Fasteners often come into contact with dissimilar metals—a recipe for corrosion. To prevent corrosion (and add some aesthetics), engineers often recommend that fasteners be plated or coated. Here is a quick look at some those platings and other finishes.
Anodizing. An acid dip provides aluminum (and aluminum only) parts with a frosty, etched appearance. It offers excellent corrosion resistance, and parts can be color dipped after being anodized.
Black oxide. A chemical treatment gives ferrous metal and stainless steel parts a black finish. The finish does not add thickness to parts, nor does it provide corrosion resistance. However, parts usually undergo an oil dip after being treated, and the oil dip does provide a fair amount of corrosion resistance.
Cadmium. Any metal can be electroplated with cadmium. The bright or dull silver-grey plating gives parts excellent corrosion resistance. The plating is often used on parts for marine and aviation applications. On the downside, the process is costly and potentially hazardous to the environment.
Zinc electroplating. Any metal can be electroplated with zinc. It is one of the most widely used plating finishes, providing an attractive, low-cost finish. Chromate is typically added to increase corrosion resistance and, if desired, add color.
Chromate. This is a secondary dipping process used on zinc or cadmium-plated fasteners. It increases corrosion resistance and can be colored. Standard clear chromate leave a bluish-white hue; other colors may also be chosen, such as yellow, olive drab or black.
Chromium. This hard, lustrous bright blue or white finish provides wear and corrosion resistance on all metals, but it is expensive.
Mechanical zinc. Parts are tumbled in zinc powder, and the peening action of the tumbling embeds powdered zinc into the surface of the part. This creates a thicker and somewhat irregular coating compared to electroplating. Parts must be sized appropriately to allow for the thickness of the coating.
Hot dip zinc. Metal parts are galvanized by being dipped in pure molten zinc. It leaves a dull grey finish that protects against corrosion. It also adds a thick, irregular coating, so the size of parts may need to be adjusted to compensate.
Iridite. If a metal part has a zinc or cadmium coating, it can also be given a finish of iridite for additional corrosion protection and/or color. This material can leave an olive drab, green, black, red, blue or bronze finish.
Nickel. This hard, stable, and relatively expensive finish leaves a silver color on all metals and good corrosion protection. However, it can be difficult to apply.
Passivating. This process is carried out by dipping stainless steel parts in nitric acid. The acid removes iron particles and brightens the finish. It also produces a passive corrosion-resistant finish.
Phosphate and oil. This low-cost coating combines zinc or manganese phosphate with a rust-inhibiting oil dip. It leaves parts with a charcoal grey or black finish.
Zinc. It is the most commonly used plating and works on all metals. It leaves a white to blue-grey finish and it cost little to apply.
David Zimmermann, President
Pivot Point Inc., Hustiford, Wisc.