Tafel Fit


Description

The Tafel Fit command is used to calculate a corrosion rate from Tafel or potentiodynamic data. Tafel analysis assumes that the corrosion reactions are under kinetic control. See the Review of Electrochemical Corrosion Theory if you’re not familiar with Tafel analysis and the mathematics describing kinetically-controlled corrosion reactions. Our Tafel Fit routine uses a non-linear chi-squared minimization to fit the data in the selected region to the Stern-Geary equation. The fit uses four adjustable parameters: Icorr, Ecorr, Beta A and Beta C. The minimization algorithm is quite complex. It makes a number of estimates for the values of the four parameters. After each estimate, a “goodness of fit” is evaluated. A new estimate for the parameter values is then made, using the well-known Marquardt algorithm. The process is repeated until the fit stops improving, or until a preset number of iterations have been made. In order to use this command, a region encompassing Ecorr must be specified on the Active trace.

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 either the mouse or keyboard curve-selection tools.
  3. After you select a region, click the Tafel Fit command on the Experiment Specific menu. A dialog box presenting the user-adjustable parameters appears. You may change these values can be changed, or you may leave them alone. The following selections are available on the Tafel Analysis Setup dialog box: Iterations, Use Seed Values, Seed Values, Weighting
  4. When you are satisfied with the input parameters, click Calculate. The algorithm now begins to fit the selected region. When the fit is complete, a new trace is added to the chart. The resulting values as adjusted by the fit are displayed in the Tafel Fit dialog box.
  5. Click the Close button to close this dialog box when you are finished reviewing the values and are satisfied with the fit.
  6. View the results for the fit by moving the mouse cursor over the fit and watching the tool-tip. The results are also presented on their own page with the Tafel caption.