Whether it’s parts for automotive customization, components for precision machinery, or metal structures for outdoor equipment, the way you handle cnc machining parts surface treatment can make all the difference. You may have noticed: some CNC machined parts stay looking new for years, while others start to rust, fade, or wear out in no time. One of the key reasons behind this difference is how the surface is finished and protected.
For many purchasing engineers and mechanical enthusiasts, surface treatment is not just about aesthetics—it directly determines corrosion resistance, wear resistance, service life, and maintenance costs. In this article, we’ll walk through the ins and outs of surface finishing for CNC machined parts, from common processes to selection tips, so you can get the full picture in one go.
Why Surface Treatment Matters for CNC Machined Parts?
In the world of CNC machining, it’s common for people to focus on tolerances, precision, and material selection. However, the real-world performance and lifespan of a part often depend heavily on the quality of its surface treatment. A good surface finish isn’t just “a protective layer on top”—it’s a functional upgrade created through mechanical, chemical, or electrochemical methods to help parts perform reliably in demanding environments.
1. Fighting Corrosion and Extending Lifespan
One of metal’s greatest enemies is corrosion. Even high-strength steel, when left unprotected, can quickly lose its strength and shine in humid, salty, chemical, or high-temperature conditions. For example, an untreated carbon steel rod end joint in a coastal or chemical plant environment may show visible rust within weeks. By contrast, the same part treated with zinc plating, nickel plating, or a Dacromet coating can survive hundreds of hours in salt spray without visible corrosion.
2. Enhancing Wear Resistance and Mechanical Properties
Many CNC machined parts endure repeated friction or impact during service—think automotive suspension components, mechanical linkage joints, or mold inserts. If surface hardness is inadequate, wear will quickly degrade precision and may lead to part failure. Treatments like PVD coatings (TiN, TiAlN, DLC) or hard anodizing can significantly increase surface hardness while reducing friction, helping parts run cooler and last longer.
3. Improving Appearance and Brand Value
For components that remain visible in the final product, surface finishing is as much about branding as it is about protection. A brushed aluminum panel or a mirror-polished stainless steel fitting not only lasts longer but also instantly communicates quality. Techniques such as painting, powder coating, or anodizing with color can also provide visual coding, making assembly and maintenance easier.
4. Adding Functionality and Special Properties
Surface treatment can go beyond protection and aesthetics to add functional value. Insulating coatings can prevent electrical shorts; UV-resistant coatings can slow outdoor aging; low-friction finishes can improve motion efficiency. In industries such as medical devices, aerospace, and marine engineering, these functional coatings can be mission-critical.
5. Balancing Cost and Value
Some buyers are tempted to skip surface finishing to save on upfront costs. While that might look good on a short-term budget, the long-term expenses—maintenance, replacements, and downtime—can be much higher. A surface treatment that accounts for just 5–10% of a part’s cost might extend its lifespan two to three times, dramatically reducing total lifecycle costs.
When it comes to CNC machined parts, surface treatment is like a building’s waterproofing layer or a car’s paint job—it’s not just about looks. It’s the shield that determines performance, longevity, and user experience. Skipping it means giving up a critical safeguard for your product’s long-term reliability.
Main Categories of CNC Parts Surface Treatment
1. Surface Modification Techniques
Surface modification is all about improving a part’s physical properties or appearance by working directly on its surface—without altering the overall dimensions or core structure. These methods are often the “first step” in a surface finishing workflow and can be performed as standalone processes or as preparation for coatings and chemical treatments.
Common surface modification techniques include:
- Surface hardening – Rapidly heating and cooling the surface layer to create a hardened shell, boosting wear resistance and fatigue strength. This is ideal for steel parts exposed to repeated impact or friction, such as shafts, gears, or connecting rods.
- Bead blasting / shot blasting – Using high-speed abrasive media to remove oxidation, burrs, or old coatings while creating a uniform matte texture. This is often done before anodizing aluminum parts to improve coating adhesion.
- Knurling – Pressing a patterned texture into the surface to increase grip, commonly seen on tool handles, knobs, or fasteners where slip resistance is important.
