A stripping solution and stripping method for off-spec zinc-nickel alloy plating

Abstract

The application discloses a stripping solution and stripping method for unqualified zinc-nickel alloy plating layer, and the stripping process is as follows: 80-150 g / L of sodium nitrite, 80-150 g / L of ammonium chloride, and the pH of the stripping solution is 6-8. When the pH of the prepared stripping solution is low, the pH is increased by using sodium carbonate solution, and when the pH is high, the pH is decreased by using dilute nitric acid. The stripping solution of the application uses sodium nitrite as an oxidizing agent for the plating layer metal and a corrosion inhibitor for the steel base body, and uses ammonium chloride as a complexing agent for zinc ions and nickel ions and a promoter for the plating layer dissolution reaction. The technical scheme of the application has no corrosion to the base metal, and overcomes the problems of high pollution and hydrogen embrittlement existing in the current stripping process for zinc-nickel alloy plating layer in the fields of aerospace and military industry.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, specifically relating to a stripping solution and method for substandard zinc-nickel alloy plating. Background Technology

[0002] Some aerospace and military components require cadmium plating to create a highly corrosion-resistant protective layer, but cadmium plating presents a significant pollution problem. To reduce cadmium pollution, the industry is developing a new process that uses zinc-nickel alloy plating to replace cadmium plating, and this process is already in mass production. [1] However, removing substandard zinc-nickel alloy plating from aerospace components presents certain difficulties, necessitating the development of new processes to address numerous challenges encountered in production.

[0003] In civilian manufacturing, hydrochloric acid is currently used to remove zinc-nickel alloy plating from steel parts. However, after stripping with dilute hydrochloric acid, nickel residue often remains on the workpiece surface. This is because metallic nickel dissolves much more slowly than zinc, and even after zinc is completely dissolved, a certain amount of nickel remains undissolved on the workpiece surface. Therefore, concentrated hydrochloric acid must be used in production to remove the zinc-nickel alloy plating and accelerate the dissolution of metallic nickel. In civilian production, steel parts are generally made of materials such as A3 steel, and removing the zinc-nickel alloy plating from their surface with concentrated hydrochloric acid will not damage the steel substrate. However, steel parts in aerospace and military fields are generally made of high-strength steel and require heat treatment to improve their strength and hardness. Removing the zinc-nickel alloy plating from the surface of such steel with hydrochloric acid will cause hydrogen embrittlement of the substrate, seriously affecting the performance of the workpiece. Production practice has shown that such steel parts must be scrapped after only two attempts at hydrochloric acid stripping. High-strength steel, after being corroded by hydrochloric acid, will develop carbon deposits on its surface. These carbon deposits adhere to and become embedded in the steel substrate. When zinc-nickel alloy is electroplated again on the stripped steel parts, the adhesion of the plating often fails to meet the technical requirements of aerospace and military products. Aerospace aluminum alloy parts use a process of chemical zinc plating followed by chemical nickel plating to prepare the base layer, and then plate with zinc-nickel alloy. The chemical nickel plating layer has a certain number of pores. When the zinc-nickel alloy plating is removed with hydrochloric acid, the stripping solution penetrates these pores and corrodes both the chemical zinc plating layer and the aluminum alloy substrate. Re-plating with zinc-nickel alloy results in blistering and pitting corrosion.

[0004] For the reasons mentioned above, aerospace and military enterprises have had to use the traditional chromic acid stripping method to remove substandard zinc-nickel alloy plating, but hexavalent chromium is highly polluting. In addition, some electroplating companies use hydrochloric acid stripping to remove substandard zinc-nickel alloy plating from some high-strength steel aerospace components, but this results in the serious problem of large-scale scrapping.

[0005] References: [1] Qin Zhili, Guo Chongwu, Application of high corrosion resistant zinc-nickel alloy protective layer in aerospace field [J], Electroplating & Finishing, 2023, 42(9): 14-17. Summary of the Invention

[0006] To address the problems of high pollution and hydrogen embrittlement associated with removing substandard zinc-nickel alloy plating using existing technologies in aerospace and military industries, this invention provides a stripping solution and method for substandard zinc-nickel alloy plating. To achieve the above objectives, this invention employs the following technical solution:

[0007] A stripping solution for a substandard zinc-nickel alloy plating includes 80-150 g / L sodium nitrite and 80-150 g / L ammonium chloride, with a pH of 6-8.

