A method for stripping off a zinc-cadmium alloy plating layer of a high-strength steel that does not meet the specifications

Abstract

The application discloses a high-strength steel unqualified zinc-cadmium alloy plating layer stripping method, and its stripping process is as follows: 80-150 g / L of sodium nitrite, 50-100 g / L of ammonium bicarbonate, 50-100 g / L of ammonium chloride, the pH of the stripping solution is 6-8, and normal temperature operation is adopted. The sodium nitrite is used as an oxidizing agent and a steel base inhibitor for the zinc-cadmium alloy plating layer, the ammonium bicarbonate is used as a complexing agent for zinc ions and cadmium ions and a pH buffer for the plating solution, and the ammonium chloride is used as a complexing agent for zinc ions and cadmium ions and a promoter for dissolving the zinc-cadmium alloy plating layer. When the pH of the stripping solution is too high, dilute nitric acid is used to reduce the pH, and when the pH is too low, sodium carbonate solution is used to increase the pH. The technical scheme of the application is used to replace the chromium acid stripping method, and the high pollution problem caused by the use of the chromium acid is overcome.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal surface treatment, and particularly relates to a stripping method for unqualified zinc-cadmium alloy plating layer of high-strength steel. BACKGROUND

[0002] The zinc-cadmium alloy plating layer has high corrosion resistance, and can be used to replace cadmium plating layer in the field of aerospace, thereby reducing the environmental pollution caused by cadmium plating. A Chinese invention with the announcement number CN 113373442 B discloses a zinc-cadmium alloy plating layer, and the mass fraction of cadmium in the plating layer is 20% to 40%. The zinc-cadmium alloy plating layer can be used as a protective layer of high-strength steel.

[0003] Carbon in the high-strength steel exists in the form of cementite (Fe3C). After heat treatment, the cementite is uniformly distributed in the steel substrate. After pickling of the workpiece, iron atoms on the surface of the workpiece dissolve in the acid, and the cementite is attached or inlaid on the surface of the workpiece, which is called carbon hanging ash in the industry. Hydrogen atoms generated during pickling of the high-strength steel can penetrate into the substrate to cause hydrogen embrittlement, which seriously affects the performance. If hydrochloric acid is used to strip the unqualified zinc-cadmium alloy plating layer of the high-strength steel workpiece, carbon hanging ash is easily formed on the surface of the workpiece after stripping. After zinc-cadmium alloy plating again, the plating layer has poor adhesion. If stripping is performed twice, the hydrogen embrittlement generated will cause the workpiece to be scrapped. Therefore, some aerospace enterprises have to use chromic acid stripping method to strip the zinc-cadmium alloy plating layer, but the use of this method has the problem of high pollution of hexavalent chromium.

[0004] The invention patent with the announcement number CN 111690933 B discloses a stripping method for unqualified cadmium plating layer. The stripping process is as follows: sulfuric acid 60 to 200 g / L, 2,4-dinitrobenzenesulfonic acid hydrate 20 to 60 g / L, and hexamethylenetetramine 10 to 50 g / L. The unqualified cadmium-plated workpiece is immersed in the stripping solution at room temperature until the plating layer is completely stripped. Although hexamethylenetetramine can play a role in corrosion inhibition, the steel substrate is still dissolved by sulfuric acid to form a small amount of carbon hanging ash on the surface. After electroplating again, the plating layer has poor adhesion. SUMMARY

[0005] In order to overcome the defects of carbon hanging ash and hydrogen embrittlement caused by the use of hydrochloric acid stripping method for unqualified zinc-cadmium alloy plating layer of high-strength steel and the high pollution problem caused by the use of chromic acid stripping method, the application provides a stripping method for unqualified zinc-cadmium alloy plating layer of high-strength steel. In order to achieve the above purpose, the application adopts the following technical scheme:

[0006] A method for stripping off unqualified zinc-cadmium alloy coating on high-strength steel, the stripping-off process comprising: 80-150 g / L of sodium nitrite, 50-100 g / L of ammonium bicarbonate, 50-100 g / L of ammonium chloride, and the pH of the stripping-off solution being 6-8, and the operation being carried out at room temperature.

