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Author: Site Editor Publish Time: 2024-12-23 Origin: Site
Nearly all commercial fasteners are made from carbon steel or alloy steel and generally require corrosion resistance. Therefore, the surface treatment coating must adhere firmly and not peel off during installation or removal. For threaded fasteners, the coating must also be thin enough to allow smooth engagement of the threads. It's important to note that the temperature limits of the coating are typically lower than those of the base material, so the working temperature conditions of the fastener should also be considered.
The primary purposes of surface treatments are aesthetics and corrosion resistance. Since the primary function of fasteners is to securely fasten components, surface treatments also significantly impact their fastening performance. Therefore, when choosing a surface treatment, factors such as torque and preload consistency must be considered.
A high-level designer must not only focus on structural design and manufacturing processes but also on assembly feasibility, environmental considerations, and cost-efficiency. Below, we provide an overview of some common fastener coatings to help industry professionals make informed choices.
Electroplated zinc is the most common coating for commercial fasteners. It is inexpensive, visually appealing (available in black, military green, etc.), and relatively easy to apply. However, its corrosion resistance is generally lower compared to other coatings, with the neutral salt spray test typically lasting 72 hours. Special sealants can improve this resistance, achieving over 200 hours, but at a significantly higher cost—5 to 8 times more expensive than standard zinc plating.
Electroplated zinc can cause hydrogen embrittlement, so it is generally not used for fasteners with a grade above 10.9. While baking can help eliminate hydrogen, this process must be done before passivation to avoid damaging the passivation film. As a result, it is not commonly performed, unless specifically required by the customer. Additionally, electroplated zinc fasteners often show poor torque-to-preload consistency, which makes them unsuitable for critical connections. Applying lubrication after plating can improve this consistency.
Phosphating is a more affordable option than zinc plating but provides lower corrosion resistance. After phosphating, an oil coating is required to improve corrosion resistance. The performance of the oil coating directly influences the corrosion resistance; for instance, using high-grade anti-rust oils can extend the neutral salt spray test to 72–96 hours, but these oils are 2–3 times more expensive.
Common phosphating types include zinc-based and manganese-based phosphating. Zinc-based phosphating offers better lubrication properties, while manganese-based phosphating is more resistant to corrosion and wear. It can withstand higher temperatures, ranging from 107°C to 204°C (225°F to 400°F), making it ideal for critical automotive components like engine connecting rod bolts, cylinder head bolts, wheel nuts, etc. Phosphating is often used for high-strength bolts (grade 10.9 and above) because it helps avoid hydrogen embrittlement.
Black oxide plus oil is a popular coating for industrial fasteners, primarily because it is inexpensive and visually appealing while the oil lasts. However, it offers minimal rust resistance, and without oil, it rusts quickly. Even with oil, the neutral salt spray test only lasts 3–5 hours.
Cadmium plating provides excellent corrosion resistance, particularly in marine environments. However, the waste treatment costs associated with cadmium plating are high, and the process is expensive—about 15–20 times more costly than zinc plating. As a result, cadmium plating is used only in specific environments, such as fasteners for oil drilling platforms or aircraft.
Chromium plating is stable in the atmosphere, resistant to discoloration, and maintains its shine. It is highly durable and offers good wear resistance. While it is typically used for decorative purposes on fasteners, it is not commonly used in industries with high corrosion resistance requirements due to its high cost. Chromium-plated fasteners are sometimes used in place of stainless steel when strength is a concern.
To prevent corrosion, copper and nickel plating are often applied before chromium plating. Chromium can withstand high temperatures (up to 650°C or 1200°F), but, like zinc plating, it is susceptible to hydrogen embrittlement.
Nickel plating is achieved through either electroplating or chemical plating and is often used in applications requiring both corrosion resistance and electrical conductivity, such as the terminals of vehicle batteries. Nickel plating provides a balance of both properties but is generally used in specialized applications rather than general industrial fasteners.
Hot-dip galvanizing involves heating zinc to a liquid state and immersing the fastener to form a thick coating. The thickness typically ranges from 15 to 100 microns, offering good corrosion resistance, especially for use in construction and other engineering projects. However, due to the thicker coating, it may lead to issues with thread engagement, particularly in high-strength fasteners (above grade 10.9), as the coating can interfere with the threading process. Additionally, the hot-dip galvanizing process generates significant pollution due to zinc fumes and waste.
Sherardizing, or zinc diffusion, is a solid-state metallurgical process that creates a uniform coating on the fastener, providing superior corrosion resistance. With a thickness range of 10 to 110 microns, the coating is well-bonded and highly durable, making it the most environmentally friendly option since it does not generate significant pollution. The coating thickness can be controlled with high precision, making it ideal for critical applications.
DACROMET is a zinc-chromium coating, also known as a zinc-aluminum-chromium coating, and is a newer anti-corrosion coating. It does not suffer from hydrogen embrittlement and provides excellent torque-to-preload consistency. Despite its higher cost, it is widely used in applications requiring high corrosion resistance, especially for high-strength fasteners.
When selecting a surface treatment for fasteners, it is crucial to consider several factors, including corrosion resistance, cost, performance under temperature conditions, and the specific requirements of the application. No one coating is universally superior; the best choice depends on the specific demands of the project. By understanding the advantages and limitations of each coating, engineers and designers can make informed decisions that ensure optimal performance, durability, and cost-effectiveness.
