A bifunctional calcium aluminate modified silica sol rebar coating and a method for preparing the same

Abstract

The present application relates to a kind of bifunctional calcium aluminate modified silica sol steel bar coating and its preparation method, by introducing calcium aluminate filler in the preparation of silica sol coating, calcium aluminate can be hydrated to form layered double hydroxide (LDH) in coating preparation, and the anion exchange work of LDH after hydration can absorb the erosive chloride ion in service environment medium.Simultaneously, the lamellar structure of LDH itself helps to relieve the internal stress generated in the curing process of coating, eliminates the cracking of coating, and can effectively improve the barrier property of coating, slow down the speed of erosive ions such as chloride ion and dissolved oxygen reaching the surface of steel bar, and then improve the protection performance of silica sol coating to steel bar corrosion.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of coating preparation, and relates to a dual-functional calcium aluminate modified silica sol steel bar coating and a preparation method thereof. BACKGROUND

[0002] Reinforced concrete is a kind of building structure material with extremely wide application, and the corrosion of steel bars will cause the destruction of concrete structures, and then shorten the service life of reinforced concrete. Therefore, the corrosion of steel bars is the primary cause of the decrease in the durability of reinforced concrete structures. When serving in a chloride-containing environment, the corrosion of steel bars will occur when the concentration ratio of [Cl - ] / [OH - ] is higher than the critical value, causing the destruction of reinforced concrete structures. Therefore, in a high-chloride environment, the corrosion of steel bars caused by chloride ion erosion is extremely common. How to reduce the concentration of chloride ions on the surface of steel bars and then reduce the influence of the corrosion of steel bars has become the key to the durability of concrete in a high-chloride environment.

[0003] For example, Chinese Patent CN103819167A discloses a self-curing cathodic protection coating for reinforced concrete and a preparation and use method thereof. The cathodic protection coating is formed by mixing a base material and a powder material in a certain proportion, the base material mainly consists of inorganic silicate, silica sol, silane coupling agent and dispersant, and the powder material mainly consists of zinc powder, magnesium powder and a small amount of superconducting metal. When used, the powder material is uniformly mixed with the base material, and then coated on the surface of the concrete, and then connected with the steel bars through a wire, so as to effectively protect the steel bars from corrosion. The patent needs to introduce superconducting metal and the like, and the implementation process is relatively complex and the use is limited. SUMMARY

[0004] The purpose of the present application is to provide a dual-functional calcium aluminate modified silica sol steel bar coating and a preparation method thereof, so as to obtain a silica sol coating with excellent protection performance for steel bars.

[0005] In the present application, water glass is used as a precursor, and calcium aluminate fillers are introduced in the coating preparation. The calcium aluminate can be hydrated to form layered double hydroxide (LDH) in the preparation of the coating, and the hydrated LDH has anion exchange function to adsorb the corrosive chloride ions in the service environment medium. At the same time, the sheet-like structure of the LDH itself helps to relieve the internal stress generated in the curing process of the coating, eliminate the cracking of the coating, effectively improve the barrier performance of the coating, slow down the speed of corrosive ions such as chloride ions and dissolved oxygen reaching the surface of the steel bars, and then improve the protection performance of the silica sol coating for the corrosion of the steel bars.

[0006] The purpose of the present application can be achieved by the following technical solutions:

[0007] One of the technical solutions of the present application provides a preparation method of a dual-functional calcium aluminate modified silica sol steel bar coating, comprising the following steps:

[0008] (1) weighing calcium aluminate powder and dispersing it in water to obtain a calcium aluminate dispersion liquid;

[0009] (2) weighing nano zinc oxide powder and dispersing it in water to obtain a zinc oxide dispersion liquid;

[0010] (3) sequentially adding the zinc oxide dispersion liquid in step (2) and the calcium aluminate dispersion liquid in step (1) to water glass, stirring and ultrasonic dispersing to obtain a calcium aluminate modified inorganic coating;

[0011] (4) brushing the calcium aluminate modified inorganic coating in step (3) on a phosphating plate and drying to obtain the target product.

[0012] Further, the addition amounts of the zinc oxide dispersion liquid, the calcium aluminate dispersion liquid and the water glass satisfy that the ratio of the addition amounts of zinc oxide, calcium aluminate and water glass is 0.5 g:(0.2-0.8) g:10 mL.

