A black passivation agent for zinc plating and a preparation method thereof

Abstract

This invention discloses an alkaline zinc plating black passivating agent and its preparation method. The alkaline zinc plating black passivating agent includes component A and component B. Component A, by weight, comprises: 20-35 parts of trivalent chromium salt, 7-15 parts of complexing agent, 5-15 parts of pH adjuster, 5-15 parts of auxiliary blackening agent, 10-20 parts of oxidant, and 30-40 parts of water. Component B, by weight, comprises: 15-25 parts of main blackening agent, 1-10 parts of film-forming accelerator, and 70-90 parts of water. This passivating agent has good stability and salt spray resistance, and is low in cost. It is suitable for galvanized screws, bolts, and other parts. The passivation film obtained using this passivating agent is black, bright, and uniform. The production process is simple and easy to operate, with minimal environmental pollution, meeting green environmental protection standards.

Description

Technical Field

[0001] This invention relates to zinc plating passivating agents, classified under C23C22 / 30, specifically to an alkaline zinc plating black passivating agent and its preparation method. Background Technology

[0002] Zinc coatings, due to their negative standard potential, can be used as anodic coatings to provide electrochemical protection. However, zinc coatings are prone to corrosion, which affects their appearance and corrosion resistance. To improve this problem, we usually passivate zinc coatings.

[0003] Passivation treatment not only improves the corrosion resistance of zinc plating but also imparts different colors (such as blue-white, multicolored, black, and military green), providing both protective and decorative functions. Most current black passivation processes contain hexavalent chromium, and the blackening agent is generally expensive silver salt. However, hexavalent chromium is highly toxic and seriously endangers human health. Trivalent chromium has much lower toxicity, only 1% of that of hexavalent chromium, making trivalent chromium black passivating agents a research hotspot. However, the stability, black appearance of the passivation film, and corrosion resistance of currently available trivalent chromium passivating agents still lag behind those of hexavalent chromium black passivating films.

[0004] Chinese patent CN 102418095A discloses a chromium-free black passivating agent for zinc plating and its preparation method. The chromium-free black passivating agent for zinc plating, prepared by molybdate, nickel chloride, ammonium sulfate and pH adjuster, solves the problem of excessive hexavalent chromium, but its salt spray resistance needs to be further improved.

[0005] Chinese patent CN1584117A discloses a trivalent chromium rainbow passivating agent for zinc plating and its manufacturing method. The trivalent chromium rainbow passivating agent prepared from raw materials such as trivalent chromium salt, chelating agent, oxidant, and accelerator has good high-temperature resistance in a 5% neutral salt spray test for 96 hours. However, it does not solve the problem of poor stability of the passivating agent. Summary of the Invention

[0006] To address the aforementioned problems, a first aspect of the present invention provides an alkaline zinc plating black passivating agent, comprising component A and component B. Component A, by weight, comprises: 20-35 parts of trivalent chromium salt, 7-15 parts of complexing agent, 5-15 parts of pH adjuster, 5-15 parts of auxiliary blackening agent, 10-20 parts of oxidant, and 30-40 parts of water. Component B, by weight, comprises: 15-25 parts of main blackening agent, 1-10 parts of film-forming accelerator, and 70-90 parts of water.

[0007] More preferably, component A, by weight, comprises: 20-30 parts trivalent chromium salt, 7-10 parts complexing agent, 10-15 parts pH adjuster, 5-10 parts auxiliary blackening agent, 10-15 parts oxidant, and 30-40 parts water; component B, by weight, comprises: 15-20 parts main blackening agent, 1-5 parts film-forming accelerator, and 70-90 parts water.

[0008] More preferably, component A, by weight, comprises: 25.1 parts trivalent chromium salt, 9.6 parts complexing agent, 10.6 parts pH adjuster, 6.5 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water; component B, by weight, comprises: 16 parts main blackening agent, 4 parts film-forming accelerator, and 80 parts water.

[0009] More preferably, the pH adjuster is liquid alkali, the oxidant is sodium nitrate, and the film-forming promoter is nitric acid.

[0010] Preferably, the trivalent chromium salt includes at least one of chromium nitrate, chromium sulfate, and chromium chloride.

