CN1219110C - Method for preventing magnesium and its alloy parts from corrosion and wearing - Google Patents

Abstract

The invention provides a method for preventing corrosion and wear resistance of magnesium and its alloy components, which is characterized in that: a composite layer of conversion film and nickel phosphorus is attached to the surface of the matrix of magnesium and its alloy components; Reticulation and cracks, the thickness is about 3-5μm; the phosphorus content in the nickel-phosphorus layer is 7-13wt%, and the deposition rate is 8-15μm/h. Firstly, the magnesium alloy is chemically converted to form a chemical conversion film on the surface of the magnesium alloy, and then electroless nickel plating is performed on the chemical conversion film. It not only solves the problem of insignificant corrosion resistance of the magnesium alloy chemical conversion film, but also solves the environmental protection problem of the direct electroless nickel plating pretreatment of the magnesium alloy. On the AZ91D and AM50 magnesium alloy examples, the coating with good corrosion resistance and wear resistance effect is obtained, which provides an effective measure for the protection of magnesium alloys.

Description

A method for preventing corrosion and wear resistance of magnesium and its alloy components

Technical field:

The invention relates to a metal surface treatment technology, specifically depositing a wear-resistant and corrosion-resistant Ni-P protective layer on the surface of magnesium and its alloys by using a method of combining chemical conversion treatment and electroless nickel plating.

Background technique:

As the lightest structural material, magnesium alloy is known as the "green engineering material" in the 21st century. It has high specific strength, specific stiffness, specific modulus of elasticity, good castability, machinability and dimensional stability. At the same time, it has good damping performance and electromagnetic shielding performance. It has been widely concerned in the automotive, aviation, electronics, communication and other industries, and the annual application growth rate exceeds 20%. However, due to the high chemical activity of magnesium alloys, it is easy to oxidize in the air, forming a loose and poorly protective oxide film, which will lead to serious corrosion of magnesium alloys in humid atmosphere, soil and seawater, hindering the development of magnesium alloys. wide application. In order to improve the corrosion resistance of magnesium alloys, surface treatment methods such as chemical conversion, anodic oxidation, and physical vapor deposition coatings are generally used, which can achieve certain effects, but the corrosion resistance and wear resistance are still not ideal. As a functional coating, electroless nickel plating has good wear resistance, corrosion resistance and other performance characteristics. If electroless nickel plating can be successfully applied to the protection of magnesium alloys, it will greatly improve the corrosion resistance of magnesium alloys. , and can also broaden its scope of application. At present, electroless nickel plating has been studied a lot in the protection of magnesium alloys.

According to ASTM B480-88, there are two main methods of electroless nickel plating on magnesium alloys: immersion zinc and direct electroless plating. The zinc dipping method is to dip zinc in a zinc salt solution containing pyrophosphate, then make a primer by cyanide copper plating, and then perform electroless plating. This process is complex and not suitable for magnesium alloys with high aluminum content. At the same time, problems such as the safety of cyanide use and waste liquid treatment need to be solved urgently. Therefore, the method of direct electroless nickel plating has been paid attention to. The process of direct electroless nickel plating is alkaline degreasing first, then chromic acid impregnation and hydrofluoric acid activation treatment, and finally electroless nickel plating. In the pretreatment process, both chromic acid and hydrofluoric acid are harmful to health and environmental protection. Therefore, improving the pretreatment process of direct electroless nickel plating on magnesium alloys and developing environmentally friendly pretreatment methods have great application value.

Technical content of the invention:

The purpose of the present invention is to provide a composite electroless nickel coating on the surface of magnesium alloy components to prevent corrosion of magnesium alloys, which not only solves the shortcomings of the insignificant protective performance of the chemical conversion coating itself, but also solves the problem of direct chemical corrosion of magnesium alloys. The problems existing in the pretreatment process of nickel plating, and the conversion coating as a transition layer, avoids the direct combination of the nickel plating layer and the magnesium alloy substrate, reduces the potential difference between the two, and avoids the strong electric shock after the nickel plating layer is destroyed. The occurrence of dual corrosion can play an ideal protective role for magnesium alloys.

