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Article

Preparation and Properties of Conductive Aluminum Powder (Al@Si@C) for Water-Borne Heavy-Duty Anticorrosive Coatings

by
Qingpeng Li
1,2,*,
Jiaxing Liu
1,
Tiancheng Jiang
1,
Xiaoyun An
1,
Na Wang
1,2,
Zhixiu Xu
3,
Wanyuan Guo
3,
Liang Zhang
4 and
Xiaofeng Liu
1,*
1
College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
2
Shenyang Key Laboratory for New Functional Coating Materials, Shenyang 110142, China
3
Shenyang Research Institute of Industrial Technology for Advanced Coating Materials, Shenyang 110300, China
4
Shenyang Hangda Technology Co., Ltd., Shenyang 110043, China
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(9), 1082; https://doi.org/10.3390/coatings14091082
Submission received: 28 June 2024 / Revised: 9 August 2024 / Accepted: 20 August 2024 / Published: 23 August 2024
(This article belongs to the Special Issue Corrosion Resistance, Mechanical Properties and Characterization of Metallic Materials and Coatings, 2nd Edition)
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Abstract

:
To improve the storage stability and conductivity of aluminum powder in an aqueous environment, the surface of aluminum powder was treated to form silica film by the sol–gel method, then was treated with conductive modification to introduce nanocarbon black particles so that conductive aluminum powder could be prepared to solve the application bottleneck of aluminum powder in water-borne heavy-duty anticorrosive coatings. The structure, surface morphology, and composition of the modified aluminum powder were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD). The corrosion resistance and electrochemical properties were measured using a hydrogen evolution test and an 2electrochemical test. The results showed that there was a compact SiO2 film formed on the surface of the prepared conductive aluminum powder, and the conductive filler nanocarbon black was uniformly grafted on the surface. According to the hydrogen evolution test at 100 h/50 °C, conductive aluminum powder with 5 wt% carbon black exhibited the best hydrogen evolution effect, with a hydrogen evolution amount of only 0.5 mL. The prepared conductive aluminum powder was applied to the water-borne coatings, and the storage stability test, electrochemical polarization test, and neutral salt spray test were further conducted. The water-borne coatings prepared with conductive aluminum powder still showed good performance and had no reaction after 6 months of storage. Compared with the coating containing SiO2-modified aluminum powder, the coating exhibited better corrosion resistance.

1. Introduction

Metal corrosion has significant impacts on the development of the social economy [1]. The economic losses caused by steel structure corrosion worldwide could amount to hundreds of billions of dollars annually, revealing the profound impact of corrosion problems on the economy and safety. The structural damage caused by corrosion also brings potential environmental risks and human casualties, especially in critical infrastructure and important industrial fields. To prevent metal corrosion problems, metal surfaces are commonly coated with solvent-based coatings. However, these coatings contain volatile organic compounds (VOCs), which can cause potential damage to the environment [2,3]. This facilitates the significantly increasing demand for environmentally friendly coatings [4,5]; prompts a shift in the coating industry towards green, environmental, and low-carbon practices; and leads to the emergence of eco-friendly water-borne aluminum coatings [6,7].
Aluminum powder serves as an essential functional additive and has been extensively utilized in various fields, including printing ink, automotive coatings, and furniture coatings [8,9,10]. Aluminum powder rich in water-borne aluminum coatings can protect metal substrates such as steel from corrosion based on a sacrificial anode protection mechanism. However, there are some problems with the direct application of aluminum powder in water-borne coating systems. Firstly, the chemical property of aluminum powder is active, and it can react with the corrosive medium at room temperature, which leads to the rapid corrosion of aluminum powder. Thus, it cannot achieve its purpose of metal corrosion protection [11]. Secondly, aluminum powder in water-borne coating systems face the problems of poor dispersion and easy agglomeration. Moreover, aluminum powder generates hydrogen during the corrosion process, which brings about potential safety hazards during industrial production and transportation. Also, the hydrogen evolution phenomenon leads to high pressure inside the storage container and the expansion of the storage barrel, which unfavorably affects the long-term storage of water-borne aluminum coatings [12]. The reaction formulas for the hydrogen evolution of aluminum powder are as follows [13,14].
2Al + 6H2O→2Al(OH)3 + 3H2↑ (neutral or alkaline conditions)
2Al + 2OH− + 6H2O→2Al(OH)−4 + 3H2↑ (strong alkaline condition)
Therefore, modifying the surface of aluminum powder is necessary to inhibit the corrosion reaction if aluminum powder is to be applied in aqueous coating systems [15,16]. A variety of surface modification methods have been described and reported [12,17], which can be summarized into three main categories: organic modification [18,19], inorganic modification [20,21], and hybrid organic–inorganic modification [22]. Ma et al. prepared an aqueous aluminum pigment using the sol–gel encapsulation method with H2O2 as an anchoring agent and demonstrated that the surface of the aluminum pigment formed a crack-free, dense, and smooth corrosion-resistant coating [23]. He et al. modified the surfaces of aqueous aluminum pigments with SiO2 and polyacrylic acid brushes and found that the dispersion and corrosion resistance of the modified aluminum powder in aqueous media were significantly improved [24]. Pi et al. encapsulated aluminum powder particles in an organic–inorganic hybrid film and found that the corrosion resistance and adhesion of the modified aluminum powder in the paint film were greatly improved [25]. However, silica has high resistivity due to its stable physical properties [26,27]. Although the above methods can improve the corrosion resistance of aluminum powder significantly, they affect the conductivity of aluminum powder so that the aluminum powder cannot play the role of sacrificial anode [28,29,30]. Therefore, it is very necessary to study a new modification method that can improve the corrosion resistance of aluminum powder without affecting its electrical conductivity.
Carbon nanomaterials (e.g., carbon black, carbon nanotubes, graphene) and metallic materials (e.g., nickel, aluminum, and copper) are widely used to improve the electrical properties of composites [31,32,33]. Among them, carbon black is more attractive due to its low cost and good electrical conductivity. Ding et al. concluded that when the concentration of carbon black in the polymer matrix reached a certain critical value, which was known as the permeation threshold, the composite material became electrically conductive due to the formation of conductive channels [34,35,36]. Therefore, researchers have explored the preparation direction of carbon black (CB)/polymer composites. Zhang et al. prepared CB-reinforced polycarbonate (PC)/acrylonitrile–butadiene–styrene (ABS) composites [37]. Liu et al. successfully prepared carbon black–polystyrene composite particles with controllable sizes by using high-speed homogenization-assisted suspension polymerization [38]. Yan et al. used an in situ suspension polymerization method to prepare PS-CB composites [39]. At present, there are two effective methods for filling CB in polymers: CB in situ modification and CB surface modification [40]. However, CB is easy to agglomerate due to high surface energy and van der Waals forces, which leads to poor dispersion and compatibility in the polymer matrix, and CB is an efficient free radical trap. All of the above limit its practical application in polymer composites [41]. Therefore, CB first needs to be treated if it is to be used in the modification of aluminum powder.
In order to solve the application bottleneck of aluminum powder in water-borne heavy anticorrosive coatings and to effectively resolve the hydrogen evolution problem of aluminum powder in the aqueous system, SiO2-modified aluminum powder was prepared using the sol–gel method. Specifically, there was a thick silane film formed on the surface of the aluminum powder using spherical aluminum powder as the source material and silane coupling agent as the precursor. Further, the carbon black was modified with KH560 and then grafted onto the surface of SiO2-modified aluminum powder to enhance its conductivity. The preparation of highly conductive and corrosion-resistant aluminum powder will provide theoretical guidance and a scientific foundation for the development of heavy-duty water-borne coatings with effective corrosion resistance.

