Scanning Kelvin Probe (SKP)

Principle

Scanning Kelvin probe (SKP) is a technique allowing a non-contact measurement of the  surface of a metal giving values of corrosion potentials that can be calibrated to a standard scale [1].  The SKP uses a scanning vibrating gold tip as a quasi-reference electrode that enables it to map electrochemical potentials, even beneath a coating, thus allowing potential measurements of corrosion processes occurring in blisters and delaminated areas as well as at defects. SKP allows an image to be constructed that shows the potential distribution on the sample and identifies anodic and cathodic regions at defects and beneath the organic coating. This technique is very useful in corrosion studies around defects as a function of time as it produces results that can be related to the mechanistic description of the process [2].

The scanning Kelvin probe is a vibrating capacitor device used to measure the work function difference between a conducting specimen and a vibrating tip. If an external electrical contact is made between the specimen and the probe, their Fermi levels equalise and the resulting flow of charge produces a potential gradient between the plates, which is equal to the difference in the electronic work function. A variable ‘backing potential’ applied to the circuit is recorded at the unique point where the (average) electric field between the plates vanishes, resulting in a null output signal. For corrosion studies, the scanning Kelvin probe gives the potential of the specimen at that point that can then be calibrated to a standard scale.

Experimental Details

 The KP Technology SKP system is shown in Figure 1. The SKP was operated using a circular silver tip with a 0.5mm flat end. Typically, the probe was set to vibrate at frequency of 60Hz with amplitude of 32μm and a gradient of 300, which gave a tip-sample separation of 64μm at closest approach. Measurements were taken using a sample backing voltage, V_b, in the range of + 5000mV. This instrument uses a non-null technique that calculates the null point from the AC current at two bias values. This also allows automatic height control. For an area scan, the tip starts back left and then moves in a meander pattern, whereas for a line scan the tip start back left and moves in a line until the scan is complete. The SKP data was plotted using Origin 7.0. The area maps are based on 625 measurements and are presented in terms of relative potentials (arbitrary scale).

Figure 1 – Schematic of SKP set up (after SKP Technology Ltd)

 Electrochemical Calibration

Calibration of the SKP was carried out by using several cylindrical metal samples (zinc, iron and copper) containing a reservoir of solution. The saturated solutions (ZnSO₄ for zinc, NaCl for iron and copper) were poured in the reservoir and connected to a saturated calomel electrode, SCE. The SCE was connected to a voltmeter so the potential reading of the respective metal sample can be recorded. The SKP was then used to take potential measurements at the surface of the solution. Both potentials measured using a voltmeter and SKP were then plotted together to determine the calibration factor. The calibration graph (Figure 2) has a gradient of 1.0434, thus confirming a 1:1 relationship between the electrochemical potential of the corroding metal and work function measured by SKP. The measured offset of 12.932mV can be used to convert the SKP raw data into electrochemical potential vs. SCE.

Figure 2 – SKP calibration graph for Zn, Fe and Cu metal standard vs. Ag probe tip

Determination of Coating Performance by SKP

One of the main significant advances of using SKP in coating studies is to better understand corrosion mechanisms under paint as well as the capability to produce maps illustrating potential and current distribution at a defect area. The mechanisms of the delamination process of organic coatings on steel have been investigated using local electrochemical and physical techniques. Stratmann et al. employed a scanning Kelvin probe to measure potential distributions at a buried coating/substrate interface. Their studies were done under accelerated corrosive conditions and minimal surface treatments, therefore the prediction of delamination rate measured was far bigger than the value from commercial technical samples [3]. Reddy, Doherty and Sykes have studied the mechanism of corrosion on a steel panel with a circular scribe defect painted with pigmented coating. They used SKP to examine the corrosion propagation at the defect and its periphery and discovered a reversal of anodes and cathodes at the scribe, beneath the coating. They have suggested that the reversal is due to the accumulation of the corrosion product at the defect. Potential mapping of electrochemical behaviour underneath the coating can lead to better understanding of the mechanism involved in coating breakdown. Later work by the same authors introduced an interpretation of the potential maps in terms of corrosion cells beneath the coating through generation of current maps, as they experienced difficulty from the former in identifying the correct region of electrochemical activity . They concluded that current mapping could reveal patterns of anodic and cathodic activity at a defect and the area surrounding it. As indicated in Figure 3 by the shaded scale next to each map, the lightest areas on the map represents anodic sites, whereas the darker areas represent cathodic sites.

(a) potential map                                                  (b) current map

Figure 3 –  SKP map for an 60 μm epoxy/talc coating on steel substrate exposed to 3% NaCl solution after 12 hours immersion [4]

References

1. M. Rohwerder, M. Stratmann,  P. Leblanc, G.S. Frankel, Application of scanning Kelvin probe in corrosion science in Analytical Methods in Corrosion Science and Engineering, P. Marcus and F. Mansfeld (editor) , Taylor & Francis, 2006.
2. I.D. Baikie, P.J.Estrup, Low cost PC based scanning Kelvin probe. Review of scientific instruments, 1998. 69(11): p. 3902-3907.
3. K. Ogle, S. Morel, N. Meddahi, An electrochemical study of the delamination of polymer coatings on galvanized steel. Corrosion Science, 2005. 47(8): p. 2034-2052.
4. B. Reddy, J.M. Sykes, Degradation of organic coatings in a corrosive environment: A study by scanning Kelvin probe and scanning acoustic microscope. Progress in Organic Coatings, 2005. 52(4): p. 280-287.

For my work on using SKP click Investigation of blister formed on coated mild steel using scanning kelvin probe

look at these beautiful colours came from data by SKP