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Article

Preparation and Properties of Silane Coupling Agent Modified Basalt Flake Polyurethane Anti-Corrosion Coatings

by
Kuoteng Sun
1,
Weichen Cai
1,
Xuemin He
1,
Hao Chen
1,
Kun Chen
2,
Tao Jiang
3,
Wenge Li
3 and
Yuantao Zhao
3,*
1
Liuzhou Bureau of EHV Transmission Company, China Southern Power Grid Co., Ltd., Liuzhou 545006, China
2
School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China
3
Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(12), 2022; https://doi.org/10.3390/coatings13122022
Submission received: 2 November 2023 / Revised: 25 November 2023 / Accepted: 28 November 2023 / Published: 29 November 2023
Browse Figures
Versions Notes

Abstract

:
Strong corrosive atmospheric environments of level C4–C5 have a serious safety impact on steel structures and industrial production. The use of anti-corrosion coatings is the most economical and efficient way of all means of anti-corrosion. To further improve the anticorrosion performance of polyurethane anticorrosion coatings, this paper adopts KH-560 silane coupling agent to modify basalt flake and add it as filler into one-component polyurethane resin with wet curing characteristics. Anti-corrosion coatings were prepared by combining pre-dispersion and high-speed dispersion. The prepared coating specimens were sprayed onto Q235 plates and tinplate plates by simple spraying. Test results show that the addition of modified basalt flakes (MBFs) helps to improve the mechanical and corrosion resistance of the coating. When 30% MBFs were added, the mechanical properties, acid and alkali resistance, and corrosion resistance of the coating reached the best, and the aging resistance was good. Among them, the adhesion of the coating could reach 15.36 MPa, and the coating surface did not show obvious flaring and peeling phenomenon after 1000 h of neutral salt spray test. The water contact angle of the coating increased from 77.77° to 81.31°. Meanwhile, the anti-corrosion performance of the base coating in 3.5 wt% NaCl solution was investigated using electrochemical impedance spectroscopy (EIS). The corrosion current density of the added modified basalt scale coating was in the order of 10−6, which was enhanced by one order of magnitude compared to that of the unmodified basalt scale. The annual corrosion rate of the MBFs coating was only 0.076588 mm·a−1. The superior anticorrosive properties of silane coupling agent-modified basalt flake composite coatings provide a new meaning for the development of anti-corrosive coatings.

