Kelvin Probes can be modified to control as well as monitor potential. The design and operation of two different Kelvin Probe Potentiostats (KPPs) are described in this paper. One approach uses a permanent magnet and double coil to oscillate the needle at a fixed frequency, an AC backing potential, and software analysis and control schemes. This technique can also control the distance between the tip and sample, thereby tracking the topography of the sample. Both KPPs were used to make measurements on Type 304L stainless steel under thin layers of electrolyte. Cathodic polarization curves exhibited a limiting current density associated with oxygen reduction. The limiting current density varied with solution layer thickness over a finite range of thickness. Anodic polarization curves on 304L in a thin layer of chloride solution resulted in pitting corrosion. The breakdown potential did not vary with solution layer thickness. However, the thin layer was observed to increase in volume remarkably during pit growth owing to the absorption of water from the high humidity environment into the layer with ionic strength increased by the pit dissolution. The open circuit potential (OCP) and solution layer thickness were monitored during drying out of a thin electrolyte layer. Pitting corrosion initiated, as indicated by a sharp drop in the OCP, as the solution thinned and increased in concentration.

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Volta potential of a material can be easily determined by using scanning Kelvin probe (SKP), but up to now there is no standard method established. This study focuses on finding an alternative or even a standard material for SKP probes. Re, Ni, and Mo are analyzed regarding their suitability for serving as tips in SKP measurements. For comparison purposes, a commonly used NiCr probe is analyzed too. Linearity, the sensitivity to humidity changes as well as the lateral and Volta potential resolution are determined for various probe materials. Linearity is excellent for all tested probes. SKP measurements carried out under humidity variations shows dissimilar results for all different metal probes. Lateral and Volta potential resolution are the best for Re and Ni tips. There may be a correlation between the work function of a material and its suitability for serving as a probe in Kelvin measurements − the higher the work function, the better the resolution of Volta potentials. Based on the results, it can be concluded that pure Re and Ni are more suitable for serving as SKP probes than NiCr alloy.

The use of the Kelvin probe (KP) to determine Ecorr values is based on the relationship Ecorr = k + ΔΨ (with ΔΨ = measured value of the KP and k = constant). The constant k is determined by calibration above a reference sample with known Ecorr. It has been shown that the constant k can vary by up to 172 +/- 18 mV with a probe of nickel and 178 µm probe diameter, when the relative humidity RH varies between 42% and 98%, which must be taken into account when performing measurements under changing climatic conditions. It has been shown theoretically and experimentally that the RH at the probe tip 40 µm above a saturated salt solution is nearly equal to its critical relative humidity. An experimental procedure is proposed to determine the dependence of k on RH. For this purpose 4 Cu /saturated Cu-salt half-cells are used. Additionally, a method is proposed to measure the critical relative humidity of a saturated salt solution using KP.