2. Materials and Methods

2.1. Materials

Aluminum powder of industrial grade was purchased from Zhangqiu Metal Pigment Co, Ltd. (Zhangqiu, Jinan, China). Tetraethyl orthosilicate (TEOS) of analytical grade was purchased from Tianjin Fuchen Chemical Reagent Factory (Tianjin, China). Triethanolamine and ethylene glycol monobutyl ether of analytical grade were purchased from Aladdin (Shanghai, China). KH560 of analytical grade was purchased from Merck (Shanghai, China). Hydroxylated carbon black and water-borne acrylic resin emulsion were purchased from the Shenyang Jiayi experimental instrument Distribution Office (Shenyang, China).

2.2. Experiments

2.2.1. Surface Treatment of Aluminum Powder

Aluminum powder (200 g) was stirred in NaOH solution (5%, 1000 mL) for 30 min. Then, the mixture was filtered and the aluminum powder was washed with deionized water and anhydrous ethanol, then dried to obtain the surface-treated aluminum powder.

2.2.2. Preparation of SiO2-Modified Aluminum Powder

The surface-treated aluminum powder was stirred vigorously at room temperature at a rate of 800 r/min for 30 min; then, the temperature was raised to 40 °C, and 100 g of silane was added. Stirring was continued at a rate of 800 r/min for 20 min; then, 100 g of deionized water was added dropwise under stirring conditions, and the pH was adjusted to 7~8 with triethanolamine. Subsequently, the reaction was carried out at 40 °C for 8 h. Finally, the product was filtered, washed with anhydrous ethanol, and dried at 30 °C for 10 h to obtain SiO2-modified aluminum powder.

2.2.3. Preparation of Conductive Aluminum Powder

Hydroxylated carbon black was mixed with anhydrous ethanol and stirred at 800 r/min for 20 min; then, an appropriate amount of silane coupling agent (KH560) was added. Subsequently, an appropriate amount of deionized water was added dropwise slowly, and the solution pH was adjusted to 7~8. The reaction was carried out at 40 °C for 4 h; then, the prepared SiO2-modified aluminum powder was added. The reaction was continued for 6 h, where the contents of hydroxylated carbon black were 1%, 3%, 5%, and 7% of that of SiO2-modified aluminum powder, respectively. Finally, the mixture was filtered and washed with anhydrous ethanol, and conductive aluminum powder was obtained after drying.

2.2.4. Preparation of Water-Borne Aluminum Coatings

Water-soluble acrylic resin (8 g) was mixed with propylene glycol monobutyl ether (5.4 g) and stirred at 1200 r/min for 10 min. Next, the prepared conductive aluminum powder (25 g) was added to the above mixture, and stirring was continued for another 10 min. Finally, 6.76 g of deionized water was added and mixed thoroughly. The obtained solution was dispersed at a speed of 2000 r/min for 0.5 h, then was sprayed uniformly onto Q-235 carbon steel sheets to prepare a water-borne aluminum coating.