1. Introduction

Corrosion is a form of deterioration or destruction of materials triggered by the reaction between the material and the service environment and is the main reason for the failure of metal materials in the electrical grid field [1,2]. According to statistics, the annual economic losses directly caused by metal corrosion account for about 3% of the total global GDP [3]. Common anti-corrosion methods are mainly material modification method [4,5] (development of weathering steel), electrochemical protection method [6,7] such as (sacrificial anode protection method, applied current protection method), the addition of retardant [8,9], coating protection method [10,11] (hot dip galvanizing, cold galvanizing, electroplating, and anticorrosion coatings and so on). Weathering steel uses alloying elements such as copper, chromium, and nickel to work with iron to form a dense oxide skin. The formation of an oxide skin on weathering steel is a relatively slow process, so its corrosion protection may be relatively low during the initial period of use. Electrochemical protection is a method of slowing or inhibiting corrosion of metals by introducing an external electric current to the metal surface. The introduction of an external current is less practical and applicable for corrosion protection of large equipment. Corrosion inhibitors can inhibit electrochemical reactions by forming a protective film on the metal surface to increase the polarization resistance. The effectiveness of corrosion inhibitors is usually influenced by environmental conditions such as temperature, pH, dissolved oxygen content, and so on. The coating protection method is to achieve the purpose of corrosion protection by forming a protective coating on the metal surface, blocking the direct contact between the metal and the external corrosive environment. Among them, the coating method is the most common corrosion prevention method, while anti-corrosion coatings are the most economical and efficient coating protection measures [12,13,14].
Heavy anti-corrosion coatings are a type of anti-corrosion coatings that serve longer in relatively harsh corrosive environments (marine, chemical, and so on.) than conventional anti-corrosion coatings, with two important characteristics: long-lasting and thick film [15,16]. “Heavy duty coatings” refers to a type of coating specifically designed to provide efficient protection in extremely corrosive environments. These types of coatings typically have a strong corrosion protection capability and can protect metal surfaces from corrosion under harsh conditions. The longevity of a coating is the ability of the coating to protect the substrate from erosion, wear and tear, corrosion, and other forms of damage over a long period of service [17]. The dry coating thickness of conventional organic anti-corrosion coatings is generally within the range of 80~150 μm, and the service life in C3 and the following corrosive environments is generally required to be 3–5 years. For the thick film of heavy-duty anti-corrosion coating thickness should be at least 200 μm or more, or even more than 1000 μm, of which the super-heavy anti-corrosion coating (mostly used in the deep sea or chemical media immersion environment) can reach 2000 μm [18,19]. Corrosion in the C4-C5 grade corrosive environment in the anti-corrosion service life of more than ten years. In addition to the above two most critical two characteristics, heavy anti-corrosion coatings also have a multi-systematic coating supporting the construction and high performance of raw materials and other characteristics, and in the C4-C5 conditions of the marine environment of the heavy anti-corrosion coatings used also need to have excellent water resistance.
Basalt flake refers to the flake basalt material with a high diameter-to-thickness ratio formed after processing by chemical or physical means, which belongs to one kind of inorganic filler [20,21]. In the research field of heavy anti-corrosion coatings, inorganic scale-type filler has a wide range of applications, and is more comprehensive and effective to meet the performance requirements of anti-corrosion coatings modifiers [20,22,23]. Compared to the traditional glass flake and other fillers, basalt flake filler type anti-corrosion coatings not only have a stronger filling effect and physical shielding effect, but their environmental economics can further reduce the cost of coatings [24,25]. Yan et al. [26] added basalt flakes to a solvent-free epoxy resin and showed that the coating with 10% basalt flakes added had the best corrosion protection. After immersion in 3.5% NaCl for 45 days, its low-frequency impedance value was still greater than 107 Ωcm2, which was at least one order of magnitude higher than that of the pure epoxy coating. Zhang et al. [27] modified the surface of basalt powder using 3-aminopropyltrimethoxysilane. The modified basalt powder was added to the epoxy resin, and the test results showed that the hardness of the coating could reach 0.25 GPa, and the elasticity of the coating could reach up to 4.97 GPa. After immersing in 3.5 wt% NaCl solution for 15 d, the low-frequency impedance value (|Z|0.01 Hz) could be up to 2.43 × 108 Ωcm2, which is 7.8 times that of the pure epoxy coating. Luo et al. [28] used nano SiO2 microspheres to modify basalt flakes and add them to epoxy resin to prepare a new epoxy anticorrosive coating. The experimental results show that the epoxy resin coating modified with 3‰ nano-SiO2 microspheres has excellent chemical durability (2.2% surface loss in alkaline solution after 480 h, and only 1.1% in acidic solution), and the tensile strength can reach 33.4 MPa.
In this paper, a one-component polyurethane resin with wet-curing properties is used as the film-forming material, basalt flakes with better weathering and acid and alkali resistance are used as the functional filler, and silane coupling agent is used to modify the basalt flakes. Test results show that the MBFs help to improve the coating performance. Adding less than 20 wt% due to the insufficient number of scales makes the overlap interlocking effect not obvious, and the weathering and corrosion resistance of the coating changes weakly. When adding the 30 wt% MBFs, the mechanical properties, acid and alkali resistance and corrosion resistance of the coating reach the best, and the aging resistance is good. Among them, the adhesion of the coating can reach 15.36 MPa, and the surface of the coating did not show obvious expansion and peeling phenomenon after 1000 h of neutral salt spray test. The corrosion current density of the added MBFs coating is in the order of 10−6, which is one order of magnitude higher than that of without modified basalt flakes. The annual corrosion rate of the MBFs coating is only 0.076588 mm·a−1. When the content of MBFs is 40 wt%, due to the agglomeration of part of the flake accumulation coating pinholes, cracks, and other defects, corrosive media can quickly through the coating to the substrate, mechanics, acid, and alkali resistance and corrosion resistance and other properties appear to be a significant decline.

2. Materials and Methods

2.1. Materials

The BFs used in the experiments were purchased from Suzhou Dexing Technology Co., Ltd. with a size distribution in the range of 25 μm−3 mm and a thickness of about 3 μm. The polyurethane resin (SWD959) was purchased from Shunjian New Materials (Shanghai, China) Co. SWD959 wet-curing polyurethane coating is synthetic by pre-polymerization of single-component polyurethane resin. Silane coupling agent (KH-560) was purchased from Dow Corning. Organic bentonite was purchased from Shanghai Beimo Industry Co. Organic bentonite is an inorganic bentonite/organic ammonium complex.

2.2. Modification of BFs

In the first step, basalt scales were put into an anhydrous ethanol solution for ultrasonic cleaning for 30 min, after which they were put into a drying oven for drying. In the second step, a mixed solution was prepared according to the mass ratio of deionized water: isopropanol = 9:1, and then KH560 coupling agent with a concentration of 1% was added, and the pH value of the solution was measured and adjusted to the range of 4–5. The silane coupling agent was fully hydrolyzed by using a constant temperature magnetic stirring water bath for more than 24 h at a continuous 40 °C water bath. Finally, the dried basalt flake was put into the hydrolysis solution and dispersed with high-speed stirring for 20 min, and then immersed for 40 min. The basalt was washed again with anhydrous ethanol and dried to obtain the MBFs.

