Active corrosion protection in Ce-modified hydrotalcite conversion coatings

Chromate conversion coatings (CCC) are noteworthy because of their ability to self-heat if mechanical or chemical damage occurs, provided the damage is not too severe. Self-healing. or active corrosion protection (ACP) involves several discrete processes: release of chromate from the coating, its transport through solution, and its action at the site of damage (typically pits). A simple exposure cell, the "simulated scratch cell," was recently devised by Zhao and coworkers to examine CCC for evidence of these processes. This cell consists of two aluminum alloy surfaces, one coated and one bare, separated by a gap several millimeters wide filled with an aggressive solution. If a coating exhibits ACP, the inhibiting agent will be released into solution, interact with the bare surface, and stifle corrosion. Each of these processes can be readily tested and examined using standard laboratory and analytical methods. These techniques were used to study a Ce-bearing conversion coating for evidence of ACP. Candidate coatings were formed on AA 2024-T3 (UNS A92024) substrates by modifying hydrotalcite (HT) coatings with Ce4+ compounds. Results of experiments conducted with the simulated scratch cell show that, when Ce4+ is introduced into HT, a classic ACP response is observed. Specifically, tetravalent Ce compounds in the coating are dissolved by an attacking solution, transported to active defect sites on bare aluminum surfaces, and then reduced and precipitated to inhibit further corrosion. This paper reviews certain aspects of ACP in CCC, summarizes prior studies on Ce-containing conversion coatings, and presents details of new experiments that show that self-healing is possible in Ce-bearing coatings.