Periodic chromium plating method for improving corrosion resistance of chromium layer of workpiece

Abstract

The invention discloses a periodic chromium plating method for improving the corrosion resistance of a workpiece chromium layer. The method sequentially comprises the following steps: (1) chemical deoiling, (2) hot water washing, (3) anodic pickling, (4) cold water washing, (5) neutralizing, (6) cold water washing, (7) mounting and hanging, (8) periodic chromium plating, (9) recycling, (10) cold water washing and clamp dismounting, # imgabs 0 # neutralizing, # imgabs 1 # cold water washing, # imgabs 2 # blow-drying and # imgabs 3 # checking. By the adoption of the process method, the thickness uniformity of the chromium plating layer is improved, the length of inherent hair cracks of the chromium plating layer is shortened, the number of the hair cracks is reduced, the probability that a corrosion medium enters a corrosion matrix through the cracks is reduced, the plating layer performance is improved, and the corrosion resistance of the chromium plating layer is improved essentially.

Description

Technical Field

[0001] The present invention relates to the technical field of electroplating processes, and in particular to a periodic chromium plating method for improving the corrosion resistance of a chromium layer on a workpiece. Background Art

[0002] In harsh working conditions such as high temperature, high salt and high humidity in marine environments, products need to have high hardness, high wear resistance and excellent corrosion resistance. Therefore, electroplating chromium is often used for surface treatment.

[0003] However, the traditional DC chromium plating process has significant technical defects: First, the chromium plating current efficiency is low (about 13%), and due to the prominent edge and tip effects, the coating thickness distribution is uneven, with a large thickness difference between the edge and the center, seriously affecting the appearance and performance of the coating; Second, during the DC chromium plating process, chromium ions in the plating solution near the surface of the plated workpiece are continuously deposited on the cathode surface, which will cause concentration polarization, causing the surface crystal formation rate to be lower than the crystal growth rate, and ultimately resulting in coarse crystals and high porosity in the coating. In addition, as the reduction reaction continues, the amount of hydrogen released on the cathode surface gradually increases, generating a large amount of chromium hydride, which causes excessive stress in the coating and leads to cracks in the coating, which are called "hairline cracks", as shown in the attached figure of the specification. Figure 2 As shown, Figure 2 This image shows the crack morphology of the chromium layer produced by conventional direct current chromium plating. Corrosive media penetrate the substrate-plating interface through hairline cracks in the coating, corroding the substrate and ultimately causing the chromium coating to flake off, rendering it ineffective in protecting the coating.

[0004] Chinese patent 201811385601.3 discloses a method for composite chromium plating of the inner hole of a pipe fitting and a chromium plating device used for the method. The method adopts a forward plating and reverse etching method to obtain a hard chromium layer, which can greatly improve the adhesion, wear resistance, and anti-firepowder ablation performance of the chromium layer of the pipe fitting. However, it cannot adapt to harsh working conditions such as high temperature, high salt, and high humidity in the marine environment.

[0005] It can be seen that the currently used DC chromium plating process has serious deficiencies in coating uniformity, density and corrosion resistance, and it is difficult to meet the high-performance protection requirements of the coating under harsh working conditions. Summary of the Invention

[0006] The purpose of the present invention is to address the corresponding deficiencies in the existing technology and provide a method for periodic chromium plating for improving the corrosion resistance of the chromium layer of a workpiece. This method for periodic chromium plating for improving the corrosion resistance of the chromium layer of a workpiece comprises the following steps: firstly performing reverse plating, electric punching, and hard chromium plating, and then performing periodic reverse chromium plating and periodic forward chromium plating in a cyclic alternating manner to achieve the desired chromium plating thickness, improve the thickness uniformity of the chromium plating, shorten the length of the inherent hairline cracks in the chromium plating, reduce the number of hairline cracks, reduce the probability of corrosive media entering the corroded substrate through the cracks, improve the plating performance, and essentially improve the corrosion resistance of the chromium plating, so as to solve the technical problems of poor thickness uniformity and poor corrosion resistance of the chromium layer.

