Research on the Application of Graphene Oxide-Reinforced SiO₂ Corrosion-Resistant Coatings in the Long-Term Protection of Water Treatment Facilities

Abstract

The reaction tank of process wastewater, as one of the key pieces of equipment for wastewater treatment, is exposed to an acidic and alkaline wastewater immersion environment for a long time and is prone to the influence of complex ions in water, resulting in concrete shedding and steel bar corrosion, which seriously affect service performance. To address the issue of ionic erosion in process wastewater reaction tanks, a silicon–oxygen grid substrate was constructed with ethyl orthosilicate, and graphene oxide was used as the corrosion-resistant functional component to prepare GO/SiO₂ corrosion-resistant films under acid-catalyzed conditions. Extreme corrosion environments were designed to evaluate the corrosion resistance of GO/SiO₂ films. The results showed that the permeability of the uncoated samples decreased significantly, and the ion concentration leached in the corrosive medium was higher. The permeability of the GO/SiO₂-coated samples did not decrease significantly, and the ion leaching concentration in the corrosive medium gradually decreased with the increase in GO content, verifying the positive correlation between GO content and corrosion resistance and GO’s use in the field of corrosion resistance in water treatment facilities.

1. Introduction

The process wastewater reaction tank, as one of the key pieces of equipment for wastewater treatment, is constantly immersed in acidic and alkaline wastewater and is prone to the influence of complex ions in water, resulting in concrete shedding and steel bar corrosion, which seriously affect service performance [1,2,3]. Industrial wastewater reaction tanks are mainly affected by factors such as sulfate corrosion, chloride ion erosion, carbonization and hydrogen ion erosion. H⁺, CI⁻ and SO₄²⁻ in the salt-containing wastewater of the reaction tank penetrate the concrete through capillary action, corrode the internal reinforcing bars and induce the gradual acidification of the surface environment of the reinforcing bars, thereby causing the passivation film to deactivate and expose the iron matrix. Subsequently, a potential difference is formed between the iron substrate and the still-intact passivation film, and electrochemical corrosion occurs on the surface of the steel bars. The anodic reaction generates Fe²⁺ and Cl⁻ to form soluble FeCl₂, and the cathodic reaction generates OH⁻ immediately to form insoluble Fe(OH)₂, which is then oxidized to Fe₂O₃ or Fe₃O₄, causing rusting of the steel bars [4,5]. After rusting, the volume of the steel bars increases, and the expansion generates a huge internal stress, which eventually leads to cracking of the concrete. Therefore, improving the corrosion resistance of industrial wastewater reaction tanks is one of the important ways to ensure their long service life.

The application of corrosion-resistant coatings on concrete surfaces is a simple, effective and widely applicable method, among which Tetraethylortho silicate (TEOS) is widely used in the process of preparing SiO₂ coatings [6]. Due to the high hydrolytic activity of TEOS, the hydrolyzed silica sol can react and condense to form a dense network structure, and the hydrolyzed SiO₂ inorganic material has excellent corrosion resistance, aging resistance, heat resistance and other advantages, which can significantly improve the corrosion resistance of the composite coating. Franzoni [7] treated the surface of concrete with TEOS, sodium silicate solution and SiO₂ water dispersion. The results showed that the coating had excellent wear resistance and performed well in terms of resistance to Cl⁻ erosion and carbonation. However, SiO₂ coatings have poor structural compactness, so more effective anti-corrosion functional units need to be added to improve the impermeability of the coatings.