[0013] Further, in the water glass, the content of silicon dioxide is greater than 25%, and the modulus is greater than 3.0.

[0014] Further, in step (3), the stirring and ultrasonic dispersing process is mechanical stirring for 3 h, followed by ultrasonic dispersing for 30 min.

[0015] Further, in step (4), the drying process is room temperature drying for 4 h, followed by vacuum drying at 120℃ for 2 h.

[0016] Further, in step (4), the phosphating plate is prepared by the following method: after polishing and cleaning a metal substrate, the substrate is put into a phosphating solution for phosphating treatment, and the preparation is completed.

[0017] Further, the components of the phosphating solution are: Zn(NO3)2 56 g / L, Mn(NO3)2 20 g / L, Mn(H2PO4)2·2H2O 24 g / L, C6H8O7·H2O 2 g / L.

[0018] Further, in step (1), the usage ratio of calcium aluminate powder to water is (0.2-0.8) g:3.5 mL.

[0019] Further, in step (2), the usage ratio of nano zinc oxide powder to water is 0.5 g:3.5 mL.

[0020] The second technical solution of the present application provides a dual-functional calcium aluminate modified silica sol steel bar coating prepared by the preparation method described in any one of the above.

[0021] Calcium aluminate (C3A) can form Ca-based LDH in the hydration process, and the interlayer OH- It is easy to exchange with other anions, and has the function of adsorbing and fixing anions. The calcium aluminate is composed of aluminum oxygen tetrahedron and calcium oxygen octahedron, and the particle spacing d is 0.76nm, and the Ca 2+ is connected by coordination, and the aluminum ion has two coordination modes, i.e. four-coordination and six-coordination, so that the aluminum oxygen tetrahedron is deformed, and the activity of the aluminum ion is relatively large, and the Ca 2+ ion has weak connection, so that the activity of the calcium aluminate is relatively high, and the OH - ion is easy to enter the crystal, so that the hydration speed of the calcium aluminate is relatively high. In the hydration process of C3A, C4AH 19 , C4AH 13 or C2AH8 compound (as shown in equation 1-1) can be formed, which is composed of positively charged calcium aluminum main layer plate and OH - anion and water molecules existing in the interlayer space. The Ca-Al-OH-LDHs main layer plate Ca, Al is combined in 2:1, and the Ca 2+ ion radius (0.10nm) is higher than that of Mg 2+ (0.072nm), so that the surface effect is higher than that of Mg-Al-LDH, and it is more conducive to improving the exchange rate of the modifier and increasing the exchange capacity. The interlayer OH - can be exchanged with CO3 2- , C1 - and other anions. The calcium-based LDHS has the characteristics of LDHS, and also has solubility and high pH in liquid phase. Therefore, C3A can be used as an inorganic anion fixing adsorbent.

[0022] 2(3CaO·Al2O3)+27H2O=4CaO·Al2O3·19H2O+2CaO·(Al2O3·8H2O (1-1)

[0023] The silica sol coating has a composition similar to that of concrete, and the application of the silica sol coating to the steel bar can improve the bonding strength between the steel bar and the concrete. Meanwhile, by introducing calcium aluminate into the silica sol coating, the hydration of the calcium aluminate forms a layered structure to increase the compactness, and the formed calcium-based LDHs can adsorb chloride ions, so that the corrosion of the steel bar by the erosive ions is effectively inhibited. The silica sol coating contains a sheet-shaped LDHs filler, which can effectively improve the barrier property of the coating, and further improve the protection performance of the silica sol coating on the steel bar corrosion.

[0024] Compared with the prior art, the present application has the following advantages:

[0025] (1) In the preparation of silica sol coating, calcium aluminate is introduced as a functional filler. During the preparation of the coating, calcium aluminate is hydrated to form a layered double hydroxide (LDH) in the form of a sheet. This two-dimensional sheet-shaped filler has a bonding relationship with the silica sol, which can inhibit the cracking of the coating caused by shrinkage during drying;

[0026] (2) The sheet-shaped LDH as a filler of the coating can improve the barrier performance of the coating, prolong the transmission route of dissolved oxygen to the substrate in service, and play a stronger anticorrosion role;

