Principles and processes of electroplating
The principle of electroplating is based on electrolysis. Its core principle is that, under the influence of direct current, metal ions in the electrolyte undergo directional migration and are reduced and deposited on the surface of the cathode (the workpiece being plated). Specifically, during the electroplating process, the workpiece being plated serves as the cathode, and the coating metal (or insoluble anode) serves as the anode. Both are immersed in an electrolyte containing the coating metal ions. When the DC power supply is applied, an oxidation reaction occurs at the anode. If the anode is soluble, the coating metal loses electrons, transforming into ions and entering the electrolyte. If the anode is insoluble, anions in the electrolyte (such as chloride ions) lose electrons and generate gases (such as chlorine). Simultaneously, a reduction reaction occurs at the cathode. The coating metal ions in the electrolyte gain electrons and are reduced to metal atoms on the workpiece surface, gradually accumulating to form a coating. During this process, factors such as the current, electrolyte concentration, and temperature influence the ion migration rate and deposition rate, thereby determining the quality of the coating.
The electroplating process is a complex system engineering process, generally consisting of three main stages: pretreatment, electroplating, and post-treatment. Pretreatment is the foundation of the electroplating process, aiming to remove impurities from the workpiece surface and ensure a strong bond between the coating and the substrate. Pretreatment steps primarily include degreasing, rust removal, and activation. Numerous degreasing methods are available. Chemical degreasing involves saponifying oils with an alkaline solution to remove saponifiable oils, while surfactants are used to emulsify unsaponifiable oils. Electrochemical degreasing, on the other hand, involves direct current (DC) to generate bubbles on the electrode surface, leveraging the agitation effect to enhance degreasing and achieve higher efficiency. Rust removal is typically achieved by pickling, where the acid reacts with the rust to dissolve and remove it. Commonly used acids include hydrochloric acid and sulfuric acid. Activation treatment typically uses a dilute acid solution to remove any residual oxide film on the workpiece surface, increasing surface activity and preparing the surface for subsequent electroplating.
The electroplating stage is a critical step in forming the coating, requiring strict control of various process parameters. The first is the electrolyte formulation. Different coating materials require different electrolyte compositions. For example, zinc plating electrolytes come in acidic and alkaline forms. Acidic zinc plating solutions produce a faster deposition rate but less uniform coatings, while alkaline zinc plating solutions produce a uniform, bright coating but at a slower deposition rate. The second factor is current density, which directly influences the coating thickness and crystallization state. Too low a current density results in a uniform coating thickness but slow deposition; too high a current density can lead to a rough, charred coating, and even defects such as pinholes and pitting. Temperature is also a crucial parameter. Increasing the temperature accelerates ion diffusion and increases the deposition rate, but excessively high temperatures can affect the electrolyte stability and coating performance. Furthermore, the electroplating time is determined by the desired coating thickness. Within a certain range, coating thickness is directly proportional to the electroplating time.
During the electroplating process, auxiliary measures are also necessary to ensure coating quality. For example, stirring the electrolyte can promote ion diffusion, reduce concentration polarization, and achieve a more uniform coating. Adding additives such as brighteners and levelers can improve the coating’s appearance and performance, making it brighter and smoother. For workpieces with complex shapes, methods such as pictographic anodes and auxiliary cathodes can also be used to improve current distribution and ensure uniform coatings in areas like grooves and deep holes. Furthermore, the electrolyte should be regularly purified to remove impurities and metal ions and maintain its stability.
Post-treatment is the final stage of the electroplating process, aimed at further improving the performance and stability of the coating. Common post-treatment methods include passivation, dehydrogenation, and sealing. Passivation involves immersing the plated workpiece in a passivation solution to form a dense oxide film on the surface of the coating, improving its corrosion resistance. An example of this is chromate passivation after zinc plating. Dehydrogenation is primarily used for post-plating treatment of materials such as high-strength steel. Heating (generally 180-220°C) allows hydrogen within the workpiece to diffuse and escape, preventing hydrogen embrittlement. Sealing involves applying an organic or inorganic coating, such as paint or oil immersion, to the surface of the coating to further isolate it from corrosive media and extend the coating’s service life. The choice of post-treatment process should be determined based on the coating material and the intended use environment to achieve the best treatment results.
In short, electroplating is based on electrolysis, and the electroplating process is a complete process encompassing pretreatment, plating, and post-treatment. Each step requires strict control to achieve a high-quality coating. With the continuous advancement of technology, electroplating processes are becoming more environmentally friendly, more efficient, and more precise, providing better surface treatment solutions for industrial production.
