Spatially resolved measurement of electrochemical activity and pH distributions in corrosion processes by scanning electrochemical microscopy using antimony microelectrode tips (original) (raw)
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Sensors and Actuators B: Chemical, 2019
Simultaneous monitoring of pH and Mg 2+ distributions above AZ63 magnesium alloy, either spontaneously corroding or galvanically coupled with iron, was achieved using SECM in the potentiometric operation. By introducing an internal micro-reference electrode in a multi-barrel arrangement of the ion-selective microelectrode used as scanning probe, superior performance was achieved compared to conventional single-barrel and double-barrel assemblies. In this way, the impact of the overestimated acidification accompanying metal dissolution using conventional tips was established from model experiments using non ion-sensitive open micropipettes and pH antimony microelectrodes. It is shown that the simultaneously acquired pH and pMg maps provide complementary information on the spontaneous and galvanic corrosion of AZ63 magnesium alloy.
Journal of the Electrochemical Society 160 (2013) C451-C459., 2013
The fabrication of a solid-contact, micropipette-based magnesium ion-selective micro-tipped electrode (ISME) suitable for scanning electrochemical microscopy is reported and compared against a conventional micro-tipped ISME having a conventional aqueous internal reference electrode. Measurements showed that the solid-contact ISME had a lower internal resistance and a faster response time than the one with a liquid-contact. These advantages increased the spatial distribution and improved 2D images depicting concentration distributions of Mg2+. The ability of the microelectrode to image local ionic concentration has been tested over magnesium surfaces freely corroding or galvanically coupled to iron in aqueous chloride-containing solution. Scans of magnesium ion distribution, in the absence of corrosion currents, were alsomade over a micro-pipette source containing a concentrated magnesium chloride gel as a source of Mg2+ and over a current source in the absence of Mg2+. From these measurements it was concluded that the potentiometric measurements over corroding surfaces were dominated by the changes in Mg2+ distributions with small electric potential contributions due to corrosion current.
Electrochimica Acta 87 (2013) 283-293., 2013
The applicability of scanning electrochemical microscopy for the local detection and quantification of relevant species participating in the corrosion of magnesium-based materials is presented. The iron–magnesium galvanic pair exposed to aqueous NaCl solution was adopted as model system for this purpose. Mg2+ ion concentration and pH profiles were investigated using ion selective electrodes, containing a liquid membrane and Sb/Sb2O3 as sensor elements, respectively. Oxygen consumption at the substrate related to the cathodic reaction was also monitored with the antimony-based electrode though operated amperometrically. Data show a major production of hydroxyl anions at the cathodic sites as result of the oxygen reduction half-reaction, whereas in the vicinity of the magnesium surface pH is greatly affected by the anodic dissolution process instead. The later produces the release of metal cations accompanied by hydrogen evolution.
Electrochimica Acta
Ion-selective microelectrodes can be employed as tips in scanning electrochemical microscopy (SECM) for chemical imaging of corrosion processes. They present higher chemical selectivity than conventional amperometric microdisks, and may be the only effective option to visualize the dissolution of metals with negative redox potentials in aqueous environments when the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions. A robust micro-sized ion selective electrode has been developed which allows the spatial distribution of Zn 2+ during galvanic corrosion of a model Fe/Zn couple to be investigated using SECM. Owing to the low internal contact potential achieved with the novel design, the resistance of the micropipette electrodes is only fractions of the resistance of conventional micropipette electrodes of the same size. As a result, no special shielding of the microelectrodes is required and higher scanning rate can be used for scanning in the potentiometric modes using these micropipette tips. Concentration profiles over corroding surfaces measured with this technique will be presented.
Journal of The Electrochemical Society
There is great interest in elucidating the corrosion mechanism of magnesium, and different experimental methods and techniques are explored with this purpose. Among the scanning probe techniques, scanning electrochemical microscopy (SECM) is delivering some promising results in recent years. In particular, the use of ion selective microelectrodes (ISME) as SECM sensing probes allow monitoring of the temporal and spatial distribution of different ionic species related to the corrosion reactions. However, a serious disturbance in the measured potential can be observed when it comes to galvanic coupling or polarization of the samples. This work explores the factors that affect the magnitude of the electrical field effects associated with the galvanic coupling, and describes an experimental arrangement for potentiometric SECM able to avoid the unwanted contribution of the potential field. The performance of a double barrel electrode assembly comprising an ion selective microelectrode and an internal reference electrode was compared to that of a conventional single barrel ISME in order to establish its applicability for the investigation of corrosion systems presenting electrical field distributions.
