Magnesium: zinc plating of magnesium alloys

After pre-treatment, the magnesium alloy materials and workpieces are immersed in a non-electrolytic chemical bath and directly coated with a thin layer of zinc, which is used as the bottom layer for electroplating. However, a layer of copper plating must be touched before electroplating to protect the thin layer of zinc plating, which is only 2.5μm thick.

Typical zinc bath solution for pyrophosphate, zinc salt, fluoride salt based aqueous solution, can also use a small amount of acid to adjust the alkalinity of the solution.Pyrophosphate can dissolve oxide film and hydroxide film and form water-soluble complex.When the pH is appropriate, when the concentration and temperature are suitable, the impurity film can be effectively removed, and a thin layer of zinc can be coated at the same time.Fluoride can control the deposition rate of zinc, too fast, will form a weak adhesion coating.Tapwater can be used to provide the solution, but it is still the best deionized water, pH 10.0 ~ 10.6, the best value 10.2, temperature 79℃ ~ 85℃.

To prepare the bath solution, first dissolve the monohydrate zinc sulfate in room temperature water, then heat the solution to 60℃ ~ 82℃, stir it while adding tetrasodium pyrophosphate, not too fast, stir it further for 5min ~ 10min, and then add fluoride and carbonate in turn after the tetrasodium pyrophosphate is completely dissolved.The amount of carbonate added should be adjusted according to the actual pH of the solution.If necessary, sulfuric acid or phosphoric acid can be added to reduce the pH.

The dipping time of galvanizing is 1min ~ 3min, but for magnesium alloy containing aluminum, it can reach 10min.The best thickness of zinc coating is 2.5μm ~ 3.8μm.The treatment tank must be stainless steel and no glass or ceramic elements are allowed to immerse in the zinc bath, as small amounts of silicate are also detrimental.The working time of the tank is quite long, and the composition to be added can be determined by analysis.

The copper plating must be touched in the cyanide bath immediately after dipping galvanizing.The workpiece must be in electrical contact with the cathode rod before it is placed into the groove.If the plating is then carried out in lye or an improved, non-corrosive nickel fluoride solution, the copper coating can be thinner than 3μm.If it is then electroplated in acid, the copper plating must be thoroughly neutralized and cleaned.

The current density of 3A/dm2 ~ 4A/dm2 was firstly applied in the melting plating of copper. Duration of 0.5min ~ 1min, and then down to 1.5A/dm2 ~ 2.5A/dm2, after the completion of copper plating and then other metal plating.

In order to prevent the dip galvanizing layer from breaking, the air bubbles should not be generated when touching the electroplated copper. In order to prevent copper plating, when the parts enter the copper solution, the voltage must be applied to be 0.5V ~ 1V. After the parts are in place, the voltage is raised to the desired value.