Chemometric Investigation of Polarization Curves: Initial Attempts

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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