An odd quirk of iron led, eventually, to the first self-healing materials. An iron bar, dipped in strong acid, was just fine. Dipped in weak acid, it was eaten away to nothing. Here's why a diluted acid will succeed when a strong acid fails.
The Industrious Michael Faraday
Michael Faraday spent the first half of the 19th century doing, roughly, everything. The amount of things, in science, that are named "Faraday," is frankly obscene and something should be done about it. What with the Faraday cage and the Faraday constant and the Faraday effect, few people know of the relatively minor phenomenon known as the Faraday paradox of electrochemistry, also called the Faraday paradox of passivation. The "electrochemistry," and "passivation," differentiate it from the more well-known Faraday Paradox which is named for Faraday, and illustrates Faraday's Law of Electromagnetic Induction - because apparently Faraday did not go on even one single vacation during his lifetime.
This particular Faraday phenomenon was noted by Michael Faraday when he dipped an iron bar in very concentrated nitric acid. He had also dipped iron in weakened nitric acid, and the acid ate away at the metal quickly, so he must have expected a huge reaction. He was disappointed. After a little bubbling, the iron bar sat there, at rest in concentrated acid. Faraday guessed that some reaction with the strong acid had caused the outer surface of the iron to form a film that protected the rest of the bar. Being Michael Faraday he was right. What he didn't realize, that he had taken the first step towards the creation of the first kind of self-healing materials.
The Process of Passivation
When he was conducting his experiment, Faraday had luck on his side. Most of the time this particular process takes a bit more rigging than lowering a metal into acid, but Faraday had stumbled on two materials that managed to go through passivation at room temperature. Passivation is the creation of a coating over a metal surface that renders it unresponsive, or passive, in the face of elements that would otherwise corrode it.
Concentrated nitric acid has an interesting property. It promotes oxidization. In biology, oxidization happens when materials combine with oxygen - in other words, when they burn. Chemistry has a slightly different definition for the same word. A material is oxidized when it gives up an electron. The more electrons it gives up, the more it is oxidized. Concentrated nitric acid is an oxidizing agent, an rips electrons off materials dipped in it. Iron is particularly susceptible to nitric acid's effects. After its electrons are ripped away, the biological sense of oxidization takes over, and oxygen does combine with the iron on the surface to create an iron oxide layer. The layer is impervious to the acid, and the interior iron is protected until the oxide layer is scraped off. Dilute nitric acid can't get the same oxidization reaction going, and so it will continue to eat away at the iron until the iron is dissolved.
Electrochemistry and Self-Healing Materials
Over time people other than Michael Faraday saw fit to involve themselves in science. They began looking at the results that Faraday had achieved with iron and acid - and electrochemistry was born. Iron, when treated only with strong nitric acid, was not particularly useful. The slightest scratch and the patina was scraped away. There was also the unfortunate fact that acid didn't work on just any metal.
What worked on lots of other metals was electricity. Nothing rips electrons away like a process designed to rip electrons away. Dipping metals in solution and applying a voltage that "pumps" electrons away oxidizes the outer layers of a metal quickly. Meanwhile, metals that receive electrons are "reduced." Oxidation and reduction are often enough to put a coating on a metal, but chemists took it one step further. They started adding chemicals to the solutions that would combine with the metals while they were in their reduced, or oxidized, state, and plate the surface. This can be done for decoration, as in "gold-plating," but there are more useful applications. This is what lead to the original self-healing materials.
As low-tech as it is, one of the most common self-healing materials was invented back in 1913, when someone figured out that adding a little chrome to iron made it less likely to get stained and corroded. Iron alone stains, and then pits, and then is eaten away. Not so when chrome is added. The iron and chrome grab oxygen atoms and form an oxide that coats and protects the surface. Scratch the surface, and the iron and chrome beneath grab on to oxygen, re-forming the protective layer. Because of Faraday's experiment in the 19th century, people at the beginning of the 20th century understood the principles of how to make a self-healing material.
In a nod to Faraday's original experiment, often the process of making or repairing stainless steel involves his original experiment. Immersing the whole thing in strong nitric acid oxidizes any free iron on the surface of the steel, allowing it to be removed, so the self-healing steel can be repaired and shine.
Top Image: Joe Sullivan