Trivalent chromium passivation solution and preparation method thereof

Abstract

The invention relates to a trivalent chromium passivation solution and a preparation method thereof, and belongs to the technical field of passivation solutions. In the trivalent chromium passivation solution, the concentration of trivalent chromium salt is 80 g / L-90 g / L, the concentration of an oxidation accelerant is 65 g / L-75 g / L, the concentration of a coordination agent is 10 g / L-15 g / L, the concentration of a pH buffering agent is 8 g / L-12 g / L, the concentration of an activating agent is 10 g / L-15 g / L, the concentration of a grain refiner is 4 g / L-6 g / L, the concentration of cobalt salt is 60 g / L-75 g / L, the concentration of nickel salt is 220 g / L-240 g / L, and the concentration of an oxidizing agent is 20 g / L-30 g / L; the concentration of the complexing agent is 50 g / L-60 g / L, and the concentration of the nano silica gel is 8 g / L-12 g / L. The problem of hexavalent chromium pollution is eliminated, the problems that a trivalent chromium passivation film is poor in corrosion resistance and uneven in blackness are solved, and the process stability can be improved.

Description

Technical Field

[0001] This invention belongs to the field of passivation solution technology, and particularly relates to a trivalent chromium passivation solution and its preparation method. Background Technology

[0002] In the field of metal surface treatment, galvanizing is widely used in many industries such as machinery manufacturing, automotive parts, and electronics because it can effectively improve the corrosion resistance of metal substrates. To further enhance the protective performance and appearance of the galvanized layer, passivation treatment is usually required on the surface of the galvanized layer, and the choice of passivation solution plays a decisive role in the passivation effect. Traditional galvanizing passivation processes mostly rely on hexavalent chromium compounds, represented by dichromates, as the core component of the passivation solution. Although such passivation solutions can form a passivation film with good protective performance, hexavalent chromium has extremely high toxicity and clear carcinogenicity. During production, use, and waste disposal, it not only poses a serious threat to the health of operators, but also causes irreversible pollution to the ecological environment such as soil and water bodies. It can no longer meet the environmental protection and safety development requirements of modern industry and has been gradually restricted or banned by environmental regulations in various countries.

[0003] With increasing environmental awareness and stricter environmental regulations, the development of environmentally friendly passivation solutions has become an important direction in the field of metal surface treatment. Trivalent chromium passivation solutions, due to their significantly lower toxicity compared to hexavalent chromium and their environmental friendliness, are gradually becoming the mainstream alternative to traditional hexavalent chromium passivation solutions. However, in the practical application of trivalent chromium passivation solutions, the black passivation process, which imparts an aesthetically pleasing black appearance to metal surfaces while also providing a certain degree of protection, still faces many unresolved technical challenges, severely restricting its widespread application in scenarios with high requirements for appearance and corrosion resistance. Among these challenges, insufficient corrosion resistance of the film is one of the most prominent issues. In the commonly used neutral salt spray test, existing trivalent chromium black passivation films often develop white rust in less than 72 hours, failing to meet the basic corrosion resistance requirements of many products and significantly limiting their service life and application range.

[0004] Besides insufficient corrosion resistance, existing trivalent chromium black passivation processes also suffer from uneven film color. In actual production, factors such as unreasonable passivation solution composition and difficulty in controlling process parameters lead to inconsistent shades of black passivation film on the metal workpiece surface. In some areas, even phenomena like mottled appearance or exposed substrate occur, severely impacting the product's appearance quality and consistency. These aesthetic defects not only reduce the product's market competitiveness but may also lead to customer doubts about product quality, causing unnecessary economic losses and reputational risks for the company. Simultaneously, poor process stability is a major problem plaguing the industry. During continuous production, the performance of the passivation solution is easily affected by external factors (such as temperature fluctuations, impurity ion accumulation, and pH changes), resulting in unstable passivation effects. This can manifest as decreased film adhesion and fluctuations in corrosion resistance, increasing the difficulty of quality control and production costs, and hindering large-scale, stable industrial production.