2.3. Pre-Dispersion of Fillers

In this experiment, the pre-dispersion of the MBFs was carried out by the pre-impregnation process. The MBFs were added into the dispersing tank, and then the polyurethane solvent was added and stirred manually until there was no obvious agglomeration of large particles and then left to be pre-dispersed for 24 h. A high-speed mixer was used to stir and disperse the flake at a speed of 1000 r/min for 15 min, and then the speed was lowered to 400 r/min, and then the dispersant and the positioning and arranging agent were added slowly under the rotary dispersing state, and the speed was raised to 1000 r/min to continue dispersing for 2 h to produce the pre-dip slurry. After 10 min of dispersion, the speed was increased to 1000 r/min and continued to disperse for 2 h to produce the prepreg slurry.

2.4. Preparation of Polyurethane Anti-Corrosion Coatings

Slowly add the prepreg slurry into the polyurethane resin under the stirring speed of 200 r/min and stir continuously for 15 min, then add the thickening agent, defoamer, and other additives in turn and accelerate the stirring speed to 800 r/min for 30 min until homogeneous dispersion, and then finally adjust the viscosity to the appropriate viscosity with the diluent and then filter out the material. The coatings were applied by air spraying to the surfaces of the Q235 steel plate with surface grade Sa3 and tinplate with phosphate treatment, respectively. The dry film thickness of the Q235 steel plate was about 70–90 μm in a single pass, with a total of two passes, and the total dry film thickness was about 160–180 μm. The tinplate was sprayed with a single pass, with a dry film thickness of about 30–40 μm, and the wet-film test panels were dried to a film at room temperature (humidity ≥ 30%) and maintained for seven days before performance tests were carried out. To further verify the effect of the MBFs content on the performance of anticorrosive coatings, five groups of experiments were set up with 10%, 20%, 30%, 40%, and blank control groups, and the specific coating formulations are shown in Table 1.

2.5. Performance Characterization

A scanning electron microscope (SEM) (TM3030, Japan) with an elemental energy spectrum analyzer (EDS) was used to analyze the microscopic morphology of BF and anti-corrosion coatings before and after modification. The micro-morphology of the anti-corrosion coating was analyzed using an optical microscope. The mechanical properties of the coatings were tested including pencil hardness, bending resistance, impact resistance, friction and abrasion resistance, and adhesion. The weathering performance of the coatings was tested and evaluated by combining xenon lamp aging artificial acceleration simulation experiment and natural exposure experiment. The water contact angle of the coatings was tested using a contact angle tester (OCA 15EC, Filderstadt, Germany). The corrosion resistance test of the coating includes acid and alkali resistance test, neutral salt spray test (JST-060, Shanghai Janto Instrument and Equipment Co., Ltd., Shanghai, China), and electrochemical test. Among them, the electrochemical test was carried out using an electrochemical workstation (Autolab PGSTAT302N, Herisau, Switzerland). The test adopts a three-electrode working system, the reference electrode is a saturated calomel electrode (SCE), the auxiliary electrode is a platinum sheet, the electrolyte solution is NaCl solution with a mass fraction of 3.5%, the scanning voltage range of polarization curve is −0.8~+0.8 V (relative open-circuit potential), the scanning speed is 1 mV/s.

3. Results

3.1. Structural Characterizations of MBFs and Polyurethane Anti-Corrosion Coatings

Figure 1a,b shows the changes in the surface micro-morphology of the BF before and after modification by the KH560 coupling agent. Before modification, the surface of the scale is smooth and even the thickness is uniform, and there are no obvious bumps or pollutants and other impurities. After the modification of the scale surface changes are obvious, attached to many uneven sizes of the white flocculent, indicating that the functional groups of the coupling agent hydrolysis and the surface of the chemical reaction with the generation of new substances, which has an effective grafting modification effect on the scale surface. Figure 1c shows the dispersion stability of the coating before and after basalt scale modification after 24 h of standing. The unmodified basalt/polyurethane coatings produced obvious delamination after 10 h of standing and completely settled after 24 h. This is because unmodified basalt flakes are inherently hydrophilic, but not organophilic. Basalt flakes also cause particle agglomeration, which accelerates the settling of the filler. The modified coating only showed weak delamination after 24 h, and the overall state of the coating still showed uniform dispersion, maintaining good dispersion stability. The KH560 coupling agent can effectively improve the dispersion stability of basalt scales in polyurethane resin, this is because the grafting effect of the coupling agent effectively forms a bridge medium between the basalt scales and the resin, connecting the basalt scales uniformly to the polymer chain of the resin, which can make the scales stably fixed in the resin.
The microscopic morphology of the prepared coatings was observed using an electron optical microscope, and the optical microscopic morphology of the coatings obtained at a magnification of 400 times is shown in Figure 2a The surface smoothness of the varnish after curing into a film is high, the reflection phenomenon is obvious, and a large number of streaks and shrinkage holes, indicating that the coating density is poor, which is due to the varnish internal no filler to weaken the cohesion generated during the curing process, and at the same time, no filler of the slip of the wet film leveling and wettability is poor, the coating is prone to the formation of wrinkles and shrinkage holes. The coating surface in Figure 2b has substantially reduced reflections and higher roughness, which is favorable to hydrophobicity, but shrinkage holes may still exist. With the increase of basalt scale content, the surface roughness gradually decreases and the distribution of scales on the surface becomes uniform, but the coating surface in Figure 2e shows large dark-colored areas, which may be the agglomeration phenomenon of scales. The overall coating surface filler distribution is more uniform, and the densification is higher.

