Corrosion behavior of lithium silicate-based Zn-5.5Al-4.5Mg-0.3Ce coating

The effects of Ce on the microstructure of Zn-5.5Al-4.5Mg alloy and the corrosion mechanism of Zn-5.5Al-4.5Mg-0.3Ce alloy and its coatings were investigated in this research. The results show that the structure of Zn-5.5Al-4.5Mg-0.3Ce alloy is composed of hcp-Zn, fcc-Al and ternary eutectic structure (Zn/Al/MgZn2/Mg2Zn11), without dendritic tissue and MgZn2. The salt spray corrosion performance of Zn-5.5Al-4.5Mg-0.5CMC-2C coating was significantly better than that of Zn-5.5Al-4.5Mg-0.3Ce coating and Zn-5.5Al-4.5Mg-0.5CMC coating. The corrosion current density of Zn-5.5Al-4.5Mg-0.5CMC-2C coating was 3.217 μA cm−2, which was significantly lower than 3.96 μA cm−2 of Zn-5.5Al-4.5Mg-0.3Ce alloy and 5.879 μA cm−2 of Zn-5.5Al-4.5Mg alloy. Sodium carboxymethylcellulose and graphene improve the film formation and electrical conductivity of water-based lithium silicate resin. During corrosion of Zn-5.5Al-4.5Mg-0.3 Ce series alloy and coatings, Zn and Mg2Zn11 dissolved preferentially to form corrosion products of Zn5(OH)6(CO3)2 and Zn5(OH)8Cl2·H2O. Mg6Al2(OH)16CO3·4H2O and Zn4Al2(OH)12CO3·3H2O bimetallic hydroxide colloidal membranes were formed, which attached and wrapped on the surface of early corrosion products. Mg6Al2(OH)16CO3·4H2O and Zn4Al2(OH)12CO3·3H2O, preventing the dissolution of soluble corrosion products, improved the corrosion resistance of Zn-5.5Al-4.5Mg-0.3Ce alloy.


Introduction
Zinc coatings have greatly contributed to improve the corrosion resistance of steels which exposed to corrosive environments [1]. Zn-Al-Mg coating is a new sacrificial anode material that does not impressed current to protect steel. At present, companies such as Roval coating company, Eckave-Werke in Germany, and Dacro in the United States have produced zinc-rich coatings. Inorganic lithium silicate resin is an environmentally friendly material but has some disadvantages, such as discontinuous film formation, slow curing speed, poor adhesion, easy cracking, peeling, and powder loss [2]. To improve its film-forming properties, sodium carboxymethyl cellulose, silane and graphene were added to the coating. The carboxymethyl cellulose aqueous solution has the advantages of thickening, film formation, and adhesion. Silane can enhance coating adhesion and improve its mechanical, water resistance and anti-aging properties. Graphene modified epoxy zinc powder primer has stable performance, good anti-corrosion performance [3,4]. In the present study Zn-5.5Al-4.5Mg-0.3Ce coatings were made of carboxymethyl cellulose, silane, graphene and so on. The corrosion properties and mechanisms of the coatings were discussed.

Materials
The metals of Zn, Al (99.99 wt%), Mg (with 99.85 wt%) and Ce were melted in medium frequency induction furnace at 600°C. The molten alloy was homogenized by mechanical stirring. The cover agent with carbon Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. powder (6 wt%), mix chlorine salt (39 wt%)with MgCl 2 , ZnCl 2 and KCl and CaF 2 (55 wt%), etc was covered the surface of melt liquid during smelting. Zn-5.5Al-4.5Mg-Ce alloy powder was prepared from molten superalloy liquid by atomization of 2Mpa argon.
Alloy powder, water-based inorganic lithium silicate resin, sodium carboxymethylcellulose (CMC-Na) and graphene (mass ratio 70:30:0.5:2) were stirred for 30 min to make slurry. The slurry was coated on the surface of the sandblasted Q235 steel and solidified for 72 h at room temperature to form coating.

