I. Introduction
It is well known amphoteric zinc metal, corrosion prone to acid, alkali solution and the coating and the substrate binding strength is remarkably decreased. Therefore, in decorative plating, pretreatment of the surface of the zinc-based alloy is critical. Parts generally by grinding, polishing, phosphorus and sodium carbonate as a main component to chemically and electrochemically oil wax, degreasing, followed by a solution of sodium cyanide leaching activated, pre-plated copper cyanide, and then plating the other Plating. However, matrix corrosion is difficult to avoid, and there are problems of dispersing ability and deep plating ability, and there are often poor plating such as grooves, inner hole deplating, corner charring, porous porosity, and even foaming and peeling.
Herein intends an improved factory testing and related technical processes, reduce environmental pollution, and good coating and the substrate binding force of a zinc alloy pressure castings decorative chrome plating process to be introduced.
Second, the process <br> blank inspection - mechanical polishing - fine polishing - gasoline scrubbing - chemical degreasing - hot water washing - washing - pickling - activated washing - alkaline chemical nickel plating - washing - neutral nickel plating - washing - Bright nickel plating - water washing - composite chrome plating - recycling - hot water washing - drying - inspection
Third, plating solution formula and process conditions
(1) Chemical degreasing and non-phosphorus washing powder (high quality) 50 ~ 80gPL, sodium carbonate 5 ~ 10gPL, 60 ~ 100 ° C, until the net.
Phosphorus-free washing powder is a unique surfactant. Its main components contain anionic surfactants, nonionic surfactants, stain suspending agents and additives such as whitening and color protection. Therefore, the degreasing liquid prepared by using the washing powder has a special function of removing wax and other dirt on the surface of the zinc die casting, and can quickly decompose various dirt to achieve the decontamination effect. The workpiece can be completely removed after about 5 minutes, and the substrate is not corroded, keeping the surface as clean as new. In addition, this washing powder does not cause phosphate eutrophication because it does not use phosphate as an auxiliary.
(2) Acid washing Activate hydrofluoric acid 15-20 mL PL, sulfuric acid 5-10 mL PL, room temperature, 3 to 5 s.
Hydrofluoric acid is a weak acid, and further addition of H ₂ SO ₄ (or HNO ₃ , H ₃ BO _{3 ,} etc.) in an aqueous HF solution can stabilize.
(3) Chemically immersed nickel nickel chloride 40～50gPL, sodium citrate 90～100gPL, ammonium chloride 45～55gPL, sodium hypophosphite 10～12gPL, pH 8.5～9.5, 80～90 °C, 5～10min.
Chemically immersed (plated) nickel is available in both acidic and basic forms. In contrast, zinc alloy die castings use alkaline chemical immersion nickel better than acid chemical immersion nickel. The latter tends to corrode the substrate and the nickel layer is fading, and the process overcomes this drawback.
(D) neutral nickel plating Nickel sulfate 160 ~ 180gPL, sodium citrate, 180 ~ 200gPL, sodium chloride 10 ~ 15gPL, boric acid 20 ~ 30gPL, tannic acid 0.4 ~ 0.6gPL, pH value of 6.5 to 7.0 at room temperature , 0.4 ~ 1.0APdm2, 15min, cathode movement.
The addition of tannic acid mainly masks Zn2 + and makes the plating crystal finer and reduces the porosity of the nickel layer. [next]
(5) Bright nickel nickel sulphate 250 ~ 300gPL, sodium chloride 15 ~ 20gPL, boric acid 30 ~ 40gPL, 1,4 butyne diol 0.4 ~ 0.5gPL, saccharin 0.8 ~ 1.0gPL, sodium lauryl sulfate 0.05 ~ 0.10 g PL, pH 4.0 to 5.1, 50 to 55 ° C, 2 to 3 APdm 2, 15 min, cathode movement.
This formulation is a general all-light nickel plating process.
