Stimuli-Responsive Intelligent Coatings With Nano/Microcarriers for Early Corrosion Sensing: Advances and Challenges

The critical threat of metal corrosion to infrastructure highlights the importance of detecting corrosion in its early stages to reduce economic impacts and safety risks. The main corrosion-sensing approaches are based on optical systems including pH indicators, metal-ligand complexation, fluorescent probes, and chromogenic agents. Corrosion causes local changes in pH and metal ion concentrations that enable these sensors to provide a clear alarm triggered by these alterations. Furthermore, emerging microcracks within the coating can initiate corrosion due to the unveiling of the metal surface; therefore, incorporation of pigments into the coatings can help the quick detection of damage and cracks by color change. However, direct incorporation of these sensors into the coating system can cause false alarms due to unintended leakage from the coating matrix. Therefore, their encapsulation inside nano/microstructures can prevent false alarms, increase the lifetime of the sensing system, and increase the mechanical strength of the coating. This review focuses on the advances in corrosion-sensing coatings reported mainly over the past five years, with particular emphasis on the encapsulation of different sensing compounds inside diverse nano/microstructures and their use as corrosion sensors after embedding into coating matrices. The advantages and limitations of each sensing strategy are discussed in detail.