Galvanic probes

The galvanic probe continuously monitors the corrosion characteristics of the drilling fluid. The probe (Figure 4-473) consists of two dissimilar metal electrodes, usually brass and steel. The electrodes are mounted on, but insulated... 

If the instrument indicates current surge in an air-free system, it generally implies hydrogen sulfide contamination, but the galvanic probe is usually best suited to detect corrosion influenced by oxygen contamination. 

Galvanic Probes. In contrast to the polarographic electrode, a galvanic probe utilizes an anode of zinc, lead, or cadmium and a cathode of silver or gold, where a silver cathode and lead anode are the most common (Linek et al., 1985). Figure 4.3b shows a schematic representation of a typical galvanic probe. The electrochemical reactions that take place at the probe surface are as follows (Linek et al., 1985 Turner and White, 1999 van Dam-Mieras et al., 1992) ... 

Similar to the polarographic probe, the galvanic probe is constrained by the rate-limiting step of oxygen diffusion across the probe membrane. Thus, the current output of the probe is linearly related to the dissolved oxygen concentration in the bulk fluid. 

Galvanic probes to monitor the effects of galvanic corrosion are much less common. Special care must be taken to consider the effect of relative areas of the different alloys in the plant as compared to those on the probe elements. 

Use of Galvanic Probe Corrosion Monitors in Oil and Gas Drilling and Production Operations, NACE Publication 1C187, NACE, Houston, TX, 1987. 

Instruments such as hydrogen probe, corrosometer, corrater, pirameter, galvanic probe, and oxygen meter... 

Direct correlations between the corrosivity of the fluid measured by a galvanic probe and the performance of the less noble constituent of an equivalent bimetallic couple that exists within the process plant should be made with care, as the surface area ratio between the two metals is critical in determining the magnitude of the galvanic effect... 

Conventional galvanic probes comprising a brass cathode and mild steel anode are sensitive to the concentration of oxidizing species in conductive fluids and may be used to monitor the level of dissolved oxygen and the effectiveness of oxygen scavengers in water injection and cooling water systems. 

Galvanic probes comprising parent metal, weld metal, and heat affected zone combinations may be used to assess the potential for preferential weldment corrosion within the process streams. 

Incorrect information can result if the probe is made of the wrong material and is not heat treated in the same way as the process equipment (as well as because of other problems). The probe must be as close as possible to the material from which the equipment of interest is made. Existence of a critical condition, such as weldments or galvanic couples or occluded cells in the eqmpment of concern, makes the fabrication, placement, and maintenance of the probes and monitoring system or critical importance, if accurate and useful data are to be obtained. 

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 . 

Galvanic current Measurement of the galvanic current between two different metals can be easily measured using a zero resistant ammeter ". This method can have specific application, e.g. to provide a signal indicating failure of a protective coating in a process vessel. Commercial probes are available for industrial monitoring. 

The concentration of dissolved oxygen in a fermenter is normally measured with a dissolved oxygen electrode, known as a DO probe. There are two types in common use galvanic... 

In the galvanic detector, the electrochemical detector consists of a noble metal like silver (Ag) or platinum (Pt), and a base metal such as lead (Pb) or tin (Sn), which acts as anode. The well-defined galvanic detector is immersed in the electrolyte solution. Various electrolyte solutions can be used, but commonly they may be a buffered lead acetate, sodium acetate and acetic acid mixture. The chemical reaction in the cathode with electrons generated in the anode may generate a measurable electrical voltage, which is a detectable signal for measurements of DO. The lead is the anode in the electrolyte solution, which is oxidised. Therefore the probe life is dependent on the surface area of the anode. The series of chemical reactions occurring in the cathode and anode is ... 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

COj/Oj in the Off-Gas CO2 evolution from a bioreactor is closely related to the physiological state and the activity of microorganisms in a bioreactor because CO, evolves as a result of catabolism and respiration by microorganisms or cells. Therefore, it is helpful to measure the content of CO2/O2 in the exhaust gas in order to understand the physiological climate of a bioreactor. The CO2 and O2 content in the exhaust gas are taken from the streamline and analyzed by infrared spectrophotometer (CO2) and galvanic cell probe (O,). The wet off-gas must be desiccated before being introduced into the gas analyzer. 

A galvanic dissolved oxygen probe measures the partial pressure of oxygen by use of an electrode based on the following reactions. 

If, however, solid electrolytes remain stable when in direct contact with the reacting solid to be probed, direct in-situ determinations of /r,( ,0 are possible by spatially resolved emf measurements with miniaturized galvanic cells. Obviously, the response time of the sensor must be shorter than the characteristic time of the process to be investigated. Since the probing is confined to the contact area between sensor and sample surface, we cannot determine the component activities in the interior of a sample. This is in contrast to liquid systems where capillaries filled with a liquid electrolyte can be inserted. In order to equilibrate, the contacting sensor always perturbs the system to be measured. The perturbation capacity of a sensor and its individual response time are related to each other. However, the main limitation for the application of high-temperature solid emf sensors is their lack of chemical stability. 

Reverse osmosis skids are typically contained within a frame of galvanized or urethane-coated steel. Skids should be designed for easy access for monitoring and maintenance. Access to controls, instruments, valves, the pump and motor, and membranes is essential. Access to the permeate from each pressure vessel is often overlooked. Without such access, profiling and probing used to troubleshoot poor performance is not possible (see Chapter 14.7). 

The geometries discussed above are relevant to laboratory scale experimental cells. The geometries discussed below are more appropriate for situations encountered in industry or practical application involving galvanic corrosion, cathodic protection, or field corrosion probes. 

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