A stripping solution and stripping method for unqualified cadmium plating layer

Abstract

The present invention discloses a stripping solution and a stripping method for unqualified cadmium plating layers. The stripping process is as follows: the oxidant is 50 - 160 g / L, the complexing agent is 50 - 160 g / L, the pH of the stripping solution is 6 - 8, and the operation is carried out at room temperature. The oxidant includes one or two of sodium nitrite and potassium nitrite, and the complexing agent includes one or two of ammonium sulfate, ammonium bicarbonate, ammonium carbonate, and ammonium chloride. When the pH of the stripping solution is too low, the pH is increased with sodium carbonate solution or sodium hydroxide solution; when the pH is too high, the pH is decreased with dilute nitric acid or dilute sulfuric acid. Nitrite in the stripping solution has a corrosion inhibition effect on the steel substrate, and there is no corrosion on the steel surface during stripping. The technical solution of the present invention overcomes the high pollution problem of the chromic acid cadmium stripping process in the fields of aerospace and military industry, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal surface treatment, and particularly relates to a stripping solution and a stripping method for unqualified cadmium plating layers. Background Art

[0002] Cadmium plating is mainly used in fields such as aerospace and military industry, and its coating has extremely high corrosion resistance. In the past, the chromic acid stripping method was usually used to strip unqualified cadmium plating layers, but hexavalent chromium is highly polluting, and the use of hexavalent chromium in China has been increasingly strictly restricted. The hydrochloric acid stripping method can also strip the cadmium plating layer, but generally it takes 5 - 20 minutes. The thickness of the cadmium plating layer on the surface of the plated parts is different, and for some plated parts with complex shapes, the thickness difference of the cadmium plating layer on the surface is relatively large. After the thinner coating is stripped, the substrate is corroded by hydrochloric acid, and when all the cadmium plating layers are stripped, the local surface of the steel substrate has been damaged. Therefore, the hydrochloric acid stripping method is not suitable for stripping unqualified cadmium plating layers on steel substrates.

[0003] The invention patent "A Stripping Solution and a Stripping Method for Unqualified Cadmium Plating Layers" with the authorization publication number CN 111690933 B discloses a method for stripping unqualified cadmium plating layers. Its stripping process is as follows: sulfuric acid 60 - 200 g / L, 2,4-dinitrobenzenesulfonic acid hydrate 20 - 60 g / L, hexamethylenetetramine 10 - 50 g / L. At room temperature, the unqualified cadmium-plated parts are immersed in the stripping solution until the coating is completely stripped. Although hexamethylenetetramine can play a corrosion inhibition role on the steel substrate by using this method, due to the long stripping time, a small amount of carbon hanging ash will still be generated due to the dissolution of the medium carbon steel substrate surface by sulfuric acid, resulting in poor coating adhesion after re-electroplating. Summary of the Invention

[0004] In order to solve the high pollution problem existing in stripping unqualified cadmium plating layers according to the chromic acid stripping process in fields such as aerospace and military industry, the present invention provides a stripping solution and a stripping method for unqualified cadmium plating layers. To achieve the above object, the present invention adopts the following technical solutions:

[0005] A stripping solution for unqualified cadmium plating layers, comprising an oxidant 50 - 160 g / L and a complexing agent 50 - 160 g / L, and the pH of the stripping solution is 6 - 8;

[0006] The oxidant includes one or two of sodium nitrite and potassium nitrite;

[0007] The complexing agent includes one or two of ammonium sulfate, ammonium bicarbonate, ammonium carbonate, and ammonium chloride.

[0008] Preferably, the pH of the stripping solution is adjusted with a 10% sodium carbonate solution or a 10% sodium hydroxide solution by mass fraction.

[0009] Preferably, the pH of the stripping solution is adjusted with dilute sulfuric acid with a mass concentration of 20% or dilute nitric acid with a mass fraction of 20%.

[0010] Preferably, the stripping solution is prepared as follows: Add 4 / 5 of water to the stripping tank according to the volume of the bath solution, add an oxidizing agent and a complexing agent, stir to dissolve them, adjust the pH of the prepared stripping solution to 6-8, and add water to the specified volume.

[0011] Preferably, the working temperature of the stripping solution is room temperature.

[0012] A method for removing an unqualified cadmium plating layer includes the following technological processes:

[0013] (1) Add water and hydrochloric acid to the film removal tank to prepare the film removal bath solution, where the mass fraction of hydrogen chloride is 3% - 10%, and the bath solution temperature is room temperature;

[0014] (2) Put the unqualified cadmium-plated parts into the film removal tank to remove the passivation film on the unqualified cadmium plating layer until the color of the passivation film fades completely;

[0015] (3) Put the unqualified cadmium-plated parts after film removal into the tank with the stripping solution for stripping until the cadmium plating layer is completely stripped.

[0016] Preferably, in production, an oxidizing agent and a complexing agent are added to the stripping solution, and the mass ratio of the added oxidizing agent to the complexing agent is 1:(0.3 - 0.8).