2.3. Experimental Mechanism

Figure 1 shows the synthesis mechanism of SiO2-modified aluminum powder and conductive aluminum powder. The surface of the aluminum powder was rich in hydroxyl groups due to the Al2O3 film on the surface, which was a kind of Lewis acid [42] and tended to adsorb water molecules in a humid environment. Firstly, TEOS was hydrolyzed to silanol in an alkaline environment constructed by triethanolamine, and silanol generated could not only condense with hydroxyl groups on the surface of the aluminum powder to form Al–O–Si bonds, but also condense with hydroxyl groups on its own surface to form Si–O–Si bonds, which were more hydrolytically stable and would be retained on the surface. Thus, SiO2-modified aluminum powder was formed [43]. Moreover, owing to the hydrophilic nature of the hydroxyl groups, the SiO2 film formed on the surface of aluminum powder augmented the density of hydroxyl functional groups present on the aluminum powder surface, which elevated the hydrophilicity of the aluminum powder and significantly improved its dispersibility in aqueous coatings. The hydroxylated carbon black was then grafted onto the surface of the SiO2-modified aluminum powder in the presence of KH560, which strengthened the linkage between the aluminum powder and the conductive material through chemical bonding between the silane coupling agent and the carbon black, as well as the aluminum powder. Then, the conductive pathway was constructed, which in turn enhanced the conductivity of the aluminum powder.
Figure 2 illustrates the mechanism of KH560 grafting to the surface of aluminum powder. During this process, the –OCH3 groups of KH560 were hydrolyzed into –OH functional groups under hydration conditions. The generated -OH groups could form hydrogen bonds with the –OH groups present on the surface of the carbon black through a dehydration synthesis mechanism. The residual KH560 further underwent self-condensation to form Si–O–Si bonds so as to create a network structure. Simultaneously, the –OH groups on the surface of the SiO2-modified aluminum powder also participated in this process. The interaction among these components resulted in the formation of an encapsulating structure, and thereby, the conductive aluminum powder was successfully prepared.
Figure 3 depicts the anti-corrosion mechanism of the prepared coatings. The conventional coatings only served as a barrier, so the corrosion medium could directly pass through the surface coating to the metal substrate. The use of aluminum powder as a filler could greatly enhance the coating’s corrosion resistance, because the addition of aluminum powder could alter the path of corrosive substances from the surface coating to the metal substrate, thereby prolonging the time for the corrosive medium to reach the substrate and achieving the purpose of anti-corrosion.
Nonetheless, the sacrificial anode effect of the aluminum powder could not be displayed due to the silica film covered on the surface to solve the problem of hydrogen evolution. However, aluminum powder modified with carbon black compensated for this drawback by creating a conductive pathway to make aluminum powder corrode preferentially, so as to prolong the corrosion time of the metal substrate and improve the anti-corrosion performance of the coatings.

2.4. Characterization Methods

Fourier transform infrared spectroscopy (FT-IR) was performed using a Nexus 670 with an Attenuated Total Reflection (ATR) accessory from Nicorette Instruments, Madison, WI, USA.
The morphology and energy spectrum mapping of the samples were performed using a scanning electron microscope (SEM) (ZEISS Gemini SEM 300, Oberkochen, Germany) supplemented with an energy-dispersive spectrometer (EDS). The powder sample was glued to the conductive adhesive, then sputtered with a gold layer by a Quorum SC7620 sputtering coater with a 10 mA power supply. The acceleration voltage was 3 kV for topography shooting and 15 kV for energy spectrum mapping with an SE2 secondary electronic detector.
In order to characterize the changes in the anticorrosive properties of the aluminum powder before and after modification, the amount of hydrogen precipitation was measured to indicate the modification effect. Samples of 1 g of the aluminum powder before and after modification were weighed, respectively, into a 250 mL Erlenmeyer flask, and 20.1 g of deionized water, 16.1 g of ethylene glycol monobutyl ether, and 8 g of aqueous acrylic resin were added to the flask to form an aqueous coating system. After connecting the apparatus, the liquid level of the measuring tube was adjusted to the same height, the temperature was raised to 50 °C, and the value of the liquid level was read and recorded at intervals of 1 h.
X-ray powder diffraction (XRD) was used to study the microstructure of the aluminum powder before and after modification, which was performed using XPERT-PRO from PANalytical, Almelo, Netherlands, with a radiation source of Cu Kα radiation (λ = 0.154 nm), a tube voltage of 40 kV, and a tube current of 40 mA.
X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental and chemical bonding compositions of the sample surfaces using PHI 5000 Versaprobe III from Ulvac-Phi, Tokyo, Japan, in which Al Kα radiation was used as the target source with an operating voltage of 12 kV and the C 1s binding energy of 284.8 eV was used as the standard. The energy distribution of each element could be determined by the binding energy of photoelectrons.
The storage stability was tested indoors at room temperature by putting the prepared coatings into bottles with a certain space left in the container. The bottle was sealed and set aside for observation at regular intervals.
The water contact angle (WCA) test was performed using a DSA30 contact angle measuring instrument from KRUSS, Hamburg, Germany, to investigate the hydrophilic properties of the sample coating surface. The droplet size was 0.5 μL, and the images were recorded and analyzed 5 s after the droplets were removed from the capillary tube to the coating surface.
The electrical conductivity of the samples was characterized by RTS-9 digital four-probe resistivity tester provided by Guangzhou Four-Probe Technology Co., Guangzhou, China. Measuring range: resistivity 10−4~105 Ω·cm, conductivity 10−5~104 Ω·cm.
The dynamic potential polarization curves of the coatings were tested using the AUTOLAB 84,362 electrochemical workstation with a scanning rate of 5 mV/s. The reference electrode of the three-electrode system was a saturated mercuric glycol electrode, the auxiliary electrode was a Pt sheet, the study electrode was the prepared coating (electrode area of 1 cm2), and the corrosion medium was 3.5% NaCl solution. The data were fitted and analyzed using the software Nova 2.1.
The neutral salt spray test was conducted in a HDYM-90 neutral salt spray test chamber manufactured by Shanghai Hengding Instrument Equipment Factory, Shanghai, China, and a 5 wt% NaCl solution was added in a timely manner. According to the GB/T10125-2012 standard [44], the corrosion resistance of samples in harsh environments was studied. We took photos to record the surface changes of the coating sample at the expected time.

