How do self-healing metal oxides protect against corrosion?
Self-healing metal oxide is a term used to describe a special category of metal oxides that are known to exhibit extraordinary corrosion resistance. This term was coined by academics at the Massachusetts Institute of Technology (MIT) when they found that specific metal oxides, when applied in adequately thin layers, possessed properties that made them superior to other metal oxides.
Metal oxides are formed when the surface of a metal reacts with air and moisture in the atmosphere to undergo an electrochemical reaction called oxidation. In some metals, this reaction results in the creation of an oxide layer that covers and adheres to the metal's surface. This layer acts as a protective barrier that prevents further air and moisture from coming in contact with the steel substrate, thus inhibiting further corrosion.
Three metal oxides in particular are known to offer barrier protection that surpasses the performance of other oxides; these are chromium oxide, aluminum oxide and silicon dioxide. While it was widely acknowledged that these three oxides provided exceptional corrosion resistance, it wasn’t until MIT researchers observed them using special instruments that their self-healing mechanism was fully understood. (You can learn more in the article A Look at Self-Healing Metal Oxides as a Corrosion Prevention Method.)
Conventional metal oxides tend to develop cracks when the metal's surface deflects due to the application of external pressure. These cracks cause air and moisture to penetrate to the level of the exposed metal substrate, resulting in further corrosion. MIT researchers found that self-healing metal oxides such as aluminum oxide, when applied in thin layers of about 2 to 3 nanometers thick, can exhibit liquid-like flow behavior in response to surface deflection.
This liquid-like attribute allows self-healing oxides to elongate or stretch. As such, cracks are prevented, and the metal's surface remains covered and protected during deformation. Researchers also found that these self-healing oxides possess no grain boundaries and are capable of being stretched up to twice their original length without developing any inconsistencies.
The flow-like properties of aluminum oxides and other self-healing oxides are only maintained when applied in sufficiently thin layers. As the thickness of the layers increases, the coating tends to become more brittle, causing it to shatter rather than flow under excessive stress and strain.