Fastener Surface Treatment Options

Surface Treatment of Fasteners: Choosing from RUSPET, Galvanization, Phosphorization, Oxidation(black) and Dacromet

    Fasteners are predominantly made from carbon steel and alloy steel, and they generally require corrosion protection. Therefore, the surface coating must adhere firmly and not peel off during installation or removal. On the other hand, for threaded fasteners, the coating must be thin enough to allow the threads to engage after plating. The primary purposes of surface treatment are aesthetics and corrosion prevention.

    Since the main function of fasteners is to secure components, and surface treatment significantly affects their fastening performance, factors such as torque and preload consistency must be considered when selecting a surface treatment process.

Below is a brief introduction to some common coatings used for fasteners.

1. RUSPET

    Ruspet is a coating developed for construction screws, characterized by strong corrosion resistance with salt spray resistance times ranging from 500 to 1500 hours. It requires a relatively low baking temperature, generally controlled below 200 degrees Celsius, particularly for construction screws or self-drilling screws.

2. Electrogalvanizing

    Electrogalvanizing is the most common coating for commercial fasteners. It is relatively inexpensive and aesthetically pleasing. However, its corrosion resistance is modest, being the lowest among zinc coatings. Standard electroplated zinc can withstand neutral salt spray tests for up to 72 hours, although special sealants can extend this to over 200 hours. Bolts of grade 10.9 and above generally do not use zinc plating.

    Electrogalvanizing fasteners have poor and unstable torque-preload consistency and are generally not used for critical connections.

3. Phosphorization

    Phosphorization is cheaper than galvanizing and has inferior corrosion resistance. After phosphating, oil should be applied, and the level of corrosion resistance depends greatly on the performance of the oil used. For instance, with general anti-rust oil, the neutral salt spray test lasts only 10 to 20 hours. With high-grade anti-rust oil, it can reach 72 to 96 hours.

    Two common types of phosphating for fasteners are zinc and manganese phosphating. 

    Zinc phosphating has better lubrication properties, while manganese phosphating offers better corrosion and wear resistance than galvanizing. It can withstand temperatures from 225 to 400 degrees Fahrenheit (107 to 204 degrees Celsius).

    High-strength bolts using phosphating can also avoid hydrogen embrittlement issues, so in the industrial field, bolts of grade 10.9 and above typically undergo phosphating surface treatment.

4. Oxidation (Blackening)

    Blackening plus oiling is a popular coating for industrial fasteners because it is the cheapest and looks decent before the oil is exhausted. Since blackening has almost no rust prevention capability, it will rust quickly once the oil is gone. Even with oil, it can only withstand neutral salt spray tests for 3 to 5 hours.

5. Nickelage

    Nickelage, achieved through electrolytic or chemical methods on metal or certain non-metals, is used where corrosion resistance and good electrical conductivity are required.

6.Hot-Dip Galvanization

    Hot-dip galvanizing has a coating thickness of 15 to 100 micrometers and offers good corrosion resistance, commonly used in engineering. Due to the processing temperature of hot-dip galvanizing (340-500°C), it cannot be used for fasteners of grade 10.9 and above.

7.Zincizing

    Zinc impregnation is a solid-state metallurgical heat diffusion coating using zinc powder. It provides uniformity, with even coverage inside threads and blind holes. The coating thickness ranges from 10 to 110 micrometers, with a controllable error margin of 10%. Its bond strength and corrosion resistance are the best among zinc coatings (electroplated zinc, hot-dip galvanizing, Dacromet). Its processing is pollution-free, making it the most environmentally friendly option.

8.Dacromet

    Dacromet, also known as zinc-chrome coating, is a new type of anti-corrosion coating primarily composed of zinc powder, aluminum powder, chromic acid, and deionized water. It is most suitable for high-strength fasteners with high corrosion resistance requirements.