Comparative Study on the Corrosion Resistance of Inorganic Zinc-Rich Coating and Thermal-Spray Zinc Coating

: The corrosion resistance of two steel coatings (inorganic zinc-rich coating and thermal-spray zinc coating) was studied in detail by using the electrochemical and salt spray test, and the salt spray corrosion thickness was calculated by the time-varying equation established from the mathematical fitting method. The result show that the corrosion mechanism of the two types of coatings was the same that based on the sacrificed of zinc through anodizing phenomenon. The main reason for the difference of corrosion resistance between the two anticorrosive coatings was that the coating density and shielding effect of corrosion products were different. The 7500-hours salt spray test showed that the corrosion rates and microstructure characteristics of both types of zinc coatings were homogeneous on the premise of ensuring coating reliability. The time-varying equation can be used to evaluate the service life of the zinc coatings and judge their corrosion resistance quickly, that providing theoretical support for the maintenance of steel structures as well as the accelerated selection and design of coating formulations. 6.5 and 7.2. The original size and mass of each test block was measured and recorded before testing. Changes in the mass of the test blocks were also monitored before and after the test.

production platform facilities, ship hulls and nuclear power plant, are designed to 70~110 years. Due to prolonged exposure to the marine or industrial atmosphere high corrosive environments for a long time, they must be protected by the anticorrosive coating system to further extend the service life [1,2]. At present, the period of anticorrosion coating system is about 25~35 years and protected by the long-acting coating is the most effective and common path In the case of comprehensive consideration of product life cycle, manufacturing cost, maintenance costs and other factors [3][4][5]. The industry practice has proved that the life of thermal spraying zinc coating can reach more than 20 years on the basis of guaranteeing the reliability of the coating [6,7]. It is a kind of heavy anticorrosive coating with excellent performance, and its life period is widely recognized in the field of heavy anticorrosive coating [8]. As a new coating technology, the inorganic zinc-rich coating has better corrosion resistance performance than the thermal spraying zinc coating in laboratory evaluation system [9]. However, due to the lack of long-term service life verification, its promotion and application in the field of heavy corrosion protection is affected to some extent [10][11][12].
The coating life prediction concerning the service life and safety is an important index that can be used to evaluate the coating durability. At present, the domestic and foreign scholars' research on zinc anticorrosive coating mainly focuses on the preparation method, synthesis process of resin and performance improvement of coating, while the research on coating life prediction is relatively few [13][14][15]. The method of coating life prediction mentioned in literatures was put forward by the National Aeronautics and Space Administration Center in the 1980s. This prediction model of service life method was limited in the laboratory conditions. Based on this prediction model, the Weibull life prediction model, fatigue curve model, theoretical prediction model and other new prediction methods have emerged [16]. However, these new prediction models are all based on the fact that thermal stress and interfacial oxidation are the key factors to determine the coating failure. The life prediction model of thermal barrier coatings was quite different from the zinc anti-corrosion failure mechanism of coatings under actual working conditions [17,18]. Therefore, it is difficult to predict the life of inorganic zinc-rich coating simply according the above model [19,20]. The coating failure will affects the safety function of equipment and drilling platform as well as the normal operation of the system. It is necessary to demonstrate and analyze the service life and microstructure of the coating in the corrosion process that providing guidance for the technical research and optimal selection of the matched engineering coating [21][22][23][24]. The homogeneous corrosion rate and microstructure characteristics of the zinc coating are important parameters for life evaluation that providing critical support for the steel structures maintenance and rapid selection and design of coating formulations [25]. The nature of corrosion resistance of zinc coating is to sacrifice anode material to protecting the steel structure [26]. Therefore, the coating life depends on the thickness of zinc layer and dissolution rate of zinc, namely the oxidation rate of the zinc coating on the premise of the reliability of the coating [27][28][29][30]. The essence of inorganic zinc-rich coating is still to prevent the corrosion of metal components through oxidation process of sacrificing zinc anode [31].
In this paper, the corrosion micromorphology and corrosion rate of hot-sprayed zinc coating and inorganic zinc rich coating were studied by the electrochemical and long-term salt spray tests. The correlative corrosion mechanism was discussed and the factors affecting the corrosion rate were determined. The interfacial corrosion thinning weight loss equation was established by mathematical fitting method and the service life of inorganic zinc-rich coating was predicted by comparing with that of hot-sprayed zinc coating through 7500-hours salt spray test. The research has practical guiding significance for the rapid selection and design of the matching scheme, and can also provide reference for the life prediction of other types of anti-corrosion coatings.

