Potential noise

Since the pressure drop is quite high, there is a possibility of approaching sonic velocity in the line. This will result in a potential noise problem. Hence, it is a good practice to limit the velocity to 60 percent of the sonic velocity or a 0.6 Mach number. 

Instruments providing simultaneous measurement of a number of parameters on multi-element probes have been developed, including potential noise , galvanic coupling, potential monitoring, and a.c. impedance . 

Electrochemical noise This is a non-perturbation method and is defined as random low frequency low amplitude fluctuations either of the potential or current in a corroding system. Analysis of the corrosion potential noise can provide information relating to both the mechanism and kinetics of the cor-... 

Keeping the feedback trace short may not always be physically feasible. We should realize that keeping it short is certainly not of the highest priority. In fact, we can often deliberately make it long, just so that we can assuredly route it away from potential noise sources. We can also judiciously cut into the quiet ground plane to pass this particular trace through, so that it is, in effect, surrounded by a sea of tranquility. ... 

Laboratory measurement procedures used for electrochemical data acquisition and analysis during the monitoring exercise are outlined, and particular emphasis is placed on the electrochemical noise techniques. Electrochemical current noise has been monitored between two identical electrodes and the potential noise between the working electrodes and a reference electrode. 

Electrochemical noise monitoring techniques have been used previously in studies of corrosion processes occurring on metals in a variety of environments. Initially, work was directed towards the monitoring of potential noise fluctua-... 

Recent work [6 has been directed towards the simultaneous monitoring of potential and current noise, where the current noise signal is generated by coupling two nominally Identical electrodes with a zero resistance ammeter (ZRA), and the potential noise of the couple is monitored with respect to a reference electrode. In this manner no externally applied signal is required. 

The potential noise signal provides information pertaining to the type of attack, whereas the current noise provides data which indicate the rate of corrosion and the type of attack. When used in parallel, the two noise measurements may be used to estimate the polarisation resistance of the interface being examined. 

The current noise signal was monitored by using a sensitive, low noise zero resistance ammeter (ZRA) to couple pairs of identical electrodes the ZRA acting as a current to voltage converter. This derived potential signal was then fed into a potential noise monitor. 

The derived value of polarisation resistance was evaluated from the ratio of the standard deviation of the potential noise signal to the standard deviation of the current noise signal, i.e. ... 

If we utilise the above equations to describe the low frequency noise signals observed with electrochemical systems, it is apparent that the potential noise signal will provide information pertaining to the value of the Stern Geary constant since ... 

Simultaneous monitoring of current and potential noise and derivation of low frequency values of in5)edance allows, in some instances, direct comparison with polarisation resistance values derived from, for example, a.c. itt jedance techniques. 

Measurement of the potential noise at an electrode can lead (though there are not a few assumptions) to the determination of the cunent passing across the electrode/so-lution interface, and hence, in a conoding electrode, to the corrosion current. It turns out that the corrosion current density is proportional to the reciprocal of the mean square of the noise. 

One way to divide the types of electrochemical noise is by the manner in which it is collected. Potential noise refers to measurements of the open circuit potential of an electrode versus either a reference electrode or a nominally identical electrode. While measurements with a conventional reference electrode have the advantage of being relatable to thermodynamic conditions, these reference electrodes have their own noise associated with them that could complicate analysis. In addition, the application of noise monitoring to field conditions would be... 

Statistical methods are the most popular techniques for EN analysis. The potential difference and coupling current signals are monitored with time. The signals are then treated as statistical fluctuations about a mean level. Amplitudes are calculated as the standard deviations root-mean-square (rms) of the variance according to (for the potential noise)... 

Figure 51 Fast Fourier transform of the potential noise from two nominally identical carbon steel electrodes exposed to 0.2 M HC1 + 0.5 M NaCl + 0.15 M NaN02. (Data courtesy of J. Yuan, M. Inman, T. Lunt, J. Hudson, University of Virginia.)...

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. 

Potential noise is measured by collecting the potential versus time record between a noisy corroding electrode and a noiseless reference electrode using a high-impedance digital voltmeter (DVM). This is essentially a measurement of... 

Area normalization of ECN data is not as straightforward as with other type of electrochemical data (140). Current and potential noise may scale differently with electrode area. For example, if it is considered that the mean current is the sum of contributions from discrete events across the electrode surface, then the variance associated with the mean value will be proportional to the electrode area. The standard deviation of the current noise, o7, a measure of current amplitude, will then scale as the square root of the area. If is assumed that potential noise originates from current noise acting on the interfacial impedance, then aE will scale with the inverse root of the area. Therefore it is inappropriate to normalize current and potential noise by electrode area linearly. On the contrary, area normalization of noise resistance does appear to be appropriate. This is so because the potential and current noise have a constant relationship with one another. As a result, it is appropriate to report noise resistance in units of T> cm2, remembering that the total area for normalization is given by the sum of the areas on both working electrodes. 

With the rapid developments of electrochemical techniques and the required instrumentation electrochemical impendance and electrochemical potential noise and current noise techniques are gaining prominence in corrosion studies. 

Figure 1.33 Apparatus used for potential noise measurements2...

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. 

More detailed information can be obtained from noise data analyzed in the frequency domain. Both -> Fourier transformation (FFT) and the Maximum Entropy Method (MEM) have been used to obtain the power spectral density (PSD) of the current and potential noise data [iv]. An advantage of the MEM is that it gives smooth curves, rather than the noisy spectra obtained with the Fourier transform. Taking the square root of the ratio of the PSD of the potential noise to that of the current noise generates the noise impedance spectrum, ZN(f), equivalent to the impedance spectrum obtained by conventional - electrochemical impedance spectroscopy (EIS) for the same frequency bandwidth. The noise impedance can be interpreted using methods common to EIS. A critical comparison of the FFT and MEM methods has been published [iv]. 

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