Electrochemical noise measurements

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

Measurements of current using same-metal electrodes are utilised for electrochemical noise measurements see section below). 

Application of Electrochemical Noise Measurements to Coated Systems... 

During the period of immersion of the samples in sodium chloride electrolyte, electrochemical noise measurements were made using the electronic apparatus previously described. The time records obtained were analysed using statistical techniques to derive mean, standard deviation and coefficient of variance. 

With electrochemical noise measurements the d.c. potential of two coupled identical electrodes is governed by the sample with the lowest in iedance. It is this lower value of inpedance which is monitored by the noise technique, i.e. that of the worst coating of the pair. 

Electrochemical noise measurements have shown great promise as a monitoring tool in studies of corroding metals in a variety of environments. 

It is common in corrosion laboratories and in field corrosion monitoring probes to immerse two vertical rods parallel to one another in an electrolyte. In the lab, one of the rods consists of a high-density graphite counterelectrode while the other is a working electrode. A reference electrode may be placed in between the two rods. In the field, polarization resistance or electrochemical noise measurements are often made between two nominally identical rods that both consist of the material of interest. The primary current distribution is nonuniform with respect to circumferential position about each electrode when the distance between the two rods is small in comparison to the radius of the rod, Fig. 10a (16). Again, the value of Ra varies from where the rods face each other to where they... 

Figure 57 Corrosion rate of carbon steel in 3% sodium chloride solution as a function of exposure time. (From J. L. Dawson, p. 3, Electrochemical Noise Measurements for Corrosion Applications, ASTM STP 1277. ASTM, Philadelphia, PA (1996).)...

Noise measurements are extensively used in the studies of metastable pits. Pistorius37 discussed several factors that can influence the proper interpretation of electrochemical noise measurements (ENM). These factors can be probe size, sampling rate, and system noise. The current measurements seem to give clearer information on the corroding system than that of the potential28 46... 

Electrochemical corrosion measurements using electrical resistance or polarization-resistance-types probes. Electrochemical noise measurements should give an interesting dimension, but this is not a routine technique. 

Electrochemical noise measurements may be performed in the potentiostatic mode (current noise is measured), the galvanostatic mode (potential noise is measured), or in the ZRA mode (zero resistance ammeter mode, whereby both current and potential noise are measured under open-circuit conditions). In the ZRA mode, two nominally identical metal samples (electrodes) are used and the ZRA effectively shorts them together while permitting the current flow between them to be measured. At the same time, the potential of the coupled electrodes is measured versus a low-noise reference electrode (or in some cases a third identical electrode). The ZRA mode is commonly used for corrosion monitoring. 

When following potential changes, eg electrochemical cell capacitance charging or discharging, ac impedance measurements, or electrochemical noise measurements, the bandwidth response of the measuring instrument may limit the application. 

Kearns, J.R., ScuUy, J.R., Roberge, P.R., Richert, D.L., Dawson, J.L., 1996. Electrochemical Noise Measurement for Corrosion Apphcations. ASTM International, West Conshohocken. 

Wood, R.J.K., Wharton, J.A., Speyer, A.J., et al., 2002. Investigation of erosion—corrosion processes using electrochemical noise measurements. Tribology Int. 35, 631—641. 

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