Process and equipment of electroless nickel platin

Process and equipment of electroless nickel plating on zinc alloy die castings

1 How to electroless nickel plating on zinc alloy castings

electroless nickel plating on zinc alloy die castings can improve its surface hardness, improve wear resistance, and broaden the scope of application. Because the amphoteric behavior system of zinc automatically exits the calibration state, special treatment must be carried out, and the following processes must be selected:

(1) determine a reasonable process flow

two typical process flows: ① weak alkali degreasing → weak corrosion (1%h2so4) → activation (pyrophosphate) → pre plating neutral nickel → chemical nickel plating → heat treatment; ② Water spray (1% ~ 20% H2SO4, 10 ~ 15s) → neutral nickel pre plating → preheating → (citric acid 1 ~ 2g/l, 80 ~ 90 ℃, 1min) → electroless nickel plating → heat treatment

(2) pre plating neutral nickel

neutral nickel formula 1#: nickel sulfate 160 ~ 180g/l, sodium citrate 192 ~ 216g/l, sodium chloride 10 ~ 16g/l, boric acid 25 ~ 30g/L; pH=7，DK=4A/dm2，45℃，≥20min。 Formula 2 we can easily see from the parameters provided by the PVC resin manufacturer #: nickel sulfate 100 ~ 180g/l, sodium citrate 130 ~ 200g/l, sodium chloride 15 ~ 20g/l, boric acid 30 ~ 40g/l, additive a3ml/l; Ph=7 ~ 7.5, dk=1 ~ 1.5a/dm2, impulse current 2 ~ 4a/dm2, impulse time 5 ~ 7min, 50 ~ 60 ℃, 20 ~ 25min, cathode movement 20 ~ 30 times/min

precautions for neutral nickel pre plating: ① use the pre plating solution with nickel sulfate as the main salt, which is close to the composition of electroless nickel plating. The pH value should be controlled at about 7 ~ 7.5. ② Ensure that the bottom layer has sufficient thickness and improve the quality of the bottom layer. If additive A is added to formula 2, the adhesion between the bottom layer and the matrix can be improved, the appearance becomes smooth and delicate, and the yield of the product reaches more than 95%. ③ Properly handle the Ni (OH) 2 Precipitation generated in the solution to restore the plating capacity of the plating solution. If the precipitation in the lower part of the plating solution is separated from the upper clear solution by siphon, then the precipitation is dissolved with dilute sulfuric acid or citric acid, and then the two parts of the solution are mixed, and the pH is adjusted to the specified value with 5% NaOH

(3) electroless nickel plating

electroless nickel plating formula 1#: nickel sulfate 20 ~ 30g/l, sodium hypophosphite 20 ~ 30g/l, boric acid 10 ~ 20g/l, lactic acid 10 ~ 30ml/l, ph=4.5 ~ 6.0, 83 ± 2 ℃. The pH of the plating solution is gradually adjusted from 6.0 to 4.5, and then the amorphous electroless nickel coating can be obtained by stable plating. If ph=4.5 at the time of starting plating, the difference between the plating layer and the pre plating layer is large, and the phosphorus content of the plating layer changes too much, resulting in internal stress and delamination of the plating layer. Formula 2#: niso4.7h2o25 ~ 30g/l, n annual output of T300, T700 Carbon Fiber 2000 tons, carbon fiber machine 2. For workpiece fabrics with a thickness of more than 5mm (300000 square meters, 10million square meters of prepreg, 8million pieces of carbon fiber series products with a thickness of less than 5mm, ah2po2.h2o25 ~ 30g/l, naac15 ~ 20g/l, na3c6h5o7.2h2o15 ~ 20g/l, appropriate amount of additive B1 (stabilizer), appropriate amount of additive B2 (accelerator), ph=4.8 ~ 5.2, 88 ± 2 ℃, 1H, Loading capacity: 1dm2/l, stirring with compressed air: 150cm2/min, plating speed: 15 ~ 20 μ m/h。

(4) heat treatment

the hardness of electroless nickel plating can be adjusted by heat treatment. The structure is still amorphous, which makes the content distribution of the bottom layer tend to be uniform, and there is a small amount of phosphorus from the substrate, which makes the adhesion between the bottom layer and the coating better, the structure more uniform and fine, the dispersion is improved, and the surface finish, corrosion resistance and wear resistance of the coating are improved. Considering the low melting point of zinc alloy, in order to prevent workpiece deformation, long-term heat treatment at bottom temperature is adopted, such as 220 ± 5 ℃, and the microhardness increases by more than 5 times in 5h. Others use 180 ℃ × 1H heat treatment conditions