[0008] The sodium nitrite is used as an oxidizing agent to oxidize the zinc and nickel plating metals, causing them to form oxides.

[0009] The ammonium chloride is used as a complexing agent to cause the oxides of zinc and nickel to form complex ions that dissolve in the stripping solution;

[0010] The ammonium chloride is used as a promoter to increase the dissolution rate of the zinc-nickel alloy plating in the stripping solution.

[0011] In some embodiments, the stripping solution is prepared as follows: 4 / 5 of the water is added to the stripping tank according to the tank volume, sodium nitrite and ammonium chloride are added, stirred to dissolve, the pH of the prepared stripping solution is adjusted to 6-8, and water is added to the specified volume.

[0012] A method for removing substandard zinc-nickel alloy plating includes the following processes:

[0013] (1) Prepare the membrane removal tank solution by adding water and sulfuric acid to the membrane removal tank, wherein the mass fraction of sulfuric acid is 5% to 10%;

[0014] (2) Place the unqualified zinc-nickel alloy plated parts into the defilm removal tank to remove the passivation film on the zinc-nickel alloy plating layer. Operate at room temperature until the color of the passivation film fades completely.

[0015] (3) Place the unqualified zinc-nickel alloy plated parts after the film removal into a stripping tank containing the stripping solution as described in claim 1 for stripping, and operate at room temperature until the zinc-nickel alloy plating is completely removed.

[0016] In some embodiments, sodium nitrite and ammonium chloride are added to the stripping tank during production, with the mass ratio of sodium nitrite to ammonium chloride being 1:(0.2 to 0.8).

[0017] In some embodiments, a filter is used in production to circulate and remove deposits from the stripping tank.

[0018] In some embodiments, when the pH of the stripping solution in the stripping bath is too high, the pH of the stripping solution is lowered to 6-8 using dilute nitric acid with a mass fraction of 15%-25%.

[0019] In some embodiments, when the pH of the stripping solution in the stripping bath is too low, the pH of the stripping solution is adjusted to 6-8 using a sodium carbonate solution with a mass fraction of 10%-20%.

[0020] It is generally believed that sodium nitrite cannot be used as an oxidizing agent in near-neutral solutions at room temperature. It can only be used as an oxidizing agent when acid is added to the sodium nitrite solution to convert it into nitrous acid. Therefore, this technical solution breaks through the understanding of the oxidizing properties of sodium nitrite by those skilled in the art. In existing theory, ammonia molecules can act as ligands, while ammonium ions do not possess liganding ability; only under alkaline conditions can some heavy metal ions form complexes with ammonia. However, this technical solution uses ammonium salts as ligands under near-neutral conditions, which also breaks through the general understanding of those skilled in the art. Furthermore, if ammonium chloride is not used as a ligand and accelerator in this invention, and ammonium sulfate is used instead, the zinc-nickel alloy plating cannot be effectively removed within an acceptable timeframe.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention provides a stripping solution and method for removing substandard zinc-nickel alloy plating. It uses nitrite as an oxidant to oxidize the zinc-nickel alloy plating and ammonium salt as a complexing agent to dissolve the oxides of zinc and nickel, thereby effectively removing the zinc-nickel alloy plating. This overcomes the corrosion problem of the traditional hydrochloric acid stripping method and the high pollution problem of the chromic acid stripping method.

[0023] 2. The stripping solution and method for unqualified zinc-nickel alloy coatings of the present invention remove unqualified zinc-nickel alloy coatings within a pH range of 6 to 8. There is no chemical reaction in which hydrogen ions react with metallic iron to generate hydrogen, thus overcoming the problems of hydrogen embrittlement and carbon ash formation on the surface of high-strength steel substrates caused by hydrochloric acid stripping methods.