[0017] Preferably, in production, the stripping solution is filtered with a filter to remove precipitates.

[0018] Preferably, after the pH of the stripping solution increases in production, the pH of the stripping solution is adjusted to 6 - 8 with dilute nitric acid with a mass fraction of 20%.

[0019] Preferably, after the pH of the stripping solution increases in production, the pH of the stripping solution is adjusted to 6 - 8 with dilute sulfuric acid with a mass fraction of 20%.

[0020] Preferably, when the pH of the stripping solution is lower than the lower limit of the process in production, the pH of the stripping solution is adjusted to 6 - 8 with a sodium carbonate solution with a mass fraction of 10%.

[0021] In the knowledge of those skilled in the art, sodium nitrite usually cannot be used as an oxidant in solutions at normal temperature and near neutrality. Only when an acid is added to the sodium nitrite solution to convert it into nitrous acid can it be used as an oxidant. Therefore, this technical solution breaks through the general understanding that nitrite cannot be used as an oxidant at normal temperature. In existing theories, ammonia molecules can act as complexing agents, while ammonium ions do not have complexing ability. Only under alkaline conditions can some heavy metal ions form complexes with ammonia. However, this technical solution uses ammonium salts as complexing agents under near-neutral conditions, which also breaks through the general understanding of those skilled in the art.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. For the stripping solution and stripping method of unqualified cadmium plating layer of the present invention, nitrite is used as an oxidant to oxidize the plated metal cadmium, and ammonium salt is used as a complexing agent to dissolve the cadmium oxide, so as to effectively strip the cadmium plating layer, overcoming the high pollution problem existing in the prior art using chromic acid stripping method;

[0024] 2. For the stripping solution and stripping method of unqualified cadmium plating layer of the present invention, the nitrite in the stripping solution has a corrosion inhibition effect on the steel substrate, and the steel substrate is not corroded during the stripping process. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a picture of the steel substrate without corrosion in the stripping solution for 8 hours in Test Example 1 of the present invention. EMBODIMENTS

[0026] The present invention will be described in detail below with reference to specific embodiments. Here, the illustrative embodiments of the present invention and the description are used to explain the present invention, but not to limit the present invention.

[0027] A stripping solution for unqualified cadmium plating layer includes 50 - 160 g / L of an oxidant and 50 - 160 g / L of a complexing agent, the pH of the stripping solution is 6 - 8, and the temperature of the stripping solution is at normal temperature;

[0028] The oxidant includes one or two of sodium nitrite and potassium nitrite;

[0029] The complexing agent includes one or two of ammonium sulfate, ammonium bicarbonate, ammonium carbonate, and ammonium chloride.

[0030] When the pH of the stripping solution is too low, the pH is increased with a solution of a weak base salt or an alkali solution. Preferably, the pH is increased with a 10% sodium carbonate solution or a 10% sodium hydroxide solution.

[0031] When the pH of the stripping solution is too high, an acid is added to lower the pH. Preferably, the pH is lowered with 20% dilute nitric acid or 20% dilute sulfuric acid.

[0032] Preferably, the stripping solution is prepared as follows: Add 4 / 5 of water to the stripping tank according to the volume of the bath solution, add an oxidizing agent and a complexing agent according to the formula requirements of the stripping solution, stir to dissolve it, adjust the pH of the stripping solution to 6-8, and add water to the specified volume.

[0033] A method for removing an unqualified cadmium plating layer includes the following technological processes:

[0034] (1) Add water and hydrochloric acid to the film removal tank to prepare the film removal bath solution, where the mass fraction of hydrogen chloride is 3%-10%, and the bath solution temperature is at room temperature;

[0035] (2) Put the unqualified cadmium-plated parts into the film removal tank to remove the passivation film on the unqualified cadmium plating layer until the color of the passivation film fades completely;

[0036] (3) Put the unqualified cadmium-plated parts after film removal into the stripping tank for stripping until the cadmium plating layer is completely stripped.

[0037] During the stripping process, yellow iron oxides appear suspended in the stripping solution in the stripping tank, and these suspended matters need to be removed.

[0038] Preferably, filter the stripping solution with a filter to remove the precipitate.

[0039] During the stripping process, as the oxidizing agent and the complexing agent are consumed and the stripping solution is carried out, it is necessary to replenish the oxidizing agent and the complexing agent to the stripping tank.

[0040] Preferably, in production, the oxidizing agent and the complexing agent are replenished to the stripping tank in a ratio of 1:(0.3-0.8) by mass ratio of the oxidizing agent to the complexing agent.

[0041] During the stripping process, hydrogen ions are consumed in the reaction of nitrite oxidizing the cadmium plating layer, and the pH of the stripping solution will gradually increase, and it is necessary to replenish acid to adjust the pH.

[0042] Preferably, adjust the pH of the stripping solution to 6-8 with dilute nitric acid with a mass fraction of 20%.

[0043] Preferably, adjust the pH of the stripping solution to 6-8 with dilute sulfuric acid with a mass fraction of 20%.