- Brushed finish – Producing fine, consistent lines with abrasive belts or wheels to hide small scratches and add decorative appeal. Frequently used on aluminum panels and stainless steel housings.
- Polishing – Smoothing the surface with abrasives and polishing compounds until it reaches a high-gloss, mirror-like finish. Perfect for parts where premium aesthetics are a selling point.
Why it matters:
- Advantages – These methods are fast, cost-effective, and highly versatile. They can dramatically improve appearance, control surface roughness, and enhance adhesion for later treatments.
- Limitations – On their own, they offer little to no corrosion protection. For long-term performance, they’re best combined with protective coatings such as plating, anodizing, or powder coating.
For example, if you have a batch of CNC-machined aluminum enclosures that need to be both visually appealing and corrosion-resistant, you might start with bead blasting to achieve a fine, even texture, followed by anodizing. This way, you get a clean, professional look plus long-term durability.
2. Coating and Plating Techniques
Among all CNC machining parts surface treatment options, coating and plating are some of the most widely used. By adding a protective layer to the surface, they not only enhance appearance but also improve corrosion resistance, wear resistance, and sometimes even add new functional properties.
2.1 Electroplating
Electroplating uses an electrolytic process to deposit a thin layer of metal onto the workpiece, creating a dense and uniform coating. The typical process involves:
- Pre-treatment – Degreasing, acid pickling, and cleaning to ensure a contaminant-free surface.
- Electroplating stage – Connecting the workpiece as the cathode, with the plating metal as the anode, and using an electrolyte solution to transfer metal ions.
- Post-treatment – Includes sealing (forming a dense protective film), passivation (reducing surface activity to slow corrosion), and hydrogen removal (preventing hydrogen embrittlement that can cause part failure).
Common plating types and properties:
- Chromium plating – Increases surface hardness, wear resistance, and provides a decorative finish. Decorative chrome is 0.5–1 μm thick, while hard chrome ranges from 5–80 μm.
- Nickel plating – Offers wear resistance, corrosion protection, and a bright finish. Typically 10–30 μm thick.
- Zinc plating – Sacrificial anode protection against rust. Thickness is usually 5–15 μm. Post-passivation can produce finishes such as white, blue-white, yellow (iridescent), or black zinc, with corrosion resistance in the order: black > yellow > blue-white > white.
Anodic vs. cathodic coatings:
- Anodic coatings (e.g., zinc plating) corrode first to protect the base metal.
- Cathodic coatings (e.g., nickel, chrome) must fully seal the base material; any defect will cause the base to corrode first.
Corrosion resistance (neutral salt spray test): Zinc > Nickel > Chrome.
2.2 Electrophoretic Coating (E-coating)
E-coating is similar to electroplating but deposits a paint-based coating instead of metal. The result is a uniform layer even on complex shapes.
- Typical thickness: 15–25 μm
- Advantages: Strong adhesion, good corrosion protection, uniform coverage.
- Applications: Automotive parts, bicycle frames, appliance housings.
- Compared to electroplating: Electroplating excels in hardness and wear resistance; E-coating offers superior corrosion resistance and better coverage on intricate geometries.
2.3 Powder Coating vs. Painting
Painting – Wide color variety and high gloss, but weaker adhesion and moderate corrosion resistance. Curing temperature ~80°C.
Powder coating – Uses electrostatic attraction to apply powder, which is then baked at ~180°C to form a strong, uniform layer. Thickness typically 60–70 μm. Strong adhesion, better corrosion resistance than paint, and more environmentally friendly.
Comparison: Powder coating is cost-effective and eco-friendly; painting delivers richer visual appeal.
2.4 Vacuum Plating (PVD Coating)
PVD (Physical Vapor Deposition) is performed under vacuum, where metals or non-metals are vaporized and condensed onto the part’s surface. Thickness ranges from 0.25–10 μm.
Decorative coatings (0.3–1 μm) – Improve appearance and color.
Functional coatings (2–5 μm) – Increase hardness, wear resistance, and corrosion protection.
Common PVD coatings:
- TiN – Gold-colored, high hardness.
- TiAlN – Suitable for high-friction machining.
- TiCN – Good impact and wear resistance.
- DLC – Low friction, high hardness.
- CrN – Balanced wear and corrosion protection.
