A new type of hot-dip Zn-5Al-0.5Mg-0.08Si and Zn-5Al alloy coatings was performed on the cold rolled common steel. The hot-dip process was executed by self-made hot-dip galvanising simulator. SEM and EDS test results demonstrated that Mg was mainly distributed in crystal boundaries. XRD test results showed that the corrosion product of Zn-5Al-0.5Mg-0.08Si alloy coating was almost Zn5(OH)8C12·H2O. The features of Zn5(OH)8C12·H2O are low electric conductivity, insolubility and good adhesion.The corrosion resistance of alloy-coated steels was detected by neutral salt spray test. The microstructural characterization of the coating surface after neutral salt spray test and removing the corrosion products revealed that the corrosion process of Zn-5Al-0.5Mg-0.08Si coating was uniform and the coating surface was almost flat. As a result, the corrosion resistance of Zn-5Al-0.5Mg-0.08Si coating has a remarkable improvement with a factor of 9.2 compared with that of Zn-5Al coating. 1. Introduction Owing to excellent mechanical properties, large production, and low-cost manufacture, steels are applied widely in nearly every field, such as machinofacture, building, and dunnage. But its resistanc to atmospheric corrosion is very low in most environments [1–7]. Hot dipping is a very efficient method to protect steels from corrosion, and zinc coating can provide a barrier and galvanic protection for steel substrate [1, 2, 5]. The corrosion resistance of zinc coating can be enhanced by adding alloying elements, for example, Al, Mg, RE, and Ni [5–12]. The effect of Mg element is the most remarkable among them. But detailed information on the structure and properties of the coating is limited in published literatures [4], and the reason for enhanced corrosion resistance has not yet been clarified definitely [2, 8, 13]. In this paper, the corrosion mechanism of hot-dip Zn-5Al-0.5Mg-0.08Si coating was studied in detail by the coating microstructure and corrosion products analysis. 2. Experimental Procedures 2.1. Materials Purities of Zn, Al, Mg, and Si were 99.99%, 99.7%, 99.94%, and 99.93%, respectively. The substrate was common cold rolled steel (DC01D). 2.2. Process of Pretreatment and Hot Dipping The process of pretreatment and hot dipping was as follows: alkaline degreasing (NaOH, 15？wt. %) at 80°C for 5 minutes with a soft brush→water flush→remove iron oxides with HCl solution (18 vol.%, 70°C)→water flush→electrolysis fuxing (the parameters was according to China patent, no. 201010296310.4)→dried at 120°C for 20 minutes in a oven→hot-dip. Table 1 shows some factors
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