Fe & sup 2; + Fe³+、 Cu²+、 Pb²+ Etc., too many impurities will cause the plating layer to be dark and black, and black stripes will occur. The laxity of the plating solution can be reduced. The edges and corners are simply burnt, the passivation is poor, and even the dull tile yellow to dark brown. The more fatal damage is the blistering of the plating layer, When it is severe, the coating will fall (most metal impurity ions are suspended in the solution as hydroxides, especially Fe & sup; 3; & Dagger; in colloidal suspension, adsorbed and accumulated on the surface of the workpiece, directly affecting the binding force between the coating and the substrate. Once the organic content increases, the internal stress will increase, making it easier to bubble and peel. ) ①Cu²+ Impurities mainly come from hangers and conductive rods, and their limit content shall not be greater than 0.15 g / L. A few Cu & sup 2+ Impurities affect the color tone of the passivation film, and slightly higher will make the passivation film foggy. More often, the nitric acid will turn black after the light is emitted. In severe cases, the coating is rough and gray black. After passivation, it is even darker. The coating with higher content is spongy ② PB & sup2+ The impurities mainly come from the zinc anode, and the limit content shall not be greater than 0.002 g / L. Pb²+ When there are many impurities, the coating is dark gray, and after nitric acid is exposed to light, it presents black spots. There is a yellowish brown film during passivation. No matter how high the content is, the zinc coating is difficult to accumulate ③ Fe & sup2+ The impurities mainly come from chemical materials and cleaning water of the previous process, and the limit content shall not be greater than 0.05 g / L. When the impurity content is high, the coating at the low current density is dim, and the passivation film is dull. Treatment methods: ① low current electrolytic treatment: corrugated iron plate is used as the cathode, 0.05-0.5 A / DM & sup2; ② zinc powder activated carbon treatment method: A. add 0.6 g / L zinc powder to the plating solution (confirm the amount by small experiment) and mix for 30 minutes; b. Participate in 1-3g / L activated carbon and continue mixing for 30 minutes; c. Stand for 3 hours and filter the plating solution; d. After analyzing and adjusting the composition of the plating solution, adjusting the pH value and adding the brightener, try to plating. ③ Sodium sulfide treatment method: A. prepare an appropriate amount of reagent grade sodium sulfide solution (the dosage shall be determined by small experiments, and the dosage shall not exceed 0.2 g / L each time) and dilute it to 100 times; b. Vigorous mixing for 1 hour while participating; c. Natural deposition day and night, trial plating (no filtration)

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