3. Results and Discussion

3.1. FT-IR Analysis

Figure 4 shows the FT-IR spectra of both non-modified carbon black and carbon black modified with KH560. Notable peaks were observed at 1100 cm−1 and 1037 cm−1 in the modified carbon black spectrum [45,46], with the former corresponding to the asymmetric stretching vibrations of Si–O–Si linkages [47] and the latter to Si–O–C stretching vibrations [48]. The hydrolyzable group OCH3 in KH560 transformed into Si–OH upon hydrolysis. The hydroxyl groups on the surface of carbon black reacted with the hydroxyl groups from Si–OH to form Si–O–C bonds. Furthermore, KH560 underwent self-condensation polymerization after hydrolysis, leading to the formation of Si–O–Si bonds. The peak at 900 cm−1 was attributed to the asymmetric stretching of epoxy groups [49], which were characteristic functional groups belonging to the silane coupling agent KH560. The presence of Si–O linkages and epoxy groups in the FT-IR spectrum of the modified carbon black conclusively demonstrated the successful grafting of KH560 onto the carbon black surface.
Figure 5 shows the FT-IR spectra of three kinds of aluminum powder. The spectrum of the Al–Si (SiO2-modified aluminum powder) presented a stretching vibration absorption peak of Si–O–Al at 1100 cm−1. Meanwhile, the absorption peaks at 780 cm−1 and 474 cm−1 were related to the symmetric stretching vibrations and the bending vibrations of the Si-O-Si bond [45,46,47,48,49]. The above findings suggest that the Si–OH group reacted with –OH on the surface of the aluminum powder after TEOS hydrolysis.
The spectrum of the Al–Si–C (conductive aluminum powder) displayed peaks at 1100 cm−1 of the Si–O–Al bond and 780 cm−1 and 474 cm−1 of the Si–O–Si bond, along with an absorption peak of the epoxy group at 900 cm−1, which met the characteristic peak of KH560 [49]. In conclusion, it could be inferred that the –OH group in the SiO2 film formed on its surface reacted with the Si–OH bond on the surface of KH560-modified carbon black; thus, the KH560-modified carbon black was grafted on the surface of SiO2-modified aluminum powder successfully.

3.2. SEM and EDS Analysis

Figure 6A,B show the morphology of aluminum powder and aluminum powder after TEOS modification. Figure 6A shows that the surface of the aluminum powder was uneven and rough due to pores. Figure 6B shows that the surface edges of the aluminum powder became smooth and clear, and the surface roughness was slightly reduced. This indicates that the uniform SiO2 film was introduced on the surface of aluminum powder by the sol–gel method, which enhanced the corrosion resistance of aluminum powder. Figure 6C–F illustrate the morphology of conductive aluminum powders that contained different contents of carbon black. It can be observed that the surface of aluminum powder began to show attachments and become rough after adding carbon black, and the amount of surface adhesion increased with the increase in the carbon black content. In Figure 6C,D, it is shown that only a few fractions of carbon black particles were attached to the surface of the aluminum powder, while carbon black particles were significantly increased and uniformly grafted onto the surface of the aluminum powder in Figure 6E. When the content of carbon black reached 7 wt% (Figure 6F), excessive carbon black particles caused agglomeration.
The above results indicate that there was a dense SiO2 film formed on the surface of aluminum powder [50,51], and it was clearly observed that carbon black had been grafted onto the surface of the aluminum powder, forming conductive pathways between the aluminum powder and carbon black. Thereby, a conductive aluminum powder with both stability and electrical properties was prepared successfully.
The sufficient content and good distribution of silicon on the surface of the aluminum powder were the key to its excellent corrosion resistance. Thus, the EDS analysis was conducted to determine the content and distribution of some special elements in the product. The results are summarized in Table 1.
It can be seen from Table 1 that the main surface elements in the aluminum powder were aluminum and carbon, and there was also a small amount of silicon, which represented the impurities from the aluminum powder processing. Meanwhile, the atomic fraction of silicon and oxygen increased significantly in the SiO2-modified aluminum powder. This indicates that the SiO2 layer was successfully coated on most of the exposed areas of spherical aluminum after the aluminum powder was treated by the sol–gel process.
For the conductive aluminum powder, with the increase in conductive filler (hydroxylated carbon black), the atomic fraction of aluminum gradually decreased, while the atomic fraction of carbon gradually increased. However, compared with SiO2-modified aluminum powder, the atomic fractions of silicon and oxygen exhibited little change. All of this indicates that the carbon black was successfully grafted onto the surface of SiO2-modified aluminum powder, and this is consistent with SEM analysis.