[0044] Adding dilute nitric acid to the stripping solution, the nitrate ion can be used as an oxidizing agent to reduce the consumption of nitrite. Therefore, this solution more preferably uses dilute nitric acid to adjust the pH of the stripping solution.

[0045] When the pH of the stripping solution is lower than the lower limit of the process, preferably, adjust the pH of the stripping solution to 6-8 with a sodium carbonate solution with a mass fraction of 10%. Example

[0046] A selective stripping process for unqualified cadmium plating layer is as follows:

[0047] The oxidizing agent sodium nitrite is 100 g / L, the complexing agent ammonium chloride is 100 g / L, the pH of the stripping solution is 7, and the operation is carried out at room temperature until the plating layer is completely stripped. The ammonium chloride also has the function of an accelerator to increase the stripping speed of the cadmium plating layer.

[0048] 1. Prepare the film removal bath solution:

[0049] Add 2 / 3 of the water to the film removal bath according to the volume of the bath solution. Add concentrated hydrochloric acid under stirring and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 3%.

[0050] 2. Prepare the stripping solution:

[0051] Add 4 / 5 of the water to the stripping bath according to the volume of the bath 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 7 with a 10% sodium carbonate solution, and then add water to the specified volume.

[0052] 3. Remove the film:

[0053] Put the unqualified cadmium plated parts into the film removal bath to remove the passivation film until the color of the passivation film completely fades. After taking out of the bath, perform 3 times of water washing.

[0054] 4. Strip the plating:

[0055] Put the unqualified cadmium plated parts after film removal into the stripping bath for stripping until the plating layer is completely stripped. After 3 times of water washing, transfer them to the cadmium plating process.

[0056] During production, add sodium nitrite and ammonium chloride to the stripping bath in a mass ratio of 1:0.4.

[0057] During production, use a filter to filter the stripping solution to remove precipitates.

[0058] After the pH of the stripping solution increases, add 20% dilute nitric acid to the stripping bath to adjust the pH of the stripping solution to 6 - 8.

[0059] When the pH of the stripping solution is lower than the lower limit of the process, use a 10% sodium carbonate solution to adjust the pH of the stripping solution to 6 - 8. Example

[0060] A selective stripping process for unqualified cadmium plating layer is as follows:

[0061] The oxidizing agent sodium nitrite is 100 g / L, the complexing agent ammonium sulfate is 100 g / L, the pH of the stripping solution is 7, and the operation is carried out at room temperature until the plating layer is completely stripped.

[0062] 1. Prepare the film removal bath solution:

[0063] Add 2 / 3 of water to the film removal tank according to the volume of the bath solution. Add concentrated hydrochloric acid with stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 4%.

[0064] 2. Prepare the stripping solution:

[0065] Add 4 / 5 of water to the stripping tank according to the volume of the bath solution. Add sodium nitrite and ammonium sulfate according to the formula requirements of the stripping solution, stir to dissolve them, adjust the pH of the stripping solution to 7 with a 10% sodium carbonate solution, and add water to the specified volume.

[0066] 3. Remove the film:

[0067] Put the unqualified cadmium-plated parts into the film removal tank to remove the passivation film until the color of the passivation film fades completely. After taking them out of the tank, wash them 3 times with water.

[0068] 4. Strip the plating:

[0069] Put the unqualified cadmium-plated parts after film removal into the stripping tank to strip the plating until the plating layer fades completely. After washing 3 times with water, transfer them to the cadmium plating process.

[0070] During production, add sodium nitrite and ammonium sulfate to the stripping tank in a mass ratio of 1:0.6.

[0071] During production, use a filter to filter the stripping solution to remove precipitates.

[0072] After the pH of the stripping solution increases, add 20% dilute nitric acid to the stripping tank to adjust the pH of the stripping solution to 6 - 8.

[0073] When the pH of the stripping solution is lower than the lower limit of the process, use a 10% sodium carbonate solution to adjust the pH of the stripping solution to 6 - 8. Example

[0074] A selective stripping process for unqualified cadmium plating layer is as follows:

[0075] The oxidizing agent sodium nitrite is 120 g / L, the complexing agent ammonium bicarbonate is 120 g / L, the pH of the stripping solution is 6.5, operating at room temperature until the plating layer fades completely; the ammonium bicarbonate also has the function of stabilizing the pH of the stripping solution.

[0076] 1. Prepare the film removal bath solution:

[0077] Add 2 / 3 of water to the film removal tank according to the volume of the bath solution. Add concentrated hydrochloric acid with stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 5%.

[0078] 2. Prepare the stripping solution:

[0079] Add 4 / 5 of water to the stripping bath based on the volume of the bath solution. Add sodium nitrite and ammonium bicarbonate according to the formula requirements of the stripping solution, stir to dissolve it, adjust the pH of the stripping solution to 6.5 with 20% dilute nitric acid, and add water to the specified volume.

[0080] 3. Film removal:

[0081] Put the unqualified cadmium-plated parts after film removal into the stripping bath for stripping until the color of the passivation film fades completely. After taking out of the bath, conduct 3 times of water washing.

