A low adsorption composite coating for aluminum alloy gas cylinders and a method for preparing the same

Abstract

This invention belongs to the field of gas standard material packaging container processing technology, specifically providing a low-adsorption composite coating for aluminum alloy gas cylinders and its preparation method. Addressing the problem of value drift in gas standard materials due to adsorption on the inner wall during long-term storage, this invention sequentially performs acid washing and polishing, trivalent chromium passivation, and silanization sealing treatment on the inner wall of the aluminum alloy gas cylinder, forming a dense, inert, and low-adsorption composite coating with very few reaction sites. This low-adsorption composite coating significantly reduces the physical and chemical adsorption of typical gaseous components such as volatile organic compounds within the cylinder, effectively ensuring the accuracy and long-term stability of ultra-low concentration, complex component gas standard materials. Testing shows that the aluminum alloy gas cylinder obtained using this method exhibits inner wall adsorption performance comparable to imported high-end coated gas cylinders, filling a gap in the application of domestically produced aluminum alloy gas cylinders in the packaging of low-concentration gas standard materials.

Description

Technical Field

[0001] This invention relates to the field of gas standard material packaging container processing technology, and in particular to a low-adsorption composite coating for aluminum alloy gas cylinders and its preparation method. Background Technology

[0002] Gas reference materials, with their stable concentration values, are the core carriers for traceability in fields such as environmental monitoring and instrument calibration. In recent years, the demand for ultra-low concentration, complex-component gas reference materials in environmental monitoring and other fields has become increasingly urgent; however, the coating of gas cylinders has become a key factor restricting the development of the gas reference material industry.

[0003] Gaseous reference materials need to be stored in stable gas cylinders, and their accuracy and long-term stability are heavily influenced by the performance of the cylinder's inner wall. Currently, aluminum alloy cylinders are the most commonly used packaging material for gaseous reference materials due to their low density and ease of molding. However, the naturally occurring alumina film on the surface of aluminum alloys is porous and easily provides adsorption sites for gas molecules, leading to physical or chemical adsorption of standard gas components, directly affecting the reliability of the measurements. To address these issues, existing technologies often employ passivation or coating treatments on the inner wall of the cylinder. It is important to note that the core purpose of such treatments is not traditional metal corrosion protection, but rather to minimize the active sites and porosity on the inner wall surface, inhibiting the adsorption behavior of gas components. Currently, commercially available coated gas cylinders exist both domestically and internationally. It is necessary to examine the adsorption performance of the cylinder coating for different concentrations and components to determine its suitability for packaging target gaseous reference materials. The adsorption performance of the coating can be evaluated by dispensing the standard gas (mother cylinder) into the cylinder to be tested (daughter cylinder) and comparing the component concentration in the daughter cylinder with that in the mother cylinder, thereby assessing the concentration loss caused by component adsorption. The Standard Samples Research Institute of the Ministry of Environmental Protection compared the adsorption effects of imported coated gas cylinders, domestically coated gas cylinders, and ordinary domestic gas cylinders on 22 chlorinated hydrocarbon gas standard substances through gas cylinder transfer experiments. The results showed that the relative deviations of the analytical values ​​from the original gas cylinders for imported and domestically coated gas cylinders were both around ±1%, while the relative deviations for ordinary domestic gas cylinders ranged from -16% to 4%. This indicates that coating the inner wall of aluminum alloy gas cylinders is a feasible solution to the adsorption problem.

[0004] Although some commercially available coated gas cylinders exist, the related processes are largely monopolized by foreign companies, with very little technical detail disclosed. Meanwhile, most publicly available metal surface treatment technologies primarily target industrial applications such as corrosion resistance and wear resistance; their coatings often contain active groups that can react with gas components, failing to meet the specific requirement of "extremely low adsorption" for gas standard substances. The following challenges need to be overcome in the development of gas cylinder coating processes: 1. Limitations of gas cylinder shape on spraying and deposition processes: The structure of gas cylinders is a "small opening, large cavity" structure, which makes it difficult for spraying and deposition processes to penetrate into the inside of the gas cylinder and apply a uniform coating on the inner wall, especially in the cylinder body and shoulder. Due to their special shape, it is difficult to build a passivation coating with complete coverage and appropriate thickness inside the gas cylinder.

[0005] 2. Limitations of Cylinder Material on Processing Temperature: In the fields of gas analysis and environmental monitoring, chemical vapor deposition (CVD) is commonly used to passivate stainless steel surfaces (such as Entech's SilcoTek process), significantly reducing adsorption losses of gas components in the gas path system. However, currently reported CVD processes typically operate at temperatures of 300-400°C or higher. Domestically produced aluminum alloy cylinder substrates, after treatment at temperatures above 300°C, exhibit "overheating," resulting in irreversible damage to their microstructure and a sharp decrease in strength, severely impacting the safety of the cylinders under subsequent high-pressure conditions. Therefore, high-temperature CVD technology is not suitable for aluminum alloy substrates. Furthermore, CVD technology relies on large equipment, requiring significant space and technical expertise, making its feasibility extremely challenging.