PVD vs. electroplating:
- PVD is ideal for high-end parts, delivering premium finish quality, gloss, and durability.
- Electroplating is better for large-volume, cost-sensitive production.
3. Conversion Coatings
Conversion coating is a chemical process that transforms the metal’s surface into a thin, protective compound layer. Unlike plating or painting, which add material on top, conversion coatings work by chemically altering the surface itself. The result is a firmly bonded layer that resists peeling and provides an excellent base for further finishes.
3.1 Black Oxide / Bluing
How it works – A controlled oxidation reaction forms a dense oxide layer on the surface, isolating the metal from air and moisture to slow rust formation.
Characteristics – Produces a black or blue-black finish, low cost, and suitable for steel components where appearance requirements are modest.
Limitations – Limited corrosion resistance; usually requires post-treatment with oil for optimal protection.
Applications – Common for mechanical components, tools, and firearm parts.
Industry note – On steel parts, black oxide is often referred to as “bluing.” While there are process differences, their protective functions are similar.
3.2 Phosphating
How it works – Immersing steel, aluminum, or zinc parts into a phosphate-containing solution to form a crystalline phosphate coating that is insoluble in water.
Characteristics – Better corrosion resistance than black oxide; porous surface structure makes it an excellent base layer for painting or powder coating.
Limitations – Limited aesthetic appeal (typically gray or dark gray); slightly higher processing cost than black oxide.
Applications – Automotive parts, hardware components, military equipment.
Black oxide vs. phosphating
- Corrosion resistance – Phosphating > Black oxide
- Appearance – Black oxide is darker and shinier; phosphating has a matte finish
- Cost – Phosphating is slightly more expensive
- Common requirement – Both require thorough cleaning and rust removal before processing to ensure effective protection.
3.3 Anodizing for Aluminum Parts
How it works – Placing aluminum in an electrolytic bath and applying current to form a dense, uniform, and hard oxide film on the surface.
Benefits – Improved corrosion resistance, wear resistance, and decorative appeal; can also increase surface friction for functional parts.
Regular anodizing vs. hard anodizing:
- Film thickness – Regular: 5–12 μm; Hard: 25–70 μm
- Hardness – Regular: HV 180–310; Hard: HV 310–550
- Appearance – Regular anodizing is translucent or colored; hard anodizing is opaque with a dull finish
UV resistance – Even hard anodizing struggles against prolonged UV exposure. Solutions include longer sealing times, UV-blocking topcoats, and using high lightfastness dyes.
Material differences – 6061 aluminum anodizes with stable results, while 7075 aluminum, due to its high zinc content, is prone to white edges, black spots, or uneven coloring—especially with bright colors like red or orange.
4. Other Protective Coatings
Beyond common plating, painting, and conversion coatings, there are specialized protective finishes designed for specific performance needs and environments. One of the best-known examples is Dacromet coating.
4.1 Dacromet Coating
How it works – Dacromet is a zinc–chromium-based coating made from zinc powder, aluminum powder, chromic acid, and deionized water. It is applied through dipping and then baked at high temperatures to create an even, protective layer.
Process steps – Solvent degreasing → mechanical polishing → dipping → baking → second dipping → baking → drying.
Typical thickness – 2–15 μm.
Advantages:
- Outstanding corrosion resistance, capable of withstanding hundreds of hours in salt spray testing
- No hydrogen embrittlement risk, making it suitable for high-strength fasteners
- Environmentally friendly and compliant with modern environmental regulations
Limitations:
- Poor wear resistance; not suitable for continuous friction or abrasion conditions
- Limited color options—most often silver-gray
Typical applications:
Bolts, nuts, springs, and brake system components in automotive, motorcycle, and machinery applications—especially load-bearing parts that require high corrosion resistance.
Industry tip: Dacromet’s strengths lie in its corrosion resistance, eco-friendliness, and zero hydrogen embrittlement, making it an excellent replacement for traditional zinc or chrome plating on high-strength fasteners. However, for parts subject to constant friction or impact, it’s recommended to apply an additional wear-resistant coating on top of Dacromet to extend service life.