[0082] 4. Stripping:

[0083] Put the unqualified cadmium-plated parts after film removal into the stripping bath for stripping until the coating fades completely. After 3 times of water washing, transfer to the cadmium plating process.

[0084] During production, add sodium nitrite and ammonium bicarbonate to the stripping bath in a mass ratio of 1:0.7.

[0085] During production, filter the stripping solution with a filter to remove precipitates.

[0086] After the pH of the stripping solution increases, add 20% dilute nitric acid to the stripping bath to adjust the pH of the stripping solution to 6 - 8.

[0087] When the pH of the stripping solution is lower than the lower limit of the process, adjust the pH of the stripping solution to 6 - 8 with 10% sodium carbonate solution. Example

[0088] A selective stripping process for unqualified cadmium coatings is as follows:

[0089] The oxidizing agent sodium nitrite is 100 g / L, the complexing agent ammonium carbonate is 100 g / L, the pH of the stripping solution is 7, operating at room temperature until the coating fades completely; the ammonium carbonate is converted into ammonium bicarbonate under the condition of pH 7, and the ammonium bicarbonate has the function of stabilizing the pH of the stripping solution.

[0090] 1. Preparation of film removal bath solution:

[0091] Add 2 / 3 of water to the film removal bath according to the volume of the bath solution. Add concentrated hydrochloric acid under stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 6%.

[0092] 2. Preparation of stripping solution:

[0093] Add 4 / 5 of water to the stripping bath according to the volume of the bath solution. Add sodium nitrite and ammonium carbonate according to the formula requirements of the stripping solution, stir to dissolve it, adjust the pH of the stripping solution to 7 with 20% dilute nitric acid, and add water to the specified volume.

[0094] 3. Film removal:

[0095] Put the unqualified cadmium-plated parts into the film-removing tank to remove the passivation film until the color of the passivation film completely fades. After taking them out of the tank, conduct three times of water washing.

[0096] 4. Stripping:

[0097] Put the unqualified cadmium-plated parts after film removal into the stripping tank for stripping until the coating completely fades. After three times of water washing, transfer them to the cadmium plating process.

[0098] During production, add sodium nitrite and ammonium carbonate to the stripping solution according to the mass ratio of 1:0.5.

[0099] During production, use a filter to filter the stripping solution to remove the precipitate.

[0100] After the pH of the stripping solution increases, add dilute nitric acid with a mass fraction of 20% to the stripping tank to adjust the pH of the stripping solution to 6 - 8.

[0101] When the pH of the stripping solution is lower than the lower limit of the process, use a sodium carbonate solution with a mass fraction of 10% to adjust the pH of the stripping solution to 6 - 8. Example

[0102] A selective stripping process for unqualified cadmium coatings is as follows:

[0103] Oxidizing agent potassium nitrite 50 g / L, complexing agent ammonium carbonate 50 g / L, pH of the stripping solution is 7.5, operate at room temperature until the coating completely fades.

[0104] 1. Prepare the film-removing bath solution:

[0105] Add 2 / 3 of water to the film-removing tank according to the volume of the bath solution. Add concentrated hydrochloric acid while stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film-removing bath solution is 7%.

[0106] 2. Prepare the stripping solution:

[0107] Add 4 / 5 of water to the stripping tank according to the volume of the bath solution. Add potassium nitrite and ammonium carbonate according to the formula requirements of the stripping solution, stir to dissolve them, adjust the pH of the stripping solution to 7.5 with dilute sulfuric acid with a mass fraction of 20%, and add water to the specified volume.

[0108] 3. Film removal:

[0109] Put the unqualified cadmium-plated parts into the film-removing tank to remove the passivation film until the color of the passivation film completely fades. After taking them out of the tank, conduct three times of water washing.

[0110] 4. Stripping:

[0111] Put the unqualified cadmium-plated parts after film removal into the stripping tank for stripping until the coating completely fades. After three times of water washing, transfer them to the cadmium plating process.

[0112] During production, potassium nitrite and ammonium carbonate are replenished into the stripping bath at a mass ratio of 1:0.4.

[0113] During production, a filter press is used to filter the stripping solution to remove precipitates.

[0114] After the pH of the stripping solution increases, dilute nitric acid with a mass fraction of 20% is replenished into the stripping bath to adjust the pH of the stripping solution to 6 - 8.

[0115] When the pH of the stripping solution is lower than the lower limit of the process, sodium carbonate solution with a mass fraction of 10% is used to adjust the pH of the stripping solution to 6 - 8. Example

[0116] A selective stripping process for unqualified cadmium plating layer is as follows:

[0117] The oxidant potassium nitrite is 160 g / L, the complexing agent ammonium sulfate is 80 g / L, the pH of the stripping solution is 8, and the operation is carried out at room temperature until the plating layer is completely stripped.

[0118] 1. Prepare the film-removing bath solution:

[0119] Add 2 / 3 of the water into the film-removing bath according to the volume of the bath solution. Add concentrated hydrochloric acid while stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film-removing bath solution is 8%.