The invention provides a method for preventing corrosion and wear resistance of magnesium and its alloy components, which is characterized in that: a composite layer of conversion film and nickel phosphorus is attached to the surface of the matrix of magnesium and its alloy components; Reticulation and cracks, the thickness is 3-5 μm; the phosphorus content in the nickel-phosphorus layer is 7-13wt%, and the deposition rate is 8-15 μm/h.

The preparation process of the magnesium alloy composite electroless nickel coating of the present invention is as follows: the chemical conversion treatment can be chromate conversion, phosphate conversion, permanganate conversion and stannate conversion treatment. The components of the chromate conversion treatment solution can be chromic acid, alkali metal chromate or dichromate and some activators such as aluminum potassium sulfate, etc.; the main components of the chromate conversion coating are trivalent chromium and hexavalent chromium Compound and magnesium chromate, the conversion coating is very thin, only about 3-5μm, and there are some lines and cracks on the surface of the coating layer, which has a certain adsorption capacity. Phosphate conversion treatment can be carried out in two types of solutions, one uses manganese, iron dihydrogen phosphate and sodium fluoride or potassium fluoride solution, the film layer is mainly composed of manganese phosphate, which is the soluble dihydrogen phosphate in the treatment solution The hydrolysis product of hydrogen salt; another solution using alkali metal phosphate, nitrate and fluoroborate, and the main composition of the conversion film is magnesium phosphate; both types of films have a porous structure and can be used as a pre-coating for electroless nickel plating. Treatment provides good adsorption conditions. The permanganate conversion is mainly treated with a mixed solution of potassium permanganate and alkali metal fluoride. The film layer is mainly composed of magnesium oxide and fluoride, with some small cracks and strong adsorption capacity. From the perspective of environmental protection, the present invention adopts stannate conversion treatment, and the main component of the conversion film is magnesium stannate. The solution composition is an alkaline solution with alkali metal stannate and pyrophosphate as the main components. Stannate is the main film-forming agent, and pyrophosphate can remove oxides or oil on the surface of the alloy very well. A relatively uniform and dense conversion film with good adsorption capacity can be obtained at a pH value. Taking sodium salt as an example, the solution composition and operating conditions are shown in Table 1.

Electroless nickel plating can adopt any known magnesium alloy nickel plating process, and the pretreatment process includes sensitization, activation and reduction processes. For the plating solution system, the main salt can be nickel acetate or basic nickel carbonate; the reducing agent can be sodium hypophosphite; the complexing agent can be citric acid. growth, the obtained coating is flat, dense, and has small porosity; the pH value is strictly controlled between 6.0-6.5 during the plating process; temperature is the most influential factor on the deposition rate of electroless nickel plating, and the plating temperature should be appropriate and controlled Evenly, keep at 80-85°C, avoid local overheating. The optimum process conditions were determined by orthogonal experiments. According to the above process, the deposition rate of electroless nickel plating on magnesium alloy can reach 8-15μm/h, and the phosphorus content can reach 7-13wt.%. The structure and composition provide a certain guarantee for the corrosion resistance of the coating.

Advantages of the present invention: chemical conversion and electroless nickel plating are combined reasonably, and electroless nickel plating is carried out on the chemical conversion film, which not only solves the shortcoming of the chemical conversion film itself that the protection effect is not obvious, but also makes full use of the chemical conversion film itself. The characteristics of porosity, it provides good adsorption conditions for the pretreatment process of electroless nickel plating, and can successfully carry out electroless nickel plating on the chemical conversion film, which greatly improves the corrosion resistance and wear resistance of magnesium alloys.

Description of drawings:

Figure 1 is the surface morphology of AZ91D alloy after stannate conversion treatment;

Figure 2 is the cross-sectional morphology of AZ91D alloy after stannate conversion treatment;

Figure 3 is the surface morphology of AM50 alloy after stannate conversion treatment;

Figure 4 is the cross-sectional morphology of AM50 alloy after stannate conversion treatment;

Figure 5 is the surface morphology of deposited nickel on the AZ91D alloy conversion film;

Figure 6 is the cross-sectional morphology of deposited nickel on the AZ91D alloy conversion film;

Figure 7 is the surface morphology of deposited nickel on the AM50 alloy conversion film;

Figure 8 is the cross-sectional morphology of deposited nickel on the AM50 alloy conversion film;

Fig. 9 is the polarization curve when the nickel-plated sample on the stannate conversion film of AZ91D and AM50 alloy is polarized to 1.0V above the self-corrosion potential in 3.5wt% NaCl solution;

Fig. 10 is the surface morphology of the electroless nickel plating layer on the stannate conversion coating of AZ91D and AM50 alloy after polarization test (the morphology of the two is similar).

