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2021.6.25
In this guide, we discuss the influence of pretreatment and pre plating process on the adhesion of zinc alloy die casting coating from the aspects of electroplating process and method.
Zinc alloy has been widely used in parts manufacturing because of its good die casting processing performance, less material consumption and high production efficiency. In order to improve its corrosion resistance in the atmosphere and other decorative properties, to meet the requirements of its use, it is often necessary to carry out surface treatment such as electroplating.
Because of the poor chemical stability of zinc alloy in acid and alkali medium, electroplating on zinc alloy die casting parts is much more difficult than electroplating on other metals such as steel, copper and copper alloy. It is easy to cause the defects of poor adhesion such as blistering and peeling of coating due to excessive corrosion of base metal. This is also the main reason of waste and modified products in zinc alloy electroplating. Therefore, it is an important way to improve the qualified rate of zinc alloy die casting and ensure the quality of products to take corresponding preventive and solving measures by pulling out the factors that affect the zinc alloy electroplating bonding force from the electroplating process.
There are many factors affecting the adhesion of zinc alloy die casting coating. However, as far as the electroplating process is concerned, there are mainly two aspects, namely, pretreatment and pre plating process.
Electroplating of zinc alloy die castings usually adopts the following process flow:
Electrochemical degreasing → hot water washing → cold water washing → corrosion → cold water washing → activation treatment (some do not carry out) → pre plating → cold water washing (plating intermediate layer or surface layer according to process requirements)
In the whole process, although each process has different degrees of influence on the electroplating quality, the pretreatment and pre plating process are the primary and key factors to determine whether a good coating adhesion can be obtained.
It is generally believed that the poor adhesion of the coating is caused by the incomplete degreasing before plating. However, another important and often neglected reason for zinc alloy die casting is improper selection and operation of corrosion process before plating.
It has been proved that a dense oxide layer is formed on the surface of zinc alloy die casting due to the heating of metal during die casting process, which must be removed before plating to reveal the crystalline structure of base metal. These processes can remove the oxide layer more smoothly. However, the crystal structure of the surface layer of zinc alloy die casting is more compact, while the structure under the surface layer is more loose, with more pores and slag loss. In the production practice, it is found that the chemical reaction between the above two kinds of corrosion solution and zinc alloy is strong, and the time control of corrosion treatment is strict during operation. If the time is too short, the oxide film can not be completely removed, which will cause unreasonable coating bonding; If the time is longer, the dense layer on the surface of the base metal will be corroded, and the loose layer under the dense layer will be exposed, and the corrosion solution will penetrate into the pores of the loose layer, which will leave hidden troubles such as blistering, falling off and continuous corrosion inside the base metal for the subsequent pre plating and electroplating.
The pre plating process is a key process in the electroplating process of zinc alloy die castings. In order to obtain a barrier layer to prevent the corrosion of the base metal in the electroplating intermediate layer or surface coating, it is an indispensable step in the electroplating process of zinc alloy die castings. The integrity of the pre coating and its adhesion to the surface of zinc alloy substrate will directly affect the adhesion of the whole electrodeposition.
Neutral electrolyte, such as neutral citrate nickel plating, which is less corrosive to zinc alloy, is used in the pre plating process. In order to reduce the stress and improve the toughness of the coating, additives should not be added or added as little as possible in the electrolyte. Ensure the adhesion between coating and substrate.