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Solution to the difficulty of zinc deposition on the surface
Pubdate date:2019-10-17Views:0

Cast iron is a kind of iron carbon alloy with carbon content of more than 2% (mass fraction). Only a small amount of carbon in the material forms solid solution with iron, mostly in the form of graphite or cementite, because graphite will reduce the potential of hydrogen precipitation, thus hindering the deposition of zinc. At the same time, because the surface of the material is too rough, full of sand holes and shrinkage cavities, and the components contain phosphorus, sulfur, silicon, graphite and titanium and other elements, the overpotential of hydrogen in the electroplating process is too low, which is very easy to cause hydrogen evolution and bring difficulties to metal deposition; while acid pickling will increase the adsorption capacity of hydrogen atoms, in addition to part of the adsorbed hydrogen atoms combined into hydrogen, hydrogen will be emitted into the air. There will also be a considerable part of hydrogen atoms diffused into the cast iron matrix, resulting in more hydrogen pressure, which will not only affect the mechanical properties of the cast iron itself, but also cause a large amount of hydrogen evolution in the electroplating process, which will not only seriously affect the combination fastness of the coating, but also cause a sharp drop in current efficiency, further increasing the difficulty of electroplating.

   The coating quality obtained by electroplating of iron castings after some improvements in the process can get satisfactory results from the appearance, but due to the particularity of its own materials, the protection of the coating to the substrate is far from the requirements. Therefore, the following suggestions are put forward for the reference of structural designers, mechanical and electroplating technologists.

    (1) As far as possible, the structural design of the plated parts uses less casting and forming technology, especially in the harsh environment.

    (2) It is recommended to use aluminum die casting or copper alloy casting.

    (3) Use other coating process instead of electroplating, such as spray painting and plastic spraying.

    (4) Sandblasting process can be added after workpiece forming and before plating.

    (5 )Potassium chloride zinc plating is used instead of zincate zinc plating.

    The author takes the following four measures to solve this problem.

    ① Brush oil removal instead of electrolytic oil removal. Because the iron castings are full of shrinkage cavity, air hole and sand hole, and the components contain many impurities, it is not suitable to use electrolytic oil removal. Otherwise, it will increase the difficulty of electroplating due to serious hydrogen permeation or oxidation of iron castings. After chemical degreasing, it is better to use cotton yarn dipped in lime water to brush instead of electrolytic degreasing. After brushing, brush again with brown brush in clear water, which can make the dirt entering the sand hole clean.

    ② Sand blasting instead of pickling. The application of sandblasting instead of pickling can avoid hydrogen permeation during pickling and reduce hydrogen pressure, so as to prevent the mechanical properties of the casting itself from being affected, which is conducive to improving current efficiency, metal deposition and enhancing the adhesion of the coating.

    ③ Adjust the process formula of activation solution. The process formulation of the activated solution is adjusted to meet the needs of the characteristics of the iron castings. The experiment shows that the activation of iron castings before plating has a strong activation ability.

    The formulation and process conditions were hydrochloric acid 50ml / L, hydrofluoric acid 50ml / L, room temperature, time 0.5-1min.

    ④ Impact current is used for initial plating. The current density which is 3-5 times higher than the normal current is used to impact plating after the plating parts are put into the bath. After 1-2 minutes, a thin coating can be formed on the surface of the casting first, and then the normal coating can be deposited on the coating with a little higher than the normal current density. The effect of this method is remarkable.

 

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