[0006] 3. Insufficient rapid characterization methods and influencing mechanisms for adsorption performance severely restrict process development and iteration: Low-adsorption treatment of the inner wall of gas cylinders differs fundamentally from traditional corrosion protection and passivation in its objectives. However, existing research largely follows the corrosion protection approach, and the targeted exploration of gas adsorption mechanisms remains insufficient and incomplete. Furthermore, internationally advanced vapor deposition technologies are restricted by commercial confidentiality, and domestic technological accumulation in this area is weak. At the evaluation level, due to the lack of direct microscopic characterization methods, the industry is forced to adopt the time-consuming and costly "typical component dispensing + standard gas analysis" method for indirect verification. This "black box" evaluation system is time-consuming and costly, making it difficult to establish a quantitative correlation between process parameters and adsorption characteristics. This results in a lack of immediate feedback for process parameter adjustments, severely slowing down the development and iteration speed of low-adsorption coating processes. Based on this, the present invention proposes a composite coating process based on low-temperature trivalent chromium passivation and silanization treatment, which is specifically designed for the low adsorption requirements of gas standard material cylinders. Under the premise of ensuring the safety of the aluminum alloy substrate, through repeated parameter optimization and process comparison, the adsorption performance of the inner wall is fundamentally improved, solving the problem of inaccurate and unstable quality of gas standard materials caused by the adsorption of components inside the cylinder. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a low-adsorption composite coating for aluminum alloy gas cylinders and a method for preparing the same. The preparation method of this invention forms a low-adsorption composite coating on the inner wall of the aluminum alloy gas cylinder, solving the problem of inaccurate and unstable quality of gas standards caused by component adsorption within the aluminum alloy gas cylinder.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing a low-adsorption composite coating for aluminum alloy gas cylinders, comprising the following steps: The inner wall of the aluminum alloy gas cylinder is subjected to pickling and polishing treatment, trivalent chromium passivation treatment and silanization sealing treatment in sequence to form a low-adsorption composite coating on the inner wall of the aluminum alloy gas cylinder. The chromium passivation solution used in the trivalent chromium passivation treatment comprises components of the following concentrations: The solvent is water. The concentrations are: 5-10 g / L of soluble trivalent chromium salt, 0.01-0.5 g / L of nitric acid, 0.5-2.0 g / L of ammonium fluoride.

[0009] Preferably, the acid pickling and polishing treatment includes the following steps: Grinding balls and acid polishing liquid are placed in the aluminum alloy gas cylinder and subjected to vibration treatment. The acid pickling and polishing solution includes sulfuric acid, phosphoric acid, hydrogen peroxide, and a brightener.

[0010] Preferably, the grinding balls are made of zirconium oxide, the volume of the grinding balls is 3 / 12 to 5 / 12 of the effective volume of the aluminum alloy gas cylinder, and the total volume of the grinding balls and the pickling and polishing liquid is 6 / 12 to 8 / 12 of the effective volume of the gas cylinder. The pickling and polishing solution contains the following components at the following concentrations: Sulfuric acid 80~120g / L, phosphoric acid 15~17g / L, hydrogen peroxide 18~22g / L, brightener 18~22g / L, solvent is water.

[0011] Preferably, the oscillation treatment is performed at a rotation speed of 200-400 r / min for 8-12 h.

[0012] Preferably, the trivalent chromium passivation treatment includes the following steps: Chromium passivation solution was filled into the aluminum alloy gas cylinder after pickling and polishing, and then allowed to stand for a period of time.

[0013] Preferably, the amount of chromium passivation solution used is sufficient to fill the aluminum alloy gas cylinder; The settling temperature is 25~35℃ and the time is 3~10min.

[0014] Preferably, the silanization sealing process includes the following steps: The inner wall of the aluminum alloy gas cylinder, which has been passivated by trivalent chromium, is fully wetted with silane hydrolysate. After the silane hydrolysate is poured out, the cylinder is then dried and cured by heating.

[0015] Preferably, the method for preparing the silane hydrolysate includes the following steps: The silanizing agent and an aqueous ethanol solution were mixed and hydrolyzed under acidic conditions to obtain a silane hydrolysate. The silanizing agent includes 1,2-bis(triethoxysilyl)ethane; The volume ratio of ethanol to water in the ethanol-water solution is 3~4:1~2; The volume ratio of the silanizing reagent to the aqueous ethanol solution is 2~4:100; The hydrolysis is carried out at a pH of 3-6, a temperature of 20-45℃, and a time of 24-72 hours.

[0016] Preferably, the soaking time is greater than or equal to 30 seconds; The heating and curing temperature is 80~100℃, and the time is 30~90min.

