A complete overview of the anodizing process for CNC machined aluminum parts
Anodizing offers a combination of durability, corrosion resistance, and aesthetic appeal. The anodizing surface treatment process strengthens CNC aluminum parts while imparting a long-lasting, fade-resistant color. This guide explains the principles of anodizing, common types, and all available colors.
What is anodizing?
Anodizing is a process that produces a strong, corrosion-resistant, and visually appealing surface. The workflow involves using an electrochemical method in multiple tanks to form an anodic layer on the surface. This process bonds a stable layer of alumina to the underlying substrate, enhancing the strength and durability of aluminum CNC parts . This additional layer is porous, making it suitable for applying paints, lubricants, dyes, and adhesives. This provides anodized aluminum parts with a unique, durable, and aesthetically pleasing surface finish. Similarly, it improves the corrosion resistance and wear resistance of CNC aluminum parts under harsh environmental conditions. Furthermore, anodizing does not affect the recyclability of aluminum and is even considered more environmentally friendly than electroplating or spraying.
Anodizing process for CNC aluminum parts
Anodizing is a strictly controlled process. In my factory, we process anodizing with the same precision as CNC machining. Below is the workflow for anodizing parts :
| Cleaning and degreasing | Parts must be completely clean. Any residual cutting fluid, grease, or even fingerprints left from the process can cause serious defects in the final surface. The anodizing process cannot penetrate these contaminants, leaving exposed or discolored spots. This is why our post-processing cleaning procedures are so rigorous. |
| Etching (or cleaning) | The parts are immersed in a solution (usually caustic soda) to remove a very thin layer of aluminum. This removes the natural oxide layer and creates a uniform, typically matte, surface. This step can conceal very fine tool marks, but it cannot repair deep scratches or rough surfaces caused by poor machining. |
| Anodizing process | This is the main event. The component becomes the anode of the circuit in the acid bath. This forms a structured porous oxide layer, which is the key to the entire process. |
| Coloring (optional) | If color is required, the parts are immersed in a dye bath. The porous oxide layer absorbs the dye like a sponge. The depth and intensity of the color depend on the quality and consistency of this porous layer, which extends all the way to the originally machined surface. |
| Sealing process | This is the final and most crucial step. The parts are treated with hot water or other chemicals to seal the pores of the oxide layer. This locks in the color and provides maximum corrosion resistance. Unsealed parts are easily stained and less durable. |
Why do CNC machined parts need to undergo anodizing surface treatment?
The primary purpose of anodizing CNC machining parts is to enhance their surface properties and durability, surpassing the performance of the base material itself. After anodizing, a controllable oxide layer forms on the metal surface, significantly improving corrosion resistance, wear resistance, and surface hardness. Furthermore, it provides a uniform and aesthetically pleasing appearance to aluminum parts and allows for coloring according to product design or brand requirements. Additionally, the anodized metal surface is sealed, preventing environmental corrosion and thus improving the stability of the part’s use, reducing maintenance needs, and extending the service life of precision-machined parts in various harsh environments.
Does the CNC machining process of aluminum parts affect the final anodizing effect?
Your anodized aluminum parts look patchy and of inconsistent quality. This is not only frustrating but also costly. The problem usually stems from the CNC machining process, not the anodizing process itself.
The CNC machining process has a significant impact on anodizing. Factors including surface roughness, tool marks, coolant contaminants, and even the choice of different aluminum alloys can affect the final surface finish. A smooth, clean, and uniformly machined surface is crucial for obtaining a uniform and durable anodized layer.
The final appearance of anodized CNC aluminum parts largely depends on the initial machining. Like painting a wall, a smooth, clean surface yields the best results. Every cut and every toolpath is crucial. In my years at Xtproto, I’ve seen countless customers dissatisfied with our products. With anodizing, we trace the problem back to the initial machining parameters. If you want to control the final result of your parts, you need to control the entire CNC machining process.
Different types of aluminum anodizing surface treatment processes
Aluminum anodizing is divided into three types, and their specifications differ due to variations in materials, electrodes, and voltage. According to the MIL-A-8625 standard, the main types of anodizing are as follows:
Type I anodizing (chromic acid anodizing)
Type I anodizing uses chromic acid to form a thin coating on aluminum parts. The coating thickness can be as thin as 0.0001 inches. Although Type I is the thinnest of the three main types, it enhances the corrosion resistance of the parts. This type appears noticeably grayer and has poorer color absorption when dyed. It may not be the best choice for decorative finishes.
Type II anodizing (sulfuric acid anodizing)
The sulfuric acid process is the primary method for anodizing, producing films with thicknesses ranging from 0.0002 to 0.001 inches. The resulting coating exhibits 67% penetration into the substrate and increases the size of the initial part by 33%. This makes it ideal for applications requiring hardness and abrasion resistance.
Type III anodizing (hard anodizing)
While hard coating anodizing typically uses a sulfuric acid-based electrolyte, it differs significantly from sulfuric acid anodizing. It produces a noticeably thicker, denser anodized layer, exceeding 0.001 inches. Compared to Type II anodizing, it provides a harder, thicker, and more durable coating.