[0024] 3. The stripping solution and method for the unqualified zinc-nickel alloy coating of the present invention have the effect of sodium nitrite inhibiting corrosion of iron-based metals, and the stripping solution does not corrode the steel substrate after the zinc-nickel alloy coating is removed.

[0025] 4. The stripping solution and method for unqualified zinc-nickel alloy coatings of the present invention use ammonium chloride as an accelerator. Chloride ions promote the dissolution of zinc-nickel alloy coatings, significantly improving the stripping speed. Attached Figure Description

[0026] Figure 1The image shows the test results of Example 8 of this invention: No rust was observed on a No. 45 steel screw after it was placed in the stripping solution for 8 hours. Implementation

[0027] The present invention will now be described in detail with reference to specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0028] A stripping solution for substandard zinc-nickel alloy plating includes 80-150 g / L of sodium nitrite oxidant and 80-150 g / L of ammonium chloride complexing agent, with a pH of 6-8.

[0029] Preferably, the stripping solution is prepared as follows: add 4 / 5 water to the stripping tank according to the tank volume, add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution, stir to dissolve, adjust the pH of the stripping solution to 6-8, and add water to the specified volume.

[0030] A method for removing substandard zinc-nickel alloy plating includes the following processes:

[0031] (1) Prepare the membrane removal tank solution by adding water and sulfuric acid to the membrane removal tank, wherein the mass fraction of sulfuric acid is 5% to 10%;

[0032] (2) Place the unqualified zinc-nickel alloy plated parts into the defilm removal tank to remove the passivation film on the zinc-nickel alloy plating layer. Operate at room temperature until the color of the passivation film fades completely.

[0033] (3) Place the unqualified zinc-nickel alloy plating parts after the film removal into the stripping tank for stripping. Operate at room temperature until the zinc-nickel alloy plating is completely removed.

[0034] During the stripping process, as sodium nitrite oxidant and ammonium chloride complexing agent are consumed and carried out by the stripping solution, sodium nitrite and ammonium chloride need to be added to the stripping tank.

[0035] Preferably, during production, oxidant and complexing agent are added to the stripping tank at a mass ratio of sodium nitrite to ammonium chloride of 1:(0.2-0.8).

[0036] During the stripping process, precipitates such as nickel hydroxide may appear suspended in the stripping solution, and these precipitates need to be removed.

[0037] Preferably, a filter is used to circulate and remove precipitates from the stripping solution.

[0038] During the stripping process, hydrogen ions are consumed in the reaction of zinc oxide and nickel with nitrite, and the pH of the stripping solution will gradually increase. It is necessary to add acid to lower the pH to the process range.

[0039] Preferably, during production, the pH of the stripping solution is lowered to 6-8 using dilute nitric acid with a mass fraction of 15%-25%.

[0040] Adding dilute nitric acid to the stripping solution allows nitrate ions to act as an oxidizing agent, reducing the consumption of nitrite ions.

[0041] When the pH of the stripping solution is lower than the lower limit of the process, preferably, the pH of the stripping solution is adjusted to 6-8 using a sodium carbonate solution with a mass fraction of 10%-20%. Example

[0042] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0043] Sodium nitrite 100g / L, ammonium chloride 100g / L, stripping solution pH 6.3, operate at room temperature until the coating is completely removed.

[0044] 1. Prepare the film removal tank solution:

[0045] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank liquid is 5%.

[0046] 2. Prepare the stripping solution:

[0047] Add 4 / 5 water to the stripping tank according to the volume of the stripping solution, and add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve.

[0048] 3. Membrane removal:

[0049] Substandard zinc-nickel alloy plated parts are placed in a stripping tank to remove the passivation film until the color of the passivation film is completely removed (after the passivation film is removed, the plated metal reacts with the acid to produce hydrogen gas, which escapes from the stripping tank). After being removed from the tank, the parts are washed with water three times.

[0050] 4. Stripping:

[0051] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0052] During production, a filter is used to circulate and filter the stripping solution.

[0053] Sodium nitrite and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.4 during production.

