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°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.