Experimental method
The microstructure was observed by OLYMPUS metalloscope(OM) and Tecnai G2 F20 field emission transmission electron microscope(TEM). X-ray diffraction (XRD) measurement was performed on a Bruker D8 x-ray diffraction. Cu Kα radiation was used as x-ray source. A step-scan mode was used at the 2-theta range from 10°to 90°with a step length of 0.1°. The operating conditions were 40 kV and 30 mA.
The electrochemical properties of alloys and coatings were investigated using CHI604E electrochemical workstation (Shanghai CH Instruments Co, China). All tests wre measured in 3.5% NaCl aqueous solutions at room temperature. The electrochemical polarization is conducted at a scanning rate of 1 mV/s using a saturated calomel electrode (SCE) as the reference electrode. A platinum flake is used as the counter electrode. The surface area of the sample exposed to the NaCl solution is 1 cm 2 .
The full-immersion corrosion test of Zn-5.5Al-4.5Mg alloys and coatings were conducted in room temperature solution pH=7, 3.5%(wt%)NaCl, and the solution was replaced every 7 days for 40 days.
Neutral salt spray test of Zn-5.5Al-4.5Mg-Ce coatings were investigated in 5%(wt%)NaCl solution with a salt spray deposition of 1.5 ml/h and an ambient temperature of 35±2°C. The samples size were 150×60×8 mm.

Results and discussion
3.1. Composition and structure of alloy and powder 3.1.1. The effect of Ce on microstructure of Zn-5.5Al-4.5Mg alloy With the addition of 0.3%Ce, Zn solid solution completely changed from heterogeneously distributed dendrites to spherical dispersion and uniform distribution in Zn-5.5Al-4.5Mg alloy (figure 1).
The microstructure of Zn-5.5Al-4.5Mg-0.3Ce alloy is homogeneous with spherical Zn solid solution, Al solid solution and ternary eutectic phase. The microstructure and composition uniformity of the alloy are completely superior to the dendrite structure of Zn-5.5Al-4.5Mg alloy (figure 2, table 1).