[0005] Therefore, developing a trivalent chromium passivation solution with excellent corrosion resistance, capable of forming a uniform black passivation film, and exhibiting high process stability, as well as its preparation method, is of great practical significance and urgent market demand for promoting the sustainable development of the metal surface treatment industry and improving product quality and market competitiveness. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a trivalent chromium passivation solution and its preparation method. This trivalent chromium passivation solution, by optimizing the synergistic film-forming mechanism of trivalent chromium salts and transition metal ions (cobalt / nickel) and combining it with silica gel pre-sealing technology to improve the density and blackening effect of the passivation film, can eliminate hexavalent chromium contamination. Furthermore, it inhibits Cr contamination through a malonic acid-ammonium thiocyanate dual complexation system. 3+ Oxidation is achieved by utilizing sodium hypochlorite for directional consumption to ensure an oxidation potential <200mV; on the other hand, it can solve the problems of poor corrosion resistance and uneven blackness of trivalent chromium passivation films, specifically through Co... 2+ / Ni 2+ The proportion of spinel structure is used to construct the structure, and a pre-reacted nano-silica network is combined to fill the pores to form a "physical-chemical" double barrier; in addition, it can improve the process stability, relying on ethylene glycol-Na + The buffer system controls pH fluctuations to be less than ±0.1, and a stepwise mixing process is used to isolate oxidation and complexation reactions.

[0007] The first objective of this invention is to provide a trivalent chromium passivation solution, wherein the concentration of trivalent chromium salt is 80 g / L-90 g / L, the concentration of oxidation promoter is 65 g / L-75 g / L, the concentration of ligand is 10 g / L-15 g / L, the concentration of pH buffer is 8 g / L-12 g / L, the concentration of activator is 10 g / L-15 g / L, the concentration of grain refiner is 4 g / L-6 g / L, the concentration of cobalt salt is 60 g / L-75 g / L, the concentration of nickel salt is 220 g / L-240 g / L, the concentration of oxidant is 20 g / L-30 g / L, the concentration of complexing agent is 50 g / L-60 g / L, and the concentration of nano-silica gel is 8 g / L-12 g / L.

[0008] In one embodiment of the present invention, the solvent of the trivalent chromium passivation solution is water.

[0009] In one embodiment of the present invention, the trivalent chromium salt is chromium trichloride;

[0010] The cobalt salt is cobalt sulfate;

[0011] The nickel salt is nickel chloride.

[0012] In one embodiment of the present invention, the oxidation accelerator is sodium nitrate;

[0013] The ligand is malonic acid;

[0014] The pH buffer is ethylene glycol;

[0015] The activator is sodium fluoride;

[0016] The grain refiner is magnesium sulfate.

[0017] In one embodiment of the present invention, the oxidant is sodium hypochlorite.

[0018] In one embodiment of the present invention, the complexing agent is ammonium thiocyanate.

[0019] A second objective of this invention is to provide a method for preparing the aforementioned trivalent chromium passivation solution, comprising the following steps:

[0020] S1, trivalent chromium salt, oxidation promoter, complexing agent, pH buffer, activator, grain refiner, cobalt salt and nickel salt are dissolved in 1 / 3-1 / 2 solvent and heated and stirred to obtain the main liquid;

[0021] S2, oxidant, complexing agent, and nano-silica gel are pre-reacted in the remaining solvent to obtain a pre-reaction solution;

[0022] S3. Add the pre-reaction solution described in S2 to the main solution described in S1, stir evenly, and obtain the trivalent chromium passivation solution.

[0023] In one embodiment of the present invention, in S1, the heating and stirring temperature is 38°C-42°C and the time is 23.5 min-24.5 min.

[0024] In one embodiment of the present invention, in S2, the temperature of the pre-reaction is 22°C-30°C and the time is 22h-26h.

[0025] In one embodiment of the present invention, in S3, the stirring speed is ≤200 rpm. Low-speed stirring can avoid the generation of bubbles and prevent residual bubbles from causing pinholes in the passivation film, which would affect the corrosion resistance.

[0026] The technical solution of the present invention has the following advantages compared with the prior art:

[0027] (1) The trivalent chromium passivation solution of the present invention achieves a comprehensive improvement in passivation film performance and process stability through the synergistic effect of a multi-dimensional technical system. Firstly, in terms of optimizing film formation and blackening effect, on the one hand, a cobalt-nickel-nano silica gel ternary synergistic film formation technology is adopted, with a specific ratio of Co 2+ / Ni 2+ Combined with pre-reacted nano-silica, Co 2+ with Ni 2+ The formation of a CoNiO4 spinel phase in the passivation film, with its dense octahedral interstitial structure, can reduce Cl... - On the one hand, relying on the optical-structural synergistic blackening technology, by combining CoNiO4 spinel with a low-roughness surface, the narrow bandgap (2.1eV) of CoNiO4 is used to achieve full absorption of visible light in the 380nm-780nm range. At the same time, through the filling of nano-silicone, the fineness of the material surface is significantly improved, and the originally relatively rough surface becomes smoother. Meanwhile, the overall color consistency of the material is greatly improved, the color difference between different areas is significantly reduced, and the visual appearance is more uniform and unified.

