Fabrication of Robust Reference Tips and Reference Electrodes for Kelvin Probe Applications in Changing Atmospheres

Abstract

The scanning Kelvin probe (SKP) is a versatile method for the measurement of the Volta potential difference between a sample and the SKP-tip (Δψ). Based on suitable calibration, this technique is highly suited for the application in corrosion science due to its ability to serve as a very sensitive noncontact and nondestructive method for determining the electrode potential, even at buried interfaces beneath coatings or on surfaces covered by ultrathin electrolyte layers, which are not accessible by standard reference electrodes. However, the potential of the reference (i.e., the SKP-tip) will be influenced by variations of the surrounding atmosphere, resulting in errors of the electrode potential referred to the sample. The objective of this work is to provide a stable SKP-tip which can be used in different or changing atmosphere, e.g., within a wide range of relative humidity (approximately 0-99%-rh) or varying O partial pressure, without showing a change of its potential (note that the work functions measured in non-UHV atmospheres are electrochemical in nature [Hausbrand et al. J. Electrochem. Soc. 2008, 155 (7), C369-C379], and hence in the following we will refer to the potential of the SKP-tip instead of its work function). In that regard, the SKP-tip is in a first approach modified with self-assembled monolayers (SAMs) in order to create a hydrophobic barrier between the metallic surface and the surrounding atmosphere. The changes in potential upon varying relative humidity (ΔE) of different bare metallic substrates are quantified, and it is shown that these potential differences cannot be minimized by SAMs. On the contrary, the ΔE increases for every examined material system modified with SAMs. The major explanation for this observation is the dipole layer at the interface metal|SAM, causing an interfacial adsorption of water molecules even in a preferred orientation of their dipole moments, which leads to a changed work function and consequently to the correlated electrode potential. However, thin paraffin coatings were found to lead to a strongly reduced ΔE, finally validated with novel robust Ag/Ag reference electrodes. It is also shown that nickel as SKP-tip material is seemingly more stable in varying atmospheric conditions compared to widely used Ni/Cr, stainless steel, or gold as SKP-tip material.

Nanoscale Electrochemical Mapping

“…There are a variety of other local electrochemical methods, including scanning kelvin probe, adapted to map the electrode potential under thin electrolyte layers and at hindered interfaces; − scanning vibrating electrode technique, capable of mapping local ionic currents in the vicinity of electrochemical interfaces; , the scanning ion selective electrode technique, designed to define local concentrations (i.e., ion flux gradients); , and local electrochemical impedance spectroscopies, , allowing for the study of the kinetics of heterogeneous electron-transfer and the properties of double layer or adsorption processes. In principle, all of these techniques could attain nanoscale resolution; however, in practice, several have often been restricted to the microscale, owing to difficulties in scaling the probe size down, as well as the working distance while immersed in an electrolyte solution.…”

Suitability of Various Materials for Probes in Scanning Kelvin Probe Measurements

“…The Cu/CuSO 4 system is challenging for using it for calibration due to high evaporating rates of the solution at low humidity conditions. Uebel et al fabricated special reference electrodes and reference tips in order to perform SKP measurements in changing atmospheres . Research in this context is very complex, in order to compare the different materials among each other, just one experiment has been done on this topic.…”

Prospect of Using Machine Learning-Based Microwave Nondestructive Testing Technique for Corrosion Under Insulation: A Review

“…The contact potential is generated between the surface of the Kelvin probe and the sample. The VCPD between the conductive vibrating tip and sample is brought into equilibrium [97]. In the case of CUI presence, the differences in VCPD occur and the probe's vibration induces Alternating Current (AC).…”