Machine Design

Finishes and Coatings

Fastener coatings or finishes improve appearance, increase corrosion resistance, and provide lubricity.

Electrodeposited coatings: Zinc, cadmium, tin, nickel, and chromium coatings are most common for fasteners. Zinc is preferred for industrial atmospheres. Cadmium is best in marine atmospheres but is more expensive than zinc. Corrosion products form a protective film on zinc and cadmium surfaces, but the film on cadmium will wash off in rain.

Tin plating does not effectively protect ferrous products from atmospheric corrosion, but it is good for fasteners in contact with food. Nickel coatings resist some chemicals and remain attractive for a long period.

Electroplating a threaded fastener can change the thread fit. Allowance must be made for this change.

Mechanical plating: Fasteners are coated by tumbling in a drum that contains powdered zinc or cadmium and glass shot.

In contrast to electrodepositing, which can be irregular, mechanical coatings are uniform on flat surfaces, except near the edges, where the plating is thinner.

Chemical-conversion coatings: Phosphate coating gives a corrosion-resistant deposit. Usually, fasteners are dipped in a solution of zinc or manganese phosphate and processed through tumbling barrels. Phosphate coating is less expensive than zinc or cadmium plating, especially when heavy deposits are required. Phosphate, like cadmium, adds lubricity.

Chromate treatment considerably increases corrosion resistance at nominal cost. It is particularly effective in drying moisture and preventing rusting between mated parts. However, chromate films should not be specified on fasteners continuously subjected to rainfall or running water.

Organic coatings: These coatings provide a tight film barrier against corrosive attack and, unlike platings, are not sacrificial. Protection is about twice that afforded by zinc and cadmium platings. Also, the organics enable a fastener manufacturer to color code different sizes and to avoid hydrogen embrittlement, which is common to platings.

Early organic finishes were based on alkyd and phenolic paints, modified with low-flash-point solvents to enable them to rapidly form a protective film in bulk application processes. New coatings based on an alloy of fluorocarbon and other polymers provide lubricity in addition to increased barrier protection. The inherent lubricity of these coatings enables users to accurately set joint preloads and to reduce runup torque in automatic equipment.

Hot-dip coatings: Aluminum (aluminized) and zinc (galvanized) are commonly used in this method to provide low-cost protective coatings for inexpensive, high-strength, ferrous fasteners. Tin coatings provide excellent corrosion resistance. Thicknesses are difficult to control, but some tinners claim excellent results, even on fine threads.

Microencapsulation: Buildup should be considered when specifying fastener dimensions. Fluids, stored in microcapsules, are released under controlled conditions to improve the corrosion resistance.

TAGS: Fasteners
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