Research on Microstructure and Corrosion Behavior of Zinc-Magnesium Coating by Powder Impregnation

In order to improve the service life of railway fasteners under various environmental conditions, the Zn and Zn-Mg coating were prepared on the railway fastener gaskets by powder impregnation. XRD and SEM were used to characterize the phase composition, microstructure and morphology before and after the salt spray. The neutral salt spray test (NSST) and the electrochemical workstation were used to characterize the corrosion resistance of the two coatings. Results show that there was a large area of red rust on Zn coating after 480 hours of NSST, while there was no red rust on Zn-Mg coating after 1920 hours of NSST. The impedance of the Zn-Mg coating reached 1015.6 Ω·cm2, about 3.65 times of the Zn coating. Zn-Mg coating’ current density was 3.27 μA·cm−2, which was only 21% of the Zn coating. MgZn2 and Mg2Zn11 phases were easier to formed in Zn-Mg coating due to the infiltration of magnesium, forming a dense and protective layer, thereby greatly improved its corrosion resistance.


Introduction
Powder impregnation is a heat treatment process that diffuses zinc and alloy elements into the surface of iron and steel components under heating to form protective layers. It has excellent bonding performance, corrosion resistance, wear resistance and other characteristics, as well as a wide application prospect in the field of anti-corrosion engineering materials [1][2][3]. At present, the commonly used process is mechanical assisted powder zincification. The cylindrical or hexagonal zincification tank rotates inside the heating body. Generally, the processing of thermal diffusion coating can be realized in a sealed container in atmospheric environment. Compared with hot dip galvanizing, powder impregnating has no hydrogen embrittlement and better wear resistance. Therefore, powder impregnating has been widely used for the anti-corrosion of high-strength fasteners [4][5][6][7][8].
Railway fasteners are required to have excellent mechanical properties and anticorrosion ability. At present, the anti-corrosion of railway fasteners mainly adopts the technology of powder zincification and composite sealing treatment. As far as the current technology of powder zincification is concerned, 2 the salt spray resistance life of pure powder zincification is less than 200 hours. Therefore, sealing treatment must be carried out on the surface after galvanizing to improve its service life. After galvanizing and sealing, the salt spray resistance life can reach more than 500 hours [9][10][11]. However, under the conditions of wind and sand, erosion and vibration in the actual service environment, the sealing layer is easy to be worn off, resulting in premature corrosion of fasteners and failure and fracture. Zn-Mg coating has attracted the attention of many large iron and steel enterprises and scientific research institutions in the world because of its excellent corrosion resistance and notch self-healing [12][13][14], Japan's Kawasaki iron and Steel Company [15], Nippon Steel Company [16], Kobe Steel Institute [17], Germany's ThyssenKrupp [18], Korea's POSCO [19] and Europe's Arcelomittal [20]. The production process and Zn-Mg coating's anticorrosion ability have been studied and reported. At present, the methods for preparing Zn-Mg coatings mainly include aqueous solution plating [21], molten salt plating [22], hot dip plating [16] and vacuum plating [23]. There are no reports on the preparation of Zn-Mg coating by powder infiltration. Therefore, it is urgent to develop a new generation of powder Zn-Mg coating technology to achieve high corrosion resistance without sealing and meet the requirements of long service life for railway fasteners under various environmental conditions. Therefore, Zn-Mg coating was prepared on the surface of railway fastener gasket in this work.

Experimental
Zn-Mg coating was prepared by using multicomponent alloy carburizing furnace (model: YCKA200-70) designed and developed by Yancheng Keao Machinery Co., Ltd. The basis material is railway fastener gasket, the material is spring steel 60Si2Mn, and the size is Φ 70 mm × 31 mm × 10 mm. See table 1 for the chemical composition of the gasket. The process flow of preparing Zn-Mg coating is as follows: shot blasting, preparation of infiltration agent, canning, powder impregnating and cooling. The penetrating agent used is self-made zinc-magnesium alloy powder, and the magnesium content is 1.8 wt%. The morphology was observed by FEI quanta FEG 650 (FE-SEM). The phase in the coatings was analyzed by XRD. YQ-25D shall be used according to the national standard GB/T 101225-2012 for corrosion test in artificial atmosphere. The salt spray solution is 5% NaCl solution, pH is 6.5~7.2, test temperature is (35±1)℃. The polarization curve and electrochemical impedance spectrum of the samples were measured by Gamry reference 600 electrochemical work station. Figure 1 shows the XRD spectra of the two coatings. The phase composition of Zn coating is mainly FeZn 10.98 、FeZn 8.87 、Fe 3 Zn 10 、FeZn 6.67 and ZnO. Figure 1 (b) shows that the Zn-Mg coating is mainly composed of FeZn 10.98 、FeZn 6.67 、MgZn 2 、Mg 2 Zn 11 、Mg 4 Zn 7 and other alloy phases, which indicates that Mg atoms have penetrated and formed into intermetallic compounds with zinc. Previous studies have shown that MgZn 2 and Mg 2 Zn 11 phases are the main reasons for the improvement of corrosion resistance [24] . Morishita [25,26] pointed out that the above two intermetallic phases have lower anodic dissolution current than pure zinc, and the formed alloy layer attached to the surface can further improve the corrosion resistance.   Figure 2 (a) shows that Zn coating is uneven. The bond between the particles is very loose and there are many pores, which is unfavorable to the anti-corrosion resistance. Figure 2 (b) shows that the particle size on the surface of the Zn-Mg coating is relatively uniform, and there are no defects such as cracks. It can be seen from figure 2 (c) that the Zn coating has no defects such as voids and skip plating, but a certain number of cracks can be observed. During the cooling process of the sherardizing process, the thermal expansion coefficient of the Zn-Fe alloy layer and the substrate are different and the residual stress in the coating is relatively large, which leads to the appearance of cracks. Figure 2 (d) shows that the cross-sectional thickness of Zn-Mg coating is relatively uniform, the alloy layer is about 50 μm, and there are no defects such as skip plating.