3.2. Effect of MBFs Content on Conventional Physicochemical Properties of Coatings

Table 2 shows the results of the conventional physical and chemical properties of the coatings. Compared to the varnish, all the physical and chemical properties of the MBFs/polyurethane coating were improved to a certain extent. With the increase of modified basalt flake content, the hardness, impact resistance, bending resistance, abrasion, and chemical resistance of the coating are enhanced, which is due to the excellent mechanical properties and corrosion resistance of basalt flake. The drying speed and adhesion of the coating show a gradual decline in the trend, the drying time increases because the content ratio between the scale and solvent is 1:1, the scale content increases the solvent content also increases, and the wet film drying time is extended. The original coating adhesion of the varnish was the highest at 17.68 MPa. The adhesion of the coating was reduced by the addition of basalt scales. The initial adhesion of the coating with the addition of the 10 wt% MBFs was 14.61 MPa. The adhesion of the coating with the addition of the 30 wt% MBFs was relatively high at 15.36 MPa. Varnish is very easy to peel off the substrate after stress, modified basalt flakes added to the resin can effectively reduce the cohesion and residual stress generated within the coating system after curing into a film, improving the coating and substrate peeling tendency. After the content of the modified basalt flake exceeds the critical value, it will affect the continuity of the combination between the coating and the surface of the substrate due to local agglomeration and settlement, and the adhesion will be reduced.

3.3. Effect of MBFs Content on Water Contact Angle of Coatings

Figure 3 shows the photos of the water contact angle test with different contents of the MBFs added to the coating. The water contact angle of the varnish coating was 77.77°, indicating that the polyurethane resin itself is hydrophobic. The water contact angle of the coating decreased to 75.28° after adding the 10 wt% MBFs, which may be due to the fact that a small number of the BFs will reduce the surface defects caused by the behavior of swirls and shrinkage of holes produced by the wet film during the drying and curing process, which will increase the smoothness of the coating. After adding the 20 wt% MBFs, the water contact angle of the coatings reached 81.31°, which was a slight increase compared to the previous two coatings, and the water contact angle of the coatings still maintained a large contact angle (80.78°) after the content was increased to 30 wt%. When the content was increased to 40 wt%, the water contact angle of the coating was 77.98°, which showed a significant decrease, indicating that the addition of the MBFs at this time exceeded the critical value for the best effect, and there was a counteraction. Overall, the addition of basalt scales results in an increase in the water contact angle, from an initial 77.77° to 77.98°. The appropriate amount of the MBFs dispersed in the resin system can be effectively wrapped by the resin to form a capsule, and the part distributed on the surface of the coating will increase the roughness of the coating. The spreading extension process at the intersection of the three-phase interface is hindered by the rough structure which cannot cross the protruding barrier and reach the thermodynamic equilibrium state, which no longer wets the surface and produces the hysteresis effect of the contact angle, which increases the value of the contact angle. In addition, the increase in roughness makes the surface micro-nano groove structure filled with more air, in the wetting process, the air will not be squeezed out, but by the liquid covered in the depression, the formation of gas-solid cross-composite surface, weakening the wettability of the coating surface, increasing the contact angle. Excessive basalt scales added will lead to incomplete resin coating, with part of the basalt scales exposed to direct contact with water molecules, resulting in a reduction of the water contact angle.

