Preparation method of high-strength crack-free ultra-fine grain chromium layer

Abstract

The invention belongs to the field of material surface treatment, and particularly relates to a preparation method of a high-strength crack-free ultra-fine grain chromium layer. A pulse electroplating method is adopted, and the method specifically comprises the following steps that (1) mechanical grinding and polishing are conducted; (2) chemical oil removal; (3) pickling and activating; step (4), slowly rising current for pre-plating; (5) formal plating is conducted, specifically, the workpiece to be plated serves as a cathode, and a lead-tin alloy is adopted as an anode; the plating solution comprises CrO3 and sulfuric acid; the duty ratio gamma of pulse electroplating is 65-80%, the conduction time is 450-600 ms, the cut-off time is 110-150 ms, the anode current density is 35-40 A / dm < 2 >, and the temperature of the plating solution is 55-62 DEG C. The invention provides the preparation method of the high-strength crack-free pulse electroplating chromium layer, the method has the characteristics of high deposition efficiency, high hardness, no crack and the like, and the prepared coating has good mechanical property and density.

Description

Technical Field

[0001] The invention belongs to the field of material surface treatment, and in particular relates to a method for preparing a high-strength crack-free ultrafine-grained chromium layer. Background Art

[0002] Wear, corrosion, and fracture are the three main forms of failure in mechanical equipment and its components. Wear, as one of the primary failure modes, leads to significant material loss and resource waste. In actual production, all operating mechanical equipment components experience relative motion with their opposing wear pairs, leading to surface wear, material loss, and even deformation, which is the primary cause of component failure.

[0003] Hard chrome electroplating is a widely used technology for improving the wear and corrosion resistance of industrial parts. It is widely used in the automotive, aerospace, mining, and general engineering industries, such as internal combustion engine components, hydraulic cylinders, rolls, and machine tools. The coating thickness varies greatly depending on the service environment, but is typically in the range of 20-500μm, significantly higher than the thickness of decorative chrome layers (1μm).

[0004] Traditional electro-deposited chromium has a very complex structure, which contains a large number of oxide, hydroxide and hydride inclusions, as well as high residual internal stress, so the electro-deposited coating has an ultra-high hardness (up to 900HV). In addition, the presence of cracks in the coating will seriously deteriorate the corrosion resistance of the coating. These cracks provide a rapid penetration path for corrosive agents to erode the interface between the substrate and the chromium layer. In order to overcome this problem, crack-free chromium plating has been proposed. "Zheng Zhimin, Zhu Di, Zhu Zengwei, et al. Experimental study on crack-free hard chromium electroplating [J]. Electromachining and Mold" discloses a method for preparing crack-free chromium coatings, which uses friction-assisted pulse electroplating to increase the hydrogen evolution potential, reduce the penetration of hydrogen atoms, reduce the internal stress of the chromium coating, and reduce cracks. The corrosion resistance of the crack-free chromium coating in a liquid salt bath is significantly higher than that of ordinary chromium layers. However, the above method still has problems such as limited corrosion resistance and limited coating thickness. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a high-strength crack-free ultrafine-grained chromium layer, which has the characteristics of high deposition efficiency, high hardness and no cracks, and the prepared crack-free ultrafine-grained chromium layer also has good mechanical properties and density.

[0006] The technical solution for achieving the purpose of the present invention is: a method for preparing a high-strength, crack-free, ultrafine-grained chromium layer using a pulse electroplating method, specifically comprising the following steps:

[0007] Step (1): mechanical grinding and polishing;

[0008] Step (2): chemical degreasing;

[0009] Step (3): pickling and activation;

[0010] Step (4): Slowly increase the current pre-plating;

[0011] Step (5): Formal plating: The workpiece to be plated is used as the cathode and the anode is a lead-tin alloy; the plating solution includes CrO3 and sulfuric acid; the duty cycle γ of the pulse plating is 65-80%, the on time is 450-600ms, the off time is 110-150ms, and the anode current density is 35-40A / dm 2 , the plating solution temperature is 55-62℃.

[0012] Furthermore, step (1) is specifically as follows: the surface to be plated of the workpiece to be plated is mechanically polished using 80#, 320#, 600# and 1500# silicon carbide sandpaper in sequence, and after polishing, the surface to be plated of the workpiece is polished using a polishing cloth and a polishing paste with a particle size of W3.5 until the surface roughness Ra≤0.4.