[0054] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 20% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0055] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 15% sodium carbonate solution. Example

[0056] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0057] Sodium nitrite 80g / L, ammonium chloride 120g / L, stripping solution pH 6.1, operate at room temperature, until the coating is completely removed.

[0058] 1. Prepare the film removal tank solution:

[0059] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank liquid is 6%.

[0060] 2. Prepare the stripping solution:

[0061] Add 4 / 5 of the water to the stripping tank according to the volume of the stripping solution. Add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve them and add water to the specified volume.

[0062] 3. Membrane removal:

[0063] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0064] 4. Stripping:

[0065] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0066] During production, a filter is used to circulate and filter the stripping solution.

[0067] Sodium nitrite and ammonium sulfate are added to the stripping tank at a mass ratio of 1:0.5 during production.

[0068] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 15% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0069] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 10% sodium carbonate solution. Example

[0070] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0071] Sodium nitrite 120g / L, ammonium chloride 80g / L, stripping solution pH 6.5, room temperature operation, until the coating is completely removed.

[0072] 1. Prepare the film removal tank solution:

[0073] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank liquid is 7%.

[0074] 2. Prepare the stripping solution:

[0075] Add 4 / 5 of the water to the stripping tank according to the volume of the stripping solution. Add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve them and add water to the specified volume.

[0076] 3. Membrane removal:

[0077] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0078] 4. Stripping:

[0079] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0080] During production, a filter is used to circulate and filter the stripping solution.

[0081] During production, sodium nitrite and ammonium chloride are added to the stripping solution at a mass ratio of 1:0.3.

[0082] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 25% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0083] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 15% sodium carbonate solution. Example

[0084] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0085] Sodium nitrite 120g / L, ammonium chloride 120g / L, stripping solution pH 6.3, operate at room temperature, until the coating is completely removed.

[0086] 1. Prepare the film removal tank solution:

[0087] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank liquid is 8%.

[0088] 2. Prepare the stripping solution:

[0089] Add 4 / 5 of the water to the stripping tank according to the volume of the stripping solution. Add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve them and add water to the specified volume.

[0090] 3. Membrane removal:

[0091] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0092] 4. Stripping:

[0093] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0094] During production, a filter is used to circulate and filter the stripping solution.

[0095] Sodium nitrite and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.4 during production.

[0096] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 20% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0097] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 10% sodium carbonate solution. Example

[0098] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0099] Sodium nitrite 150g / L, ammonium chloride 150g / L, stripping solution pH 6.3, operate at room temperature, until the coating is completely removed.

[0100] 1. Prepare the film removal tank solution:

[0101] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank liquid is 9%.

[0102] 2. Prepare the stripping solution:

[0103] Add 4 / 5 of the water to the stripping tank according to the volume of the stripping solution. Add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve them and add water to the specified volume.

[0104] 3. Membrane removal:

[0105] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0106] 4. Stripping:

[0107] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0108] During production, a filter is used to circulate and filter the stripping solution.

[0109] Sodium nitrite and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.6 during production.

[0110] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 20% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0111] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 15% sodium carbonate solution. Example

[0112] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0113] Sodium nitrite 150g / L, ammonium chloride 80g / L, stripping solution pH 6, operate at room temperature until the coating is completely removed.

[0114] 1. Prepare the film removal tank solution:

[0115] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume. Add concentrated sulfuric acid according to the process requirements while stirring. Then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank liquid is 10%.

[0116] 2. Prepare the stripping solution:

[0117] Add 4 / 5 water to the stripping tank according to the volume of the stripping solution. Add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve them. Adjust the pH of the stripping solution to 6 with 15% dilute nitric acid. Add water to the specified volume.

[0118] 3. Membrane removal:

[0119] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0120] 4. Stripping:

[0121] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0122] During production, a filter is used to circulate and filter the stripping solution.

[0123] During production, potassium nitrite and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.2.

[0124] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 20% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0125] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 15% sodium carbonate solution. Example

[0126] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0127] Sodium nitrite 80g / L, ammonium chloride 150g / L, stripping solution pH 7.8, operate at room temperature until the coating is completely removed.

