Choosing the “Coating” for Bolts

I. Core Principles

Zinc plating is the most common and economical surface treatment method for commercial fasteners. Typically, a passivation process is performed after zinc plating; depending on the passivation solution used, the resulting finish can appear white, blue, yellow, black, green, or other colors.

Nickel plating is divided into two types: electroplating and chemical plating. The finish appears silvery-white with a slight yellowish tint, featuring an attractive luster and excellent decorative properties.

Chromium plating is divided into two categories: decorative chromium plating, which produces a mirror-like finish; and functional hard chrome plating, designed to enhance the hardness and wear resistance of metal parts. It is important to note that to achieve effective corrosion protection, decorative chromium plating typically requires a preliminary layer of copper and nickel, making the process relatively complex.

II. Corrosion Resistance

Zinc plating falls under “sacrificial anode” protection. Since zinc is more chemically reactive than iron, in corrosive environments, zinc oxidizes first, thereby “sacrificing” itself to protect the bolt’s base metal. Even if the coating is slightly damaged, the exposed iron remains protected. Its corrosion resistance is moderate; generally, electrogalvanized bolts pass the neutral salt spray test for approximately 72 hours.

Nickel and chrome plating provide “physical barrier” protection. Their function is to isolate the bolt substrate from external moisture and air. However, if the coating is too thin or becomes damaged, corrosion will spread inward from the damaged area. The corrosion resistance of nickel plating alone is relatively limited. While chromium plating is extremely stable in the atmosphere, if the sole purpose is rust prevention, direct chromium plating is less effective than zinc plating and is also costly; therefore, it is used less frequently in industrial corrosion protection.

III. Hardness and Wear Resistance

This is the “trump card” of chromium plating. Chromium coatings possess extremely high hardness and a low coefficient of friction, resulting in excellent wear resistance. This is why many hydraulic cylinder piston rods and shock absorber rods are plated with “hard chrome.”

In contrast, zinc coatings have lower hardness and poor wear resistance. Although nickel coatings are harder than zinc and offer some wear resistance, they are far inferior to chromium coatings.