CN1237206C - Method for preparing corrosion-resisting wearing-resisting coat used for magnesium and alloy thereof - Google Patents

Abstract

A method for preparing an anti-corrosion and wear-resistant coating for magnesium and its alloys, comprising pretreatment and electroless nickel plating process, characterized in that the pretreatment process is carried out in the order of alkali washing, alkali etching, and activation steps, wherein: alkali etching : The solution components are alkali metal pyrophosphate 40-60g/L, nitrate 10-20g/L, carbonate 10-20g/L, treatment temperature 60-80℃, time 5-15min, medium stirring; activation: The solution components are HF 80-150ml/L, ZnO 25-50g/L, activated at a temperature of 20-40°C for 5-10min, with moderate stirring. The present invention solves the anti-corrosion problem of magnesium alloy by utilizing the electroless nickel plating method, the pretreatment utilizes alkali etching instead of impregnating with chromic acid, and the components of the plating solution are replaced by NH ₄ HF ₂ , which is beneficial to environmental protection, and the operation is safer; Activation by HF+ZnO is a combination of zinc dipping method and direct electroless nickel plating. The corrosion resistance of Ni-P coating and the bonding force with the substrate are further improved through the modification of zinc.

Description

A preparation method for anti-corrosion and wear-resistant coating of magnesium and its alloys

Technical field:

The invention relates to a metal surface coating technology, in particular to depositing a layer of wear-resistant and corrosion-resistant Ni-P protective layer on the surface of magnesium and its alloys by means of electroless nickel plating.

Background technique:

As the lightest structural material, magnesium alloy is hailed as the "green engineering material" of the 21st century by the material industry. It has high specific strength, specific stiffness, specific elastic modulus, good castability, and machinability. Sex and dimensional stability, as well as good damping performance and electromagnetic shielding performance, have been widely concerned by industries such as automobiles, aviation, electronics, and communications. Among them, AZ91D magnesium alloy has good casting performance and mechanical properties, and is most widely used in magnesium alloy castings. 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 painting are generally used, which can achieve certain effects, but the corrosion resistance and wear resistance are still not ideal. If high corrosion-resistant electroless nickel plating can be used for the surface treatment of magnesium alloys, the corrosion resistance of magnesium alloys will be greatly improved, and its application range will also be broadened.

Electroless nickel plating is the process of using the reducing agent in the plating solution to reduce nickel ions and deposit them on the surface of the substrate. The main reason for the rapid development of the electroless nickel plating process since the advent of the electroless nickel plating process in 1945 is that the coating has excellent corrosion resistance and wear resistance. and electromagnetic properties. According to ASTM B480-68, there are mainly two 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 complicated 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. However, for the AZ91D magnesium alloy with a high aluminum content, since the intermetallic compound β phase (Mg ₁₇ Al ₁₂ ) segregates along the grain boundary and forms a galvanic couple with the matrix magnesium, it is difficult to obtain good corrosion resistance. , Coating with wear-resistant effect.

Technical content:

The purpose of the present invention is to provide a method for electroless nickel plating on the surface of magnesium alloys. By adjusting the solution composition and process parameters in the pretreatment of electroless nickel plating and the plating process, it is difficult to solve the problem of AZ91D magnesium alloys with higher aluminum content. Plating problems, while making the operation safer and more environmentally friendly.

The invention provides a preparation method for anti-corrosion and wear-resistant coatings of magnesium and its alloys, including pretreatment and electroless nickel plating process, characterized in that the pretreatment process is carried out in sequence according to the steps of alkali washing, alkali etching and activation, wherein :

Alkali etching: The composition of the solution is 40-60g/L of alkali metal pyrophosphate, 10-20g/L of nitrate, 10-20g/L of carbonate, the treatment temperature is 60-80℃, the time is 5-15min, medium stirring ;

Activation: The composition of the solution is HF 80-150ml/L, ZnO 25-50g/L, activated at a temperature of 20-40°C for 5-10min, with medium stirring.

