Zero resistance ammeter monitoring

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

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. 

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 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. 

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. 

A variety of techniques based on noise exists, but the most common uses two identical working electrodes and a noise-free reference electrode situated between the two working electrodes. The current flowing between the two working electrodes is measured by a zero-resistance ammeter, and their potential is monitored versus the reference electrode. 

This recently reported [26] technique is similar to the Synthetic Crevice approach. In this method, two parallel flow channels are operated at different water flow rates. The low flow channel becomes an active anode, which is connected through a zero resistance ammeter to the high flow channel, which acts as a cathode. The current flow then indicates the corrosion rate of the anodic surface. This technique shows promise as a continuous on-line instrumental monitor that will show changes in corrosion rate under deposits. 

A zero-resistance ammeter is connected between two metals, and the galvanic current is directly measured as a function of time. At the same time, a reference electrode can also be used to monitor the galvanic couple potential, which can be used to determine the galvanic corrosion if a third metal is to be connected with this couple. Most commercial potentiostats can be used as a zero-resistance ammeter by changing the electrode connections. 

With this electrochemical technique galvanic currents between dissimilar electrode materials are measured with a zero resistance ammeter. The design of dissimilarities between sensor elements may be made to target a feature of interest in the system being monitored (e.g., different compositions, heat treatments, stress levels, or surface conditions). Zero resistance ammetry (ZRA) may also be applied to nominally identical electrodes in order to reveal changes occurring in the corrosivity of the environment. 

If we were to place a zero-resistance meter between the two electrodes, we could monitor the amount of charge that flows. Such a meter would be called an ammeter if it measured the current, or a coulometer if it measured the charge. (In practice, most modem meters are multi-function devices and can measure both, changing from one function to another at the flick of a switch.)... 

The advantage of these techniques is that they are permanently set up and can be used to monitor the total charge passing with time. This is a clearer indication of the total metal loss or total oxide produced than an instantaneous measure of the corrosion rate. Also the measurement equipment is fairly straight forward, being a voltage measurement across a resistor or a direct zero (or low) resistance ammeter measurement of the current. 

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