Impedance measurement, corrosion monitoring

The first attempts to use impedance measurements for monitoring the corrosion of metals in cathodic polarization conditions were made in the U.S.A. in the 1980s. Ruck et al. (1985) and Thompson et al. (1988) analyzed the nature of impedance spectra obtained for steel exposed in soil and concrete at various levels of cathodic polarization (from (.orr to -1.2 V vs. CSE). They observed significant quality differences be-... 

Neufeld, P. and Queenan, E. D., Frequency Dependence of Polarisation Resistance Measured with Square Wave Alternating Potential , Br. Corros. J., 5, 72-75, March (1970) Fontana, M. G., Corrosion Engineering, 3rd edn., McGraw-Hill, pp 194-8 (1986) Dawson, J. L., Callow, L. M., Hlady, K. and Richardson, J. A., Corrosion Rate Determination By Electrochemical Impedance Measurement , Conf. On-Line Surveillance and Monitoring of Process Plant, London, Society of Chemical Industry (1977)... 

ENA was recently used for remote on-line corrosion monitoring of carbon steel electrodes in a test loop of a surge water tank at a gas storage field. An experimental design and system for remote ENA and collection of electrochemical impedance spectroscopy (EIS) data (Fig. 13) have been presented elsewhere. In the gas storage field, noise measurements were compared with electrode weight loss measurements. Noise resistance (R ) was defined as... 

Wherever possible, there should be a continuous secondary barrier for the entire pipeline system, in the form of jacket pipes, collecting basins, and channels. In some cases, this is not possible because it would impede cathodic corrosion protection or because differential expansion of the pipe and outer jacket threaten the systan. The secondary barrier can then only consist of intensive organizational and operative backup measures shorter inspection periods, shorter distances between shutoff points to limit possible losses, more elaborate provisions for leak detection, continual ronote monitoring of the pipeline systan, both internally and externally, equipment for tackling anergency events and catastrophes, contingency plans, and so forth. 

Dawson, J. L., Callow, L. M., Hlady, K. and Richardson, J. A., Corrosion Rate Determination By Electrochemical Impedance Measurement, Conf. On-Line Surveillance and Monitoring of Process Plant, London, Society of Chemical Industry (1977)... 

Kihira et al. [118] applied EIS to investigate the condition of the rust film formed on the weathering steel, and proposed new corrosion monitoring method based on rust film resistance. Nishimura et al. [119] measured the electrochemical impedance of a carbon steel covered with rust film formed in a wet/dry environment containing chloride ions. They reported that the charge transfer resistance (/ p) increased with the wet-dry cycles of exposure. 

Electrochemical impedance. spectro.scopy, AC probe.s. EIS, although around since the 1960s, has primarily been a laboratory technique. Commercially available probes and monitoring systems that measure EIS are becoming more widely used, especially in plants that have on-staff corrosion experts to interpret the data or to train plant personnel to do so. 

Impedance spectroscopy This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed above. The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes, which are taking place. 

A simplification of the polarization resistance technique is the linear polarization technique in which it is assumed that the relationship between E and i is linear in a narrow range around E . Usually only two points ( , 0 are measured and B is assumed to have a constant value of about 20 mV. This approach is used in field tests and forms the basis of commercial corrosion rate monitors. Rp can also be determined as the dc limit of the electrochemical impedance. Mansfeld et al. used the linear polarization technique to determine Rp for mild steel sensors embedded in concrete exposed to a sewer environment for about 9 months. One sensor was periodically flushed with sewage in an attempt to remove the sulfuric acid produced by sulfur-oxidizing bacteria within a biofilm another sensor was used as a control. A data logging system collected Rp at 10-min intervals simultaneously for the two corrosion sensors and two pH electrodes placed at the concrete surface. Figure 2 shows the cumulative corrosion loss (Z INT) obtained by integration of the MRp time curves as ... 

In some cases, the physical size of a reference electrode can be important when space is constrained either in a laboratory cell or in a field monitoring application. The success of microdevices for monitoring environmental, physiological, and corrosivity variables has been greatly impeded by the lack of a robust, inexpensive RE. As discussed in Chapter 5, a RE placed too close to a surface can affect the current distribution and lead to erroneous potential measurements. 

Electrochemical noise consists of low-frequency, low-amplitude fluctuations of current and potential due to electrochemical activity associated with corrosion processes. ECN occurs primarily at frequencies less than 10 Hz. Current noise is associated with discrete dissolution events that occur on a metal surface, while potential noise is produced by the action of current noise on an interfacial impedance (140). To evaluate corrosion processes, potential noise, current noise, or both may be monitored. No external electrical signal need be applied to the electrode under study. As a result, ECN measurements are essentially passive, and the experimenter need only listen to the noise to gather information. 

Among the electrochemical techniques available for the study of corrosion of metals, potential monitoring is the most simple and inexpensive of all. Its nondestructive character is also an advantage. The measurements are made using only a reference electrode and a high impedance voltmeter (Fig. 9). The readings correspond to the open circuit... 

AC impedance relies on the ability to monitor the behavior of a coating that absorbs water by modeling it as a capacitor. By measuring capacitance increase as a function of immersion time, the diffusion coefficient of water and the amount of saturated water within the coating film can be calculated. In addition, any severe under-film corrosion can be monitored by a drastic decrease in the coating resistance and charge transfer resistance. 

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