## 1 Introduction  
Aluminum alloys are widely used in rail transit vehicles, aircraft, and other manufacturing industries because of their high strength and low density [1; 2; 3; 4]. In the manufacturing of high-speed electric multiple units (EMU), the commonly used 7050 aluminum alloy has low electrode potential; indeed, when it is in contact with other metals, its corrosion as an anode gets easily accelerated. Unfortunately, its wear resistance is also poor. Therefore, one or more types of coatings are often sprayed on the surface of the aluminum alloy for protection against corrosion [5; 6; 7]. Polyurethane (PU) coating used as the topcoat has the advantages of high gloss, excellent weather resistance, and excellent wear resistance [8; 9]. However, the adhesion of PU coating on light metals such as aluminum alloy is poor and their anti-corrosion performance is also not good. In general, an epoxy coating with excellent adhesion and good anti-corrosion performance is selected as the primer to form the epoxy-polyurethane (EP-PU) coating system [10]. However, the commonly used epoxy primer is a two-component coating; therefore, it needs a suitable pot life in order to maintain the chemical reaction speed to be moderate, thus the drying speed of the EP coating is slow, which eventually affects the manufacturing efficiency [11]. In conclusion, it is crucial to explore alternative technologies to improve the corrosion resistance and adhesion strength of PU on aluminum alloy substrates. As a new surface treatment technology, micro-arc oxidation (MAO) can significantly improve the corrosion resistance of the substrate by direct in situ formation of ceramic coating with alumina as the main component on the surface of aluminum and its aluminum alloy materials [12; 13; 14; 15; 16]. After MAO treatment, two different layers are grown on the aluminum alloy: a dense layer with excellent corrosion resistance and a porous layer capable of improving the adhesion strength of organic coating on the surface of aluminum alloy [17; 18]. Thus, it can be speculated that by comparing the aluminum alloy substrates with and without MAO treatment, it can be verified whether spraying PU coating on the aluminum alloy after MAO treatment can improve the corrosion resistance and the adhesion strength between the PU coating and the aluminum alloy substrate. For example, [PERSON] et al. [19] studied the effect of MAO pretreatment on the protection performance of epoxy magnesium (Mg)-rich primer on a AZ91D Mg alloy. They demonstrated that the adhesion strength of epoxy resin to the Mg substrate increased due to the surface morphology endowed by MAO. [PERSON] et al. [15] showed that the adhesion strength of aluminum alloy samples after MAO treatment to epoxy coating was about twice that of samples without MAO treatment. [PERSON] et al. [20] investigated the effect of baking coating on the corrosion resistance of the MAO layer. It was found that the corrosion current density of Mg alloy samples sprayed with baking coating after MAO decreased by two or three orders of magnitude compared to the samples treated only by MAO; furthermore, no corrosion phenomenon was observed during the long-term immersion test for about 500 h.  
The molecular structure formula of the silane coupling agent 3-aminopropyltriethoxysilane (APTES) is NH\({}_{2}\)(CH\({}_{2}\))\({}_{3}\)Si(OCH\({}_{2}\)CH\({}_{3}\))\({}_{3}\). Silyloxy groups are not only reactive toward inorganic compounds, but also reactive or compatible with organic compounds. Therefore, when the APTES is present between the inorganic and the organic interface, the organic matrix/silane coupling agent/inorganic matrix bonding layer can be formed, which benefits the organic coating to effectively improve the adhesion on the inorganic substrate [21; 22]. [PERSON] et al. [23] used APTES as an adhesive promotion layer in epoxy resin/stainless steel composites to increase the adhesion strength of epoxy resin to stainless steel by more than 20 MPa.