The aluminum die-casting rear frame of electric bicycles is exposed to the outdoors for a long time, requiring resistance to corrosive media such as rainwater, dust, and salt. At the same time, it must cope with friction and impact during riding. The surface anti-corrosion process not only determines its service life but also affects the appearance quality and assembly adaptability. Aluminum die-casting parts inherently contain tiny pores, which are prone to localized corrosion. Therefore, a protective layer must be formed through scientific surface treatment. The mainstream process revolves around the core of 'isolating corrosive media and enhancing surface performance', forming a diversified adaptation system that is suitable for different grades of models and usage scenarios.
Chemical passivation is a fundamental and economical anti-corrosion process, often used as a pretreatment step or independently in low-demand scenarios. This process involves immersing the aluminum die-casting frame in chemical solutions such as chromate or chromium-free passivation solutions, forming a dense oxide film on the surface with a thickness of only 0.5 - 2μm, which quickly isolates the air and moisture. Its advantages include simple process, low cost, short treatment time, suitable for mass production, and no impact on the dimensional accuracy of the workpiece, while retaining the natural color of the aluminum parts. However, when used alone, its anti-corrosion ability is limited and its weather resistance is weak. It is mostly used in concealed parts of economical electric bicycles, or as a pretreatment before anodizing and spraying to enhance the adhesion of subsequent coatings.
Anodizing is a mainstream choice for enhancing corrosion resistance and wear resistance, especially for applications requiring surface hardness. Through electrolysis, the process forms an alumina film with a thickness of 5-20μm on the surface of aluminum die-casting, with a hardness of HV300-500. This film not only resists corrosion but also withstands daily friction. The process can achieve various colors through dyeing, meeting basic appearance requirements. Moreover, the film layer is tightly bonded to the substrate and is not prone to peeling off. However, due to the influence of porosity in die-casting parts, sealing treatment is required first, otherwise residual corrosion media may remain, reducing the protective effect. It is commonly used for exposed rear frame components of vehicle models.
The electrostatic spraying process, due to its diverse appearance and comprehensive protective properties, has become one of the widely used processes in the market. The process is divided into powder spraying and liquid spraying. First, the surface of the workpiece is degreased and phosphated. Then, through the principle of electrostatic adsorption, powder coating or liquid coating is evenly attached to the surface, and a coating with a thickness of 20-60μm is formed after high-temperature curing. Its advantages lie in the strong compactness of the coating, excellent impact resistance and UV resistance, rich color options, and compatibility with different vehicle design styles. Powder spraying produces no volatile pollutants, making it more in line with environmental protection requirements, while liquid spraying provides a more delicate texture. Both are suitable for complex outdoor environments and are commonly used processes for mass-market vehicles and various vehicle models.
The cathode electrophoresis process emphasizes high adhesion and uniformity, making it suitable for scenarios demanding high precision in protection. In this process, the aluminum die-casting rear frame serves as the cathode in water-soluble electrophoretic paint. Through the influence of an electric field, the paint is uniformly deposited to form a coating, with a film thickness ranging from 10 to 30μm and an adhesion level up to Grade 1. This coating can penetrate into concealed areas such as gaps and corners of the workpiece, achieving comprehensive protection. The coating exhibits strong salt spray resistance, passing salt spray tests lasting over 500 hours, making it suitable for electric bicycles used in coastal areas with high salt spray and humidity. However, this process offers limited color options, mostly basic colors like black and gray, and requires high equipment investment costs. It is predominantly used for heavy-duty electric bicycle rear frames with stringent anti-corrosion requirements.
The composite anti-corrosion process is an advanced solution for complex environments, with the core being the combination of 'pretreatment + main protection'. Common combinations include chemical passivation + electrostatic spraying, and chemical passivation + anodic oxidation. The pretreatment process can seal the pores of die-casting parts and remove surface impurities, while the main protective layer enhances anti-corrosion performance, making the rear frame both have excellent weather resistance and improve the service life of the coating. Compared to a single process, the protective effect is improved by more than 30%, making it suitable for outdoor environments and high-frequency usage scenarios.
All processes must comply with relevant industry standards to ensure protective effectiveness and environmental compliance. In summary, the surface anti-corrosion process for aluminum die-casting rear frames of electric bicycles needs to be selected based on the model positioning, usage environment, and cost budget. Single processes focus on basic needs, while composite processes enhance multi-faceted protection, jointly ensuring the long-term stable use of the rear frame.
