The scanning Kelvin probe (SKP) is a device that measures the difference in work function between a sample and probe tip on the instrument, and has proven to be a useful research tool over the past several decades. With this technique, the Volta potential difference, also known as the contact potential difference (CPD), can be determined which can characterize the corrosion, contamination, and coating condition of a surface of interest. The recently developed Field Deployable Scanning Kelvin Probe (FDSKP) allows for a variety of new uses as it allows the SKP technique to be used in a field environment rather than samples or test components being brought to a lab. However, additional challenges come with the increased portability and versatility of the FDSKP, as the set-up and tear down nature of a field deployable unit necessitates a rapid determination that the system is properly set up and functioning. A calibration electrode solves this problem while further demonstrating the underlying connections between electrical potential and the work function as measured by the SKP.

The absolute potential difference, also called the Galvani potential (Φ), can be described as the sum of two components: the Volta potential (Ψ) and the dipole potential (χ)¹. The Volta potential is directly related to the electrode potential and, therefore, corrosion potential². This relationship has been used in previous methods of SKP calibration that included relating the Volta potential differences measured by the SKP to the electrode potentials of bare metals and the corrosion potentials of immersed metals³. The effort described here develops a new approach to SKP calibration by applying an external potential onto the calibration electrode surface to further modify the Volta potential and, therefore, the measured Volta potential difference between the polarized sample and the probe tip. The design of the calibration electrode includes flat conductive regions across which a potential can be applied. By applying an external potential to a sample, the measured work function is changed by that amount, thus sharp voltage changes between regions of different applied potential can be observed.

The calibration electrode described here is a flexible Kapton substrate with vacuum deposited gold regions produced through a mask and UV lithography fabrication process (Figure 1). Three different geometric designs of matching shapes with a 10 um gap between them are present: bars, wedges, and a puzzle-piece type design. By electrically connecting one region from each of the three designs into two groups, a potential difference could then be applied between the two regions of each design group by means of a potentiostat or battery. With this applied potential, an SKP area scan was performed above the sample. The average work function for each of the two regions was determined. The difference in average work function correlates very well with applied potential across a range of voltage from -1 V to 1 V at steady-state. Similar results were recorded on both coated and uncoated electrodes. As the correlation between observed work function difference and applied potential is very good, this can be used to calibrate an SKP system and demonstrate its proper functioning. A system check can include determining that the measured voltage difference on the calibration electrode matches the applied potential. Additionally, the sharp distinction on the sample between regions of different work function can allow for the calibration and tuning of SKP parameters, such as electrometer and proportional integral derivative (PID), in order to optimize the system’s measurement capability. As SKP techniques move from the lab into the field, a calibration electrode is very useful as it will greatly assist in system set-up and facilitate parameter optimization to allow for better and clearer results.

References

1. Grunmeir, G., Juttner, K. and Stratmann, M. (2000) Novel Electrochemical Techniques in Corrosion Research. In R.W. Cahn, P. Haasen & E.J. Kramer (Eds.), Material Science and Technology: Corrosion and Environmental Degradation (Volume I, p. 285-382). Wiley-VCH.
2. Leng, H. Streckel and M. Stratmann “The Delamination of Polymeric Coatings from Steel. Part I: Calibation of the Kelvin Probe and Basic Delamination Mechanism,” Corrosion Science 41 (1999): p. 547-578.
3. B. Cook et al. “Calibration of the Scanning Kelvin Probe Force Microscope under Controlled Environmental Conditions,” Electrochimica Acta 66 (2012), p. 100-105.