The present invention is used in the preparation method of magnesium and its alloy anti-corrosion and wear-resistant coating, and the key lies in the pretreatment process, electroless Ni-P plating can adopt comparatively conventional plating solution and method, preferably select following technology for use certainly:

(1) Plating solution composition

Ni(CH ₃ COOH) ₂ 4H ₂ O 15-30g/L, or NiCO ₃ 2Ni(OH) ₂ 4H ₂ O 10-20g/L;

NaH ₂ PO ₂ ·H ₂ O 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 0.01-0.02g/L, or thiourea 0.001-0.002g/L;

_CH3COONa 5-15g/L;

Sodium dodecylbenzenesulfonate 0.01-0.025g/L;

(2) The plating process conditions are: the pH value of the plating solution is between 5.0-6.5, and the plating temperature is 80-90°C.

In the preparation method of the present invention for the anti-corrosion and wear-resistant coating of magnesium and its alloys, the alkali washing in the pretreatment process is a relatively conventional step, specifically: lye components NaOH 10-20g/L, Na ₂ CO ₃ 15- 25g/L, wash at 80-95°C for 5-15min.

In addition, when the present invention is used in the preparation method of the anti-corrosion and wear-resistant coating of magnesium and its alloys, it can also be ultrasonically cleaned in acetone for 5-10 minutes after alkaline cleaning.

Since magnesium alloys corrode faster in solutions containing Cl ^- and SO ₄ ^2-, Cl ^- and SO ₄ ^2- should not be added to the plating solution, so the main salt of the electroless nickel plating system of the present invention is Ni(CH ₃ COO) ₂ 4H ₂ O, which can control the corrosion of the substrate, increase the life of the plating solution, and reduce the internal stress of the coating; the reducing agent is sodium hypophosphite, which is cheap and the plating solution is easy to control; the complexing agent is citric acid, The PH value is 6.9, the complexation ability is strong, the coating grows rapidly along the two-dimensional direction parallel to the substrate, and the resulting coating is flat, dense, and small in porosity; PH is an important factor affecting the quality of the coating, and its change affects the deposition rate of the coating , phosphorus content, stress distribution and stability of the plating solution, etc. During the plating process, the pH must be strictly controlled between 6.0-6.5; temperature is the most influential factor on the deposition rate of electroless nickel plating, and the temperature is low, the deposition rate is slow, and even Plating cannot be applied, the temperature is high, the plating speed is fast, the phosphorus content of the coating decreases, and the square and porosity increase, which affects the performance of the coating. Therefore, the plating temperature should be appropriate and uniformly controlled, and kept at 80-85 ° C to 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 10-25μm/h, and the phosphorus content can reach 7-13wt%. corrosion resistance of the coating.

Advantages of the present invention: use the method of chemical nickel plating to solve the anti-corrosion problem of magnesium alloys, use alkali etching instead of chromic acid impregnation for pretreatment, replace HF with NH ₄ HF ₂ in the plating solution, which is beneficial to environmental protection, and the operation It is safer; the activation of HF+ZnO is the combination of zinc dipping method and direct electroless nickel plating, and the corrosion resistance of Ni-P coating and the bonding force with the substrate are further improved through the modification of zinc.

Description of drawings:

Fig. 1 is the surface morphology of embodiment 1;

Fig. 2 is the surface morphology of embodiment 2;

Fig. 3 is the surface morphology of embodiment 3;

The potentiodynamic polarization curve of Fig. 4 embodiment 1 and substrate;

The potentiodynamic polarization curve of Fig. 5 embodiment 2 and substrate;

Fig. 6 is the potentiodynamic polarization curve of Example 3 and the substrate.

Detailed ways:

The embodiments of the present invention are specifically carried out according to the following steps, which of course do not limit the present invention.

1) Alkaline washing of the sample: the alkaline solution consists of NaOH 10-20g/L and Na ₂ CO ₃ 15-25g/L. Wash at 80-95°C for 5-15min to remove oil and impurities on the surface.

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

3) Alkali etching: the solution composition is alkali metal pyrophosphate 40-60g/L, nitrate 10-20g/L, carbonate 10-20g/L, the treatment temperature is 60-80°C, the time is 5-15min, Agitate moderately to remove surface oxides, hydroxides, etc.

4) Rinse with distilled water.

5) Activation: The composition of the solution is HF 80-150ml/L, ZnO 25-50g/L, activate at a temperature of 20-40°C for 5-10min, stir moderately, activate the surface, and try to equipotentially.

6) Rinse with distilled water.

