Scanning Kelvin Probe SKP

Fundamentals. The surface (or contact) potential of a solid or a liquid film covered solid can be measured with a Kelvin probe [204, 205]. Essentially, the Volta potential difference between the two employed surfaces, as described below, is measured. In common abbreviation, this is also called measurement of a Volta 

Instrumentation. Reported examples are mostly related to corrosion studies the particular problems in relating Volta potential differences as measured with an SKP and local corrosion potentials have been treated in detail [209]. The effect of barrier layers and metal surface pretreatment has been investigated [214,215]. In a study of the effect of a corrosion protection primer that contains the intrinsically conductive 

In a study of zinc-coated steel covered with a polymer topcoat, the mechanism of topcoat delamination was elucidated with high spatial resolution [216]. Depending on the details of the defect and the composition of the corroding atmosphere, the rate and type of delamination could be described. A similar study with a coated iron surface has been reported [217]. A comparison of results obtained with SKP, electrochemical impedance measurements and cyclic voltammetry with respect to validity as a corrosion prediction tool has been reported [218]. 

Differences in detected Volta potentials between pristine and corroded Al-Mg alloy surfaces could be related to the factors influencing thickness and conductivity of the corrosion product layers [219]. Corrosion layers developed in the presence of ion-containing solutions yielded lower Volta potentials and showed higher conductivity. Cathodic delamination of poly aniline-based organic coatings on iron have been studied with SKP [220]. The role of dioxygen reduction and of the poly aniline fraction in the coating were included in a proposed corrosion mechanism. 

In a setup similar to the Evans drop experiment, the height and the potential of an iron surface with a drop of an aqueous solution of 0.5 M NaCl were scanned results are displayed in Fig. 7.24. 

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In order to study the effect of the aging after atmospheric exposure on the potential at the inner buried interface of plasma polymer-coated iron, two different plasma-pretreated iron samples were used. For each pretreatment, different TMS plasma polymer thicknesses were studied 20, 50, and 70 nm for Ar + H2 plasma pretreatment, 20, 55, and 115nm for O2 plasma pretreatment. The Scanning Kelvin Probe (SKP) data shown in Figure 33.18 for (Ar + H2) plasma-pretreated and O2 plasma-pretreated samples depicts a correlation between the potential at the inner buried interface and the polymer thickness for respective sample. SKP results showed that the plasma polymer effectively inhibits reoxidation of the interface... 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

The Kelvin probe is a noncontact, nondestructive, vibrating capacitor technique for measuring work functions, or more precisely the difference between the work function of sample and probe. It was first used by Thomson, later Lord Kelvin, in 1862 [93]. This method has been further improved throughout the following decades [94] and is now a well-established method for measuring work functions, or, from a more electrochemical point of view, Volta potentials. Whereas in traditional Kelvin probes, the probe is a small gold plate or mesh of several square millimeters or centimeters, in Scanning Kelvin Probes (SKP), the probe is a small metal tip with a diameter of typically several tens of micrometers, which can be scaimed across the surface of the sample. 

The work functions of ITO, PEDT, and PAni layers were determined using a scanning Kelvin probe (SKP, UBM Messtechnik GmbH) in a chamber equipped with silica gel giving a relative humidity of 0%. A Cr or Ni wire with a tip diameter of 80 p,m was used as a vibrating reference electrode. The tip was positioned about 20 p,m above the specimen, the vibration amplitude was +10 p,m, and the vibration frequency of the needle was 1.75 kHz. As measurements could not be performed in ultrahigh vacuum, gold was used as reliable reference material. 

The scanning Kelvin probe (SKP,. similar to the EFM technique) permits determination of the work function of CP films. This technique has been used to characterize delamination of a polyaniline containing primer combined with an epoxy topcoat [63] and also to probe the doping-level distribution (from the work function variation) in a conducting poly(2,2 -bithiophene) film [64]. 

The Scanning Kelvin Probe (SKP) serves to measure changes in the corrosion potential of film covered metals [15]. The same method is applicable to the study of... 

The scanning Kelvin probe (SKP) provides a measure of the Volta potential (work function) that is related to the corrosion potential of the metal, withont touching the corroding surface [24]. The technique can give a corrosion potential distribution, with a spatial resolution of 50 to 100 pm, below highly isolating polymer films. The SKP is an excellent research tool to study the initiation of corrosion at the metal/poly-mer interface. 

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