3.4. Effect of MBFs Content on the Weathering Properties of Coatings

The xenon lamp aging test was used to accelerate the weathering test of the coating under sunlight, and Figure 4 shows the surface state of the coating after the 500 h test of the specimen. The yellowing phenomenon of the varnish coating is more serious, the aging area is more than 80% of the whole coating, with more chalking and spots, a little blistering, and cracks. Add 10 wt% content of the coating also caused part of the yellowing and chalking phenomenon and a small number of blistering spots, did not find other defects, mainly polymer resin itself is prone to age degradation. With the increase of basalt flake content, add 20 wt% and 30 wt% of the surface of the coating has no significant changes, no yellowing, blistering or chalking phenomenon. At this time, the MBFs filled in the coating horizontal arrangement can effectively block the light intake and absorb most of the UV light. The 40 wt% coating showed weak corrosion, but no other defects, mainly due to the MBFs part of the agglomeration affecting the coating density, a long-time hot and humid environment will have water molecules penetrate the coating and even reach the substrate to make it corrode.
The polymer coatings undergo weight loss behavior during aging degradation due to changes in the polymer chain, and Figure 5 shows the change in the mass of the coating after the 500 h aging test. From the distribution of weight loss data in Figure 5, it can be seen that the higher the content of the MBFs, the lower the loss of coating mass, and it can be assumed that the coating aging degradation behavior is slightly slower. Clear coat has the largest weight loss value, and the weight loss of the coating with the MBFs is significantly reduced, indicating that basalt scales are beneficial to the coating’s weathering performance. The weathering resistance gradually leveled off after the content exceeded 20 wt%, with the least mass loss at 30 wt%, and the increase in mass loss with the addition of 40 wt% of the coating may be due to localized corrosion, with the corrosion products increasing the mass of the specimen.

