1. Coating preparation
In order to facilitate the later electrochemical test, 30mm is selected × 4 mm 304 stainless steel as the base. Polish and remove the residual oxide layer and rust spots on the surface of the substrate with sandpaper, put them into a beaker containing acetone, treat the stains on the surface of the substrate with bg-06c ultrasonic cleaner of Bangjie electronics company for 20min, remove the wear debris on the surface of the metal substrate with alcohol and distilled water, and dry them with a blower. Then, alumina (Al2O3), graphene and hybrid carbon nanotube (mwnt-coohsdbs) were prepared in proportion (100: 0: 0, 99.8: 0.2: 0, 99.8: 0: 0.2, 99.6: 0.2: 0.2), and put into a ball mill (qm-3sp2 of Nanjing NANDA instrument factory) for ball milling and mixing. The rotating speed of the ball mill was set to 220 R / min, and the ball mill was turned to
After ball milling, set the rotation speed of the ball milling tank to be 1 / 2 alternately after the ball milling is completed, and set the rotation speed of the ball milling tank to be 1 / 2 alternately after the ball milling is completed. The ball milled ceramic aggregate and binder are mixed evenly according to the mass fraction of 1.0 ∶ 0.8. Finally, the adhesive ceramic coating was obtained by curing process.
2. Corrosion test
In this study, the electrochemical corrosion test adopts Shanghai Chenhua chi660e electrochemical workstation, and the test adopts a three electrode test system. The platinum electrode is the auxiliary electrode, the silver silver chloride electrode is the reference electrode, and the coated sample is the working electrode, with an effective exposure area of 1cm2. Connect the reference electrode, working electrode and auxiliary electrode in the electrolytic cell with the instrument, as shown in Figures 1 and 2. Before the test, soak the sample in the electrolyte, which is 3.5% NaCl solution.
3. Tafel analysis of electrochemical corrosion of coatings
Fig. 3 shows the Tafel curve of uncoated substrate and ceramic coating coated with different nano additives after electrochemical corrosion for 19h. The corrosion voltage, corrosion current density and electrical impedance test data obtained from electrochemical corrosion test are shown in Table 1.
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When the corrosion current density is smaller and the corrosion resistance efficiency is higher, the corrosion resistance effect of the coating is better. It can be seen from Figure 3 and table 1 that when the corrosion time is 19h, the maximum corrosion voltage of bare metal matrix is -0.680 V, and the corrosion current density of matrix is also the largest, reaching 2.890 × 10-6 A/cm2 。 When coated with pure alumina ceramic coating, the corrosion current density decreased to 78% and PE was 22.01%. It shows that the ceramic coating plays a better protective role and can improve the corrosion resistance of the coating in neutral electrolyte.
When 0.2% mwnt-cooh-sdbs or 0.2% graphene was added to the coating, the corrosion current density decreased, the resistance increased, and the corrosion resistance of the coating was further improved, with PE of 38.48% and 40.10% respectively. When the surface is coated with 0.2% mwnt-cooh-sdbs and 0.2% graphene mixed alumina coating, the corrosion current is further reduced from 2.890 × 10-6 A / cm2 down to 1.536 × 10-6 A / cm2, the maximum resistance value, increased from 11388 Ω to 28079 Ω, and the PE of the coating can reach 46.85%. It shows that the prepared target product has good corrosion resistance, and the synergistic effect of carbon nanotubes and graphene can effectively improve the corrosion resistance of ceramic coating.
4. Effect of soaking time on coating impedance
In order to further explore the corrosion resistance of the coating, considering the influence of the immersion time of the sample in the electrolyte on the test, the change curves of the resistance of the four coatings at different immersion time are obtained, as shown in Figure 4.
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At the initial stage of immersion (10 h), due to the good density and structure of the coating, the electrolyte is difficult to immerse into the coating. At this time, the ceramic coating shows high resistance. After soaking for a period of time, the resistance decreases significantly, because with the passage of time, the electrolyte gradually forms a corrosion channel through the pores and cracks in the coating and penetrates into the matrix, resulting in a significant decrease in the resistance of the coating.
In the second stage, when the corrosion products increase to a certain amount, the diffusion is blocked and the gap is gradually blocked. At the same time, when the electrolyte penetrates into the bonding interface of the bonding bottom layer / matrix, the water molecules will react with the Fe element in the matrix at the coating / matrix junction to produce a thin metal oxide film, which hinders the penetration of the electrolyte into the matrix and increases the resistance value. When the bare metal matrix is electrochemically corroded, most of the green flocculent precipitation is produced at the bottom of the electrolyte. The electrolytic solution did not change color when electrolyzing the coated sample, which can prove the existence of the above chemical reaction.
