Steel is commonly used in marine engineering. However, despite their remarkable mechanical properties, steel-based marine structures are highly susceptible to corrosion-induced damages, which has become a serious concern considering its potential influence on the environment, structural security as well as financial loss. Research attributes 20% of the world’s corrosion losses to microbiologically influenced corrosion (MIC). Special attention has been given to sulfate reducing bacteria (SRB), a corrosive microorganism prevalent in the marine environment. SRB converts sulfate into sulfide ions during respiration, which accelerated the localized steel corrosion. Therefore, the production of sulfide ions plays a critical role in the existence of SRB and should be the main focus in the efforts to reduce microbiologically influenced corrosion of steel structures.
Among the several methods for reducing steel corrosion, coating technology has been considered a promising solution. For instance, water-borne coatings and the addition of biocides can increase material resistance to MIC by preventing the settlement of microorganisms. However, the antibacterial agents released causes significant environmental issues that reduce the coating service lifespan. These issues can be addressed by intelligent coatings capable of delivering the antibacterial agents on demand and in response to environmental stimuli. Additionally, smart self-defensive coatings have demonstrated remarkable antibacterial effects suitable for protecting metals from SRB-induced MIC due to their high sulfide ion sensitivity. Unfortunately, developing structurally stable sulfide ion-responsive self-defensive coatings with effective controlled release and high loading capacity is challenging.
Controlled release technology has been extensively studied. In self-healing coatings, the use of stimuli-responsive hollow mesoporous silica nanoparticles (HMSNs) as nanocontainers for delivering healing agents can be replicated by incorporating biocide-loaded HMSNs into water-borne coatings. Consequently, research findings have revealed that besides sacrificial templates, zeolite imidazolate framework-8 (ZIF-8) can be a facile sulfide ion responsive nanovalves for biocide-loaded HMSNs, despite lack of experimental evidence. Inspired by these results, Chinese Academy of Sciences researchers: Dr. Haoyuan Cai, Dr. Peng Wang, Professor Dun Zhang, Dr. Yu Wang and Dr. Ee Li developed an intelligent self-defensive coating based on sulfide ion responsive nanocontainers for suppressing MIC induced by SRB. Their work is published in the journal, Corrosion Science.
In their approach, ZIF-8 was used as a sacrificial template to fabricate HMSNs. The release system comprised sulfide ion-responsive containers ([email protected]) based on HMSNs loaded with biocide metronidazole (MNZ), a bacterial agent. The nanocontainers were prepared using insoluble ZIF-8 nanovalves formed at the orifice of silica mesopores. Subsequently, the nanocontainers were passed through water-borne alkyd resin and deposited on Q235 carbon steel. The anticorrosion and antiadhesive performances of pure coating and the intelligent self-defensive coatings were evaluated in an SRB-dominated corrosive environment. In addition, the bacterial settlement and their subsequent resistance to MIC were analyzed using various techniques such as electrochemical impedance spectroscopy.
Results showed that the nanocontainers could control biocide release due to the degradation of the ZIF-8 nanovalves when exposed to sulfide ions. The responsive release was initiated at sulfide ion concentration higher than 0.04 mM due to the reaction of the sulfide and zinc ions to form zinc sulfide that slowly dissolved the ZIF-8 gate valve. Furthermore, after 14 days of immersion, the smart coating with [email protected] exhibited the lowest corrosion density, highest impedance, and fewest bacterial settlement than other coating systems. Thus, 14 days represented the optimal resistance capability to SRB-induced MIC.
In summary, the authors developed a smart self-defensive coating consisting of water-borne alkyd resin incorporated with [email protected] sulfide ion-responsive nanocontainers to suppress SRB-induced MIC. The presented coating demonstrated superior performance than other types of coating systems. The excellent anti-adhesion and anti-corrosion performance were attributed to the effective response of the intelligent coating to the variation in the SRB environment and subsequent responsive biocide release. In doing so, it inhibited the SRB settlement, thus decelerating the corrosion rate. In a statement to Advances in Engineering, the author explained their findings significantly contributed to the broader field of controlled release and would specifically guide the development of self-defensive coatings for precise delivery of biocides to suppress MIC.
Cai, H., Wang, P., Zhang, D., Wang, Y., & Li, E. (2021). An intelligent self-defensive coating based on sulfide ion responsive nanocontainers for suppression of microbiologically influenced corrosion induced by sulfate reducing bacteria. Corrosion Science, 188, 109543.