Detailed ways:

1) Alkaline washing of the sample: the alkaline solution consists of NaOH 10-20g/L and Na ₂ CO ₃ 15-25g/L, and wash at 85-95°C for 5-15min to remove oil and impurities on the surface.

2) Ultrasonic cleaning with acetone for 5-10 minutes.

3) Chemical conversion treatment: chemical conversion treatment can use a solution of chromate, phosphate, permanganate, and stannate as a film-forming agent. The solution components for chromate treatment can be selected from Na ₂ Cr ₂ O ₇ 2H ₂ O 150-180g/L, H ₃ NO ₃ 120-180g/L, NH ₄ HF ₂ 5-15g/L, temperature 20-35°C, The treatment time is 2-5min, with medium stirring; the solution components of stannate conversion treatment can be selected from Na ₂ SnO ₃ 3H ₂ O 30-50g/L, Na ₄ P ₂ O ₇ 30-50g/L, NaOH 5-15g/L L. NaCH ₃ COO·3H ₂ O 5-15g/L, temperature 70-90°C, treatment time 50-70min, medium stirring.

4) Rinse with distilled water.

5) Ultrasonic cleaning with acetone for 5-10 minutes to remove residual substances on the surface during chemical conversion.

6) Sensitization treatment: the purpose is to adsorb a layer of reducing Sn ²⁺ on the chemical conversion film, so that during the activation treatment, palladium ions can be reduced to catalytic palladium atoms, and active centers can be generated during electroless plating. The solution components are SnCl ₂ 5-15g, HCl 5-10mL, H ₂ O 1000mL, the treatment temperature is room temperature, the time is 1-5min, and the stirring is moderate.

7) Activation treatment: the purpose is to produce a thin layer of catalytic noble metal Pd on the surface of the chemical conversion film as a catalyst for the oxidation-reduction reaction during electroless nickel plating. The solution components are PdCl ₂ 0.5-1.5g, C ₂ H ₅ OH 500mL, H ₂ O 500mL, the treatment temperature is room temperature, the time is 1-5min, and moderately stirred.

8) Reduction treatment: After the sample is activated, it is necessary to perform reduction treatment to reduce the excess noble metal ions adsorbed, so as to prevent these ions from destroying the stability of the electroless nickel plating solution. The components of the solution are NaH ₂ PO ₂ ·H ₂ O 20-50g/L, H ₂ O 1000mL, the treatment temperature is room temperature, the time is 10s-2min, and the stirring is moderate.

9) Rinse with distilled water and dry with cold air.

10) Electroless nickel plating: The basic components of the plating solution of electroless nickel plating are as follows:

①Main salt: For magnesium and its alloys, nickel sulfate or nickel chloride is not suitable for the main salt due to its high activity. The main salt should be nickel acetate or basic nickel carbonate.

② The reducing agent can be sodium hypophosphite, sodium borohydride, alkyl ammonia boron and hydrazine, etc. Their common feature in structure is that they contain two or more active hydrogens, and the reduction of Ni ²⁺ depends on the catalytic desorption of the reducing agent. performed by hydrogen. Because sodium hypophosphite is cheap, the plating solution is easy to control, and the Ni-P alloy coating has excellent performance, sodium hypophosphite is selected as the reducing agent.

③The complexing agent is the most important part of the plating solution except the main salt and the reducing agent. Its main function is to prevent the precipitation of the plating solution, increase the stability of the plating solution and prolong the service life, and increase the pH range of the plating bath. and improve coating quality. Complexing agent selects citric acid.

④ The role of the stabilizer is to inhibit the spontaneous decomposition of the plating solution, so that the plating process can be carried out in an orderly manner under controlled conditions. Choose KIO ₃ or Thiuria as a stabilizer.