[0027] (3) In the service of the coating, the anion exchange function of LDH can be used to absorb the corrosive chloride ions reaching the substrate surface from the service environment, avoid the promoting effect of corrosive chloride ions on the corrosion of the substrate, and thus improve the corrosion protection performance of the silica sol coating on the steel bar. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The change of the exchange capacity of C3A for chloride ions with time;

[0029] Figure 2 The SEM morphology of C3A before and after ion exchange and the EDS analysis of C3A after ion exchange;

[0030] Figure 3 The infrared spectra of C3A before and after ion exchange;

[0031] Figure 4 The XRD of C3A before and after ion exchange;

[0032] Figure 5 The XRD of the coating with and without the addition of C3A;

[0033] Figure 6 The SEM images of the coating before and after the addition of C3A;

[0034] Figure 7 The polarization curves of the coating samples with different addition amounts;

[0035] Figure 8 The electrochemical impedance spectroscopy bode plots of different addition amounts;

[0036] Figure 9 The equivalent circuit diagram for analyzing electrochemical impedance spectroscopy;

[0037] Figure 10 The coating R f and R ct change with time. DETAILED DESCRIPTION

[0038] The application will be described in detail below with reference to the drawings and specific embodiments. The embodiments are implemented on the premise of the technical solutions of the application, and detailed implementation modes and specific operation processes are given, but the protection scope of the application is not limited to the following examples.

[0039] In the following examples, the nano-zinc oxide powder was purchased from the McKin Company, with a purity of 99.9% and a particle size of 30±10 nm.

[0040] The remaining raw materials or processing technologies, if not specifically stated, are all conventional commercially available raw materials or conventional processing technologies in the art.

[0041] 2.1 Characterization and analysis of calcium aluminate and coating

[0042] 2.2.1 Characterization and analysis of calcium aluminate

[0043] (1) Composition and structure characterization

[0044] The morphology of calcium aluminate (99%, 5000 mesh, Haorui Chemical (Shanghai) Co., Ltd.) was characterized by scanning electron microscopy (FESEM, JSM-6510, Japan), the composition and structure of the sample were characterized by X-ray diffractometer (Burker, D8 Advance, Germany), and the scanning speed was 2.4 (2θ) min-1 under monochromatic CuKα radiation at 15 kV and 20 mA, and the scanning range was 10° to 40°. The composition of the calcium aluminate sample was analyzed by FT-IR analyzer (PerkinElmer, America), and the scanning range was 4000-400 cm -1 .

[0045] (2) Ion exchange capacity determination

[0046] The calcium aluminate 0.2 g was ion exchanged with 100 mL of 0.01 mol / L sodium chloride solution and saturated Ca(OH)2 solution, sealed and stirred, and the sample was taken out at a set time, the supernatant after centrifugation was used to determine the chloride ion concentration in the liquid by ion chromatography, and the exchange capacity of C3A per unit mass of chloride ion was calculated according to the difference in chloride ion concentration before and after the experiment, and the calculation formula is as follows:

[0047]

[0048] In the formula: q e is the ion exchange capacity, C0 is the initial concentration of sodium ions in the exchange solution, C E is the concentration of sodium ions in the exchange solution after equilibrium, m is the mass of titanium phosphate used for exchange, and V is the volume of the solution used for exchange.

[0049] Preparation of phosphating layer on steel bar matrix

[0050] The adhesion between coating and metal substrate is an important factor affecting the anticorrosion efficiency of the coating. One of the main problems in coating sol-gel coating on the surface of steel bar is that the coating formed on the substrate lacks adhesion. The adhesion of the coating can be improved by preparing a phosphating layer on the surface of carbon steel.

[0051] A 5x7cm Q235 steel plate was polished with 400#, 600#, 800#, 1000# and 1500# in sequence, washed with petroleum ether and acetone to remove oil, ultrasonicated in alcohol for 10min and deionized water for 5min, pickled for 5min, and the residual acid on the surface was washed away with deionized water, then the steel plate was placed in a phosphating solution (Table 1) at 70℃ for 15min, and after phosphating, the surface was rinsed with deionized water and dried to prepare a phosphating layer.

