Metals are one of the most durable and versatile materials known to man. For centuries, different metals and alloys have been used in a broad spectrum of applications in numerous sectors including the automotive, biomedical, military, electronics, and oil and gas industries. Their popularity is mainly attributed to their ability to withstand harsh conditions such as high temperatures and pressure.
Despite their numerous desirable properties, their susceptibility to corrosion continues to be a significant concern. This corrosion can substantially deteriorate metals, resulting in loss of thickness, decreased load carrying capacity, cracking and structural failure. (For more on these topics, read Why Understanding the Stress Concentration Factor (Kt) is Important When Evaluating Corrosion in Metal Structures.)
However, researchers at the Massachusetts Institute of Technology (MIT) have discovered that certain metal oxides, when applied in sufficiently thin layers, possess distinctive properties that offer superior levels of corrosion resistance compared to other oxides. These special oxides have been termed self-healing metal oxides.
How do Self-healing Metal Oxides Work?
To appreciate how self-healing metal oxides work, it is essential to first understand the metal oxidizing process and how it ties into corrosion resistance.
Most metals, with the exception of gold, when exposed to moisture and oxygen undergo an electrochemical reaction called oxidation. This process causes an oxide layer to form and cover the surface of the metal. In steel, this oxide layer is hydrated iron (III) oxide, commonly referred to as rust. In many metals, the oxide layer is brittle and flakes away easily, thus exposing more of the metal to air and moisture to create more rust. This continuous cycle gradually consumes and deteriorates the metal.
However, in specific metals, this oxide layer is durable and tightly adheres to the surface, thus forming a protective barrier. This protective barrier acts as a shield that blocks the intrusion of air and moisture, thereby preventing further corrosion. Three metal oxides known to offer barrier protection are aluminum oxide, chromium oxide and silicon dioxide. These oxides are commonly used as the primary compounds in several anti-corrosion paints and coatings.
Although it was widely accepted that these oxides provided superior corrosion resistance, it wasn’t until MIT scientists observed them using highly specialized instruments that the behavior of these self-healing oxides was fully understood, particularly when subjected to stresses due to pressure.
The MIT team used a modified version of a transmission electron microscope (TEM), called an environmental transmission electron microscope (E-TEM). This apparatus allowed the team to observe the test samples in the presence of gases or liquids as opposed to conventional test methods that require vacuum conditions. Using the E-TEM, researchers were able to simulate conditions that promote stress corrosion cracking and to test how these oxides protect metal substrates exposed to oxygen while placed under mechanical stress.
In most cases, the deflection of the metal's surface due to the presence of pressure can cause cracks to form in the protective layer. As a result, oxygen penetrates the oxide barrier and comes into contact with the bare metal substrate to cause further corrosion. The MIT researchers, using the microscopic level resolution of the E-TEM, determined that the well-known coating material, aluminum oxide, could address the shortcomings of traditional protective coatings. It was found that in thin layers of about 2 to 3 nanometers thick, aluminum oxide, despite being solid, can exhibit liquid-like flow behavior.
This attribute allows the aluminum oxide layer to elongate as the metal deforms under pressure, keeping the metal substrate covered and preventing the intrusion of oxygen. It was discovered that this thin oxide coating consists of no grain boundaries and can be stretched to up to twice its length without developing any cracks, even under the strain of stretching. This is in contrast to thicker applications of aluminum oxide, which is known to shatter under stress due to its brittle nature.
Applications for Self-healing Oxides
Self-healing metal oxides, such as aluminum oxide, can help prevent corrosion in applications where the deformation of metal surfaces can cause brittle metal oxides to crack, leaving the substrate vulnerable to corrosion attack. In adequately thin layers, aluminum oxide stretches to fill gaps and cracks as they form. This is especially useful in reactor vessels, where the structure’s walls are constantly exposed to superheated steam and are under stress from internal pressures.
Since thin layers of aluminum oxide do not easily crack and do not possess any grain boundaries, they can also be used to prevent the diffusion of molecules through metals, such as would occur with hydrogen in fuel-cell-powered cars or radioactive tritium in the cores of nuclear power plants. This application can create safer working environments because such leakages can create hazardous conditions for plant operators and consumers. Additionally, self-healing aluminum oxides can increase the cost efficiency of plant operations by reducing the need to frequently replace expensive machinery and equipment.
Self-healing aluminum oxide must, however, be applied in a layer thin enough to achieve this liquid/ductile behavior. Thicker layers, though effective against other types of corrosion, can be brittle and crack when deformed due to stress corrosion cracking. (Metallic coatings in general are discussed in How Metallic Coatings Protect Metals from Corrosion.)
What sets self-healing metal oxides apart from typical metal oxides is their ability to exhibit fluid-like flow properties. Contrary to the brittle behavior of many metal oxides, self-healing oxides stretch or elongate along with the coated metal, keeping the metal substrate covered without developing any cracks. This makes it especially useful in applications where metal components are susceptible to intergranular corrosion and stress corrosion cracking.