⑤ The function of the buffer is to ensure that the pH value of the plating solution does not change too much during the plating process and can be maintained within a certain pH range. Sodium acetate is selected as the buffer agent.

⑥Surfactant helps the escape of gas (H ₂ ) and reduces the porosity of the coating; in addition, because the surfactant also acts as a foaming agent, the plating solution is agitated by the escaped gas during the plating process. A layer of white foam can be formed on the surface, which can keep warm, reduce the evaporation loss of the plating solution, and also help to remove dirt and keep the plating solution and plated parts clean. Sodium dodecylbenzenesulfonate was selected as the surfactant.

⑦The pH value is an important factor affecting the quality of the coating. Its change affects the deposition rate, phosphorus content, stress distribution and stability of the plating solution. Between 5.0-6.5.

⑧Temperature is the most influential factor on the deposition rate of electroless nickel plating. The plating temperature should be appropriate and evenly controlled, and kept at 80-85°C to avoid local overheating. The overall composition is shown in Table 1.

Table 1 Element content Ni(CH ₃ COOH) ₂ 4H ₂ O or NiCO ₃ 2Ni(OH) ₂ 4H ₂ O 15-30g/L or 10-20g/L NaH ₂ PO ₂ ·H2O 15-30g/L NH ₄ HF ₂ 10-20g/L C ₆ H ₈ O ₇ H ₂ O 5-15g/L NH ₄ ·H ₂ O 20-30ml/L KIO ₃ or Thiuria 0.01-0.02g/L or 0.001-0.002g/L CH ₃ COONa 5-15g/L SDBS 0.01-0.025g/L pH value 5.0-6.5 temperature 80-90℃

Example 1

The material is cast AZ91D alloy, the sample size is 15mm×10mm×3mm, and it is polished with 1000grit sandpaper to ensure that the substrate has the same surface roughness, and then alkaline degreasing → acetone ultrasonic cleaning → chemical conversion treatment → water washing → acetone ultrasonic Cleaning → chemical nickel plating pretreatment → chemical nickel plating. The solution composition and operating conditions of electroless nickel plating are the same, as shown in Table 2. The solution composition and operating conditions of the chemical conversion treatment and the solution composition and operating conditions of the electroless nickel plating pretreatment process are shown in Table 3.

Example 2

The material is cast AM50 alloy, the sample size is 15mm×10mm×3mm, and it is polished with 1000grit sandpaper to ensure that the substrate has the same surface roughness, and then alkaline degreasing → acetone ultrasonic cleaning → chemical conversion treatment → water washing → acetone ultrasonic Cleaning → chemical nickel plating pretreatment → chemical nickel plating. The solution composition and operating conditions of electroless nickel plating are the same, as shown in Table 2. The solution composition and operating conditions of the chemical conversion treatment and the solution composition and operating conditions of the electroless nickel plating pretreatment process are shown in Table 3.

Table 2 Ni(CH ₃ COOH) ₂ 4H ₂ O 20g/LNaH ₂ PO ₂ H2O 20g/LNH ₄ HF ₂ 20g/LC ₆ H ₈ O ₇ H ₂ O 6.5g/LNH ₄ H ₂ O 30ml/LKIO ₃ 0.015g/LCH ₃ COONa 10g/L sodium dodecylbenzene sulfonate 0.02g/L pH value 6.5 temperature 85℃

table 3 Embodiment 1AZ91D alloy Solution composition and operating conditions for chemical conversion treatment Na ₂ SnO ₃ ·3H ₂ O 30G/L Temperature 90℃Na ₄ P ₂ O ₇ 30g/L Time 50minNaOH 5g/L Medium stirring NaCH ₃ COO·3H ₂ O 5g/L Components and Operating Conditions of Pretreatment Solution for Electroless Nickel Plating Sensitization SnCl ₂ 5g room temperature, 2.5min, stirring HCl 5mLH ₂ O 1000mL activation PdCl ₂ 0.5g room temperature, 25min, stirring C ₂ H ₅ OH 500mLH ₂ O 500mL reducing NaH ₂ PO ₂ H ₂ O 20g/L room temperature, 1min ,agitation Embodiment 2AM50 alloy Solution composition and operating conditions for chemical conversion treatment Na ₂ SnO ₃ ·3H ₂ O 50g/L Temperature 70℃Na ₄ P ₂ O ₇ 50g/L Time 70minNaOH 15g/L Medium stirring NaCH ₃ COO·3H ₂ O 15g/L Components and Operating Conditions of Pretreatment Solution for Electroless Nickel Plating Sensitization SnCl ₂ 15g room temperature, 4min, stirring HCl 10mLH ₂ O 1000mL activation PdCl ₂ 1.5g room temperature, 4min, stirring C ₂ H ₅ OH 500mLH ₂ O 500mL reducing NaH ₂ PO ₂ H ₂ O 50g/L room temperature, 2min, agitation