[0128] 1. Prepare the film removal tank solution:

[0129] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank solution is 7.5%.

[0130] 2. Prepare the stripping solution:

[0131] Add 4 / 5 water to the stripping tank according to the tank volume, add sodium nitrite and ammonium chloride according to the stripping solution formula requirements, stir to dissolve, adjust the pH of the stripping solution to 7.8 with a 15% sodium carbonate solution, and add water to the specified volume.

[0132] 3. Membrane removal:

[0133] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0134] 4. Stripping:

[0135] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0136] During production, a filter is used to circulate and filter the stripping solution.

[0137] Sodium nitrite and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.8 during production.

[0138] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 20% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0139] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 15% sodium carbonate solution. Example

[0140] The following is a stripping process for a substandard zinc-nickel alloy plating:

[0141] Sodium nitrite 150g / L, ammonium chloride 150g / L, stripping solution pH 7, operate at room temperature until the coating is completely removed.

[0142] 1. Prepare the film removal tank solution:

[0143] Add 2 / 3 water to the membrane removal tank according to the tank liquid volume, add concentrated sulfuric acid according to the process requirements while stirring, and then add water to the specified volume. The mass fraction of sulfuric acid in the prepared membrane removal tank solution is 7.5%.

[0144] 2. Prepare the stripping solution:

[0145] Add 4 / 5 water to the stripping tank according to the volume of the stripping solution. Add sodium nitrite and ammonium chloride according to the formula requirements of the stripping solution. Stir to dissolve. Adjust the pH of the stripping solution to 7 with sodium carbonate (15% by mass). Add water to the specified volume.

[0146] 3. Membrane removal:

[0147] Substandard zinc-nickel alloy plated parts are placed in a decoction tank to remove the passivation film until the color of the passivation film is completely removed. After removing the parts from the tank, they are washed with water three times.

[0148] 4. Stripping:

[0149] The defective zinc-nickel alloy plated parts, after the film removal process, are placed in the stripping tank for stripping until the coating is completely removed. After three water rinsings, they are transferred to the zinc-nickel alloy electroplating process.

[0150] During production, a filter is used to circulate and filter the stripping solution.

[0151] Sodium nitrite and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.4 during production.

[0152] If the pH of the stripping solution rises during production, dilute nitric acid with a mass fraction of 20% is added to the stripping tank to adjust the pH of the stripping solution to within the process range.

[0153] When the pH of the stripping solution is lower than the lower limit of the process during production, the pH of the stripping solution is adjusted to the process range using a 15% sodium carbonate solution.

[0154] Experimental Example 1:

[0155] A 50mm × 100mm × 2mm No. 45 steel plate was heat-treated to form sorbitic steel, followed by ultrasonic degreasing, sanding, chemical degreasing, and activation with 10% dilute sulfuric acid for 1 minute. Then, an experimental sample was prepared by electroplating an alkaline zinc-nickel alloy with a coating thickness of 10μm, using a hexavalent chromium black passivation process. Following the process in Example 1, the experimental sample was placed in a film removal tank to remove the passivation film, then immersed in a stripping solution for 30 minutes. After removal, it was rinsed three times with water, dried, and the surface was observed and wiped. The zinc-nickel alloy coating had been completely removed, and there was no carbon residue on the steel plate surface.

[0156] Experimental Example 2:

[0157] A 50mm×100mm×2mm aluminum plate of model 2024 was subjected to ultrasonic degreasing, chemical degreasing, alkaline etching, brightening, and micro-etching. Then, it underwent a first chemical zinc plating, zinc stripping, a second chemical zinc plating, chemical nickel plating, and alkaline zinc-nickel alloy plating. The chemical nickel plating layer was 5μm thick, and the zinc-nickel alloy plating layer was 10μm thick. Trivalent chromium blue-white passivation was used. Following the process in Example 2, the test sample was placed in a film removal tank to remove the passivation film, then immersed in a stripping solution for 30 minutes. After removal, it was rinsed three times with water, dried, and the surface of the sample was observed. The zinc-nickel alloy plating layer had been completely removed, and the chemical nickel plating layer and the aluminum alloy substrate remained intact.