[0028] (2) The trivalent chromium passivation solution described in this invention constructs a dual-stabilization system to ensure the stability of its components and performance. One is a coordination-oxidation dual-stabilization system, with a malonic acid-ammonium thiocyanate dual complexation system as the core. Malonic acid reacts with Cr through dicarboxyl groups. 3+ The formation of a five-membered ring chelate will reduce Cr 3+ The reduction potential inhibits its oxidation to Cr. 6+ The first is a coordination-buffered bistable system. Through the coordination of ethylene glycol and sodium nitrate, the carboxyl and hydroxyl groups of ethylene glycol form a buffer pair, which reduces the pH fluctuation range of the working solution to ±0.1. Sodium nitrate provides ionic strength and inhibits the hydrolysis and precipitation of components.

[0029] (3) The trivalent chromium passivation solution described in this invention further improves overall reliability through synergistic optimization of process steps. Specifically, it adopts a combination of stepwise mixing and precise temperature-time control: on the one hand, by using sodium hypochlorite for pre-reaction, it avoids its reaction with Cr. 3+ Direct contact significantly reduces Cr 3+ The oxidation rate is reduced, thus achieving an effective oxidation isolation effect. On the other hand, by controlling the temperature conditions during the preparation of the main solution, the activation energy of the complexation reaction is reduced, making the reaction easier to proceed fully. This not only ensures that the difference in the degree of reaction completion between different batches is minimal, but also effectively controls the kinetics of the entire reaction. Attached Figure Description

[0030] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0031] Figure 1 This is a photograph of the cold-rolled galvanized steel sheet after passivation solution treatment according to Example 1 of the present invention.

[0032] Figure 2 This is a physical image of the cold-rolled galvanized steel sheet after passivation solution treatment according to Comparative Example 3 of the present invention. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0034] In this invention, unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0035] In this invention, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0036] In this invention, unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0037] Example 1

[0038] The trivalent chromium passivation solution and its preparation method in this embodiment specifically include the following steps:

[0039] S1. Chromium trichloride, sodium nitrate, malonic acid, ethylene glycol, sodium fluoride, magnesium sulfate, cobalt sulfate, nickel chloride and half of the water are added sequentially to the reaction vessel and stirred at 40°C for 30 minutes to obtain the main liquid.

[0040] S2, sodium hypochlorite, ammonium thiocyanate, nano silica gel and remaining water are mixed evenly and allowed to stand at 25°C for 24 hours to pre-react, thus obtaining a pre-reaction solution.

[0041] S3. Add the pre-reaction solution to the main solution and stir evenly at 180 rpm to obtain a trivalent chromium passivation solution. The trivalent chromium passivation solution contains 86 g / L chromium trichloride, 70 g / L sodium nitrate, 13 g / L malonic acid, 10 g / L ethylene glycol, 14 g / L sodium fluoride, 5 g / L magnesium sulfate, 69 g / L cobalt sulfate, 230 g / L nickel chloride, 25 g / L sodium hypochlorite, 54 g / L ammonium thiocyanate, and 10 g / L nano silica gel.

[0042] Comparative Example 1

[0043] The basic principle is the same as in Example 1, except that no nano-silica gel is added; instead, water is used instead of nano-silica gel.

[0044] Comparative Example 2

[0045] The basic formula is the same as in Example 1, except that ammonium thiocyanate is replaced with malonic acid.

[0046] Comparative Example 3

[0047] The basic formula is the same as in Example 1, except that sodium hypochlorite is replaced with sodium molybdate.

[0048] Comparative Example 4

[0049] The commercially available trivalent chromium passivation solution was purchased from Guangdong Dazhi, model HKS-916.

[0050] Test Example 1

[0051] Cold-rolled galvanized steel sheets were selected as the experimental substrate, ensuring the zinc layer thickness was controlled at 10±2μm. The steel sheet surface was pretreated to remove oil, oxide layers, and other impurities, ensuring consistent surface cleanliness. The trivalent chromium passivation solutions from Examples 1 and 1-4 were diluted with pure water at a ratio of 100ml / L. The pretreated cold-rolled galvanized steel sheets were then immersed in the passivation solution at a temperature of 25℃ for 30 seconds. After immersion, the cold-rolled galvanized steel sheets were dried with hot air at 60℃ for 10 minutes.

[0052] According to the neutral salt spray test standard ASTM B117, the corrosion resistance of cold-rolled galvanized steel sheets treated with passivation solution in Example 1, Comparative Examples 1 and 4 was tested. Under continuous spraying conditions of 35℃±1℃ and 5% NaCl solution, the time for the appearance of white rust (judgment criterion: white rust area ≥5%) and the time for the appearance of red rust were recorded for each group of samples. The corrosion rate was also measured. The results are shown in Table 1.

