Polarisation resistance probes

Choice of an appropriate reference electrode remains an enigma for non-corrosion-aware personnel-although commercially available polarisation-resistance probes can be adapted. An interesting aspect concerns localised corrosion in that, for some materials, localised corrosion only occurs within characteristic potential ranges. 

Inhibitor control can be effected by conventional methods of chemical analysis, inspection of test specimens or by instrumentation. The application of instrumental methods is becoming of increasing importance particularly for large systems. The techniques are based on the linear (resistance) polarisation method and the use of electrical resistance probes. They have the advantage that readings from widely separated areas of the plant can be brought together at a central control point. (See Section 18.1.)... 

Polarisation resistance This technique, sometimes referred to as linear polarisation resistance (LPR), has been applied widely in industrial monitoring because of its ability to react instantaneously to a corrosion situation or change in corrosion rate " " . The limitation of the technique arises from the necessity to have a defined electrolyte as the corrosive (the author has seen an LPR probe installed in a dry gas-line in an oil refinery). 

A scratch was created on the coated specimen using a rod with a diamond tip on a scratch tester (IMC-1552, Imoto Machinery Co. Ltd.). The load for the scratch was 30 g. The scratched specimen was put in a holder and connected to a conductive wire, followed by immersion in a 0.0005 M NaCl corrosive solution, maintained at 40°C. The electrochemical impedance spectroseopy of the scratched specimen was measured at intervals of 4 h, or more, for a total of 24 h. Measurement data were analysed to calculate the polarisation resistance after the test. Scratched specimens were also immersed in a 0.0005 M NaCl corrosive solution at 40°C for 7 days. The surface appearance of the specimens was observed after the corrosion test to confirm the self-healing properties of the coatings. The seratched part of the specimen was analysed after the corrosion test by Electron Probe Micro-Analysis (EPMA). 

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