[0017] The present invention also provides a low-adsorption composite coating obtained by the preparation method described above, wherein the low-adsorption composite coating is attached to the inner wall of an aluminum alloy gas cylinder; the low-adsorption composite coating includes a trivalent chromium passivation layer attached to the inner wall of the aluminum alloy gas cylinder, and a silane sealing layer attached to the trivalent chromium passivation layer; the trivalent chromium passivation layer is composed of a mixture of chromium oxide, aluminum oxide, chromium hydroxide and aluminum hydroxide, and the silane sealing layer is composed of crosslinked dehydration condensation products after hydrolysis of silanizing reagent.

[0018] This invention provides a method for preparing a low-adsorption composite coating for aluminum alloy gas cylinders. The method involves trivalent chromium passivation treatment to form a dense passivation film on the substrate surface; and silanization sealing treatment to further seal the surface active sites on the basis of the dense passivation film to construct a low-adsorption composite coating.

[0019] The preparation method of the present invention has at least the following advantages: (1) The preparation method of the present invention forms a high-performance low-adsorption composite coating on the inner wall of the aluminum alloy gas cylinder: based on trivalent chromium passivation treatment and silanization sealing treatment, the reaction sites on the inner wall of the aluminum alloy gas cylinder are effectively reduced, the adsorption of components is reduced, and the stability of gas standard substances is significantly improved. The adsorption performance of untreated aluminum alloy gas cylinders, aluminum alloy gas cylinders treated by the method of the present invention, and imported coated gas cylinders with leading performance in the industry were compared through dispensing experiments. It is proved that the aluminum alloy gas cylinders treated by the method of the present invention can effectively improve the adsorption loss of four typical components of volatile organic compounds (VOCs) in the aluminum alloy gas cylinder. The performance of aluminum alloy gas cylinders with low-adsorption composite coating is significantly better than that of untreated aluminum alloy gas cylinders and reaches a similar level to imported coated gas cylinders.

[0020] (2) The preparation method provided by this invention is highly feasible: This invention mainly uses solution dip coating, and employs trivalent chromium passivation treatment and silanization sealing treatment. Compared with electrochemical and hexavalent chromium passivation technologies, it has the advantages of generating less waste liquid and being environmentally friendly. Compared with chemical vapor deposition, this invention does not require large-scale vapor deposition equipment, is simple to operate, has lower cost, and a higher safety factor.

[0021] (3) The preparation method provided by the present invention has wide applicability: the aluminum alloy gas cylinder with low adsorption composite coating produced by the method of the present invention performs well in the use of standard gases for monitoring volatile organic compounds, fluorine-containing greenhouse gases and 57 kinds of photochemical pollutants, effectively improving the accuracy and long-term stability of the relevant gas standard substances. Attached Figure Description

[0022] Figure 1 This is a schematic diagram illustrating the principle of silanization sealing in this invention; Figure 2 These are SEM images of the inner wall of the A6061 type aluminum alloy gas cylinder before and after treatment in Embodiment 1 of the present invention; Figure 3 The relative reduction of the four typical volatile organic compounds (styrene, methylbenzyl chloride, and dodecane) in sub-cylinders of different aluminum alloy gas cylinders; Figure 4 The relative reduction of the four typical volatile organic compounds (styrene, methylbenzyl chloride, benzyl chloride, and dodecane) in the sub-cylinders of different aluminum alloy gas cylinders is shown. Detailed Implementation

[0023] This invention provides a method for preparing a low-adsorption composite coating for aluminum alloy gas cylinders, comprising the following steps: The inner wall of the aluminum alloy gas cylinder is subjected to pickling and polishing treatment, trivalent chromium passivation treatment and silanization sealing treatment in sequence to form a low-adsorption composite coating on the inner wall of the aluminum alloy gas cylinder. The chromium passivation solution used in the trivalent chromium passivation treatment comprises components of the following concentrations: The solvent is water. The concentrations are: 5-10 g / L of soluble trivalent chromium salt, 0.01-0.5 g / L of nitric acid, 0.5-2.0 g / L of ammonium fluoride.

[0024] Unless otherwise specified, the raw materials used in this invention are preferably commercially available products.

[0025] This invention involves acid washing and polishing the inner wall of an aluminum alloy gas cylinder.

[0026] In this invention, the pickling and polishing treatment preferably includes the following steps: placing grinding balls and pickling and polishing liquid into the aluminum alloy gas cylinder and performing an oscillation treatment.

[0027] In this invention, the aluminum alloy gas cylinder is preferably formed by compressing and plasticizing aluminum ingots using a mold. During the mold compression process, lubricating oil is used. Generally, aluminum alloy gas cylinders undergo internal wall degreasing before leaving the factory. If an undegreased aluminum alloy gas cylinder is encountered, this invention preferably performs a degreasing operation before acid pickling and polishing. This invention does not specifically limit the degreasing reagent; commercially available degreasing agents well-known to those skilled in the art can be used.