Differences between Type I, Type II and Type III aluminum anodizing processes
The main difference between the two aluminum anodizing processes lies in the different materials, electrodes, and energy used. The table below summarizes the differences between them:
| characteristic | Type I (chromic acid) | Type II (Sulfuric Acid) | Type III (Hard Anodizing) |
| application | decorative | decorative | Corrosion resistant |
| Corrosion Resistance | OK | OK | Excellent |
| Coating thickness | 0.08-0.25 micrometers | 2.54-25 micrometers | 12.7-80 micrometers |
| abrasion resistance | poor | easing | Excellent |
| Porosity | low | Medium to High | high |
| Environmentally friendly | No | Yes | Yes |
| appearance | primer | Transparent or stained | Hard Clear or Hard Black |
How to test whether the anodizing process of CNC machined aluminum parts is successful
The success of the anodizing process for CNC machined aluminum parts can be tested using the following four methods .
| Check the matte finish | The absence of a matte finish may indicate that the anodizing process of the part was incomplete or failed. However, a matte finish alone cannot be used as the final test criterion. |
| Scratch resistance test | Scratch the parts with a sharp object . Fully anodized CNC parts are more scratch-resistant than uncoated aluminum parts. However, their hardness depends on the aluminum alloy and the type of anodizing. |
| Eddy current testing | An eddy current thickness gauge can be used to display the completion status of the anodizing process and the coating thickness. Components that are not successfully anodized will generate unstable electromagnetic fields. |
| Conductivity test | A basic voltmeter can be used to check whether the anodizing process was successful. If there is a significant deviation in the conductivity of the component, it indicates a problem with the process. |
Recommended equipment for measuring the thickness of anodized coatings
Eddy current meters are ideal for non-destructive measurements of non-conductive coatings on non-ferrous metal substrates. The PosiTector 6000 NAS probe is designed for high-resolution measurements of anodized CNC aluminum parts. While the PosiTector 6000 NAS probe has a measurement range up to 625 μm (25 mils), it offers the highest accuracy, with a maximum resolution below 100 μm (4 mils), which is within the expected range for most anodizing applications.
Furthermore, the PosiTector 6000 N microprobe series is an ideal choice for measuring coating thickness in small or hard-to-reach areas. Thickness readings can be taken from deep holes, small mountings, or inner diameters using 0°, 45°, or 90° probe tips. When using a clamp or quick-release adapter, the N-type microprobes are identical in specifications to the NAS probes.
aluminum alloy materials for anodizing machined parts
The following are common types , compositions, properties, and applications of anodized aluminum alloy materials for CNC machined parts .
| 1000 Series (Pure Aluminum) | 1000 series alloys contain 99% aluminum and impurities such as iron and silicon , giving them high electrical conductivity, excellent corrosion resistance, and flexibility. Anodizing pure aluminum alloys can create transparent or translucent coatings on parts and improve their hardness and corrosion resistance. |
| 2000 Series (Aluminum-Copper Alloy) | The 2000 series alloys consist of aluminum and copper (2% to 7%) , and may contain small amounts of manganese and magnesium. This series of alloys offers high strength and moderate corrosion resistance. Anodizing can improve the corrosion resistance and wear resistance of the parts . |
| 3000 Series (Aluminum-Manganese Alloy) | The 3000 series contains aluminum and manganese (1% to 1.5%) , giving it good formability, moderate strength and excellent corrosion resistance. |
| 5000 Series (Aluminum-Magnesium Alloy) | The 5000 series alloys are made of aluminum and magnesium (2% to 6%), giving them a high strength-to-weight ratio and excellent corrosion resistance. Anodizing further enhances the corrosion resistance and durability of machined parts . |
| 6000 Series (Aluminum-Magnesium-Silicon Alloy) | The 6000 series alloys are composed of aluminum, magnesium (0.6% to 1.2%) and silicon (0.4% to 1.2%) , giving them good extrusion properties, a high strength-to-weight ratio and excellent corrosion resistance. |
| 7000 Series (Aluminum-Zinc Alloy) | Alloy 7000 is a blend of aluminum and zinc (5% to 8%) , giving it a high strength-to-weight ratio, fatigue resistance, and corrosion resistance. Anodizing improves the corrosion resistance of CNC parts and enhances their surface finish . |
Coloring methods for anodized CNC aluminum parts
Anodizing of CNC aluminum parts involves a variety of techniques that determine the final appearance, durability, and color stability of the parts. Each method works differently at the microscopic level, resulting in effects ranging from subtle metallic hues to vibrant and long-lasting colors.
