Formation and corrosion inhibition mechanisms of chromate conversion coatings on aluminum and AA2024-T3

Chromate conversion coatings (CCCs) are applied to aluminum alloys to enhance their resistance to localized corrosion and to increase paint adhesion. However, chromate is toxic and suspected carcinogen. To develop environmentally friendly alternative coatings, a detailed and accurate understanding of CCC formation and breakdown is needed. Several studies on CCC formation and breakdown were conducted in this regard.; A first set of experiments was aimed at studying CCC formation and breakdown on 25-element Al electrode arrays. Results from coating formation experiments show that the coating process occurs in two stages. The first stage is characterized by intense electrochemical activity on the array and last from 20 to 30 seconds. The second stage occurs under electrochemical quiescence and little measurable current flows among elements in the electrode array. Raman spectroscopy shows that the coating continues to adsorb Cr6+. Anodic polarization of conversion coated arrays in chloride solutions led to several important findings. First, it was found that pitting potential increases as coating time increases through both stage one and stage two coating formation. These results also show that changes in coating structure and chemistry occur during the electrochemically quiescent second stage of coating formation. Further analysis showed that pitting potentials were higher on electrode elements that were net cathodes during first stage CCC formation than on electrode elements that were net anodes. Results also showed that the supplemental ingredients, NaF and K₃Fe(CN)₆, are essential to CCC formation and contribute greatly to increasing the corrosion protection provided by the coating.; A second set of experiments was aimed at characterizing the effect of aging on CCC structure and properties. CCCs are dynamic due to the fact that they continue to polymerize after they are removed from the coating bath. Using cathodic polarization experiments carried out in aerated chloride solutions, it was found that CCCs less than 48 hours old inhibited cathodic reactions. With increased aging time in ambient lab air, cathodic inhibition was lost. This loss of cathodic inhibition was attributed to shrinkage cracking resulting from coating dehydration and continued polymerization. The relative humidity of the environment in which coatings aged was also found to have a significant effect on the CCC aging process. After aging in dry air (RH ∼ 0%), CCCs contained significant shrinkage cracking. Cr6+ release from such coatings was severely inhibited and corrosion resistance was poor in electrochemical testing. CCCs aged in ambient lab air (RH ∼ 50%) exhibited less shrinkage cracking, a 2-order of magnitude increase in Cr6+ release, and considerably greater corrosion resistance. These results support the notion that CCC aging is strongly influenced by coating dehydration.*; *This dissertation is multimedia (contains text and other applications not available in printed format). The accompanying CD requires the following system requirements: Windows MediaPlayer or RealPlayer.