxide layer growth without any significant effect on the dissolution process. On the other hand, Zr suppressed dissolution rate under cathodic potentials. Although applying cathodic potential resulted in a rapid increase of dissolution rate, cross-section transmission electron microscopy (TEM) analysis did not reveal significant influence of the short cathodic polarization on the oxide film growth during further prolonged exposure at free corrosion conditions.

The influence of alloying elements Co, Fe, Ni on the spontaneous passivation behavior of Cr-containing alloys (Cr = 25 at %) in 0.1 M H2SO4 was investigated using element-resolved electrochemistry. A bottom-up strategy was used from pure metals to quaternary alloys. The dissolution - polarization curves of pure metals showed strong correlation with that of alloys containing the corresponding elements. For most alloys, the dissolution curve closely aligns with that of the pure metal with the lowest dissolution rate at a given potential. For CoCrFeNi, Ni limits dissolution in the cathodic domain and the critical dissolution rate is determined by the overlap of Ni and Cr dissolution curves.

The electrochemical reactivity of the high entropy Cantor alloy (CoCrFeMnNi) was investigated via potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), and surface analysis as a function of chloride content, in sulfuric acid solutions. The results revealed that corrosion mechanism of the CoCrFeMnNi alloy at the corrosion potential, is a two-step mechanism, involving one adsorbed intermediate, similar to that of pure iron in the H2SO4, and this model provided a good description of the impedance data. X-ray photoelectron spectroscopy (XPS) surface characterization and impedance data fitting revealed that chloride ions do not alter the corrosion mechanism or the thickness of the surface oxide film. Instead, they affect the corrosion resistance by changing the composition of the oxide film. Additionally, this work demonstrated that the corrosion mechanism of the CoCrFeMnNi alloy varies with the applied potential, as evidenced by impedance measurements under potentiostatic polarization. Surface observations using scanning electron microscopy (SEM) and XPS analysis provided insights into the morphology and composition of the surface oxide film at different applied potentials, explaining the observed changes in impedance.

Nickel-based alloys are among the suitable candidates to design industrial equipments containing corrosive nitric acid solutions, particulary the commercial alloy Hastelloy G35 (Ni33Cr8Mo wt%) alloy which shows a better corrosion resistance compared to some austenitic stainless steels in these conditions. To understand the mechanisms involved behind this improved corrosion behaviour, conventional electrochemical (linear sweep voltammetry, electrochemical impedance spectroscopy, etc .) and gravimetric measurements have been used in various [HNO 3 ] and temperature conditions. To gain further insight into the role of the individual elements, atomic emission spectroelectrochemistry (AESEC), was used. This relative novel technique [1] allows the real-time measurements by ICP-AES of the elemental dissolution rates of the material induced by an electrochemical polarization. It gives direct insights of the specific role of each alloying elements in corrosion processes. In this study, this methodology has been developed for this use in strong oxidative conditions ([HNO 3 ] = 2, 4, and 6 mol.L -1 at temperatures up to 70 °C). The passive and transpassive domains have been investigated and described in Figure 1. In the passive domain , an increase in corrosion current is observed with [HNO 3 ] (Figure 1a). Linear sweep voltammetries and the elemental dissolution rates associated showed oxides formations at all temperatures. The selective dissolution of nickel indicates that the oxides formed are essentially composed of chromium and molybdenum. Moreover, the selective dissolution of molybdenum is also observed only at [HNO 3 ] = 4 mol.L -1 at 70 °C. In the transpassive domain , chronoamperometries showed a congruent dissolution of nickel, chromium, and molybdenum in nitric acid medias. The results suggest also that the dissolution rate decreases when the nitric acid concentration increases (Figure 1b). This phenomenum was observed at from 25 °C to boiling temperature In order to explain the evolution of the dissolution rate with [HNO 3 ] in the transpassive domain, experiments were carried out on pure alloying elements (Ni, Cr and Mo) at different concentrations and temperatures. It is shown that the dissolution rate of chromium also decreases when the nitric acid concentration increases in contrast to nickel and molybdenum (Figure 1c). This phenomenon could be attributed to the chromium: chromium would set the dissolution rate of the alloy in the transpassive domain (setting the elemental dissolution rates of Ni and Mo due to congruent dissolution). The dissolution of the Ni33Cr8Mo alloy in the transpassive domain would therefore be driven by oxidation/dissolution in chromium VI. Finally, AESEC chronoamperometry in the passive and transpassive domains coupled with ex situ XPS surface analyses Ni-Cr model alloys with different proportions of Mo were carried out to provide further insights into the corrosion behavior of the Ni-Cr-Mo alloy, especially the role of molybdenum and nickel. [1] K. Ogle, Atomic emission spectroelectrochemistry: real-time rate measurements of dissolution, corrosion, and passivation, Corrosion 75 (2019) 1398– 1419, doi:10.5006/3336. Figure 1

An element-resolved electrochemical database of a ZnAlMg alloy coating is presented, obtained via atomic emission spectroelectrochemistry (AESEC) linear sweep voltammetry (LSV). Nominally pure Zn, Al and Mg metals as well as MgZn2, ZnAl intermetallic phases, and commercial ZnAl alloy coatings were investigated using AESEC-LSV to understand the complex electrochemical response of multi-phase ZnAlMg alloys. The elemental dissolution rates extrapolated from AESEC-LSV curves showed a linear relationship with spontaneous elemental dissolution rates. This demonstrates the possible use of AESEC-LSV for determining long-term elemental corrosion rates, as well as the use of element-specific electrochemical data as input parameters for more accurate machine learning based corrosion resistant alloy design. Element-resolved electrochemistry reveals important corrosion phenomena not detectable in conventional electrochemistry such as cathodic dissolution, chemical dissolution, cathodic dealloying, negative correlation effects, and anomalous hydrogen evolution. These phenomena may be significant and should be taken into account in the rate equations used for numerical modeling.