5. Aluminum Anodizing and Special Considerations
Anodizing is one of the most common—and most versatile—surface treatments for CNC-machined aluminum parts. By running an electrical current through the aluminum in an electrolytic bath, a dense and uniform oxide layer forms on the surface. This not only boosts corrosion resistance but also enhances wear resistance and allows for a wide range of decorative finishes.
5.1 Process Steps
Cleaning – Use alkaline or acidic solutions to remove oils and surface contaminants.
Pre-treatment – Deburring, bead blasting, grinding, or polishing to ensure an even surface.
Anodizing – Immersing the part in an electrolyte bath and applying current; film thickness depends on electrolyte type, current density, and processing time.
Coloring (optional) – Immersion in a dye bath to achieve the desired color.
Sealing – Treating in hot water or a sealing agent to close the pores of the oxide layer, increasing corrosion resistance and hardness.
Rinsing and drying – Removing residual electrolyte and sealing agent.
5.2 Regular Anodizing vs. Hard Anodizing
Film thickness – Regular: 5–12 μm; Hard: 25–70 μm
Hardness – Regular: HV 180–310; Hard: HV 310–550
Surface appearance – Regular anodizing is translucent or can be dyed; hard anodizing is opaque with a dull finish
Roughness – Hard anodizing slightly increases surface roughness
5.3 UV Resistance Challenges and Solutions
While anodized coatings protect against corrosion, prolonged UV exposure can degrade the oxide layer, causing color fading and performance loss.
Possible solutions:
- Apply a UV-blocking topcoat (similar to a clear protective spray)
- Extend sealing time to improve coating density
- Use dyes with high lightfastness (rare in domestic supply)
5.4 Material-Specific Behavior
6061 aluminum – Stable anodizing results, uniform color, suitable for a wide range of dye colors
7075 aluminum – High zinc content makes it prone to white edges, black spots, or uneven color, especially with bright dyes like red or orange
Industry tip: If aesthetics are critical for 7075 parts, run a small test batch first and consider using darker shades to minimize visible color variation.
How to Choose the Right Surface Treatment for CNC Machining Parts
Selecting the right surface treatment for CNC machined parts is all about finding the balance between material compatibility, operating environment, functional requirements, cost, and lead time. A coating or finish that works perfectly for one application might be a poor choice for another. Here’s a step-by-step guide to help you make an informed decision.
1. Start with Material Compatibility
Different materials respond very differently to surface treatments, so this should always be your first filter.
Aluminum and aluminum alloys (6061 / 7075)
- 6061 – Stable anodizing results, even color. Works well with regular anodizing (5–12 μm) or hard anodizing (25–70 μm).
- 7075 – High zinc content increases the risk of white edges, black spots, or uneven color after anodizing—especially with bright colors like red or orange. Test a sample first, opt for darker shades, and consider extending sealing time to improve density.
Carbon steel / alloy steel
- Zinc plating (anodic coating, 5–15 μm) sacrifices itself to protect the base metal; great for high-corrosion environments.
- Nickel plating (10–30 μm) and chrome plating (0.5–80 μm) are cathodic coatings that must fully seal the base metal to prevent corrosion; ideal when hardness, wear resistance, or decorative appeal are priorities.
- Dacromet (2–15 μm) offers outstanding corrosion resistance without hydrogen embrittlement, making it perfect for high-strength fasteners.
Stainless steel
Naturally corrosion-resistant; can be brushed or polished for better aesthetics, or enhanced with PVD coatings (TiN, DLC, CrN) for added wear resistance and decorative finishes.
2. Match the Operating Environment
Indoor, stable conditions – Nickel or chrome plating provide a good combination of appearance and rust prevention; painting or powder coating can be chosen based on budget and style.
High humidity, salt spray, or chemical exposure – Zinc plating outperforms nickel for corrosion resistance; Dacromet and zinc–nickel alloys extend part lifespan.
Outdoor + UV exposure – Anodizing should be paired with a UV-blocking topcoat or extended sealing time; thick powder coating (60–70 μm) offers reliable protection.
High-strength fasteners – Prioritize Dacromet or zinc–nickel alloy plating with passivation.
3. Define Functional Priorities
Corrosion resistance – Zinc plating (black > yellow > blue-white > white), Dacromet, E-coating (15–25 μm), hard anodizing.