Materials and Instruments
The coated steel samples were provided by CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd. All the interfaces of the steel plate were covered with a zinc coating. The sample of thermal-spray zinc coating and inorganic zinc-rich primer were obtained by self-produced. The sample specification was 90×50×6 mm and 90×50×6 mm and the thickness of coating was 80-150 μm and 200-350 μm, respectively. The reagents needed in the experiment (Distilled water, Ammonium chloride, Acetic acid and Ammonium acetate) were all at industrial grade and purchased from Sinopharm Chemical Reagent Co., Ltd. The electrochemical performance of coating were measured using a electrochemical workstation (Auto Lab PGPSTA302, Eco.chemine company). The test conditions are as follow: pH=7(5 wt. % NaCl aqueous solution adjusted by 0.5 M NaOH solution); temperature: 25±2 ℃; sweep rate: 1 mVs −1 ; scanning range: -400 mV~+400 mV. The corrosion resistance performance of coating was tested by the salt spray test chamber (ATLAS BCX3000(850L), ATLAS Engineering Machinery Co., Ltd.). The relevant test conditions are: testing area: 191×74×63 cm; pressurized gas flow: 0.1 m 3 /min; pressure of dry and oil-free pressurized gas: 103 kPa. The phases identifications and microstructures of thermal-spray zinc and inorganic zinc-rich coating were observed by using the X-ray diffractometer (D8 Focus, Bruker company of Germany) and scanning electron microscope (SU3500/X-act, Hitachi company of Japan), respectively.

Accelerated corrosion test
Before spraying, the steel plate was pretreated by sandblasting roughening. The technological parameters of sandblasting are as follows: sandblasting pressure: 0.5-0.8 Mpa; quartz sand size: 12-24 mesh. After sandblasting, the surface must be clean and free of moisture, iron oxides and other dirt, and meet the requirements of Sa2.5. A zinc wire (purity>99.9%) with a diameter of 3 mm was used as thermal-spray zinc, while an inorganic zinc-rich coating was made by the high-pressure airless spraying technology.
After coating formation, the zinc content was>80%, while the content of other organic and inorganic substrate (ethyl silicate, silica powder, etc.) was<20%.
Accelerated cyclic corrosion tests according to procedure ASTM B 117. The salt solution pH stayed between 6.5 and 7.2. The original size and mass of each test block was measured and recorded before testing. Changes in the mass of the test blocks were also monitored before and after the test.
After they were placed well according to the regulations, the thermal-spray zinc coating and inorganic zinc-rich coating samples underwent a salt spray test for intervals of 500 h, 1200 h, 2000 h, 3500 h, 5000 h, and 7500 h, respectively. Test blocks were taken out at each specified time point, with 5 test blocks for testing, 3 for an experiment and 2 for later use. The corrosion products were removed, and 2 samples taken out at each time point were weighed. After the removal of the corrosion products, mass monitoring was performed and the change in the mass of each test block was recorded.
The corrosion products were removed at each salt spray time point. After sampling, the corrosion test blocks were weighed and recorded. Then, the corrosion products on the test blocks were removed on the specialized cleansing table. After being cleaned, the test blocks were dried left idle for 10 min. The test blocks were then weighed again with the weight data recorded. The corrosion weight loss per unit area at each sampling time point was calculated by comparing with the initial mass and the corrosion area of the test blocks. Finally, the corrosion weight loss rate of each test block was calculated.
The calculation method of corrosion rate of the salt spray sample is as follows formula [32,33]: Where the v, w1, w2, s and t represents the corrosion rate of the sample, mass loss of the salt spray and blank sample, surface area and salt spray time, respectively.
According to the calculation formula, the weight loss data and thinning thickness at each time point were calculated and fitted for plotting. The whole accelerated corrosion test process was shown in Fig.1.