Due to the short soaking time and large external influence factors, in order to further obtain the accurate change relationship of electrochemical parameters, the Tafel curves of 19 h and 19.5 h are analyzed. The corrosion current density and resistance obtained by zsimpwin analysis software are shown in Table 2. It can be found that when soaked for 19 h, compared with the bare substrate, the corrosion current density of pure alumina and alumina composite coating containing nano additive materials are smaller and the resistance value is larger. The resistance value of ceramic coating containing carbon nanotubes and coating containing graphene is almost the same, while the coating structure with carbon nanotubes and graphene composite materials is significantly enhanced, This is because the synergistic effect of one-dimensional carbon nanotubes and two-dimensional graphene improves the corrosion resistance of the material.
With the increase of immersion time (19.5 h), the resistance of bare substrate increases, indicating that it is in the second stage of corrosion and metal oxide film is produced on the surface of substrate. Similarly, with the increase of time, the resistance of pure alumina ceramic coating also increases, indicating that at this time, although there is the slowing effect of ceramic coating, the electrolyte has penetrated the bonding interface of coating / matrix, and produced oxide film through chemical reaction.
Compared with the alumina coating containing 0.2% mwnt-cooh-sdbs, the alumina coating containing 0.2% graphene and the alumina coating containing 0.2% mwnt-cooh-sdbs and 0.2% graphene, the coating resistance decreased significantly with the increase of time, decreased by 22.94%, 25.60% and 9.61% respectively, indicating that the electrolyte did not penetrate into the joint between the coating and the substrate at this time, This is because the structure of carbon nanotubes and graphene blocks the downward penetration of electrolyte, thus protecting the matrix. The synergistic effect of the two is further verified. The coating containing two nano materials has better corrosion resistance.
Through the Tafel curve and the change curve of electrical impedance value, it is found that the alumina ceramic coating with graphene, carbon nanotubes and their mixture can improve the corrosion resistance of metal matrix, and the synergistic effect of the two can further improve the corrosion resistance of adhesive ceramic coating. In order to further explore the effect of nano additives on the corrosion resistance of the coating, the micro surface morphology of the coating after corrosion was observed.
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Figure 5 (A1, A2, B1, B2) shows the surface morphology of exposed 304 stainless steel and coated pure alumina ceramics at different magnification after corrosion. Figure 5 (A2) shows that the surface after corrosion becomes rough. For the bare substrate, several large corrosion pits appear on the surface after immersion in electrolyte, indicating that the corrosion resistance of the bare metal matrix is poor and the electrolyte is easy to penetrate into the matrix. For pure alumina ceramic coating, as shown in Figure 5 (B2), although porous corrosion channels are generated after corrosion, the relatively dense structure and excellent corrosion resistance of pure alumina ceramic coating effectively block the invasion of electrolyte, which explains the reason for the effective improvement of the impedance of alumina ceramic coating.
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Surface morphology of mwnt-cooh-sdbs, coatings containing 0.2% graphene and coatings containing 0.2% mwnt-cooh-sdbs and 0.2% graphene. It can be seen that the two coatings containing graphene in Figure 6 (B2 and C2) have flat structure, the binding between particles in the coating is tight, and the aggregate particles are tightly wrapped by adhesive. Although the surface is eroded by electrolyte, less pore channels are formed. After corrosion, the coating surface is dense and there are few defect structures. For Figure 6 (A1, A2), due to the characteristics of mwnt-cooh-sdbs, the coating before corrosion is a uniformly distributed porous structure. After corrosion, the pores of the original part become narrow and long, and the channel becomes deeper. Compared with Figure 6 (B2, C2), the structure has more defects, which is consistent with the size distribution of coating impedance value obtained from electrochemical corrosion test. It shows that the alumina ceramic coating containing graphene, especially the mixture of graphene and carbon nanotube, has the best corrosion resistance. This is because the structure of carbon nanotube and graphene can effectively block the crack diffusion and protect the matrix.
5. Discussion and summary
Through the corrosion resistance test of carbon nanotubes and graphene additives on alumina ceramic coating and the analysis of the surface microstructure of the coating, the following conclusions are drawn:
(1) When the corrosion time was 19 h, adding 0.2% hybrid carbon nanotube + 0.2% graphene mixed material alumina ceramic coating, the corrosion current density increased from 2.890 × 10-6 A / cm2 down to 1.536 × 10-6 A / cm2, the electrical impedance is increased from 11388 Ω to 28079 Ω, and the corrosion resistance efficiency is the largest, 46.85%. Compared with pure alumina ceramic coating, the composite coating with graphene and carbon nanotubes has better corrosion resistance.
