Surface treatment of fasteners can enhance their corrosion resistance, improve their appearance, and give them specific properties according to different usage requirements. The following are common surface treatment methods and their characteristics:
1. Electroplating
Electroplating is a process that uses electrochemical methods to deposit metal ions onto the surface of Screws. Common types include:
Zinc Electroplating
• Economical: low cost, beautiful appearance, and can be provided in a variety of colors such as black and military green.
• Corrosion Resistance: The corrosion resistance time of the neutral salt spray test is usually 72 hours; it can be increased to more than 200 hours with the use of special sealants, but the cost is relatively high.
• Hydrogen Embrittlement: Not suitable for high-strength Bolts (grade 10.9 and above) due to the risk of hydrogen embrittlement. • Torque–Preload Consistency: Typically inconsistent but can be improved by applying lubricating materials.
Nickel Plating
• Used in applications requiring both corrosion resistance and excellent electrical conductivity, such as battery terminals in vehicles.
Chrome Plating
• Chrome layers are stable in the atmosphere, resistant to discoloration, hard, and wear-resistant.
• Primarily used for decorative fasteners. In industrial applications, it is less common due to high cost, unless stainless steel lacks sufficient strength.
2. Hot-Dip Galvanizing
Hot-dip galvanizing involves immersing fasteners in molten zinc to form a diffusion coating.
• Coating Characteristics: Thickness ranges from 15–100 μm, offering excellent corrosion resistance but difficult to control precisely.
• Environmental Concerns: The process generates zinc waste and vapor, leading to pollution.
• Thread Engagement Issues: Thick coatings may cause internal and external threads to bind.
Solutions include:
1. Post-Galvanizing Thread Tapping: Addresses thread binding but reduces corrosion resistance at the thread area.
2. Oversized Threading: Threads are cut oversized (by 0.16–0.75 mm for M5–M30) before galvanizing. This solves the binding issue but may reduce strength.
• High-Strength Limitations: Not suitable for high-strength fasteners (grade 10.9 and above) due to the high processing temperature.
3. Phosphating
Phosphating creates a phosphate conversion coating on the surface of fasteners through chemical treatment.
• Cost and Corrosion Resistance:
• Cheaper than zinc plating but offers lower corrosion resistance.
• Requires oiling after treatment to improve corrosion resistance, which depends on the quality of the anti-rust oil:
• Regular anti-rust oil provides 10–20 hours of neutral salt spray resistance.
• Premium anti-rust oil can extend this to 72–96 hours but costs 2–3 times more.
• Types of Phosphating:
1. Zinc Phosphating: Excellent lubricity, suitable for applications requiring reduced friction.
2. Manganese Phosphating: Better corrosion and wear resistance, with a working temperature range of 107–204°C (225–400°F).
• Industrial Applications:
• Widely used in industrial fields for its good torque–preload consistency, ensuring assembly meets design requirements.
• Commonly used for critical connections, such as:
• Steel structure assembly.
• Engine connecting rod bolts, cylinder heads, main bearings, and flywheel bolts.
• High-Strength Bolts: Preferred for grade 10.9 and above to avoid hydrogen embrittlement.
4. Oxidation (Blackening)
Creates a black oxide film on the surface through chemical oxidation, providing basic rust protection, commonly used in mechanical applications.
5. Dacromet Coating
A zinc and chrome-based coating offering exceptional corrosion resistance without hydrogen embrittlement, making it suitable for high-strength fasteners.
6. Spraying
Applies paint or powder coatings to fasteners, providing both corrosion protection and aesthetic enhancement.
7. Mechanical Plating
Deposits zinc particles onto the surface through mechanical friction and chemical reactions, eliminating hydrogen embrittlement risks, and is ideal for high-strength fasteners.
8. Stainless Steel Passivation
Passivation improves the corrosion resistance of stainless steel fasteners, making them suitable for extreme environments.
9. Sandblasting and Shot Blasting
Removes surface oxides and rust while enhancing coating adhesion, serving as a critical pre-treatment step.
Conclusion
Each surface treatment method offers unique advantages and is suited for specific performance, cost, and application needs. Selecting the right method ensures optimal performance and cost-effectiveness for fasteners.