3.3. XPS Analysis

Figure 7 shows the XPS spectra of aluminum powder. The spectra illustrated that the aluminum powder had prominent peaks of Al 2p, Al 2s, C 1s, and O 1s, whereas the spectra of the two modified aluminum powders revealed three additional new peaks with binding energies of 102.7 eV (Si 2p), 154.4 eV (Si 2s), and 403.06 eV (N 1s). The Si element originated from the hydrolysis products of TEOS and KH560, while the N element was derived from triethanolamine; 119.0 eV (Al 2s) and 74.21 eV (Al 2p) of the Al element corresponded to Al2O3 [52] and Al [53], respectively. After modification, the Al2O3 peak strength of the two modified aluminum powders significantly weakened at 119.0 eV due to the formation of Si–O–Al bonds between SiO2 and aluminum powder. Moreover, the Al element peak at 74.21 eV was not detected after modification. This indicated that the SiO2 film exhibited a good coating effect on aluminum powder and formed a uniform and dense coating on the surface of aluminum powder [54].
Figure 8 shows the Si 2p peak diagram of SiO2-modified aluminum powder. From the diagram, it can be seen that the peak of Si 2p contained the peak of Si–O–Si with binding energy near 103.2 eV [53] and the peak of Si–O–Al with binding energy near 103.9 eV [54]. The presence of Si–O–Al and Si–O–Si groups indicated that the silane coupling agent could not only condense with hydroxyl groups on the surface of aluminum powder to form Al–Si–O bonds, but also condense with itself to form more stable Si–O–Si bonds, which is consistent with FT-IR results. The above analysis fully showed that the surface of the aluminum powder had been successfully coated with a layer of SiO2 film.
Figure 9 displays the Si 2p peak diagram of conductive aluminum powder, the binding energy of which was slightly different from that of Si 2p of SiO2-modified aluminum powder. Figure 9 shows the peak of Si–O–C with binding energy near 102.7 eV [50] and the peak of Si–O–Al with binding energy near 103.62 eV [54]. The existence of the Si–O–C group fully proved that carbon black reacted with hydroxyl groups on the surface of aluminum powder and was grafted onto the aluminum powder.
The element composition and atomic percentage of the aluminum powder surface from the XPS results are summarized in Table 2. It is evident from Table 2 that there were small amounts of the elements Si and N besides Al, C, and O on the surface of the aluminum powder, which was due to the presence of impurities in the aluminum powder. Further analysis showed that the Al content on the surface of the SiO2-modified aluminum powder decreased to 0.99%, while the Si and N contents increased to 14.07% and 1.36% compared with the original aluminum powder. The decrease in Al content and increase in Si content indicated a successful coating of SiO2 film onto the aluminum powder surface. For the conductive aluminum powder, the surface with a reduced Al content of 0.76% increased the Si and C contents of 9.55% and 55.98%, indicating that grafting carbon black onto the surface of aluminum powder led to a significant increase in the C content and a decrease in the Si content.
XPS analysis showed that the content changes of the elements such as Al, Si, and C, fully demonstrating that KH560 served as a bridge and connected carbon black with SiO2-modified aluminum powder through chemical bonding to construct a conductive pathway, which was consistent with the SEM results.

3.4. XRD Analysis

Figure 10 displays the X-ray diffraction patterns of aluminum powder (Al), SiO2-modified aluminum powder (Al-Si), and conductive aluminum powder with 5 wt% C (Al–Si–C). In the pattern of the aluminum powder, the diffraction peaks for the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystal planes were observed at 38.4°, 44.6°, 65°, and 78.1°, respectively, and there was no other impurity peak [55]. In the XRD patterns of modified aluminum powders Al–Si and Al–Si–C, there was no significant change compared to the aluminum powder. Both maintained the characteristic peaks of Al and exhibited the typical Al structure. This indicated that the modification of aluminum powder by silica and hydroxyl carbon black only generated grafting reaction and intermolecular interaction on the surface of aluminum powder, and did not change the crystal structure of the aluminum powder, so the sacrificial anode effect of aluminum powder would not be damaged.

3.5. Resistivity Test

Table 3 displays the resistivity data of the three kinds of aluminum powders characterized by four-probe measurement. The average resistivity of the aluminum powder was 3.24 Ω·cm, and that of the SiO2-modified aluminum powder was 12.95 Ω·cm, with an increase of 400% compared to the aluminum powder. There was a uniform and dense SiO2 film on the surface of the SiO2-modified aluminum powder, in which the SiO2 was extremely resistive, so it would lead to a sharp increase in the resistivity. However, the average resistivity of the conductive aluminum powder was 2.3 Ω·cm, which was reduced by one order of magnitude compared with the SiO2-modified aluminum powder, while it decreased by 29% compared with the aluminum powder. This proved that the KH560-modified carbon black had been successfully grafted onto the surface of the aluminum powder, and the conductive aluminum powder constructed a conductive pathway, which was constructed by the interconnection of carbon black nanoparticles [56], so the carbon black nanoparticles significantly enhanced the electrical conductivity of the conductive aluminum powder, resulting in a significant decrease in the resistivity.

3.6. Hydrogen Evolution Behavior

To evaluate the stability and corrosion resistance of the modified aluminum powder, the behavior of hydrogen evolution of the modified aluminum powder in the water-borne coating system (pH 7–8, 50 °C) was measured, and the results are shown in Figure 11.
As depicted in the Figure 11, the aluminum powder in the aqueous system exhibited high reactivity, releasing a considerable amount of hydrogen within 3 h. With the extension of the reaction time, the amount of hydrogen evolution continued to increase rapidly until the reaction was completed with 58 h and the total amount of hydrogen evolution was 685.5 mL. The amount of hydrogen evolution of the SiO2-modified and conductive aluminum powder exhibited a significant reduction compared with the aluminum powder. The amount of hydrogen evolution within 58 h was below 4 mL, indicating that the hydrogen evolution inhibition effect was evident. For the SiO2-modified aluminum powder, the hydrogen evolution reaction began after 2 h and the amount of hydrogen evolution increased slowly with the increase in reaction time. The reaction was complete within 10 h, and the amount of hydrogen evolution was only 3.0 mL, which accounted for only 0.44% of the aluminum powder. The above data and analysis showed that the SiO2 film on the surface of the aluminum powder was an effective barrier against the corrosive medium to help prevent surface corrosion of the aluminum powder.
Under the same reaction conditions, conductive aluminum powders with carbon black contents of 1%, 3%, and 7% all experienced hydrogen evolution corrosion within 10 h. The amount of hydrogen evolution increased with the prolongation of reaction time, with a total hydrogen evolution of 1.3–1.6 mL after 100 h, which was lower than that of SiO2-modified aluminum powder. It was remarkable that conductive aluminum powder with a carbon black content of 5% exhibited a hydrogen evolution phenomenon at 46 h and was completed at 70 h. The total hydrogen evolution amount was only 0.5 mL, and its inhibition efficiency was 99.93%. This fully proved that the grafting of carbon black did not damage the inhibition effect of the hydrogen evolution of aluminum powder by the SiO2 layer, and even the silicon alcohol on the carbon black could react with the Si–OH on the surface of aluminum powder to further repair the gaps and defects on the surface, so the conductive aluminum powders presented a better inhibition effect on the hydrogen evolution phenomenon of aluminum powder.
To compare the changes in the micro-morphology of the aluminum powder before and after hydrogen evolution, the aluminum powder and conductive aluminum powder with 5 wt% C and with the best hydrogen evolution inhibition effect were subjected to SEM tests, and the results are shown in Figure 12.
Figure 12 showed that, for the aluminum powder after the hydrogen evolution test, there was an obvious columnar crystalline structure on the surface (as shown in the red square in Figure 12A). The surface became rough, large voids appeared, and even the edge contour became rough and fuzzy, which was due to the fact that Al had been oxidized to alumina after the hydrogen evolution corrosion. And the surface of the aluminum powder showed exfoliation and porosity due to the evolution of hydrogen. However, there was a dense SiO2 film on the surface of the conductive aluminum powder, and the corrosion medium had no way to penetrate the barrier to corrode aluminum powder. Therefore, the surface of the conductive aluminum powder still maintained its original structure.