3.5. Effect of MBFs Content on Corrosion Resistance of Coatings

The macroscopic morphology of different coatings after 1000 h of salt spray test is shown in Figure 6. The uncoated part of the substrate of the scribed specimen has already shown obvious corrosion. The coating around the scratch line of the varnish specimen has been partially detached from the substrate, so that a large area of serious corrosion occurs and spreads rapidly, and the coating is completely detached, indicating that the basic protective function of the coating fails. The reason for the detachment during the scribing process is presumed to be the brittleness of the varnish coating and the uniform cohesion produced by polymer cross-linking during the curing process of the resin, while a part of the coating breaks the stress balance caused by the coating detachment from the substrate after the damage occurred. The specimen with basalt scales also showed obvious signs of corrosion, but there was no obvious phenomenon of spreading and peeling, and the surface of the coating had no defects such as blisters, cracks, or pockmarks. The polar end of the resin in the coating can be combined with the active surface of the inorganic flake to generate a cross-linked closed structure, which helps to eliminate the interlayer stress. At the same time, basalt scales evenly dispersed in the coating can be intertwined with the resin polymer chain cross, layer by layer lap to form a partition effect that will be coated into a mesh space, reducing the cohesion generated in the process of resin film, so that the resin deformation is closer to the filler.
Unilateral spreading behavior tests characterize the rate at which corrosion spreads internally along the coating-substrate interface after a coating has been damaged. Figure 7a shows the results of the salt spray test 500 h and 1000 h unilateral flaring measurements. After 500 h, the varnish coating has been peeled off in a large area, and the unilateral flaring is much more than 2 mm, and the coating has completely failed. The coatings with the 10 wt% and 40 wt% MBFs showed weak corrosion expansion, while the coatings with 20 wt% and 30 wt% MBFs showed no significant corrosion expansion and the periphery of the scribe line remained intact. After 1000 h of the experiment, all the coatings with different degrees of spreading corrosion, in which the amount of spreading corrosion of the coating with the 40 wt% MBFs is more than 1 mm, but less than 2 mm, and it can still effectively play a protective role on the substrate. Adhesion is the most basic requirement for a coating to have corrosion resistance, and it is of great significance to examine the change of wet adhesion during the salt spray test for the anti-corrosion performance of the coating. Figure 7b shows the adhesion of different coatings after 1000 h of salt spray test. According to the test results, it can be seen that during the salt spray test, the varnish coating has the fastest decrease in adhesion due to the lack of corrosion resistance, from the initial 17.68 MPa to 11.1 MPa after 1000 h, with a decrease of 37.16%, which indicates that the varnish coating is affected by the corrosion of salt spray. The decrease in coating adhesion with the addition of the MBFs was also significant, while the decrease in coating adhesion with the addition of the 30 wt% MBFs was the smallest, indicating relatively good corrosion resistance.
In summary, the MBFs have an important influence on the salt spray resistance of coatings. Varnish has the weakest salt spray resistance, on the one hand, due to the role of no functional filler inside the varnish, the coating does not have a good physical shielding effect, resulting in sodium ions, water molecules, and other strong corrosive media can quickly penetrate the substrate to cause corrosion. On the other hand, the varnish itself belongs to the polymer material, the inner minister molecular chain in high temperature and high humidity conditions is prone to aging and deterioration, at the same time in the cohesion of the role of very easy to brittle fracture, a serious impact on the performance of the coating. Basalt flakes joining can effectively lap to form a “lap interlocking—labyrinth effect”, to extend the corrosive medium penetrating through the coating to contact the substrate path, the corrosive medium to play a more excellent barrier effect. At the same time, basalt flakes can effectively slow down the aging and deterioration of the coating due to its high-temperature resistance and aging resistance, thus improving the salt spray resistance of the coating.
To verify the acid and alkali resistance effect of the coatings, basalt flakes coatings with different contents were immersed in 5% NaOH and H2SO4, and the macroscopic morphology of the coating surface was observed. The changes in the coating surface before and after immersion are shown in Figure 8(a1,d2). To facilitate the observation of coating changes, 2/3 of the specimen was immersed under the liquid surface, and 1/3 was exposed to the atmosphere. The red dotted line in the figure shows the position of the immersed liquid surface, the upper part is the original coating, and the lower part is the immersed coating. The coatings with the 30 wt% and 40 wt% MBFs showed significant discoloration after 168 h of immersion in acid-alkali solution, but the coatings with the 10 wt% and 20 wt% MBFs did not show any change. Figure 7e shows the adhesion of different coatings after 168 h of immersion. The coatings with the 10 wt% and 40 wt% MBFs showed a significant decrease in adhesion due to the lack of corrosion resistance, indicating that the coatings were more affected by the corrosion of acid and alkali solution immersion. The adhesion of the coatings with the 20 wt% and 30 wt% MBFs was only slightly changed and remained high. From the adhesion data, the coating with the 30 wt% MBFs had the best acid and alkali resistance. Basalt flakes have a low content of alkaline oxides, which can be added to the coating to significantly enhance the acid and alkali resistance and effectively prolong the ion penetration process. The addition of excessive MBFs will make part of the scales exposed in the surface layer due to incomplete coating, at this time, the hydrophilic property of basalt makes the solution further wet the coating, and the corrosive medium enters the interior of the coating. There will be obvious filler agglomeration behavior inside the coating, forming pinholes and other defects on the surface and inside the coating, which will reduce the airtightness of the coating, accelerate the penetration of corrosive media to corrode the substrate and affect the performance of the coating.
Figure 9 shows the polarization curve of Q235 bare material and different coatings after 7 d immersion in 3.5% NaCl solution. Tafel fitting was used to obtain the corrosion parameters in Table 3. Compared to the Q235, the corrosion current density of the coated specimen was significantly reduced, and the corrosion potential was significantly positively shifted, indicating that the coating can provide effective corrosion protection for the substrate. This is because the organic polymer resin is cured into a film after the structure of a dense, scale-like structure and can play a shielding role, impeding the penetration and erosion of corrosive media. With the increase of basalt flakes content, the corrosion current density of the coating gradually decreases, adding the 20 wt% and 30 wt% MBFs coating corrosion current density is in the order of 10−6, the best protective effect. The annual corrosion rate of the 30 wt% MBFs coating was only 0.076588 mm/year. The corrosion current density of the 20 wt% MBFs coating increased to the order of 10−5, and the protective effect was weakened. Mainly due to basalt flakes over the critical value of uniformly dispersible part of the formation of agglomeration effect, the coating cracks, shrinkage, and other defects, affecting the coating denseness and water absorption.
The absolute impedance modulus |Z|0.1Hz in the low-frequency region (0.1 Hz) in the electrochemical test results can effectively evaluate the corrosion resistance of the coating. Figure 10 shows the Bode impedance curves of different coatings after immersion in 3.5% NaCl solution at different times. After 7 d of immersion, the |Z|0.1Hz value of the coating with the addition of the 30 wt% MBFs was 1.88 × 106 Ω/cm2 and the coating had the best corrosion resistance. All other coatings have an impedance between 104~105 and have poor corrosion protection. This is mainly due to the penetration of corrosive media such as H2O and Cl− into the coating after prolonged immersion, causing weak corrosion. The |Z|0.1Hz values of all coatings showed different degrees of decrease with increasing immersion time. Among them, the varnish coatings, the coatings with the 20 wt% and 40 wt% MBFs showed a larger decrease at 24 d, indicating that the coatings had completely failed at this time. The addition of the 30 wt% MBFs coating still maintains effective protection with |Z|0.1Hz values around 1.0 × 106 Ω/cm2 at 24 d. The coating only starts to fail at 31 d, but the impedance values are still significantly higher than the other coatings. Comprehensive electrochemical test results analyzed that the appropriate amount of basalt scales helps to improve the corrosion resistance of the coating, but more than the critical content can be uniformly dispersed will affect the densification of the coating, so that the performance of the performance is greatly reduced, or even lower than the varnish.

3.6. Analysis of Coating Corrosion Resistance Mechanism

Figure 11 is a diagram of the mechanism of adding basalt flakes. Basalt flakes are an inorganic material whose lamellar structure in the coating can be lapped to form a “lap interlocking—labyrinth” effect, increasing the corrosive media penetrating through the coating to contact the substrate path, playing a good shielding effect [37], thus effectively slowing down the process of corrosion and improve the service life of the coating. At the same time, the formation of the lap network can be divided into several small independent spaces within the coating grid, the more the content of filler, the physical and chemical properties of the coating closer to the filler’s own properties, effectively reducing the coating in the process of curing the film produced by the contraction or expansion of stress on the performance of the coating.