The surface and cross-sectional morphology of the chemical conversion coatings of the two embodiments are shown in Figures 1 to 4. It can be seen that the stannate conversion coating is formed by densely packing fine spherical particles, and there are gaps between the particles, which will provide a solid foundation for the subsequent electroless nickel plating. The pretreatment process provides good adsorption conditions, and the combination of the conversion coating and the substrate is good. The main composition of the conversion coating is MgSnO ₃ ·3H ₂ O. The surface and cross-sectional morphology of the electroless nickel plating on the conversion coating are shown in Figures 5-8. It can be seen that the microstructure of the electroless nickel plating layer deposited on the AZ91D and AM50 alloy chemical conversion coatings is very dense without surface defects. From the cross-sectional morphology It can be seen that the bonding condition of the electroless nickel plating layer is good, and there is an excessive layer of magnesium stannate between the electroless nickel plating layer and the substrate. Energy spectrum analysis shows that the phosphorus content of the coating reaches 7-13wt%, which provides a good guarantee for the corrosion resistance of the coating from the structure and composition. The result of potentiodynamic polarization test in 3.5wt% NaCl solution shows that the electroless nickel plating layer of the two alloys has obvious passivation phenomenon in the process of anodic polarization, as shown in Fig. 9 . The surface morphology after the polarization test is shown in Figure 10. Even when the anodic polarization is 1.0V above the self-corrosion potential, the nickel plating layer only produces some extremely small pits, but it does not corrode or crack. Plays an ideal protective role on the substrate.

Claims (10)

1. A method for preventing corrosion and wear of magnesium and its alloy components, characterized in that: a composite layer of conversion film and nickel-phosphorus is attached to the surface of the substrate of magnesium and its alloy components; there are some textures on the surface of the conversion film layer and cracks, the thickness is 3-5 μm; the phosphorus content in the nickel-phosphorus layer is 7-13wt%, and the deposition rate is 8-15 μm/h. 2. The method for preventing corrosion and wear resistance of magnesium and its alloy components according to claim 1, characterized in that: the conversion coating is a chromate conversion coating, and its main components are compounds of trivalent chromium and hexavalent chromium and Magnesium chromate. 3. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 1, characterized in that: the conversion coating is a phosphate conversion coating, and the main component is manganese phosphate or magnesium phosphate. 4. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 1, characterized in that: the conversion coating is a permanganate conversion coating, and its main components are magnesium oxide and fluoride. 5. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 1, characterized in that: the conversion coating is a stannate conversion coating, the main component of which is magnesium stannate. 6. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to any one of claims 1-5, characterized in that the composite layer is firstly obtained through chemical conversion treatment, and then electroless nickel plating treatment. 7. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 6, characterized in that: for chromate conversion coating, the components of the treatment solution are chromic acid, alkali metal chromate or dichromate and activators. 8. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 6, characterized in that: for the phosphate conversion coating, the components of the treatment solution are dihydrogen phosphate of manganese and iron and sodium fluoride or fluorine solutions of potassium chloride, or solutions of phosphates, nitrates and fluoroborates using alkali metals. 9. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 6, characterized in that: for the permanganate conversion coating, the composition of the treatment solution is a mixture of potassium permanganate and alkali metal fluoride solution. 10. The method for making magnesium and its alloy components anti-corrosion and wear-resistant according to claim 6, characterized in that: for the stannate conversion coating, the components of the treatment solution use alkali metal stannate and pyrophosphate as the main components alkaline solution.

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