[0011] More preferably, the trivalent chromium salt is chromium nitrate.

[0012] Preferably, the complexing agent is at least one selected from citric acid, malonic acid, malic acid, gluconic acid, tartaric acid, and disodium ethylenediaminetetraacetate.

[0013] More preferably, the complexing agent is malonic acid.

[0014] Preferably, the weight ratio of the trivalent chromium salt to the complexing agent is (23-26):(8-10).

[0015] More preferably, the weight ratio of the trivalent chromium salt to the complexing agent is (24-26):(9-10).

[0016] More preferably, the weight ratio of the trivalent chromium salt to the complexing agent is 25.1:9.6.

[0017] Preferably, the main blackening agent is an organic sulfide.

[0018] Preferably, the organosulfur compound is diammonium dithiodiglycolic acid.

[0019] Preferably, the diammonium dithiodiglycolic acid is added in the form of an aqueous solution, and its mass fraction in the aqueous solution is 43-48%.

[0020] More preferably, the diammonium dithiodiglycolic acid has a mass fraction of 44-46% in the aqueous solution.

[0021] More preferably, the diammonium dithioglycolic acid has a mass fraction of 45% in the aqueous solution.

[0022] Preferably, the auxiliary blackening agent is a cobalt salt.

[0023] More preferably, the cobalt salt includes at least one of cobalt nitrate, cobalt sulfate, and cobalt chloride.

[0024] More preferably, the cobalt salt is cobalt nitrate.

[0025] Preferably, the weight ratio of the main blackening agent to the auxiliary blackening agent is (14-17):(5-7).

[0026] More preferably, the weight ratio of the main blackening agent to the auxiliary blackening agent is (15-17):(6-7).

[0027] More preferably, the weight ratio of the main blackening agent to the auxiliary blackening agent is 16:6.5.

[0028] A second aspect of this invention provides a method for preparing an alkaline zinc plating black passivating agent, comprising the following steps:

[0029] After mixing water, trivalent chromium salt, and complexing agent of component A in parts by weight, a pH adjuster is added, and the mixture is kept at 40-50℃ for 1.5-2.5 hours. Then, auxiliary blackening agent and oxidizing agent are added in parts by weight, stirred, and kept at the same temperature for 1.5-2.5 hours. After cooling, solution A is obtained. Water, main blackening agent, and film-forming accelerator are added in parts by weight, mixed, and stirred for 0.5-1.5 hours to obtain solution B. Finally, 80-120 mL / L solution A and 80-120 mL / L solution B are mixed to obtain the passivating agent.

[0030] The present invention also provides a method for using an alkaline zinc plating black passivating agent, with the following specific parameters: temperature 25℃, pH 2.0, and immersion time 90s.

[0031] When the weight ratio of malonic acid to chromium nitrate is higher, the passivation film forms faster, but the passivation film is prone to floating dust, and the passivating agent has poor stability and is prone to precipitation. When the weight ratio of chromium nitrate to malonic acid is limited to (23-26):(8-10), the passivating agent has better stability and corrosion resistance is improved under the combined action of oxidant sodium nitrate and film-forming promoter nitric acid.

[0032] When the content of diammonium dithioglycolate is too low, the passivation appearance changes from a slightly yellowish surface to a glossy black, and its corrosion resistance increases accordingly. However, when the content of diammonium dithioglycolate is too high, its corrosion resistance deteriorates. This may be because when the mass fraction of diammonium dithioglycolate is low, the content of metal sulfides deposited in the film is low, the blackness is poor, the film thickness is thin, and the corrosion resistance is poor. When the mass fraction is high, the reaction rate is fast, the film surface is loose, and the corrosion resistance is poor. This limits the effectiveness of diammonium dithioglycolate in aqueous solutions. When the mass fraction is 44-46%, the film is black and shiny with enhanced corrosion resistance, but the adhesion of the passivation film is weak. Based on this, cobalt nitrate is added, and the weight ratio of the two is limited to (14-17):(5-7). This avoids the problems of reduced corrosion resistance and decreased stability of the passivation agent caused by excessive cobalt nitrate content. Furthermore, cobalt nitrate can form a Zn-Co alloy layer on the coating surface, thereby adjusting and stabilizing the color of the passivation film and improving its corrosion resistance. Simultaneously, Co... 2+ The addition of [a substance] can strengthen the bonding force between the passivation film and the zinc plating layer. The two work synergistically to form black metal sulfide deposits in the film layer, further improving its corrosion resistance, while making the passivation layer black and shiny.

