The development of water-based chromium-free zinc aluminum anti-corrosion coating

Chen Gang, Tao Qi, Liu Xuemei
(Wuhan Engineering University, Key Laboratory of the Ministry of Green Chemical Process, Wuhan 430073)
Abstract: Replace the traditional chromium hydrate adhesive with high-module silicate, achieve chromium-free bonding, and use silicone coupled agent to coat the surface of the pigment aluminum powder, so as to modifie aluminum powder to prepare a water-based zinc aluminum coating, while adding a certain concentration of sodium phosphate corrosion inhibitor. The effects of the type and dosage of silane coupled agent on the dispersion stability of coated aluminum powder in the water-based system were studied, and the effect of corrosion inhibitor concentration on the corrosion resistance of the coating was also studied. The performance of pigment aluminum powder and coating film is indicated by SEM, EDS, EIS and other analytical methods
the
. The results showed that the dispersion stability of pigment aluminum powder after 5% WD-60 coating was good, and the corrosion resistance of zinc aluminum coating prepared by adding 1% sodium phosphate corrosion inhibitor was significantly improved.
keywords: silane coupled agent; corrosion inhibitor; zinc aluminum coating
0 Introduction
steel structure has high strength, excellent mechanical processing properties, is widely used in the construction field, but steel exposure to the atmosphere is very prone to corrosion, seriously affecting its mechanical properties and life cycle. Therefore, it is of great significance to study the technology to prevent corrosion of steel structure. The traditional Dacro coating is a chemical conversion film with high corrosion resistance, which is widely used because of its hydrogen-free brittleness, good weather resistance, strong corrosion resistance and thermal corrosion resistance. The main raw material in the Dacro coating is chromium, due to passivation, bonding needs, the solution contains 2% to 5% of hexavalent chromium ions (Cr6 plus), seriously polluting the atmosphere and endangering human health. In recent years, with the increase of national environmental protection efforts, the application prospects of Dacro coating more and more narrow, the future green water-based chromium-free zinc aluminum coating will replace the organic solvent-based Dacro coating.
This experiment replaced the traditional chromium-free adhesive with a high-mode potassium silicate solution to achieve chromium-free bonding, and focused on the effect of the type and dosage of the silicane coupled agent on the performance before and after the aluminum powder cladding and the corrosion inhibitor concentration on the corrosion resistance of the prepared zinc-aluminum coating.
1 Test part
1.1 Raw materials and instruments
high-module potassium silicate solution: industrial grade, Hubei Tieshin Chemical Co., Ltd.; scaly zinc powder (15 to 20 m): analysis pure, Jiangsu Science and Technology non-ferrous metal new materials Co., Ltd.; scale aluminum powder (15 to 20 m): analysis pure, Zhangqiu City Yellow River silver powder plant; 3PO4.12MoO4): Analysis pure, Tianjin Shengxin Weiye Trading Co., Ltd.; silane couple agent KH-550, KH-560, KH-570: analysis pure, Jinan Jinhui Chemical Co., Ltd.; silane coupled agent WD-60: analysis pure, Hubei Wuda silicone new material Co., Ltd.; methanol, waterless ethanol: analysis pure, Tianjin Fuyu Fine Chemical Co., Ltd.
GS28-B Electronic Constant Speed Agitator: Shanghai Anting Electronic Instrument Factory; DF-101S Intelligent Heating Heating Magnetic Mixer: Shanghai Dongxuan Refrigeration Equipment Co., Ltd.; SHZ-D (III.) Circulating Water Vacuum Pump: Wuhan Kohl Instruments and Equipment Co., Ltd. DHG-9123A type electric blower drying box: Shanghai Jinghong Test Equipment Co., Ltd.; JSM-6510 LV scanning electron microscope, X-ray energy spectrum analyzer: Jintan Wenhua Science and Education Test Instrument Factory; Electrochemical Workstation: AMETEK Corporation of the United States.
1.2 Preparation of water-based chromium-free zinc aluminum coating
1.2.1 preparation of modified aluminum powder
1) surface washing: said to take quantitative sheet aluminum powder placed in a 250 mL beost, add waterless ethanol to fully stir, filter drying, in order to remove the pigment surface residue of organic solvents, to be used.
2) Silane hydrolysing: silane, deionized water, alcohol (bethoxysilane KH-550 select ethanol hydrolytic, methoxysilane KH-560, KH-570, WD-60 choose methanol hydrolysing) by a certain mass ratio evenly mixed, water bath heated to 40 degrees C, static hydrolytic 5 to 10 min.
3) Cladding modified: said to take 2 g washed flaky pigment aluminum powder, added to the silane hydrolytic solution, stirring at room temperature 0.5 to 1.0 h, with waterless ethanol washing filtration, put into the 80 degrees C oven drying, that is, to make the coating modified treatment of aluminum powder pigment.
1.2.2 Preparation of water-based chromium-free zinc aluminum coating
1) claims to take a certain quality pigment metal powder, so that Zn and Al (modified) mass ratio of 4:1, mixed evenly.
2) The metal mixture is slowly added to the high-mode potassium silicate solution, and the appropriate amount of desolation agents and other additives are added, so that the potassium silicate solution and metal mixture powder mass ratio of 1.0: 2.5,500 r/min fully stirred 1 h.
3) add a certain concentration gradient corrosion inhibitor, continue stirring 1 h, to make water-based chromium-free zinc aluminum coating.