[0053] Table 1

[0054] Group White rust time (h) Rusting time (h) <![CDATA[Corrosion rate (g / m 2 ·h)]]> Example 1 120 360 0.002 Comparative Example 1 75 210 0.015 Comparative Example 4 48 160 0.028

[0055] As shown in Table 1, the white rust appeared in Example 1 after 120 hours, the red rust appeared after 360 hours, and the corrosion rate was only 0.002 g / m³. 2 •h, its performance far surpasses the other two groups. This is because the nano-silicone in Example 1 can fill the pores of the passivation film, while Co 2+ with Ni 2+ The formation of a CoNiO4 spinel structure, together with the formation of a "physical-chemical double barrier," reduces Cl... - Corrosion ion penetration reduces the diffusion coefficient. Comparative Example 1, lacking nano-silica gel, has a high porosity in its passivation film, allowing for rapid penetration of corrosive media. The time to white rust and red rust appearance is shortened to 75 hours and 210 hours, respectively, and the corrosion rate increases to 0.015 g / m³. 2 •h; Comparative Example 4, without optimized cobalt-nickel ratio and without silica gel filler, exhibited poor passivation film density, with white rust appearing after 48 hours and red rust after 160 hours, reaching a corrosion rate of 0.028 g / m. 2 ·h.

[0056] Test Example 2

[0057] Referring to Test Example 1, the corrosion resistance (white rust time) of cold-rolled galvanized steel sheets after passivation solution treatment in Example 1 and Comparative Example 2 was tested.

[0058] Using a Gamry Interface 1010E electrochemical workstation, the electrochemical corrosion current density of cold-rolled galvanized steel sheets treated with passivation solution in Example 1 and Comparative Example 2 was tested using the Tafel polarization curve test method.

[0059] The hexavalent chromium content of cold-rolled galvanized steel sheets treated with passivation solution in Example 1 and Comparative Example 2 was determined using the ISO 3613 point test method.

[0060] Table 2 shows the relevant test results:

[0061] Table 2

[0062] Group White rust time (h) <![CDATA[Electrochemical corrosion current density (μA / cm 2 )]]> Hexavalent chromium content (ppm) Example 1 120 0.12 Not detected (<0.1) Comparative Example 2 52 2.35 0.9±0.2

[0063] As shown in Table 2, Example 1 used a malonic acid-ammonium thiocyanate dual complexation system, with a white rust time of 120 hours and an electrochemical corrosion current density of 0.12 μA / cm². 2 Furthermore, hexavalent chromium was not detected. However, in Comparative Example 2, the use of malonic acid as a single complexing agent failed to consume excess sodium hypochlorite, leading to the partial release of Cr. 3+ Oxidized to Cr 6+ Cr 6+ / Cr 3+ Oxidizing micro-batteries accelerate localized corrosion, resulting in white rust formation in just 52 hours, with the electrochemical corrosion current density rising to 2.35 μA / cm². 2 Furthermore, 0.9 ± 0.2 ppm of hexavalent chromium was detected. This is because in the dual-complex system, ammonium thiocyanate can directionally consume sodium hypochlorite, controlling the redox potential to <200 mV and preventing Cr from being detected. 3+ Oxidation ensures corrosion resistance and environmental friendliness.

[0064] Test Example 3

[0065] The actual images of the cold-rolled galvanized steel sheets after passivation solution treatment in Example 1 and Comparative Example 3 are shown below. Figures 1-2 As shown. From Figures 1-2 As can be seen, the surface of the cold-rolled galvanized steel sheet treated in Example 1 is a uniform and full pure black color, without any mottled appearance or exposed substrate. This is because sodium hypochlorite has strong oxidizing properties, which can quickly oxidize Zn on the substrate surface and promote the oxidation of Cr. 3+ Co 2+ Ni 2+ Oxidative polymerization forms a complete CoNiO4 spinel structure with a narrow bandgap (2.1 eV) that can fully absorb visible light. Combined with nano-silica gel to reduce surface roughness and light reflection, it presents a high-quality black appearance. In contrast, the surface of the cold-rolled galvanized steel sheet treated in Comparative Example 3 is grayish-black with light gray spots or streaks. This is because sodium molybdate has weak oxidizing properties, and the oxidation and polymerization of Zn and metal ions are insufficient, failing to form a complete spinel structure. Some areas are dominated by low-valence metal compounds, which cannot fully absorb visible light. Furthermore, uneven oxidation rates lead to inconsistent film thickness, resulting in poor final appearance quality.