Removal process of corrosion product
A corrosion product removal technique was selected prior data acquisition (stripping refers to removing the salt stains, rust, and other corrosion products on the coating without damaging the coating or matrix themselves). To ensure the reliability of the data, the test was conducted repeatedly until the weight loss was minimized and stabilized [34,35]. After repeated tests, the sodium-hydroxide method was selected as the corrosion product removal method for the thermal-spray zinc coating, while the ammonium chloride method was selected as the corrosion product removal method for the inorganic zinc-rich coating. The optimal process parameters determined for the above stripping technique were adopted for cleaning. This test was also conducted based on reference to HB 5257-1983 (the determination of the weight loss from the corrosion test results and the removal of corrosion products) and GB/T 16545-2015" (the removal of the corrosion products on the metal and alloy corrosion samples).
(1) The Removal process of the corrosion products of thermal-spray zinc coating are as follow: 1) Gently brush the test blocks to remove corrosion products weakly attached to the surface.
2) Soak all test blocks in distilled water, then remove them after five minutes.
3) Mixing 100 g of sodium hydroxide (NaOH) with distilled water, producing a 1000 mL solution.
4) Place the test plates vertically into 10% NaOH solution, and take them out 10 min later (specific soaking duration may be extended or shortened depending on the length of salt spray).

5)
Rinse the test blocks with running water while gently cleaning the surface of the blocks with a soft brush to remove the residual NaOH solution and corroded materials from the surface.
6) Soak the test blocks in distilled water for 30s, then remove them.
7) Bake the test blocks in an air-dry oven for 15min at 80°C. (2) The Removal Process of the corrosion products of inorganic zinc-rich coating are as follow: 1) Gently brush the test blocks to remove corrosion products weakly attached to the surface.
2) Soak all test blocks in distilled water and remove them 5min later.
3) Mix 100g of NH4Cl with distilled water to produce a 1000 mL solution.

Reliability tests and appearance of corrosion samples
It is obvious that after the salt spray acceleration tests, both types of coatings show flat surface, but salt stains are more prone to adhering to the thermal-spray zinc coating surface, while the inorganic zinc-rich coating had a smoother surface, where there are only a few salt stains.
3500/5000/7500 hours of salt spray (from left to right respectively；the upper part of the pictures shows the thermal-spray zinc coating, while the lower part of the picture shows the inorganic zinc-rich).
As can be seen from Fig.3, after the salt spray test and the removal of the corrosion products from the surface, corrosion stains appeare on the surface of the thermal-spray zinc coating, which was whitened, while there were basically no changes on the surface of the inorganic zinc-rich coating, whose color remains unchanged. The surface of the two zinc coatings is free of defects such as bubbles, cracks, rust, etc., and the coating samples are intact, meeting the requirements of coating reliability.

Electrochemical characteristics
As shown in Fig.4, the corrosion potential of the steel substrate is -0.563V, while the corrosion potential of the thermal-spray zinc and inorganic zinc-rich coating is -1.078V and -0.971V, respectively. The corrosion potential of both coatings is much lower than that of the substrate, suggesting that it can provide cathodic protection for the substrate. The corrosion potential of zinc coatings is mainly determined by the Zn/Fe area ratio. The thermal-spray zinc coating has a loose surface and high roughness, and compared with the inorganic zinc-rich zinc coating, its specific surface area is larger. Due to the addition of a polymer to the inorganic zinc-rich coating, the electrical conductivity is deteriorated, and the Zn/Fe area ratio also decreases. From the perspective of corrosion tendency, the inorganic zinc-rich coating is more resistant to corrosion. The Nyquist curves of AC impedance in Fig.6 and Fig.7 show that the samples  that the posivtive shift of self-corrosion potential of the inorganic zinc-rich coating is small, from -1.2 V to -0.8 V; within the test time range, the self-corrosion current changes slightly, indicating that the effect of cathodic protection with sacrificial anode is relatively stable, and there is no rapid decline in the coatings' protective performance.