[0007] The sodium nitrite is used as an oxidizing agent to oxidize zinc and cadmium into zinc oxide and cadmium oxide.

[0008] The ammonium bicarbonate is used as a complexing agent to react with the zinc and cadmium oxides to form complex ions and dissolve in the stripping-off solution, and also as a pH buffer to keep the pH of the stripping-off solution stable.

[0009] The ammonium chloride is used as a complexing agent to react with the zinc and cadmium oxides to form complex ions and dissolve in the stripping-off solution, and also as an accelerator to increase the stripping-off speed of the zinc-cadmium alloy coating.

[0010] In some embodiments, the stripping-off solution is prepared according to the stripping-off process: 4 / 5 of water is added to the stripping-off tank according to the volume of the tank, and the sodium nitrite, ammonium bicarbonate and ammonium chloride are added and stirred to dissolve, and then water is added to the specified volume.

[0011] In some embodiments, the stripping-off method comprises the following processes:

[0012] (1) A film-removing solution is prepared in a film-removing tank, wherein the mass concentration of sulfuric acid is 50-80 g / L;

[0013] (2) A stripping-off solution is prepared in a stripping-off tank;

[0014] (3) The unqualified zinc-cadmium alloy plated part is put into the film-removing tank to remove the passivation film, and the operation is carried out at room temperature until the passivation film is completely removed;

[0015] (4) The film-removed unqualified zinc-cadmium alloy plated part is put into the stripping-off tank to strip off the zinc-cadmium alloy coating until the zinc-cadmium alloy coating is completely removed.

[0016] In some embodiments, the mass ratio of the sodium nitrite, ammonium bicarbonate and ammonium chloride added during production is 1:(0.2-0.5):(0.2-0.5).

[0017] In some embodiments, the pH of the stripping-off solution is adjusted to 6-8 by using dilute nitric acid with a mass concentration of 160-240 g / L during production.

[0018] In some embodiments, the pH of the stripping-off solution is adjusted to 6-8 by using sodium carbonate solution with a mass concentration of 160-240 g / L when the pH of the stripping-off solution is too low.

[0019] In some of the embodiments, the filter is used to circulate the stripping solution in production to remove the precipitates generated during the stripping process.

[0020] In the knowledge of the skilled in the art, ammonia molecules can be used as complexing agent, but ammonium ions have no complexing ability, and some heavy metal ions can form complexes with ammonia under alkaline conditions. However, the present technical solution uses ammonium salt as complexing agent under nearly neutral conditions, which breaks the general knowledge of the skilled in the art. It is well known that nitrite has oxidizing property under acidic conditions, but cannot be used as oxidizing agent under normal temperature and neutral conditions. The present technical solution uses sodium nitrite as oxidizing agent under normal temperature and nearly neutral conditions, which also breaks the general knowledge of the skilled in the art.

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

[0022] 1. The stripping method of the unqualified zinc-cadmium alloy coating of high-strength steel in the present application uses nitrite as oxidizing agent to oxidize the coating metals zinc and cadmium, and uses ammonium bicarbonate and ammonium chloride as complexing agent to dissolve the oxides of zinc and cadmium, so that the zinc-cadmium alloy coating is effectively removed in the nearly neutral stripping solution, and the problems of carbon hanging gray on the surface of the workpiece and hydrogen embrittlement of the substrate caused by the use of hydrochloric acid for stripping are overcome, and the high pollution problem caused by the use of chromic acid for stripping is also overcome;

[0023] 2. The stripping method of the unqualified zinc-cadmium alloy coating of high-strength steel in the present application has corrosion inhibition effect of nitrite on the steel substrate, and the stripping solution has no corrosion on the steel substrate during the stripping process;

[0024] 3. The stripping method of the unqualified zinc-cadmium alloy coating of high-strength steel in the present application uses ammonium bicarbonate as pH buffer, and the bicarbonate ion can release hydrogen ions to supplement the hydrogen ions consumed in the stripping reaction, so that the pH of the stripping solution remains stable; when the pH is too low, the bicarbonate ion reacts with hydrogen ions to form carbonic acid, and carbon dioxide gas is generated after the decomposition of carbonic acid and escapes from the stripping solution;