3.1.2.
Effect of Ce on phase structure of Zn-5.5Al-4.5Mg alloy The main phase structure of Zn-5.5Al-4.5Mg-0.3Ce alloy consists of Zn-rich solid solution, Al-rich solid solution and Zn/Al/Mg 2 Zn 11 ternary eutectic structure, without MgZn 2 . It can be known that Ce is beneficial for the formation of Mg 2 Zn 11 , or promote the transformation of MgZn 2 to Mg2Zn 11 . It shows that MgZn 2 phase is firstly generated at high temperature and then transformed into Mg 2 Zn 11 phase in the solidification process of Zn-5.5Al-4.5Mg alloy [5,6].
In order to further analyze the effect of Ce on the phase structure of Zn-5.5Al-4.5Mg alloy, TEM was used for STEM analysis of Zn-5.5Al-4.5Mg and Zn-5.5Al-4.5Mg-0.3Ce alloys, respectively. The diffraction spots in the 1, 2, 3 regions analyzed in figure 3.
Then the corresponding crystal face index respectively are (112) Compared with Zn-5.5Al-4.5Mg alloy, Zn-5.5Al-4.5Mg-0.3Ce alloy has of hcp-Zn, fcc-Al and ternary eutectic structure without MgZn 2 phase. The microstructure and composition distribution of the alloy are more uniform.     coatings with different conditions are presented in figure 7. While the addition of Ce makes the structure and composition of Zn-5.5Al-4.5Mg alloy more uniform and has not obvious effect on its corrosion potential. The electrode potential of the Zn-5.5-4.5Mg-0.3Ce coatings are higher than that of alloys. In the alloy system, Ce can make the corrosion potential of the alloy positively shifted. Ce plays the role of refining grain, increasing the repair speed of surface film and reducing the corrosion tendency [7]. The anodic polarization curve of Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC alloy coating is at the top. The Zn-5.5Al-4.5Mg-0.3Ce coating with graphene showed lower corrosion current under the same condition. The addition of graphene and CMC reduced the corrosion current of Zn-5.5Al-4.5Mg-0.3Ce coatings.   The self-corrosion potential of the Zn-5.5Al-4.5Mg-0.3Ce, Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC and Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC-2C coatings have been improved to some extent, but they were still at the protective potential of steel in sea water (−0.85∼−1.0 V vs. SCE) (table 3). On the other hand, the corrosion current density of Zn-5.5Al-4.5Mg alloy decreased significantly from 5.879 μA/cm 2 to 3.96 μA/cm 2 with the addition of Ce. However, compared with the alloy, the corrosion potential and corrosion current density of Zn-5.5Al-4.5Mg-0.3Ce coating increased slightly. CMC and graphene reduced the corrosion current density of Zn-5.5Al-4.5Mg-Ce coatings to different degrees. Zn-5.5Al-4.5Mg-0.5CMC-2C coating has the lowest corrosion current density of 3.217 μA/cm 2 .
3.3. Salt spray corrosion characteristics of Zn-5.5Al-4.5Mg-0.3Ce coatings Corrosion test of surface topography of coatings as shown in figures 8, 9 and 10. There was no sign of rust at the scratch of the Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC-2C coating after 700 h. Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC-2C coating had better cathodic protection and corrosion resistance than Zn-5.5Al-4.5Mg-0.3Ce, Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC coatings. It was found that CMC-Na and graphene could significantly improved the film formation performance of the coating that made the film formation more continuous and the coatings conductivity more excellent.
3.4. Zn-5.5Al-4.5Mg-0.3Ce alloy and Coating corrosion mechanism The corrosion rate of Zn-5.5Al-4.5Mg-0.3Ce coating was lower than that of Zn-5.5Al-4.5Mg alloy. The SEM morphologies of corrosion products on the surfaces of Zn-5.5Al-4.5Mg alloy, Zn-5.5Al-4.5Mg-0.3Ce alloy and Zn-5.5Al-4.5Mg-0.3Ce coating were observed. The corrosion morphologies of Zn-5.5Al-4.5Mg alloy are shown in figures 11(a) and (b). The corrosion products were loose straw pellets or blade -like aggregates, which gradually form into a flocculent soluble product. The corrosion products were mainly composed of Zn 5 (OH) 6 (CO 3 ) 2 and Zn 5 (OH) 8 Cl 2 ·H 2 O, which were consistent with the results in relevant literatures [8]. The corrosion morphologies of Zn-5.5Al-4.5Mg-0.3Ce alloy and coating are shown in figures 11(c) and (d). The corrosion products were spherical in relatively dense structure, and the spheres were stacked on top of each other. The surface of the sphere is enveloped by a denser membrane. The surface of the sphere is a gastric wall structure with a larger specific surface area. Further XRD analysis was carried out on the corrosion products ( figure 12). In addition to Zn 5 (OH) 6 (CO 3 ) 2 and Zn 5 (OH) 8 Cl 2 ·H 2 O, the corrosion products also contained  . Zn-5.5Al-4.5Mg-0.3Ce alloy had lower corrosion rate than Zn-5.5Al-4.5Mg. Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC-2C coating had lower corrosion current density than Zn-5.5Al-4.5Mg-0.3Ce coating. It was found that Ce could made the structure and composition of Zn-5.5Al-4.5Mg alloy more uniform and electrochemical corrosion process more stable. CMC-Na could improved the film forming performance of Zn-5.5Al-4.5Mg-0.3Ce coating, and the corrosion performance of the coating was better than that of Zn-5.5Al-4.5Mg-0.3Ce coating. The combined action of CMC-Na and graphene not only improved the film forming performance of Zn-5.5Al-4.5Mg-0.3Ce coating, but also greatly improved the electrical conductivity of the coating. When Zn-5.5Al-4.5Mg-0.3Ce alloy coating was corroded, hcp-Zn and Mg 2 Zn 11 phases are preferentially corroded and   dissolved to form Zn 5 (OH) 6 (CO 3 ) 2 and Zn 5 (OH) 8 Cl 2 ·H 2 O. With the increase of Mg 2+ and Al 3+ concentration in the corrosive environment, the layered double hydroxide(LDH) of Mg 6 Al 2 (OH) 16 CO 3 ·4H 2 O and Zn 4 Al 2 (OH) 12 CO 3 ·3H 2 O are formed [9]. The LDH film wraps around the surface of spherical Zn 5 (OH) 6 (CO 3 ) 2 and Zn 5 (OH) 8 Cl·H 2 O, forming a relatively dense corrosion product film. The corrosion process of Zn-5.5Al-4.5Mg-0.3Ce alloy was slow down, and the transformation of corrosion products Zn 5 (OH) 6 (CO 3 ) 2 and Zn 5 (OH) 8 Cl·H 2 O from spherical to soluble flocs was slow down [10]. Graphene also forms a physical barrier layer in the coating, improving the corrosion resistance of the coating [11]. So that the corrosion rate of Zn-5.5Al-4.5Mg-0.3Ce-0.5CMC-2C alloy coating is the lowest.