[0013] Furthermore, the chemical degreasing in step (2) is specifically as follows: heating the chemical degreasing liquid to 70-80°C, and then immersing the workpiece after mechanical grinding and polishing in the degreasing liquid for 8-10 minutes; the degreasing liquid includes NaOH, Na2CO3 and Na2SiO3, and the specific components are NaOH: 80-120g / L, Na2CO3: 15-25g / L, Na2SiO3: 8-16g / L. After the degreasing is completed, rinse it with deionized water, and use a hot air blower to blow the surface of the workpiece to be plated dry and then set it aside.

[0014] Furthermore, step (3) is specifically as follows: immersing the workpiece after chemical degreasing in a 10-15% hydrochloric acid solution for pickling and activation for 15-30 seconds, rinsing with deionized water, and drying for later use.

[0015] Furthermore, step (4) is specifically as follows: at 0.25-0.4A / dm per second 2 The current density is increased to the set current value at a speed of 55-62°C, and the plating solution temperature is 55-62°C; the set current value = current density * area to be plated.

[0016] Furthermore, in step (5), the CrO3 content in the plating solution is 180-260 g / L, the sulfuric acid content is 2.2-3.0 g / L; the tin content in the lead-tin alloy is 6-13%; the distance between the cathode and the anode is 30-50 mm, and the plating solution is stirred during the formal plating, and the stirring rate is 30r-60r / min.

[0017] The material of the further workpiece to be plated is steel.

[0018] A high-strength, crack-free ultrafine-grained chromium layer is prepared by the above method.

[0019] Compared with the prior art, the present invention has the following significant advantages:

[0020] The structure of the chrome coating is the primary factor affecting the damage and degradation of the interface between the chrome coating and the substrate, and determines the formation timing of the main cracks in the chrome coating. Under the same service conditions, the earlier the main cracks form, the earlier the damage and degradation behavior of the interface between the chrome coating and the steel substrate will occur. Traditional hard chrome coatings are prone to form main cracks that penetrate the chrome coating, causing damage and degradation at the interface between the chrome coating and the steel substrate. This is because the hard chrome coating has high hardness and dense network-like inherent cracks, which can easily expand and extend to the substrate under thermal and chemical effects. Figure 1 As shown in Figure a. However, the rate of crack formation and expansion in the crack-free chromium layer is significantly lower than that in the hard chromium layer. Therefore, the cracks in the surface hard chromium layer are hindered from expanding toward the substrate, delaying the expansion of the main crack from the chromium layer to the substrate. In the present invention, by using a pulsed electrodeposition process, chromium ions are fully diffused near the cathode, increasing the potential of the secondary hydrogen evolution reaction, and promoting hydrogen ion oxidation in the anode potential range, thereby reducing the hydride content in the deposited layer and preventing cracking of the chromium layer. The coating has a fast deposition rate, no cracks on the coating surface, and better corrosion resistance. It can effectively protect the metal substrate for a long time, has better wear resistance and corrosion resistance, and can significantly extend the service life of the workpiece.

[0021] Conventional hard chromium columnar crystal coatings have significant variations in mechanical properties due to grain growth perpendicular to the substrate. Columnar crystal coatings also have a single grain boundary structure, which allows dislocations to accumulate along the grain boundaries, forming crack sources. The equiaxed crystals formed by the present invention disperse stress through grain refinement, and their grain boundary network effectively hinders dislocation motion, reducing the risk of crack initiation and propagation.

[0022] The grains of traditional hard chromium columnar crystal coatings grow along specific directions, resulting in anisotropic mechanical properties (such as large differences in strength and toughness in different directions); while the ultrafine equiaxed crystals formed by the present invention have disordered and randomly oriented grains, giving the coating uniform mechanical properties in multiple directions; this isotropic property is particularly suitable for complex stress environments (such as aircraft engine blades, precision bearings, etc.), and can avoid local stress concentration caused by directional differences. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Comparison of cracked and crack-free chromium layers; (a) optical micrograph of cracked coating, (b) optical micrograph of crack-free coating;

[0024] Figure 2 These are SEM images of the high-strength, crack-free, ultrafine-grained electroplated chromium layer of Example 2; (a) SEM image of the crack-free coating, (b) a partial magnified image of (a);

[0025] Figure 3This is a composition diagram of the high-strength, crack-free, ultrafine-grained electroplated chromium layer of Example 2;

[0026] Figure 4 This is a hardness diagram of the high-strength, crack-free, ultrafine-grained electroplated chromium layer of Example 2;

[0027] Figure 5 This is the EBSD image of the high-strength, crack-free, ultrafine-grained electroplated chromium layer of Example 2. DETAILED DESCRIPTION

[0028] The present invention will be described in detail below with reference to specific examples. The following examples will help those skilled in the art further understand the present invention but are not intended to limit the present invention in any way. It should be noted that variations and modifications are possible within the scope of the present invention, without departing from the spirit of the present invention. These modifications and improvements are all within the scope of protection of the present invention.

