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Speaking at the recent ISA conference in Chicago, Tyler Redslob ofPepperl+Fuchs said that although corrosion costs industries $236 billion annually, most manufacturers continue to use old technologies to attempt to combat the problem. The costs could be managed better if corrosion could be monitored like other process variables such as temperature, pressure or flow.
Three different methods for detectingcorrosion are available, said Mr. Redslob, and now it can be done on-line in a real time format.
The three technologies are Linear Polarisation Resistance (LPR), Harmonic Distortion Analysis (HDA), andElectrochemical Noise (ECN). LPR involves the measurement of the polarisation resistance of a corroding electrode to determine the corrosioncurrent. Since the voltage-current response of a corroding element tends to be linear over a small range, determination of the polarisationresistance allows the corrosion current to be determined. The slope of the response, the polarisation resistance, is inversely proportionalto the corrosion current, thus a corrosion rate can be calculated.
HDA is similar to LPR and measures the resistance of the corrosive solution by applying a low frequency sine wave to the measurement current. Using harmonic analysis, the solution resistance is determinedand combined with the polarisation resistance of the LPR method to calculate a more accurate general corrosion rate.
Developed in theearly 1980s, ECN evaluates the fluctuation in current and voltage noise generated at the corroding metal-solution interface. This techniqueis can be used to detect non-uniform or localized corrosion.
Out of the laboratory
All three methods have been used in laboratoryequipment for many years, however the instruments used required extrasoftware and hardware to interpret data to provide corrosion rates. Incorporating these technologies into a field transmitter enable processes to be controlled, as they are no longer tied to laboratory equipment.
Steps have been made to make corrosion monitoring more automated with LPR and ER probes, however these probes typically output the information into a hand held device for occasional readings taken inthe field. Some allow the probe to be mounted in the field with occasional readings and the electronics get connected to the probe when a reading is necessary. Alternatively, the data are transmitted to special electronics requiring special software to decipher the information.
The issue in the past has not been that corrosion rates could not be measured electronically. The issue has been that this information was contained in manufacturer specific products and software outsideof the normal process control.
With the current Pepperl+FuchsCorrTran technology, corrosion rates can be monitored electronically and transmitted in the industry standard 4-20mA. General corrosion or localised corrosion can be monitored in an on-line, real time format without additional hardware and software. The addition of HART protocolenhances the functionality of this standard signal. As a result of this innovation, corrosion can now be monitored and controlled like otherprocess variables, such as temperature, pressure and flow.
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