Common Surface Treatment Methods for Fasteners

Common Surface Treatment Methods for Fasteners

As technology advances and environmental protection requirements increase, more and more products are taking the route of environmental protection and high-tech. Stainless steel fasteners, regarded as the "industrial staple", are now widely used in numerous industries. Here are the commonly adopted surface treatment methods for fasteners:

1. Electro-galvanizing

Electro-galvanizing is the most prevalent coating for commercial fasteners. It's cost-effective and has an appealing appearance, available in colors like black or army green. However, its anti-corrosion ability is mediocre, being the weakest among zinc coatings. Typically, electro-galvanized fasteners can endure a neutral salt spray test for up to 72 hours. Special sealants can extend this to over 200 hours, but they're pricey, costing 5 - 8 times more than regular galvanizing. The electro-galvanizing process often leads to hydrogen embrittlement. Hence, bolts of grade 10.9 or above usually don't undergo this treatment. Although hydrogen can be removed using an oven after plating, the passivation film gets damaged above 60 °C. So, hydrogen removal must be done after electroplating and before passivation, which is operationally challenging and raises processing costs. In practice, manufacturers seldom remove hydrogen voluntarily unless specifically requested by clients. Moreover, the torque-preload consistency of electro-galvanized fasteners is poor and unstable, making them unsuitable for crucial connections. To enhance this consistency, applying lubricants after plating can be an option.

2. Phosphating

Phosphating is cheaper than galvanizing but has inferior corrosion resistance. After phosphating, oil application is necessary, and the corrosion resistance depends largely on the quality of the oil. For instance, with ordinary anti-rust oil after phosphating, the neutral salt spray test lasts only 10 - 20 hours, while high-quality oil can extend it to 72 - 96 hours, though the latter is 2 - 3 times more expensive. Fasteners commonly use two types of phosphating: zinc phosphating and manganese phosphating. Zinc phosphating offers better lubrication, while manganese phosphating excels in corrosion resistance and wear resistance, and can withstand temperatures from 225 to 400 °F (107 to 204 °C). Many industrial fasteners are treated with phosphating and oil application due to its excellent torque-preload consistency, ensuring that the fastening meets the design expectations. It's thus widely used in industry, especially for connecting important components like steel structure connection sets, engine connecting rod bolts and nuts, cylinder head, main bearing, and flywheel bolts, as well as wheel bolts and nuts. Additionally, high-strength bolts are often phosphated to avoid hydrogen embrittlement, so bolts of grade 10.9 or above in the industrial field usually undergo phosphating surface treatment.

3. Oxidation (Blackening)

Blackening + oil application is a popular coating for industrial fasteners as it's the cheapest and looks good until the oil runs out. However, blackening has almost no rust-proof ability, so fasteners will rust quickly once the oil is depleted. Even with oil, the neutral salt spray test can only last 3 - 5 hours. The torque-preload consistency of blackened fasteners is also poor. To improve it, grease can be applied to the internal threads before assembly.

4. Electroplating Cadmium

The cadmium coating has excellent corrosion resistance, especially in a marine atmospheric environment, outperforming other surface treatments. However, the electroplating cadmium process incurs high costs for waste liquid treatment, making it about 15 - 20 times more expensive than electro-galvanizing. Thus, it's only used in specific environments, such as for fasteners on oil drilling platforms and naval aircraft.

5. Electroplating Chromium

The chromium coating is stable in the atmosphere, resistant to discoloration and loss of luster, and has high hardness and good wear resistance. On fasteners, the chromium coating is mainly used for decorative purposes. In industrial fields with high anti-corrosion requirements, it's seldom used because good chromium electroplated fasteners are as expensive as stainless steel. Only when the strength of stainless steel is insufficient will chromium-plated fasteners be used instead. To prevent corrosion, copper and nickel should be plated first before chromium plating. The chromium coating can withstand high temperatures up to 1200 °F (650 °C), but like electro-galvanizing, it also has the problem of hydrogen embrittlement.