[0158] Experimental Example 3:

[0159] After pickling and activation, the steel plate sample from Example 1 was re-electroplated with an alkaline zinc-nickel alloy to a thickness of 10 μm, using hexavalent chromium black passivation. The adhesion of the plating was tested according to GB / T 5270-2005 "Review of Test Methods for Adhesion Strength of Electrodeposited and Chemically Deposited Coatings on Metallic Substrates," using the thermal shock method. The sample was heated to 300°C in a furnace and held for 30 minutes, then removed and rapidly cooled in room temperature water. No blistering or peeling occurred, indicating good adhesion. The test shows that the method for removing substandard zinc-nickel alloy plating of this invention does not reduce the surface quality of medium carbon steel workpieces.

[0160] Experimental Example 4:

[0161] The aluminum alloy electroless nickel-plated parts obtained after stripping in Example 2 were acid-washed and activated, then electroless nickel-plated again, followed by electroplating with an alkaline zinc-nickel alloy. The zinc-nickel alloy coating thickness was 10 μm, and trivalent chromium was used for blue-white passivation. The coating adhesion of the prepared samples was tested using the thermal shock method according to GB / T 5270-2005 "Review of Test Methods for Adhesion Strength of Electrodeposited and Chemically Deposited Metallic Coatings on Metal Substrates". The samples were heated to 220°C in a furnace and held for 30 min, then removed and rapidly cooled in room temperature water. No blistering or peeling of the coating occurred, indicating good adhesion. The test shows that the method for removing substandard zinc-nickel alloy coatings of the present invention does not reduce the surface quality of the aluminum alloy electroless nickel-plated parts.

[0162] Experimental Example 5:

[0163] The test solution was prepared using 120 g / L sodium nitrite and 120 g / L ammonium chloride, with a pH of 6.3. It was divided into seven portions. Test solutions 1 and 2 were adjusted to pH 5.5 and 6.0 respectively using 10% dilute sulfuric acid. Test solutions 3 through 7 were adjusted to pH 6.5, 7.0, 7.5, 8.0, and 8.5 respectively using 10% sodium hydroxide solution. The zinc-nickel alloy sample prepared in Example 1 was used for stripping tests at room temperature. Within the tested pH range, the zinc-nickel alloy plating was effectively removed. However, at pH 5.5, the test solution tended to slowly dissolve the steel substrate. At pH 8.5, ammonia gas was released from the test solution. Based on this, the pH process parameters of this invention are specified as 6–8.

[0164] Experimental Example 6:

[0165] Prepare test solution A: sodium nitrite 120 g / L, ammonium chloride 120 g / L, and adjust the pH to 4.5 with 10% (w / w) dilute sulfuric acid. At pH 4.5, ammonium ions cannot undergo hydrolysis to produce ammonia molecules.

[0166] Prepare test solution B: 120 g / L sodium nitrite, 120 g / L sodium chloride, and adjust the pH to 4.5 with 10% dilute sulfuric acid.

[0167] The experiment was conducted using a zinc-nickel alloy-plated sample on a steel substrate, specifically an M8×40 screw. After removing the passivation film in 8% dilute sulfuric acid, the sample was immersed in test solution A for 30 minutes, resulting in the complete disappearance of the zinc-nickel alloy plating. After removing the passivation film in 8% dilute sulfuric acid, the sample was immersed in test solution B for 30 minutes, and no observable change was observed in the zinc-nickel alloy plating.

[0168] In test solution A, sodium nitrite oxidizes the zinc-nickel alloy into zinc and nickel oxides. At the same time, ammonium ions react with zinc oxide and nickel oxide to generate zinc complex ions and nickel complex ions, which dissolve in test solution A. Finally, the zinc-nickel alloy coating is removed.

[0169] In test solution B, sodium nitrite oxidizes the zinc-nickel alloy into zinc oxide and nickel oxide, forming a protective film on the zinc-nickel alloy coating and preventing further reaction between sodium nitrite and metallic zinc and nickel. The experiment shows that, in the absence of ammonium ion coordination, sodium nitrite alone cannot remove the zinc-nickel alloy coating.