[0120] 2. Prepare the stripping solution:

[0121] Add 4 / 5 of the water into the stripping bath according to the volume of the bath solution. Add potassium nitrite and ammonium sulfate according to the formula requirements of the stripping solution, stir to dissolve it, adjust the pH of the stripping solution to 8 with potassium hydroxide solution with a mass fraction of 10%, and add water to the specified volume.

[0122] 3. Remove the film:

[0123] Put the unqualified cadmium-plated parts into the film-removing bath to remove the passivation film until the color of the passivation film completely fades. After taking out of the bath, carry out 3 times of water washing.

[0124] 4. Strip the plating:

[0125] Put the unqualified cadmium-plated parts after film removal into the stripping bath for stripping until the plating layer is completely stripped. After 3 times of water washing, transfer to the cadmium plating process.

[0126] During production, potassium nitrite and ammonium sulfate are replenished into the stripping bath at a mass ratio of 1:0.3.

[0127] During production, a filter press is used to filter the stripping solution to remove precipitates.

[0128] After the pH of the stripping solution increases, dilute nitric acid with a mass fraction of 20% is replenished into the stripping bath to adjust the pH of the stripping solution to 6 - 8.

[0129] When the pH of the stripping solution is lower than the lower limit of the process, adjust the pH of the stripping solution to 6 - 8 with a 10% sodium carbonate solution by mass fraction. Example

[0130] A selective stripping process for unqualified cadmium plating layers is as follows:

[0131] Oxidant sodium nitrite 80 g / L, complexing agent ammonium sulfate 80 g / L, complexing agent ammonium bicarbonate 80 g / L, pH of the stripping solution is 7.5, operate at room temperature until the plating layer is completely stripped.

[0132] 1. Prepare the film removal bath solution:

[0133] Add 2 / 3 of the water to the film removal bath according to the bath solution volume, add concentrated hydrochloric acid under stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 9%, and the bath solution is at room temperature.

[0134] 2. Prepare the stripping solution:

[0135] Add 4 / 5 of the water to the stripping bath according to the bath solution volume, add sodium nitrite, ammonium sulfate and ammonium bicarbonate according to the formula requirements of the stripping solution, stir to dissolve it, adjust the pH of the stripping solution to 7.5 with a 10% sodium carbonate solution by mass fraction, and add water to the specified volume.

[0136] 3. Remove the film:

[0137] Put the unqualified cadmium plated parts into the film removal bath to remove the passivation film until the color of the passivation film completely fades, and perform 3 - time water washing after taking them out of the bath.

[0138] 4. Strip the plating:

[0139] Put the unqualified cadmium plated parts after film removal into the stripping bath for stripping until the plating layer is completely stripped, and transfer them to the cadmium plating process after 3 - time water washing.

[0140] During production, add sodium nitrite, ammonium sulfate, and ammonium bicarbonate to the stripping bath according to the mass ratio of 1:0.4:0.4.

[0141] During production, use a filter to filter the stripping solution to remove the precipitate.

[0142] After the pH of the stripping solution increases, add 20% dilute nitric acid by mass fraction to the stripping bath to adjust the pH of the stripping solution to 6 - 8.

[0143] When the pH of the stripping solution is lower than the lower limit of the process, adjust the pH of the stripping solution to 6 - 8 with a 10% sodium carbonate solution by mass fraction. Example

[0144] A selective stripping process for unqualified cadmium plating layers is as follows:

[0145] The oxidant sodium nitrite is 100 g / L, the complexing agent ammonium sulfate is 50 g / L, the complexing agent ammonium carbonate is 50 g / L, the pH of the stripping solution is 6.8, and the operation is carried out at room temperature until the coating is completely stripped.

[0146] 1. Prepare the film removal bath solution:

[0147] Add 2 / 3 of the water to the film removal bath according to the volume of the bath solution. Add concentrated hydrochloric acid while stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 10%, and the bath solution is at room temperature.

[0148] 2. Prepare the stripping solution:

[0149] Add 4 / 5 of the water to the stripping bath according to the volume of the bath solution. Add sodium nitrite, ammonium sulfate and ammonium carbonate according to the formula requirements of the stripping solution, stir to dissolve them, adjust the pH of the stripping solution to 6.8 with 20% dilute sulfuric acid, and add water to the specified volume.

[0150] 3. Remove the film:

[0151] Put the unqualified cadmium-plated parts into the film removal bath to remove the passivation film until the color of the passivation film completely fades. After taking out of the bath, perform 3 times of water washing.

[0152] 4. Strip the coating:

[0153] Put the unqualified cadmium-plated parts after film removal into the stripping bath for stripping until the coating is completely stripped. After 3 times of water washing, transfer them to the cadmium plating process.

[0154] During production, sodium nitrite, ammonium sulfate and ammonium carbonate are replenished to the stripping bath in a mass ratio of 1:0.2:0.2.

[0155] During production, use a filter to filter the stripping solution to remove precipitates.