[0028] In this invention, the grinding balls are preferably made of zirconium oxide. The particle size of the grinding balls is preferably 3-8 mm, specifically 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, or 8 mm. The volume of the grinding balls is preferably 3 / 12 to 5 / 12 of the effective volume of the aluminum alloy gas cylinder, more preferably 4 / 12 (1 / 3).

[0029] In this invention, the pickling and polishing solution preferably comprises sulfuric acid, phosphoric acid, hydrogen peroxide, and a brightener. In this invention, the pickling and polishing solution preferably comprises the following components at the following concentrations: sulfuric acid 80-120 g / L, phosphoric acid 15-17 g / L, hydrogen peroxide 18-22 g / L, brightener 18-22 g / L, and water as the solvent. In this invention, the concentration of sulfuric acid in the pickling and polishing solution is specifically preferably 80 g / L, 90 g / L, 100 g / L, 110 g / L, or 120 g / L. In this invention, the concentration of phosphoric acid in the pickling and polishing solution is specifically preferably 15 g / L, 16 g / L, or 17 g / L. In this invention, the concentration of hydrogen peroxide in the pickling and polishing solution is specifically preferably 18 g / L, 19 g / L, 20 g / L, 21 g / L, or 22 g / L. In this invention, the concentration of the brightener in the pickling and polishing solution is preferably 18 g / L, 19 g / L, 20 g / L, 21 g / L, or 22 g / L; the brightener is preferably a commercially available brightener, specifically Dongsheng QXGL-7. In this invention, the water is preferably deionized water.

[0030] In this invention, the total volume of the grinding balls and the acid polishing liquid is preferably 6 / 12 to 8 / 12 of the effective volume of the gas cylinder, and more preferably 6 / 12 (1 / 2), 7 / 12 or 8 / 12.

[0031] In this invention, the rotational speed of the oscillation treatment is preferably 200-400 r / min, specifically preferably 200 r / min, 300 r / min, or 400 r / min; the time is preferably 8-12 h, more preferably 10 h. In this invention, the oscillation treatment is preferably performed on a mixer.

[0032] After the oscillation treatment is completed, the present invention preferably further includes: after pouring out the grinding balls and acid polishing liquid, sequentially rinsing the inner wall of the aluminum alloy gas cylinder with tap water and then with deionized water. The present invention does not specifically limit the number of tap water rinsings, as long as no bubbles are generated on the inner wall of the aluminum alloy gas cylinder. In the present invention, the number of deionized water rinsings is preferably 2 to 4 times, more preferably 3 times.

[0033] In this invention, the pickling and polishing process can remove the oxide layer and uneven burrs, protrusions, dust, etc. from the inner wall of the aluminum alloy gas cylinder, making the substrate smoother and flatter, and making the subsequent passivation film more firm and uniform.

[0034] After the acid pickling and polishing treatment, the present invention performs trivalent chromium passivation treatment.

[0035] In this invention, the chromium passivation solution used for the trivalent chromium passivation treatment comprises the following components at the following concentrations: 5-10 g / L of soluble trivalent chromium salt, 0.01-0.5 g / L of nitric acid, 0.5-2.0 g / L of ammonium fluoride (NH4F), and water as the solvent. In this invention, the soluble trivalent chromium salt preferably includes chromium sulfate, and more preferably chromium sulfate hexahydrate (Cr2(SO4)3·6H2O). Specifically, the concentration of the soluble trivalent chromium salt in the chromium passivation solution is preferably 5 g / L, 5.5 g / L, 6 g / L, 6.5 g / L, 7 g / L, 7.5 g / L, 8 g / L, 8.5 g / L, 9 g / L, 9.5 g / L, or 10 g / L. In this invention, the concentration of nitric acid in the chromium passivation solution is preferably 0.01 g / L, 0.05 g / L, 0.1 g / L, 0.2 g / L, 0.3 g / L, 0.4 g / L, or 0.5 g / L. In this invention, the concentration of ammonium fluoride in the chromium passivation solution is preferably 0.5 g / L, 1.0 g / L, 1.5 g / L, or 2.0 g / L. In this invention, the water is preferably deionized water.

[0036] In this invention, the trivalent chromium passivation treatment preferably includes the following steps: filling an aluminum alloy gas cylinder after acid pickling and polishing with chromium passivation solution and allowing it to stand. In this invention, the volume of the chromium passivation solution filled is preferably sufficient to completely fill the aluminum alloy gas cylinder. In this invention, the temperature of the standing treatment is preferably 25~35℃, more preferably 30℃; the time is preferably 3~10 min, specifically 3 min, 5 min, 7 min, or 10 min.

[0037] After the settling process, the present invention preferably further includes: after pouring out the chromium passivation solution, sequentially rinsing the inner wall of the aluminum alloy gas cylinder with tap water and then with deionized water. In the present invention, the tap water rinsing is preferably performed 2 to 4 times, more preferably 3 times; the deionized water rinsing is preferably performed 2 to 4 times, more preferably 3 times.