[0025] 4. The stripping method of the unqualified zinc-cadmium alloy coating of high-strength steel in the present application uses ammonium chloride as accelerator, which effectively improves the stripping speed of the zinc-cadmium alloy coating. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 Figure 6 is a photo of a 45 steel plate placed in the stripping solution for 8 hours without rust. EMBODIMENTS

[0027] The present application will be described in detail below with specific examples, and the illustrative examples and descriptions of the present application are used to explain the present application, but are not intended to limit the present application.

[0028] A method for stripping off unqualified zinc-cadmium alloy coating from high-strength steel, which uses sodium nitrite as oxidant to oxidize zinc and cadmium into zinc oxide and cadmium oxide; uses ammonium bicarbonate and ammonium chloride as complexing agent to react with zinc and cadmium oxide to form complex ions and dissolve in the stripping solution; uses ammonium bicarbonate as pH buffer to stabilize the pH of the stripping solution; and uses ammonium chloride as accelerator to increase the dissolution rate of the stripping solution to zinc-cadmium alloy coating.

[0029] Preferably, the process parameters of the stripping method are as follows: sodium nitrite 80-150 g / L, ammonium bicarbonate 50-100 g / L, ammonium chloride 50-100 g / L, and the pH of the stripping solution is 6-8, and the operation is at room temperature.

[0030] Preferably, the stripping solution is prepared according to the stripping process: 4 / 5 of water is added to the stripping tank according to the volume of the tank, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added, and stirring is performed to dissolve them, and water is added to the specified volume.

[0031] Preferably, the stripping method comprises the following processes:

[0032] (1) Preparing a film removal solution in a film removal tank, wherein the mass concentration of sulfuric acid is 50-80 g / L;

[0033] (2) Preparing a stripping solution in a stripping tank;

[0034] (3) Placing the unqualified zinc-cadmium alloy plated part into the film removal tank to remove the passivation film, and the operation is at room temperature until the passivation film is completely removed;

[0035] (4) Placing the unqualified zinc-cadmium alloy plated part after film removal into the stripping tank for stripping until the zinc-cadmium alloy coating is completely removed.

[0036] During the stripping process, sodium nitrite, ammonium bicarbonate, and ammonium chloride need to be added to the stripping tank as they are consumed. Preferably, the mass ratio of sodium nitrite, ammonium bicarbonate, and ammonium chloride added is 1:(0.2-0.5):(0.2-0.5).

[0037] During the stripping process, hydrogen ions participate in the oxidation reaction of sodium nitrite with zinc and cadmium, and the pH of the stripping solution continuously increases, so acid needs to be added to reduce the pH. Preferably, dilute nitric acid with a mass concentration of 160-240 g / L is used to adjust the pH of the stripping solution to 6-8, and the added nitric acid also serves as an oxidant for the zinc-cadmium alloy coating.

[0038] The pH of the stripping solution may also be too low, and preferably, a sodium carbonate solution with a mass concentration of 160-240 g / L is used to adjust the pH of the stripping solution to 6-8.

[0039] During the stripping process, some precipitates will be produced in the stripping solution. Preferably, a filter is used to recycle and filter the stripping solution to remove the precipitates. Example 1

[0040] A selective stripping process for a high-strength steel unqualified zinc-cadmium alloy coating is as follows:

[0041] Sodium nitrite 125 g / L, ammonium bicarbonate 75 g / L, ammonium chloride 75 g / L, the pH of the stripping solution is 7.2, normal temperature operation, and the coating is stripped until it is completely removed.

[0042] 1. Preparation of the film removal solution:

[0043] In the film removal tank, 4 / 5 of water is added according to the volume of the tank solution, concentrated sulfuric acid is added according to the process requirements, and after stirring, water is added to the specified volume. The mass concentration of sulfuric acid in the prepared film removal solution is 50 g / L.

