The influence and treatment method of impurities in zincate galvanizing solution

Impurities in zincate galvanizing solutions are classified into two categories: inorganic and organic. The common inorganic impurities are copper and lead, the organic ones are mainly the decomposition products of organic additives in the bath liquid, and some dirt such as grease and other impurities that are not clean and brought in by the pretreatment. Too many organic impurities will make the coating foggy and flowery, and in severe cases, it will bring brittleness to the coating. The treatment of organic impurities is more convenient, as long as activated carbon treatment is carried out when necessary according to the situation, or treatment is carried out regularly. Let's talk about the hazards of copper and lead impurities and how to deal with them.

The metal impurities in the zincate galvanizing solution should not be treated with sodium sulfide, because the complexation of sodium hydroxide to zinc ions is relatively weak. It is precipitated by the formation of milky white zinc sulfide; the zinc sulfide particles are relatively coarse, and deposition on the cathode will cause burrs in the coating. Because the concentration is difficult to control, and if it is not added properly, it will cause disadvantages. Therefore, sodium sulfide is generally not used as a treatment agent for heavy metal impurities. Also because the zincate bath does not contain the strong complexing agent sodium cyanide, the zincate zinc solution should be more sensitive to impurities than the cyanide zinc solution. This is also the case. We can also see from the passivation color of the parts after galvanizing. If the zincate galvanizing solution is not high in purity, it is difficult to obtain a blue-white appearance by using the white passivation process. According to our test, the copper ion impurities in the bath should not be greater than 15mg/L. A small number of copper impurities will affect the color tone of the passivation film, slightly higher - some will make the passivation film foggy, and if it is long, it will turn black after the nitric acid emits light. Copper impurities can be treated by electrolysis at low current density, or by zinc powder or alkaline galvanizing impurity remover, but prevention should be given priority.

Lead impurities are mainly brought by poor-quality zinc sheets. Lead is more harmful than copper, especially for barrel plating. When the lead impurities are high, the zinc coating will not be deposited. If this kind of plating solution is tested with the Hull cell, it is possible that the plating layer is normal, which often makes the technicians who deal with the fault on the spot feel like they are in a fog, because - - they can't figure it out. There are many lead impurities. In addition to the non-deposition of the barrel plating layer, the plating layer will appear yellowish-brown after light nitric acid is emitted. After passivation in a low-chromium passivation solution, the film layer will show a yellow interference color. Get beautiful shades of blue and white. The lead content should be controlled below 2mg/L. Lead can also be electrolytically treated at low current densities or treated with alkaline galvanizing remover, or CK-778. The effect of these two impurities removers is better than that of zinc powder. They also reduce hexavalent chromium and adsorb organic impurities. When treating with these two kinds of impurity removers, it is necessary to stir vigorously while pouring them in. At this time, some organic impurities adsorbed by some impurity removers will float to the surface of the plating solution, which can be scooped off with a spoon. Precipitated and suspended impurities, along with impurities, are removed by filtration.