Nine Major Surface Treatment Options for Fasteners
1. Electro-galvanizing
Electro-galvanizing is the most common coating for commercial fasteners. It is cost-effective and visually appealing, available in black or military green. However, its corrosion resistance is relatively low among zinc-based coatings, typically lasting up to 72 hours in neutral salt spray tests. Special sealants can extend this to over 200 hours, but at a cost 5–8 times higher than standard electro-galvanizing.
Electro-galvanizing may induce hydrogen embrittlement, making it unsuitable for bolts above grade 10.9. Post-plating hydrogen removal via baking is possible but challenging due to the risk of damaging the passivation layer above 60°C (140°F). Consequently, hydrogen removal is rarely performed unless mandated.
Electro-galvanized fasteners exhibit inconsistent torque-preload performance and are generally avoided in critical applications. Lubricant coatings post-plating can improve consistency.
2. Phosphating
Phosphating is more economical than electro-galvanizing but offers lower corrosion resistance. Post-phosphating oil application significantly impacts performance—standard rust-proof oil achieves 10–20 hours in salt spray tests, while premium oils extend this to 72–96 hours (at 2–3 times the cost).
Common types include zinc and manganese phosphate. Zinc phosphate provides better lubrication, while manganese phosphate offers superior corrosion and wear resistance, withstanding temperatures up to 107–204°C (225–400°F).
Phosphating is widely used in industrial fasteners (e.g., engine bolts, structural joints) due to excellent torque-preload consistency. It also avoids hydrogen embrittlement, making it ideal for high-strength bolts (grade 10.9+).
3. Oxidation (Blackening)
Blackening + oil coating is a low-cost, visually acceptable option for industrial fasteners. However, it provides minimal corrosion resistance (3–5 hours salt spray test even with oil). Torque-preload consistency is poor but can be improved by applying grease during assembly.
4. Cadmium Plating
Cadmium plating excels in marine environments but involves high waste treatment costs (15–20 times more expensive than zinc plating). It is restricted to specialized applications like offshore drilling platforms and naval aircraft.
5. Chromium Plating
Chromium plating offers high hardness, wear resistance, and aesthetic appeal but is rarely used in corrosive environments due to cost (comparable to stainless steel). Pre-coating with copper/nickel is required, and it withstands temperatures up to 650°C (1200°F). Hydrogen embrittlement remains a concern.
6. Silver/Nickel Plating
Silver plating acts as a solid lubricant and corrosion inhibitor, ideal for high-temperature applications (up to 871°C/1600°F). Nickel plating combines corrosion resistance with electrical conductivity, commonly used in battery terminals.
7. Hot-dip Galvanizing
This process creates a thick zinc coating (15–100 μm) with excellent corrosion resistance, widely used in engineering. Challenges include thread fit issues post-coating. Solutions include post-coating thread re-tapping (reduces corrosion resistance) or oversizing threads (weakens strength). SPL (Spiralock) internal threads offer a viable alternative by accommodating thicker coatings without compromising performance.
Not suitable for fasteners above grade 10.9 due to high processing temperatures.
8. Zinc Diffusion Coating
A solid-state metallurgical process producing uniform coatings (10–110 μm ±10%) with superior adhesion and corrosion resistance. Environmentally friendly, it outperforms electro-galvanizing and hot-dip coatings.
9. Dacromet Coating
Dacromet eliminates hydrogen embrittlement and ensures excellent torque-preload consistency. Despite environmental concerns over hexavalent chromium, it is highly effective for corrosion protection.
