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Zinc alloy die - casting parts electroplating decorative chromium process
Publisher:admin Add time:2018/6/7 Viewed:549
As is known to all, zn is a kind of amphoteric metal, which is easy to corrode in acid and alkali solution, so that the binding force between coating and substrate is significantly decreased. Therefore, in decorative electroplating, the pretreatment of zinc-based alloy surface is the key. Usually parts after grinding, polishing, sodium carbonate and sodium phosphate as the main components to the oil, chemical and electrochemical dewaxing and degreasing, activation, and then sodium cyanide leaching solution cyanide copper plating, and then with other kinds of plating. Matrix corrosion, however, is difficult to avoid, and the spread and deep plating ability problems, tend to appear grooves, the inner hole of plating, corner burning, produce porous loose, even is bubbly desquamate bad plating, etc.
In this paper, the technology and process flow of zinc-alloy die casting are improved in a factory to reduce environmental pollution.
2 process flow
Blank check to mechanical polishing, fine polishing and gasoline scrub to chemistry in addition to the oil to hot water washing, water washing, acid activation - water - alkaline chemical leaching of nickel - water - neutral nickel - plating bath to bright nickel - plating bath, composite plating chromium to recycling, hot water washing to dry, inspection
Formula and process conditions of plating solution
3. 1 chemical oil removal
No phosphorus detergent (quality) 50 ~ 80 g/L, 5 ~ 10 g/L sodium carbonate, 60 ~ 100 ℃, to go to the net.
Phosphorus free detergent is a unique surfactant. Its main ingredients include anionic surfactant, non-ionic surfactant, stain suspension, whitening and color protection. Therefore, the oil removal solution prepared with this detergent powder has the special function of removing wax and other dirt on the surface of zinc die casting parts, and can decompose all kinds of dirt quickly to achieve the decontamination effect. The workpiece can be completely cleaned in about 5 minutes after it is put into the groove, and it will not cause the corrosion of the substrate and keep the surface smooth and clean. In addition, this washing powder does not cause water eutrophication because it does not use phosphate as an auxiliary.
Hydrofluoric acid is a weak acid, which can be stabilized by adding H2SO(or HNO, HBO, etc.) to HF aqueous solution.
3. 3 chemically impregnated nickel
Nickel chloride is 4-50g/L, sodium citrate is 90-100g/L, ammonium chloride is 45-55g/L, sodium hypophosphate is 1-12g/L, pH is 8. 5 ~ 9. Reached 5 ~ 9 ℃, o 5 ~ 10 min.
Compared with the two types of acid and alkaline nickel plating, the effect of alkaline nickel plating on zinc alloy die casting is better than that of acid nickel plating. The latter is easy to corrode the matrix, and the nickel layer flowers, but this process has overcome this drawback.
3. 4 neutral nickel plating
Nickel sulphate 160 ~ 180g/L, sodium citrate 180 ~ 200g/L, sodium chloride 1O ~ 15g/L, boric acid 20 ~ 30g/L, tannic acid 0. 4 to 0. 6g/L, pH 6. 5 to 7. Zero, room temperature, zero. 4-1. 0A/dm, 15min, cathode move.
The addition of tannic acid mainly obscures zn "and makes the plating crystallization more refined and reduces the porosity of nickel layer.
3. 5 bright nickel plating
Nickel sulphate 250 ~ 300g/L, sodium chloride 15 ~ 20g/L, boric acid 3O ~ 40g/L, 1,4 butanediol 0. 4 to 0. 5g/L, saccharin 0. 8 to 1. 0g/L, sodium dodecyl sulfate 0. 05 ~ 0. 10g/L, pH 4. 0 ~ 5. 1, 50 ~ 55 ℃, 2 ~ 3 a/dm, 15 min, the cathode.
This recipe is a general bright nickel plating process.
3. 6 compound chrome plating
Chromic anhydride 250 ~ 300g/L, fluorosilicic acid 4 ~ 6g/L, sulfuric acid 1. 0 to 1. 5 g/L, trivalent chromium 2 ~ 6 g/L, 45 ~ 55 ℃, 15 ~ 30 a/dm, 1. 0 to 1. 5min, the anode is lead tin alloy.
Compound chromium plating, also known as fluorosilicate chromium plating. Using this process is to overcome the zinc alloy die casting due to the complicated shape (that is, the groove hole) set of ordinary chrome often appear "dew yellow" phenomenon, and fluorosilicic acid solution of cathode colloid film than ordinary containing sulfuric acid chromium plating liquid cathode colloid film much thinner, chromium passivation trend is lesser, can obtain under low temperature, low current density light coating, the liquid temperature is high, the brightness range is wider. Therefore, the composite chromium plating process is especially suitable for decorative plating of zinc alloy die casting parts.
4 operating points
(1) for small parts, the polishing process can be carried out (no need for grinding and polishing). Suitable for grinding and polishing parts, apply uniform force during operation. The thickness of the surface of zinc casting should not exceed the thickness of its dense layer, because the underneath part is porous and loose. If too much polish is applied, the dense layer on the surface of the substrate will be damaged. As a result, the polishing paste and groove fluid will enter the exposed pores and cracks, which will affect the coating quality.
(2) when polishing, the yellow polishing paste shall be used for rough polishing, and then the white polishing paste shall be used for fine polishing. Use polishing paste should use a small amount, frequently for the principle, usage, polishing paste will stick to the workpiece grooves, the inner hole, bring difficulties to decontamination, but also cannot too little, otherwise, can cause a small local overheating and surface pitting, so make the coating produces blister, peeling, etc.
(3) after the above grinding and polishing, the parts should be cleaned with gasoline as soon as possible to remove the residue of the polishing paste, so as not to solidify the polishing paste and affect the quality of chemical oil removal.
(4) high quality washing powder (type B) should be used, with small particles, high density and little foam. It can be dissolved quickly in water. Pay special attention to: enzymatic detergent water temperature cannot exceed 60 ℃, otherwise, will lose its enzyme activity, influence the oil removal effect. In addition, for large zinc die-casting pieces, you can use a brush or cloth to adhere to the washing powder hand wipe oil.
(5) the activation time of pickling should not be too long. When hydrogen bubbles appear on the surface of the workpiece, it should be taken out and cleaned quickly into the water. Otherwise, the surface is easy to produce corrosion products adhesion, that is, hanging ash, loose film. The film is difficult to be washed off, which is an important reason for low electroplating yield of zinc alloy die casting parts. If this is the case, a 15L organic acid complex cleaning solution of oxalic acid and sulfosalicylic acid can be used to soak at room temperature for about 5min to remove the corrosion film.
(6) after the parts are chemically dipped in nickel, the current density should not be too high when electroplating neutral nickel (about 0. 5A/dm2), otherwise, the coating is easy to produce nickel-base salt inclusion, resulting in the coating porous and loose. Attention to uniform distribution of the neutral nickel layer at the same time, no burning phenomenon, and has low porosity, otherwise, bright nickel artifacts into the trough points which were liable to occur, coating and the substrate, and appearance quality influence.
(7) bright nickel plating and sleeve composite chromium plating shall be carried out in accordance with the conventional plating process.
5 conclusion
In conclusion, the preplating of copper cyanide was eliminated in this process, and the alkaline chemical nickel immersion was used instead. The key factor of chrome-plating decoration process of zinc alloy die casting is that the substrate is clean, no over-corrosion and the product adhesion before plating. As long as you pay attention to each process, the yield is relatively high.
3. 2 pickling activation
Hydrofluoric acid 15 ~ 2OmL, sulfuric acid 5 ~ 10mIJ, L, room temperature, 3 ~ 5s.

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