[0029] Example 1

[0030] The high-strength, crack-free, ultrafine-grained chromium layer described in this embodiment is electroplated on 304 steel as a substrate through the following steps:

[0031] a. Mechanical grinding and polishing: Use silicon carbide sandpaper to grind the surface of the part to be plated flat, and use a grinder and polishing cloth to fine-polish the surface of the workpiece until the surface roughness Ra is 0.2;

[0032] b. Chemical degreasing: Heat the chemical degreasing liquid to 80°C, then immerse the mechanically polished workpiece in it, operate at 80°C, and process for 10 minutes;

[0033] c. Pickling and activation: Immerse the chemically degreased workpiece in a 15% hydrochloric acid solution for pickling and activation for 25 seconds, rinse with deionized water, and dry for later use;

[0034] d. Slow rising current pre-plating: In order to improve the bonding strength between the coating and the workpiece surface, the current is increased at 0.25A / dm per second. 2 Increase the current slowly to the set value (current value = current density * area to be plated);

[0035] e. Formal plating: CrO3 content in the plating solution is 180g / L, sulfuric acid content is 2.8g / L, pulse plating is used, duty cycle (γ) is 80%, on time is 650ms, off time is 100ms, anode current density is 35A / dm 2 The plating solution temperature is 53°C.

[0036] The degreasing solution includes NaOH, Na2CO3 and Na2SiO3, with the specific composition being NaOH: 80g / L, Na2CO3: 15g / L, and Na2SiO3: 10g / L. During the formal plating process, the workpiece to be plated serves as the cathode and the lead-tin alloy (10% tin content) serves as the anode. A water bath is used for temperature control during the electroplating process, with the cathode and anode spaced 50mm apart. The plating solution is stirred at a rate of 30r / min to ensure sufficient flow in the bath. See the optical microscope image for details. Figure 1 (b).

[0037] Example 2

[0038] The high-strength, crack-free, ultrafine-grained chromium layer described in this embodiment is electroplated on 45 steel using the following steps:

[0039] a. Mechanical grinding and polishing: Use silicon carbide sandpaper to grind the surface of the part to be plated flat, and use a grinder and polishing cloth to fine-polish the surface of the workpiece until the surface roughness Ra is 0.3;

[0040] b. Chemical degreasing: Heat the chemical degreasing liquid to 80°C, then immerse the mechanically polished workpiece in it, operate at 80°C, and process for 10 minutes;

[0041] c. Pickling and activation: Immerse the chemically degreased workpiece in a 10% hydrochloric acid solution for pickling and activation for 30 seconds, rinse with deionized water, and dry for later use;

[0042] d. Slow rising current pre-plating: In order to improve the bonding strength between the coating and the workpiece surface, the current is increased at 0.3A / dm per second. 2 Increase the current slowly to the set value (current value = current density * area to be plated);

[0043] e. Formal plating: CrO3 content in the plating solution is 230 / L, sulfuric acid content is 2.5g / L, pulse plating is used, duty cycle (γ) is 73%, on time is 400ms, off time is 150ms, anode current density is 35A / dm 2 .

[0044] The degreasing solution includes NaOH, Na2CO3 and Na2SiO3, with the specific composition being NaOH: 100g / L, Na2CO3: 10g / L, and Na2SiO3: 8g / L. During the formal plating process, the workpiece to be plated serves as the cathode and the lead-tin alloy (10% tin content) serves as the anode. A water bath is used for temperature control during the electroplating process, with the cathode and anode spaced 50mm apart. The plating solution is stirred at a rate of 30r / min to ensure sufficient flow in the bath. The SEM morphology is shown in the figure. Figure 2 , ingredient picture see Figure 3 、Hardness diagram Figure 4 .