| Electrolytic coloring | Electrolytic coloring is a process that uses alternating current or direct current electrolyte to deposit metal ions into the pores of the anode layer. |
| Dye coloring | The dyeing process involves immersing the anodized parts in an organic dye bath. |
| Overall coloring (integral coloring) | The overall coloring process uses a specific aluminum alloy during the anodizing process, allowing the color to form naturally within the oxide layer. |
| Interference coloring | Interference coloring technology generates color by changing the thickness of the anodic film and utilizing optical interference. |
Optional colors for anodized CNC aluminum parts
From clear and black to bold custom shades , every anodized color balances aesthetics and durability. Understanding the performance of each color will help you choose the right finish based on your brand image, function, and environmental exposure.
| Transparent or silver | Transparent anodizing retains the natural metallic luster of aluminum while giving it excellent corrosion resistance and wear resistance. |
| black | CNC aluminum parts exhibit the highest UV stability after black anodizing. Because the dye is sealed deep within the oxide layer, it maintains excellent fade resistance even after more than 1,000 hours of UV testing. |
| blue | Blue anodizing offers a range of shades from soft pastels to deep cobalt blue. Its fade resistance depends on the type of dye and the quality of the seal, so a well-sealed blue coating can retain its color for 5-10 years or more under controlled conditions. |
| red | Red anodized parts have a striking appearance, but because red dyes are more sensitive to ultraviolet light, precise process control is required. |
| green | Compared to vibrant red anodizing, green anodizing typically performs better outdoors with its inorganic dye system. |
| Gold or copper | Gold and bronze finishes are colored using electrolytic metal salts, resulting in extremely high stability and durability compared to most organic dyes. |
| Custom colors | Custom anodized colors allow for precise branding, gradient effects, and special hues. Durability depends on the dye chemistry, oxide layer thickness, and sealing process. |
What are the applications of anodized CNC aluminum parts in various industries?
Anodized aluminum CNC parts are favored across various industries for their durability, aesthetics, and superior corrosion resistance. They also offer sustained performance and customizable color options, enhancing both functionality and design.
| Aerospace Industry | Aircraft skin, interior panels, and structural supports. |
| automotive industry | Engine hood, trim pieces, suspension components, heated housing. |
| Electronics and consumer devices | Mobile phone bezels, laptop casings, speaker grilles, smartwatch bezels. |
| Architecture and Construction | Curtain walls, window frames, facade panels, and signage. |
| Medical devices and laboratory equipment | Surgical instruments, dental equipment, medical casings. |
| Industrial and mechanical components | Gears, pistons, pneumatic components, and robotic arm parts. |
| Sports equipment and outdoor gear | Bicycle parts, rock climbing tools, camping equipment, and accessories. |
in conclusion
Anodizing is a highly regarded surface treatment process because it ensures the durability and aesthetic appeal of finished aluminum CNC parts . When you require durable and aesthetically pleasing custom aluminum parts , you can request anodizing from a custom CNC parts manufacturer . However, for high-quality anodizing results, consider choosing a professional anodizing and CNC machining service provider.
Choose Xtproto and you’ll get quality-focused anodizing processes, fast delivery cycles, convenient quoting, and highly competitive pricing. Are you considering getting high-quality anodized CNC aluminum parts at a low price ? Contact us!
FAQ
anodized CNC aluminum parts rust?
No, anodized CNC aluminum parts will not rust because of the aluminum oxide coating that is unaffected by oxidation and corrosive agents.
anodized CNC aluminum parts wear out?
No. Under normal use, the coating will not wear down because it is part of the aluminum base material and occurs at the microscopic level. However, wear can cause the coating to deteriorate, and exposure to sunlight can cause fading. The degree of wear depends on the type of anodizing. For example, Type III anodized parts retain their color and abrasion resistance better than Type II and Type I anodized parts.
What happens if aluminum parts are not anodized?
Ordinary aluminum or unanodized aluminum parts are susceptible to corrosion, wear, and abrasion. Therefore, certain products, especially those exposed to seawater and other extreme conditions, require anodizing.
anodized aluminum CNC parts better?
Yes, anodized aluminum CNC parts have better mechanical properties than ordinary alumina CNC parts . They are more durable, harder, more corrosion-resistant, and offer a wider range of color options, resulting in better aesthetics.
aluminum parts fade after anodizing ?
Yes, continuous wear and tear, exposure to sunlight (especially organic dyes), and harsh environmental conditions can all cause anodized parts to fade.
What can damage anodized aluminum?
Using highly abrasive materials (materials with high hardness) or corrosive chemicals may damage anodized parts.
How long does the color from anodizing last?
High-quality anodized coatings typically last 10-20 years outdoors and 20-40 years indoors. In my experience, a well-sealed Type II coating can withstand 2,000-3,000 hours of salt spray testing without significant fading. Color durability depends on UV exposure, dye quality, sealing method, and anodized layer thickness.
How to anodize aluminum CNC parts to different colors?
The colored anodizing process comprises four controlled steps: oxide layer formation, dye absorption, optional electrolytic coloring, and sealing. I use a sulfuric acid bath at 12–18°C to adjust the pore size, followed by coating with organic dyes or metal salts. After coloring, sealing is performed at 96–100°C to fix the pigment, ensuring excellent UV resistance and abrasion resistance.