[0156] After the pH of the stripping solution increases, replenish 20% dilute nitric acid to the stripping bath to adjust the pH of the stripping solution to 6 - 8.

[0157] When the pH of the stripping solution is lower than the lower limit of the process, use 10% sodium carbonate solution to adjust the pH of the stripping solution to 6 - 8. Example

[0158] A selective stripping process for unqualified cadmium coatings is as follows:

[0159] The oxidant sodium nitrite is 160 g / L, the complexing agent ammonium bicarbonate is 50 g / L, the complexing agent ammonium chloride is 50 g / L, the pH of the stripping solution is 7.5, and the operation is carried out at room temperature until the coating is completely stripped.

[0160] 1. Prepare the film removal bath solution:

[0161] Add 4 / 5 of water to the film removal tank according to the volume of the bath solution. Add concentrated hydrochloric acid under stirring, and then add water to the specified volume. The mass fraction of hydrogen chloride in the prepared film removal bath solution is 5.5%, and the bath solution is at room temperature.

[0162] 2. Prepare the stripping solution:

[0163] Add 4 / 5 of water to the stripping tank according to the volume of the bath solution. Add sodium nitrite, ammonium bicarbonate, and ammonium chloride according to the formula requirements of the stripping solution, stir to dissolve them, adjust the pH of the stripping solution to 7.5 with a 10% sodium carbonate solution, and add water to the specified volume.

[0164] 3. Remove the film:

[0165] Put the unqualified cadmium-plated parts into the film removal tank to remove the passivation film until the color of the passivation film fades completely. After taking them out of the tank, perform 3 times of water washing.

[0166] 4. Strip the coating:

[0167] Put the unqualified cadmium-plated parts after film removal into the stripping tank to strip the coating until the coating is completely removed. After 3 times of water washing, transfer them to the cadmium plating process.

[0168] During production, add sodium nitrite, ammonium bicarbonate, and ammonium chloride to the stripping tank according to the mass ratio of 1:0.2:0.2.

[0169] During production, use a filter to filter the stripping solution to remove precipitates.

[0170] After the pH of the stripping solution increases, add 20% dilute sulfuric acid to the stripping tank to adjust the pH of the stripping solution to 6 - 8.

[0171] When the pH of the stripping solution is lower than the lower limit of the process, use a 10% sodium carbonate solution to adjust the pH of the stripping solution to 6 - 8.

[0172] Test Example 1:

[0173] Heat treat a 45 steel plate of 50mm×100mm×2mm to form sorbite steel, then perform ultrasonic degreasing, sandpaper grinding, chemical degreasing, and activate it with 10% dilute sulfuric acid for 1 min. Then prepare a cadmium-plated sample by potassium chloride cadmium plating, with a coating thickness of 10 μm. The cadmium-plated sample is passivated with low-chromium hexavalent chromium color. Put the cadmium-plated sample into the film removal tank to remove the passivation film according to the process in Example 1, then hang it in the stripping solution for 15 min, take it out, perform three times of water washing, dry it, and observe the surface of the sample. The cadmium plating layer has been completely removed, and there is no carbon hanging ash on the surface of the steel plate.

[0174] Put the stripped steel plate sample into the newly prepared stripping solution for 8 h, and there is no observable change on the surface of the steel plate. See Figure 1, there is also no observable change in the stripping solution.

[0175] Test Example 2:

[0176] A 50mm×100mm×2mm aluminum plate of type 2024 was subjected to ultrasonic degreasing, chemical degreasing, alkaline etching, brightening, and micro-etching, and then underwent the first chemical zinc deposition, zinc stripping, the second chemical zinc deposition, electroless nickel plating, and potassium chloride cadmium plating. The thickness of the electroless nickel plating layer was 5μm, and the thickness of the cadmium plating layer was 10μm. The cadmium plating layer was passivated with low-chromium hexavalent chromium color. According to the process of Example 1, the cadmium-plated sample was placed in the film removal tank to remove the passivation film, and then hung in the stripping solution for 15 minutes. After taking it out, it was washed three times with water and dried. After observing the surface of the sample, the cadmium plating layer had been completely stripped, and the electroless nickel plating layer and the aluminum alloy substrate were intact.

[0177] Test Example 3:

[0178] The steel plate sample after stripping in Test Example 1 was pickled and activated and then re-plated with potassium chloride cadmium plating. The thickness of the plating layer was 10μm, and it was passivated with low-chromium hexavalent chromium color. According to GB / T 5270-2005 "Review of Test Methods for Adhesion of Metallic Coatings on Metallic Substrates - Electrodeposited and Chemically Deposited Coatings", the adhesion of the coating was tested by the thermal shock method. The sample was placed in a heating furnace and heated to 250°C for 30 minutes, then taken out and suddenly cooled in water at room temperature. The coating did not show blistering or peeling, indicating that the coating had good adhesion. The test shows that the method for removing the unqualified cadmium plating layer of the present invention does not reduce the surface quality of medium carbon steel workpieces.

