What Is Surface Treatment?
Surface treatment is any post-machining process that alters the surface of the part to achieve a specific property. The properties that surface treatment can improve:
Corrosion resistance: protect the part from oxidation and corrosion.
Wear resistance: protect the part from abrasion and wear.
Appearance: provide a specific color, gloss, or texture.
Electrical conductivity: provide a conductive surface (e.g., for electrical contact).
Solderability: provide a solderable surface (e.g., for electronic components).
Biocompatibility: provide a biocompatible surface (e.g., for medical implants).
Lubricity: provide a low-friction surface (e.g., for sliding parts).
The choice depends on the substrate material, the operating environment, and the functional requirements.
What Is Anodizing?
Anodizing is an electrochemical process that converts the surface of aluminum (and other non-ferrous metals) into a protective oxide layer. The oxide layer is hard, corrosion-resistant, and can be dyed in various colors.
Types of anodizing:
Type I (chromic acid anodizing): thin oxide layer; used for aerospace and corrosion resistance.
Type II (sulfuric acid anodizing): medium-thickness oxide layer; the most common type; can be dyed in various colors.
Type III (hardcoat anodizing): thick, hard oxide layer; used for wear resistance.
Anodizing is performed by immersing the part in an acid bath and applying an electrical current. The oxide layer grows from the substrate, so it cannot peel or chip. The thickness is controlled by the current and the time.
The Yuqing processing of mechanical parts capability includes anodizing through qualified sub-suppliers.
What Is Powder Coating?
Powder coating is a dry finishing process in which a polyester or epoxy powder is electrostatically applied to the part and then cured in an oven. The cured coating is hard, durable, and available in a wide range of colors and textures.
Advantages of powder coating:
Durability: the coating is more durable than wet paint.
Environment: no solvents or VOCs; environmentally friendly.
Color range: wide range of colors, textures, and gloss levels.
Coverage: excellent coverage of edges and corners.
Cost: cost-effective for medium to high volumes.
Limitations of powder coating:
Heat: requires curing at 160–200°C; not suitable for temperature-sensitive parts.
Thickness: typical thickness is 50–100 µm; thicker coatings are possible but may crack.
Repair: difficult to repair if damaged; the part may need to be re-coated.
What Is Plating?
Plating is an electrochemical process that deposits a thin layer of metal onto the part. Common plating types:
Zinc plating: provides corrosion resistance for steel parts; typically 5–25 µm thick.
Nickel plating: provides corrosion resistance and wear resistance; typically 5–25 µm thick.
Chrome plating (decorative): provides a bright, decorative finish; typically 0.2–0.5 µm thick.
Chrome plating (hard): provides wear resistance; typically 25–250 µm thick.
Tin plating: provides solderability and corrosion resistance; typically 5–25 µm thick.
Gold plating: provides conductivity and corrosion resistance; typically 0.05–0.5 µm thick.
Silver plating: provides conductivity; typically 1–25 µm thick.
Electroless nickel plating: provides uniform thickness on complex geometries; typically 5–50 µm thick.
Plating is used for steel, aluminum, copper, and other metals. The Yuqing processing of various structural parts capability includes plating through qualified sub-suppliers.
What Is Passivation?
Passivation is a chemical treatment that removes free iron from the surface of stainless steel and enhances the formation of the passive oxide layer. Passivation is required for stainless steel parts used in food, medical, pharmaceutical, and aerospace applications.
Passivation standards:
ASTM A967: standard for chemical passivation of stainless steel.
AMS 2700: aerospace material specification for passivation.
ASTM A380: standard for cleaning and passivation of stainless steel parts.
Passivation does not change the dimensions of the part. The process is performed in an acid bath (nitric or citric acid), followed by rinsing and drying.
What Is Electropolishing?
Electropolishing is an electrochemical process that removes a thin layer of metal from the surface of the part, leaving a smooth, bright, corrosion-resistant finish. Electropolishing is commonly used for stainless steel parts in food, medical, and pharmaceutical applications.
Advantages of electropolishing:
Smooth surface: reduces surface roughness, improving cleanability.
Corrosion resistance: enhances the passive oxide layer.
Appearance: produces a bright, mirror-like finish.
Deburring: removes small burrs and sharp edges.
Limitations:
Cost: more expensive than passivation.
Dimensional change: removes 0.0005–0.002 inches of material; not suitable for tight-tolerance parts.
Size: limited by the size of the electropolishing bath.
What Is Black Oxide?
Black oxide is a conversion coating that produces a black finish on steel, stainless steel, and copper. The coating provides mild corrosion resistance and appearance.
Black oxide standards:
MIL-DTL-13924: military specification for black oxide coating.
ISO 11408: international standard for black oxide coating.
Black oxide is used for tools, machine parts, and decorative applications. The coating is porous and is often supplemented with oil or wax for improved corrosion resistance.
What Is Painting?
Painting (wet painting) is a liquid finishing process in which paint is applied by spray, dip, or brush. Painting is used for parts that require a specific color, gloss, or texture that cannot be achieved with other surface treatments.
Types of painting:
Spray painting: most common; applies a uniform coating.
Dip painting: immerses the part in paint; used for complex geometries.
Powder painting: see powder coating section above.
