These instances have sparked intense research in the field of smart anti-corrosion coatings on a worldwide scale. The general idea is to enable a safe storage of active substances combined with an intelligent release of them only when needed. This means equipping the coating with the ability for a case sensitive self-healing of corrosive damages. Possible trigger signals exclusively associated to active corrosion that initiate the release of inhibitors encapsulated in microcapsules are: change in ionic strength, increase of pH and change of electrochemical potential. It will be shown that the latter one is the most reliable trigger signal, and moreover, that its spreading velocity laterally within the coating can be controlled by the addition of a continuous conductive polymer film deposited in between the metal and the microcapsules. This trigger signal spreading velocity represents a key factor to overcome the main challenge for smart corrosion coatings to self-heal even macroscopic defects, which is to activate a great number of microcapsules under atmospheric corrosion conditions (triggering and transportation of inhibitors only possible laterally within the coating – no full immersion). It will be demonstrated that for redox-responsive coatings the reduction rate of microcapsules (release of inhibitors) indeed is in tandem with the delamination rate manipulated by the underlying polypyrrole interlayer. Additionally, it is possible to incorporate the microcapsules within the polypyrrole interlayer thus forming an addressable capsular network thereby further increasing the number of released inhibitors (for one additional layer of microcapsules incorporated by a factor of approx. 2.5 compared to microcapsules applied directly on zinc (see 7)). Besides, the electrodeposited polypyrrole on zinc insures steady passivity under non-active conditions serving a stable basis for microcapsules deposition or incorporation.
1 Stratmann, M. 2005 W.R. Whitney Award Lecture: Corrosion stability of polymer-coated metals - New concepts based on fundamental understanding. Corrosion 2005, 61 (12), 1115-1126.
2 Simões, A., Grundmeier, G. Corrosion Protection by Organic Coatings. In Encyclopedia of Electrochemistry; Wiley-VCH, 2003; Vol. 4, pp 500-566.
3 Plieth, W., Bund, A. Mechanism of Corrosion Protection by Metallic Coatings. In Encyclopedia of Electrochemistry; Wiley-VCH, 2003; Vol. 4, pp 571-572.
4 Markley, T.; Dligatch, S.; Trinchi, A.; Muster, T. H.; Bendavid, A.; Martin, P.; Lau, D.; Bradbury, A.; Furman, S.; Cole, I. S. Multilayered coatings: Tuneable protection for metals. Corros Sci 2010, 52 (12), 3847-3850.
5 REACH Regulation, 2006R1907 — EN — 09.10.2012 — 014.001 2006.
6 Sinko, J. Challenges of chromate inhibitor pigments replacement in organic coatings. Prog Org Coat 2001, 42 (3-4), 267-282.
7 Vimalanandan, A.; Lv, L.-P.; Tran, T. H.; Landfester, K.; Crespy, D.; Rohwerder, M. Redox-Responsive Self-Healing for Corrosion Protection. Advanced Materials 2013, 25 (48), 6980-6984.