[0045] Example 3

[0046] The high-strength, crack-free, ultrafine-grained chromium layer described in this embodiment is electroplated on 45 steel using the following steps:

[0047] a. Mechanical grinding and polishing: Use silicon carbide sandpaper to grind the surface of the part to be plated flat, and use a grinder and polishing cloth to fine-polish the surface of the workpiece until the surface roughness Ra is 0.3;

[0048] b. Chemical degreasing: Heat the chemical degreasing liquid to 80°C, then immerse the mechanically polished workpiece in it, operate at 80°C, and process for 10 minutes;

[0049] c. Pickling and activation: Immerse the chemically degreased workpiece in a 10% hydrochloric acid solution for pickling and activation for 30 seconds, rinse with deionized water, and dry for later use;

[0050] d. Slow rising current pre-plating: In order to improve the bonding strength between the coating and the workpiece surface, the current is increased at 0.3A / dm per second. 2 Increase the current slowly to the set value (current value = current density * area to be plated);

[0051] e. Formal plating: The composition of the plating solution is CrO3 content of 230 / L, sulfuric acid content of 2.5g / L, pulse plating is used, duty cycle (γ) 73%, on time 400ms, cut-off 150ms, anode current density 35A / dm 2 .

[0052] The degreasing solution is composed of NaOH, Na2CO3, and Na2SiO3, with the specific composition being NaOH: 100g / L, Na2CO3: 10g / L, and Na2SiO3: 8g / L. During the formal plating process, the workpiece to be plated serves as the cathode, and the lead-tin alloy (10% tin content) serves as the anode. A water bath is used for temperature control during the electroplating process, with a 50mm distance between the cathode and the anode. The plating solution is stirred at a rate of 30r / min to ensure sufficient flow in the bath. The EBSD image is shown in the figure. Figure 5 .

[0053] Comparative Example 1

[0054] The only difference between this comparative example and Example 1 is that the temperature of the plating solution is 65° C. The high-strength, crack-free pulse electroplated chromium coating described in this example is electroplated on 304 steel as a substrate through the following steps:

[0055] a. Mechanical grinding and polishing: Use silicon carbide sandpaper to grind the surface of the part to be plated flat, and use a grinder and polishing cloth to fine-polish the surface of the workpiece until the surface roughness Ra is 0.2;

[0056] b. Chemical degreasing: Heat the chemical degreasing liquid to 80°C, then immerse the mechanically polished workpiece in it, operate at 80°C, and process for 10 minutes;

[0057] c. Pickling and activation: Immerse the chemically degreased workpiece in a 15% hydrochloric acid solution for pickling and activation for 25 seconds, rinse with deionized water, and dry for later use;

[0058] d. Slow rising current pre-plating: In order to improve the bonding strength between the coating and the workpiece surface, the current is increased at 0.25A / dm per second. 2 The current density is slowly increased to the set value at a speed of

[0059] e. Formal plating: The composition of the plating solution is 180g / L CrO3 content and 2.8g / L sulfuric acid content. Pulse plating is used with a duty cycle (γ) of 80%, an on time of 650ms, a cut-off time of 100ms, and an anode current density of 35A / dm 2， The plating solution temperature is 65°C.

[0060] The coating prepared in this comparative example has a tested hardness of 486 HV, which is lower than the coating prepared by the method of the present invention.

[0061] Comparative Example 2

[0062] The chromium layer was prepared according to the method of Example 2, except that the anode current density was a pulse process with a duty cycle of 60%. The high-strength, crack-free pulse electroplated chromium layer described in this embodiment was electroplated on 304 steel as a substrate through the following steps:

[0063] The high-strength, crack-free, ultrafine-grained chromium layer described in this embodiment is electroplated on 45 steel using the following steps:

[0064] a. Mechanical grinding and polishing: Use silicon carbide sandpaper to grind the surface of the part to be plated flat, and use a grinder and polishing cloth to fine-polish the surface of the workpiece until the surface roughness Ra is 0.3;

[0065] b. Chemical degreasing: Heat the chemical degreasing liquid to 80°C, then immerse the mechanically polished workpiece in it, operate at 80°C, and process for 10 minutes;

[0066] c. Pickling and activation: Immerse the chemically degreased workpiece in a 10% hydrochloric acid solution for pickling and activation for 30 seconds, rinse with deionized water, and dry for later use;

[0067] d. Slow rising current pre-plating: In order to improve the bonding strength between the coating and the workpiece surface, the current is increased at 0.3A / dm per second. 2 Increase the current slowly to the set value (current value = current density * area to be plated)

[0068] e. Formal plating: The composition of the plating solution is CrO3 content of 230 / L, sulfuric acid content of 2.5g / L, pulse plating is used, duty cycle (γ) 60%, on time 300ms, cut-off 200ms, anode current density 35A / dm 2 .