Therefore, graphene oxide is used to enhance the SiO₂ corrosion-resistant coating in this study. Graphene oxide (GO), the oxidation product of graphene, has the characteristics of high strength, good toughness, good ductility [8,9], structural stability [10,11] and excellent barrier properties, and can be used as a new type of corrosion-resistant material. The anti-corrosion effect of graphene is related to its physical and chemical properties [12]. There are a large number of functional groups (hydroxyl, carboxyl, epoxy, etc.) on the surface of its two-dimensional, sheet-like structure, which causes its own agglomeration and stacking when applied in corrosion-resistant coatings. Its three-dimensional structure is more complex in the depth direction of the film layer, forming complex “maze” pathways, which can extend the path of the corrosive medium and delay the occurrence of corrosion [13,14,15,16]. Duan [17] prepared graphene oxide-modified isobutyltriethoxysilane (IBTS/GO) emulsion and coated it on aluminum sulfate salt concrete, and the results showed that the impermeability of the coating was significantly improved. Fu [18] directly incorporated graphene oxide into concrete, and the results showed that the distribution of graphene oxide in the concrete voids could effectively reduce the seepage capacity. To sum up, the use of graphene oxide to enhance SiO₂ corrosion-resistant coatings can significantly improve the corrosion resistance of the coatings and has broad application prospects.

In summary, the GO/SiO₂ coating was prepared by sol-gel method under acidic catalytic conditions in this study, and its corrosion resistance was further verified under acidic corrosion, alkaline corrosion and salt spray corrosion conditions.

2. Materials and Methods

The main reagents required for preparing GO-SiO₂ are shown in

Table 1. Main reagents.

Names	Supplier	Specifications
GO	Shanghai Macklin Biochemical Co., Ltd., Shanghai, China	99%
TEOS	Shanghai Macklin Biochemical Co., Ltd., Shanghai, China	99.9%
Acetone	Shanghai Macklin Biochemical Co., Ltd., Shanghai, China	99%
Ethyl Alcohol	Shanghai Macklin Biochemical Co., Ltd., Shanghai, China	99.6 vol %
HCl	Shanghai Macklin Biochemical Co., Ltd., Shanghai, China	1 mol/L
NaOH	Shanghai Macklin Biochemical Co., Ltd., Shanghai, China	5 wt%
Deionized Water	Self-Made	Self-Made

.

GO/SiO₂ coating samples were prepared by sol-gel method, as shown in Figure 1. A total of 0.05 g of GO powder and 15 mL of GO water dispersion with a concentration of 0.00333 g/mL were, respectively, magnetically stirred with 10 mL of TEOS hydrolysate at 30 °C for 7 h to thoroughly mix and prepare the GO/TEOS solution. For the preparation of GO/SiO₂ composite films, the glass substrate was ultrasonically treated in ethanol, acetone and deionized water for 15 min. The coating solution was spin-coated on the glass surface at 2500 r/min for 20 s at room temperature on a spin coater, and this process was repeated twice. Then, it was placed in a tubular atmosphere furnace and heated to 100 °C at a rate of 2 degrees per minute under nitrogen protection to completely remove the residual ethanol, hydrochloric acid and water; the solution was maintained at this temperature for 1 h. Then, the solution was heated to 600°C at a rate of 2 °C/min and held for 2 h, and subsequently cooled down with the furnace to room temperature.

GO/SiO₂-coated samples with different film thicknesses were prepared by changing the rotational speed and subjecting them to corrosion resistance tests, with SiO₂-coated glass as the control group. All samples were placed in 1 mol/L hydrochloric acid solution, 5 wt% sodium hydroxide solution and a neutral salt spray test chamber for a period of 504 h. The salt spray test was set at 34.99 °C, the conductivity of deionized water was no more than 20 μS/cm, the concentration of NaCl solution was 46.99 g/L and the pH was 6.7–7.1. Corrosion resistance was verified by variations in glass transmittance, SEM and ion leaching concentration under different coating conditions. The names of the samples and the designs of the corrosion resistance tests are shown in

Table 2. Corrosion resistance tests of GO/SiO₂ coating samples.

Sample Name	Coating Condition	Thickness of Membrane	Corrosive Medium
L1	2500 rpm	2.1 μm	1. HCl 1 mol/L 2. NaOH 5 wt % 3. NaCl 46.99 g/L
L2	3500 rpm	1 μm
L3	4500 rpm	0.4 μm
L4	SiO2 coating (2500 rpm)	1.2 μm
L5	Uncoated	0

.

The main equipments are shown in

Table 3. Main equipments.

