8 Common Surface Treatments for CNC Machined Brass Parts
Introduction
Brass, known for its corrosion resistance and aesthetic luster, is a widely used material in CNC machining, especially in industries such as plumbing, architecture, electronics, and consumer products. However, surface treatment is essential to enhance its performance in harsh environments and meet specific aesthetic or functional requirements.
Surface finishes for CNC machined brass parts help improve oxidation resistance, enhance surface durability, reduce friction, and deliver polished or matte appearances. This blog details eight of brass components' most commonly applied surface treatments.
Surface Treatment Technologies for Brass Components
Scientific Principles & Industrial Standards
Definition: Surface treatments modify the exterior of brass parts through mechanical, chemical, or electrochemical processes to improve properties like corrosion resistance, surface hardness, adhesion, electrical conductivity, or appearance.
Governing Standards:
ASTM B456: Specification for electrodeposited coatings (nickel, chrome, silver, gold) on copper alloys.
ASTM B912: Passivation methods for copper and brass.
ISO 4525: Guidelines for decorative and functional metal plating.
Process Function and Cases
Performance Dimension | Technical Parameters | Application Cases |
|---|---|---|
Corrosion Resistance | - Teflon coating resists pH 1–14 and up to 260°C - Chrome plating: HV 800–1000, 0.5–2.5 µm thick - Passivation increases surface energy >72 mN/m | Faucet components, outdoor lighting, marine hardware |
Aesthetic Enhancement | - Polishing to Ra ≤ 0.2 µm - Brushing with #400–#600 grit belts - PVD coatings in gold, bronze, or black | Jewelry, luxury watch casings, architectural fittings |
Wear Resistance | - PVD hardness HV 2000–3000 - Powder coating thickness: 60–120 µm - Teflon coating friction: 0.05–0.20 | Valve bushings, bearing sleeves, moving mechanical parts |
Functional Coating | - Electroplated nickel or silver: 5–25 µm - Powder coating with ASTM B117 >1000hr salt spray resistance - Chrome plating: Reflective and hydrophobic | Electrical connectors, heat exchangers, appliance trims |
Surface Treatment Process Classification
Technical Specification Matrix
Treatment Type | Key Parameters & Metrics | Advantages | Limitations |
|---|---|---|---|
- Thickness: 5–25 µm - Metals: Nickel, Silver, Gold, Chrome | - Improves conductivity and corrosion resistance - Decorative and functional | - Requires tight current and chemical control | |
- Surface finish: Ra ≤ 0.2 µm - Buffing compound or electropolish | - High-gloss finish - Smooths imperfections | - Adds no protective layer | |
- Grit: #320–#600 - Uniform matte texture | - Reduces glare - Aesthetic appeal for exposed parts | - Requires sealing to prevent tarnish | |
- Thickness: 1–5 µm - Hardness: HV 2000–3000 | - Decorative with excellent hardness - Wear and corrosion-resistant | - Higher cost and vacuum deposition required | |
- Acid bath at 40–60°C - Time: 10–30 min (HNO₃ or citric) | - Improves corrosion resistance - Retains natural brass tone | - No visible layer added | |
- Thickness: 60–120 µm - Curing: 180–200°C for 15–25 min | - UV-resistant, color versatile - Corrosion-proof barrier | - Reduces conductivity | |
- Friction: 0.05–0.20 - Temperature range: –200°C to +260°C | - Non-stick, chemical-resistant - Ideal for dynamic components | - Thick layers may affect tolerance | |
- Thickness: 0.5–2.5 µm - Mirror gloss finish (Ra < 0.05 µm) | - Highly aesthetic - Wear and tarnish resistant | - Contains hazardous hexavalent chromium |
Selection Criteria & Optimization Guidelines
Electroplating
Selection Criteria: Ideal for decorative and functional brass components like connectors, trim, and precision fasteners requiring enhanced corrosion resistance or conductivity.
Optimization Guidelines:
Use current density of 2–4 A/dm² and bath temperatures of 50–60°C for nickel or silver.
