Chromate conversion coatings are noted for their ability to self-heal if damaged by mechanical or chemical action, provided the damage is not too severe. Self-healing, or active corrosion protection (ACP), involves release of chromate from the coating, its transport through solution, and its action at the site of damage; namely pits. A simple exposure cell, named the "simulated scratch cell", was recently devised by Zhao and co-workers to reproductibly examine chromate coversion coatings for evidence of these elements. This cell consists of two aluminum alloys surfaces, one coated and one bare, seprated by a few millimeters of an aggressive solution. If a coating exhibits ACP, the inhibiting agent will be released into solution, interact with bare surface, and stifle corrosion. Each of these elements can be readily tested by standard laboratory methods. Given these testable elements, and methods of examining for them, several chromate-free conversion coatings were studied for evidence of ACP. Candidate coatings were formed on AA 2024-T3 substrates by modifying hydrotalcite coatings with Ce and Mn oxides. Results of experiments conducted with the simulated scratch cell show that when high oxidation state oxides are introduced to hydrotalcite, they can be dissolved by a contacting solution transported to defect sites on bare aluminum samples where they are reduced and precipitated elsewhere to inhibit further corrosion. These experiments show that self-healing is possible with conversion coatings other than those based on chromates.
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