The influence of curing process on the performance of insulation coating for non-oriented environmentally friendly silicon steel

　　Curing is a key process in the coating of silicon steel. This paper formulates an environmentally friendly silicon steel insulating coating liquid and uses different curing process conditions to treat non-oriented silicon steel insulating coatings. By observing the surface morphology differences, the rusted area of the neutral salt spray test, the size of the insulation resistance value, and the quality of the electrochemical parameters, the surface state, corrosion resistance, electrochemical performance, and insulation performance of the coating are evaluated, and then the quality of the coating curing process is evaluated, which has guiding significance for the actual production and application of silicon steel coatings.

　　With the increasing demand for electrical steel in China, the production technology of electrical steel has been continuously improved. Non-oriented silicon steel accounts for a huge market share in China's silicon steel production, and silicon steel coating is an indispensable key technology in the production of silicon steel products, which will inevitably affect the production and development of silicon steel products. With the development and growth of China's silicon steel industry, silicon steel coating has huge market potential. Curing is a key process in the coating of silicon steel. This paper formulates an environmentally friendly silicon steel insulating coating liquid and uses different curing process conditions to treat non-oriented silicon steel insulating coatings. By observing the surface morphology differences, the rusted area of the neutral salt spray test, the size of the insulation resistance value, and the quality of the electrochemical parameters, the surface state, corrosion resistance, electrochemical performance, and insulation performance of the coating are evaluated, and then the quality of the coating curing process is evaluated, which has guiding significance for the actual production and application of silicon steel coatings.

 

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　　1.1 Coating Preparation

　　A phosphate-based non-oriented environmentally friendly silicon steel insulating coating liquid prepared in the laboratory is applied to the treated silicon steel substrate using a patterned roller, ensuring that the coating amount is between 1-1.5 g/cm2. Temperatures of 300, 350, 400, and 450 ℃ and times of 10, 20, and 30 s are selected for coating sintering and curing to obtain non-oriented environmentally friendly silicon steel insulating coatings under different process conditions, and their performance is tested and characterized.

 

　　1.2 Test Characterization

　　A Shanghai Chenghua CHI660E electrochemical workstation is used to test the polarization curve and AC impedance of the coating. A traditional three-electrode system is used, with a working area of 1 cm2. The corrosive medium is 3.5% NaCl solution, the scan rate is 5 mV/s, the scan frequency range of the AC impedance spectrum is 10 mHz-100 kHz, and the scan rate is 0.05 mV/s. An Italian Angelantoni DCTC1200P neutral salt spray test chamber is used to test the corrosion resistance of the coating according to GB/T10125-1997. According to GB/T2522-2007, an HT-2007 coating insulation resistance meter is used to test the insulation resistance of the coating to evaluate the insulation performance of the coating. A Hitachi S-3400N scanning electron microscope is used to observe the surface micromorphology of the cured environmentally friendly silicon steel insulating coating. A German Netzsch STA-449C thermal analyzer is used to perform thermal analysis on the non-oriented environmentally friendly silicon steel coating liquid under nitrogen protection at a heating rate of 10 ℃/min. A Fourier transform infrared spectrometer from PerkinElmer, USA, is used for infrared analysis of the coating.

 

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　　2.1 Corrosion Resistance

　　A neutral salt spray test is used to evaluate the corrosion resistance of the coating. The percentage of rusted area after a 5-hour neutral salt spray test is shown in Figure 1.

　　Figure 1 shows that in the neutral salt spray test, at the same curing time, as the curing temperature increases (300-450 ℃), the corrosion resistance of the coating generally shows a gradually increasing trend. When the curing temperature is 300 ℃, the corrosion percentage of the coating under the three curing times is significantly higher than that under other process conditions, and the corrosion resistance of the coating is relatively poor, indicating that the coating is not completely cured at this temperature. When the curing temperature is 350 ℃, as the curing time increases, the corrosion percentage gradually decreases, indicating that as the curing time increases (10-30 s), the corrosion resistance of the coating continuously improves. When the curing temperature is 400 ℃, the corrosion percentage of the coating under the three curing times is significantly lower than that under other process conditions, and the corrosion resistance is higher, and the corrosion resistance is particularly excellent when the curing time is 20 s and 30 s. At 450 ℃, the corrosion percentage at 10 s and 20 s is low, but when the curing time reaches 30 s, the corrosion percentage of the coating increases significantly, indicating that the corrosion resistance of the coating is poor at this time.

 

　　2.2 Insulation Performance

　　The insulation performance of the environmentally friendly silicon steel insulating coating is characterized by the measured insulation resistance value. The insulation resistance of the coating under different process conditions is shown in Figure 2.