[0007] The purpose of the present invention is to adopt the following scheme to achieve:

[0008] A method for periodic chromium plating for improving the corrosion resistance of a chromium layer on a workpiece comprises the following steps:

[0009] 1) Pre-plating treatment of the workpiece surface;

[0010] 2) Periodic chrome plating

[0011] Place the workpiece in a chromium plating solution containing 130-150 g / L chromic anhydride and 1.3-2.0 g / L sulfuric acid solution at a temperature of 63-66°C and perform the following steps in sequence:

[0012] 2-1) Reverse plating: current density 25-40A / dm 2 , time 0.5-1min;

[0013] 2-2) Electric impulse: current density 50-80A / dm 2 , time 1-2min;

[0014] 2-3) Hard chromium plating: current density 30-40A / dm 2 , time 60min;

[0015] 2-4) Cycle reverse chromium plating: current density is 80% J 正 , J 正 is the forward chromium plating current density, holding time 5s, and soft start time 0s;

[0016] 2-5) Cyclic forward chromium plating: current density 30-40A / dm 2 , hold time 180s, soft start time 0s;

[0017] 2-4) and 2-5) are repeated alternately until the required chromium layer thickness is reached and then ends with 2-5).

[0018] In step 1), the process of pre-plating treatment of the workpiece surface is as follows: the workpiece surface is chemically degreased, then washed with hot water, anodic acid washed, washed with cold water, neutralized, and washed with cold water.

[0019] The chemical degreasing adopts 80-120 g / L sodium hydroxide solution, the temperature is greater than 90° C., and the soaking time is 30-40 minutes.

[0020] The washing time of the hot water washing and the cold water washing is 1 to 3 minutes.

[0021] The anode pickling adopts a sulfuric acid solution with a concentration of 1000-1250g / L, a temperature of less than 35°C, a pickling time of 3-5min, and a current density of 25-40A / dm 2 .

[0022] The neutralization adopts 40-60g / L sodium carbonate solution, and the soaking is continued for 1-3 minutes.

[0023] The method described in the present invention uses periodic chromium plating to replace the direct current chromium plating process for electroplating chromium production. Periodic chromium plating includes five steps: reverse plating, electric impulse, hard chromium plating, periodic reverse chromium plating, and periodic forward chromium plating. First, the oxide on the surface of the workpiece is eliminated by reverse plating; then the electric impulse is used to increase the bonding strength between the workpiece and the chromium layer; the purpose of hard chromium plating is to lay a chromium layer on the substrate to ensure the normal plating of positive and reverse periodic chromium plating. Periodic reverse chromium plating and periodic forward chromium plating are a cyclic alternating process, first reverse chromium plating, then forward chromium plating... and so on, and finally the entire periodic chromium plating process is ended with forward chromium plating. The reverse current dissolves the coating, which can rapidly increase the concentration of metal ions on the cathode surface, increase the cathode current density, and the nucleation rate of the cathode grains is much higher than the growth rate of the grains.

[0024] In addition, the reverse current can interrupt the growth of dendritic grains and obtain fine equiaxed grains, thereby obtaining a low-porosity, high-density coating, shortening the length of the inherent hairline cracks in the chromium coating and reducing the number of hairline cracks. Forward electroplating is to deposit and obtain the chromium coating required by the process on the workpiece. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a process flow chart of a method for periodic chromium plating to improve the corrosion resistance of the chromium layer of a workpiece.

[0026] Figure 2 This is the crack morphology of the chromium layer of traditional DC chromium plating.