In this study, a single pit was successfully generated on the surface of a CoCrFeMnNi alloy using a home-made device consisting of an electrochemical work station coupled with a microflow-cell. The three-dimensional morphology of the single pit was observed by optical microscopy, revealing that the pit radius increased with the polarization of the material, while at the same time the pit depth remained constant. Additionally, the various stages of pit evolution, including pit initiation, pit propagation, and pit repassivation were investigated as a function of the alloy polarization using electrochemical methods, in particular, electrochemical impedance spectroscopy (EIS). The incubation and initiation rates of a single pit increased with the anodic polarization of the electrode, which was attributed to an increase in the dissolution kinetics. The mechanism during the propagation phase was shown to be independent of the polarization potential. Interestingly, it was also shown that the repassivation step occurred in distinct stages, potentially due to a limited ion diffusion, which warrants further investigation.

The influence of at% Mn on the polarization and spontaneous passivation of (CoCrFeNi)100-xMnx (at%) alloys in aerated 0.1 M H2SO4 was investigated using atomic emission spectroelectrochemistry. At 0 - 6.5 at% Mn, spontaneous passivation occurred but at >= 10 at% Mn, the beneficial effect of Cr on passive film stability was completely negated. Spontaneous passivation could be predicted from Evans diagrams in which total current was decoupled into anodic dissolution and the cathodic components. Passivation occurred in two stages: rapid reaction with significant Cr enrichment; slower reaction with less Cr enrichment. Differences between spontaneous and potentiodynamic passivation are discussed and interpreted with elemental dissolution rate vs. open circuit potential curves.

This study offers a real-time monitoring of titanium dissolution in H2O2-enriched phosphate buffer solution (PBS) during polarization of Ti-6Al-4V at cathodic potentials using atomic emission spectroelectrochemistry (AESEC). Polarization had a deleterious effect on titanium dissolution when it was conducted at potentials between -0.5 V and -1 V. AESEC results demonstrate a non-linear and time-dependent relation between titanium dissolution and applied potential. Scanning and transmission electron microscopy (SEM) and (TEM) observations revealed that the vanadium-rich (beta) phase is more sensitive to polarization-induced corrosion, which continues during subsequent Ti-6Al-4V exposure at open circuit potential.

Al0.3Cr0.5Fe2MoxNi1.5Ti0.3 (x = 0, 0.05, 0.15) FCC+L21 compositionally complex alloys are investigated using electrochemical and surface science methods in 0.1 M Na2SO4 and 0.1 M NaCl at pH 4 and 10. Mo is mainly found in the FCC phase and amplifies Al and Ti partitioning. The passive film is enriched in Cr(III), Ti(IV), Mo(IV)/Mo(VI), and (at pH 4) Al(III) with elemental fates tracked during film formation and dissolution. Mo presence does not enhance Cr or Ni passivation, instead promoting Ti(IV). Pitting and repassivation potentials increase at higher Mo concentrations, suggesting improved resistance to localized corrosion often initiated at FCC-L21 interfaces.

A conversion coating is an inorganic or organic–inorganic hybrid surface film conveying specific desirable functions to a material, formed by a spontaneous reaction between the material and an electrolyte. This chapter reviews the fundamental physical chemistry of the conversion coating process and overviews the variety of different conversion coatings. Emphasis is placed on the conventional phosphating and chromating process while reviewing alternatives such as trivalent chromium, titanate, zirconate, and vandate, rare earths, layered double hydroxides, and hybrid inorganic-organic species. The chemistry and formation mechanisms of each conversion coating are discussed in this chapter. Analytical techniques to understand the formation mechanism of the conversion coatings are also summarized.

The objective of this work was to determine the in-situ correlation of the elemental dissolution of Zn with the potentiodynamic polarization in deaerated and naturally aerated 1 M Na2SO4 (aq). A custom-built in-situ atomic emission spectroelectrochemistry (AESEC) electrochemical flow cell, which incorporates an inductively coupled plasma-atomic emission spectrometer (ICP-AES), was used for this purpose. Major findings include (i) O2 reduction is the dominant cathode reaction, (ii) Zn spontaneously forms a slowly dissolving oxide corrosion product film and (iii) Zn dissolution occurs under cathode polarization in the presence of dissolved O2 reduction. Postexposure imaging shows the polarized surface is covered by a solid oxide corrosion product that consists of a porous outer layer residing on a compact inner layer.