[0066] Test Example 4

[0067] Select relevant samples after passivation treatment in Example 1 (such as passivation solution residue, treated substrate, etc.) and prepare standard reference materials that meet the EU RoHS limit requirements.

[0068] Total chromium content detection: The sample was digested according to the EPA 3050B digestion method; the total chromium content (mg / L) in the digested sample was determined using an appropriate detection instrument and compared with the EU RoHS limit (1000mg / L);

[0069] Hexavalent chromium detection: The sample was tested according to the ISO 3613 point test method; the hexavalent chromium content (μg / L) in the sample was determined and compared with the EU RoHS limit (100μg / L);

[0070] COD emission testing: The sample was processed according to the method specified in GB11914-89 standard; the COD emission of the sample (mg / L) was tested and compared with the EU RoHS limit (500mg / L);

[0071] Table 3 shows the relevant test results:

[0072] Table 3

[0073] Testing items Example 1 EU RoHS limits Total chromium content (mg / L) 0.38 1000 Hexavalent chromium (μg / L) <0.1 100 COD emissions (mg / L) 85 500

[0074] As shown in Table 3, Example 1 demonstrates excellent environmental compliance, with all indicators significantly lower than the EU RoHS limits. The total chromium content is 0.38 mg / L, due to the presence of Cr in the passivation solution. 3+ Highly efficient participation in film formation, free Cr 3+ Low concentration and no additional chromium source introduced; hexavalent chromium <0.1 μg / L, thanks to the dual complexation system and pre-reaction process, effectively suppressing Cr. 3+ Oxidation; COD emissions are 85 mg / L. Due to the high participation of organic components in the formula in the reaction, the residue is low and easily degraded. These data indicate that Example 1 will not cause excessive accumulation of chromium during use, has no risk of hexavalent chromium toxicity, causes minimal organic pollution to water bodies, and meets environmental protection requirements.

[0075] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A trivalent chromium passivation solution, characterized in that, The trivalent chromium passivation solution contains trivalent chromium salts at concentrations of 80 g / L-90 g / L, oxidation promoters at concentrations of 65 g / L-75 g / L, ligands at concentrations of 10 g / L-15 g / L, pH buffers at concentrations of 8 g / L-12 g / L, activators at concentrations of 10 g / L-15 g / L, grain refiners at concentrations of 4 g / L-6 g / L, cobalt salts at concentrations of 60 g / L-75 g / L, nickel salts at concentrations of 220 g / L-240 g / L, oxidants at concentrations of 20 g / L-30 g / L, complexing agents at concentrations of 50 g / L-60 g / L, and nano-silica gel at concentrations of 8 g / L-12 g / L.

2. The trivalent chromium passivation solution according to claim 1, characterized in that, The solvent for the trivalent chromium passivation solution is water.

3. The trivalent chromium passivation solution according to claim 1, characterized in that, The trivalent chromium salt is chromium trichloride; The cobalt salt is cobalt sulfate; The nickel salt is nickel chloride.

4. The trivalent chromium passivation solution according to claim 1, characterized in that, The oxidation accelerator is sodium nitrate; The ligand is malonic acid; The pH buffer is ethylene glycol; The activator is sodium fluoride; The grain refiner is magnesium sulfate.

5. The trivalent chromium passivation solution according to claim 1, characterized in that, The oxidant is sodium hypochlorite.

6. The trivalent chromium passivation solution according to claim 1, characterized in that, The complexing agent is ammonium thiocyanate.

7. A method for preparing a trivalent chromium passivation solution as described in any one of claims 1-6, characterized in that, Includes the following steps: S1, trivalent chromium salt, oxidation promoter, complexing agent, pH buffer, activator, grain refiner, cobalt salt and nickel salt are dissolved in 1 / 3-1 / 2 solvent and heated and stirred to obtain the main liquid; S2, oxidant, complexing agent, and nano-silica gel are pre-reacted in the remaining solvent to obtain a pre-reaction solution; S3. Add the pre-reaction solution described in S2 to the main solution described in S1, stir evenly, and obtain the trivalent chromium passivation solution.

8. The method for preparing the trivalent chromium passivation solution according to claim 7, characterized in that, In S1, the heating and stirring temperature is 38℃-42℃, and the time is 23.5min-24.5min.

9. The method for preparing the trivalent chromium passivation solution according to claim 7, characterized in that, In S2, the pre-reaction temperature is 22℃-30℃ and the time is 22h-26h.

10. The method for preparing the trivalent chromium passivation solution according to claim 7, characterized in that, In S3, the stirring speed is ≤200 rpm.

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