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Summary of Screw Surface Treatment Processes and Corrosion Resistance: Comprehensive Overview
Author: Site Editor Publish Time: 2025-02-25 Origin: Site
Summary of Screw Surface Treatment Processes and Corrosion Resistance: Comprehensive Overview
Black Coated Screws
Common black screws are primarily used to fix the base or rear shells of dark-colored electrical appliances. To minimize inventory management, black screws are also widely used internally in machinery. The main processes for black coatings include black oxide, black nickel, black zinc, and electrophoretic black.
Black Oxide
Process: Chemical degreasing → Hot water rinse → Cold water rinse → Rust removal and acid etching → Cleaning → Blackening → Cleaning → Oil coating or sealing.
Mechanism: A black oxide film is formed at temperatures above 100°C using sodium hydroxide and sodium nitrite.
Composition: The main component of the oxide film is magnetite (Fe₃O₄), with a thickness of only 0.6-1.5 micrometers. It has relatively poor corrosion resistance, with neutral salt spray resistance of only 1-2 hours without oil coating or sealing, and 3-4 hours with oil coating.
Appearance: Black oxide has a similar color to black zinc and electrophoretic black but is not as bright.
Composition: The black nickel layer contains approximately 40%-60% nickel, 20%-30% zinc, 10%-15% sulfur, and 10% organic compounds.
Performance: Industry standards show neutral salt spray resistance of 6-12 hours. Enhanced corrosion resistance can be achieved by increasing the thickness of the copper underlayer. However, oil coating is generally avoided due to potential corrosion of plastic components.
Identification: Black nickel has a deeper color compared to other black coatings, making it easily distinguishable.
Mechanism: Zinc is chemically active and tends to oxidize and darken in the atmosphere, forming "white rust." After plating, a chromate conversion coating is applied to passivate the zinc layer.
Performance: Black zinc screws have a neutral salt spray resistance of over 20 hours after oil coating, with some reaching up to 12 hours in industry standards. The color is darker than black nickel, closer to black oxide and electrophoretic black.
Identification: Grinding on white paper reveals the light blue-white color of zinc.
Mechanism: Organic resin particles are deposited on the part using electrochemical methods to form a black organic coating.
Performance: Neutral salt spray resistance exceeds 300 hours, comparable to the corrosion resistance of the Dacromet process.
Advantages: Better application properties and reduced environmental impact compared to paint processes.
White Coated Screws
White-coated screws are also widely used, especially for external applications. The main types include white nickel, white zinc, and stainless steel screws without surface treatment.
Performance: White passivation forms a transparent zinc oxide film with relatively poor corrosion resistance compared to black or blue zinc. Industry standards show neutral salt spray resistance of 6-12 hours, with some reaching up to 20 hours with improved passivation formulations.
Identification: White zinc has a light blue-white appearance, distinct from white nickel.
Performance: Similar corrosion resistance to black nickel (6-12 hours), with potential improvements through oil coating or sealing.
Identification: White nickel has a brighter appearance compared to white zinc.
Stainless Steel Screws
Materials: Commonly made from SUS304, with some applications using SUS202, SUS201, or SUS410 (martensitic stainless steel).
Performance: Corrosion resistance varies by material. SUS304 can withstand neutral salt spray for 48-96 hours, while SUS202 can exceed 48 hours with proper surface treatment. SUS410 has a lower corrosion resistance of around 20 hours.
Recommendation: Avoid using martensitic stainless steel (SUS410) due to higher costs compared to ferritic stainless steel with surface treatments.
Other Coated Screws
Other colored coatings are generally used for internal applications, including blue zinc, green zinc, color zinc, and Dacromet.
Blue Zinc and Green Zinc
Process: Similar to white zinc, with blue zinc containing trivalent chromium (0.5-0.6 mg/dm²) in the passivation film. Green passivation (also called pentavalent passivation) forms a thick, olive-green film.
Performance: Blue zinc offers better corrosion resistance than white zinc, while green zinc is superior to blue zinc. Neutral salt spray resistance can exceed 48 hours for green zinc.
Identification: Blue zinc has a light blue appearance, similar to white zinc, while green zinc has a distinct olive-green color.
Performance: Color zinc offers excellent corrosion resistance, with neutral salt spray resistance exceeding 48 hours and up to 100 hours with proper control.
Mechanism: Passivation temperature should be controlled around 25°C to ensure uniform color and film adhesion.
Mechanism: A solution containing flake zinc and aluminum, chromium trioxide, and organic compounds is applied to the metal surface. After sintering at around 300°C, a corrosion-resistant coating is formed.
Performance: Neutral salt spray resistance exceeds 300 hours. However, the coating thickness is uneven (5-10 micrometers to 40 micrometers), which may affect the thread depth of screws.
Recommendation: Avoid using Dacromet for machine screws or those with small thread diameters.
Conclusion
This article provides a comprehensive overview of screw surface treatment processes and their impact on corrosion resistance. Various common surface treatments, such as zinc plating, nickel plating, chromium plating, and blackening, are discussed in detail, highlighting their principles, procedures, and characteristics.
In terms of corrosion resistance, different surface treatments offer varying levels of protection. Zinc plating effectively isolates air and moisture but performs differently in various environments. Nickel plating forms a dense nickel layer, significantly enhancing corrosion resistance, especially in chemically demanding conditions. Chromium plating not only improves appearance but also increases hardness and corrosion resistance. Black oxide treatment, while less effective in corrosion resistance, is suitable for specific applications with lower requirements.
Additionally, the article explores other factors influencing screw corrosion, such as environmental humidity, temperature, and pH levels. It emphasizes the importance of selecting the appropriate surface treatment based on specific usage conditions to ensure optimal corrosion resistance, prolong service life, and maintain product quality and safety.