[0027] Figure 3 This is the crack morphology of the chromium layer after periodic chromium plating. DETAILED DESCRIPTION

[0028] like Figure 1As shown, a method for periodic chrome plating to improve the corrosion resistance of the chrome layer of a workpiece is carried out in the following steps: ① chemical degreasing → ② hot water washing → ③ anodic pickling → ④ cold water washing → ⑤ neutralization → ⑥ cold water washing → ⑦ hanging → ⑧ periodic chrome plating → ⑨ recycling → ⑩ cold water washing and disassembly of the fixture → Neutralize → Cold water wash → Blow dry → test.

[0029] The specific steps are as follows:

[0030] 1) Pre-plating treatment of the workpiece surface:

[0031] ①Chemical degreasing: Use an electric hoist to completely place the porous barrel containing the workpiece in an 80-120g / L sodium hydroxide degreasing solution above 90°C and soak it for 30-40 minutes.

[0032] ② Hot water washing: Use the electric hoist to completely immerse the porous barrel containing the workpiece in hot water and wash for 1 to 3 minutes.

[0033] ③ Anodic pickling: Use an electric hoist to completely place the porous barrel containing the workpiece into a sulfuric acid solution with a concentration of 1000-1250g / L and a temperature below 35°C. Anodic pickling is performed for 3-5 minutes at a current density of 25-40A / dm 2 .

[0034] ④Cold water washing: Use the electric hoist to completely immerse the porous barrel containing the workpiece into the water washing tank and wash for 1 to 3 minutes.

[0035] ⑤ Neutralization: Soak the porous barrel containing the workpiece in 40-60g / L sodium carbonate solution at room temperature for 1-3 minutes.

[0036] ⑥Cold water washing: The working steps are carried out according to the contents of “④Cold water washing”.

[0037] ⑦ Hanging: For workpieces with dedicated tooling, hang them as required. For smaller workpieces, a custom hanger made of copper wire can be used depending on the workpiece shape. The area of ​​the custom hanger immersed in the solution should be wrapped with PVC plastic sheeting and tied tightly. Workpieces with sharp corners or pointed points should be shielded with a custom-made wire or lead wire tamper-evident loop.

[0038] A thief ring, also known as an auxiliary anode, optimizes current distribution and ensures a uniform chromium layer on the workpiece, especially around corners or sharp points, as well as on workpieces with complex shapes or deep holes. It acts as an auxiliary anode, placed at specific locations on the workpiece (such as holes or sharp points), to absorb the excessive current generated by the dense electric field lines at the workpiece tip, thereby suppressing excessive chromium deposition at the edge.

[0039] 2) Periodic chrome plating:

[0040] ⑧ Place the workpiece in a chrome plating bath with a chromic acid concentration of 130-150g / L, a sulfuric acid concentration of 1.3-2.0g / L, and a bath temperature of 63-66°C for chrome plating. The chromium plating cycle involves five steps: reverse plating → electric punching → hard chrome plating → reverse chrome plating → forward chrome plating. The key points for controlling the parameters of each step are as follows:

[0041] a) Reverse plating: current density 25-40A / dm 2 , time 0.5-1min.

[0042] b) Electric impulse: current density 50-80A / dm 2 , time 1-2 minutes.

[0043] c) Hard chromium plating: current density 30-40A / dm 2 , time 60 minutes.

[0044] d) Cyclic reverse chromium plating: the current density is 80% of Jpositive (Jpositive is the current density of forward chromium plating), the holding time is 5s, and the soft start time is 0s.

[0045] e) Cyclic forward chromium plating: current density 30-40A / dm 2 , hold time 180s, soft start time 0s.

[0046] d) and e) are repeated alternately until the required chromium layer thickness is reached and ends with e).

[0047] The number of cycles of cyclic reverse chromium plating and cyclic forward chromium plating is determined by the thickness of the chromium layer.

[0048] The process conditions are as follows:

[0049] Hard chrome plating: current density 40A / dm 2 , time: 60 minutes.

[0050] Cyclic reverse chromium plating: the current density is 80% of J positive (J positive is the current density of forward chromium plating), that is, 32A / dm 2 , hold time 5s, soft start time 0s.