Wear resistance – PVD coatings (TiN, TiAlN, DLC, CrN), hard anodizing, hard chrome plating.
Appearance – Anodizing with color, high-gloss painting, smooth powder coating, brushed finish, mirror polishing.
Special properties – Insulation, UV resistance, low friction—achieved with functional coatings such as DLC.
4. Balance Cost and Lead Time
Cost trend (per PDF): Chrome plating > Nickel plating > Zinc plating.
Batch processing – Electroplating, powder coating, and E-coating are great for mass production; PVD and hard chrome are better for high-value, smaller batches.
Lead time impact – Combination processes (e.g., bead blasting + anodizing) require extra processing time.
Post-treatment essentials – For steel parts, hydrogen relief (200–250°C for ≥3 hours) after plating prevents hydrogen embrittlement; sealing or passivation greatly improves corrosion resistance.
5. Consider Combination Processes
Sometimes, the best results come from layering processes:
- Bead blasting + anodizing – Improves adhesion and ensures a uniform appearance (ideal for aluminum).
- Phosphating + painting/powder coating – Porous phosphate layer boosts coating adhesion and corrosion resistance.
- Zinc plating + passivation – Adds color coding and extends salt spray life.
- Dacromet + wear-resistant coating – Balances corrosion resistance and durability under friction.
6. Quick Selection Reference Matrix
| Material / Environment | Corrosion | Wear | Appearance | Recommended Process Combination |
|---|---|---|---|---|
| 6061 Aluminum (outdoor) | ★★★ | ★★ | ★★★ | Bead blasting + hard anodizing + sealing + UV topcoat |
| 7075 Aluminum (decorative) | ★★ | ★★ | ★★★ | Dark-colored anodizing + sealing |
| Carbon steel (marine/chemical) | ★★★ | ★★ | ★★ | Zinc plating (black/yellow) + passivation |
| High-strength bolts | ★★★ | ★★ | ★ | Dacromet / Zinc–nickel alloy + passivation |
| Stainless steel (premium look) | ★★ | ★★★ | ★★★ | Brushed/polished + PVD coating |
Cost and Lead Time Considerations
When choosing a surface treatment, performance and appearance are important—but cost and lead time can be just as critical. In large-scale production or tight delivery schedules, the right (or wrong) decision on surface finishing can directly determine whether a project stays on budget and on time.
1. Cost Trends and Breakdown
In common electroplating processes, the PDF clearly states the unit cost trend:
Chrome plating > Nickel plating > Zinc plating
The differences come from:
- Material costs – Chrome plating requires costly chromium compounds; PVD coatings need vacuum equipment and high-purity materials.
- Process complexity – Chrome plating, PVD, and hard anodizing involve more steps and longer processing times, raising costs.
- Coating thickness & performance requirements – Thicker layers or higher performance specs require more time and materials.
Example:
- For indoor decorative parts, zinc plating or regular anodizing may be enough, saving 30–50% compared to premium finishes.
- For high-end molds or wear-resistant components, PVD coatings have a higher upfront cost but can greatly extend lifespan and reduce replacement expenses.
2. Lead Time Factors
Single vs. combination processes – A single surface treatment may take only 1–3 days (depending on production load), while a combination process (e.g., bead blasting + anodizing + sealing) will require extra time.
Post-treatment steps:
- Steel parts after plating require hydrogen relief (200–250°C for ≥3 hours) to prevent hydrogen embrittlement.
- Anodized parts often need sealing (hot water/sealant) to improve corrosion resistance—this adds half a day to a full day.
Equipment capacity & queue time – High-end PVD and vacuum coating equipment are limited and may require booking during peak season; plating and powder coating generally have more stable capacity.
3. Environmental Regulations and Compliance Costs
For exports to markets with strict environmental standards (e.g., RoHS, REACH), certain substances like hexavalent chromium and some nickel salts are restricted:
- This may require switching to trivalent chromium zinc plating or chrome-free passivation, which increases raw material costs.
- E-coating and powder coating offer stronger environmental compliance but may require longer pre-treatment steps.
4. Balancing Cost and Lead Time
- Set priorities – Is this a cost-sensitive project or a high-performance premium market product?