Corrosion Resistance of Two Coatings
The NSST was carried out to study the anti-corrosion resistance of Zn-Mg coating, and the sample was taken out after 1920 hours. The results are shown in figure 3. No passivation was performed before the NSST.   It can be seen from figure 4 that after 180h of NSST, red rust began to appear on the surface of Zn coating; after 480h of NSST, red rust appeared on the surface of Zn coating in a large area; after 960 hours of NSST, its surface was full of red rust. White rust on Zn-Mg coating began to appear after 180 hours of NSST; after the NSST for 1200h, the white rust on surface of Zn-Mg coating increased, and the others hardly changed. After 1920 hours of NSST, there was no red rust on Zn-Mg coating. It can be seen that the anti-corrosion resistance has been greatly improved compared to Zn coating, and anti-corrosion resistance of Zn-Mg coating is better.
The sectional morphology of the two coatings after 1000 hours of NSST was observed by SEM, and the photos are as follows: A comparative analysis of the cross-section of the Zn-Mg coating after 1000h of NSST is carried out. Pictures in figure 5 shows that corrosion of Zn coating after 1000 hours is more serious, while Zn-Mg coating is just infiltrated for 1000h. There is basically no change in the sectional morphology. Studies have shown that [18,26,27] the chemical composition of the corrosion products of the Zn-Mg coating are ZnO, Zn 4 CO 3 (OH) 6  insoluble colloidal substance. Its existence effectively cuts off the transmission of the permeated layer and external substances, and enhances the corrosion resistance of the Zn-Mg coating. Mg can promote the formation of protective ZnO in the Zn-Mg coatings, the alkaline zinc chloride and alkaline zinc carbonate are filled in the corrosion cracks to prevent further corrosion, which causes Zn-Mg coating to corrode. Magnesium in the alloy layer reacts with water to form hydroxides, which absorbs carbon dioxide in the air to form carbonates, reducing the pH of electrolyte solution on which and promotes the formation of dense protective corrosion products, thereby slowing down the corrosion process. The corrosion product is Zn 5 (OH) 8 Cl 2 ·H 2 O with low conductivity, which covers the free surface and is perpendicular to the surface to form a dense and effective protective layer and effectively hinders the transfer of Cl -.

Electrochemical Characteristics
Two coatings were immersed in 3.5% NaCl neutral solution for 20 minutes and then subjected to AC impedance spectroscopy. The obtained AC impedance spectrum is shown in figure 4.  It can be seen from figure 6 (a) and table 2 that the impedance modulus |Z| of Zn coating is 1034Ω·cm 2 , and the impedance modulus value of Zn-Mg coating |Z| is 3241Ω·cm 2 , The capacitive reactance arc of Zn-Mg coating is larger than that of the Zn coating, which means the greater the resistance of the electrochemical reaction, that is, the greater the charge transfer resistance during the corrosion process, the stronger the corrosion resistance, indicating that the electrochemical reaction resistance of the Zn-Mg coating is higher. Anti-corrosion resistance of the Zn-Mg coating is far better than that of the Zn. Anti-corrosion resistance is mainly due to its existence environment and other dynamic factors. Therefore, its corrosion resistance mainly depends on its corrosion rate, and the current density of the coating in this corrosive environment can be obtained through the polarization curve. Thus, the corrosion rate can be calculated, as shown in table 3. Figure 6 (b) shows the polarization curves of two coatings in 3.5% NaCl solution. The corrosion potential of Zn-Mg coating is more positive compared with that of Zn coating because of the addition of magnesium; the Zn coating has a tendency to be more corrosive from the thermodynamics. However, magnesium ions tend to form compounds and are difficult to stably exist in the electric double layer, the addition of magnesium not only increases the overpotential of the cathode process, but the overpotential of the anode dissolution process is also higher than that of the Zn coating. Zn-Mg coating's corrosion current density reduced, so the corrosion rate reduced, and the corrosion resistance is improved, so that the kinetic analysis of the Zn-Mg coating is less prone to corrosion. Comprehensive AC impedance spectroscopy and polarization curve analysis show that the corrosion resistance of Zn-Mg coating is far greater than that of Zn coating. The results of the salt spray corrosion test and the electrochemical test are consistent.

Conclusions
(1) Zn-Mg coatings have been prepared by powder infiltration. After 1920 hours of NSST, there was no red rust on its surface, which indicates that Zn-Mg coating has extremely high anti-corrosion resistance.
(2) XRD analysis results show that the main phases of Zn-Mg coating are MgZn 2 , Mg 2 Zn 11 and other alloy phases, and the corrosion products form a dense and protective layer, which greatly improves anti-corrosion resistance of Zn-Mg coating.
(3) Based on electrochemical results, Zn-Mg coating has a smaller corrosion current density than Zn coating. Combining with the experiment results of NSST, Zn-Mg coating has better anti-corrosion resistance than Zn coating.