[0038] In this invention, the process and mechanism of trivalent chromium passivation treatment are mainly divided into the following three steps: (1) The chromium passivation solution is weakly acidic, and its contact with the aluminum alloy substrate surface causes aluminum to dissolve (metallic aluminum reacts with hydrogen ions to generate trivalent aluminum ions and hydrogen gas, and fine bubbles can be observed to be generated on the inner wall of the gas cylinder during the process), and at the same time, the dissolution of aluminum causes the pH of the local area on the surface of the gas cylinder to rise. (2) Cr in the chromium passivation solution 3+ In the complexing agent (ammonium fluoride) F - Under certain conditions, it remains stable and does not form precipitates. However, when the interfacial pH locally increases, trivalent chromium ions undergo a co-deposition reaction with dissolved aluminum ions, hydroxide ions, and other components in the solution, forming a mixed hydroxide precipitate. Finally, as the pH increases, the precipitate gradually thickens until it forms a continuous and complete passivation layer on the aluminum alloy surface. Traditional hexavalent chromium passivation treatment has significant advantages in metal corrosion protection, but its main component, Cr... 6+ Ions pose significant risks to human health and the environment and have been banned in the EU, the US, and other countries. They are gradually being replaced by trivalent chromium passivation.

[0039] After the trivalent chromium passivation treatment, the present invention performs silanization sealing treatment.

[0040] In this invention, the silanization sealing process preferably includes the following steps: The inner wall of the aluminum alloy gas cylinder, which has been passivated by trivalent chromium, is fully wetted with silane hydrolysate. After the silane hydrolysate is poured out, the cylinder is then dried and cured by heating.

[0041] In this invention, the method for preparing the silane hydrolysate preferably includes the following steps: The silanizing reagent and an aqueous ethanol solution are mixed and hydrolyzed under acidic conditions to obtain a silane hydrolysate.

[0042] In this invention, the silanizing agent preferably includes 1,2-bis(triethoxysilyl)ethane (BTSE). In this invention, the 1,2-bis(triethoxysilyl)ethane has the structure shown in Formula 1. Its advantage lies in the fact that a single BTSE molecule can hydrolyze to produce 6 times the amount of Si-OH, providing more dehydration condensation sites than traditional single silanizing agents (which hydrolyze to produce 3 times the amount of Si-OH), thus forming a more compact cross-linked structure on the inner wall of the gas cylinder.

[0043] Formula 1.

[0044] In this invention, the volume ratio of ethanol to water in the ethanol-water solution is preferably 3-4:1-2, more preferably 3:1. In this invention, the volume ratio of the silanizing reagent to the ethanol-water solution is preferably 2-4:100, more preferably 3:100. In this invention, mixing the silanizing reagent and the ethanol-water solution is preferably done by adding the silanizing reagent to the ethanol-water solution. In this invention, the silanizing reagent is preferably added dropwise. In this invention, the addition is preferably carried out under stirring conditions. In this invention, the pH value of the hydrolysis is preferably 3-6, more preferably 4.5; the temperature is preferably 20-45℃, more preferably 35℃; and the time is preferably 24-72h, more preferably 48h. In this invention, the hydrolysis is preferably carried out under closed and constant temperature water bath conditions. In this invention, the specific process of mixing the silanizing reagent and the ethanol-water solution and hydrolyzing under acidic conditions preferably includes the following steps: adding the silanizing reagent to the ethanol-water solution, then adjusting the resulting mixture to acidic conditions, and then carrying out hydrolysis. In this invention, the reagents used to adjust the resulting mixture to be acidic are preferably glacial acetic acid solution and glacial sodium acetate solution; the concentration of the glacial acetic acid solution is preferably 0.1 mol / L, and the concentration of the glacial sodium acetate solution is preferably 0.1 mol / L.

[0045] This invention does not specifically limit the method by which the silane hydrolysate fully wets the inner wall of the trivalent chromium passivated aluminum alloy gas cylinder, as long as the bottom, body, and shoulder of the cylinder are in contact with the silane hydrolysate. Specifically, this can be done by filling or by rolling coating, with the preferred rotation speed of the rolling coating being 300 r / min. In this invention, the wetting time is preferably greater than or equal to 30 s, more preferably 30 to 60 s, and specifically preferably 30 s, 40 s, 50 s, or 60 s. In this invention, the wetting time specifically refers to the time during which the bottom, body, and shoulder of the aluminum alloy gas cylinder are in contact with the silane hydrolysate.

[0046] This invention does not specifically limit the operation of pouring out the silane hydrolysate and controlling its drying; any operation well-known to those skilled in the art can be used. In this invention, the heating and curing temperature is preferably 80~100℃, more preferably 90℃; the time is preferably 30~90min, more preferably 60min. After adding and curing, this invention preferably also includes installing a bottle valve and evacuating the vacuum for later use.

