Influence of Water Immersion Post-Treatment Parameters on Trivalent Chromium Conversion Coatings Formed on AA2024-T351 Alloy

In this work, the influence of water immersion post-treatment, with consideration of temperature and pH values, on trivalent chromium conversion coating morphologies, compositions and corrosion protection has been investigated, using high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and electrochemical impedance spectroscopy. Compared to the coating formed without the water immersion post-treatment, the post-treatment in the deionized water bath at 40 degrees C evidently reduced the concentrations of fluorine-containing constituents and increased the extent of oxides/hydroxides across the coating. Consequently, the enhanced corrosion protection property was investigated in the post-treated coatings during full immersion in naturally-aerated NaCl solution with mass fraction of 3.5%. In terms of water pH effects, the coatings displayed comparable thicknesses after post-treatment in the as-prepared deionized water bath (pH 5) and neutral water bath (pH 7); However, a considerable shrinkage was observed in the caustic water bath (pH 9). It was found that the resultant corrosion resistance of the coatings was greatest after post-coating treatment in the water bath of pH 5 and the least for pH 9. Inductively coupled plasma-atomic emission spectroscopy was used to trace small amounts of Zr and Cr elements in the used water baths. A robust chemical titration, adding 1, 5-diaminonaphthalene into the used water baths, revealed that Cr species were only in trivalent form. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.