[0044] 2. Preparation of the stripping solution:

[0045] 4 / 5 of water is added to the stripping tank according to the volume of the tank solution, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added according to the formula requirements of the stripping solution, and water is added to the specified volume after stirring to dissolve them.

[0046] 3. Film removal:

[0047] The unqualified zinc-cadmium alloy plated parts are placed in the film removal tank to remove the passivation film. The surface of the parts is obviously bubbling with hydrogen gas, and after being taken out of the tank, they are washed with water for 3 times.

[0048] 4. Stripping:

[0049] After the film removal, the unqualified zinc-cadmium alloy plated parts are placed in the stripping tank for stripping until the coating is completely removed. After being washed with water for 3 times, they are transferred to the zinc-cadmium alloy plating process.

[0050] In production, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added to the stripping tank at a mass ratio of 1:0.3:0.3. When the pH rises, dilute nitric acid with a mass concentration of 200 g / L is added to the stripping tank to adjust the pH of the stripping solution to 6-8. When the pH is too low, a sodium carbonate solution with a mass concentration of 200 g / L is added to the tank to adjust the pH of the stripping solution to 6-8. The stripping solution is recycled and filtered using a filter. Example 2

[0051] A selective stripping process for a high-strength steel unqualified zinc-cadmium alloy coating is as follows:

[0052] Sodium nitrite 80 g / L, ammonium bicarbonate 50 g / L, sodium chloride 50 g / L, the pH of the stripping solution is 7, normal temperature operation, and the coating is stripped until it is completely removed.

[0053] 1. Preparation of the film removal solution:

[0054] In the film removal tank, 4 / 5 of water is added by volume of tank liquid, concentrated sulfuric acid is added according to process requirements, after stirring uniformly, water is added to the specified volume, the mass concentration of sulfuric acid in the prepared film removal liquid is 60 g / L.

[0055] 2. Preparation of stripping solution:

[0056] In the stripping tank, 4 / 5 of water is added by volume of tank liquid, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added according to the formula of stripping solution, stirring to dissolve, and then water is added to the specified volume.

[0057] 3. Film removal:

[0058] The unqualified zinc-cadmium alloy plated parts are put into the film removal tank to remove the passivation film, until the passivation film is removed, and then washed with water for 3 times.

[0059] 4. Stripping:

[0060] After the film removal, the unqualified zinc-cadmium alloy plated parts are put into the stripping tank to remove the plating layer, until the plating layer is removed, and then washed with water for 3 times and transferred to the zinc-cadmium alloy plating process.

[0061] In production, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added to the stripping tank in a mass ratio of 1:0.3:0.3; when the pH is increased, dilute nitric acid with a mass concentration of 200 g / L is added to the stripping tank to adjust the pH of the stripping solution to 6-8; when the pH is low, sodium carbonate solution with a mass concentration of 200 g / L is added to the tank to adjust the pH of the stripping solution to 6-8; and the stripping solution is filtered by a filter. Example 3

[0062] A high-strength steel unqualified zinc-cadmium alloy plating layer selection stripping process is as follows:

[0063] Sodium nitrite 150 g / L, ammonium bicarbonate 100 g / L, ammonium chloride 100 g / L, the pH of the stripping solution is 7, and the operation is at room temperature, until the plating layer is removed.

[0064] 1. Preparation of film removal solution:

[0065] In the film removal tank, 4 / 5 of water is added by volume of tank liquid, concentrated sulfuric acid is added according to process requirements, after stirring uniformly, water is added to the specified volume, the mass concentration of sulfuric acid in the prepared film removal liquid is 70 g / L.

[0066] 2. Preparation of stripping solution:

[0067] In the stripping tank, 4 / 5 of water is added by volume of tank liquid, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added according to the formula of stripping solution, stirring to dissolve, and then water is added to the specified volume.

[0068] 3. Film removal:

[0069] The unqualified zinc-cadmium alloy plated parts are put into the film removing tank to remove the passivation film until the passivation film is removed completely, and then are washed with water for 3 times.

