Different Surface Treatment Options for Fasteners

1. Electric Galvanizing

Electric galvanizing is the most commonly used coating for commercial fasteners. It's relatively inexpensive and aesthetically pleasing, available in white or yellow. However, its corrosion resistance is average, the lowest among zinc plating (coatings). Electric galvanizing typically passes a neutral salt spray test within 72 hours.

Electric galvanizing is prone to hydrogen embrittlement, so bolts grade 10.9 and above are generally not treated with galvanizing. Electric galvanized fasteners have poor torque-to-preload consistency and are unstable, making them generally unsuitable for critical connections. To improve torque-preload consistency, a lubricant can also be applied after plating to improve and enhance torque-preload consistency.

2. Phosphating

Phosphating is cheaper than galvanizing, but its corrosion resistance is inferior. Phosphating should be followed by an oil coating, and the corrosion resistance is closely related to the properties of the oil. For example, applying a standard rust-preventive oil after phosphating will only last 10-20 hours in a neutral salt spray test. Applying a high-quality rust-preventive oil can last up to 72-46 hours.

Many industrial fasteners are treated with phosphating oil. Because it offers excellent torque-to-preload consistency, ensuring the desired tightening requirements are met during assembly, it is widely used in industry, particularly for connecting critical components. Examples include steel structures, engine connecting rod bolts and nuts, cylinder heads, main bearings, flywheel bolts, and wheel bolts and nuts.

Phosphating high-strength bolts can also help prevent hydrogen embrittlement, so in industry, bolts grade 10.9 and above generally use a phosphate surface treatment.

3. Oxidation Black

Oxidation black combined with oiling is a popular coating for industrial fasteners because it is the cheapest and looks good until the oil depletes. However, since Oxidation black offers little rust protection, it rusts quickly without oil. Even with oil, it can only withstand a neutral salt spray test of 3-5 hours.

Oxidation black fasteners also exhibit poor torque-to-preload consistency. To improve this, grease the internal threads before assembly.

4. Electroplating Chromium plating

Chromium plating is very stable in the atmosphere, resists discoloration and tarnishing, and possesses high hardness and excellent wear resistance. Chromium plating on fasteners is generally used for decorative purposes. It is rarely used in industrial applications requiring high corrosion resistance, as high-quality chromium-plated fasteners are as expensive as stainless steel. Chromium-plated fasteners are only used when stainless steel is not strong enough.

To prevent corrosion, copper and nickel plating should be applied before chromium plating. Chromium plating can withstand temperatures up to 1200°F (650°C). However, like zinc plating, it also suffers from hydrogen embrittlement.

5. Silver and Nickel Plating

Silver plating provides both corrosion protection and serves as a solid lubricant for fasteners. For cost reasons, silver plating is used on nuts but not bolts. Sometimes, small bolts are also silver-plated. Silver tarnishes in air, but can function at temperatures up to 1600°F. Therefore, its high-temperature resistance and lubricating properties are utilized in fasteners operating at high temperatures to prevent oxidation and galling of nuts and bolts. Nickel plating of fasteners is primarily used in applications requiring both corrosion resistance and good electrical conductivity, such as battery terminals on vehicles.

6. Hot-dipped Galvanizing

Hot-dip galvanizing involves the thermal diffusion of zinc into a liquid state. The coating thickness ranges from 15 to 10 μm and is difficult to control, but it offers excellent corrosion resistance and is often used in engineering applications. The hot-dip galvanizing process is highly polluting, producing zinc waste and zinc vapor.

The thick coating can cause difficulty in threading internal and external threads in fasteners. There are two solutions to this problem. One is to tap the internal threads after plating. While this solves the thread engagement problem, it also reduces corrosion resistance. The other is to tap the nut to a thread size approximately 0.16 to 0.75 mm (M5 to M30) larger than the standard thread, followed by hot-dip galvanizing. This also solves the thread engagement problem, but at the expense of reduced strength. Currently, a new anti-loosening thread, the American "Spiroc" internal thread, can address this issue. Because the clearance between the internal and external threads is large when not tightened, it can accommodate thick coatings, thus maintaining proper threadability while also maintaining corrosion resistance and strength.

Due to the temperatures involved in hot-dip galvanizing, it cannot be used on fasteners with a grade of 10.9 or higher.

7. Dacromet

Dacromet surface treatment is a technique used to improve the corrosion resistance of metal products. The process includes cleaning, pretreatment, coating with Dacromet coating, and baking and curing. Dacromet coatings, composed of zinc and aluminum powders, offer strong corrosion resistance, are hydrogen-resistant, and are resistant to high temperatures. They are widely used in the automotive, construction, and power industries.