The most commonly used coating for commercial fasteners is electroplating zinc. It is cost-effective and has a good appearance, with various colors such as black and army green. However, its corrosion resistance is mediocre compared to other zinc coatings, and its anti-corrosion ability is the worst. Normally, the neutral salt spray test of electroplating zinc can last for 72 hours, but with special sealing agents, it can reach more than 200 hours. However, the cost is 5 to 8 times higher than regular galvanizing. Electroplating zinc is susceptible to hydrogen embrittlement during the production and processing process. However, bolts of grade 10.9 or higher are not treated with galvanizing. Even if they are dehydrogenated after plating, the passivation film will be damaged when the temperature is higher than 60℃, so dehydrogenation must be carried out after electroplating and before passivation. It is not easy to operate and has a high processing cost. However, in everyday life, production plants do not actively dehydrogenate unless specifically required by certain customers. The consistency of torque-preloading force of electroplated zinc fasteners is poor and unstable, and it is not normally used for important connections. In order to improve the consistency of torque-preloading force, a lubricating substance can be coated after plating to change and strengthen the consistency.
2. Oxidation (Blackening)
Blackening and oiling is a popular coating for industrial fasteners because it is low cost and looks good before the oil runs out. However, since blackening has no rust prevention ability, it will rust quickly without oil. Even in the presence of oil, its neutral salt spray test can only last for 3-5 hours. The consistency of torque-preloading force of fasteners with blackened surfaces is also very poor. If you want to improve it, you can apply grease on the internal threads during assembly before tightening.
3. Electroplating Cadmium
Although the cadmium coating has good corrosion resistance, its cost of waste liquid treatment during processing is high-approximately 15-20 times that of electroplating zinc. However, it is not widely used in general industries and can only be used in certain fixed conditions. For example, it can be used for fasteners in oil drilling platforms and sea-aircraft.
4. Electroplating Chromium
Chromium coatings are stable in the atmosphere, not prone to discoloration or losing their luster, and have high hardness and good wear resistance. Chromium coatings are generally used in the fastener industry for decorative purposes. They are not often used in industrial industries with high corrosion resistance requirements because the cost of good chromium electroplated fasteners is high, comparable to that of stainless steel. Only when the strength of the stainless steel is insufficient is it generally replaced with chromium-plated fasteners. In order to resist corrosion, copper and nickel plating should be applied before chromium plating. Chromium coatings can withstand extremely high temperatures up to 1200 degrees Fahrenheit (650℃). However, they also suffer from hydrogen embrittlement issues as electroplated zinc.
5. Zinc Diffusion
Zinc diffusion is a solid-state metallurgical thermal diffusion coating with good uniformity. It can achieve uniform layering on threads and blind holes. The coating thickness is generally 10-110 microns, with an error of 10% under control. Its bonding strength with the substrate and corrosion resistance are the best among zinc coatings (electroplating zinc, hot-dip galvanizing, and DACROMET). The processing process is environmentally friendly and non-polluting.
DACROMET does not have hydrogen embrittlement issues and has good torque-preloading force consistency. If the environmental protection issue of DACROMET's hexavalent chromium is not considered, it is actually the most suitable for high-strength fasteners with high anti-corrosion requirements.