(2) With the increase of immersion time of electrolyte, the electrolyte penetrates into the joint surface of coating / substrate to produce metal oxide film, which hinders the penetration of electrolyte into the substrate. The electrical impedance first decreases and then increases, and the corrosion resistance of pure alumina ceramic coating is poor. The structure and synergy of carbon nanotubes and graphene blocked the downward penetration of electrolyte. When soaked for 19.5 h, the electrical impedance of the coating containing nano materials decreased by 22.94%, 25.60% and 9.61% respectively, and the corrosion resistance of the coating was good.
6. Influence mechanism of coating corrosion resistance
Through the Tafel curve and the change curve of electrical impedance value, it is found that the alumina ceramic coating with graphene, carbon nanotubes and their mixture can improve the corrosion resistance of metal matrix, and the synergistic effect of the two can further improve the corrosion resistance of adhesive ceramic coating. In order to further explore the effect of nano additives on the corrosion resistance of the coating, the micro surface morphology of the coating after corrosion was observed.
Figure 5 (A1, A2, B1, B2) shows the surface morphology of exposed 304 stainless steel and coated pure alumina ceramics at different magnification after corrosion. Figure 5 (A2) shows that the surface after corrosion becomes rough. For the bare substrate, several large corrosion pits appear on the surface after immersion in electrolyte, indicating that the corrosion resistance of the bare metal matrix is poor and the electrolyte is easy to penetrate into the matrix. For pure alumina ceramic coating, as shown in Figure 5 (B2), although porous corrosion channels are generated after corrosion, the relatively dense structure and excellent corrosion resistance of pure alumina ceramic coating effectively block the invasion of electrolyte, which explains the reason for the effective improvement of the impedance of alumina ceramic coating.
Surface morphology of mwnt-cooh-sdbs, coatings containing 0.2% graphene and coatings containing 0.2% mwnt-cooh-sdbs and 0.2% graphene. It can be seen that the two coatings containing graphene in Figure 6 (B2 and C2) have flat structure, the binding between particles in the coating is tight, and the aggregate particles are tightly wrapped by adhesive. Although the surface is eroded by electrolyte, less pore channels are formed. After corrosion, the coating surface is dense and there are few defect structures. For Figure 6 (A1, A2), due to the characteristics of mwnt-cooh-sdbs, the coating before corrosion is a uniformly distributed porous structure. After corrosion, the pores of the original part become narrow and long, and the channel becomes deeper. Compared with Figure 6 (B2, C2), the structure has more defects, which is consistent with the size distribution of coating impedance value obtained from electrochemical corrosion test. It shows that the alumina ceramic coating containing graphene, especially the mixture of graphene and carbon nanotube, has the best corrosion resistance. This is because the structure of carbon nanotube and graphene can effectively block the crack diffusion and protect the matrix.
7. Discussion and summary
Through the corrosion resistance test of carbon nanotubes and graphene additives on alumina ceramic coating and the analysis of the surface microstructure of the coating, the following conclusions are drawn:
(1) When the corrosion time was 19 h, adding 0.2% hybrid carbon nanotube + 0.2% graphene mixed material alumina ceramic coating, the corrosion current density increased from 2.890 × 10-6 A / cm2 down to 1.536 × 10-6 A / cm2, the electrical impedance is increased from 11388 Ω to 28079 Ω, and the corrosion resistance efficiency is the largest, 46.85%. Compared with pure alumina ceramic coating, the composite coating with graphene and carbon nanotubes has better corrosion resistance.
(2) With the increase of immersion time of electrolyte, the electrolyte penetrates into the joint surface of coating / substrate to produce metal oxide film, which hinders the penetration of electrolyte into the substrate. The electrical impedance first decreases and then increases, and the corrosion resistance of pure alumina ceramic coating is poor. The structure and synergy of carbon nanotubes and graphene blocked the downward penetration of electrolyte. When soaked for 19.5 h, the electrical impedance of the coating containing nano materials decreased by 22.94%, 25.60% and 9.61% respectively, and the corrosion resistance of the coating was good.
(3) Due to the characteristics of carbon nanotubes, the coating added with carbon nanotubes alone has a uniformly distributed porous structure before corrosion. After corrosion, the pores of the original part become narrow and long, and the channels become deeper. The coating containing graphene has flat structure before corrosion, the combination between particles in the coating is close, and the aggregate particles are tightly wrapped by adhesive. Although the surface is eroded by electrolyte after corrosion, there are few pore channels and the structure is still dense. The structure of carbon nanotubes and graphene can effectively block the crack propagation and protect the matrix.
Post time: Mar-09-2022