3.7. Water Contact Angle Test

The wetting ability of the aluminum powder surface was a crucial parameter reflecting the compatibility between the filler and coating, which affected the solid content of water-borne aluminum coatings so as to affect the coating performance. To further assess the wettability of modified aluminum powder, the water contact angle (WCA) was measured, and the results are presented in Figure 13.
The WCA image of the aluminum powder is depicted in Figure 13A with 97° and hydrophobic characteristics. This is also an important reason why aluminum powder cannot be directly added to the water-borne coating system. Figure 13B,C showed that Al-Si and Al-Si-5wt% C exhibited contact angles of 54° and 40° with hydrophilic characteristics. On the one hand, the large production of Si–OH from hydrolysis of TEOS reacted with the surface of aluminum powder, which increased the hydrophilic groups on the surface of aluminum powder and improved its hydrophilicity. On the other hand, KH560 may have been hydrolyzed into silanol so that the quantity of hydroxyl groups on the surface of conductive aluminum powder was intensified. Moreover, the epoxy groups present in KH560 also promoted the diffusion rate of water on the surface of aluminum powder, thereby elevating the hydrophilicity of conductive aluminum powder. In turn, the enhancement of the surface wettability of conductive aluminum powder considerably boosted its compatibility with the water-borne coating system [57,58].

3.8. Storage Stability Test

In the hydrogen evolution experiment, it was observed that the hydrogen evolution reaction was prone to occur for the aluminum powder in the water-borne coating system, which would result in coating failure. The storage stability of the prepared water-borne aluminum coating from the conductive aluminum powder was measured, and the changes with time were observed. They are presented in Figure 14.
As shown in Figure 14A, there was a notable difference between the water-borne aluminum coating prepared by the aluminum powder and the conductive aluminum powder before storage. The water-borne aluminum coating prepared with the aluminum powder displayed evident delamination and divided into two layers: a milky resin layer and a silver-grey aluminum powder layer. This phenomenon was attributed to the poor dispersion of aluminum powder in the water-borne coating system. However, this phenomenon was not observed in the water-borne aluminum coating prepared with conductive aluminum powder.
Figure 14B revealed that there was no significant change for the water-borne aluminum coating prepared with conductive aluminum powder after six months of static storage, in which the aluminum powder still remained uniform in the water-borne coating, indicating that the conductive aluminum powder maintained high stability within the water-borne coating. In contrast, the water-borne aluminum coating prepared with the aluminum powder displayed obvious discoloration and swelling phenomena after six months of storage. This finding suggested that some of the aluminum powder had reacted with the corrosive medium to form white aluminum oxide, indicating poor compatibility between the aluminum powder and the water-borne system.
The above results demonstrate that carbon black was grafted onto the surface of aluminum powder without damaging the original SiO2 film layer, which significantly enhanced the corrosion resistance and storage stability of the conductive aluminum powder while also improving its dispersion and compatibility in water-borne coating systems. The results of the storage stability test were consistent with those of the hydrogen evolution behavior test.

3.9. Electrochemical Polarization Test

The previous experiment showed that the conductive aluminum powder with a carbon black content of 5 wt% showed a greater advantage for the preparation of water-borne heavy-duty aluminum coatings, which was based on carrying out the electrochemical test. The polarization curve and corrosion electrochemical parameters of the coating are shown in Figure 15 and Table 4, and the results were compared with those of the coating based on the aluminum powder and SiO2-modified aluminum powder.
Compared with the aluminum powder coating, the corrosion potential of the SiO2-modified aluminum powder coating increased by 0.10 V, and the corrosion current density decreased by one order of magnitude, from 2.12 × 10−6 A/cm2 to 3.47 × 10−7 A/cm2, and the polarization resistance increased from 1.22 × 104 Ω·cm2 to 7.51 × 104 Ω·cm2. For the conductive aluminum powder coating, the corrosion potential increased by 0.07 V, and the corrosion current density decreased to 4.08 × 10−8 A/cm2, with a decrease of two orders of magnitude, and the polarization resistance increased to 6.38 × 105 Ω·cm2.
It could be seen from above that the reduction in corrosion current density and the increase in polarization resistance indicated that the film layer successfully inhibited the penetration of corrosive ions. At the same time, it hindered the charge movement within the coating, thus slowing down the corrosion rate and better protecting the carbon steel substrate. There are few studies on aluminum powder in heavy-duty anticorrosive coating, so the fitting data of the potentiodynamic polarization curve was compared with that of the aluminum powder in a self-curing water-based zinc potassium silicate coating (Z-A) with a corrosion current density of 1.58 × 10−7 A/cm2 and a polarization resistance of 9.77 × 103 Ω·cm2 [59]. In Table 4, it can be seen that the corrosion current density of the prepared conductive aluminum powder coating was reduced by one order of magnitude, and the polarization resistance increased by nearly two orders of magnitude. All of this indicates that the prepared coating exhibited good protective properties [29,60,61].

