Polarization Resistance


Description

Polarization Resistance calculates a corrosion rate from polarization resistance data (a small-signal I-versus-E curve near Eoc). See the Review of LPR if you’re not familiar with polarization resistance experiments and the typical calculations done on their data.

The macro calculates a linear least-squares fit of the current-versus-voltage curve over a selected region. The slope of the fitted line has units of ohms. This slope is the polarization resistance from which the technique takes its name. The fit also yields an estimate for Ecorr. The polarization resistance value is then used to calculate Icorr and corrosion rate. In order to use this command, you must select a region on the Active trace using the mouse selection tool .

Typically the region selected is ±10 mV of the Eoc point. The format of the graph must also be such that voltage is on the x-axis and current is on the y-axis. The axes must also be in a linear format.

Method

  1. Make the trace on which to perform the fit the Active Trace. Do this by right-clicking on the trace and clicking Activate Trace.
  2. Select a region on this trace using the Select Portion of Curve tool . To use this tool, left-click with the mouse near one bounding point of the region. Next, left-click again near the second bounding point of the region. The region becomes highlighted. To re-select a region, toggle the Select Portion of Curve tool, and try again.
  3. When a region is selected, click the Polarization Resistance command in the Experiment Specific menu. If there is an error, a dialog box specifying the type of error appears. Follow these directions and repeat the steps. If the graph is in the correct format, a dialog box presenting the user-adjustable parameters (Beta A and Beta C) appears. You may change these values, or you may leave them alone. The units for the β values are entered in volts/decade.
  4. When you are satisfied with the input parameters, click Calculate. The algorithm now begins to fit the selected region.
  5. Results are presented both on their own PolRes page, and in the QuickView area at the bottom of the chart. You can hide the QuickView window by clicking the X on the side-bar of the QuickView.
  6. View the results for the fit by moving the mouse cursor over the fit and watching the tool-tip.