Surface Treatment Processes for Screws And Corrosion Resistance

Screws are commonly subjected to various surface treatment processes to improve corrosion resistance, appearance, and durability. These processes are primarily categorized by color, and include oxidation, electrophoresis, electroplating, and Dacromet. Below is a summary of these treatments based on their colors and characteristics.

1. Black Coated Screws

Black-coated screws are frequently used in appliances or parts with dark or black casings, and to minimize management, black screws are often used inside machines. Common black treatments include black oxidation, black nickel, black zinc, and electrophoretic black coatings.

1.1 Black Oxidation

• Process: Chemical de-oiling → hot water rinse → cold water rinse → rust removal → cleaning → blackening → cleaning → oiling/sealing.

• Corrosion Resistance: Poor, with salt spray resistance around 1-2 hours without oil or sealing. After oiling, it improves to 3-4 hours.

• Appearance: The black color is less glossy compared to black zinc or electrophoretic black.

1.2 Electroplating (Black Zinc and Black Nickel)

• Black Zinc:

• Process: De-oiling → cleaning → mild acid etching → zinc plating → cleaning → passivation → drying → black oil treatment.

• Corrosion Resistance: Neutral salt spray test lasts 12 hours. After black oil treatment, resistance can improve to over 20 hours.

• Appearance: Deep black, compared to black nickel, black zinc is slightly lighter and can be distinguished by a slight whitish zinc hue when rubbed on paper.

• Black Nickel:

• Process: De-oiling → cleaning → mild acid activation → copper base plating → black nickel plating → passivation → drying.

• Corrosion Resistance: Generally 6-12 hours in neutral salt spray tests. To improve, oiling or sealing is common, but plastic parts must avoid oiling.

• Appearance: Darker compared to black zinc and more uniform.

1.3 Electrophoretic Black Coating

• Process: De-oiling → cleaning → phosphating → electrophoretic painting → drying.

• Corrosion Resistance: Excellent, with salt spray resistance of over 300 hours.

• Appearance: Glossy black with uniform coating. Cost and corrosion resistance are comparable to Dacromet.

2. White Coated Screws

White-coated screws are commonly used in exposed applications. The common white coatings include white zinc, white nickel, and untreated stainless steel.

2.1 White Zinc

• Process: De-oiling → cleaning → mild acid activation → zinc plating → cleaning → white passivation → drying.

• Corrosion Resistance: Lower than black zinc or blue zinc, generally 6-12 hours in neutral salt spray. High-quality passivation can extend resistance to 20 hours.

• Appearance: Light, pale zinc color, distinct from white nickel which has a brighter, silvery appearance.

2.2 White Nickel

• Process: Similar to black nickel but without the sulfur-based treatments.

• Corrosion Resistance: Generally 6-12 hours in salt spray tests, similar to black nickel.

• Appearance: Bright, silvery white, distinct from white zinc's duller finish.

2.3 Stainless Steel Screws

• Materials: Commonly made from SUS304, SUS202, or SUS201 stainless steel, with higher grades like SUS410 used for more demanding applications.

• Corrosion Resistance: SUS304 offers excellent resistance with salt spray tests lasting 48-96 hours, while SUS202 offers about 48 hours. SUS410 (stainless iron) has lower resistance, around 20 hours, making it less suitable for high corrosion environments.

3. Other Colored Coatings for Screws

These are typically used for internal machine parts but can also be used externally for specific needs.

3.1 Blue Zinc & Green Zinc

• Process: Similar to white zinc but with additional chromate treatments for coloration.

• Corrosion Resistance: Blue zinc is more resistant than white zinc, and green zinc offers the highest resistance.

• Appearance: Blue zinc has a light blue tint, while green zinc has a distinct grass green appearance. Green zinc is less commonly used but offers superior corrosion protection.

3.2 Color Zinc (Yellow/Gold)

• Process: Zinc plating followed by light acid cleaning, then low-chromium passivation.

• Corrosion Resistance: Generally lasts over 48 hours, and with proper control, can exceed 100 hours in salt spray tests.

• Appearance: Yellow to gold-colored finish, often used for aesthetic purposes.

3.3 Dacromet

• Process: Involves coating with a zinc-aluminum chromate solution, followed by baking at 300°C.

• Corrosion Resistance: Excellent, with salt spray resistance exceeding 300 hours.

• Appearance: The coating is uneven, which can affect the screw's thread dimensions. This treatment is less suitable for small threads or screws that require precise measurements.

Conclusion

Surface treatment processes for screws significantly impact their performance and corrosion resistance. Black and white coatings are commonly used, with black coatings generally offering better resistance, while white coatings are typically more aesthetic. For more demanding applications, stainless steel screws or treatments like Dacromet provide superior corrosion resistance. When choosing a surface treatment, factors like the screw's exposure to corrosion, its aesthetic requirements, and compatibility with the materials it interacts with should all be considered.