6. Silver Plating and Nickel Plating

The silver coating can not only prevent corrosion but also serve as a solid lubricant for fasteners. Due to cost considerations, nuts are often silver-plated while bolts usually aren't, although small bolts may sometimes be plated. Silver tarnishes in the air but can function at 1600 °F. People take advantage of its high-temperature resistance and lubricating properties for fasteners working at high temperatures to prevent oxidation and seizing of bolts and nuts. Fastener nickel plating is mainly used in places where both anti-corrosion and good electrical conductivity are required, such as the terminal leads of vehicle batteries.

7. Galvanizing (Hot-dip Galvanizing and Sherardizing)

Hot-dip galvanizing involves heating zinc to a liquid state for thermal diffusion coating. The coating thickness ranges from 15 to 10 μm and is difficult to control precisely, yet it has good corrosion resistance and is widely used in engineering projects. However, the hot-dip galvanizing process causes serious pollution, generating zinc waste and zinc vapor. Due to the thick coating, there's a problem with the difficulty of screwing internal and external threads together in fasteners. There are two solutions. One is to tap the internal thread after plating, which solves the screwing problem but reduces the anti-corrosion performance. The other is to make the thread of the nut larger than the standard thread by about 0.16 - 0.75 mm (for M5 - M30) before hot-dip galvanizing. Although this can also solve the screwing problem, it sacrifices strength. Currently, there's an anti-loosening thread - the American "Spiralock" internal thread that can solve this issue. Its internal thread has a large clearance with the external thread when not tightened, allowing for thick coatings without affecting the screwing performance while maintaining the original anti-corrosion performance and strength. Due to the temperature involved in the hot-dip galvanizing process, it can't be used for fasteners of grade 10.9 or above.

8. Sherardizing The thickness of the sherardizing layer can be controlled within the range of 10 - 190 μm according to user requirements, with an error of no more than ±10%. For workpieces with threads and fitting requirements, interchangeability can be ensured, which is difficult for the hot-dip galvanizing process as it can't control the thickness well and ensure interchangeability, and requires larger fitting clearances for workpieces, weakening the bonding strength of mating parts. Vacuum powder sherardizing overcomes defects like zinc nodules and burrs in hot-dip galvanizing, resulting in smooth and flat sherardized parts that look good even without any post-treatment. The sherardizing layer has a uniform thickness, and it remains consistent even in threaded, blind hole, and corner areas. During use, there won't be premature local rusting, and it can prevent the hazard of tip discharge in high-voltage power transmission, which is an issue that hot-dip galvanizing and other processes can't solve, especially in threaded and blind hole areas. The sherardizing layer is a zinc-iron alloy layer with a dense structure, firmly bonded to the steel substrate, having an electrode potential lower than iron and higher than zinc. It has high surface hardness and good wear resistance, with its hardness being more than twice that of steel and over four times that of hot-dip galvanizing. During transportation, assembly, and disassembly, the sherardizing layer is less likely to be worn, scratched, or peeled off. It also has good painting performance and can be directly painted, coated with plastic, or wrapped with polymer materials without any pretreatment. The coating adhesion is 3 - 4 levels higher than that of hot-dip galvanizing, reaching the first level of the national standard. The sherardizing process doesn't change the mechanical properties of the steel substrate. Since the temperature for forming the sherardizing layer is lower than the phase transformation temperature of steel, it doesn't damage the mechanical properties of the steel substrate, has no hydrogen embrittlement, and can be used to process calcined parts, assemblies, irregular parts, high-strength steel parts, cast iron and cast steel parts (without cracking), spring steel (without losing elasticity), and section steel (without deformation).

9. Dacromet

Dacromet has no hydrogen embrittlement problem and excellent torque-preload consistency. If the environmental issue of hexavalent chromium is not considered, it's actually most suitable for high-strength fasteners with high anti-corrosion requirements.