I. Electroplating

Through electrolytic action in the fastener surface deposition of metal or alloy layer, the formation of a protective layer.

1. Galvanized (Zn)

Type:

Electro zinc plating: low cost, widely used, common colors are blue-white zinc, colored zinc, black zinc, etc.

Hot dip galvanizing (hot dip galvanizing): thick coating (60-120μm), stronger corrosion resistance, suitable for outdoor and high humidity environment.

Features:

Better corrosion resistance, blue-white zinc resists salt spray for about 24-72 hours, hot-dip galvanizing can reach more than 500 hours.

Hot-dip galvanized surface is rougher, which may affect assembly precision.

Applications: architectural hardware, automotive parts, general machinery, etc.

2. Nickel plating (Ni)

Characteristics: Bright surface, better corrosion resistance than galvanized, high temperature resistance (about 200℃), both wear-resistant and decorative.

Applications: electronic equipment, bathroom hardware, optical instruments and other scenes that require high appearance and precision.

3. Chrome plating (Cr)

Characteristics: high hardness (HV 800-1000), strong wear resistance, bright and beautiful surface, but the chromium plating layer is brittle, need to be plated with copper or nickel primer first.

Application: tools, molds, decorative parts (e.g. automobile wheel bolts), etc.

4. Cadmium plating (Cd)

Characteristics: excellent corrosion resistance, especially suitable for maritime climate, but cadmium is toxic, strict environmental requirements, the application is gradually limited.

Applications: aerospace (once used), military and other special areas (now mostly replaced by cadmium-free process).

Second, hot dip plating class

Formation of coatings through high-temperature molten metal dip plating.

1. hot dip plating aluminum (Al)

Characteristics: high temperature resistance (up to 600 ℃), oxidation and corrosion resistance, the surface of the formation of aluminum oxide protective layer.

Application: High temperature environment (such as boilers, exhaust pipes), chemical equipment, etc. 2.

2. Hot-dip tin plating (Sn)

Characteristics: good weldability, smooth surface, commonly used in electrical connection parts.

Application: electronic components, wire connectors and other fasteners that require tin soldering.

Third, the surface conversion film treatment

Generate a non-metallic film layer on the metal surface through chemical reaction.

1. Blackening (bluing)

Principle: Immerse steel parts in a strong oxidizing solution to form a black Fe₃O₄ film layer.

Characteristics: low cost, simple process, but weak corrosion resistance (need to be coated with oil to enhance protection), thin film layer (0.5-1μm).

Application: standard parts, tools, internal parts with low appearance requirements.

2. Phosphating

Principle: Formation of iron phosphate / zinc / manganese film layer, porous surface, can absorb lubricants.

Characteristics:

Corrosion resistance is better than blackening, often used as a coating base layer to enhance adhesion.

Manganese phosphating has outstanding abrasion resistance and is used for high-strength bolts (e.g. automobile chassis bolts).

Applications: automotive parts, fastener pretreatment, cold process lubrication (such as thread rolling before phosphating).

3. Chromate Passivation (Chromate Conversion Coating)

Principle: After galvanizing, it is treated by chromate solution to form passivation film and enhance corrosion resistance.

Features:

Colored zinc (colorful passivation) can resist salt spray for more than 200 hours, blue-white zinc for about 72 hours.

Contains hexavalent chromium which is toxic and is gradually replaced by chromium-free passivates (e.g. trivalent chromium passivation, silane treatment).

Application: Post-treatment of galvanized parts, widely used in hardware, automotive and other fields.

Fourth, coating technology

Through spraying, plating and other ways to form non-metallic or metal composite coating on the surface.

1. Dacromet (Dacromet)

Principle: zinc, aluminum scales and chromate solution mixed coating, high temperature curing to form an amorphous film layer.

Characteristics:

Salt spray resistance up to 1000 hours or more, no hydrogen embrittlement, suitable for high-strength bolts.

Environmental protection is better than traditional galvanizing (but still contains a small amount of chromium), in recent years there are chrome-free Dacromet (such as Geomet).

Applications: automotive fasteners (e.g. chassis bolts), marine engineering, highly corrosive environments. 2.

2. Nickel-phosphorus plating (Ni-P)

Characteristics: amorphous coating, high hardness (HV 500-1000), excellent corrosion and wear resistance, poor weldability.

Applications: petrochemical, aerospace, molds, etc.

3. Spray organic coating

Type: electrostatic spraying epoxy resin, polyurethane, etc.

Characteristics: rich colors, good corrosion resistance, controllable film thickness (50-200μm), but general temperature resistance (<150℃).

Applications: architectural hardware, outdoor furniture, decorative fasteners.

V. Other treatments

1. Carburizing / Nitriding (Thermalization)