1.3 Coating preparation
substrate surface treatment: sandpaper → alkaline de-oiling→ drying → ethanol cleaning.
the resulting water-based chrome-free zinc aluminum coating is evenly brushed on a surface-treated Q235 steel base and baked in two places: 20 min pre-baked at 80 degrees C and 40 min baked at 280 degrees C.
1.4 Testing and Characterizing
uses JSM-6510LV scanning electroscopes (SEMs) and X-ray energy spectrograph analyzers (EDS) to analyze the surface appearance and surface elements of modified aluminum powders.
the use of a rowing method adhesion meter, according to GB/T 9286-1998 to determine the film adhesion, according to GB/T 6739-2006 to determine the hardness of the coating pencil, according to GB/T 1731-1993 to determine the flexibility of the coating, according to GB/T 1732-1993 test coating resistance.
Using electrochemical impedance spectrum (EIS) to evaluate the corrosion resistance of water-based chromium-free zinc aluminum coating, measuring frequency range from 10-2 to 105 Hz, measuring signal for amplitude 10 mV AC sine wave, with platinum electrode, saturated glycomer electrode, zinc aluminum coated iron plate as working electrode, electrolyte solution with mass fraction of 3.5% of sodium chloride solution.
2 Results and discussion of the effect of
2.1 silicane coupled agent types and dosages on the performance of modified aluminum powders

As can be seen from Tables 1 and 1, the unalvested pigment aluminum powder is hydrophobic and cannot be dispersed in the water-based system, all floating on the surface of the water; In this study, WD-60 silane coupled agent was initially selected as the surface modifier of aluminum powder pigment.
effect of the amount of silicone coupled agent on the performance of aluminum powder pigment after modified treatment is listed in Table 2, and the dosage of silane coupled agent is expressed by its concentration in the hydrolyzed solution.
As can be seen from Table 2, when the content of silane coupled agent is low, the pigment aluminum powder partially floats on the surface of the water, because the WD-60 is not fully coated on the aluminum powder surface to form a dense hydro-hydro cladding layer; The experimental phenomenon shows that the dispersion stability of pigment aluminum powder after 5%WD-60 coating is significantly improved in the water-like system, and the appearance of pigment has not changed.
2.2 Sheet aluminum powder surface analysis and profiling
2.2.1 scanning electroscope (SEM) analysis
microscopic deformation of flaky aluminum powder pigments before and after surface changes by scanning electron microscopes (SEMs), as shown in Figure 2. As can be seen from Figure 2, a is a pre-modified flaky aluminum powder pigment, pigment surface is more smooth, the appearance of irregular and different sizes of scales; The results show that the coating film layer is formed on the surface of the pigment aluminum powder modified by the silicone coupler, and the coating correction of the flaky aluminum powder is realized.
2.2.2 EDS spectrometry
analyzed the surface elements of the cladding sheet aluminum powder pigment by X-ray energy spectrometer, with the results shown in Figures 3 and 3.
As can be seen from Figures 3 and 3, the peak of Si element appears on the surface of aluminum powder modified by WD-60, indicating that Si element is contained on the surface of sheet aluminum powder, indicating that the surface of aluminum powder after coating the modified is indeed covered with a silicone film.
2.3 Effect of corrosion inhibitor concentration on coating properties
The effect of the concentration of sodium phosphate corrosion inhibitor in the coating fluid on the performance of the prepared zinc-aluminum coating was examined, and the results are listed in Table 4.
as can be seen from Table 4, the concentration of sodium phosphate corrosion inhibitor in the coating has no significant effect on the pencil hardness and adhesion of the coating, but with the increase of the concentration, the mechanical strength of the coating decreased sharply. This may be because the addition of sodium phosphate reunites the pigment and destroys the integrity of the coating. The test shows that when the concentration of sodium phosphate corrosion inhibitor in the coating is 0.5% to 1.0%, the combined performance of the resulting coating is the best.
2.4 Effect of corrosion inhibitor concentration on corrosion resistance of the coating
The quality score of sodium phosphate corrosion inhibitor was 0, 0.5%, 1.0%, 1.5%, 2.0% zinc aluminum coating was placed in 3.5% sodium chloride aqueous solution, and the results were shown in Figure 4.
As can be seen from Figure 4, after adding different concentrations of sodium phosphate corrosion inhibitor, the tolerance arc radius of each group of coating samples has changed to varying degrees, but they are greater than the blank group size anti-arc radius, and (turn down page 8) (page 3 above) and with the increase of corrosion inhibitor concentration in the coating fluid, the tolerance anti-arc radius shows a tendency to increase and then decrease. When the concentration of sodium phosphate corrosion inhibitor was 1.0%, the maximum arc-resistant radius was added, indicating that the corrosion inhibitor corrosion inhibitor with the highest corrosion reduction efficiency and the best corrosion resistance.
3 Conclusion
1) After 5% WD-60 modified treatment of aluminum powder, its surface formed a hydro-water cladding layer, in the water-like system dispersion stability is better, to achieve the cladding of flaky aluminum powder.
2) to high-mode potassium silicate instead of traditional chromium adhesive, zinc powder and aluminum powder (modified) (quality ratio 4: 1) as a mixture pigment, potassium silicate solution and metal mixture powder mass ratio of 1.0: 2.5, while adding a concentration of 1.0% of sodium phosphate corrosion inhibitor, in order to prepare a water-based zinc aluminum coating corrosion resistance significantly improved.
(see Modern Coatings and Coatings 2018-6 for details)