Effect of Mg and Nano-TiO 2 on the Marine Protective 2 Properties of Zn-Al Coatings 3

: According to research, we have learned that Mg and TiO 2 are new types material of 12 marine protective coatings. We found that the addition of Mg can improve the performance of 13 Zn-Al coating passivation film, and TiO 2 has excellent photocatalytic self-cleaning performance. In 14 this paper Zn-Al pseudo alloy coating was prepared by cold spray technique, and Zn-Al-Mg-TiO 2 15 pseudo alloy composite coating was prepared by adding Mg and nano-TiO 2 . The effects of Mg and 16 TiO 2 to the marine protective properties of Zn-Al coatings were studied by friction and wear test, 17 dynamic salt water corrosion test, electrochemical test, scanning electron microscope(SEM), energy 18 dispersive spectrometer(EDS) and super deep scene 3D microscope. The results show that the 19 addition of Mg and nano-TiO 2 not only fills the gap of the coating and improves the density of the 20 coating, but also generates grid-like flocculent corrosion products on the surface of the coating 21 which can gather other corrosion products to improve the density of corrosion products, reduce the 22 friction coefficient and corrosion rate of the coating surface, effectively prevent the invasion of Cl - in 23 solution, and improve the wear and corrosion resistance of the coating.


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In the field of anticorrosive coatings for marine equipment, Zn-Al series coatings are the focuses 29 of current research [1]. As early as the 1920s, research on Zn-Al protective coatings has been carried 30 out abroad and applied in practice [2]. It was found that the Zn-Al alloy coating combines the 31 advantages of Zn coating and Al coating. Zn-Al coating not only has excellent electrochemical 32 cathodic protection performance of pure Zn coating, but also forms corrosion passivation film on the 33 surface of the coating when addition of Al element which can effectively slows down the corrosion 34 rate [3][4][5].

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In recent years, people have tried to add Mg element to Zn-Al coatings for research which can 36 significantly improve the corrosion resistance of the coatings [6][7][8]. Yao et al through the study of 37 Zn-Al-Mg coating prepared by hot dipping, It was found that the addition of magnesium can not 38 only improve the microhardness of the coating, but also form special corrosion products, block the 39 passage of oxygen and water, and reduce the corrosion rate [9]. Joung et al, study Zn-Mg coatings 40 and found addition of magnesium resulted in the change of coating structure and corrosion product 41 film which had better corrosion resistance than pure zinc coatings[10].

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TiO2 is a newly marine antifouling and self-cleaning material with excellent chemical 43 stability [11][12][13]. TiO2 has excellent self-cleaning effect because it can react to produce hydroxyl 44 groups under light which can form a physical adsorption layer on the surface. In addition, its own 45 nanostructure has a certain shielding effect. Therefore, it has a good application prospect in the field 46 of marine equipment anti-fouling [14][15][16][17]. Nano-TiO2 was successfully added to varnish in Ford 47 Central Laboratory, USA, to obtain coatings with good weatherability and scratch resistance [16].

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Therefore, this paper intends to prepare Zn-Al-Mg-TiO2 coating by cold spraying. Then study 49 the effect of Mg and TiO2 addition to the corrosion resistance of Zn-Al coating with experiments, and 50 discuss the anti-corrosion mechanism of Zn-Al-Mg-TiO2 coating, so as to provide some reference for 51 further research of marine metal anti-corrosion coating.

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The sample is cut into 10 x 10 x 3 mm pieces by wire electrical discharge machining (

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NaCl solution for different time periods. In addition the initial potential, the final potential, and the 77 scan rate of the tafel polarisation curves plot were -1.8 V, -0.8 V, and 10 mv / s, respectively.

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Using super deep scene 3D microscope, electrochemical test, scanning electron 79 microscope(SEM) and energy dispersive spectrometer(EDS) to characterize the composition and 80 surface micro-morphology of sample when the test resule was analysised.  Fig.1. We can see that the coating materials are distributed in groups.

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The brighter colour is the zinc-rich group and the darker colour is the aluminium-rich group. As 86 shown in fig.1 (a), there are many holes and pits on the surface of Zn-Al coating, and the distribution 87 of material groups is concentrated and not uniform enough. As depicted by Fig.1

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Compared with friction and wear, the biggest damage of steel components in the marine 120 environment is the electrochemical erosion of the matrix by halogen ions in sea water [18][19]. In 121 order to understand the corrosion mechanism of the coating in seawater more intuitively, we 122 observe and analyze the sample immersed different stages in the continuously fluctuating 3.5%

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NaCl solution. Fig. 4 shows the surface macro-morphology of Zn-Al coating and Zn-Al-Mg-TiO2 124 composite coating immersed in dynamic salt water for 144 h, 240 h, 480 h and 720 h respectively. As 125 shown in Fig. 4a, the coating of Zn-Al is covered with white corrosive product after immersing 144h.

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As the immersion time increases, the area of white corrosive material covers gradually increase and 127 thick, and the phenomenon of material falling off occurs. When the white corrosive material falls off, 128 the coating peels off and pits are generated.

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As shown in Fig.4(b). The corrosion resistance of Zn-Al-Mg-TiO2 composite coating is obviously  the coating is relatively low. As the reaction continues, the exposed coating area gradually increases,

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the mainly reaction at this time is anodic corrosion of zinc, aluminum and magnesium active metals.

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In this stage the reaction speed of coating is faster and the potential is rapidly move to negative

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The protective mechanism of Zn-Al-Mg-TiO2 coating is similar to the Zn-Al coating. Fig.10 (b) is 239 a schematic diagram of the protective mechanism of Zn-A-Mg-TiO2 coating. At the initial stage of 240 corrosion, the active metal zinc in the coating acts intense reaction as an anode, providing cathodic 241 protection for substrate by consuming itself. At this time, the corrosion products of Al and Mg in the 242 coating act together with the corrosion products of Zn to form a dense passivation film, the 243 nanostructure of TiO2 blocks the corrosion hole of the coating surface, so they can effectively blocks 244 the intrusion channel of halogen ions and slows down the corrosion rate. In contrast, the protective 245 mechanism of Zn-Al coating is shown in Fig. 3.10(a). Since there is no Mg to improve the passivation 246 film and no TiO2 to close the gaps, the compactness of passivation film structure is relatively poor.

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Therefore, a large number of corrosive media can still enter from the surface voids of the coating to 248 corrode the internal materials of the coating with a faster rate. So the corrosion resistance is poor.

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(1) The surface of Zn-Al-Mg-TiO2 composite coating with Mg and nano-TiO2 is smoother. There 258 are fewer pores and pits and the friction coefficient is 0.3629 which is lower than Zn-Al coatin. In 259 addition, the scratch of Zn-Al-Mg-TiO2 is more dense and uniform, so the coating has good wear 260 resistance.

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(2) The change trend of the corrosion rate of Zn-Al-Mg-TiO2 coating in 3.5% NaCl solution is 262 similar to the Zn-Al coating. Corrosion rate of the coating is gradually increased at the beginning of