　　Figure 2 shows that different curing processes have different effects on the insulation performance of the coating. When the curing temperature is 300 ℃, the insulation resistance of the coating increases slightly with the increase of curing time. At 300 ℃, the maximum insulation resistance is only 175 Ωmm2. When the curing temperature reaches 350 ℃, the insulation resistance of the coating increases significantly with the increase of curing time. When cured for 30 s, the insulation resistance can reach more than 300 Ωmm2. When the curing temperature is 400 ℃, the insulation resistance at 20 s and 30 s is significantly higher than that at 10 s, but the difference between the two is not large. However, when the curing temperature is 450 ℃, it can be found that the insulation resistance value changes parabolically with the increase of curing time, and the value is lower than that at 400 ℃ under various curing times.

 

　　2.3 Surface Morphology

　　In order to explore the micromorphology of the coating surface under different process conditions, combined with the test results of neutral salt spray test and insulation performance, four representative process conditions of silicon steel coating at 300 ℃ + 10 s, 350 ℃ + 30 s, 400 ℃ + 20 s, and 450 ℃ + 30 s are selected to observe the surface micromorphology, as shown in Figure 3. SHAPE*MERGEFORMAT

　　Figure 3 clearly shows the differences in the micromorphology of the coating surface under the four process conditions: When the curing process is 300 ℃ + 10 s, the coating color is lighter, translucent, and the uneven surface of the silicon steel substrate can be seen, and the coating is basically not cured; when the curing process is 350 ℃ + 20 s, the color of the coating is significantly darker, but the morphology of the substrate can still be vaguely seen, and the degree of curing increases; when the curing process is 400 ℃ + 20 s, the coating surface is uniform and dense, and the coverage is good, and the degree of curing is good; when the curing process is 450 ℃ + 30 s, the coating shows obvious defects, the surface is burnt and damaged, the silicon steel matrix is exposed, and blocky and powdery morphologies are aggregated, indicating that the temperature is too high at this time, causing the coating to be over-cured. The micromorphology of the coating under different processes can well explain the relationship between the corrosion resistance and insulation performance of the coating.

 

　　2.4 Electrochemical Performance Test

　　2.4.1 Tafel Polarization Curve

　　Combining the experimental results of neutral salt spray test and insulation performance test, the Tafel polarization curves of the substrate and four process condition samples are selected, as shown in Figure 4, and the electrochemical parameters are listed in Table 1.

　　From Figure 4 and Table 1, it can be seen that: under four process conditions, the corrosion current is basically the same, belonging to the same order of magnitude, and the corrosion potential difference is not large. However, when the curing temperature is 400℃ and the curing time is 20s and 30s, the polarization resistance is twice that of other conditions, indicating that the electrochemical parameters of the coating under this process condition are relatively superior. In this case, the coating can effectively hinder the corrosion of chloride ions on the sample, reduce the transfer rate of electrons between the anode and cathode, and reduce the corrosion rate. Under different process conditions, the protection rate of the coating on the substrate is shown in Table 1. Under the four process conditions, the protection rate of the coating is all greater than 98%, which can basically meet the protection of the coating on the substrate, which is consistent with the experimental results of the neutral salt spray test and insulation resistance. Among them, the corrosion protection rate of the coating at 400℃+20s is as high as 99.53%, indicating that the coating has the best protection effect on the substrate under this process condition.

 

　　2.4.2 Electrochemical Impedance Spectroscopy

　　Electrochemical impedance spectroscopy of the new environmentally friendly silicon steel insulation coating under different curing process conditions.

　　From Figure 5, it can be seen that: the impedance arcs under the four process conditions are quite different. When the curing temperature is 400℃ and the curing time is 20s, the radius of the coating impedance arc is the largest, and it is significantly larger than the impedance arc radius of the other three process conditions.

　　Table 2 lists the values of the components of the equivalent circuit fitting. Rs is the solution resistance, RPo is the coating pore resistance, Rt is the coating corrosion reaction polarization resistance, Cc is the coating capacitance, and Cdl is the double layer capacitance. The larger the Rt value and the smaller the Cc value, the better the protective effect of the coating. When the curing process is 400℃+20s, the Rt value is as high as 70561Ωcm2, and the Cc value is one order of magnitude smaller than that under other conditions, indicating that the protective effect of the coating is the best at this time. Combining the electrochemical parameter experimental analysis in the Tafel polarization curve, it can be concluded that when the curing condition is 400℃+20s, the electrochemical performance of the coating is superior, and the corrosion resistance of the coating is significantly improved.

 

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　　The effects of different curing processes on the various properties of the coating are different. When the curing process is 400℃+20s, the coating has the best performance. The non-oriented environmentally friendly silicon steel insulation coating covers the silicon steel substrate well, the coating surface is dense and uniform, and the coating protection rate is as high as 99.53%. In the 5h neutral salt spray test, the percentage of rusted area is only 6%, the electrochemical parameters are significantly better than other conditions, and the insulation resistance reaches 300Ω·mm2.