Capsular Networking and Accelerated Trigger Signal Spreading Velocity in Smart Redox-Responsive Coatings for Corrosion Protection
Authors: Uebel Matthias , Rohwerder Michael 
Published: February 2020

The protection mechanisms provided by current industrial coatings are still based on technical developments dating well back into the 20th century.1 These are mainly predicated on providing a barrier between the underlying metal and the environment or on sacrificial consumption.2,3 In practice usually combinations of these approaches are applied in order to maximize the overall corrosion protection performance since each of them has got its limitations – e.g. the barrier achievable by organic coatings never will be able to totally prevent diffusion of water, oxygen and ions and to fully block reactions at the interface metal/coating. Sacrificial coatings, such as zinc alloy coatings, will be used up after some time of corrosion, and corrosion inhibitors released from pigments will even without corrosion be continuously depleted by leaching and hence have to be added in large quantity to ensure sufficient corrosion protection in the long-term perspective. The uncontrolled release of inhibitors by leaching from organic coatings upon water uptake not only presents a loss of active substances but also is associated with environmental pollution - among other consequences.4 As a result the use of the most potent inhibitors, considering e.g. hexavalent chromium, are banned from the European market by law due to their environmental and health concerns.5 Most likely the number of restricted inhibitors will increase with time and consequently might also include several which are nowadays still in use.6Furthermore, the ban is expected to expand to countries beyond Europe in the future.
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.
References:
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