[0070] 4. Deposition:

[0071] The unqualified zinc-cadmium alloy plated parts are put into the film removing tank to remove the passivation film until the passivation film is removed completely, and then are washed with water for 3 times.

[0072] In production, the sodium nitrite, ammonium bicarbonate and ammonium chloride are added to the deposition tank in a mass ratio of 1:0.3:0.3. When the pH is increased, the pH of the deposition tank is adjusted to 6-8 by adding dilute nitric acid with a mass concentration of 200 g / L. When the pH is decreased, the pH of the deposition tank is adjusted to 6-8 by adding sodium carbonate solution with a mass concentration of 200 g / L. The deposition solution is recycled and filtered by a filter. Example 4

[0073] A high-strength steel unqualified zinc-cadmium alloy plated layer is selected by the following deposition process:

[0074] The sodium nitrite is 150 g / L, the ammonium bicarbonate is 50 g / L, the ammonium chloride is 50 g / L, the pH of the deposition solution is 7.3, the operation is at room temperature, and the deposition is stopped until the plated layer is removed completely.

[0075] 1. Preparation of film removing solution:

[0076] In the film removing tank, 4 / 5 of water is added according to the volume of the tank solution, concentrated sulfuric acid is added according to the process requirements, and then water is added to the specified volume after stirring uniformly. The mass concentration of sulfuric acid in the prepared film removing solution is 80 g / L.

[0077] 2. Preparation of deposition solution:

[0078] In the deposition tank, 4 / 5 of water is added according to the volume of the tank solution, the sodium nitrite, ammonium bicarbonate and ammonium chloride are added according to the formula requirements of the deposition solution, and then water is added to the specified volume after stirring to dissolve.

[0079] 3. Film removing:

[0080] The unqualified zinc-cadmium alloy plated parts are put into the film removing tank to remove the passivation film until the passivation film is removed completely, and then are washed with water for 3 times.

[0081] 4. Deposition:

[0082] The unqualified zinc-cadmium alloy plated parts are put into the film removing tank to remove the passivation film until the passivation film is removed completely, and then are washed with water for 3 times.

[0083] In production, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added to the stripping tank in a mass ratio of 1:0.2:0.2; after the pH is increased, dilute nitric acid with a mass concentration of 200 g / L is added to the stripping tank to adjust the pH of the stripping solution to 6-8; when the pH is low, a sodium carbonate solution with a mass concentration of 200 g / L is added to the tank to adjust the pH of the stripping solution to 6-8; the stripping solution is filtered by a filter machine. Example 5

[0084] A selective stripping process for a high-strength steel zinc-cadmium alloy plating layer that is out of specification is as follows:

[0085] Sodium nitrite 80 g / L, ammonium bicarbonate 100 g / L, ammonium chloride 100 g / L, pH of the stripping solution 6.8, normal temperature operation, until the plating layer is completely stripped.

[0086] 1. Preparation of the film removal solution:

[0087] In the film removal tank, 4 / 5 of water is added based on the volume of the tank solution, concentrated sulfuric acid is added according to the process requirements, and after stirring, water is added to the specified volume. The mass concentration of sulfuric acid in the prepared film removal solution is 75 g / L.

[0088] 2. Preparation of the stripping solution:

[0089] Based on the volume of the tank solution, 4 / 5 of water is added to the stripping tank, and sodium nitrite, ammonium bicarbonate, and ammonium chloride are added according to the requirements of the stripping solution formula, and water is added to the specified volume after stirring to dissolve them.

[0090] 3. Film removal:

[0091] The out-of-specification zinc-cadmium alloy plated parts are placed in the film removal tank to remove the passivation film, until the passivation film is completely removed, and then they are washed with water for 3 times after being taken out of the tank.

[0092] 4. Stripping:

[0093] After the film removal, the out-of-specification zinc-cadmium alloy plated parts are placed in the stripping tank for stripping, until the plating layer is completely stripped, and then they are washed with water for 3 times before being transferred to the zinc-cadmium alloy plating process.