[0052] Table 1 Composition of phosphating solution

[0053] [Zn(NO3)2] Mn(NO3)2 Mn(H2PO4)2.2H2O [C6H8O7.H2O] 56 g / L 20 g / L 24 g / L 2 g / L

[0054] Coating characterization process:

[0055] The morphology and elemental composition of the coating were analyzed by scanning electron microscopy (FESEM, JSM-6510, Japan) and energy dispersive spectroscopy (EDS), the composition and structure of the coating were characterized by X-ray diffractometer (Burker, D8 Advance, Germany), and the composition of the coating was analyzed by FTT-IR analyzer (PerkinElmer, America).

[0056] The electrochemical workstation (Interface 1000, Gamry Electrochemical Instrument Company, USA) was used for electrochemical test of the coating, and the solution used was saturated calcium hydroxide solution containing 3.5wt.% NaCl (pH = 12.6). A three-electrode system was used, with platinum mesh as the counter electrode, saturated calomel electrode as the reference electrode, and the coating sample as the working electrode. The test was carried out in a convenient detection device for testing the performance of anticorrosion coating, and the exposed area was 7cm 2 . Before testing, the sample was immersed in the solution for 30min to obtain a stable open circuit potential, and then electrochemical impedance spectroscopy (EIS) and polarization curve test were carried out. The range of potentiodynamic polarization scanning was -100mV~+100mV (vs. OCP), and the scanning rate was 0.1667mV / s; the frequency range of EIS measurement was 10mHz~100kHz, and the sinusoidal disturbance amplitude was 10mV. The results of potentiodynamic polarization test were fitted by least squares method using Cview software, and the electrochemical impedance spectroscopy data were analyzed by equivalent circuit using Zsimpwin software.

[0057] Example 1:

[0058] (1) Dispersion of fillers: 0.5g of calcium aluminate powder was dispersed in 3.5ml of water and ultrasonically dispersed for 30min, and 0.5g of nano zinc oxide powder was dispersed in 3.5ml of water and ultrasonically dispersed for 30min. Adding an appropriate amount of zinc oxide can fill the gaps in the coating, increase the toughness and density of the coating, and also help reduce the precipitation of sodium ions and increase the water resistance of the coating.

[0059] (2) Preparation of coating: Take 10 ml of water glass (silicon dioxide content 27.5%, sodium oxide content 8.78%, modulus 3.23, Foshan Zhongfa Water Glass Factory), add each well dispersed filler dropwise to the water glass in the order of (zinc oxide, calcium aluminate) under magnetic stirring, stir mechanically for 3 h, and ultrasonically disperse for 30 min to prepare calcium aluminate modified inorganic coating.

[0060] (3) Coating preparation: Take 0.7 ml of coating and brush it evenly onto a 5×7 cm phosphate plate. Hydrate at room temperature for 24 h and dry in a vacuum drying oven at 120℃ for 2 h. The resulting coating thickness is about 20 μm.

[0061] In addition, based on the above implementation process conditions, only the amount of C3A filler was adjusted. The coating without C3A filler was designed as pure, and the coatings with different amounts of C3A filler were designated as 0.2g, 0.5g, and 0.8g, respectively.

[0062] Figure 1 The curve showing the change in the exchange capacity of C3A for chloride ions over time is given by... Figure 1 It can be seen that C3A can rapidly hydrate and exchange chloride ions under alkaline conditions within a short time. The reaction quickly reaches equilibrium within one hour, and the adsorption capacity tends to stabilize after one hour. The maximum adsorption capacity is 160 mg / g.

[0063] Figure 2 The image shows the SEM morphology of C3A before and after ion exchange, and the EDS analysis of C3A after exchange. Figure 2 As shown in Figure a, the C3A particles in the calcium aluminate powder that have not undergone ion exchange are irregularly shaped, with a particle size between 1 and 3 μm. After ion exchange, the irregular, blocky C3A gradually peels off to form sheet-like LDH, such as... Figure 2 As shown in b and 2c, this exfoliated lamellar structure can be dispersed within the coating. Based on the EDS results of the exchanged samples, the presence of Cl in the exchanged samples indicates that C3A hydrates to form lamellar structures, and chloride ions are adsorbed onto Ca2(Al(OH)6)2. + The interlayer structure proves that C3A has transformed into a Ca2Al(OH)6Cl(H2O)2 compound with a layered structure.