3.10. Neutral Salt Spray Test

Figure 16 shows the results of the neutral salt spray test on water-borne aluminum coatings prepared with different aluminum powders. The test was evaluated at 0, 24, and 100 h in the 5 wt% NaCl solution. It is obvious from Figure 16A that the water-borne aluminum coating prepared with the aluminum powder showed obvious corrosion after 24 h, while Figure 16B,C show no obvious change after 24 h of the salt spray test. After 100 h, the corrosion phenomenon in Figure 16A was further aggravated, and the whole coating surface was covered with red rust. In Figure 16B, a large number of corrosion traces were also found, accompanied by a certain degree of foaming phenomenon, which indicated that the corrosive medium had penetrated into the coating and led to the corrosion of the substrate. Figure 16C does not show any corrosion during the whole experiment, and the coating still had a good protective effect on the substrate after 100 h.
The results indicated that the conductive pathway between carbon black and aluminum powder could enhance the conductivity of aluminum powder, thereby improving its sacrificial anode effect. In comparison to the aluminum powder and SiO2-modified aluminum powder, the water-borne aluminum coating prepared using conductive aluminum powder exhibited good corrosion resistance.

4. Conclusions

SiO2-modified aluminum powder (Al-Si) and conductive aluminum powder (Al-Si-C) were prepared by the sol–gel approach based on a silane coupling agent and hydroxylated carbon black. FT-IR, SEM, EDS, XPS, and XRD all proved that the aluminum powder was successfully modified with Si and C, and there was a dense SiO2 film and nano-conductive carbon black on the surface of the aluminum powder. Four-probe resistance measurements demonstrated that the resistivity of conductive aluminum powder was reduced by 82% compared to that of SiO2-modified aluminum powder, indicating a significant enhancement in electrical conductivity. Hydrogen evolution tests confirmed that the hydrogen evolution rate of the conductive aluminum powder decreased by over 99%, significantly improving the powder’s corrosion resistance. Water contact angle tests indicated that the compatibility of the conductive aluminum powder with water-borne coatings had been markedly enhanced.
Further investigations focused on the application of conductive aluminum powder in water-borne coatings. Storage stability tests revealed that water-borne coatings prepared with conductive aluminum powder maintained high stability at room temperature for over 180 days. Electrochemical assessments further confirmed the significant reduction in corrosion current density and increase in polarization resistance compared to SiO2-modified aluminum powder. Neutral salt spray tests proved that the coating prepared with conductive aluminum powder still had a good protective effect on the substrate after 100 h. Therefore, the prepared conductive aluminum powder possessed excellent electrical conductivity and corrosion resistance, effectively solving the hydrogen evolution stability of aluminum powders in water-based coating systems and broadening the applications of water-borne coatings in anti-corrosion fields.