[0094] In production, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added to the stripping tank in a mass ratio of 1:0.5:0.5; after the pH is increased, dilute nitric acid with a mass concentration of 200 g / L is added to the stripping tank to adjust the pH of the stripping solution to 6-8; when the pH is low, a sodium carbonate solution with a mass concentration of 200 g / L is added to the tank to adjust the pH of the stripping solution to 6-8; the stripping solution is filtered by a filter machine. Example 6

[0095] A selective stripping process for a high-strength steel zinc-cadmium alloy plating layer that is out of specification is as follows:

[0096] Sodium nitrite 120 g / L, ammonium bicarbonate 100 g / L, ammonium chloride 50 g / L, the pH of the stripping solution is 6.2, and the operation is carried out at room temperature until the coating is completely stripped.

[0097] 1. Preparation of the stripping solution:

[0098] In the stripping tank, 4 / 5 of water is added according to the volume of the tank solution, concentrated sulfuric acid is added according to the process requirements, and after stirring, water is added to the specified volume. The mass concentration of sulfuric acid in the prepared stripping solution is 75 g / L.

[0099] 2. Preparation of the stripping solution:

[0100] According to the volume of the tank solution, 4 / 5 of water is added to the stripping tank, and sodium nitrite, ammonium bicarbonate, and ammonium chloride are added according to the formula requirements of the stripping solution. After stirring to dissolve, the pH of the stripping solution is adjusted to 6.2 with dilute nitric acid with a mass concentration of 200 g / L, and water is added to the specified volume.

[0101] 3. Stripping:

[0102] The unqualified zinc-cadmium alloy plated parts are placed in the stripping tank to remove the passivation film until the passivation film is completely removed. After being taken out of the tank, they are washed with water for 3 times.

[0103] 4. Stripping:

[0104] After the unqualified zinc-cadmium alloy plated parts are stripped, they are placed in the stripping tank for stripping until the coating is completely stripped. After being washed with water for 3 times, they are transferred to the zinc-cadmium alloy plating process.

[0105] In production, sodium nitrite, ammonium bicarbonate, and ammonium chloride are added to the stripping tank in a mass ratio of 1:0.4:0.2. When the pH rises, dilute nitric acid with a mass concentration of 200 g / L is added to the stripping tank to adjust the pH of the stripping solution to 6-8. When the pH is too low, a sodium carbonate solution with a mass concentration of 200 g / L is added to the tank to adjust the pH of the stripping solution to 6-8. The stripping solution is filtered by a filter machine.

[0106] Test Example 1:

[0107] A 45 steel plate with a size of 50 mm x 100 mm x 2 mm is quenched and tempered to form sorbite steel. The heat-treated steel plate is subjected to ultrasonic oil removal, sandpaper polishing, chemical oil removal, and activation with dilute sulfuric acid with a mass concentration of 100 g / L for 1 min. Then, a zinc-cadmium alloy plated sample is prepared according to the zinc-cadmium alloy plating process disclosed in the invention patent with a publication number of CN 111690933 B. The thickness of the coating is 10 μm. The zinc-cadmium alloy plated sample is subjected to low-chromium color passivation with hexavalent chromium. After being stripped according to the process of Example 1, the sample is placed in the stripping solution for 20 min, and then washed with water for 3 times. After being blown dry, the surface of the sample is observed. The zinc-cadmium alloy coating has been completely stripped, and there is no carbon dust on the surface of the steel plate.

[0108] The steel plate after stripping is placed in the stripping solution for 8h, and the surface of the steel plate is free of rust, as shown in Figure 1 The test shows that the sodium nitrite in the stripping solution has good slow-release effect on the steel and iron base.

[0109] Test Example 2

[0110] After the steel plate after stripping in the test example 1 is pickled and activated, the zinc-cadmium alloy layer is re-plated, and the thickness of the plating layer is 10μm. The low-chromium passivation with hexavalent chromium is used. According to GB / T 5270-2005 “Metallic Coatings on Metal Surfaces - Evaluation of the Adhesion by Measuring the Resistance to Thermal Shock”, the plating layer adhesion is tested by the thermal shock method. The plated piece is heated to 200℃ in a heating furnace for 30min, and then taken out and placed in water at room temperature for sudden cooling. The plating layer does not appear to bubble and fall off, and the plating layer has good adhesion. The test shows that the stripping method of the unqualified zinc-cadmium alloy plating layer does not reduce the surface quality of the medium carbon steel workpiece.

