1. What is Surface Treatment?
Surface treatment uses mechanical, physical, or chemical methods to alter the surface condition, chemical composition, microstructure, and stress state of a workpiece, or to apply various coatings, to impart specific properties to the workpiece surface that differ from those of the substrate, thereby achieving specific application requirements.
It can impart the following functions to the partial or entire surface of a product or component:
● Improve Corrosion Resistance
By forming a protective layer on the material surface (such as electroplating, anodizing, or painting), it isolates the material from air, moisture, and corrosive media, preventing rust and aging, and extending its service life. For example, hot-dip galvanizing or electrogalvanizing steel parts allows them to withstand long-term use in harsh environments.
● Improve Wear Resistance
Surface treatment can increase the surface hardness and wear resistance of materials, reducing frictional loss. For example, processes such as carburizing, nitriding, and laser quenching can create a hard layer on the surface of parts, making them suitable for high-wear environments (such as mechanical parts and molds).
● Improve Decorativeness
Processes such as electroplating, spraying, brushing, and polishing can achieve a variety of colors, glosses, and textures to meet aesthetic needs. For example, electronic products and furniture are enhanced visually through surface treatment.
● Improve Adhesion
Strengthening the adhesion between coatings, platings, and substrates ensures that the coatings or platings resist detachment during use. For example, pretreatment processes such as sandblasting and phosphating can increase surface roughness and improve coating adhesion.
● Achieve Special Functional Needs
Based on the application, materials can be given specific functionalities such as insulation, electrical conductivity, thermal conductivity, and radiation resistance. For example, electrophoretic coatings provide excellent insulation properties, and certain platings can be used for conductive connections in electronic products.
● Improve Fatigue Strength
By introducing residual compressive stress, increasing surface strength, or improving surface topography, fatigue crack initiation and propagation are delayed, extending the service life of components. Processes such as shot peening and rolling can effectively improve component fatigue strength.
2. Types of Metal Parts Surface Treatment
2.1 Physical / Mechanical Finishing
Improve surface finish, reduce burrs, and achieve a decorative effect.
| Table 1 – Physical / Mechanical Finish | |||||
| Process | Description | Features | Metal | Cost | Applications |
| Polishing | Polishing with a grinding wheel, sandpaper, or polishing paste | Produces a glossy, mirror-like finish, but high cost | Stainless steel, copper, and aluminum | Medium | Decorative / precision parts |
| Sand / Shot Blasting | High-speed abrasive impacts the part surface | Removes scale, enhances adhesion, and creates a matte finish | Steel, aluminum, zinc | Low | Pre-treatment before painting, matte finish |
| Vibratory Finishing | Vibratory friction between the grinding stone and the workpiece | High-volume deburring and polishing | Stainless steel,steel, copper, zinc, aluminum | Low | Small parts such as screws and hardware |
| Brushing | Fine lines formed by mechanical friction | Excellent decorative effect, concealing minor defects | Stainless steel, copper, zinc, aluminum | Low | Doors, windows, and home appliance panels |
2.2 Chemical Treatment
Remove oxide layers, passivate for rust prevention, and enhance adhesion.
| Table 2 – Chemical Treatment | ||||||
| Process | Description | Features | Metal | Cost | Applications | Coating Thickness |
| Pickling | Acid dissolves oxide scale | Cleans the surface | Stainless steel, aluminum, steel | Low | Post-weld treatment | / |
| Passivation | Chemical oxidation creates a dense protective film | Improves corrosion resistance | Stainless steel, aluminum, titanium, nickel | Low | Stainless steel, aluminum | 0.01–0.1 µm |
| Phosphating | Forms a phosphate film | Improves rust prevention and coating adhesion | Steel, aluminum, zinc | Medium | Pre-treatment | 2–10 µm |
| Chemical Oxidation | Chemical solution creates an oxide film | Various colors, corrosion protection and aesthetics | Aluminum, copper | Medium | Oxidation colors aluminum, blackens copper | 0.5–5 µm |
2.3 Electrochemical Treatment
Improves corrosion resistance, aesthetics, and functionality.
| Table 3 – Electrochemical Treatment | ||||||
| Process | Description | Features | Metal | Cost | Applications | Coating Thickness |
| Electroplating | Electrolytic deposition of a metal layer on a metal surface | Aesthetics, corrosion resistance, conductivity | Steel, copper, aluminum, zinc | Medium | Fasteners, electronic parts | 5–25 µm |
| Anodizing | Electrochemical oxidation to create an oxide film | High hardness, corrosion resistance, colorability | Aluminum, titanium | Medium | Building profiles, 3C shells | 10–25 µm |
| Electropolishing | Electrolytic dissolution of peaks | Mirror finish, deburring | Stainless steel, aluminum, copper | Medium | Medical and food equipment | / |
| Electrophoretic Coating (E-coating) | Electric field-driven paint deposition | Strong adhesion and uniform coating | Stainless steel, aluminum, nickel | Medium | Automotive parts, appliance housings | 15–35 µm |
2.4 Thermo-Chemical Treatment
Improves surface hardness, wear resistance, and corrosion resistance.
