A particularly ingenious remedy for the problem of rust may be available soon.
Containers made of a conductive polymer only open in the presence of corrosion and release anticorrosive payloads.
Scientists from the Max-Planck-Institut für Eisenforschung GmbH in Düsseldorf and the Max Planck Institute for Polymer Research in Mainz have succeeded in making two enormous strides towards developing a self-healing anticorrosion coating. In one study, they embedded a few 100-nanometre-sized polymer capsules containing anticorrosion payloads in a coating. They applied the coating to a metal and exposed the metal to corrosion through a crack in the coating. Thereupon, the capsules opened and released the protective payloads. As soon as the corrosive attack ended, the containers closed again. In the second study, the researchers encapsulated substances in nanocontainers that can heal small cracks and holes in the protective metal coating. The researchers thereby demonstrated that the containers were chemically altered and released the healing payloads when the corrosion process started. The containers then closed again at the end of the corrosive attack.
Human and animal skin is exemplary in many respects. Materials scientists are impressed above all by the way in which it heals itself when damaged. They would like to endow anticorrosion coatings with this very capacity, so that fine cracks and small holes in coatings do not spell disaster in the short or long term for the underlying metal. “We have made two breakthroughs in the quest for intelligent corrosion protection,” reports Michael Rohwerder, Leader of a Research Group at the Max-Planck-Institut für Eisenforschung.
Together with their colleagues from the Max Planck Institute for Polymer Research, the Düsseldorf-based researchers tested capsules made from the conductive polymer polyaniline as containers for anticorrosive substances. They had decorated the nanocapsules with metal nanoparticles to generate suitable electrical contact between the containers and the metal to which they applied the capsules as components in a coating. Through a defect in the protective coating, they exposed the metal to corrosion by trickling a drop of salty water onto the opening in the protective coating. The corrosive attack, however, had no effect, as the walls of the polymer capsules became porous, allowing the substances contained in them to escape, which then blocked the oxygen reduction process.
The electrochemical potential is the most reliable key for opening the capsules
“What is crucial here is to select the correct signal for opening the capsule wall,” says Michael Rohwerder. The capsules can thus be opened purely mechanically when the protective coating is scratched. Or they can react to a rising pH value, which can accompany the process of corrosion. However, the Max Planck team opted to exploit the electrochemical potential as a capsule opener that punctured the polyaniline cover through a process of chemical conversion. “This potential always falls when corrosion starts,” explains Rohwerder. “So it provides the most reliable signal for the capsules to open.” Moreover, electrical contact is required for the capsules to recognise the electrochemical alarm as well. This is provided by the metal nanoparticles between the capsule wall and the metal. The capsules detect when the corrosion has stopped through the same information channel, as the potential rises constantly at this point. The capsule wall then restructures itself and the pores are re-sealed.
The containers, in which the researchers enclosed payloads can be used to form a polymer skin. These payloads can polymerise in a defect and seal the crack or hole. However, in this study the scientists did not apply the capsules to a metal using a coating to test them for corrosion. They replicated the chemical conditions that exist at the beginning and end of the corrosion process with reducing and oxidising substances and opened or closed the capsules in this way. “We were able to repeat this redox process with the polyaniline capsules over 80 times,” says Daniel Crespy, a Research Group Leader at the Max Planck Institute for Polymer Research, who supervised the study.