## 2 Silane Treatment Technology  
As a new technology, silane treatment technology can significantly improve the corrosion resistance of metal by preparing a layer of silane coating on the metal surface [73; 74]. The dense silane layer not only undertakes the charge transfer from the metal surface to the electrolyte solution interface as a conductive layer but also restricts the direct contact between metal and electrolyte solution as an effective isolation layer, which is considered \begin{table}
\begin{tabular}{c c c c} \hline
**Bipolar Plate Materials** & **Polymer Coatings** & **Nitride Coatings** & **Precious Metal Coatings** \\ \hline Coating methods & silica sol–gel, Nanocasting & Thermal nitriding process & Pulse current \\  & & & electrodeposition, PVD \\ Materials & Conductive polymers & Nitrides & Rare and precious metals \\ Chemical stability & good & good & good \\ Electrical conductivity & fair & fair & good \\ Thermal conductivity & good & fair & good \\ Corrosion resistance & good & good & good \\ Scratch resistance & good & fair & fair \\ Pitting corrosion resistance & good & fair & fair \\ Process complexity & easy & complex & fair \\ Cost & low & low & high \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of different coatings. to be the reason why the silane layer can effectively improve the corrosion resistance of metal [75; 76]. In addition, organic silane coating can be used as a \"bridge\" for the close connection between metal and other polymer coatings, greatly improving the adhesion between the organic coating and the metal. Silane treatment has the advantages of simple operation, strong adhesion, and environmental safety. Generally, the silane coating has a wide application prospect, and it is affected by the polarity of functional groups of silane itself. Silane coupling agents are generally divided into hydrophobic and hydrophilic. Silanes with non-polar functional groups are hydrophobic and need to use a cosolvent to make them soluble in water for hydrolysis. The structure of silane is generally Y-R-SiX\({}_{3}\) or R\({}^{*}\)-(CH\({}_{2}\))m-Si(OR)\({}_{3}\) where R is an aliphatic carbon chain, R\({}^{*}\) (or Y) is a group that can react with organic compounds (such as epoxy group, amino group, etc.) to improve the compatibility and reactivity of polymer and silane, and OR (or X) is a hydrolytic group (such as acetoxy, alkoxy, etc. ), which can bond some metals, SiO\({}_{2}\), etc. Because of these two functions, silane molecules can connect organic materials and inorganic materials to form an \"organic silane chain inorganic\" intermediate layer with a certain network structure. [PERSON] et al. [77] pointed out that hydroxysilane with a carbon functional group can be used for metal surface treatment. If the silane coupling agent does not have an organic group, it can generally be used to prepare a single protective film. For example, compounds with epoxy silane that can match the coating generally have specific carbon functional groups, which can improve the adhesion between the substrate and the organic coating [78]. There are many causes for silane molecules to adsorb and form films on the surface of metal substrates. At present, most people believe that the chemical bond theory proposed is the closest to the actual situation [79; 80]. Figure 3 shows the theoretical model of the silane chemical bond, which summarizes the action process of silane on the substrate surface in three steps:  
(1) Activation of silane molecules. Hydrolysis reaction will occur during the preparation of silane. Under the action of water molecules, the Si-X group will hydrolyze to generate Si-OH, as shown in formula (4). With the gradual hydrolysis of (Si-OR)n, a large amount of Si-OH will be generated in the solution, which will activate the silane molecules for further reaction. \[Si-OR+H_{2}O\to Si-OH+R-OH \tag{4}\]  
Figure 3: Theoretical model of chemical bonding of silane coupling agent [78]. (2) Adsorption and condensation of silane molecules.