Surface Treatment Processes for Hardware Parts
The core of hardware surface treatment is corrosion resistance, wear resistance, decoration, and improved adhesion. Mainstream methods include mechanical, chemical, electrochemical, spraying, conversion coating, and special processes, selected according to specific needs.
I. Mechanical Processes (Pre-treatment/Finishing)
1. Sandblasting/Shot Blasting
Principle: High-pressure air/centrifugal force is used to blast quartz sand, glass beads, and steel shot to clean rust and roughen the surface.
Features: Strong adhesion, fatigue resistance, adjustable roughness; environmentally friendly and efficient.
Applications: Pre-treatment before coating/plating, deburring and cleaning of castings and forgings.
2. Polishing (Mechanical/Chemical/Electrolytic)
Mechanical: Abrasion with sandpaper/wool wheel, achieving a mirror finish (Ra 0.008μm).
Chemical: Immersion in acidic solution, smoothing complex parts, burr-free.
Electrolytic: Electrochemical dissolution, commonly used for stainless steel/aluminum, high gloss, good corrosion resistance.
Applications: Bathroom fixtures, kitchenware, decorative parts. II. Electrochemical Applications (Corrosion Protection + Decoration)
1. Electroplating (Zinc/Nickel/Chromium/Copper)
Zinc plating: High cost-effectiveness, sacrificial anode corrosion protection; salt spray resistance 48–120h; commonly used in bolts/building materials.
Nickel plating: Corrosion resistance + decoration, commonly used as a base layer; dense coating, high hardness.
Chromium plating: Mirror finish, extremely wear-resistant; bathroom faucets, automotive trim (hard chrome used in molds/shafts).
Disadvantages: High cost, wastewater treatment required; hydrogen embrittlement should be noted for high-strength steel.
2. Anodizing (Aluminum/Aluminum Alloy Exclusive)
Principle: Electrolysis generates a dense Al₂O₃ film (5–25μm), high hardness, insulation, and dyeable.
Applications: Mobile phone cases, laptop casings, aluminum profiles.
3. Electrophoretic Coating
Principle: Electric field drives coating deposition, uniform film thickness, good edge coverage, environmentally friendly.
Applications: Home appliances, automotive chassis, fasteners; both black and colored are acceptable.
III. Spray Coating (Low Cost + Multiple Colors)
1. Powder Coating (Electrostatic Powder Coating)
Principle: Electrostatic adsorption of powder → high-temperature curing (180–220℃); solvent-free, wear-resistant and corrosion-resistant, rich colors.
Applications: Cabinets, appliance housings, outdoor furniture; high cost-performance ratio, environmentally friendly.
2. Liquid Spray Paint (Solvent-based/Water-based)
Features: Low cost, flexible application, suitable for small batches; prone to sagging, poor wear resistance, solvent pollution.
Applications: Automotive parts, non-standard parts, temporary protection.
IV. Conversion Coating (Thin Coating + Corrosion Protection/Priming)
1. Phosphating (Zinc-based/Manganese-based/Iron-based)
Principle: Generates a phosphate film (5–20μm), improving coating adhesion, short-term rust prevention, and reducing friction.
Zinc-based: Primer before coating (automotive/home appliances).
Manganese-based: Wear-resistant (gears/bearings).
2. Blackening/Blueing (Steel)
Principle: Alkaline oxidation generates a Fe₃O₄ film, lowest cost, aesthetically pleasing, and thin; weak corrosion resistance (salt spray 1–2h).
Applications: General machinery, standard parts, springs; requires oil seals.
Blackening treatment
3. Passivation (Chromate/Chromium-free)
Principle: Chemically generates a passivation film, improving coating corrosion resistance, sealing pores, and low cost.
Applications: Zinc-plated/aluminized parts, stainless steel; chromium-free passivation is more environmentally friendly.
V. Special Processes (High Corrosion Resistance/High-end Decoration)
1. Dacromet (Zinc-Chromium Coating)
Principle: Zinc powder + chromic acid coating → sintering at 300℃; no hydrogen embrittlement, salt spray 1000h+, high temperature resistance.
Applications: High-strength bolts, outdoor parts, automotive chassis; poor conductivity, thick coating.
1. Dacromet Treatment
2. PVD Vacuum Plating (Titanium/Zirconium/Chromium)
Principle: Ion deposition under vacuum, extremely wear-resistant, highly corrosion-resistant, metallic texture, environmentally friendly.
Applications: High-end bathroom fixtures, mobile phone frames, watch cases; high cost, expensive equipment.
PVD Vacuum Plating
3. Micro-arc Oxidation (Aluminum/Magnesium/Titanium)
Principle: High-pressure plasma generates a ceramic film, extremely hard, wear-resistant, corrosion-resistant, and insulating.
Applications: Aerospace, military, high-end aluminum parts; high energy consumption, high cost.
VI. Process Selection Overview (by Requirements)
General Corrosion Protection + Low Cost: Blackening, Phosphating, Zinc Plating
Outdoor High Corrosion Resistance: Dacromet, Powder Coating, PVD
Decorative High Gloss: Chrome Plating, Polishing, PVD
Aluminum Parts Specific: Anodizing, Micro-arc Oxidation
Pre-coating Primer: Sandblasting, Phosphating, Passivation
Environmentally Friendly: Powder Coating, Water-based Paint, Chromium-free Passivation, PVD
VII. Common Issues and Precautions
Hydrogen Embrittlement: Prone to hydrogen embrittlement in zinc plating and electroplating; use with caution on high-strength steel (≥10.9 grade); prioritize Dacromet/blackening.
Adhesion: Pre-treatment (sandblasting/degreasing/phosphating) is crucial; otherwise, it is prone to peeling.
Salt Spray Standards: General protection ≥24h, industrial ≥96h, harsh environments ≥500h.