Names	Manufacturer
SHCM-200 spectral haze clarity meter	EVERFINE, Zhejiang, China
Zeiss Ultra Plus field emission scanning electron microscope	Carl Zeiss AG, Oberkochen, Germany
Prodigy 7 full-spectrum direct reading plasma emission spectrometer	Leeman Labs, Hudson, NY, USA

. The transmittance changes in the samples were measured using the SHCM-200 spectral haze clarity meter. The same sample was measured three times in succession and the median value was taken. The Zeiss Ultra Plus field emission scanning electron microscope with an X-Max 50 X-ray energy spectrometer was used to observe the microscopic morphology of the film surface and cross-section. Three regions of images were captured for the same sample. The Prodigy 7 full-spectrum direct reading plasma emission spectrometer was used to measure the concentrations of B³⁺ and Si⁴⁺ ions in the corrosion medium solution. The same sample was measured three times in succession and the average value was taken.

3. Results

3.1. Transmittance

Figure 2a shows the transmittance curve of a GO/SiO₂-coated sample, tested in the ultraviolet-visible light range of 200 nm to 800 nm. It can be seen from the figure that the maximum transmittance of the uncoated sample is 91.5%, which is basically the same as 91.24% of the SiO₂-coated sample, indicating that only the SiO₂ film has a small effect on the transmittance of the sample. The maximum transmittance of the GO/SiO₂ film sample increases gradually with spin-coating speed because the thickness of the film layer is inversely proportional to the spin-coating speed. As the spin-coating speed increases, the thickness of the film layer decreases gradually, and the interlaced graphene oxide layer structure in the depth direction decreases, reducing the refraction and reflection of light. The maximum transmittance of sample L1 is only 75.43%.

Figure 2b shows the transmittance curve of the GO/SiO₂-coated sample after acid corrosion. It can be seen from the figure that the transmittance of sample L1 does not change significantly, only decreasing by 0.53%, indicating that the corrosion of sample L1 in an acidic environment has a relatively small effect on the transmittance. The permeability of samples L2 and L3 decreased by 2.24% and 1.78%, respectively, indicating that the samples were less affected by the hydrochloric acid solution. The transmittance of samples L4 and L5 decreased significantly over the period, especially that of uncoated sample L5, which dropped from a maximum of 91.5% to 87.85%, a decrease of 3.65%. This indicates that the uncoated sample was more affected by the corrosion of the hydrochloric acid solution, proving that the stacking of graphene oxide in the film layer played an effective anti-corrosion role and effectively reduced the corrosion effect of hydrochloric acid on the coated sample.

Figure 2c shows the transmittance curve of the GO/SiO₂-coated sample after alkali corrosion. It can be seen that sample L1 has a significant change in transmittance, from 75.43% to 85.47%, an increase of 10.04%, because alkali corrosion of SiO₂ is more intense compared to acid corrosion. Since the SiO₂ network structure in the film layer structure is severely damaged in the alkali solution, which leads to the shedding of graphene in the film layer and the thinning of the film layer, the sample transmittance gradually increases, indicating that the corrosion of sample L1 in a sodium hydroxide solution has a greater impact on its transmittance. The transmittance of samples L4 and L5 decreased significantly over the period, especially that of uncoated sample L5, which decreased by 6.47%, indicating that the uncoated sample was more affected by the corrosion of the sodium hydroxide solution. Experiments showed that the GO/SiO₂ film did not significantly improve the alkali resistance of the glass due to the greater damage to the SiO₂ structure caused by alkali corrosion, which led to the shedding of graphene oxide sheets.

Figure 2d shows the transmittance curve of the GO/SiO₂-coated samples after salt spray corrosion. It can be seen that the transmittance of all samples has decreased. This is because, compared with acid and alkali corrosion, the corrosion of the GO/SiO₂-coated samples by salt spray test is partly due to the diffusion of Cl⁻, water molecules and H⁺ within the film layer and the damage [19] to the glass substrate and interface. On the other hand, corrosion was due to the deposition [20] of salt particles on the surface, and the deposition of salt particles had a greater impact on the permeability, resulting in a decrease in the permeability of all samples during the test period. But the permeability of sample L1 decreased by 2.61%, and that of sample L5 decreased by 7.56%, proving that graphene oxide films can improve salt spray corrosion resistance.

