# Recent Trends and Progress in Corrosion Inhibitors and Electrochemical Evaluation
## 3 Types of Corrosion Inhibitors
### Composite-Based Corrosion Inhibitors
#### 3.4.4 Sol-Gel-Based Composites  
The migration of water and electrolytes through the sol-gel topcoat can cause corrosion processes at the metal-oxide contact. Pre-emptive healing occurs when water and other corrosives are trapped, and/or when the hybrid sol-gel releases inhibitors that specifically target and stop the corrosion reactions. As shown in Figure 8, self-healing chemicals could also repair damage to the topcoat caused by weathering and/or other factors [83].  
The slow condensation processes of molecular precursors in a liquid medium that produce an oxide network are known as the sol-gel method. Sol-gel protective coatings have been shown to offer higher corrosion resistance, better oxidation control, and superior chemical stability for metal substrates. Additionally, the sol-gel process has the potential to replace hazardous pre-treatments and coatings that have typically been used to increase metals' corrosion resistance and is an environmentally beneficial method of surface protection [84]. [PERSON] et al. [85] developed eco-friendly sol-gel-based saline silane-based composite coatings reinforced with praseodymium cations (Pr\({}^{3+}\)) and benzimidazole that have a synergistic self-healing effect. In aluminum matrix composites supplemented with SiC particles, [PERSON] [86] and colleagues developed mono- and multi-layered silica coatings achieved through a sol-gel technique for their corrosion protection. To replace the current three- or multi-coat methods, a colored sol-gel topcoat that [PERSON] and his coworkers developed over a zinc primer enhanced production while preserving the protective sacri  
Figure 8: Schematic diagram illustrating the mechanism of the sol-gel composite-based coating [83].  
ficial properties. However, the cerium (III) system in the sol-gel coating finally attained the maximum impedance and proved to be the most effective choice. Molybdate and cerium (III), two corrosion inhibitors, were added to sol-gel coatings to provide inhibitive protection in addition to the barrier protection [87]. For the purpose of preventing corrosion in the AZ91D magnesium alloy, [PERSON] and his team [88] developed a unique composite film using the sol-gel method. Electrochemical tests were performed in a 3.5 wt.% NaCl neutral solution to evaluate the anticorrosion properties of the sol-gel-coated and untreated samples. The outcomes showed that composite films with ace inner film and a mag outer film had improved corrosion resistance.  
Sol-gel-based composites have drawn interest due to their adaptability and capacity to produce customized materials with a variety of uses. They do, however, have some restrictions that must be taken into account. When numerous drying and annealing phases are necessary, the sol-gel process can be time-consuming. This might affect turnaround time and production efficiency [89]. Sol-gel materials may shrink significantly while drying and curing, which can cause cracks and other flaws in the finished composite. In order to reduce these problems, proper processing methods are necessary [90].