7) Electroless nickel plating system:

①Main salt: The main salt of electroless nickel plating is nickel salt, such as the most commonly used nickel sulfate NiSO ₄ .6H ₂ O, nickel chloride NiCl ₂ 6H ₂ O was used as the main salt in the early days, but due to the existence of Cl ^- not only It will reduce the corrosion resistance of the coating and produce tensile stress, so it is no longer used. For magnesium and its alloys, due to its own high activity, nickel sulfate or nickel chloride should not be selected as its main salt, and the main salt of the present invention is defined as nickel acetate or basic nickel carbonate.

②Reducing agents include sodium hypophosphite, sodium borohydride, alkylammonium boron and hydrazine, etc. Their common feature in structure is that they contain two or more active hydrogens, and the reduction of Ni ²⁺ is carried out by the catalytic dehydrogenation of the reducing agent of. Because sodium hypophosphite is cheap, the plating solution is easy to control, and the performance of Ni-P alloy coating is excellent, so the reducing agent of the present invention selects sodium hypophosphite.

③The complexing agent is the most important component 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, increase the deposition rate, and increase the pH of the plating bath. range and improve coating quality. Commonly used complexing agents are mainly aliphatic carboxylic acids and their substituted derivatives, such as succinic acid, citric acid, lactic acid, malic acid and glycine. Due to the strong complexing ability of citric acid, the coating grows along the two-dimensional direction parallel to the substrate, and the coating is smooth and compact, so the complexing agent of the present invention 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 control. At present, the commonly used stabilizers in electroless nickel plating can be divided into four categories: A: Compounds of the VIA elements S, Se, Te, such as thiourea, thiohydrogen salt, etc. B: Some oxygen-containing compounds, such as IO ₃ ^- , BrO ₃ ^- , NO ₂ ^- , MoO ₄ ^2- and H ₂ O ₂ etc. C: Heavy metal ions, such as Pb ₂ ⁺ , Sn ₂ ⁺ , Sb ₃ ⁺ and Ca ₂ ⁺ , Zn ₂ ⁺ , Bi ₃ ⁺ etc. D: Water-soluble organic matter such as unsaturated fatty acid maleic acid, methylene succinic acid, etc.

The present invention selects KIO ₃ or thiourea as a stabilizer.

5. buffering agent effect is to guarantee that the pH value of plating bath will not change too much in the plating process, can maintain the normal value in certain pH range, and the present invention selects sodium acetate as buffering 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 is formed on the surface, which can not only keep warm, reduce the evaporation loss of the plating solution, but also help to remove dirt and keep the plating solution and plating parts clean. Surfactants commonly used in electroless nickel plating are anionic surfactants, such as sodium dodecylbenzenesulfonate or sodium dodecylsulfate, and sodium dodecylbenzenesulfonate is selected in the present invention.

⑦PH 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, etc. During the plating process of magnesium and its alloys, the pH range must be strictly controlled, generally between 4.8- Between 6.5.

⑧Temperature is the factor that has the greatest influence on the deposition rate of electroless nickel plating. Low temperature and slow deposition rate can even prevent plating. High temperature and fast plating speed will decrease the phosphorus content of the coating, increase stress and porosity, and affect the performance of the coating. Therefore, the plating temperature should be appropriate and evenly controlled, and kept at 80-85°C to avoid local overheating.