3.2. SEM Images

3.2.1. Before Corrosion

To investigate the effect of spin-coating rate on the film-forming effect of the GO/TEOS coating solution on the samples, and to characterize the distribution of SiO₂ and graphene oxide in the film layer, scanning electron microscopy analysis was performed on samples with different spin-coating rates (L1, L2, L3), as shown in Figure 3, all of which were treated with Pt spray. It can be seen from Figure 3a that the GO/TEOS coating solution formed a uniform film on the sample surface at a spin coating rate of 2500 r/min, and the film did not crack after heat treatment at 600 °C, indicating that SiO₂ was evenly distributed during spin coating and the film was heated evenly during heating. There was no -Si-O-Si- fracture due to thermal stress concentration, and the film formation was good. This is because the acid catalyzes the rapid hydrolysis of TEOS, and the silanolate formed by the hydrolysis of ethyl orthosilicate dehydrates or dealcoholizes to form polymers that interpenetrate, cross, entwine and eventually lead to the formation of gelation products, during which hydrolysis and polymerization occur almost simultaneously. If SiO₂ is unevenly distributed or agglomerates with each other in the GO/TEOS coating solution and during spin coating, it will cause thermal stress to concentrate at the edges of the film layer as the temperature rises, leading to cracking of the film layer. In addition, it can be seen from the figure that graphene oxide is evenly distributed in the film layer, with a relatively tight distribution, while SiO₂ presents a disordered and interlaced distribution form, which can make the internal structure of the film layer complex; as well, due to the maze effect, it is difficult to form good pathways, thereby blocking the invasion and corrosion of the corrosive medium. Figure 3b shows sample L2 prepared at a spin-coating rate of 3500 r/min. It can be seen from the figure that no cracking occurred in the film layer after heat treatment at this spin-coating rate, indicating a good film-forming effect from the GO/TEOS coating solution. In addition, it can be clearly seen in the figure that the distribution of graphene is relatively uniform. It also presents a complex conformation of interlaced distribution, which can improve its corrosion resistance. Figure 3c shows sample L3 prepared at a spin-coating rate of 4500 r/min. The graphene distribution is also clearly shown in the figure. The film layer does not crack after heat treatment, indicating that the GO/TEOS coating solution can still maintain good film-forming performance at this spin-coating rate, with the graphene evenly distributed in the film layer.

A cross-sectional SEM image is shown in Figure 4. Figure 4a shows sample L1 at a spin-coating rate of 2500 r/min. It can be seen from the figure that the cross-sectional structure of the film layer is clear, the film layer is uniform and complete, and the thickness is approximately 2.1 μm. Figure 4b shows sample L2, with a spin-coating rate of 3500 r/min. It can be seen that the cross-section of the film layer is clear and intact, the film layer is tightly bonded to the sample substrate and the film layer thickness is approximately 1 μm. After the spin-coating rate is increased by 1000 r/min, the thickness of the film layer is reduced by 1.1 μm compared to sample L1. This is because, as the spin-coating rate increases, the increased centrifugal force during spin coating would cause more GO/TEOS coating solution to be flung off the sample substrate due to centrifugal force at a higher spin-coating rate compared to a lower spin-coating rate, resulting in a decrease in film thickness. Figure 4c shows sample L3, with a spin-coating rate of 4500 r/min. It can be seen that the cross-section of the film layer is clear and closely bonded to the sample substrate, and the film layer is evenly distributed with a thickness of approximately 0.4 μm, which is 1.7 μm thinner than sample L1 and 0.6 μm thinner than sample L2. It is indicated that increasing the spin-coating rate would reduce the thickness of the film when the GO/TEOS coating solution forms the film, resulting in a thinner film layer.