Apply a pre-flash copper strike for complex geometries.
Monitor coating thickness with XRF (±0.1 µm accuracy).
Polishing
Selection Criteria: Used for ornamental brass components like jewelry, plaques, or interior fixtures that demand a high-luster finish.
Optimization Guidelines:
Buff using cotton wheels with tripoli or rouge compounds.
Final polish with diamond paste for Ra ≤ 0.1 µm on luxury-grade parts.
Clean and seal with lacquer to delay tarnish.
Brushing
Selection Criteria: Perfect for products with a contemporary matte look such as handles, signage, or appliance knobs.
Optimization Guidelines:
Apply linear brushing with #400–#600 grit belts.
Ensure unidirectional motion to maintain uniform texture.
Finish with clear passivating or polymer-based sealant.
PVD Coating
Selection Criteria: Chosen for luxury hardware and high-contact parts needing both aesthetics and hardness, e.g., door handles and watch cases.
Optimization Guidelines:
Pre-clean to <10° contact angle (ASTM D7334).
Chamber vacuum below 1×10⁻² Pa during deposition.
Preheat part to 200°C, rotate for uniform coating.
Passivation
Selection Criteria: Essential for untreated brass components in humidity-prone or reactive chemical environments like laboratory or HVAC fittings.
Optimization Guidelines:
Use 20% citric acid at 60°C for 20 minutes.
Rinse in deionized water and dry in filtered air.
Verify surface energy (>72 mN/m) using dyne pens.
Powder Coating
Selection Criteria: Recommended for architectural brass panels and industrial housings that require scratch resistance and outdoor durability.
Optimization Guidelines:
Degrease and blast brass with #80 Al₂O₃ grit.
Apply 80–100 kV electrostatic charge for coating adhesion.
Cure at 190°C for 20 minutes per ASTM D2454.
Teflon Coating
Selection Criteria: Applied to moving components, threaded inserts, and sealing rings where non-stick and chemical resistance are essential.
Optimization Guidelines:
The surface roughness Ra ~1.0 µm before coating.
Spray layers of 25–30 µm; bake at 370°C for PTFE.
Test coefficient of friction (<0.2) using ASTM D1894.
Chrome Plating
Selection Criteria: Best for polished architectural parts and wear-prone decorative items such as switch plates and luxury appliance handles.
Optimization Guidelines:
Maintain bath temperature at 50–55°C, current density at 25–35 A/dm².
Polish brass to mirror finish before plating.
Finish with a neutralizer rinse to remove acid residues.
Material-Coating Compatibility Chart
Brass Grade | Recommended Surface Treatment | Performance Gain | Industrial Validation Data |
|---|---|---|---|
Chrome Plating | Corrosion protection, mirror finish | Faucet bodies used in high-humidity bathrooms | |
Powder Coating | UV and abrasion resistance | Decorative panels tested 1000+ hrs salt spray (ASTM B117) | |
PVD Coating | Surface hardness, decorative finish | Luxury cabinet handles with >HV2000 durability | |
Electroplating | Electrical conductivity | Terminal pins in electric motor assemblies | |
Teflon Coating | Chemical and friction resistance | Sealing rings and connectors in fluid systems |
Comprehensive Process Control and Quality Assurance
Preparation and Quality Standards
Pre-Treatment: To optimize surface adherence, brass parts are degreased, etched, or mechanically finished.
Process Control: Temperature, chemical composition, and electrical parameters are closely monitored per industry protocols.
Post-Treatment: Coated parts undergo inspection for thickness, adhesion, gloss level, hardness, and corrosion resistance.
Expert Insights and Common Inquiries
Which surface treatment is best for brass parts used outdoors?
Can decorative treatments like polishing be combined with protective coatings like powder or lacquer?
What finishes help maintain electrical conductivity in brass terminals?
How does PVD coating compare to electroplating in terms of wear resistance?
What’s the most cost-effective treatment for high-volume brass parts needing aesthetic finishes?