[0064] Figure 3The FTIR spectra of C3A before and after ion exchange were compared. It was found that there were obvious differences between the FTIR spectra of C3A before and after ion exchange. In the FTIR spectrum of the product after ion exchange, there was an absorption peak at 3500 cm -1 There was a relatively obvious absorption peak, which was mainly related to the stretching vibration of hydrogen bond in interlayer hydroxyl; the absorption peak at 530 cm -1 was the vibration absorption peak of Al-O bond in the layer. In the FTIR spectrum of C3A before ion exchange, only the vibration absorption peak of Al-O bond appeared, and the vibration peak of O-H bond did not appear. The absorption peak of Al-OH bond near 800 cm -1 was obviously enhanced after ion exchange, and the content of Al-OH bond was increased, which further proved the LDH structure of C3A after ion exchange.

[0065] Figure 4 The XRD patterns of C3A before and after ion exchange were compared. In the standard PDF card 78-1532, there were characteristic peaks of Ca3(Al(OH)6)2 at 31.8°, 34.8°, 36.4°, 44.6° and 47.3°. After ion exchange, the sample had characteristic peaks of Ca2Al(OH)6Cl(H2O)2layered structure at 11.2° and 31.14° (Ca-Al-Cl-LDH, pdf: 78-1219). There were diffraction peaks near 31.8°, 36.4° and 44.6° in the sample after ion exchange, which were the unexchanged calcium aluminate hydrate and the product after ion exchange, which was Ca-Al-Cl-LDH. The layered structure was formed in the calcium aluminate hydrate product, and the positively charged middle layer structure unit [Ca2Al(OH)6·2H2O]+was balanced due to the charge change caused by the replacement of Ca 3+ by Al 2+ Most of the free chloride ions were directly adsorbed into the layered structure unit [Ca2Al(OH)6·2H2O] + from the NaCl solution.

[0066] Figure 5XRD patterns of the coating before and after modification of C3A. From the figure, it can be seen that the main components of the coating added with 0.5 g of C3A are amorphous silicon dioxide, zinc oxide, a small amount of silicon dioxide crystals and calcium-based LDH compounds, and the characteristic peaks of amorphous silicon dioxide are at 15-30°, two characteristic peaks of zinc oxide appear at about 2θ = 35°, and the characteristic peaks of Ca-LDH are at about 2θ = 30° and 52°. Taking the peak of zinc oxide at 36.2° as a reference, the intensity of the diffraction peak of sodium compounds in the sample introduced by C3A is reduced, and the peak ratio at 44.6° to 36.2° of the unadded coating is 1.43, while the peak ratio of the added coating is 0.66. Compared with the unadded calcium aluminate, the content of free sodium ions in the coating added with C3A is reduced. C3A in the coating can form calcium-based LDH, which not only plays a role in fixing chloride ions, but also increases the density of the coating and reduces the precipitation of sodium ions.

[0067] Figure 6 SEM images of the cross-section of the coating before and after adding 0.5 g of C3A. From the figure, it can be seen that a large number of sheet-like substances with a size of about 1 μm and a thickness of about 100 nm are distributed in the cross-section of the coating added with C3A, and they are orderly distributed in the coating, while the unadded sample has only a small amount of zinc oxide particles on the surface which are not uniformly dispersed. Combined with the XRD results, these sheet-like substances in the coating are calcium-based LDH formed by the hydration of C3A. They can act as two-dimensional materials to increase the density of the coating, and can also relieve the internal stress generated by the solidification shrinkage of the coating.

[0068] Figure 7 Potentiodynamic polarization curves of the coatings with different amounts of addition in a 3.5% sodium chloride saturated calcium hydroxide solution, and the corresponding electrochemical data analysis results are shown in Table 2. It can be seen that the corrosion current density of the coating added with C3A is significantly lower than that of the unadded coating. With the increase of the addition amount, the corrosion current density of the coating decreases and the corrosion potential shifts positively. Among them, the corrosion current density corresponding to the polarization curve of the sample added with 0.5 g of C3A is the smallest, and the corrosion potential is the highest, but when the addition amount increases to 0.8 g, the corrosion current density increases slightly and the corrosion potential no longer shifts positively.