[0179] Test Example 4:

[0180] The electroless nickel-plated aluminum alloy workpiece obtained after stripping in Test Example 2 was pickled and activated and then re-plated with electroless nickel plating, and then potassium chloride cadmium plating was carried out. The thickness of the cadmium plating layer was 10μm, and it was passivated with low-chromium hexavalent chromium color. According to GB / T 5270-2005 "Review of Test Methods for Adhesion of Metallic Coatings on Metallic Substrates - Electrodeposited and Chemically Deposited Coatings", the adhesion of the coating was tested by the thermal shock method. The sample was placed in a heating furnace and heated to 220°C for 30 minutes, then taken out and suddenly cooled in water at room temperature. The coating did not show blistering or peeling, indicating that the coating had good adhesion. The test shows that the method for removing the unqualified cadmium plating layer of the present invention does not reduce the surface quality of electroless nickel-plated aluminum alloy workpieces.

[0181] Test Example 5:

[0182] The stripping solution was prepared according to Example 1, and a cadmium-plated steel substrate that had been placed for half a year was used for the stripping test. The plated part was an M8×40 screw. During the stripping process, it was observed that a small amount of yellow iron oxide was suspended in the stripping solution.

[0183] After cadmium plating on iron and steel parts, a small amount of iron atoms at the junction of the iron and steel substrate and the coating can diffuse into the cadmium plating layer, and a small amount of cadmium atoms can also diffuse into the iron and steel substrate. The iron oxide is formed after the iron atoms in the cadmium plating layer are oxidized.

[0184] Test Example 6:

[0185] Prepare test solutions: sodium nitrite 100 g / L, ammonium sulfate 100 g / L, and its pH is 6.3. The pH of Test Solutions No. 1 and No. 2 is adjusted to 5.5 and 6.0 with 10% dilute sulfuric acid, and the pH of Test Solutions No. 3-7 is adjusted to 6.5, 7.0, 7.5, 8.0, and 8.5 with 10% sodium hydroxide solution respectively. Use the cadmium-plated samples prepared in Test Example 1 for stripping tests. Operate at room temperature. Within the tested pH range, the cadmium plating layer can be effectively stripped. However, when the pH is 5.5, the test solution has a tendency to slowly dissolve the iron and steel substrate. When the pH is 8.5, ammonia gas escapes from the test solution. Based on this, the present invention stipulates the pH process parameter as 6-8.

[0186] Test Example 7:

[0187] Prepare test solution A: sodium nitrite 100 g / L, ammonium sulfate 100 g / L, and adjust the pH to 4.5 with 10% dilute sulfuric acid. Under this pH condition, ammonium ions cannot undergo hydrolysis reaction to produce ammonia molecules.

[0188] Prepare test solution B: sodium nitrite 100 g / L, sodium sulfate 100 g / L, and adjust the pH to 4.5 with 10% dilute sulfuric acid.

[0189] Use cadmium-plated samples of iron and steel materials for testing. The cadmium-plated samples are M8×40 screws. After removing the passivation film of the cadmium-plated samples in 5% dilute hydrochloric acid, put them into test solution A for 25 minutes, and the cadmium plating layer completely disappears. After removing the passivation film of the cadmium-plated samples in 5% dilute hydrochloric acid, put them into test solution B for 25 minutes, and no observable change occurs to the cadmium plating layer.

[0190] In test solution A, sodium nitrite oxidizes metallic cadmium to cadmium oxide, and at the same time ammonium ions react with cadmium oxide to form a cadmium complex ion and dissolve in test solution A. Finally, the cadmium plating layer is stripped.

[0191] In test solution B, sodium nitrite oxidizes metallic cadmium to cadmium oxide, and cadmium oxide forms a protective film on the cadmium plating layer, preventing further reaction between sodium nitrite and metallic cadmium. The test shows that in the absence of the coordination effect of ammonium ions, sodium nitrite alone cannot strip the cadmium plating layer.

[0192] Comparative Example 1:

[0193] Prepare a cadmium - plated sample according to Test Example 1, with the thickness of the cadmium - plated layer being 10 μm. Remove the cadmium - plated layer according to the stripping process of Example 2. The time required to completely remove the cadmium - plated layer is 25 min. Compared with Test Example 1, it can be seen that when ammonium chloride is used as a complexing agent, ammonium chloride also has the function of an accelerator at the same time, which can significantly improve the dissolution rate of the cadmium - plated layer.

[0194] Comparative Example 2:

[0195] Prepare 1 L of hydrochloric acid stripping solution in a glass beaker, with the volume ratio of concentrated hydrochloric acid to water being 1:1.

[0196] Conduct a comparative test using the cadmium - plated sample prepared in Test Example 1. Place the cadmium - plated sample in the above - mentioned hydrochloric acid stripping solution for observation. After the cadmium - plated layer is completely removed, take out the sample, rinse it with water, dry it, and observe that there is a layer of gray - black carbon hanging ash on the surface of the sample.

