Metal materials directly exposed to air, water or other corrosive media are prone to damaged due to various physical and chemical changes, causing huge resource waste and environmental problem. A protective layer can effectively slow down the corrosion of the matrix material by isolating the metal from environmental medium, and its compactness and corrosion resistance are two key factors for the final protective effect.

Compared with inorganic coatings such as enamel and ceramics, organic coatings are more likely to form denser cover on the metal surface, with wide range of sources and good corrosion resistance, have been widely used nowadays. But a critical fact is that microcracks are easily to form on the surface of organic coating under different natural application conditions, allowing water and other corrosive media to penetrate.

To solve this problem, the scientific research team of Zhifeng Lin and Weihua Li from Sun Yat-sen University prepared an epoxy-based coating (thickness about 200 μm) with Fe₃O₄ nanoparticles and tetradecanol which exhibited self-healing ability under solar irradiation and could protect the underlying carbon steel (CS) substrate from erosion. This study has just been published in Frontiers of Chemical Science and Engineering.

Self-healing coating (SHC) is a new type coating which has extended life expectation by repairing microcracks or damage to form a repairable physical barrier, was firstly reported only 20 years ago. Although several ways can be applied to achieve the self-healing coatings, including external repair/corrosion inhibitors addition, polymer resin self-dynamic bonding and thermoplasticity/shape-memory materials usage, UV irradiation or high temperature conditions are necessary to induce chemical reactions, so self-healing coatings achieved under mild conditions are urgently demand.

They found that the SHC coating could be heated to 70 °C in only 200 s under simulated solar irradiation due to the photothermal effect of Fe₃O₄ nanoparticles, which greatly exceeded the melting point of tetradecanol at 37.6 °C. And the perforations and scratches caused by scalpel on the coating surface were repaired only after the simulated solar irradiating for 3 minutes. Its wettability was also proved to increases with the addition ratio of tetradecanol, attributed to a large number of polar hydroxyl groups on it. To testify the performance of SHC coating in real natural environment, researchers took the coating scratch-heal experiment under natural outdoor light as well and finally achieved ideal results.

The corrosion protection efficiency of SHC coating on the underlying metal substrate is verified can be maintained above 99% regardless of the repair. It can effectively prevent the infiltration of O₂, H₂O, and Cl^–, and shows good stability and protective performance even after immersion in 3.5 wt% NaCl solution for 168 hours.

By selecting appropriate liquefied phase change materials, photothermal particles and mix them with suitable content, scientists achieved a novel self-healing of anti-corrosion coating under sunlight in simple way. The SHC coating possesses good photothermal property and can transform into liquid flow under sunlight to fill the defects. Its impermeability and anti-corrosion property are comparable to commercial coatings, expected to be mass-produced and used for corrosion protection in outdoor facilities.

Journal Reference:

1. Zhentao Hao et al (2022). Anticorrosive composite self-healing coating enabled by solar irradiation, Frontiers of Chemical Science and Engineering DOI: 10.1007/s11705-022-2147-1