[0111] Test Example 3

[0112] The test solution is prepared: sodium nitrite 120g / L, ammonium bicarbonate 80g / L, and ammonium chloride 80g / L. 7 parts of the test solution are taken, and the pH of the test solution is adjusted to 5.5, 6.0, 6.5, 7.0, 7.5, 8.0 and 8.5 respectively by using dilute sulfuric acid with a mass concentration of 50g / L and sodium hydroxide solution with a mass concentration of 50g / L. The zinc-cadmium alloy sample prepared in the test example 1 is used for stripping test. In the tested pH range, the zinc-cadmium alloy plating layer can be effectively stripped. However, when the pH is 5.5, carbon dioxide gas is emitted in the test solution, and when the pH is 8.5, ammonia gas is emitted in the test solution. Based on this, the pH process parameter is specified as 6-8.

[0113] Test Example 4

[0114] Test solution A is prepared: sodium nitrite 100g / L, ammonium bicarbonate 120g / L, and ammonium chloride 60g / L. The pH is adjusted to 5 by using dilute sulfuric acid with a mass concentration of 50g / L. Under this condition, the ammonium ion cannot generate ammonia molecules by hydrolysis reaction.

[0115] Test solution B is prepared: sodium nitrite 120g / L, sodium bicarbonate 80g / L, and sodium chloride 80g / L. The pH is adjusted to 5 by using dilute sulfuric acid with a mass concentration of 50g / L.

[0116] The zinc-cadmium alloy sample for plating on the surface of the steel and iron without passivation treatment is tested, and the zinc-cadmium alloy sample is a screw with M8x40. The zinc-cadmium alloy plating layer completely disappears after the sample is placed in the test solution A for 20min. The zinc-cadmium alloy plating layer does not have observable changes after the sample is placed in the test solution B for 20min.

[0117] In the test solution A, sodium nitrite oxidizes the metal zinc and cadmium into zinc oxide and cadmium oxide, and at the same time, the ammonium ions react with the zinc and cadmium oxides to form complex ions of zinc and cadmium to be dissolved in the test solution A, and finally the zinc-cadmium alloy plating layer is removed.

[0118] In the test solution B, sodium nitrite oxidizes the metal zinc and cadmium into zinc oxide and cadmium oxide, and the oxides form a protective film on the zinc-cadmium alloy plating layer to prevent further reaction of sodium nitrite with the plating metal. Tests show that, without the coordination of ammonium ions, the zinc-cadmium alloy plating layer cannot be removed by using sodium nitrite alone.

[0119] Comparative Example 1

[0120] The test plating removal solution is prepared by adding 100 g / L of sodium nitrite and 120 g / L of ammonium bicarbonate into water, and the pH is 7.3.

[0121] A zinc-cadmium alloy sample prepared in the test example 1 is used for a comparative test. The sample is placed in the above plating removal solution for 20 minutes, and no steel substrate is exposed. After being placed in the plating removal solution for 40 minutes, the zinc-cadmium alloy plating layer is completely removed. It can be seen from the comparison between the comparative example 1 and the test example 1 that the addition of ammonium chloride in the plating removal solution of the present application significantly improves the removal speed of the zinc-cadmium alloy plating layer.

[0122] Comparative Example 2

[0123] The hydrochloric acid plating removal solution is prepared by mixing concentrated hydrochloric acid and water in a volume ratio of 1:1.

[0124] A zinc plating sample prepared in the test example 1 is used for a comparative test. The zinc-cadmium alloy sample is placed in the above hydrochloric acid plating removal solution. The metal zinc and cadmium react with the hydrochloric acid to generate hydrogen bubbles which escape from the solution. After the bubbling disappears, the sample is taken out and washed with water, and then observed after being blown dry. It is found that the surface of the sample has a layer of gray-black carbon dregs.

