The purposes of surface treatment are aesthetic enhancement and corrosion protection. Given that the main function of a fastener is to secure components, the selected surface finish significantly impacts its fastening performance. When choosing a surface treatment process, critical factors such as the fastener’s torque and clamping force consistency must be considered. A high-caliber design engineer must account not only for structural design and manufacturing processes but also for assembly feasibility, environmental compliance.
Electroplating
Electroplating involves depositing a metallic layer onto the fastener’s surface to alter its properties, mainly to inhibit oxidation and corrosion, often using a corrosion-resistant metal. Beyond enhancing corrosion resistance, plating can increase hardness to prevent wear, improve electrical conductivity and heat resistance, and provide a smoother, more attractive finish. The electroplating process uses electrolysis to apply a metal film onto the surface of metal or other material components.
Zinc plating is the most widely used category. Depending on the passivating agent, the color can range from silver-white, iridescent or black. Zinc is a relatively inexpensive and easily plated metal, though its standard corrosion resistance is moderate, typically achieving less than 72 hours in a Neutral Salt Spray (NSS) test for standard electro-galvanized zinc. However, special sealants can extend NSS performance beyond 200 hours, though this comes at a higher cost, often 5 to 8 times that of standard zinc plating. Zinc-nickel alloy fasteners are generally used where high corrosion resistance and good electrical conductivity are both required, but their cost is higher, around 8 to 10 times that of zinc plating.
Tin plating shares very similar characteristics with zinc-nickel, offering excellent electrical conductivity and good adhesion for organic topcoats. Like zinc-nickel, tin plating is more costly. It is primarily applied in areas requiring grounding and subsequent soldering. Chrome plating is highly stable in the atmosphere, resisting tarnishing and loss of luster, and offers high hardness and good wear resistance. On fasteners, chrome plating is used for decorative purposes. It is rarely employed in industrial sectors with high anti-corrosion requirements because chrome-plated fasteners are as expensive as stainless steel; they are only used as a substitute when stainless steel does not meet the necessary strength specifications. To ensure sufficient corrosion protection, an underlayer of copper and nickel should be applied before chromium plating. A chrome layer can withstand high temperatures up to 1200℉. However, chrome plating is susceptible to the risk of hydrogen embrittlement. Electroplated fasteners are commonly found in vehicle electronics, electrical systems, and interior/exterior trims, where they provide mechanical fastening along with conductivity and decorative appearance.
Black Oxide and Phosphating
Black Oxide is essentially a conversion coating for steel, protected by a light oil application. Its primary function is aesthetic improvement without altering the dimensional tolerances. The only corrosion protection comes from the protective oil; therefore, it is only recommended for bolts requiring strict dimensional control and used in non-exposed, closed environments.
Phosphating is a chemical process that forms a phosphate chemical conversion film on the metal surface. This resulting layer is known as the phosphate coating. The main objectives of phosphating are to protect the base metal and inhibit corrosion to a certain degree, to serve as a pre-paint base to enhance paint adhesion and anti-corrosion ability, to improve aesthetics, and to provide lubrication during some metal cold working processes. A post-phosphating oil treatment is essential, and the resulting corrosion resistance is highly dependent on the properties of the applied oil. For instance, phosphating followed by a general anti-rust oil may only achieve 10 to 20 hours in the NSS test. Using high-grade anti-rust oil, however, can extend performance to 72 to 96 hours, albeit at 2 to 3 times the cost of the general process. The two common types of phosphating for fasteners are zinc phosphate and manganese phosphate. Zinc phosphate offers better lubricating properties than manganese phosphate, while manganese phosphate provides superior corrosion resistance and wear resistance compared to zinc plating. It can be used in temperatures ranging from 225 to 400 ℉. Due to the non-flaking nature of the phosphate film, phosphating is suitable for securing critical components such as connecting rod bolts, cylinder head bolts, main bearing bolts, and flywheel bolts—key engine connection points. In fastener manufacturing, the phosphate layer often acts as an intermediate layer, providing some wear resistance and protection against galvanic corrosion, but primarily prepares the surface for subsequent treatments like oiling or waxing, where the phosphate layer facilitates adhesion and offers limited corrosion protection. Fasteners with black oxide or phosphate finishes are commonly used in engines, transmissions, and other sealed environments where cleanliness requirements are high.
Zinc-Aluminum Coating
Zinc-aluminum coating, often referred to as “chrome-free Dacromet” or a similar proprietary name, is composed of non-electrolytically applied, micro-layered zinc and aluminum flakes, along with organic binders. The formulation typically contains not less than 70% zinc flakes and 10% aluminum flakes, cured by baking at a specific temperature and duration to form a protective, anti-corrosion coating.
Commercially available zinc-aluminum coatings are primarily offered in black and silver-gray. This technology represents a new generation of surface treatment for fasteners, considered a green plating alternative to traditional electroplating processes. Its key advantages include:
- Superior corrosion resistance,
- Freedom from hydrogen embrittlement,
- High heat resistance,
- Excellent adhesion and recoating capability and being environmentally non-polluting and hazard-free.
Fasteners with zinc-aluminum coating are specified for vehicle chassis and exterior exposed connections. Due to the severe operating conditions in these areas, extremely high corrosion protection is necessary. The absence of hydrogen embrittlement and its excellent torque-to-clamping-force consistency make it the most suitable choice for high-strength fasteners requiring superior corrosion resistance.