[0051] Cyclic forward chromium plating: current density 40A / dm 2 , hold time 180s, soft start time 0s.

[0052] Among them, the hard chromium plating rate is 19.5μm / h; the chromium plating rate of alternating reverse and forward cycles is 12.5μm / h.

[0053] The cyclic reverse chromium plating and cyclic forward chromium plating were cycled 68 times, and the average thickness of the chromium layer on the round rod specimen processed by the cyclic chromium plating process was 63 μm.

[0054] 3) Post-plating treatment of the workpiece surface:

[0055] ⑨ Recovery: Operate the electric hoist to completely immerse the workpiece in pure water at room temperature, and shake it repeatedly in the water for 0.5-2 minutes. After the surface is clean, take the workpiece out of the recovery tank.

[0056] ⑩ Wash with cold water and remove the fixture: The steps for cold water washing are as per “④Cold water washing”. After cleaning, remove the fixture and ensure that there is no solution residue on the surface.

[0057] Neutralization: Soak the porous barrel containing the workpiece in 40-60g / L sodium carbonate solution at room temperature for 1-3 minutes.

[0058] Cold water washing: The working steps are as per “④Cold water washing”.

[0059] Blow dry: Open the compressed air valve and aim the air blowpipe at the workpiece until it is completely dry to avoid residual liquid in gaps and inner holes.

[0060] Inspection: Inspect the appearance and thickness of the chromium layer of the workpiece according to the process requirements.

[0061] The method of the present invention adopts periodic chromium plating to replace the DC chromium plating process for electroplating chromium production, and the obtained chromium layer crack morphology is shown in the accompanying figure of the specification. Figure 3 As shown. This method can improve the thickness uniformity of the chromium plating layer, shorten the length of the hairline cracks inherent in the chromium plating layer, reduce the number of hairline cracks, reduce the probability of corrosive media entering the corrosive substrate through cracks, improve the coating performance, and essentially enhance the corrosion resistance of the chromium plating layer. Under the condition of an average chromium layer thickness of 90μm, compared with DC chromium plating, the corrosion resistance of the workpiece processed by periodic chromium plating is improved by about 8 times, and the circumferential thickness deviation of shaft workpieces is reduced by 50%. The periodic chromium plating process has the advantages of strong practicality and wide application, and can guide the processing of chromium-plated workpieces with various specifications and thicknesses.

[0062] Periodic chromium plating, or periodic reversing electroplating, utilizes a traditional chromium plating process formula. This process involves a period of forward chromium plating followed by a short reverse chromium plating (anodic etching) followed by another chromium plating cycle, achieving chromium deposition. The core concept is to achieve excellent overall performance by periodically alternating different states of the same material (chromium layer) to form a multi-layer or composite chromium coating on the surface of the substrate.

[0063] When the current is in the forward direction, the base material acts as the cathode, and the hexavalent chromium ions in the solution are reduced to metallic chromium and deposited on the cathode surface. The reaction formula is summarized as follows:

[0064]

[0065] 2H + +2e=H2↑

[0066] Due to the precipitation of hydrogen, a large amount of H+ is consumed, which gradually increases the pH value near the cathode surface. Convert to so Discharge produces metallic chromium, the reaction formula is

[0067]

[0068] When the current is reversed, the base material briefly becomes the anode, causing a slight dissolution of the deposited chromium layer (anodic etching), which eliminates the tip effect at the protruding parts and makes the coating more uniform. The key point in the cyclic chromium plating process is that the positive cathode duration is much longer than the reverse anodic duration, otherwise no coating will be deposited on the part surface.