Electrostatic painting: uses an electrical charge to attract paint to the part; reduces overspray.
Painting is used for steel, aluminum, plastic, and other materials. The paint can be acrylic, polyurethane, epoxy, or enamel, depending on the application.
How Are Surface Treatments Selected?
Surface treatment selection is based on:
|
Factor |
Consideration |
|
Substrate material |
Aluminum, steel, stainless, etc. |
|
Operating environment |
Corrosion, wear, temperature |
|
Appearance requirement |
Color, gloss, texture |
|
Functional requirement |
Conductivity, solderability, biocompatibility |
|
Cost |
Per-part cost, tooling cost |
|
Lead time |
Process lead time |
|
Volume |
Production volume, batch size |
|
Regulatory |
FDA, aerospace, automotive standards |
For example, a medical device part made of stainless steel may require passivation + electropolishing. An automotive bracket made of steel may require zinc plating. An aluminum enclosure may require anodizing or powder coating.
What Are the Common Defects in Surface Treatment?
Common defects in surface treatment:
|
Defect |
Cause |
Prevention |
|
Pinholes in coating |
Contamination, poor surface prep |
Clean parts before coating |
|
Uneven coating thickness |
Poor racking, low current |
Optimize racking and process parameters |
|
Color variation |
Inconsistent process parameters |
Control process parameters |
|
Poor adhesion |
Contamination, wrong surface prep |
Proper surface prep |
|
Hydrogen embrittlement |
Acid pickling, plating |
Bake parts after plating |
|
Burning (anodizing) |
Too high current |
Reduce current, improve cooling |
|
Orange peel (paint) |
Wrong viscosity, application |
Adjust viscosity, application technique
|
Each defect is preventable with discipline. The first batch should be inspected thoroughly to identify issues before scaling up.
How Are Surface Treatments Quoted?
Surface treatment is quoted based on:
Process cost: the per-part cost of the surface treatment process.
Setup cost: the cost of setting up the process for the specific part.
Tooling cost: the cost of racking, masking, or special fixtures.
Inspection cost: the cost of inspecting the surface treatment.
Lead time: the process lead time.
For a typical custom part, surface treatment cost is 5–100 per part, depending on the process and the part size. Anodizing is typically 5–30; powder coating is 10–50; plating is 5–50; passivation is 5–20.
What Is the Future of Surface Treatment?
Trends in surface treatment:
Environmentally friendly processes: water-based paints, chromium-free pretreatments.
Improved durability: new coatings with higher wear and corrosion resistance.
Functional coatings: anti-microbial, anti-fingerprint, self-healing coatings.
Digital color matching: precise color matching using spectrophotometers.
In-line inspection: automated inspection of surface treatment quality.
For a custom machine shop, the trend toward environmentally friendly processes and functional coatings is creating new opportunities for value-added surface treatment.
Conclusion
Surface treatment is the post-machining process that improves the corrosion resistance, wear resistance, appearance, or other surface properties of a
custom CNC part. The common surface treatments include anodizing, powder coating, plating, passivation, and painting. Each treatment has different cost, lead time, and applicability. The choice depends on the substrate material, the operating environment, the appearance requirement, and the budget. Yuqing's processing of mechanical parts, processing of various structural parts, and export machined parts capabilities include surface treatment through qualified sub-suppliers.
Frequently Asked Questions
What is the most common surface treatment for aluminum?
Anodizing (Type II) is the most common surface treatment for aluminum. It provides corrosion resistance, wear resistance, and a wide range of color options.
What is the most common surface treatment for steel?
Zinc plating is the most common surface treatment for steel. It provides corrosion resistance at a low cost. Powder coating is also common for higher-end applications.
What is the most common surface treatment for stainless steel?
Passivation is the most common surface treatment for stainless steel. It enhances the natural passive oxide layer. Electropolishing is used for higher-end applications.
What is the difference between Type II and Type III anodizing?
Type II (sulfuric acid) is medium thickness (5–25 µm) and can be dyed. Type III (hardcoat) is thicker (25–150 µm) and is used for wear resistance.
What is the cost of anodizing?
Anodizing typically costs 5–30 per part, depending on the part size, the quantity, and the color. Small parts in standard colors are at the low end; large parts in custom colors are at the high end.
What is the cost of powder coating?
Powder coating typically costs 10–50 per part, depending on the part size, the quantity, and the color/texture. Small parts in standard colors are at the low end; large parts in custom textures are at the high end.
What is the cost of zinc plating?
Zinc plating typically costs 5–30 per part, depending on the part size and the quantity. Zinc plating is one of the most cost-effective surface treatments for steel.
What is the lead time for anodizing?
Anodizing lead time is typically 3–7 days, depending on the part size, the quantity, and the anodizing type.
What is the lead time for powder coating?
Powder coating lead time is typically 3–10 days, depending on the part size, the quantity, and the color.
What is the lead time for plating?
Plating lead time is typically 5–10 days, depending on the part size, the quantity, and the plating type.
What is the lead time for passivation?
Passivation lead time is typically 3–5 days, depending on the part size and the quantity.
Can multiple surface treatments be applied to one part?
Yes, parts can receive multiple surface treatments. For example, a steel part may be zinc-plated and then powder-coated. The treatments must be compatible.