3.2.2. Acid Corrosion

Figure 5 shows the surface SEM images of samples L1, L2, L3, L4 and L5 after etching for 504 h in 1 mol/L HCl solution. Figure 5a shows sample L1, from which it can be seen that as the acid corrosion time increases, the integrity of the film layer is damaged and some cracks appear in the film layer, indicating that the acid corrosion is aggravated. Figure 5b shows sample L2. It can be observed from the figure that the film cracks are larger and the pitting phenomenon has intensified, forming small corrosion pits. At the same time, as the corrosive medium acts, the pitting holes expand in depth and horizontally, as shown in the SEM image, with pitting holes that are aggregated. Figure 5c shows sample L3. It can be observed from the figure that the film layer is severely damaged. In some areas, the film layer surface is completely damaged, exposing the internal structure of the film layer. The change is obvious compared with samples L1 and L2, indicating that the corrosion resistance of the GO/SiO₂ coating sample decreases with the reduction in the film layer thickness. Figure 5d shows sample L4. It can be observed from the figure that there is extensive loss and cracking of the film layer, indicating that the corrosive medium corrodes in multiple places within the film layer, causing severe damage to the integrity of the film layer. Figure 5e shows sample L5, which shows many pits after immersion in 1 mol/L HCl solution for 504 h, and partial cracking on the glass surface, demonstrating that the GO/SiO₂ coating has obvious acid resistance, and its corrosion resistance gradually decreases with a reduction in film thickness.

3.2.3. Alkali Corrosion

Figure 6 shows the surface SEM images of samples L1, L2, L3, L4 and L5 after etching in 5 wt % NaOH solution for 504 h. Figure 6a shows sample L1, from which it can be seen that the film structure is damaged, with a large number of crack defects on the film surface; the corrosion is further intensified in the depth direction, making it difficult to observe a large intact film surface. Figure 6b shows sample L2, from which it can be observed that the film is significantly corroded, with the surface of the film basically corroded and cracks appearing in the film. Figure 6c shows sample L3, from which it can be observed that the structure of the film layer has been significantly damaged, with obvious film layer notches, indicating that its corrosion resistance decreases with a reduction in film layer thickness. Figure 6d shows sample L4, from which it can be observed that there is extensive corrosion of the film layer, extensive cracking of the film layer surface, and obvious film layer shedding. Figure 6e show sample L5. It can be observed that there are obvious cracks on the glass surface, indicating that the surface corrosion of the sample is severe under alkaline corrosion. Experiments show that the GO/SiO₂-coated sample is more susceptible to alkali corrosion, and the corrosion intensifies as the film thickness decreases.

3.2.4. Salt Spray Corrosion

Figure 7 shows the surface SEM images of samples L1, L2, L3, L4 and L5 after corrosion in a salt spray test chamber for 504 h. Figure 7a is sample L1. It can be seen from the figure that after 504 h of the salt spray test, there were a few cracks in the film layer and obvious salt grain deposition, indicating that increasing the salt spray test time caused less corrosion to the film layer and shallower corrosion in the depth direction, which is attributed to the complex stacking of graphene oxide sheets. Figure 7b shows sample L2, from which it can be observed that the film layer is cracked, which is basically the same as the SEM image of sample L3 in Figure 7c, from which it can be observed that some salt grains are concentrated and settled. Figure 7d shows sample L4, which has more salt grain deposition compared to Figure 7d, with salt spray water droplets polymerizing on the surface of the film layer to form salt grain patches. There are partial cracks on the surface of the film layer, indicating that H⁺ and Cl⁻ have damaged part of the film layer structure. Figure 7e shows sample L5, from which it can be observed that the corrosion has intensified, indicating that the GO/SiO₂ coating has obvious salt spray corrosion resistance.

3.3. ICP Ion Leaching Analysis

To further investigate the acid corrosion resistance performance of GO/SiO₂ coatings, the concentrations of B³⁺ and Si⁴⁺ in the acid solution were collected by a full-spectrum direct reading plasma spectrometer, as shown in

Table 4. ICP data analysis of acid resistance tests of GO/SiO₂-coated samples.