Fig.8 Polarization curve of the inorganic zinc-rich coating
As can be seen from Fig.9 and Fig.10, at 0h, since the inorganic zinc-rich coating shows a single capacitance arc, indicating that the inorganic zinc-rich coating is more compact, and at the initial stage, the electrolyte solution can't pass through the coating to the surface of the coating/metal. After 500h of salt spraying, a Warburg impedance diffusion arc appears at low frequency end, indicating that the electrolyte solution penetrates into the coating/metal interface, with an electrochemical reaction occurring at the interface, leading to an increase in the corrosion dissolution rate of the coating [18][19] . With the extension of salt spray time, the capacitance arc radius of the coating increases significantly after 2000h. The oxidation reaction of metallic zinc in the interface area would generate corrosion products, which would cover the coating surface, thereby restricting the diffusion of dissolved oxygen in the corrosion products, finally leading to a decrease in the dissolution rate of the coating. This is consistent with the results of the polarization curve. According to the comparison of electrochemical test results between the thermalspray zinc coating and the inorganic zinc-rich coating, the self-corrosion potential of the thermal-spray zinc coating decreases more subsequently than that of the inorganic zinc-rich coating, indicating that the performance of the thermal-spray zinc coating decline faster than that of the inorganic zinc-rich coating. The self-corrosion current of the thermal-spray zinc coating was higher than that of the inorganic zinc-rich coating, indicating that the reaction rate of the inorganic zinc-rich coating was lower than that of the thermal-spray zinc coating. In short, the inorganic zinc-rich coating featured better performance than the thermal-spray zinc coating in terms of galvanic anode protection.

Coatings microstructure and corrosion products identification
As shown by the results of SEM and EDS, in the blank sample applied with thermal-spray zinc coating, due to the construction principle and technology, zinc failed to form a continuous and homogeneous coating in the cooling process, resulting in uneven coating surface. After formation, the inorganic zinc coating was continuous and uniform. As can be seen, spherical zinc is closely arranged and flat, with very low roughness. The computer grayscale method was used to measure the porosity of the zinc coatings. The porosity of the thermal-spray zinc coating is 6.3% while the porosity of the inorganic zinc-rich coating is 1.6%. The high density of the inorganic zinc-rich coatings might be due to the presence of Si-OH in the coating, complexing with zinc atoms, forming a highly dense physical cross-linking network. Therefore, for thermal-  To further analyze the change in the material structure on the coating surface before and after salt spray treatment, we analyzed the substance composition of the two coatings before and after corrosion. Fig.13  The approximate content ratio of the three substances being as follows: pure zinc: zinc oxide: quartz=90: 5: 5. It also contains a small amount of impurities (Zn5(OH)8Cl2, C30H14N4O4Zn, etc.). This is because the inorganic zinc-rich coating can react with the water and CO2 in the air, generating a new chemical compound or complex during its own solidification.   a new substance with a crystal structure similar to zinc oxide might be generated during this process; 2) when the coating surface is formed, the grain orientation is not randomly distributed, but preferentially arranged around some special orientations, forming a preferred orientation and resulting in an increase in the diffraction peak intensity at 2θ=34.4°. The content ratio of the two substances is as follows: pure zinc: zinc oxide=20:80. It contains a little new corrosion product. The position and relative intensity of the diffraction peaks appearing at 2θ=33.5°, 32.8° and 37.9° in Fig.16

Thinning rate comparison and service life prediction
Considering that the two coatings are the same in corrosion mechanism, it is the difference in their composition and structure that caused a difference in the corrosion rate. Although the previous SEM and electrochemical analysis show that the inorganic zinc-rich coating has higher corrosion resistance, more detailed data remain required for the prediction of its service life. After 7500 h of salt spray test, the thinning rate of the two coating samples was compared，the relevant data are shown in Table 1. Marquardt algorithm (LMA) was adopted for curve fitting). The sets of data were fitted, deriving corrosion rate equations for the two coatings, as shown in Fig.17. The thinning rate of thermal-spray zinc coating was fitted as the following: The thinning rate of inorganic zinc-rich coating was fitted as the following: Thinning thickness (μm)

CONCLUSIONS
Based on coating reliability, the corrosion mechanism of the thermal-spray zinc and inorganic zinc-rich anti-corrosion coatings was compared using analytical methods including electrochemical testing, XRD, SEM, etc., concluding that insoluble corrosion products are generated in both coatings during the corrosion process. The corrosion products at the interface changes the micro-current coupling capability and exerts a shielding effect on the diffusion of chloride ions, thereby reducing the corrosion rate of zinc and improving the service life and corrosion resistance of the coating. Due to its denser structure and the existence of Si-OH, the corrosion rate of the inorganic zincrich coating is further decreased. After 7500 h of salt spray test, the corrosion rate equations of the two coatings were fitted, and according to the known service life of the thermal-spray zinc coating, the inorganic zinc-rich coating proves to have much better corrosion resistance, i.e., its service life is more than 40 years under the same condition.