[0033] Beneficial effects: The alkaline zinc plating passivating agent formula provided by this invention combines the components in specific amounts and prepares them using a specific process, achieving synergy between the components. The prepared passivating agent is stable, and the resulting passivation film still exhibits good corrosion resistance even after up to 3 months. It does not contain hexavalent chromium, is harmless to humans and the environment, and does not use expensive silver salts as blackening agents, thus greatly reducing the cost of the passivating agent. It is suitable for galvanized screws, bolts, and other parts. At the same time, the passivation film obtained using this passivating agent is black, bright, and uniform. The production process is simple and easy to operate, with minimal environmental pollution, meeting green and environmentally friendly standards. Example

[0034] Example 1

[0035] An alkaline zinc plating black passivating agent comprises component A and component B. Component A, by weight, comprises: 25.1 parts trivalent chromium salt, 9.6 parts complexing agent, 10.6 parts pH adjuster, 6.5 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water. Component B, by weight, comprises: 16 parts main blackening agent, 4 parts film-forming accelerator, and 80 parts water.

[0036] The trivalent chromium salt is chromium nitrate.

[0037] The complexing agent is malonic acid.

[0038] The weight ratio of chromium nitrate to malonic acid is 25.1:9.6.

[0039] The pH adjuster is a liquid alkali, added in the form of an aqueous solution, with a mass fraction of 30% in the aqueous solution.

[0040] The auxiliary blackening agent is cobalt nitrate.

[0041] The oxidant is sodium nitrate.

[0042] The main blackening agent is diammonium dithiodiglycolate, which is added in the form of an aqueous solution with a mass fraction of 45% in the aqueous solution, and is purchased from Lana White.

[0043] The weight ratio of diammonium dithioglycolate to cobalt nitrate is 16:6.5.

[0044] The film-forming accelerator is nitric acid.

[0045] This embodiment also provides a method for preparing an alkaline zinc plating black passivating agent, including the following steps:

[0046] After mixing water, trivalent chromium salt, and complexing agent of component A in parts by weight, a pH adjuster is added, and the mixture is kept at 45°C for 2 hours. Then, auxiliary blackening agent and oxidizing agent are added in parts by weight, stirred, and kept at the same temperature for 2 hours. After cooling, solution A is obtained. Water, main blackening agent, and film-forming accelerator are added in parts by weight, mixed, and stirred for 1 hour to obtain solution B. Finally, 100 mL / L solution A and 100 mL / L solution B are mixed to obtain the passivating agent.

[0047] The present invention also provides a method for using an alkaline zinc plating black passivating agent, with the following specific parameters: temperature 25℃, pH 2.0, and immersion time 90s.

[0048] Example 2

[0049] An alkaline zinc plating black passivating agent comprises component A and component B. Component A, by weight, comprises: 20 parts trivalent chromium salt, 7 parts complexing agent, 8 parts pH adjuster, 6.5 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water. Component B, by weight, comprises: 16 parts main blackening agent, 3 parts film-forming accelerator, and 80 parts water.

[0050] The trivalent chromium salt is chromium nitrate.

[0051] The complexing agent is malonic acid.

[0052] The weight ratio of chromium nitrate to malonic acid is 20:7.

[0053] The pH adjuster is a liquid alkali, added in the form of an aqueous solution, with a mass fraction of 30% in the aqueous solution.

[0054] The auxiliary blackening agent is cobalt nitrate.

[0055] The oxidant is sodium nitrate.

[0056] The main blackening agent is diammonium dithiodiglycolate, which is added in the form of an aqueous solution with a mass fraction of 45% in the aqueous solution, and is purchased from Lana White.

[0057] The weight ratio of diammonium dithioglycolate to cobalt nitrate is 16:6.5.

[0058] The film-forming accelerator is nitric acid.