[0069] The coating prepared in this comparative example has a tested hardness of 513 HV, which is lower than the coating prepared by the method of the present invention.

[0070] Comparative Example 3

[0071] The coating was prepared according to the method of Example 1, except that the current density was 55A / dm 2 The high-strength crack-free pulse electroplated chromium coating described in this embodiment is electroplated on 304 steel as a substrate through the following steps:

[0072] a. Mechanical grinding and polishing: Use silicon carbide sandpaper to grind the surface of the part to be plated flat, and use a grinder and polishing cloth to fine-polish the surface of the workpiece until the surface roughness Ra is 0.2;

[0073] b. Chemical degreasing: Heat the chemical degreasing liquid to 80°C, then immerse the mechanically polished workpiece in it, operate at 80°C, and process for 10 minutes;

[0074] c. Pickling and activation: Immerse the chemically degreased workpiece in a 15% hydrochloric acid solution for pickling and activation for 25 seconds, rinse with deionized water, and dry for later use;

[0075] d. Slow rising current pre-plating: In order to improve the bonding strength between the coating and the workpiece surface, the current is increased at 0.25A / dm per second. 2 Increase the current slowly to the set value (current value = current density * area to be plated)

[0076] e. Formal plating: The composition of the plating solution is 180g / L CrO3 and 2.8g / L sulfuric acid. Pulse plating is used with a duty cycle (γ) of 80%, an on time of 650ms, a cut-off time of 100ms, and an anode current density of 55A / dm 2， The plating solution temperature is 70℃.

[0077] Obvious cracks appeared on the surface of the coating prepared in this comparative example, and the hardness was tested to be 374 HV, which was lower than the hardness of the coating prepared by the method of the present invention.

[0078] Table 1

[0079]

[0080] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0081] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations. In addition, the various different embodiments of the present invention can also be arbitrarily combined, as long as they do not violate the concept of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a high-strength, crack-free, ultrafine-grained chromium layer, characterized in that: The pulse plating method includes the following steps: Step (1): mechanical grinding and polishing; Step (2): chemical degreasing; Step (3): pickling and activation; Step (4): Slowly increase the current pre-plating; Step (5): Formal plating: The workpiece to be plated is used as the cathode and the anode is a lead-tin alloy; the plating solution includes CrO3 and sulfuric acid; the duty cycle γ of the pulse plating is 65-80%, the on time is 450-600ms, the off time is 110-150ms, and the anode current density is 35-40A / dm 2 , the plating solution temperature is 55-62℃.

2. The method according to claim 1, characterized in that Step (1) is specifically as follows: the surface to be plated of the workpiece to be plated is mechanically polished using 80#, 320#, 600# and 1500# silicon carbide sandpaper in sequence, and after polishing, the surface to be plated of the workpiece is polished using a polishing cloth and a polishing paste with a particle size of W3.5 until the surface roughness Ra≤0.

4.

3. The method according to claim 2, characterized in that The chemical degreasing in step (2) is specifically as follows: heating the chemical degreasing liquid to 70-80°C, then immersing the workpiece after mechanical grinding and polishing in the degreasing liquid for 8-10 minutes; the degreasing liquid includes NaOH, Na2CO3 and Na2SiO3, and the specific components are NaOH: 80-120g / L, Na2CO3: 15-25g / L, Na2SiO3: 8-16g / L. After degreasing, rinse with deionized water, and use a hot air blower to blow the workpiece surface to be plated dry before use.

4. The method according to claim 3, characterized in that Step (3) is specifically as follows: immersing the workpiece after chemical degreasing in a 10-15% hydrochloric acid solution for pickling and activation for 15-30 seconds, rinsing with deionized water, and drying for later use.

5. The method according to claim 4, characterized in that Step (4) is as follows: 0.25-0.4A / dm per second 2 The current density is increased to the set current value at a speed of 55-62°C, and the plating solution temperature is 55-62°C; the set current value = current density * area to be plated.

6. The method according to claim 5, characterized in that In step (5), the CrO3 content in the plating solution is 180-260 g / L, the sulfuric acid content is 2.2-3.0 g / L; the tin content in the lead-tin alloy is 6-13%; the distance between the cathode and the anode is 30-50 mm, and the plating solution is stirred during the formal plating, and the stirring rate is 30r-60r / min.

7. The method according to claim 6, characterized in that The material of the workpiece to be plated is steel.

8. A high-strength, crack-free, ultrafine-grained chromium layer, characterized in that: The method according to any one of claims 1 to 7 is used for preparation.

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