Local Electrochemical Measurements Research Articles

Incentives for using local electrochemical impedance methods in the investigation of organic coatings

The assessment of changes in organic coatings in a corrosive environment has traditionally utilized global electrochemical methods, such as electrochemical impedance spectroscopy (EIS), in which a relatively large area of the sample is examined. Recently, more localized methods of impedance measurement have been developed and used to map heterogeneities across a sample surface, and the temporal changes of these regions. These localized methods are essential to further understand the breakdown process of coated alloys that inevitably occurs at discrete sites. This paper will examine the incentives for making local electrochemical impedance measurements, the advantages and limitations of this method, some of the lessons learned, and the present understanding of local breakdown processes on model-coated substrates.

Local electrochemical impedance measurement: scanning vibrating electrode technique in ac mode

The paper gives a new insight into the meaning of local impedances and the related measurement techniques. By numerical simulation it is shown that the only fully relevant quantity is obtained through local values of both the current density and potential. A recently developed equipment based on a commercial SVET device modified for ac polarization is applied to corrosion situations. Under oxygen reduction control of the corrosion rate, serious artifacts are found on the dc current density mapping and local impedance. The drawback is explained by the contribution of a chemical term (redox potential) in the probe potential. A model is elaborated accounting for the resulting side effects in both dc and ac measurements.

Local Measurements in Electrochemistry and Corrosion Technology

The first models describing uniform corrosion were based on the assumed presence of microscopically small local anodes and cathodes. Later this theory was replaced by the mixed potential theory where the corroding surface has the same potential allover even at submicroscopical level. Nowadays local anodes and cathodes are however are very relevant for many models that describe specific forms of localised corrosion. Modern equipment enables measuring local potential and current differences. Amongst these techniques are the Kelvin probe, the Scanning Reference Electrode (SRET), and the Scanning Vibrating Electrode (SVET). Besides these basically electrochemical methods in-situ Atomic Force Microscopy is available to produce sub-microscopic images of local differences in morphology of corroding surfaces. In theory AFM is also be able to measure some other parameters like potential differences, magnetic field and mechanical properties. This paper gives a short theoretical comparison of the different methods along with some experimental data. This' will lead to the conclusion that local electrochemical measurements and AFM are powerful tools for the analysis of many corrosion problems. Even some formerly inaccessible problems like underfilm corrosion can be accurately studied with these techniques. They should however, as any method, be used with care and in in most cases combination with other techniques. In addition it will be shown that the production of equipment for localised measurements is relatively simple. In our laboratories both a Kelvin probe and SVET were built from a simple XYZ stage, a lockin amplifier and some simple home made amplifiers and probes. The experimental data presented will come from this equipment and from a commercially available Kelvin probe.

Adaptation of the Scanning Vibrating Electrode Technique to ac Mode: Local Electrochemical Impedance Measurement

A novel method of measuring local electrochemical impedance using a single vibrating probe electrode has been developed by modification of a commercial device of scanning vibrating electrode technique (SVET). The small size of the vibrating probe used by the SVET, increases the space resolution of the current distribution. Furthermore, the local impedance as defined here takes into account not only the local current density but also the local electrode potential, as opposed to the measurements proposed up to now in the literature. Simulation calculations demonstrate that this definition is the only one which leads to understandable results. The signal processing used to obtain this impedance is explained in detail and tested on a dummy cell including a phototransistor to simulate a vibrating electrode. The local impedances of a corroding Ni disk and of an electrode simulating pitting corrosion are also measured with this system.

Microscopic properties of passive films on Ti and Zr from optical, electrochemical and SXM-measurements

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