4. Conclusions

In this paper, basalt flakes are modified with silane coupling agent type KH-560 and added as filler to one-component polyurethane resin with wet curing characteristics. Through the experimental design and performance assurance, the following conclusions can be obtained. The excellent mechanical properties and segmentation behavior of the MBFs effectively enhanced the mechanical properties of the coatings, such as hardness, adhesion, abrasion wear resistance, and so on. The addition of the MBFs increased the water contact angle of the coatings, which increased from 77.77° to 81.31°. The weathering resistance of the coatings was tested by artificially simulating accelerated aging and natural sun exposure tests, and the weathering resistance of the coatings was greatly enhanced after basalt flakes were added to the resin. The 500 h accelerated aging test results showed that the phenomena of coating yellowing, cracking, and chalking were significantly improved, and the coating weight loss was reduced. The weathering performance of the coatings with the 30 wt% MBFs was good, while the coatings with the 40 wt% MBFs showed slight corrosion. The water contact angle of modified basalt flake increases and hydrophilicity is improved, which makes the coating resistant to water molecules, acid and alkali solutions, and other corrosive media wetting and penetration ability is enhanced. Compared to the varnish, the modified basalt scale coating in the salt spray test 1000 h corrosion phenomenon is weak, has no obvious defects and erosion behavior, still maintains high adhesion, the annual corrosion rate is greatly reduced, and excellent corrosion resistance. Among them, the corrosion current density of the 30 wt% MBFs coating can reach 1.88 × 106 Ω/cm2, which is 2 orders of magnitude higher than that of pure Q235 matrix. The annual corrosion rate of the 30 wt% MBFs coating is only 0.076588 mm/year. The superior anticorrosive properties of silane coupling agent-modified basalt flake composite coatings provide a new meaning for the development of anti-corrosive coatings.

Author Contributions

Conceptualization, K.S. and W.C.; methodology, K.S.; software, K.S.; validation, K.S., W.C., and X.H.; formal analysis, K.S. and W.C.; investigation, X.H. and Y.Z.; resources, H.C.; data curation, K.S., Y.Z., T.J., and K.C.; writing—original draft preparation, K.S.; writing—review and editing, K.S. and Y.Z.; visualization, W.L. Funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by China Southern Power Grid Co., Ltd. Liuzhou Bureau of EHV Transmission Company grant number CGYKJXM20220151, the Science & Technology Commission of Shanghai Municipality and Shanghai Engineering Research Center of Ship Intelligent Maintenance and Energy Efficiency under (20DZ2252300) and Shanghai High-level Local University Innovation Team (Maritime safety & technical support).

Data Availability Statement

All data, models, and code generated or used during the study appear in the submitted article.

