Electrochemical noise (the spontaneous fluctuations in current or potential associated with corroding electrodes) has been studied for about 37 years, but it is only relatively recently that a reasonably sound theoretical basis for the technique has been derived. It is now clear that the technique provides a method, the determination of electrochemical noise resistance (Rn) that provides a reasonable estimate of the corrosion rate. It also seems probable that it is able to provide information about the type of corrosion occurring, although the optimal technique for doing this is not yet established. Furthermore, there are indications that although the estimate of corrosion rate is relatively robust, indicators of corrosion type are very susceptible to extraneous influences. However, as an estimator of corrosion rate, electrochemical noise measurement is relatively poor compared to conventional techniques, but its potential ability to identify the type of corrosion occurring remains its major advantage.
The measurement of electrochemical noise, EN, for the study of corrosion processes is now relatively well-established. In this paper the properties of the technique and the methods of analysis will be reviewed, and its capabilities compared with those of other electrochemical methods.
MEASUREMENT OF EN
Initial measurements of EN recorded the fluctuation in potential, usually measured relative to a conventional reference electrode. This is known as the electrochemical potential noise, EPN. Subsequently it was appreciated that it was also possible to measure the fluctuation in current or electrochemical current noise, ECN. This can either be done by maintaining a constant potential with a potentiostat, or by coupling two electrodes through a zero resistance ammeter (also know as a current amplifier). This then led to the now conventional three-electrode method wherein the ECN is measured between two nominally identical working electrodes, while the EPN is measured with a third electrode, which may either be a reference electrode or a third electrode that is nominally identical to the two working electrodes.
It is important to appreciate that there are a number of artefacts that can be introduced during the measurement process, including instrument noise, aliasing and quantization noise. These are detailed in. An important characteristic of the measurement process is the sampling rate of the measurement. This is frequently of the order of 1 sample per second, as is it often found that the power present in the EN signal is very small at higher frequencies. It is also much simpler to obtain reliable measurements when the sampling frequency is well below the power line frequency, since this presents a major noise source at higher frequencies. In addition, the only well-established theoretical analysis depends on the properties at very low frequencies. Despite this, it may be useful to make measurements at higher frequencies in some situations, and this probably merits more careful examination.
Specimen area has a somewhat counter-intuitive effect on measured EN. Workers who are used to normal measurements of electrochemical potential and current tend to assume that electrochemical noise can be treated in the same way, so that the potential noise, measured as the standard deviation of potential, öE , so that it has units of volts, is independent or area, while the standard deviation of current, öI , is proportional to the specimen area, A. However, this is not what is expected for ?normal? noise behaviour; instead óE is expected to be proportional to A-1/2, while öI is expected to be proportional to A1/2. Note that there are some situations, notably wh