2. Pretreatment method of electroless nickel plating on magnesium alloy

according to the conventional coating process, it is difficult to obtain a electroless nickel coating with good adhesion on the surface of magnesium alloy. Because the magnesium alloy is also easy to oxidize during the conversion of the pretreatment process of electroless nickel plating. In this process, an alkali activation process is added before electroless nickel plating, which dissolves the fluoride and oxide film generated during the pretreatment to activate the surface and then electroless nickel plating, so as to obtain a electroless nickel coating with good adhesion

pretreatment process flow: degreasing (alkali boiling) → alkali etching (na4p2o7.10h2o100 ~ 200g/l, nano330 ~ 100g/l, room temperature, 1 ~ 2min) → acid activation (h3po4.200ml/l, nh4hf100g/l, room temperature, 1 ~ 2min) → alkali activation (na2hpo2.h2o25g/l, chelating agent 15g/l, ph=9 ~ 10, room temperature, 1 ~ 2min)

chemical nickel plating formula: ni3k6h5o7, nahpo4 H2o25g/l, na4p2o7.10h2o50g/l, nh4f5 ~ 10g/l, =9 ~ 10, the activation reaction hardly occurs at 6. Under other normal conditions, the longer the activation time is, the more sufficient the activation is. When the surface of the basic course of activation is exposed, a layer of uniform yellowish brown film with catalytic activity is formed on the surface, and the replacement reaction between nickel and titanium occurs. Until the titanium alloy matrix is completely covered by the nickel layer, the conversion reaction stops, so the longer the activation time is, the better. If the activation is not sufficient, the subsequent electroless nickel plating will be partially free of plating or bubbling

activation solution: ethylene glycol 700ml/l, acidic ammonium fluoride 35g/l, nickel chloride 20g/l, boric acid 50g/l, lactic acid 20ml/l, acetic acid (99%) 180ml/l, ph50 ℃, 30min

4 Electroless nickel plating

process acid formula: nickel sulfate 20g/l, sodium hypophosphite 25g/l, complexing agent 15g/l, lactic acid 18g/l, sodium propionate 18g/l, pb2+ trace, ph=5, 85%, cathode movement. This formula is applicable to Ti-6Al-4V, ti-15v-3al-3cr and other titanium alloys, and the coating has good adhesion

5. Pretreatment method of electroless nickel plating on powder metallurgy parts

(1) drying oil

drying oil temperature 250 ~ 300 ℃, until the oil smoke is removed. Baking oil is one of the key processes of electroless nickel plating, which plays a great role in the subsequent oil and rust removal

(2) washing

hot water washing temperature>85 ℃, higher than the hot water washing temperature of general material surface treatment, which is conducive to cleaning the lye, soap, dirt, etc. infiltrated into the workpiece hole

(3) anode degreasing

in order to reduce the chance of alkali liquor penetrating into the inner hole of the workpiece to a greater extent, anode degreasing is used. The degreasing fluid is: naoh30 ~ 50g/l, na2co320 ~ 30g/l, na3po4.10h2o20 ~ 30g/l, nisio33 ~ 5g/l, 60 ~ 90 ℃, da=3 ~ 10,3 ~ 5min

(4) weak corrosion

hcl 100 ~ 150g/l, room temperature, 0.5 ~ 2min, the time should not be too long, and the washing should be thorough

it is worth pointing out that powder metallurgy parts should not be derusted by sand blowing. Although the surface is smooth after sand blowing, the sand dust can penetrate into the pores of the workpiece. During electroless nickel plating, the sand dust will escape and attach to the surface of the workpiece, affecting the coating quality. In addition, the workpiece should be washed thoroughly to avoid the "blooming" phenomenon of the coating

6. What are the formulas for removing electroless nickel coating?

the removal of electroless nickel coating is much more difficult than that of electroplated nickel coating, especially for high corrosion-resistant electroless nickel coating. Unqualified electroless nickel plating should be removed before heat treatment, otherwise it is more difficult to passivate and remove the plating. It is required that the stripping solution must have no corrosion to the substrate, and then factors such as plating thickness, stripping speed and stripping cost should be considered

(1) chemical stripping method

chemical stripping method does not corrode the workpiece, is suitable for workpieces with complex geometry, and can achieve uniform stripping

formula 1#: concentrated HNO3, 20 ~ 60 ℃. This liquid has low cost and fast speed (30 ~ 40 μ M/h), low toxicity. It is applicable to the deplating of workpieces with low requirements for precision size, so as to prevent water from being brought in and quickly enter the running water for cleaning after deplating

formula 2#: HNO3 (1:1), 20 ~ 40 ℃, fast regression (10 μ M/5 ~ 6min), suitable for stainless steel

formula 3#: concentrated HNO3 1000ml/l, NaCl 20g/l, urea 10g/l (inhibiting the formation of NOx gas), hexamethylene tetramine 5g/l, room temperature, regression rate 20 μ m/h。