[0069] Table 2 Polarization curve fitting data of coatings with different amounts of addition

[0070] Coating addition amount / g Blank 0.2 0.5 0.8 E corr ,V vs SCE]]> -0.667 -0.550 -0.375 -0.665 [I0,A / cm 2 ]]> 2.27 x 10 -6 ]] 1.36 x 10 -8 ]]> 1.29 x 10 -9 ]]> 1.58 x 10 -8 ]]

[0071] Figure 8The (a2) (b2) (c2) (d2) figure in (a2) (b2) (c2) (d2) represents the phase angle diagram of different C3A addition amount samples, it can be observed that the phase angle value of the coating sample at high frequency is close to 80°, the high frequency response of the phase angle is related to the barrier property of the coating, the closer the high frequency phase angle is to 90°, the closer the coating is to pure capacitance, the larger the phase angle value, the better the barrier property of the coating, the low frequency response is related to the corrosion process occurring at the interface between the coating and the substrate. When the addition amount is 0.8g, the response of the coating at medium and low frequency indicates that the electrolyte enters the inside of the coating and contacts the metal interface to produce a charge transfer process, resulting in a low frequency response, and there is also a response corresponding to the diffusion process produced at 0.01Hz to 0.1Hz. With the increase of immersion time, the response produced by charge transfer gradually shifts from low frequency to high frequency. The 0.2g and 0.5g samples do not produce low frequency response at the beginning of immersion, which represents that the coating has good barrier property, with the increase of immersion time, the 0.2g sample begins to appear obvious low frequency response at the 7th day, with the further increase of immersion time, the high frequency response of the 0.2g sample coating almost disappears at the 13th day, the barrier property of the coating rapidly decreases, and the change of the modulus value also indicates this. For the sample with an addition amount of 0.5g, low frequency phase angle appears at the 9th day, but there is still a response at high frequency at this time. It indicates that the coating is relatively complete and has certain barrier property. Until the immersion time reaches 15 days, the high frequency phase angle of the coating disappears. In summary, when the addition amount is 0.5g, the barrier property and corrosion resistance of the coating are the best. From the measurement results of EIS, the C3A modified coating effectively improves the corrosion resistance of the coating and significantly improves the protection performance of the coating on the metal substrate, which plays a role in isolating the erosive ions. By comparing the corrosion resistance of the samples through the impedance modulus value |Z| 0.01Hz , the impedance modulus value of the coating with an addition amount of 0.5g is the largest, which is 2 orders of magnitude higher than that of the sample with an addition amount of 0.8g, which indicates that when the addition amount is certain, the addition of calcium aluminate can improve the corrosion resistance of the coating.

[0072] Figure 9 The corresponding equivalent circuit is proposed, which is commonly used for the impedance spectrum of silicate coating, and the fitting results are shown in Table 3. In the equivalent circuit, R f and R ct are the film resistance and charge transfer resistance of the coating respectively, and W represents the warburg impedance. Due to the non-ideality of the capacitor, a constant phase element CPE is used instead, in which CPE1 and CPE2 represent the capacitance of the coating and the capacitance of the substrate / coating interface respectively, and the warburg impedance reflects the diffusion process at the interface. The equivalent circuit diagrams of 0, 0.8g coating are a, the equivalent circuit of 0.2g, 0.5g coating in the early stage of immersion is b, and the diffusion equivalent circuit occurs with the increase of immersion time. According to the fitting data, the changes of film resistance and charge transfer resistance with time are obtained Figure 10 , Figure 10(a) The membrane resistance of 0.2 g and 0.5 g samples on the first day of immersion is greater than 10 8 Ω·cm 2 An order of magnitude higher than that of the 0.8 g sample, and 4 orders of magnitude higher than that of the sample without the coating, indicating that the addition of C3A significantly improves the barrier performance of the coating. With the increase of the immersion time, the membrane resistance of the coating decreases. After one week of immersion, the membrane resistance of the 0.5 g sample remains at 10 7 Ω·cm 2 , and the coating has good barrier performance. The membrane resistance of the 0.2 g sample is close to 10 7 Ω·cm 2 on the 5th day of immersion, and less than 10 7 Ω·cm 2 on the 7th day, corresponding to the appearance of low-frequency response of the 0.5 g sample in the phase angle diagram on the 7th day. According to Figures 4-10 (b) The charge transfer resistance of the samples after addition is 3 orders of magnitude higher than that of the blank sample, and the charge transfer resistance of the coating with C3A after 7 days of immersion is higher than 10 7 Ω·cm 2 . From the change of Rct of several samples over time, it can also be found that with the increase of the addition amount of C3A, the decrease of Rct over time is smaller, which may be because the addition of C3A can effectively exchange the chloride ions entering the coating, and at the same time, increase the pH value of the substrate surface, which can reduce the [Cl-] / [OH-] ratio, and delay the effect of chloride ions on the corrosion of the substrate.

