Characterization of Chromate Conversion Coating Formation and Breakdown Using Electrode Arrays

Chromate conversion coating (CCC) formation and breakdown was examined using 25 element Al wire electrode arrays. Arrays were interrogated using a multichannel analyzer capable of separately recording currents from each electrode. During CCC formation, electrodes exhibited a 30 s period of intense electrochemical activity characterized by large net currents. On any given element, net current polarity was found to be predominantly anodic, predominately cathodic, or mixed. After 30 s, net currents decayed to small values, which remained small out to 300 s of exposure. Raman spectroscopy showed that concentrations in the coating continued to increase during this electrochemically quiescent period, suggesting continued CCC evolution. Conversion-coated arrays were subject to anodic potentiodynamic polarization in 0.5 M NaCl until all elements on the array exhibited coating breakdown and substrate pitting. Breakdown potentials were found to increase with coating time up to 120 s, indicating anodic inhibition in CCC corrosion protection. Breakdown was found to be more difficult on electrodes that were net cathodes during coating formation. Results also showed that the NaF and in commercial CCC bath formulations strongly contributed to coating corrosion resistance. Without Fv, the Al surface passivated quickly during coating formation, and a nonprotective film formed. Without CCCs exhibited lower breakdown potentials. © 2002 The Electrochemical Society. All rights reserved.