[0047] In this invention, the schematic diagram of the silanization sealing process is as follows: Figure 1As shown, specifically: a single 1,2-bis(triethoxysilyl)ethane molecule hydrolyzes to generate 6 Si-OH groups, which can dehydrate and condense with the metal hydroxyl bonds on the inner wall of the aluminum alloy gas cylinder to form a film on the aluminum alloy gas cylinder. At the same time, adjacent molecules can also dehydrate and condense to form a film, which can further shield the metal hydroxyl groups. The free hydroxyl groups after the hydrolysis of the silanizing reagent are bonded and cross-linked on the surface of the trivalent chromium coating to form a film, further reducing the active adsorption sites on the surface of the gas cylinder, thereby achieving the goal of reducing the adsorption of gas molecules inside the gas cylinder.

[0048] In the preparation method provided by this invention, the trivalent chromium passivation treatment and silanization sealing treatment effectively reduce the loose structure of oxides and metal hydroxyl bonds on the inner wall of the aluminum alloy gas cylinder, reduce the physical and chemical adsorption of components, and significantly improve the stability of the gas standard material.

[0049] The present invention also provides a low-adsorption composite coating obtained by the preparation method described above. The low-adsorption composite coating is attached to the inner wall of an aluminum alloy gas cylinder. The low-adsorption composite coating includes a trivalent chromium passivation layer attached to the inner wall of the aluminum alloy gas cylinder and a silane sealing layer attached to the trivalent chromium passivation layer. The trivalent chromium passivation layer is composed of a mixture of chromium oxide, aluminum oxide, chromium hydroxide and aluminum hydroxide. The silane sealing layer is formed by crosslinking of the dehydration condensation product after hydrolysis of the silanizing agent.

[0050] The following detailed description, in conjunction with embodiments, of the low-adsorption composite coating for aluminum alloy gas cylinders provided by the present invention and its preparation method thereof, should not be construed as limiting the scope of protection of the present invention.

[0051] Example 1 A method for preparing a low-adsorption composite coating for aluminum alloy gas cylinders, particularly suitable for A6061 type aluminum alloy gas cylinders widely used in my country's gas standard material production field.

[0052] The specific process is as follows: The aluminum alloy gas cylinder used in this embodiment (where the aluminum alloy type is A6061) is formed by compressing and plasticizing aluminum ingots using a mold. The aluminum alloy gas cylinder has undergone degreasing treatment on the inner wall before leaving the factory, so this embodiment directly prepares the low-adsorption composite coating.

[0053] 1. Acid pickling and polishing treatment: Prepare an acid pickling and polishing solution using sulfuric acid 100g / L, phosphoric acid 16g / L, hydrogen peroxide 20g / L, commercial brightener Dongsheng QXGL-7 20g / L, and deionized water as the solvent. Remove the valve from the aluminum alloy gas cylinder and add zirconia microspheres (4mm in diameter) as abrasive material to the cylinder, filling it to 1 / 3 of the effective volume. Then pour in the acid pickling and polishing solution until it reaches 1 / 2 of the effective volume. Place the aluminum alloy gas cylinder containing the zirconia microspheres and the acid pickling and polishing solution on a mixer and vibrate it at 300r / min for 12 hours. After that, pour out the microspheres and the acid pickling and polishing solution, and repeatedly wash the inner wall of the cylinder with tap water until no bubbles are generated. Then wash the inner wall with deionized water three times.

[0054] 2. Trivalent chromium passivation treatment: The chromium passivation solution consists of the following components at the following concentrations: chromium sulfate hexahydrate (Cr2(SO4)3·6H2O) 8g / L, nitric acid 0.01g / L, ammonium fluoride (NH4F) 1.0g / L, and water as the solvent. The chromium passivation solution is then filled into the acid-washed and polished aluminum alloy gas cylinder and allowed to stand at 30℃ for 7 minutes. After the treatment, the chromium passivation solution is poured out, and the inner wall is rinsed three times each with tap water and deionized water.

[0055] 3. Silanization Sealing Treatment: Prepare a 500 mL ethanol-water solution by mixing 375 mL ethanol and 125 mL deionized water. While stirring, slowly add 15 mL of the silanizing reagent 1,2-bis(triethoxysilyl)ethane to the ethanol solution. Then, adjust the pH of the solution to approximately 4.5 using 0.1 mol / L glacial acetic acid and 0.1 mol / L sodium acetate solution. Pour the acidic mixture into a 500 mL reagent bottle, seal it, and place it in a constant temperature water bath at 35°C for 48 hours to hydrolyze the solution, obtaining a silane hydrolysate for later use. Add the silane hydrolysate to a trivalent chromium passivated aluminum alloy gas cylinder, ensuring that the bottom, body, and shoulder of the cylinder are in contact with the silanizing reagent for 30 seconds. Pour out the silane hydrolysate, invert the cylinder to allow it to dry, and then heat it at 90°C for 1 hour to solidify. Finally, install the valve and evacuate the cylinder for later use.