[0073] Table 3 Fitting values of coatings with different addition amounts

[0074]

[0075]

[0076] In summary, by analyzing C3A before and after ion exchange, introducing C3A into the silica-based inorganic sol-gel coating with water glass as the precursor, observing the cross-sectional morphology of calcium aluminate and the coating by scanning electron microscopy, performing phase analysis of C3A and the coating composition by XRD and infrared, and studying the performance of the coating modified by different addition amounts of C3A, the corrosion resistance of the coating is evaluated by electrochemical measurement. The following conclusions are obtained:

[0077] (1) C3A can form a calcium-based LDH in a saturated calcium hydroxide and 3.5% sodium chloride solution, which can exchange chloride ions to generate Ca2Al(OH)6Cl(H2O)2.

[0078] (2) C3A can be hydrated to form a lamellar structure calcium-based LDH in the preparation of the coating, and this lamellar structure can effectively fix the free chlorine ions in the environment containing chlorine ions. When the concrete is in service in the environment containing chlorine ions, the C3A hydration product in the coating can adsorb the chlorine ions reaching the surface of the steel bar, delaying the promotion of the corrosion of the steel bar by the chlorine ions. At the same time, the introduction of C3A significantly improves the barrier property of the coating.

[0079] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. Those skilled in the art can obviously make various modifications to the embodiments and apply the general principles described herein to other embodiments without having to go through creative labor. Therefore, the invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art according to the disclosure of the invention without departing from the scope of the invention should be within the protection scope of the invention.

Claims

1. A process for the preparation of a bifunctional calcium aluminate modified silica sol rebar coating characterized in that, It comprises the following steps: (1) take the calcium aluminate powder and disperse it in water to obtain a calcium aluminate dispersion liquid; (2) take the nano zinc oxide powder and disperse it in water to obtain a zinc oxide dispersion liquid; (3) add the zinc oxide dispersion liquid in step (2) and the calcium aluminate dispersion liquid in step (1) to water glass in turn, stir and ultrasonic disperse to obtain a calcium aluminate modified inorganic coating; (4) brush the calcium aluminate modified inorganic coating in step (3) on the phosphating plate and dry to obtain the target product of bifunctional calcium aluminate modified silica sol steel bar coating; The addition amount of the zinc oxide dispersion liquid, the calcium aluminate dispersion liquid and the water glass satisfies that the ratio of the addition amount of zinc oxide, calcium aluminate and water glass is 0.5g:(0.2-0.8)g:10mL; In the water glass, the content of silicon dioxide is greater than 25%, and the modulus is greater than 3.0; In step (4), the phosphating plate is prepared by the following method: after polishing and cleaning the metal substrate, it is put into the phosphating liquid for phosphating treatment, and the process is completed; The component composition of the phosphating liquid is: Zn(NO3)2 56g / L, Mn(NO3)2 20g / L, Mn(H2PO4)2·2H2O 24g / L, C6H8O7·H2O 2g / L; In step (1), the dosage ratio of calcium aluminate powder to water is (0.2-0.8)g:3.5mL; In step (2), the dosage ratio of nano zinc oxide powder to water is 0.5g:3.5mL.

2. A process for the preparation of a dual functional calcium aluminate modified silica sol rebar coating according to claim 1, characterized in that, In step (3), the stirring and ultrasonic dispersion process is: mechanical stirring for 3h, and then ultrasonic dispersion for 30min.

3. The method of claim 1, wherein the method is characterized by: In step (4), the drying process is: room temperature drying for 4h, and then vacuum drying at 120℃ for 2h.

4. A bifunctional calcium aluminate modified silica sol steel bar coating prepared by the preparation method of any one of claims 1-3.

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