Chemometric Investigation of Polarization Curves: Initial Attempts
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Outline Polarization Curves Data Methods Results Future Work Definition and Terms Motivation for Chemometric Approach Differences from Spectroscopic Data Data Methods Results Future Work Acknowledgements
Chemometric Investigation of Polarization Curves: Initial Attempts
Christopher A. Marks. Center for Electrochemical Science and Engineering. University of Virginia. Charlottesville, VA – USA.
Definition and Terms. Motivation for Chemometric Approach. Differences from Spectroscopic Data. Data. Methods. Results. Future Work. Acknowledgements.
Electrochemical measurements of net current density (i (A cm-2)) as a function of potential (E (V)) for a given electrolyte and working electrode. E versus some reference electrode. Importance of net current. i and log(|i|)
E versus some reference electrode. i and log(|i|) Exchange current density can change. Diffusion limited current density changes as a function of [O2], pH and stirring, etc.
Exchange current density can be a function of electrolyte. Alloys are more complicated than a pure metal, non-stoichiometric dissolution. Passivity is more complex than indicated.
inet = ianodic + icathodic. M Mn+ + ne- 2H20 O2 + 4H+ + 4e- O2 + 4H+ + 4e- 2H2O. Only a small fraction of what is of interest can be measured experimentally. Net curve is offset slightly for clarity. E versus some reference electrode. Importance of net current. Define Eoc, Ep, and ipass.
Resolve net current data into components which are simple functions of pH, [Cl-], [O2], [Mn+], etc. so that the important parameters, Eoc, Ep, and ipass, can be optimized or interpolated. Compare to anova, etc. of values picked from curves.
Non-constant domain (non-random missing data) Variable uncertainty in i, depends on i, not E. How to calculate 2 Discuss how uncertainty in i is a function of i,
240 (partial) polarization curves – 8 shown.
No mean-centering or scaling. Missing values replaced by estimates (Xestij=tipj) Iteratively re-weighted least squares. Estimate loadings (p=(t’t)-1t’X) Calculate variable weights (v) based on p (vi = # obs / (a priori uncertainty for pi)2. Estimate scores (t=Xdiag(v)p’(pdiag(v)p’)-1) Go to 1, until convergence. Orthogonalize p with respect to the previous P.
Several outliers identified and removed. Algorithm is slow to converge. First factor.
Results 6 component residuals are bothersome, not making progress but still large residuals
Verify target function and weighting. Non-orthogonal P. Simulated data. Smaller/simpler data sets. Non-negative T (P ) and/or Rotations. PLS and other techniques. Time series. EIS, 3-way Spatial electrode arrays.
B.A. Kehler, G.O. Ilevbare, J.R. Scully, Comparison of the Crevice Corrosion Resistance of Alloys 625 and 22, CORROSION/2000, paper no. 182, NACE, B.A. Kehler, Crevice Corrosion Electrochemistry of Alloys 625 And C22, University of Virginia, Charlottesville, January, John Scully, Rob Kelly, et al. – CESE. Jack McArdle – UVa Psychology. WSC1 presenters and participants.
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