Electroanalysis, 2016
Scanning Electrochemical Microscopy (SECM) is presented as an essential tool for the local characterization of the still uncertain mechanism for magnesium corrosion. The reaction leading to magnesium release and hydrogen evolution from separated magnesium cathodes and anodes has been imaged using an adequate combination of the operation modes available in SECM. Magnesium ion selective microelectrodes (Mg‐ISME's) were used for the visualization of the heterogeneously distributed release of magnesium (II) species. Antimony microelectrodes detected the pH gradients in the adjacent electrolyte resulting from either water or magnesium electrolysis, whereas platinum microdiscs were used to monitor the concomitant local evolution of hydrogen. Alkalization and H2 generation were observed over the magnesium strip polarized as cathode, whereas a small local acidification was observed above the strip polarized anodically, at which extensive heterogeneous magnesium release was also imaged.
Possibilities and limitations of scanning electrochemical microscopy of Mg and Mg alloys
Magnesium (Mg) and its alloys undergo corrosion at high rates in aqueous electrolytes of essentially any concentration, releasing copious amounts of hydrogen (H-2) into the electrolyte, thereby making conventional scanning electrochemical microscopy (SECM) measurements difficult upon Mg. Examples of the issues associated with Mg SECM, and strategies to counter these, are discussed in this review. SECM has been employed in Sample Generation-Tip Collection (SG-TC) mode to image cathodic sites on the Mg surface, wherein, the platinum (Pt) tip oxidizes H-2 emanating from the specimen surface. This mode of SECM does not clearly reveal a close correspondence between the microstructure and its associated reactivity; the H-2 currents measured by the Pt tip found to vary by over an order of magnitude with time. Convective fluxes due to H-2 bubbles, local acidification from protons released during H oxidation on the Pt tip, and the negative difference effect of Mg may contribute to these high tip currents. SECM has also been performed on Mg using Mg2+ ion selective electrodes (ISEs), where an Mg2+ ionophore cocktail serves as a Mg2+ ion permeable membrane. The Mg2+ ions diffuse/migrate through the membrane which eventually settles at the equilibrium membrane potential (Donnan potential). This potential varies with the logarithm of Mg2+ ion concentration in the electrolyte, and thus, could be used to ascertain the local Mg2+ ion concentrations (or anodic sites) on the surface. This technique needs to be complemented with other modes of SECM to obtain holistic electrochemical micrographs of a corroding Mg surface.
Electrochemistry Communications 26 (2013) 25-28., 2013
The spatial resolution of Mg2+ release from magnesium and its alloys during exposure to aqueous environments has been imaged using a new, solid contact, micropipette-based magnesium-ion selective electrode employed as potentiometric tip in SECM. The detection of metal dissolution is a crucial factor to detect the local microelectrodes established on the surface of the metal, and distinguish the processes related to anodic and cathodic half-cell reactions. Concentration distribution images have been obtained for the magnesium-based alloy AZ63 when galvanically coupled to pure iron during exposure to 1 mM NaCl solution.
In: “Current microscopy contributions to advances in science and technology”. Volumen 2. A. Méndez-Vilas (editor). Formatex Research Center, Badajoz, pp. 1407-1415 (2012)., 2012
Scanning electrochemical microscopy (SECM) with coupled potentiometric and amperometric measurement modes is used to study corrosion reactions in aqueous environments. Whereas conventional electrochemical techniques lack spatial resolution and provide little information on behaviour at sites of corrosion initiation or at defects, the advent of scanning electrochemical microscopy is contributing to overcome these limitations when applied to the investigation of corrosion processes in situ. Ion-selective microelectrodes (ISME) and dual amperometric/potentiometric probes have been developed that can be employed as SECM tips as they exhibit sufficiently low resistances and response times. The applicability of this experimental approach is demonstrated by chemically imaging the behaviour of an iron-magnesium galvanic couple immersed in a diluted aqueous electrolyte. The dissolution of metal ions from anodic sites, the consumption of oxygen at the cathodic sites, and the local pH changes associated with both half-cell reactions are thus effectively monitored.
ChemPhysChem, 2000
Scanning electrochemical microscopy (SECM) with coupled potentiometric and amperometric measurement modes is used to study corrosion reactions in aqueous environments. Whereas conventional electrochemical techniques lack spatial resolution and provide little information on behaviour at sites of corrosion initiation or at defects, the advent of scanning electrochemical microscopy is contributing to overcome these limitations when applied to the investigation of corrosion processes in situ. Ion-selective microelectrodes (ISME) and dual amperometric/potentiometric probes have been developed that can be employed as SECM tips as they exhibit sufficiently low resistances and response times. The applicability of this experimental approach is demonstrated by chemically imaging the behaviour of an iron-magnesium galvanic couple immersed in a diluted aqueous electrolyte. The dissolution of metal ions from anodic sites, the consumption of oxygen at the cathodic sites, and the local pH changes associated with both half-cell reactions are thus effectively monitored.