[0059] This embodiment also provides a method for preparing an alkaline zinc plating black passivating agent, including the following steps:

[0060] After mixing water, trivalent chromium salt, and complexing agent of component A in parts by weight, a pH adjuster is added, and the mixture is kept at 45°C for 2 hours. Then, auxiliary blackening agent and oxidizing agent are added in parts by weight, stirred, and kept at the same temperature for 2 hours. After cooling, solution A is obtained. Water, main blackening agent, and film-forming accelerator are added in parts by weight, mixed, and stirred for 1 hour to obtain solution B. Finally, 100 mL / L solution A and 100 mL / L solution B are mixed to obtain the passivating agent.

[0061] The present invention also provides a method for using an alkaline zinc plating black passivating agent, with the following specific parameters: temperature 25℃, pH 2.0, and immersion time 90s.

[0062] Example 3

[0063] An alkaline zinc plating black passivating agent comprises component A and component B. Component A, by weight, comprises: 25.1 parts trivalent chromium salt, 9.6 parts complexing agent, 10.6 parts pH adjuster, 10 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water. Component B, by weight, comprises: 20 parts main blackening agent, 4 parts film-forming accelerator, and 80 parts water.

[0064] The trivalent chromium salt is chromium nitrate.

[0065] The complexing agent is malonic acid.

[0066] The weight ratio of chromium nitrate to malonic acid is 25.1:9.6.

[0067] The pH adjuster is a liquid alkali, added in the form of an aqueous solution, with a mass fraction of 30% in the aqueous solution.

[0068] The auxiliary blackening agent is cobalt nitrate.

[0069] The oxidant is sodium nitrate.

[0070] The main blackening agent is diammonium dithiodiglycolate, which is added in the form of an aqueous solution with a mass fraction of 45% in the aqueous solution, and is purchased from Lana White.

[0071] The weight ratio of diammonium dithiodiglycolate to cobalt nitrate is 20:10.

[0072] The film-forming accelerator is nitric acid.

[0073] The rest is the same as in Example 1.

[0074] Comparative Example 1

[0075] An alkaline zinc plating black passivating agent comprises component A and component B. Component A, by weight, comprises: 25.1 parts trivalent chromium salt, 9.6 parts complexing agent, 10.6 parts pH adjuster, 6.5 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water. Component B, by weight, comprises: 21 parts main blackening agent, 4 parts film-forming accelerator, and 80 parts water.

[0076] The trivalent chromium salt is chromium nitrate.

[0077] The complexing agent is malonic acid.

[0078] The weight ratio of chromium nitrate to malonic acid is 25.1:9.6.

[0079] The pH adjuster is a liquid alkali, added in the form of an aqueous solution, with a mass fraction of 30% in the aqueous solution.

[0080] The auxiliary blackening agent is cobalt nitrate.

[0081] The oxidant is sodium nitrate.

[0082] The main blackening agent is diammonium dithiodiglycolate, which is added in the form of an aqueous solution with a mass fraction of 40% in the aqueous solution. It was purchased from Daicel Japan.

[0083] The weight ratio of diammonium dithiodiglycolate to cobalt nitrate is 21:6.5.

[0084] The film-forming accelerator is nitric acid.

[0085] The rest is the same as in Example 1.

[0086] Comparative Example 2

[0087] An alkaline zinc plating black passivating agent comprises component A and component B. Component A, by weight, comprises: 25.1 parts trivalent chromium salt, 9.6 parts complexing agent, 10.6 parts pH adjuster, 16 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water. Component B, by weight, comprises: 19 parts main blackening agent, 4 parts film-forming accelerator, and 80 parts water.

[0088] The trivalent chromium salt is chromium nitrate.

[0089] The complexing agent is malonic acid.

[0090] The weight ratio of chromium nitrate to malonic acid is 25.1:9.6.

[0091] The pH adjuster is a liquid alkali, added in the form of an aqueous solution, with a mass fraction of 30% in the aqueous solution.

[0092] The auxiliary blackening agent is cobalt nitrate.

[0093] The oxidant is sodium nitrate.

[0094] The main blackening agent is diammonium dithiodiglycolate, which is added in the form of an aqueous solution with a mass fraction of 45% in the aqueous solution, and is purchased from Lana White.