formula 4#: sodium m-nitrobenzene sulfonate 60 ~ 70g/l, sulfuric acid 100 ~ 120g/l, potassium thiocyanate 0.5 ~ 1g/l, 80 ~ 90, suitable for the deplating of copper and copper alloy workpieces. When the deplated surface is dark brown, take it out and clean it fully, and then remove the brown film (nacn30g/l, naoh30g/l, room temperature)

formula 5#: HNO3 ∶ hf=4 ∶ 1 (volume ratio), with proper heating in winter, fast retreat speed and no corrosion of iron matrix. However, HF must be analytically pure (Industrial HF tank is easy to explode)

formula 6#: ammonium nitrate 100g/l, aminotriacetic acid 40g/l, hexamethylene tetramine 20g/l, ph=6, room temperature, regression rate 1/5min, low cost

in addition to the above acid chemical nickel removal method, there is also alkaline chemical nickel removal method, which has no corrosion effect on ordinary carbon steel workpiece, but the temperature is high, the time is long, and the efficiency is low. For example, sodium m-nitrobenzene sulfonate 100g/l, naoh100g/l, ethylenediamine 120ml/l, sodium dodecyl sulfate 0.1g/l, 60 ~ 80 ℃. When adding sodium m-nitrosulfonate during adjustment, the regression rate can be restored to 80% of the maximum regression rate

(2) electrolytic plating method

the formula is: nano3100g/l, aminotriacetic acid 15g/l, citric acid 20g/l, thiourea 2g/l, sodium gluconate 1g/l, sodium dodecyl sulfate 0.1g/l, ph=4, room temperature, da=2 ~ 10a/dm2, cathode 10# steel, SK ∶ sa=23 ∶ 1

7. How to choose the equipment for electroless nickel plating

(1) avoid the pollution of tank material

the advantages and disadvantages of tank material and inner wall coating have a certain impact on electroless nickel plating. It is generally believed that polypropylene tank, soft PVC lined steel tank, acid resistant enamel tank and heat-resistant and acid resistant ceramic tank all have the disadvantages of poor thermal conductivity and tank strength, and only stainless steel tank (1Cr18Ni9Ti) has a good effect. Because stainless steel is inert in electroless nickel plating solution, nickel is not easy to reduce and deposit on its surface. It has good thermal conductivity, high strength and convenient manufacture. However, the stainless tank needs to be protected by 0.5a/dm2 anode current, and the structure is complex. It is best to dip a layer of ZD-1 sol or TZ anti-corrosion coating on the inner wall of the stainless steel tank, and also spray Teflon with appropriate thickness, such as Teflons, F46 and other coatings. The situation is very ideal, and there will be no signs of metal reduction deposition on the tank wall. Moreover, the groove has simple structure and convenient maintenance (the coating can be repaired)

practice also shows that soft and hard PVC (polyvinyl chloride), soft and hard PP, and acid resistant rubber have varying degrees of influence on the electroless nickel plating solution, as follows: the stability of the plating solution is extremely poor; The dispersion ability decreases; Plating speed decreases or plating stops; The brightness of the coating is improved, but the brittleness is increased. Among them, the influence of soft PVC is significant, and its harm is persistent, that is, no matter how many times the plating solution is replaced, the plating solution will be polluted by soft PVC. Other materials have little influence on the plating solution, and they are all one-time effects. In order to eliminate their influence, it is recommended not to use soft PVC, but to soak other materials in hot water above 80 ℃ for more than 10B in advance, and the effect is good

(2) improve the heat transfer of the heating tube

use the electric furnace outside the water jacket to heat, avoid the rapid decomposition of the tank liquid components, and have the function of regulating the temperature (equipped with a pressure regulator). It can also be heated directly with steam, which not only improves the utilization rate of steam, but also accelerates the heating rate. The heating pipe shall be polished 316 stainless steel pipe, and air stirring (oil-free air) shall be set below the heating pipe, so as to accelerate heat transfer and prevent local overheating. In order to reduce the noise caused by steam flushing, a silencing pipe with a diameter twice that of the steam pipe can be welded on the steam pipe. There are 20 small holes parallel to the bottom of the groove on the silencing pipe. The total number of these air vents is 2 times larger than the diameter of the silencing pipe, and the silencing effect is excellent. Generally, immersion heaters are also allowed to be used for direct heating, such as quartz heating tubes, polytetrafluoroethylene heating tubes, etc

(3) the rationality of improving the drum structure

small workpieces should be loaded into the drum for barrel plating. The barrel should be larger and have good water permeability (dense workpieces, extremely high load). The loading of workpieces by the drum should not exceed 1/3 of the volume, and the loading of small workpieces should also