Author Contributions

Conceptualization, Q.L. and N.W.; methodology, Q.L., X.L., Z.X. and W.G.; validation, J.L., T.J. and X.A.; formal analysis, T.J.; investigation, J.L., T.J. and X.A.; resources, Q.L. and N.W.; data curation, Q.L.; writing—original draft preparation, T.J.; writing—review and editing, X.L.; visualization, T.J.; supervision, Q.L. and L.Z.; project administration, Q.L. and N.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Fundamental Research Program for Applications from Liaoning Provincial Department of Science and Technology—Environmentally friendly bio-based polyurethane coatings, funder Na Wang, grant number 2023JH2/101300229; “Jie Bang Gua Shuai” of Science and technology Projects of Liaoning Province in 2021, funder Na Wang, grant number 2021JH1/10400091; Sino-Spain Joint Laboratory on Material Science, funder Na Wang, grant number 2022JH2/10700005; The 2021 Scientific Research Foundation of Education Department of Liaoning Province, funder Xiaofeng Liu, grant number LJK Z0463.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Author Liang Zhang was employed by the company Shenyang Hangda Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Modification mechanism diagram of aluminum powder.
Figure 1. Modification mechanism diagram of aluminum powder.
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Figure 2. Mechanism diagram of KH560 grafting to aluminum powder surface.
Figure 2. Mechanism diagram of KH560 grafting to aluminum powder surface.
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Figure 3. Anti-corrosion mechanism of the prepared coatings.
Figure 3. Anti-corrosion mechanism of the prepared coatings.
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Figure 4. FT-IR spectra of KH560-modified carbon black and non-modified carbon black.
Figure 4. FT-IR spectra of KH560-modified carbon black and non-modified carbon black.
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Figure 5. FT-IR spectra of modified aluminum powder: Al: aluminum powder; Al–Si: SiO2-modified aluminum powder; Al–Si–C: conductive aluminum powder.
Figure 5. FT-IR spectra of modified aluminum powder: Al: aluminum powder; Al–Si: SiO2-modified aluminum powder; Al–Si–C: conductive aluminum powder.
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Figure 6. SEM images of various aluminum powders (×1000): (A) Al, (B) Al-Si, (C) Al-Si-1wt% C, (D) Al-Si-3wt% C, (E) Al-Si-5wt% C, and (F) Al-Si-7wt% C.
Figure 6. SEM images of various aluminum powders (×1000): (A) Al, (B) Al-Si, (C) Al-Si-1wt% C, (D) Al-Si-3wt% C, (E) Al-Si-5wt% C, and (F) Al-Si-7wt% C.
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Figure 7. XPS spectra of aluminum powder.
Figure 7. XPS spectra of aluminum powder.
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Figure 8. Si peak fitting diagram of SiO2-modified aluminum powder.
Figure 8. Si peak fitting diagram of SiO2-modified aluminum powder.
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Figure 9. Si peak fitting diagram of conductive aluminum powder.
Figure 9. Si peak fitting diagram of conductive aluminum powder.
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Figure 10. XRD patterns of aluminum powder.
Figure 10. XRD patterns of aluminum powder.
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Figure 11. Hydrogen evolution experiment of different aluminum powders.
Figure 11. Hydrogen evolution experiment of different aluminum powders.
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Figure 12. SEM images of aluminum powder after undergoing the hydrogen evolution test: (A)Al; (B) Al-Si-5wt% C.
Figure 12. SEM images of aluminum powder after undergoing the hydrogen evolution test: (A)Al; (B) Al-Si-5wt% C.
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Figure 13. Water contact angle of different aluminum powders (A) Al, (B) Al-Si, (C) Al-Si-5wt% C.
Figure 13. Water contact angle of different aluminum powders (A) Al, (B) Al-Si, (C) Al-Si-5wt% C.
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Figure 14. Storage stability test: (A) standing for 0 h, (B) standing for 6 months.
Figure 14. Storage stability test: (A) standing for 0 h, (B) standing for 6 months.
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Figure 15. Potentiodynamic polarization curves of different aluminum powder coatings in 3.5 wt% NaCl solution.
Figure 15. Potentiodynamic polarization curves of different aluminum powder coatings in 3.5 wt% NaCl solution.
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Figure 16. Neutral salt spray test at different times: (A) Al; (B) Al-Si; (C) Al-Si-5wt% C.
Figure 16. Neutral salt spray test at different times: (A) Al; (B) Al-Si; (C) Al-Si-5wt% C.
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Table 1. The atomic fractions of surface elements in various aluminum powders.
Table 1. The atomic fractions of surface elements in various aluminum powders.
SamplesAlSiCO
Al90.870.068.130.95
Al-Si88.880.987.822.32
Al-Si-1wt% C84.040.9612.122.89
Al-Si-3wt% C79.241.0416.663.06
Al-Si-5wt% C77.480.7719.312.43
Al-Si-7wt% C73.490.4823.582.45
Table 2. Element composition and atomic percentage of aluminum powder surface from XPS.
Table 2. Element composition and atomic percentage of aluminum powder surface from XPS.
SamplesAlSiCON
Al20.230.9735.4342.670.90
Al-Si0.9914.0743.1640.421.36
Al-Si-C0.769.5555.9832.351.36
Table 3. Four-probe test results of different aluminum powders.
Table 3. Four-probe test results of different aluminum powders.
SamplesV23+
(mV)		V23−
(mV)		V24+
(mV)		V24−
(mV)		Resistivity (Ω·cm)Conductivity
(S·cm)		Average Resistivity
Al3.066.332.215.834.10.24393.24
3.196.566.032.214.40.2273
2.856.125.721.924.00.2500
3.777.146.662.974.40.2273
Al–Si9.8613.296.9610.6612.20.082012.95
15.7912.6213.509.7313.50.0741
15.4212.2512.528.8314.40.0694
15.8612.6714.3910.6711.70.0855
Al–Si–C3.790.490.962.741.80.55562.30
3.890.453.760.061.80.5556
4.671.274.680.712.20.4545
5.041.804.520.903.40.2941
Table 4. Fitting data of potentiodynamic polarization curves of different coatings.
Table 4. Fitting data of potentiodynamic polarization curves of different coatings.
Samplesφcorr (V)jcorr (A/cm2)Rp (Ω·cm2)
Al coating−0.53 ± 0.03(2.12 ± 0.09) × 10−6(1.22 ± 0.08) × 104
Al-Si coating−0.43 ± 0.02(3.47 ± 0.13) × 10−7(7.51 ± 0.21) × 104
Al-Si-C coating−0.46 ± 0.02(4.08 ± 0.15) × 10−8(6.38 ± 0.17) × 105
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Li, Q.; Liu, J.; Jiang, T.; An, X.; Wang, N.; Xu, Z.; Guo, W.; Zhang, L.; Liu, X. Preparation and Properties of Conductive Aluminum Powder (Al@Si@C) for Water-Borne Heavy-Duty Anticorrosive Coatings. Coatings 2024, 14, 1082. https://doi.org/10.3390/coatings14091082

AMA Style

Li Q, Liu J, Jiang T, An X, Wang N, Xu Z, Guo W, Zhang L, Liu X. Preparation and Properties of Conductive Aluminum Powder (Al@Si@C) for Water-Borne Heavy-Duty Anticorrosive Coatings. Coatings. 2024; 14(9):1082. https://doi.org/10.3390/coatings14091082

Chicago/Turabian Style

Li, Qingpeng, Jiaxing Liu, Tiancheng Jiang, Xiaoyun An, Na Wang, Zhixiu Xu, Wanyuan Guo, Liang Zhang, and Xiaofeng Liu. 2024. "Preparation and Properties of Conductive Aluminum Powder (Al@Si@C) for Water-Borne Heavy-Duty Anticorrosive Coatings" Coatings 14, no. 9: 1082. https://doi.org/10.3390/coatings14091082

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