(F) Composite chromium plating chromic anhydride 250 ~ 300gPL, fluorosilicate 4 ~ 6gPL, sulfate, 1.0 ~ 1.5gPL, trivalent chromium 2 ~ 6gPL, 45 ~ 55 ℃ , 15 ~ 30APdm2,1.0 ~ 1. 5min, tin-lead anode alloy.
Composite chrome plating, also known as fluorosilicate chrome plating. The process is designed to overcome the phenomenon that the zinc alloy die-casting part is complicated by the shape (that is, the inner hole of the groove is large), and the ordinary chromium often exhibits a "yellow-yellow" phenomenon, and the cathode colloid film of the fluorosilicic acid plating solution is more common than the ordinary chrome-plating solution containing only sulfuric acid. The cathode colloid film is much thinner, the chrome layer has a less passivation tendency, and the bright plating layer can be obtained at a low temperature and a low current density, and the liquid temperature is increased, and the brightness range is wider. Therefore, the composite chrome plating process is particularly suitable for decorative plating of zinc alloy die castings.
Fourth, the operation points (1) For smaller parts, can be rolled (no need to grind). Suitable for grinding and throwing parts, the force should be uniform during operation. The thickness of the surface of the zinc casting should not exceed the thickness of the dense layer, because the bottom is porous and loose. If the excessive force of polishing will destroy the dense layer on the surface of the substrate, the polishing paste and the bath will enter the exposed pores and cracks, which will affect the quality of the coating.
(2) When polishing, it should be coarsely polished with a yellow polishing paste, and then polished with a white polishing paste. The use of polishing paste should be a small amount, diligent use principle. When the dosage is large, the polishing paste will stick to the groove and inner hole of the workpiece, which will bring difficulty to the decontamination, but it should not be too little. Otherwise, the surface will be partially overheated. Small pitting, causing the coating to cause blistering, peeling and other diseases.
(3) After the parts have been ground and polished, they should be cleaned with gasoline as soon as possible to remove the polishing paste residue, so as to avoid the curing of the polishing paste and affect the quality of chemical degreasing.
(4) High-quality washing powder (type B) should be used. The particles are small, the density is high, the foam is small, the water is dissolved quickly, the hand contact solution has no burning feeling, the solution is clear and smooth, and the decontamination effect is obvious. Pay special attention: the water temperature of the enzyme washing powder should not exceed 60 ° C, otherwise, the enzyme will lose its activity and affect the degreasing effect. In addition, for large-volume zinc die-casting parts, you can use a brush or a rag to apply detergent powder to manually remove the oil.
(5) The pickling activation time should not be too long. When the hydrogen bubbles appear on the surface of the workpiece, it should be taken out and quickly washed into the running water. Otherwise, the surface is prone to corrosion product adhesion, that is, ash and loose film. The film is difficult to wash off, which is an important reason for the low plating yield of zinc alloy die castings. If this occurs, an organic acid complex cleaning liquid of 15 g PL of each of oxalic acid and sulfosalicylic acid may be used, and this etching film may be removed by immersing at room temperature for about 5 minutes.
(6) When the parts are chemically immersed in nickel, the current density should not be too high (about 0.5APdm2) when plating neutral nickel. Otherwise, the plating layer is likely to be mixed with nickel basic salt, and as a result, the plating layer is porous and loose. At the same time, attention should be paid to the uniform distribution of neutral nickel layer, no scorch phenomenon, and low porosity. Otherwise, the workpiece enters the bright nickel bath and is prone to corrosion, which affects the bonding strength and appearance quality of the coating.
(7) Bright nickel plating and sleeve composite chromium operation, according to the conventional plating process.
V. Conclusion <br> In summary, the process eliminates the pre-plated copper cyanide and replaces it with alkaline chemical immersion nickel. The key factor in the process of chrome plating of zinc alloy die-casting parts is that the substrate before plating is clean, free of over-corrosion and adhesion of its products. As long as you pay attention to each process, the yield is relatively high.

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