New techniques for inhibiting corrosion are needed because of the challenge in detecting this phenomenon as it initiates in a specific location. Once localized corrosion starts, it can be difficult to detect and stop what potentially can lead to catastrophic failure of a specific machine.

The potential for corrosion to occur at the metal surface means that coatings applied must be robust. But coatings can be damaged by scratches and cracks leading to exposure of metal to corrosion-inducing conditions. Development of self-healing coatings is one approach to minimize the initiation of localized corrosion.

Jiaxing Huang, professor of materials science and engineering at Northwestern University in Evanston, Ill., indicates that there have been two main approaches in the past for preparing self-healing coatings. He says: “The first method involves the preparation of a polymeric coating that contains reversible bonding. Once a small crack forms in the metal surface that causes bonds within the coating to break, heating can be done to reestablish the bonding, which enables the polymeric coating to fill the small crack. 

“The second method is similar to how clotting minimizes the flow of blood when human skin is punctured. Small capsules or capillary tubes containing monomers can be imbedded in a coating. Once a crack forms, the capillary tubes break, enabling the monomers to polymerize in the presence of oxygen to form a new coating that quickly solidifies over the crack.”

In a previous TLT article (1), the second technique was used by researchers to produce a self-healing coating. Microcapsules containing two monomers (hydroxyl end-functionalized polydimethylsiloxane and polydiethoxysiloxane) and a tin-based catalyst were embedded in an epoxy vinyl ester matrix. After a razor blade was used to expose a steel substrate, the coating self-healed, enabling the metal to remain protected even in the presence of a sodium chloride solution for 24 hours at 50 C.”

Huang says, “A self-healing coating must deliver new material to fill cracks through use of a mass transport or delivery system. This process must be done without the need for human intervention.”

While fluids have excellent self-healing properties, can they be used directly as coating materials? One challenge in developing a fluid-based, self-healing coating is finding the right viscosity of the material to fill the crack yet also produce a stable coating. Huang says, “Low-viscosity corrosion-inhibitor fluids can flow readily to a crack, but they will not form stable coatings once the crack is sealed. In contrast, higher viscosity fluids can form stable coatings but cannot readily move to seal a crack.”

A new strategy combines the beneficial characteristics of low- and high-viscosity fluids to produce a self-healing coating.

Graphene oxide-based microcapsules
Huang and his colleagues developed a self-healing coating where an oil is thickened through the inclusion of hollow microcapsules derived from reduced graphene oxide. He says, “A recent trend has been to incorporate thickeners into self-healing oils. We found that the use of reduced graphene oxide that has a very low density of 0.12 grams per cubic centimeter is effective in producing self-healing coatings.”

The microcapsules are prepared by spray drying a mixture of graphene oxide and polystyrene followed by annealing. The microcapsules contain interconnected voids between 200 and 250 nanometers with thin graphene walls less than 10 nanometers thick. The reduction process makes the graphene oxide hydrophobic so it is compatible with materials such as silicone oil. A high-viscosity oil can be prepared through introduction of these microcapsules at a treat rate of 5 weight %. 

The researchers conducted a series of stability and performance tests using silicone oil impregnated with the reduced graphene oxide microcapsules. Figure 2 shows testing done by coating an aluminum foil boat loaded with a methylene blue dye solution and placing the boat in a two molar hydrochloric acid solution. 


Figure 2. The top figure shows an uncoated aluminum foil boat loaded with methylene blue dye solution degrading after only eight minutes in a two molar hydrochloric acid solution. In contrast, the same aluminum foil boat coated with the graphene oxide microcapsule-thickened oil shows no evidence of degradation in the middle figure. The bottom figure shows an uncoated aluminum wire reacting with hydrochloric acid as evidenced by the hydrogen gas bubbles on the left and a coated aluminum wire on the right that is protected against degradation. (Figure courtesy of Northwestern University.)

As the top figure shows, after only eight minutes the uncoated aluminum foil boat leaking methylene blue dye in the acid solution. The entire aluminum foil boat took 20 minutes to completely dissolve.

When coated with the graphene oxide microcapsule-thickened oils, the aluminum foil boats did not show any evidence of degradation after being placed in the acid solution for 24 hours, as shown in the middle figure. In the bottom figure, the self-healing coating was applied directly to a bare aluminum wire immersed in the hydrochloric acid solution. Once applied, the coating acts as an effective barrier as shown by the wire on the right. The left wire was left uncoated and reacts with the hydrochloric acid to generate hydrogen gas that is shown bubbling away from the metal surface.

The researchers found that some wires remained protected under these severe conditions for more than a year. Self-healing testing showed that the coating was capable of healing submillimeter to millimeter scale scratches within seconds. The researchers raised the concentration of the hydrochloric acid solution to 10%, and still the self-healing coating was effective. Huang says, “Experiments with 20% hydrochloric acid solution proved to be too difficult for the self-healing coating to overcome because the rapid release of hydrogen gas prevented the oil film from sealing the crack.”

Huang believes this self-healing coating can be developed with other lightweight materials besides reduced graphene oxide. “This type of approach could be used to enhance the corrosion inhibition of marine coatings,” he says. “Another application is protecting coatings from microbial contamination. A mobile surface should make it more difficult for microbes to become established on surfaces and form biofilms.”

Additional information can be found in a recent article (2) or by contacting Huang at jiaxing-huang@northwestern.edu. 

REFERENCES
1. Canter, N. (2009), “Coating: Heal Thyself,” TLT, 65 (6), pp. 16-17.
2. Lim, A., Cui, C., Jang, H. and Huang, J. (2019), “Self-Healing Microcapsule-Thickened Oil Barrier Coatings,” Research, Article ID: 3517816.