| Table 4 – Thermo-Chemical Treatment | ||||||
| Process | Description | Features | Metal | Cost | Applications | Coating Thickness |
| Carburizing | Carbon infiltration at high temperatures | Improves surface hardness | Low carbon steel , low carbon alloy steel | High | Gears and shafts | 0.2–2 mm |
| Nitriding | Nitrogen infiltration | High hardness, minimal deformation | Carbon structural steel , alloy structural steel | High | Molds, precision parts | 0.1–0.6 mm |
| Carbonitriding | Carbon and nitrogen infiltration | Balances hardness and toughness | Low carbon steel | High | Automotive parts | 0.2–1 mm |
| Black Oxide | Low-temperature oxidation produces a black film | Beautiful, rust-resistant | Steel, aluminum, copper | Low | Tools and hardware | 0.2–1 µm |
2.5 Coating and Painting
Decoration, corrosion protection, and insulation.
| Table 5 – Coating / Painting | ||||||
| Process | Description | Features | Metal | Cost | Applications | Coating Thickness |
| Powder Coating | Electrostatic adsorption followed by high-temperature curing | Strong adhesion, environmentally friendly | Steel, aluminum, zinc | Medium | Home appliances, aluminum profiles | 40–120 µm |
| Painting | Paint spraying | Diverse colors | Various metals | Low | Steel structures, hardware | 20–60 µm |
| PVD (Physical Vapor Deposition) | Vacuum-deposited metal film | High hardness, wear resistance, and aesthetics | Stainless steel, aluminum, titanium | High | Knives, jewelry | 0.1–5 µm |
| Teflon / PTFE coating | Fluoroplastic coating | Anti-stick, high temperature resistance | Steel, aluminum, copper | High | Molds, mechanical parts | 10–50 µm |
2.6 Functional Treatments
| Table 6 – Functional Treatments | ||||||
| Process | Description | Features | Metal | Cost | Applications | Coating Thickness |
| Gold/Silver/Copper Plating | Electrolytic plating of conductive metal layers | Conductive and aesthetically pleasing | Copper, steel | High | Electronic connectors and decorative parts | 1–10 µm |
| Hard anodizing | Thick oxide layer for enhanced wear resistance | Extremely high hardness | Titanium, aluminum | High | Aviation and cylinders | 25–100 µm |
| Plasma spraying | Spraying ceramic or metal layers | Excellent wear and heat resistance | Steel, aluminum | High | Aerospace, machinery | 50–500 µm |
3. How to Choose the Right Finish for Your Project/Product
3.1 Why Should You Do Surface Treatment?
| Table 7 – Purpose of Surface Treatment | ||
| Functions | Description | Common Processes |
| Corrosion Protection | Prevents rust and extends service life | Electrogalvanizing/nickel plating, anodizing, powder coating, phosphating, blackening |
| Improve hardness and wear resistance | Reduce wear and increase surface life | Carburizing, nitriding, hard anodizing, PVD coating |
| Improve appearance | Bright, beautiful, and decorative | Polishing, brushing, electroplating, anodizing, painting |
| Improving electrical / thermal conductivity | Conductivity requirements | Gold, silver, copper plating |
| Enhanced adhesion or bonding | Facilitates subsequent painting or bonding | Sandblasting, phosphating, pickling, passivation |
| Friction reduction or anti-stick | For sliding or mold release applications | Teflon coating, PVD, hard anodizing |
| Insulation or anti-fingerprint | Electronic products, decorative parts | Anodizing, spray-on insulating paint, nano-film coating |
3.2 Select the Surface Treatment Based on the Material
| Table 8 – Recommended Surface Treatment for Different Materials | ||
| Material | Recommended | Non-Recommended |
| Carbon Steel / Alloy Steel | Electrogalvanizing, Blackening, Carburizing, Phosphating, Powder Coating | Anodizing |
| Stainless Steel | Polishing, Electropolishing, Passivation, PVD, Sandblasting | Carburizing (prone to discoloration) |
| Aluminum and Aluminum Alloys | Anodizing, Hard Anodizing, Painting, Powder Coating | Electroplating (Poor Adhesion) |
| Copper and Copper Alloys | Nickel Plating, Chrome Plating, Chemical Oxidation (Blackening/Coloring) | Carburizing, Nitriding |
| Titanium and Titanium Alloys | Anodizing, PVD, Passivation | Electroplating (Difficult Adhesion) |
| Zinc/Magnesium Alloy | Chromate Passivation, Powder Coating, Painting | High-Temperature Heat Treatment (Prone to Deformation) |
3.3 Compare Cost and Performance
| Table 9 – Compare Cost and Performance | |||
| Cost | Features | Common Processes | Typical Applications |
| Low | Simple, fast, average appearance | Blackening, pickling, sandblasting, painting | Hardware, built-in components |
| Medium | Balanced Appearance and Protection | Electroplating, Anodizing, Powder Coating, Phosphating | Structural Components, Mechanical Parts |
| High | Excellent Performance, Long Life | PVD, Nitriding, Hard Anodic, Teflon | Medical, Aerospace, Molds, Tools |
3.4 Environment Considerations
Indoor Dry Environment: Blackening, Painting, Phosphating
Outdoor General Environment: Electrogalvanizing, Powder Coating, Anodizing
High Humidity or Marine Environment: Nickel Plating, PVD, Hard Anodic, Teflon
High Temperature Environment: Hard Anodic, Nitriding, Plasma Spraying
Medical/Food Environment: Electropolishing, Passivation, Anodizing (Non-Contamination)
3.5 Conclusions
● What functions need to be achieved (corrosion protection, decorative design, wear resistance, electrical conductivity, etc.)
● Understand the material properties (steel, aluminum, stainless steel, etc.)
● Evaluate the work environment (indoor/outdoor/corrosive environment)
● Evaluate the budget and production volume (one-time cost + unit cost)
● Compare the results with the sample or customer requirements (appearance, color, thickness, etc.)