[0125] It can be seen from the test example 1 and the comparative example 1 that, for the plating removal of the zinc-cadmium alloy plating layer on the surface of high-strength steel, the technical solution of the present application overcomes the defect of carbon dregs generated by the traditional hydrochloric acid plating removal method.

[0126] Comparative Example 3

[0127] The steel plate with carbon dregs after plating removal in the comparative example 2 is pickled and activated, and then re-plated with zinc-cadmium alloy with a plating layer thickness of 10 μm. A low-chromium hexavalent chromium color passivation is used. According to GB / T 5270-2005 "Metallic Coatings on Metal Substrates - Test Methods for Adhesion - Evaluation of Electrodeposited and Chemically Deposited Coatings", the plating layer adhesion is tested by a thermal shock method. The sample is heated to 200°C in a heating furnace for 30 minutes, and then taken out and placed in water at room temperature for sudden cooling. A small amount of bubbles appears on the plating layer, and the plating layer adhesion does not meet the standard requirements.

[0128] The above embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the protection scope of the present application. The protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for stripping off a zinc-cadmium alloy plating layer of a high-strength steel nonconforming to a specification, characterized by, The stripping process is as follows: 80-150 g / L of sodium nitrite, 50-100 g / L of ammonium bicarbonate, and 50-100 g / L of ammonium chloride, and the pH of the stripping solution is 6-8, and the operation is carried out at room temperature; The sodium nitrite is used as an oxidizing agent to oxidize zinc and cadmium into zinc oxide and cadmium oxide; The ammonium bicarbonate is used as a complexing agent to react with the zinc and cadmium oxides to form complex ions and dissolve in the stripping solution, and also as a pH buffer to keep the pH of the stripping solution stable; The ammonium chloride is used as a complexing agent to react with the zinc and cadmium oxides to form complex ions and dissolve in the stripping solution, and also as an accelerator to increase the stripping speed of the zinc-cadmium alloy coating.

2. The method of claim 1, wherein the high-strength steel unqualified zinc-cadmium alloy plated layer is removed by, According to the volume of the solution in the tank, 4 / 5 of water is added to the stripping tank, and the sodium nitrite, ammonium bicarbonate, and ammonium chloride are added according to the stripping process, and stirred to dissolve, and then water is added to the specified volume.

3. The method for stripping substandard zinc-cadmium alloy coatings on high-strength steel as described in claim 1, characterized in that: The stripping method comprises the following processes: (1) preparing a film removal solution in a film removal tank, wherein the mass concentration of sulfuric acid is 50-80 g / L; (2) preparing a stripping solution in a stripping tank; (3) placing the unqualified zinc-cadmium alloy plated parts into the film removal tank to remove the passivation film, and operating at room temperature until the passivation film is completely removed; (4) placing the film-removed unqualified zinc-cadmium alloy plated parts into the stripping tank to strip the zinc-cadmium alloy coating until the zinc-cadmium alloy coating is completely removed.

4. The method of claim 1, wherein the high-strength steel unqualified zinc-cadmium alloy plating layer is removed by: In production, the mass ratio of sodium nitrite, ammonium bicarbonate, and ammonium chloride is 1:(0.2-0.5):(0.2-0.5).

5. The method of claim 1, wherein the high-strength steel unqualified zinc-cadmium alloy plating layer is removed by: In production, dilute nitric acid with a mass concentration of 160-240 g / L is used to adjust the pH of the stripping solution to 6-8.

6. The method of claim 1, wherein the high-strength steel unqualified zinc-cadmium alloy plating layer is removed by: When the pH of the stripping solution is too low, sodium carbonate solution with a mass concentration of 160-240 g / L is used to adjust the pH of the stripping solution to 6-8.

7. The method of claim 1, wherein the high-strength steel unqualified zinc-cadmium alloy plating layer is removed by: In production, a filter is used to circulate and filter the stripping solution to remove the precipitates generated during the stripping process. In production, a filter is used to circulate and filter the stripping solution to remove the precipitates generated during the stripping process.

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