Acknowledgments

This work was financially supported by China Southern Power Grid Co., Ltd. Liuzhou Bureau of EHV Transmission Company, grant number CGYKJXM20220151, the Science & Technology Commission of Shanghai Municipality and Shanghai Engineering Research Center of Ship Intelligent Maintenance and Energy Efficiency under (20DZ2252300) and Shanghai High-level Local University Innovation Team (Maritime safety & technical support). All authors agreed to acknowledgements.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Coatings 13 02022 g001
Figure 1. Characterization of basalt flakes. Microscopic morphology of basalt flakes (a), Microscopic morphology of basalt flakes after KH560 treatment (b), Dispersion stability of basalt flakes before and after KH560 treatment (c).
Figure 1. Characterization of basalt flakes. Microscopic morphology of basalt flakes (a), Microscopic morphology of basalt flakes after KH560 treatment (b), Dispersion stability of basalt flakes before and after KH560 treatment (c).
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Figure 2. Optical photographs of the coatings. Polyurethane coating (a), coating with 10% MBFs added (b), coating with 20% MBFs added (c), coating with 30% MBFs added (d), coating with 40% MBFs added (e).
Figure 2. Optical photographs of the coatings. Polyurethane coating (a), coating with 10% MBFs added (b), coating with 20% MBFs added (c), coating with 30% MBFs added (d), coating with 40% MBFs added (e).
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Figure 3. Water contact angle test: (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating.
Figure 3. Water contact angle test: (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating.
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Figure 4. Surface morphology of xenon lamp artificially accelerated aging specimens. (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating.
Figure 4. Surface morphology of xenon lamp artificially accelerated aging specimens. (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating.
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Figure 5. Quality change of each coating after 500 h of xenon lamp aging test.
Figure 5. Quality change of each coating after 500 h of xenon lamp aging test.
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Figure 6. Corrosion morphology of the coating surface in salt spray test 1000 h. (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating.
Figure 6. Corrosion morphology of the coating surface in salt spray test 1000 h. (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating.
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Figure 7. Amount of unilateral flaring of scratch coatings in salt spray test (a). Change in coating adhesion after salt spray test (b).
Figure 7. Amount of unilateral flaring of scratch coatings in salt spray test (a). Change in coating adhesion after salt spray test (b).
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Figure 8. Surface morphology of coatings immersed in the acid-alkali solution for 168 h, (a1,a2) 10 wt%-MBFs/Polyurethane coating. (b1,b2) 20 wt%-MBFs/Polyurethane coating. (c1,c2) 30 wt%-MBFs/Polyurethane coating. (d1,d2) 40 wt%-MBFs/Polyurethane coating. (The red line represents the liquid level demarcation line; below the red line is the test surface.) Adhesion of the coating after 168 h of acid and alkali resistance (e).
Figure 8. Surface morphology of coatings immersed in the acid-alkali solution for 168 h, (a1,a2) 10 wt%-MBFs/Polyurethane coating. (b1,b2) 20 wt%-MBFs/Polyurethane coating. (c1,c2) 30 wt%-MBFs/Polyurethane coating. (d1,d2) 40 wt%-MBFs/Polyurethane coating. (The red line represents the liquid level demarcation line; below the red line is the test surface.) Adhesion of the coating after 168 h of acid and alkali resistance (e).
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Figure 9. Polarization curves for Q235 and different coatings.
Figure 9. Polarization curves for Q235 and different coatings.
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Figure 10. Bode curves for different coatings with different immersion times. (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating. (f) Impedance curves of different specimens immersed for 7 days.
Figure 10. Bode curves for different coatings with different immersion times. (a) Varnish. (b) 10 wt%-MBFs/Polyurethane coating. (c) 20 wt%-MBFs/Polyurethane coating. (d) 30 wt%-MBFs/Polyurethane coating. (e) 40 wt%-MBFs/Polyurethane coating. (f) Impedance curves of different specimens immersed for 7 days.
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Figure 11. Schematic diagram of basalt flakes anti-corrosion mechanism.
Figure 11. Schematic diagram of basalt flakes anti-corrosion mechanism.
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Table 1. Ratio of the MBFs/polyurethane anti-corrosion coatings.
Table 1. Ratio of the MBFs/polyurethane anti-corrosion coatings.
SamplePolyurethanePolyurethane SolventsMBFsDispersantPermutation AgentDefoamers
110000001.5
29010101.511.5
3802020321
47030304.531
5604040640.5
Table 2. General physical and chemical properties of coatings.
Table 2. General physical and chemical properties of coatings.
Test ProjectsVarnish10 wt%-MBFS20 wt%-MBFS30 wt%-MBFS40 wt%-MBFSTesting Standards
Surface dry h/real dry h4/184/225/225/246/24[29]
Pencil hardness/H22334[30]
Adhesion/Mpa17.6814.6115.1515.3615.04[31]
Impact resistance/cm5859657070[32]
Bending resistance/level32123[33]
Oil resistance/level00001[34]
Saltwater resistance/level21113[35]
Resistance to frictional wear/g0.220.180.190.230.27[36]
Table 3. Parameters for polarization curve fitting.
Table 3. Parameters for polarization curve fitting.
SamplesCorrosion Current Density (A/cm2)Corrosion Potential (V)Polarization Resistance (kΩ)Corrosion Rate (mm/Year)
Q2352.9474 × 10−4−0.7662.361303.42920
X05.3961 × 10−5−0.6126.872400.39370
X17.8156 × 10−5−0.6097.238100.32851
X28.90670 × 10−6−0.54933.04100.10350
X36.59110 × 10−6−0.32332.74300.076588
X43.44351 × 10−5−0.60312.815100.254610
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Sun, K.; Cai, W.; He, X.; Chen, H.; Chen, K.; Jiang, T.; Li, W.; Zhao, Y. Preparation and Properties of Silane Coupling Agent Modified Basalt Flake Polyurethane Anti-Corrosion Coatings. Coatings 2023, 13, 2022. https://doi.org/10.3390/coatings13122022

AMA Style

Sun K, Cai W, He X, Chen H, Chen K, Jiang T, Li W, Zhao Y. Preparation and Properties of Silane Coupling Agent Modified Basalt Flake Polyurethane Anti-Corrosion Coatings. Coatings. 2023; 13(12):2022. https://doi.org/10.3390/coatings13122022

Chicago/Turabian Style

Sun, Kuoteng, Weichen Cai, Xuemin He, Hao Chen, Kun Chen, Tao Jiang, Wenge Li, and Yuantao Zhao. 2023. "Preparation and Properties of Silane Coupling Agent Modified Basalt Flake Polyurethane Anti-Corrosion Coatings" Coatings 13, no. 12: 2022. https://doi.org/10.3390/coatings13122022

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