[0069] The chromium plating solution has poor dispersion capabilities, and the presence of free space in the plating solution, as well as the length of the anode, prevents the electric lines from being evenly distributed between the cathode and the cathode. This results in a larger current at the cathode tip and surrounding areas than in the middle, causing the coating thickness at the tip and surrounding edges of the part to be too thick. This phenomenon is inevitable during the chromium plating process, and it becomes increasingly apparent as the coating thickness gradually increases. However, the current for periodic chromium plating is not applied continuously. Reverse electroplating eliminates the tip effect at the raised areas of the coating, and then the chromium coating is deposited in the forward direction, and this process is repeated. This entire process can improve the shortcomings of conventional direct current chromium plating, which has a fast initial deposition rate but slows down as the coating thickens. Ultimately, the fluctuation range of the chromium coating thickness is reduced, and the uniformity of the chromium coating is significantly improved.

[0070] There are many cracks in the DC chromium plating layer. The crack morphology of the chromium layer is as follows: Figure 2 , slender in length, and even a through crack. The number of cracks in the periodic chromium plating layer is significantly less than that in the DC chromium plating layer. The crack morphology of the chromium layer is shown in the figure below. Figure 3 The cracks are relatively short, and there are no through-going cracks. Most cracks extend only to the middle of the coating, and rarely reach the substrate interface. In addition, the periodic chromium coating has obvious stratification. The special layered structure can prevent the longitudinal expansion of cracks, reducing the risk of corrosive media entering the substrate through micro cracks and corroding the product. The different crack morphology characteristics of the chromium coatings of the two have been intuitively and numerically verified through neutral salt spray tests. Figure 2 and Figure 3, which clearly states that the use of periodic chromium plating method can significantly shorten the length of hairline cracks and reduce the number of hairline cracks.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications made to the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for periodic chromium plating to improve the corrosion resistance of the chromium layer of a workpiece, characterized in that: The following steps are involved: 1) Pre-plating treatment of the workpiece surface; 2) Periodic chrome plating Place the workpiece in a chromium plating solution containing 130-150 g / L chromic anhydride and 1.3-2.0 g / L sulfuric acid solution at a temperature of 63-66°C and perform the following steps in sequence: 2-1) Reverse plating: current density 25-40A / dm 2 , time 0.5-1min; 2-2) Electric impulse: current density 50-80A / dm 2 , time 1-2min; 2-3) Hard chromium plating: current density 30-40A / dm 2 , time 60min; 2-4) Cycle reverse chromium plating: current density is 80% J 正 , J 正 is the forward chromium plating current density, holding time 5s, and soft start time 0s; 2-5) Cyclic forward chromium plating: current density 30-40A / dm 2 , hold time 180s, soft start time 0s; 2-4) and 2-5) are repeated alternately until the required chromium layer thickness is reached and then ends with 2-5).

2. The method according to claim 1, wherein: In step 1), the process of pre-plating treatment of the workpiece surface is as follows: the workpiece surface is chemically degreased, then washed with hot water, anodic acid washed, washed with cold water, neutralized, and washed with cold water.

3. The method according to claim 2, wherein: In step 1), the chemical degreasing is carried out using 80-120 g / L sodium hydroxide solution at a temperature greater than 90° C. for immersion for 30-40 minutes.

4. The method according to claim 2, wherein: In step 1), the washing time of the hot water washing and the cold water washing is 1 to 3 minutes.

5. The method according to claim 2, wherein: In step 1), the anodic pickling is carried out using a sulfuric acid solution with a concentration of 1000-1250 g / L, a temperature of less than 35°C, a pickling time of 3-5 min, and a current density of 25-40 A / dm 2 .

6. The method according to claim 2, wherein: In step 1), the neutralization is carried out using a 40-60 g / L sodium carbonate solution, and the immersion is continued for 1-3 minutes.

7. The method according to claim 1, wherein: After the chromium plating cycle is completed, the workpiece surface needs to be post-plated. The process includes: cold water washing, neutralization, cold water washing, blow drying and inspection.

8. The method according to claim 7, wherein: The neutralization is carried out using a 40-60 g / L sodium carbonate solution and the mixture is immersed for 1-3 minutes at room temperature.

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