[0197] Comparative Example 3:

[0198] Pickle and activate the steel plate sample with carbon hanging ash after stripping in Comparative Example 2, and then re - plate it with potassium chloride cadmium plating. The thickness of the plating layer is 10 μm, and hexavalent chromium low - chromium color passivation is used. According to GB / T 5270 - 2005 "Review of Test Methods for Adhesion of Metallic Coatings Electrodeposited and Chemically Deposited on Metallic Substrates", the adhesion of the plating layer is tested by the thermal shock method. Place the prepared cadmium - plated sample in a heating furnace, heat it to 250 °C and keep it warm for 30 min, then take it out and suddenly cool it in water at room temperature. Blistering occurs on the plating layer, and its adhesion does not meet the requirements of the industry standard.

[0199] Comparative Example 4:

[0200] Prepare 1 L of hydrochloric acid stripping solution in a glass beaker, with the volume ratio of concentrated hydrochloric acid to water being 1:1.

[0201] Conduct a comparative test using the aluminum alloy cadmium - plated sample prepared in Test Example 2. Place the cadmium - plated sample in the above - mentioned hydrochloric acid stripping solution for observation. After the cadmium - plated layer is completely removed, take out the sample and rinse it with water. Then re - plate it with electroless nickel plating and potassium chloride cadmium plating. The thickness of the cadmium - plated layer is 10 μm, and hexavalent chromium low - chromium color passivation is used. According to GB / T 5270 - 2005 "Review of Test Methods for Adhesion of Metallic Coatings Electrodeposited and Chemically Deposited on Metallic Substrates", the adhesion of the plating layer is tested by the thermal shock method. Place the above - mentioned cadmium - plated sample in a heating furnace, heat it to 220 °C and keep it warm for 30 min, then take it out and suddenly cool it in water at room temperature. Blistering occurs on the plating layer, and its adhesion does not meet the requirements of the industry standard. Tests show that when using hydrochloric acid to strip the cadmium - plated layer of aluminum alloy parts, after removing the cadmium - plated layer, hydrochloric acid will corrode the chemically deposited zinc layer and the aluminum alloy matrix through the pores of the electroless nickel plating layer. Therefore, blistering occurs on the cadmium - plated layer at the corroded parts of the chemically deposited zinc layer and the aluminum alloy matrix after the thermal shock test.

[0202] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the protection scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. The protection scope of the patent for the present invention shall be subject to the appended claims.

Claims

1. A stripping solution for unqualified cadmium plating layer, characterized in that: It includes 50 - 160 g / L of oxidant and 50 - 160 g / L of complexing agent, and the pH of the stripping solution is 6 - 8; The oxidant includes one or two of sodium nitrite and potassium nitrite; The complexing agent includes one or two of ammonium sulfate, ammonium bicarbonate, ammonium carbonate, and ammonium chloride.

2. The stripping solution for unqualified cadmium plating layer according to claim 1, characterized in that: Adjust the pH of the stripping solution with 10% sodium carbonate solution or 10% sodium hydroxide solution by mass fraction.

3. The stripping solution for unqualified cadmium plating layer according to claim 1, characterized in that: Adjust the pH of the stripping solution with 20% dilute sulfuric acid or 20% dilute nitric acid by mass fraction.

4. The stripping solution for unqualified cadmium plating layer according to claim 1, wherein, The stripping solution is prepared as follows: Add 4 / 5 of water to the stripping tank according to the volume of the bath solution, add the oxidant and the complexing agent, stir to dissolve them, adjust the pH of the prepared stripping solution to 6 - 8, and add water to the specified volume.

5. The stripping solution for unqualified cadmium plating layer according to claim 1, characterized in that, The working temperature of the stripping solution is normal temperature.

6. A method for removing an unqualified cadmium plating layer, characterized in that, It includes the following technological processes: (1) Add water and hydrochloric acid to the film removal tank to prepare the film removal bath solution, where the mass fraction of hydrogen chloride is 3% - 10%, and the bath solution temperature is normal temperature; (2) Put the unqualified cadmium - plated parts into the film removal tank to remove the passivation film on the unqualified cadmium - plated layer until the color of the passivation film completely fades; (3) Put the unqualified cadmium - plated parts after film removal into the stripping tank containing the stripping solution as described in claim 1 for stripping until the cadmium - plated layer is completely stripped.

7. The method for removing the unqualified cadmium plating layer according to claim 6, wherein: During production, add oxidant and complexing agent to the stripping solution, and the mass ratio of the added oxidant to the complexing agent is 1:(0.3 - 0.8).

8. The method for removing the unqualified cadmium plating layer according to claim 6, characterized in that: During production, filter the stripping solution with a filter to remove precipitates.

9. The method for removing the unqualified cadmium plating layer according to claim 6, wherein: During production, after the pH of the stripping solution increases, adjust the pH of the stripping solution to 6 - 8 with 20% dilute nitric acid or 20% dilute sulfuric acid by mass fraction.

10. The method for removing the unqualified cadmium plating layer according to claim 6, characterized in that: During production, when the pH of the stripping solution is lower than the lower limit of the process, adjust the pH of the stripping solution to 6 - 8 with 10% sodium carbonate solution by mass fraction.

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