Example 1, 2, 3

The selected materials are all cast AZ91D, the sample size is 15mm×10mm×3mm, polished with 1000# water sandpaper to ensure that the sample has the same surface finish, and then ultrasonic cleaning with acetone→alkali etching→water washing→hydrofluoric acid+ZnO activation→ Washing → electroless nickel plating. The pretreatment and plating conditions of the three embodiments are shown in the table below. The surface morphology of the plated samples under the three conditions was observed by scanning electron microscope as shown in Figures 1 to 3, the surface is smooth and the structure is compact; the corrosion resistance performance was tested using the potentiodynamic polarization test method, using a standard three-electrode system, the sample It is the working electrode, the platinum sheet is the auxiliary electrode, and the saturated calomel electrode is the reference electrode. It is tested in a NaCl solution with a concentration of 3.5 wt%. Compared with the test results of the matrix alloy, it shows that the process of the present invention has played an ideal role in AZ91D magnesium alloy. The anti-corrosion effect, the open circuit potential is greatly increased, the tendency of corrosion is reduced, and the corrosion current density is reduced, as shown in Figure 4-6. Example 1 Example 2 Example 3 Alkali etching Na ₄ P ₂ O ₇ 40g/LNaNO ₃ 10g/LNa ₂ CO ₃ 10g/L Temperature 50℃ Time 15min Medium stirring Na ₄ P ₂ O ₇ 50g/LNaNO ₃ 15g/LNa ₂ CO ₃ 15g/L Temperature 60℃ Time 10min Medium stirring Na ₄ P ₂ O ₇ 60g/LNaNO ₃ 20g/LNa ₂ CO ₃ 20g/L Temperature 7℃ Time 5min Medium stirring activation HF 80ml/LZnO 25g/L temperature 40℃ time 5min medium stirring HF 100ml/LZnO 35g/L temperature 30℃ time 10min medium stirring HF 150ml/LZnO 50g/L temperature 20℃ time 10min medium stirring Electroless Nickel Plating System Ni(CH ₃ COOH) ₂ 4H ₂ O 15g/LNaH ₂ PO ₂ H ₂ O 15g/LNH ₄ HF ₂ 10g/LC ₆ H ₈ O ₇ H ₂ O 5g/LNH ₄ H ₂ O 20ml/LKIO ₃ 0.01g/LCH ₃ COONa 5g/L Sodium dodecylbenzenesulfonate 0.01g/LPPH value 5.0 Temperature 80℃ Ni(CH ₃ COOH) ₂ 4H ₂ O 20g/LNaH ₂ PO ₂ H ₂ O 20g/LNH ₄ HF ₂ 15g/LC ₆ H ₈ O ₇ H ₂ O 10g/LNH ₄ H ₂ O 25ml/LKIO ₃ 0.015g/LCH ₃ COONa 10g/L sodium dodecylbenzenesulfonate 0.02g/LPPH value 6.0 temperature 85℃ Ni(CH ₃ COOH) ₂ 4H ₂ O 30g/LNaH ₂ PO ₂ H ₂ O 30g/LNH ₄ HF ₂ 20g/LC ₆ H ₈ O ₇ H ₂ O 15g/LNH ₄ H ₂ O 30ml/LKIO ₃ 0.02g/LCH ₃ COONa 15g/L sodium dodecylbenzenesulfonate 0.025g/LPPH value 6.5 temperature 90℃

Claims (4)

1, a kind of preparation method who is used for magnesium and alloy corrosion-resisting wearing-resisting coat thereof comprises pre-treatment and nickel process, it is characterized in that the pre-treatment process carries out in proper order according to alkali cleaning, alkaline etching, activation step, wherein:

Alkaline etching: solution composition is alkali metal pyrophosphate 40-60g/L, nitrate 10-20g/L, carbonate 10-20g/L, and treatment temp 60-80 ℃, time are 5-15min, medium stirring;

Activation: solution composition is HF 80-150ml/L, ZnO 25-50g/L, activates 5-10min under 20-40 ℃ temperature, medium stirring.

2, according to the described preparation method who is used for magnesium and alloy corrosion-resisting wearing-resisting coat thereof of claim 1, it is characterized in that in the described nickel process:

(1) solution composition

Ni (CH ₃COOH) ₂4H ₂O 15-30g/L, or NiCO ₃2Ni (OH) ₂4H ₂O 10-20g/L;

NaH ₂PO ₂·H ₂O 15-30g/L；

NH ₄HF ₂ 10-20g/L；

C ₆H ₈O ₇·H ₂O 5-15g/L；

NH ₄·H ₂O 20-30ml/L；

KIO ₃0.01-0.02g/L, or thiocarbamide 0.001-0.002g/L;

CH ₃COONa 5-15g/L；

Sodium dodecylbenzene sulfonate 0.01-0.025g/L;

(2) the plating technology condition is: the plating bath pH value between 5.0-6.5, plating temperature 80-90 ℃.

3, according to claim 1 or the 2 described preparation methods that are used for magnesium and alloy corrosion-resisting wearing-resisting coat thereof, it is characterized in that the alkali cleaning in the pre-treatment process: the alkali lye composition is NaOH 10-20g/L, Na ₂CO ₃15-25g/L cleans 5-15min under 80-95 ℃ of temperature.

4,, it is characterized in that after the alkali cleaning ultrasonic cleaning 5-10min in acetone according to the described preparation method who is used for magnesium and alloy corrosion-resisting wearing-resisting coat thereof of claim 3.

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