Sample Name	B3+/(mg·L−1)	Si4+/(mg·L−1)
L1	0.568	5.6224
L2	0.732	7.9415
L3	0.9451	7.2308
L4	0.6693	7.582
L5	2.6739	9.1251

. The concentrations of B³⁺ and Si⁴⁺ in the hydrochloric acid solution of sample L1 were significantly lower than those in the hydrochloric acid solution of uncoated sample L5. Comparing GO/SiO₂-coated samples L2 and L3 prepared at different spin-coating rates, as the spin coating rate increased, the thickness of the GO/SiO₂ film decreased, and the concentrations of B³⁺ and Si⁴⁺ in the solution increased significantly compared to sample L1. The B³⁺ and Si⁴⁺ leaching amounts of sample L1 were 0.568 mg·L⁻¹ and 5.6224 mg·L⁻¹, respectively, which were both lower than those of sample L2 (0.732 mg·L⁻¹ and 7.9415 mg·L⁻¹). This indicates that the corrosion resistance performance is inversely proportional to the film thickness. Compared with the ion concentration of uncoated sample L5, the difference was significant, suggesting that due to the complex conformation of GO dispersion, the penetration path of the corrosive medium was greatly increased. The layered graphene oxide could seal the pores formed during the curing process of the film layer, playing a physical barrier role. Meanwhile, its sheet structure was chemically stable and not easily penetrated by the corrosive medium, thereby delaying the occurrence of a corrosion reaction. The ion concentration changes in the acid immersion experiment showed a clear pattern. Combined with the changes in transmittance and SEM images, it was proved that the GO/SiO₂ coating could effectively improve acid corrosion resistance performance. Moreover, the influence of GO on the acid corrosion resistance effect was directly proportional to the film thickness and inversely proportional to the spin-coating rate.

To further study the alkali corrosion resistance performance of GO/SiO₂-coated samples, the concentrations of B³⁺ and Si⁴⁺ in the alkali solution were collected by a full-spectrum direct reading plasma spectrometer, as shown in

Table 5. ICP data analysis of alkali resistance tests of GO/SiO₂-coated samples.

Sample Names	B3+/(mg·L−1)	Si4+/(mg·L−1)
L1	0.8689	17.0238
L2	1.3441	15.6285
L3	1.3682	18.8478
L4	1.1424	19.0785
L5	1.6278	21.3935

. The concentrations of B³⁺ and Si⁴⁺ in the sodium hydroxide solution of sample L1 were significantly lower than those of samples L2 and L3 prepared at different spin-coating rates. The B³⁺ and Si⁴⁺ leaching amounts of sample L1 were 0.8689 mg·L⁻¹ and 17.0238 mg·L⁻¹, respectively, which were both lower than those of sample L3 (1.3682 mg·L⁻¹ and 18.8478 mg·L⁻¹). This indicates that the corrosion resistance performance of the GO/SiO₂ coating decreased with the decrease in film thickness. In addition, the ion concentrations in the sodium hydroxide solutions of samples L1 were significantly lower than those of the SiO₂ film glass in sample L4 and uncoated glass sample L5, indicating that the GO/SiO₂ coating had a relatively obvious alkali corrosion resistance performance.

4. Discussion

(1)

In this study, GO/SiO₂-coated samples with different film thicknesses were prepared by sol-gel method. The permeability of the GO/SiO₂-coated samples and the corrosion of the film layers were investigated under acidic, alkaline and salt spray conditions, respectively. Corrosion resistance was further verified by ion leaching concentration.

(2)

The results of the sample transmittance changes and SEM images indicated that the GO/SiO₂ coating had strong corrosion resistance in acidic and salt spray corrosion environments, but low corrosion resistance in alkaline conditions.

(3)

The results of ion leaching in the acid–base leaching test indicated that corrosion resistance gradually increased with the increase in film thickness, proving that GO/SiO₂ coating can effectively improve corrosion resistance and can be used in the field of corrosion resistance with respect to water treatment facilities.