Surface treatment of die castings
Surface treatment of die-cast parts involves forming a protective film or functional layer on the surface of die-cast parts through physical, chemical, or electrochemical methods to improve corrosion resistance, wear resistance, decorative properties, or impart specific functions (such as conductivity or insulation). Because die-cast alloys (such as aluminum and zinc alloys) are susceptible to oxidation in air, and die-cast parts may exhibit surface defects such as pores and shrinkage, surface treatment not only improves appearance quality but is also crucial for extending product life. The choice of surface treatment process depends on the die-cast part’s material, operating environment, and performance requirements. Common methods include electroplating, anodizing, spraying, phosphating, and passivation.
Electroplating is a process that deposits a metal coating (such as chromium, nickel, zinc, or copper) on the surface of a die-cast part. Electrolysis reduces and precipitates metal ions on the workpiece surface, forming a uniform, dense coating. Electroplating not only improves the corrosion resistance of die-cast parts (for example, the zinc coating offers salt spray resistance of over 500 hours), but also enhances surface gloss and hardness, making it suitable for decorative and wear-resistant parts. For example, chromium plating can achieve a surface hardness exceeding HV800 on zinc alloy die-cast parts, increasing wear resistance by 5-10 times and creating a mirror-like finish. These parts are widely used in bathroom accessories, automotive trim, and other applications. Prior to electroplating, die-cast parts must undergo rigorous pretreatment (such as degreasing, pickling, and activation) to ensure the surface is free of oil, scale, and impurities, which can cause blistering and flaking of the coating. Aluminum alloy die-cast parts, due to the tendency for oxide films to form on the surface, require zincate treatment (zinc immersion) to improve the adhesion between the coating and the substrate.
Anodizing is primarily used for aluminum alloy die-castings. The workpiece, acting as the anode, is placed in an electrolyte (such as sulfuric acid or oxalic acid). Electrolysis forms a porous oxide film ( Al₂O₃ ) on the surface. The oxide film, typically 5-20μm thick , offers excellent corrosion resistance and adsorption properties. It can be dyed to various colors, providing both protective and decorative properties. For example, architectural aluminum profiles, after anodizing and dyeing, can withstand salt spray for over 1,000 hours, with a durable, non-fading color. Depending on the electrolyte and process parameters, anodizing can be categorized as conventional anodizing (sulfuric acid method), hard anodizing (thick film anodizing), and porcelain anodizing. Hard anodizing can produce an oxide film as thick as 50-150μm with a hardness exceeding HV500 , making it suitable for wear-resistant components such as engine pistons and cylinders. The anodized oxide film requires sealing (such as hot water sealing or nickel salt sealing) to block pores and further improve corrosion resistance.
Spray coating involves applying a coating (such as paint, powder coating, or fluorocarbon coating) to the surface of a die-cast part through atomization using a spray gun. The coating is then cured to form a continuous coating. This process is suitable for large-area, complex-shaped die-cast parts. Powder spraying utilizes electrostatic attraction to adhere the powder coating to the workpiece surface. Curing occurs at 180-220°C, resulting in a 50-150μm thick coating. This coating offers excellent weather and corrosion resistance, making it suitable for outdoor applications such as streetlight housings and air conditioner units. Fluorocarbon spray coating utilizes a fluorine-containing resin coating, offering excellent UV resistance and a service life exceeding 20 years. It is widely used in architectural curtain walls and high-end home appliances. Surface pretreatment (such as phosphating and passivation) is required before spraying to improve coating adhesion and prevent flaking and cracking. For example, aluminum alloy automotive wheels treated with phosphating and powder spraying can achieve coating adhesion (cross-cut test) of Class 0 and salt spray resistance exceeding 1000 hours.
Phosphating and passivation are chemical processes that form a conversion film (such as a phosphate film or chromate film) on the surface of die-cast parts. These films are primarily used to improve corrosion resistance and serve as a base layer for coatings. Phosphating is suitable for steel and aluminum alloy die-castings. The resulting phosphate film is porous, enhancing the adhesion of subsequent coatings and is commonly used as a pre-painting treatment for automotive chassis components. Passivation treatments (such as chromate passivation and chromium-free passivation) chemically oxidize the metal surface to form a dense oxide film, preventing further corrosion. For example, chromate passivation can improve the salt spray resistance of zinc alloy die-castings from a few hours to hundreds of hours. With increasing environmental protection requirements, chromium-free passivations (such as zirconate and titanate passivations) are gradually replacing traditional chromate passivation, ensuring corrosion resistance while reducing environmental pollution.
Surface treatment quality testing includes visual inspection, film thickness measurement, adhesion testing, and corrosion resistance testing. The exterior must be free of defects such as pinholes, bubbles, and scratches. Film thickness (e.g., plating, oxide film, and coating) must meet design requirements (typically measured using an eddy current thickness gauge or magnetic thickness gauge). Adhesion tests (e.g., cross-hatch and bend tests) ensure that the coating does not fall off. Corrosion resistance tests (e.g., salt spray and damp heat tests) verify the protective effect. For example, automotive die-castings must withstand a salt spray test of at least 500 hours without showing any white rust. By selecting the right surface treatment process and strictly controlling quality, the overall performance of die-castings can be significantly improved to meet the needs of diverse applications.