[0170] Experimental Example 8:

[0171] Prepare a stripping solution containing 100 g / L sodium nitrite and 100 g / L ammonium chloride. Prepare electroplated zinc-nickel alloy samples using M8×40 screws made of 45# steel, employing a trivalent chromium blue-white passivation film. After removing the film in 8% dilute sulfuric acid, immerse the samples in the prepared stripping solution for stripping. After 30 minutes, the plating is completely removed. Continue immersing the samples in the stripping solution for 8 hours, then remove, wash with water, and dry. No rust was observed on the steel substrate surface. Figure 1 Experiments show that the sodium nitrite used in the stripping solution of this invention has good corrosion inhibition ability on the steel substrate.

[0172] Comparative Example 1:

[0173] The stripping solution was prepared using ammonium sulfate instead of ammonium chloride, with sodium nitrite at 100 g / L and ammonium sulfate at 100 g / L. Test samples were prepared according to Example 1. The samples were placed in a stripping tank to remove the passivation film, and then immersed in the stripping solution for 180 minutes until the zinc-nickel alloy plating on the sample surface was completely removed. Compared to Example 1, the stripping time in Comparative Example 1 was six times longer. The experiment shows that the use of ammonium chloride as a accelerator in this invention significantly improves the stripping speed of the zinc-nickel alloy plating.

[0174] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. The protection scope of this invention patent should be determined by the appended claims.

Claims

1. A stripping solution for off-spec zinc-nickel alloy plating, characterized by: The stripping solution comprises 80-150 g / L of sodium nitrite and 80-150 g / L of ammonium chloride, and has a pH of 6-8; The sodium nitrite is used as an oxidant to oxidize the zinc and nickel in the plating layer to form oxides; The ammonium chloride is used as a complexing agent to form complex ions of the zinc and nickel oxides, which are dissolved in the stripping solution; The ammonium chloride is used as an accelerator to increase the dissolution rate of the zinc-nickel alloy plating layer in the stripping solution.

2. The plating solution for stripping off a non-conforming zinc-nickel alloy plating layer according to claim 1, wherein The stripping solution is prepared as follows: 4 / 5 of the water in the stripping tank is added, and the sodium nitrite and ammonium chloride are added and stirred to dissolve, and the pH of the prepared stripping solution is adjusted to 6-8, and then water is added to the specified volume.

3. A method for removing a nonconforming zinc-nickel alloy coating, characterized by, The process comprises the following steps: (1) water and sulfuric acid are added to prepare a film removal tank solution, wherein the mass fraction of sulfuric acid is 5-10%; (2) the unqualified zinc-nickel alloy plated parts are placed in the film removal tank to remove the passivation film on the zinc-nickel alloy plating layer, and the operation is carried out at room temperature until the color of the passivation film is removed; (3) the unqualified zinc-nickel alloy plated parts after film removal are placed in the stripping tank containing the stripping solution according to claim 1, and the operation is carried out at room temperature until the zinc-nickel alloy plating layer is removed.

4. The method for removing a disqualification zinc-nickel alloy plating layer according to claim 3, characterized by: In production, the sodium nitrite and ammonium chloride are added to the stripping tank, and the mass ratio of the added sodium nitrite and ammonium chloride is 1:(0.2-0.8).

5. The method for removing a disqualification zinc-nickel alloy coating according to claim 3, characterized by: In production, the filter is used to recycle and filter the precipitates in the stripping tank.

6. The method for removing a disqualification zinc-nickel alloy coating according to claim 3, characterized by: When the pH of the stripping solution in the stripping tank is too high, dilute nitric acid with a mass fraction of 15-25% is used to reduce the pH of the stripping solution to 6-8.

7. The method for removing a disqualification zinc-nickel alloy coating according to claim 3, characterized by: When the pH of the stripping solution in the stripping tank is too low, a sodium carbonate solution with a mass fraction of 10-20% is used to adjust the pH of the stripping solution to 6-8.

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