- Batch production advantage – Processing parts in bulk helps spread setup and equipment prep costs.
- Include buffers – Combination processes or export compliance testing require extra lead time.
- Think lifecycle costs – A low upfront price may lead to higher long-term maintenance costs.
Common Mistakes and How to Avoid Them
When it comes to selecting and applying surface treatments for CNC machined parts, many problems can be avoided with a bit of foresight. A wrong choice or poor execution can hurt performance, cause rework, delay deliveries, and even result in customer complaints. Here are some common mistakes and how to steer clear of them.
1. Focusing Only on Appearance, Ignoring Performance
Some buyers prioritize color, gloss, and visual appeal but overlook the part’s operating environment and functional needs.
Example: Decorative chrome-plated fasteners installed in a coastal area looked great initially but failed within months due to salt spray corrosion.
Tip: Strike a balance between aesthetics and performance. In high-humidity or salt-spray environments, prioritize corrosion resistance—consider zinc plating (black/yellow), Dacromet, or hard anodizing.
2. Mismatching Process and Material
Different materials react very differently to surface treatments, and ignoring compatibility can lead to color inconsistency, poor adhesion, or insufficient protection.
Example: Bright-colored anodizing on 7075 aluminum resulted in white edges and black spots, leading to batch rejection.
Tip:
- 6061 aluminum works well with a variety of anodizing colors and offers stable results.
- 7075 aluminum requires testing and is best suited to darker colors.
- For carbon steel, anodic coatings like zinc plating provide more reliable corrosion protection.
3. Skipping or Rushing Post-Treatment Steps
Even with a proper finish applied, inadequate post-treatment can drastically reduce corrosion resistance.
Example: Steel parts plated without hydrogen relief treatment failed due to hydrogen embrittlement in service.
Tip:
- For steel parts, always perform hydrogen relief at 200–250°C for ≥3 hours after plating.
- For anodized parts, extend sealing time to increase coating density.
- Verify coating thickness and adhesion before shipment.
4. Underestimating Environmental Impact
Failing to consider the part’s real-world environment is a frequent cause of premature finish failure.
Example: Painted parts used outdoors faded and peeled in under a year.
Tip:
- Outdoor use: Choose thick protective layers (powder coating, E-coating, hard anodizing) plus a UV-blocking topcoat.
- High-humidity chemical environments: Dacromet or zinc–nickel alloy plating works best.
- High-impact mechanical environments: Add a wear-resistant coating (PVD, hard chrome).
5. Ignoring Lifecycle Cost
Cutting upfront costs by choosing a cheaper process can backfire when maintenance and replacements are frequent.
Tip:
For critical parts, calculate total lifecycle cost, including replacements, downtime, and maintenance.
When budget allows, invest in a process with stronger protection to reduce long-term expenses.
Final Thoughts
Whether it’s an aluminum housing, a stainless steel decorative component, or a carbon steel structural part, choosing the right surface treatment is never just an optional extra. It directly impacts a part’s corrosion resistance, wear resistance, appearance quality, and even its total lifecycle cost.
In this guide, we’ve mapped out the entire surface finishing landscape:
- Surface modification techniques – Fast, effective ways to improve appearance and coating adhesion.
- Coating and plating techniques – From zinc plating to PVD coatings, balancing protection with decorative appeal.
- Conversion coatings – Chemically transforming the base material into a stable protective layer.
- Special protective coatings – Like Dacromet, offering reliable barriers for high-strength fasteners and extreme environments.
- Aluminum anodizing details – Ensuring lightweight parts perform consistently across different conditions.
As a purchasing engineer or a mechanical enthusiast, your goal isn’t just to make a part “look good today,” but to ensure it remains reliable and functional for months or even years to come. That means factoring in material, environment, function, cost, and lead time, and—when necessary—selecting combination processes to achieve optimal performance.
One simple takeaway:
Don’t treat surface finishing as a last-minute fix—integrate it into your design stage planning.
That way, your CNC machined parts can wear the right “armor” and perform under any condition without compromise.
Surface treatment is critical to durability. To see how it fits into the larger selection process, read our Heim Joints Buyers’ Guide.
