In the electroplating production line, pretreatment plays a crucial role in ensuring a high-quality coating. One of the key steps in this process is chemical polishing, which enhances surface finish and prepares the workpiece for subsequent plating operations. Unlike electropolishing, which requires an external power source, chemical polishing relies on a chemical reaction to smooth the metal surface.
During chemical polishing, the metal surface interacts with a solution that dissolves the material unevenly. The formation of a non-uniform passivation layer or a viscous film leads to faster dissolution of surface protrusions compared to depressions. This results in a smoother, brighter finish. Chemical polishing is commonly used for stainless steel, copper, and aluminum alloys, and it can also serve as a decorative finishing step.
Compared to electropolishing, chemical polishing offers advantages such as no need for electricity or conductive fixtures, making it suitable for complex-shaped parts. However, it has some drawbacks, including shorter solution life, difficulty in concentration adjustment, and the release of harmful gases. Additionally, its surface quality is generally lower than that of electropolishing because the lack of applied voltage can lead to localized corrosion and micro-batteries.
The effectiveness of chemical polishing depends on several factors, including the composition of the polishing solution, time, and temperature. The basic components of a chemical polishing solution typically include an etchant, oxidizer, additives, and water. Strong acids like sulfuric, nitric, and phosphoric acid are often used, while additives like glycerin or gelatin help increase viscosity and improve film formation. For aluminum, strict control of nitric acid concentration is essential to avoid issues like squamous corrosion or poor gloss.
Polishing time must be carefully controlled—too short and the surface remains dull; too long and defects like spots or stains may appear. Temperature also plays a critical role, as higher temperatures accelerate dissolution but can cause pitting or uneven results if not managed properly. For copper and its alloys, solutions containing phosphoric, nitric, or chromic acids are commonly used, while aluminum requires precise control of acid concentrations and the addition of certain salts to enhance surface quality.
After chemical polishing, parts are often rinsed in nitric acid or chromic anhydride to remove any residual copper or impurities. Overall, chemical polishing is a versatile and efficient method when properly optimized for each material and application.
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