SECM Imaging of Interfacial Processes in Defective Organic Coatings Applied on Metallic Substrates Using Oxygen as Redox Mediator

Quantitative Feedback Referencing for Improved Kinetic Fitting of Scanning Electrochemical Microscopy Measurements

Scanning electrochemical microscopy (SECM) has matured as a technique for studying local electrochemical processes. The feedback mode is most commonly used for extracting quantitative kinetic information. However, approaching individual regions of interest, as is commonly done, does not take full advantage of the spatial resolution that SECM has to offer. Moreover, fitting of experimental approach curves remains highly subjective due to the manner of estimating the tip-to-substrate distance. We address these issues using negative or positive feedback currents as a reference to calculate the tip-to-substrate distance directly for quantitative kinetic fitting of approach curves and line profiles. The method was first evaluated by fitting simulated data and then tested experimentally by resolving negative feedback and intermediate kinetics behavior in a spatially controlled fashion using (i) a flat, binary substrate composed of Au and SiO₂ segments and (ii) a dual-mediator system for live-cell measurements. The methodology developed herein, named quantitative feedback referencing (QFR), improves fitting accuracy, removes fitting subjectivity, and avoids substrate-microelectrode contact.

Uses of Scanning Electrochemical Microscopy (SECM) for the Characterization with Spatial and Chemical Resolution of Thin Surface Layers and Coating Systems Applied on Metals: A Review

Scanning Electrochemical Microscopy (SECM) is increasingly used in the study and characterization of thin surface films as well as organic and inorganic coatings applied on metals for the collection of spatially- and chemically-resolved information on the localized reactions related to material degradation processes. The movement of a microelectrode (ME) in close proximity to the interface under study allows the application of various experimental procedures that can be classified into amperometric and potentiometric operations depending on either sensing faradaic currents or concentration distributions resulting from the corrosion process. Quantitative analysis can be performed using the ME signal, thus revealing different sample properties and/or the influence of the environment and experimental variables that can be observed on different length scales. In this way, identification of the earlier stages for localized corrosion initiation, the adsorption and formation of inhibitor layers, monitoring of water and specific ions uptake by intact polymeric coatings applied on metals for corrosion protection as well as lixiviation, and detection of coating swelling—which constitutes the earlier stages of blistering—have been successfully achieved. Unfortunately, despite these successful applications of SECM for the characterization of surface layers and coating systems applied on metallic materials, we often find in the scientific literature insufficient or even inadequate description of experimental conditions related to the reliability and reproducibility of SECM data for validation. This review focuses specifically on these features as a continuation of a previous review describing the applications of SECM in this field.

Contributions of Microelectrochemical Scanning Techniques for the Efficient Detection of Localized Corrosion Processes at the Cut Edges of Polymer-Coated Galvanized Steel

Spatially resolved information on corrosion reactions operating at the cut edges of coated metals can be obtained using microelectrochemical scanning techniques using a suitable selection of operation modes and scanning probes. The scanning vibrating electrode technique (SVET) provides current density maps with a spatial resolution of the order of the dimensions of the sample, which allows the temporal evolution of the corrosion reactions to be followed over time. This leads to the identification and localization of cathodic and anodic sites, although the technique lacks chemical specificity for the unequivocal identification of the reactive species. The application of scanning electrochemical microscopy (SECM) was previously limited to image cathodic reaction sites, either due to oxygen consumption in the amperometric operation or by the alkalinisation of the electrolyte in potentiometric operation. However, it is shown that anodic sites can be effectively monitored using an ion-selective microelectrode (ISME) as a probe. The ISME probes detected differences in the local concentrations of Zn²⁺ and OH⁻ ions from the cut edges of a complete coil coating system compared to the same system after the polymeric layers were removed. In this way, it has been shown that the inhibitor loading in the polymer layers effectively contributes to reducing the corrosion rates at the cut edge, thus helping to extend the useful life of the sacrificial galvanized layer bonded directly to the steel matrix. Additionally, these two probe configurations can be integrated into a multi-electrode tip for potentiometric operation to simultaneously monitor localized changes in pH values and metal ion dissolution in a single scan. Spatial and temporal distributions were further investigated using different rastering procedures, and the potential of constructing pseudomaps for 2D-imaging is described.

Use of Amperometric and Potentiometric Probes in Scanning Electrochemical Microscopy for the Spatially-Resolved Monitoring of Severe Localized Corrosion Sites on Aluminum Alloy 2098-T351

Amperometric and potentiometric probes were employed for the detection and characterization of reactive sites on the 2098-T351 Al-alloy (AA2098-T351) using scanning electrochemical microscopy (SECM). Firstly, the probe of concept was performed on a model Mg-Al galvanic pair system using SECM in the amperometric and potentiometric operation modes, in order to address the responsiveness of the probes for the characterization of this galvanic pair system. Next, these sensing probes were employed to characterize the 2098-T351 alloy surface immersed in a saline aqueous solution at ambient temperature. The distribution of reactive sites and the local pH changes associated with severe localized corrosion (SLC) on the alloy surface were imaged and subsequently studied. Higher hydrogen evolution, lower oxygen depletion and acidification occurred at the SLC sites developed on the 2098-T351 Al-alloy.

Scanning electrochemical microscopy with conducting polymer probes: Validation and applications

Scanning electrochemical microscopy (SECM) allows spatially and temporally resolved measurements of a broad range of reactive surfaces and specimens, typically using electrochemically active metal probes. While conducting polymers (CPs) present several analytical properties of interest due to their chemical versatility, potentially enabling the measurement of ionic fluxes as well as redox processes, they have not been widely used as probe materials for SECM. CPs can be modified and fine-tuned to improve experimental parameters and they can be easily prepared by electrodeposition. In this paper, we show a new type of CP probe for SECM that retains the spatial resolution of conventional metal probes and introduces the possibility to exploit a wide range of ionic and redox systems. Poly-3,4-ethylenedioxythiophene (PEDOT) was electrochemically deposited on flat and recessed Pt microdisks to generate CP SECM probes. To demonstrate their usefulness, an insulating substrate with conducting features was imaged. Well-defined SECM feedback images were observed for both the CP well-probe and the Pt probe, proving the efficiency of the new electrode to image redox reactions. Additionally, an organosulfur compound was used as mediator taking advantage of the electrocatalytic effect PEDOT has on the molecule's kinetics. Finally, these probes were also used in a mediator-less fashion, taking advantage of the ion flux required to electrochemically oxidize the PEDOT deposit. We investigated the impact of anion size and concentration on current-distance relationships for SECM probe positioning. CP probes pose exciting prospects for the imaging and measurement of combined redox and ionic processes in energy materials.

Scanning Electrochemical Microscopy as a Tool for the Characterization of Dental Erosion

When the tooth is exposed to acidic environments, an irreversible loss of dental hard tissue occurs in a process called dental erosion. In this work, the scanning electrochemical microscopy (SECM) was used to probe the consumption of protons at the vicinity of a tooth surface with a platinum microelectrode fixed at −0.5 (V) versus Ag/AgCl/KCl(sat). SECM approach curves were recorded to assess the extent of diffusion in the solution close to the tooth substrate. SECM images clearly demonstrated that the acid erosion process is very fast at solution pH values in the range between 3 and 4.