Electrochemical methods noise

In general, low level detection is masked by the noise level inherent in any measuring device. Electrochemical methods are susceptible to electrical interference from external sources, variations in reference electrode parameters resulting from aging or contamination, and interference from redox... 

Developments in electrochemical methods since 1976 for measurement of corrosion have been rapid. Research and development has produced several new techniques, e.g. a.c. impedance and electrochemical noise. These methods require corrosion expertise for both operation and interpretation. Industry generally prefers instrumentation that can be operated by process... 

In spite of its limited sensitivity, colorimetry is still useful in determination of elemental concentrations in the g range or higher (Seiler, 1988). Its main advantage is that the needed instrument, a spectrophotometer, is common in every laboratory. Colorimetric trace metal determinations are based, commonly after sample decomposition, on selective separations from interfering ions (Abbasi et al., 1988). Automated colorimetric procedures are described for the determination of N and P in trees (Stewart et al., 1990). Modern spectrophotometers provide high stability, low noise, and the advantages of computerised background control. However, for total metal determinations in environmental samples, this method is less frequently applied and has been replaced by atomic spectroscopic and electrochemical methods (Stoeppler, 1991). 

It is preferable to carry out laboratory corrosion tests and to validate the data with service tests for the selection of materials. It is needless to note that the chosen test method be reliable and cost effective. Some of the test methods in use in industry are service tests, field tests, laboratory tests, and rapid electrochemical methods such as potentiodynamic polarization, linear polarization, electrochemical impedance and electrochemical noise. 

Flicker-noise spectroscopy — The spectral density of - flicker noise (also known as 1// noise, excess noise, semiconductor noise, low-frequency noise, contact noise, and pink noise) increases with frequency. Flicker noise spectroscopy (FNS) is a relatively new method based on the representation of a nonstationary chaotic signal as a sequence of irregularities (such as spikes, jumps, and discontinuities of derivatives of various orders) that conveys information about the time dynamics of the signal [i—iii]. This is accomplished by analysis of the power spectra and the moments of different orders of the signal. The FNS approach is based on the ideas of deterministic chaos and maybe used to identify any chaotic nonstationary signal. Thus, FNS has application to electrochemical systems (-> noise analysis). 

Potential step methods have emerged as valuable electrochemical methods due to the highly sensitive nature of the technique. The waveform employed in potential step methods, also referred to as pulsed methods, have some advantages over potential sweep methods. The main advantage is that the steplike waveform can discriminate and separate the capacitive current versus the faradaic current, the current due to the reduction or oxidation undergone by the analyte, increasing signal to noise. Capacitive versus faradaic current discrimination is the basis for all of the pulsed techniques. The rate of decay of the capacitive current and the faradaic current is not the same. The capacitive current has an exponential decay whereas the faradaic current decays as a function of t Since the rate of decay of the capacitive current is much... 

Measurement of the electrochemical current noise is aimed at correlating the observed current fluctuations with breakdown and repair events that might lead to the formation of stable growing pits [53, 54], In view of this mechanistic interpretation, the application of statistical methods to the occurrence of current spikes and the observed probability of pit formation lead to a stochastic model for pit nucleation. The evaluation of current spikes in the time and frequency domain yields parameters such as the intensity of the stochastic process X and the repassivation rate r [53]. They depend on parameters such as the potential, state of the passive layer, and concentration of aggressive anions. 

Electrochemical methods are not as widely used in process analysis as spectroscopic methods and methods based on measurement of physical rather than chemical properties of the material to be analyzed or characterized. Calibration of the analyzer is always a problem and no general procedure can be recommended or applied because it depends on the particular application and analytical method. Poisoning and deactivation of the active surface of the electrode shortens the lifetime of the sensors. Accumulation of static electricity on the body of the sensor creates extra noise. 

For corrosion in environments with high-temperature gases, the ability to detect corrosion in real time suffers from the lack of a good (sufficiently conductive) electrolyte for electrochemical measurements. However, electrochemical methods, including noise and impedance measurements, have been and are being... 

The veirious types of measurement technologies for assessment of corrosion may be summarized as shown in Tables 2 to 5. These techniques cover both laboratory Jind field use. However, many of the direct methods, particularly the electrochemical methods of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are generally more suited to laboratoiy evaluation. In the laboratory, test conditions are clean and more controlled. Consequently, more sophisticated measurement electrode systems can be used that take advantage of their more sophisticated measurements technologies. In the field, practicalities of changing process conditions, high flow rates, debris, electrical noise, and electrical safety limit their use. 

Iverson, W. P. and Heverly, L. F., "Electrochemical Noise as an Indicator of Anaerobic Corrosion, Proceedings, Symposium on Nondestructive Testing and Electrochemical Methods of Monitoring Corrosion in Industrial Plants, ASTM STP 908, ASTM International, West Conshohocken, PA, 1984, pp. 459-471. 

This potentiostatic test method involves the monitoring and analysis of electrochemical current noise signals emitted by copper tubes immersed in a water that is linked to cold water pitting corrosion. It is claimed that the method provides a rapid and reliable metuis to distinguish between satisfactory and unsatisfactory tubes, especially in the borderline region. 

Optical sensors offer some advantages over electrochemical methods [10, 11]. First, no reference electrode is required however, reference intensity is necessary to minimize environmental effects on the system. Second, fiberoptic sensors are immune to electrical noise, but ambient light can be a problem. Finally optical sensors have the potential for higher information content than electrical sensors because there is a complete spectrum of information available. However, the linearity is usually limited to a very narrow range. 

The laboratory development of sophisticated electrochemical methods, including impedance zero resistance ammetry (ZRA) and electrochemicat noise. 

On the other hand. Little et al., who did not use polarisation methods but instead one of the safest electrochemical methods, electrochemical noise analysis (to be discussed later in Chapter 6) reported the corrosion-enhancing effects of another type of IRB, Shewanella purefaciens [32]. 

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... 

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