[0056] Figure 2 The images show SEM images of the inner wall of an A6061 type aluminum alloy gas cylinder before and after the process described in Example 1. The left image is the SEM image before processing, and the right image is the SEM image after processing. Figure 2 It can be seen that compared with the aluminum alloy surface before treatment, the scratches on the aluminum alloy surface after treatment are significantly repaired, and dust particles are also clearly covered.

[0057] Comparative Example 1 The difference from Example 1 is that the trivalent chromium passivation treatment in step 2 is replaced with hexavalent chromium passivation treatment. Specifically, the chromium passivation solution is replaced with commercially available hexavalent chromium passivation solution Alodine 1500, and the other parameters are the same as in Example 1.

[0058] Comparative Example 2 The difference from Example 1 is that the trivalent chromium passivation treatment and silanization sealing treatment are replaced with nickel plating, while the other parameters are the same as in Example 1.

[0059] Test Example 1 The 57 components of the PAMS standard gas are listed by the Photochemical Assessment Monitoring Stations (PAMS) in the United States, including 57 typical hydrocarbons such as alkanes, alkenes, and benzene series compounds. These compounds undergo a series of complex changes in the atmospheric environment, including photochemical reactions, ultimately forming smog pollution, which harms the environment and human health. During the development of PAMS gas standard materials, it was found that the preparation of high-boiling-point components (such as n-dodecane) was quite difficult, and the concentration analysis values ​​were repeatedly significantly lower than the weight-based preparation values: for example, in one bottle of standard gas, the preparation value of n-dodecane was 0.952 μmol·mol⁻¹. -1 However, the analysis result was only 0.868 μmol·mol⁻¹ -1 The relative deviation is -8.86%.

[0060] To ensure the accuracy of the values ​​of gaseous standard substances, passivation treatment is required to reduce component adsorption, thereby reducing the loss of high-carbon components (such as n-dodecane) during the preparation process.

[0061] The method of Example 1 was used to treat aluminum alloy gas cylinders (specifically, A6061 aluminum alloy), and the changes in the amount of high-carbon components in PAMS were tested before and after treatment. The amount of loss was evaluated using a mother-daughter bottle dispensing experiment. The amount of loss = relative decrease in daughter bottle concentration = (daughter bottle concentration - mother bottle concentration) / mother bottle concentration × 100%. The results are shown in Table 1 when the dispensing pressure is 5 MPa.

[0062] Table 1. Changes in the magnitude of high-carbon components in PAMS components.

[0063] As shown in Table 1, when the aluminum alloy gas cylinder is treated using the method of the present invention, the high carbon components hardly undergo significant adsorption within the aluminum alloy gas cylinder, thus solving the problem of component concentration loss during the preparation process and improving the accuracy of the values ​​of gas standard substances.

[0064] Test Example 2 Twenty different aluminum alloy gas cylinders (specifically, A6061 aluminum alloy, involving three batches) were processed using the method of Example 1, and compared with untreated aluminum alloy gas cylinders, imported A8 gas cylinders, and gas cylinders obtained from Comparative Examples 1 and 2.

[0065] The relative adsorption of four typical volatile organic compounds (VOCs)—styrene, methylbenzyl chloride, benzyl chloride, and dodecane—in different aluminum alloy gas cylinders was investigated using a dispensing experiment. Dispensing is a common method for evaluating the adsorption performance of gas cylinders using standard gases. Specifically, a portion of a standard gas of known concentration (called the mother cylinder) is transferred through a pipeline or gas distribution device to the aluminum alloy gas cylinder under investigation (called the daughter cylinder). This transfer process is the dispensing process. Assuming no adsorption occurs in the daughter cylinder, the concentration of each component in the daughter cylinder after dispensing should be completely consistent with that in the mother cylinder. If adsorption occurs in the daughter cylinder, the concentration of the gas component in the daughter cylinder will be lower than that in the mother cylinder. By comparing the concentration difference between the daughter and mother cylinders after dispensing, the strength of adsorption on the inner wall of the daughter cylinder can be evaluated: the greater the deviation between the daughter and mother cylinders, the stronger the adsorption of the component in the daughter cylinder; the smaller the difference between the daughter and mother cylinders, or even if no significant difference is observed, the smaller or negligible the adsorption in the daughter cylinder.

[0066] Since the adsorption sites on the inner wall of the gas cylinder are limited, the total amount of target molecules adsorbed... n a It is finite. During the dispensing process, the lower the dispensing pressure, the smaller the total amount of gaseous substance in the bottle. n The lower the value of 0, the greater the total number of molecules that undergo adsorption. n a exist n The higher the proportion of 0, the greater the difference in concentration between the daughter bottle and the mother bottle due to adsorption. To facilitate observation of the difference between the coated gas cylinder of this invention and the imported gas cylinder, in this embodiment, 10% of the gas cylinder operating pressure (usually 10 MPa) is selected as the dispensing pressure, i.e., the dispensing pressure is 1 MPa. By reducing the dispensing pressure, the difference in adsorption performance between different gas cylinders can be amplified, thereby obtaining more intuitive and reliable measurement results under the same precision analytical method.

