Scanning probes & coating researchLatest updated: June 2, 2023
When studying coatings systems, it is important to understand processes occurring locally for a full understanding of the system. The family of techniques known as scanning probe electrochemistry is well-suited to these studies, allowing local electrochemistry to be correlated with sample features, such as damage to the coating. Scanning probe electrochemistry can be used to answer a number of questions about the coating system as outlined in the table below.
|Does the smart coating self-heal to offer an improvement in corrosion protection?
|LEIS measures the local admittance of a sample. A higher admittance is generally seen over the scratched regions in coated surfaces due to exposure of the underlying sample. Decrease in this admittance over time would indicate a decrease in the activity of the scratched region, reflecting the smart coating is healing.
|R. Raj, et al. Electrochimica Acta 319 (2019) 801-812
|Cathodic corrosion processes use dissolved oxygen, causing a depletion in its concentration. dc-SECM can be used to follow this by biasing the probe to interact with oxygen. Over defective sites, which use the dissolved oxygen, the current measured by the probe will be lower than coated regions. If the coating is effectively healing an increase in current should be measured at the probe due to the increased availability of oxygen.
|Y. González-García, et al. Electrochemistry Communications 13 (2011) 1094–1097
|SKP measures the local Volta potential of a surface which is directly related to the corrosion potential. An SKP map with a peak in local Volta potential indicates and active region, which is not protected by the coating. When multiple SKP maps are measured over time changes in the Volta potential can be tracked. A decrease in the Volta potential peak height and width reflects the healing of the coating. When the coating is fully healed a homogeneous Volta potential is expected.
|W. Fan, et al., Chemical Engineering Journal 368 (2019) 1033-1044
|SVET measures the local ionic flux associated with anodic and cathodic processes. Regions with a high magnitude current density indicate an active region due to corrosion. A scratch in a self-healing coating can be measured repeatedly over an extended period. Reduced current densities indicate reduced activity associated with coating healing.
|M. D. Wang, M. Y. Liu, J. J. Fu, Journal of Materials Chemistry A 3 (2015) 6423-6431
|Does the coating protect the sample surface?
|dc-SECM measures Faradaic current related to the interaction of the redox mediator with the sample. Lower currents measured by dc-SECM indicate reduced interaction between the sample and mediator. This in turn indicates increasing passivation of the sample.
|A. Doublet, et al. RSC Advances 9 (2019) 24043–24049
|SVET measures the local current density of a sample. High current densities reflect active regions of the sample. Uniform current density indicates that a galvanic protection coating is corroding uniformly, protecting the sample.
|F. Andretta, et al. Materials and Corrosion 70 (2019) 793-801
|Can the breakdown of the coating be detected?
|The Volta potential measured by SKP is highly sensitive to surface changes. The breakdown of an epoxy coating occurs with the breaking of bonds from hydrolysis. This breakdown is reflected in a change in the measured Volta potential compared to the original coating.
|D. J. Borth, et al., CORROSION 75 (2019) 457-464
|Breakdown of epoxy coatings can be associated with the release of OH– which can interact with the redox mediator used in an SECM measurement, consuming it before it can interact with the biased SECM probe. Regions which are actively decomposing will show a decrease in measured current due to depletion of the mediator. By using ic-SECM the probe to sample distance is kept constant removing ambiguity from differences in topography.
|T. Morimoto, et al., AIAA Scitech 2019 Forum, 7-11 January 2019, San Diego, California
|Is underfilm corrosion occurring?
|LEIS area mapping is performed at a single frequency with the resulting |Z| plotted. Areas with reduced |Z| reflect corrosion sites. The lowest |Z| value is measured over defect sites, like scratches, with other low |Z| regions reflecting underfilm corrosion. When mapped over time changes to the low |Z| region can indicate expanding underfilm corrosion.
|L.V. S. Philippe, G. W. Walter, S. B. Lyon, Journal of The Electrochemical Society 150 (2003) B111-B119
Scanning ElectroChemical Microscopy (SECM): Measurement of local electrochemical activity of a sample with chemical selectivity. The local impedance of a sample can also be measured.
Localized Electrochemical Impedance Spectroscopy (LEIS): Measurement of local impedance differences and point-by-point Electrochemical Impedance Spectroscopy
Scanning Kelvin Probe (SKP): Local non-contact measurement of sample work function. Topography can also be measured.
Scanning Vibrating Electrode Technique (SVET): Measurement of local current distribution of a sample in electrolyte. Also known as Vibrating Probe.
Scanning Droplet Cell (SDS): Local electrochemical and impedance measurements of a sample within a droplet.
Optical Surface Profiler (OSP): Non-contact measurement of local sample topography.
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