LPR, EIS, HDA and IMD are powerful techniques developed originally for short-term laboratory evaluation of metallic corrosion, and are finding increased use in field applications for the determination of general corrosion rate behavior.
With knowledge of icorr, the corrosion rate (CR) is calculated according to:
CR = icorr x seconds per year x atomic mass x electrode area
No. of electrons transferred x Faraday's constant x density
The corrosion rate is expressed in either mils per year (mpy) or millimeters per year (mmpy), according to preferred units of measurement.
Electrochemical Noise (ECN) was initially developed to fill a huge gap left by the more conventional techniques discussed above, particularly the area of identifying when localized corrosion phenomena (such as pitting, crevice corrosion, stress corrosion cracking etc) are occurring. The “steady state” analogy for these phenomena is not applicable, since the localized attack occurs in a stochastic, or random, manner. Electrochemical noise is used pro-actively to identify periods when the corrosion processes become unstable, and to recognize when the probability of localized corrosion is high. ECN differs from LPR in that no electrical perturbation of the electrodes is required. The technique measures small, naturally occurring variations in current (current noise, typically microamps) and potential (potential noise, typically millivolts). When general corrosion is occurring the potential and current fluctuations can be processed to provide a general corrosion rate using a similar data treatment to that outlined above for LPR except using a resistance noise (RECN) in place of Rp. When localized corrosion phenomena occur, the characteristics of the fluctuations change dramatically, and these characteristics can be used to identify the most probable mode of attack. From a plant corrosion control perspective, this is important, since the technique provides an early warning of incipient localized corrosion.
Electrochemical Noise has been widely applied to the evaluation of localized corrosion phenomena (pitting, crevice corrosion, environmentally assisted cracking, microbiologically influenced corrosion, etc.). Localized corrosion can occur on many metals, in fact surveys have shown that some 70-90% of all corrosion failures are due to localized corrosion. The difference between general and localized corrosion is in the distribution of anodic and cathodic sites on the metal surface. In the case of general corrosion, the distribution is quite even. Localized corrosion results from situations where a local anodic area can be supported by a relatively large cathodic area. Examples may include where a local breakdown in a protective film occurs on the metal surface, e.g. in passive materials, where scale has partially formed, where a local coating holiday occurs. The potentially catastrophic nature of localized corrosion is realized often through the appearance of pinholes in the metal which, e.g. in the case of pressurized systems, can lead to explosive failure of the equipment.
It is important to note that the electrochemical evaluation of localized corrosion is necessarily limited to a qualitative assessment of the corrosion mechanism, as opposed to the quantitative rate information available for general corrosion attack. This is because it is impossible to predict the number, size and distribution of pits or cracks. Accordingly, a pitting probability or 'Pitting Factor' provides an easily understood representation of localization, where a low value close to 0 (zero) represents general corrosion and a high value closer to 1 (one) represents that localized corrosion is occurring.
Electrochemical noise data has been studied for more than twenty years across a broad range of industrial field applications and a comprehensive assessment of the localized corrosion data resulted in a preferred method of statistical evaluation, namely skewness and kurtosis. In brief, these techniques allow the distribution of current noise and potential noise data to be analyzed statistically as compared with the normal distribution of signals anticipated in the case of general corrosion. The kurtosis shows whether the distribution is more peaky or flat than 'normal', whereas the skewness indicates a shift of the distribution to the left or right of 'normal'. Further information about this data appraisal can be obtained from Reference 4.
Taking Electrochemical Monitoring Into the Field: Some of the considerations to be given to migrating electrochemical techniques to a plant environment are:
1. The choice of a suitable location to insert a probe. Ideally the corrosion measurement should be made in an area where the corrosion processes will be most active, such that the “worst case” scenario is covered. Occasionally this may present some difficulties, particularly in case where the corrosion zone may fluctuate as a consequence of local conditions, for example in condensing environments.
2. The type of probe to use. What will be the maximum operating temperature and pressure? Are the insulating materials suitable for the environment? Can a conventional “off the shelf” finger style probe be used, is a flush mounted probe or a “flow through” probe more appropriate? What exposed surface area of the material should be used (this has a direct impact on the sensitivity of the measurement techniques – low corrosion rates require larger surface areas exposed than high corrosion rates). Are there special temperature control requirements? Consideration should be given to the expected probe lifetime, and the complexity of the design of the probe (the more robust solutions are usually the simplest).
3. Identifying the most appropriate automated monitoring technique Is there a requirement for quick response? Is localized corrosion an issue? Can the measurement be made autonomously on a regular basis without operator control? Fortunately the electrochemical techniques are generally suited to providing a measurement within a period of minutes, the issue really becomes one of quality of data.