[0067] The packaging results are as follows: Figure 3 and Figure 4 As shown. Figure 3 This section describes the relative reduction of four typical volatile organic compounds (styrene, methylbenzyl chloride, and dodecane) in sub-cylinders of different aluminum alloy gas cylinders. This is used to characterize the adsorption performance of the gas cylinders. A greater relative decrease in the component concentration in a sub-cylinder indicates more component adsorption and a poorer cylinder coating effect. Figure 3It can be seen that, based on the average level of 20 aluminum alloy gas cylinders treated by the method of the present invention, the adsorption loss of the four typical volatile organic compounds in the cylinders was significantly reduced, reaching a level comparable to that of imported A8 gas cylinders.

[0068] from Figure 4 It can be seen that the adsorption of typical components in the gas cylinder of the present invention is less than that of gas cylinders processed by the other two types of processes (hexavalent chromium and nickel plating), and the gas cylinder of the present invention is more suitable for packaging gas standard substances.

[0069] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a low-adsorption composite coating for aluminum alloy gas cylinders, characterized in that, Includes the following steps: The inner wall of the aluminum alloy gas cylinder is subjected to pickling and polishing treatment, trivalent chromium passivation treatment and silanization sealing treatment in sequence to form a low-adsorption composite coating on the inner wall of the aluminum alloy gas cylinder. The chromium passivation solution used in the trivalent chromium passivation treatment comprises components of the following concentrations: The solvent is water. The concentrations are: 5-10 g / L of soluble trivalent chromium salt, 0.01-0.5 g / L of nitric acid, 0.5-2.0 g / L of ammonium fluoride.

2. The production method according to claim 1, characterized by, The acid pickling and polishing process includes the following steps: Grinding balls and acid polishing liquid are placed in the aluminum alloy gas cylinder and subjected to vibration treatment. The acid pickling and polishing solution includes sulfuric acid, phosphoric acid, hydrogen peroxide, and a brightener.

3. The preparation method according to claim 2, characterized in that, The grinding balls are made of zirconium oxide. The volume of the grinding balls is 3 / 12 to 5 / 12 of the effective volume of the aluminum alloy gas cylinder. The total volume of the grinding balls and the pickling and polishing liquid is 6 / 12 to 8 / 12 of the effective volume of the gas cylinder. The pickling and polishing solution contains the following components at the following concentrations: Sulfuric acid 80~120g / L, phosphoric acid 15~17g / L, hydrogen peroxide 18~22g / L, brightener 18~22g / L, solvent is water.

4. The production method according to claim 2 or 3, characterized by, The oscillation process is performed at a speed of 200-400 r / min for 8-12 h.

5. The preparation method according to claim 1, characterized in that, The trivalent chromium passivation treatment includes the following steps: Chromium passivation solution was filled into the aluminum alloy gas cylinder after pickling and polishing, and then allowed to stand for a period of time.

6. The production method according to claim 1 or 5, characterized by, The amount of chromium passivation solution used is based on filling the aluminum alloy gas cylinder; The settling temperature is 25~35℃ and the time is 3~10min.

7. The preparation method according to claim 1, characterized in that, The silanization sealing process includes the following steps: The inner wall of the aluminum alloy gas cylinder, which has been passivated by trivalent chromium, is fully wetted with silane hydrolysate. After the silane hydrolysate is poured out, the cylinder is then dried and cured by heating.

8. The preparation method according to claim 7, characterized in that, The preparation method of the silane hydrolysate includes the following steps: The silanizing agent and an aqueous ethanol solution were mixed and hydrolyzed under acidic conditions to obtain a silane hydrolysate. The silanizing agent includes 1,2-bis(triethoxysilyl)ethane; The volume ratio of ethanol to water in the ethanol-water solution is 3~4:1~2; The volume ratio of the silanizing reagent to the aqueous ethanol solution is 2~4:100; The hydrolysis is carried out at a pH of 3-6, a temperature of 20-45℃, and a time of 24-72 hours.

9. The preparation method according to claim 7, characterized in that, The soaking time is greater than or equal to 30 seconds; The heating and curing temperature is 80~100℃, and the time is 30~90min.

10. The low-adsorption composite coating obtained by the preparation method according to any one of claims 1 to 9, wherein the low-adsorption composite coating is attached to the inner wall of an aluminum alloy gas cylinder; the low-adsorption composite coating comprises a trivalent chromium passivation layer attached to the inner wall of the aluminum alloy gas cylinder, and a silane sealing layer attached to the trivalent chromium passivation layer; the trivalent chromium passivation layer is composed of a mixture of chromium oxide, aluminum oxide, chromium hydroxide and aluminum hydroxide, and the silane sealing layer is composed of crosslinked dehydration condensation products after hydrolysis of a silanizing agent.

Images