[0095] The weight ratio of diammonium dithiodiglycolate to cobalt nitrate is 19:16.

[0096] The film-forming accelerator is nitric acid.

[0097] The rest is the same as in Example 1.

[0098] Comparative Example 3

[0099] An alkaline zinc plating black passivating agent comprises component A and component B. Component A, by weight, comprises: 32 parts trivalent chromium salt, 17 parts complexing agent, 10.6 parts pH adjuster, 6.5 parts auxiliary blackening agent, 11 parts oxidant, and 36.4 parts water. Component B, by weight, comprises: 16 parts main blackening agent, 4 parts film-forming accelerator, and 80 parts water.

[0100] The trivalent chromium salt is chromium nitrate.

[0101] The complexing agent is malonic acid.

[0102] The weight ratio of chromium nitrate to malonic acid is 32:17.

[0103] The pH adjuster is a liquid alkali, added in the form of an aqueous solution, with a mass fraction of 30% in the aqueous solution.

[0104] The auxiliary blackening agent is cobalt nitrate.

[0105] The oxidant is sodium nitrate.

[0106] The main blackening agent is diammonium dithiodiglycolate, which is added in the form of an aqueous solution with a mass fraction of 45% in the aqueous solution, and is purchased from Lana White.

[0107] The weight ratio of diammonium dithioglycolate to cobalt nitrate is 16:6.5.

[0108] The film-forming accelerator is nitric acid.

[0109] The rest is the same as in Example 1.

[0110] Performance Evaluation

[0111] The galvanized screws that needed passivation were immersed in the passivating agent prepared in the examples and comparative examples at a temperature of 25°C and a pH of 2.0 for 90 seconds, followed by rinsing with water and drying. The coating thickness was 6 μm.

[0112] (1) Salt spray resistance: The test was conducted in accordance with the national standard GB / T 10125-2012 Artificial Atmosphere Corrosion Test Salt Spray Test, with a test temperature of 35±2℃ and a NaCl concentration of 50±5g / L. The time when the area of ​​white rust on the galvanized screws coated with passivating agent was greater than 5% was recorded.

[0113] (2) After the passivating agents prepared in the examples and comparative examples were placed at 25°C for 90 days, the salt resistance performance was tested again.

[0114] Table 1

[0115]

[0116]

[0117] Compared with Example 1, the mass fraction of diammonium dithiodiglycolate in aqueous solution was reduced in Comparative Example 1, which greatly affected its corrosion resistance. The content of cobalt nitrate in Comparative Example 2 was too high, which greatly affected its stability. The amount of malonic acid used in Comparative Example 3 was too large, resulting in poor stability of the passivating agent.

Claims

1. An alkaline zinc plating black passivating agent, characterized in that, The product comprises component A and component B. Component A, by weight, comprises: 23-26 parts trivalent chromium salt, 8-10 parts complexing agent, 5-15 parts pH adjuster, 5-7 parts auxiliary blackening agent, 10-20 parts oxidant, and 30-40 parts water. Component B, by weight, comprises: 15-17 parts main blackening agent, 1-10 parts film-forming accelerator, and 70-90 parts water. The main blackening agent is an organic sulfide; the organic sulfide is diammonium dithiodiglycolic acid, which is added in the form of an aqueous solution, and its mass fraction in the aqueous solution is 43-48%.

2. The alkaline zinc plating black passivating agent according to claim 1, characterized in that, The trivalent chromium salt includes at least one of chromium nitrate, chromium sulfate, and chromium chloride.

3. The alkaline zinc plating black passivating agent according to claim 1, characterized in that, The complexing agent is at least one of citric acid, malonic acid, malic acid, gluconic acid, tartaric acid, and disodium ethylenediaminetetraacetate.

4. A method for preparing the alkaline zinc plating black passivating agent according to claim 1, characterized in that, The steps are as follows: Mix water, trivalent chromium salt, complexing agent, and pH adjuster in component A and keep it warm. Then add auxiliary blackening agent and oxidant, mix and keep warm to obtain solution A. Mix the main blackening agent, film-forming promoter, and water in component B and stir to obtain solution B. Finally, mix solution A and solution B to obtain the passivating agent.

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