Smart Composite Coatings for Corrosion Protection
Authors: Venkatachalam Durgambika , Singh Aakyat , Govindaraj Yoganandan , Jothi Manickam Manoj Prabhakar , Ganapathi Arulkumar , Rohwerder Michael , Neelakantan Lakshman 
Published: February 2024

Corrosion is a major problem faced by the automobile industry. To overcome corrosion, the industry relies on surface coatings in which metallic (Zn, Cr etc.) and polymeric coatings (polyurethane, epoxy, etc.) are widely recommended. However, currently smart, self-healing coatings are considered to be the state of the art option to inhibit cut-edge corrosion and to enhance the life-time. The direct introduction of inhibitors into any matrix creates undesirable effects, majorly the integrity of the inhibitor with the matrix and spontaneous release. Hence, loading inhibitors into a suitable container and subsequent release of the same when the coating is damaged favors the self-healing property of a coating, which results in high durability.

Such a smart coating can be achieved by the synthesis of porous silica particles, which can be an effective corrosion inhibitor container. They possess exceptional properties like nontoxicity, high surface area, and capability to be filled with different types of active materials, including organic, inorganic, and hybrid inhibitors. Hence, these can be the equivalent and non-toxic alternative to chromates which are toxic and carcinogenic to the environment and the living system.

Among the different porous containers reported in the literature, mesoporous silica nanoparticles have large surface area which is beneficial to load the required quantity of corrosion inhibitors. The corrosion inhibitor-loaded porous silica containers are compatible with different matrices, including metals, polymers, and ceramics, to prepare the composite materials. To control the instant release of the loaded inhibitors, encapsulation with a polyelectrolyte is required. As a result, improved self-healing properties can be achieved as compared to the particles without encapsulation.

Among the various types of porous silica, Santa Barbara Amorphous-15 (SBA-15) has several benefits, including the ease of preparation, nontoxic precursors, hexagonal structure to carry a high quantity of material, size range from micro to nanometer, controllable pore size (20-500Å), large specific surface area (∼1000m2/g), porosity, surface morphology and huge pore volume. This work describes the synthesis of mesoporous silica (SBA-15) using a three-step solvo-thermal process. The process includes template preparation (synthesis of mesoporous silica), loading with benzotriazole (BTA) an organic corrosion inhibitor, followed by Poly(diallyldimethylammonium chloride) (PDDA) encapsulation.

The synthesised mesoporous silica was characterised for morphological features, porosity, and surface charge. Based on the BET analysis, successful modification of the mesoporous silica particles at different stages shows a reduction in the surface area and pore volume. The zeta (ζ)-potential measurements showed a significant variation in surface charge after the modification of the mesoporous silica particles. The releasing nature of BTA before and after the PDDA encapsulation was studied using UV-Visible technique at different pH levels. Based on the characterisation of the PDDA encapsulated inhibitor-loaded mesoporous silica particles, appropriate coating was prepared by incorporating these particles into eg. metallic (Zn coating) and/or polymeric (epoxy coating) matrices. The release and corrosion inhibition performance of these coatings were studied using electrochemical techniques. The multi-functional performance of these coatings would be highlighted.