In the CNC machining industry, anodizing is the most core and commonly used surface treatment process for aluminum alloy parts. Its core function is to optimize the performance and appearance of parts through electrochemical action, thus enhancing the capabilities of CNC precision-machined products.

From a performance perspective, anodizing generates a dense aluminum oxide film on the surface of aluminum parts. This film significantly improves the corrosion resistance of the parts, effectively resisting the erosion of external corrosive media such as acids, alkalis, and moisture. It also enhances the wear resistance of the parts, extending their service life and addressing the inherent weakness of aluminum alloys—their softness and susceptibility to wear.

From an aesthetic perspective, anodized parts have a finer surface texture and can be dyed in various colors to meet the design requirements of different scenarios, making CNC-machined precision parts both practical and beautiful.

What is anodizing?

The core principle of anodizing is based on an electrochemical reaction. Its operation logic is clear and the process is controllable, specifically divided into three key steps:

• Determining the electrode setup: The CNC-machined aluminum part is used as the anode, and corrosion-resistant materials such as lead plates are selected as the cathode. This ensures the electrode materials are compatible with the electrolyte, preventing electrode wear from affecting the reaction effect.
• Preparing the electrolyte and applying current: The anode and cathode are immersed together in a specific electrolyte such as sulfuric acid, and a stable direct current is applied to provide energy for the oxidation reaction, ensuring a continuous and uniform reaction.
• Forming a double-layer oxide film: Under the influence of an electric field, an oxidation reaction occurs on the surface of the aluminum part, generating an aluminum oxide film that is metallurgically bonded to the substrate. This oxide film consists of two layers: an inner layer that is dense and robust, effectively isolating external corrosive media and providing core protection; and an outer layer that is porous, allowing for subsequent processes such as dyeing and sealing to impart rich colors and more stable properties to the parts, thus adapting to the needs of different industries.

Mainstream Technology

Currently, the most widely used anodizing processes in the CNC industry are mainly divided into two categories, both adhering to the MIL-A-8625 industrial standard. The appropriate choice must be made based on product requirements.

The core differences between the two mainstream processes lie in process parameters, performance indicators, and application scenarios, as detailed below:

Ordinary anodizing

• Ordinary Anodizing: As the most widely used type, it boasts low process barriers, high cost-effectiveness, and suitability for most conventional product needs. The process operates at room temperature, with voltage controlled at 12–18V. The resulting oxide film thickness is between 5–25μm, and the hardness can reach HV 300–400, meeting the wear resistance requirements of general applications. Its core advantage is excellent coloring performance, allowing for the formulation of various colors. It is widely used in consumer electronics, swimming goggles and diving equipment housings, architectural hardware, and other scenarios requiring high aesthetics and mild corrosive environments, making it the preferred choice for mass production.
• Hard Anodizing: Designed specifically for high wear resistance and high corrosion resistance requirements, this process has more stringent requirements. It must be carried out at a low temperature of 0–5°C using a high-voltage treatment of 60–100V. The resulting oxide film thickness can reach 25–100μm, with a hardness exceeding HV 500 (approximately 60–70 HRC). Its wear resistance and impact resistance far surpass those of ordinary anodizing. Its shortcomings are limited colorability (mostly black/dark gray) and weak appearance decoration. It is mainly used in high-end precision products such as aerospace parts and wear-resistant mechanical structures, focusing on performance protection.

Quality Control

The final quality of anodizing is not determined solely by the oxidation process, but rather by the entire process from material selection to CNC machining and oxidation treatment. Detail control at each stage is crucial, as any oversight may lead to product defects.

Material selection

Material selection is the primary key to determining the oxidation effect. The differences in composition between different series of aluminum alloys directly affect the oxidation compatibility, and precise selection is required based on the product application.

• 6xxx Series (Aluminum-Magnesium-Silicon Alloy): Represented by 6061 and 6063, this is a “universal” grade for CNC anodizing, offering the best compatibility. After anodizing, the surface is uniformly bright with no obvious texture, and the color is stable and not prone to color difference. It is the first choice for various structural components and outdoor parts, and also the most widely used aluminum alloy grade in CNC machining.
• 5xxx Series (Aluminum-Magnesium Alloy): Represented by 5052, its core advantage is its outstanding corrosion resistance, especially against seawater corrosion. After anodizing, it presents a natural silver-white color, meeting basic appearance requirements without complex dyeing. Suitable for marine, diving equipment, and corrosion-resistant shells, effectively resisting the erosion of harsh environments.
• 7xxx Series (Aluminum-Zinc Alloy): Represented by 7075, its anodizing compatibility is moderate. After anodizing, it easily shows grain texture and a grayish-brown color, resulting in a poorer appearance. However, this series has high strength and is often used for high-load structural components. Hard anodizing processes should be prioritized to balance appearance improvement and performance enhancement.

CNC machining pre-requirements

CNC machining is a crucial pre-anodizing step, and its quality directly affects the subsequent anodizing effect. Three core controls need to be implemented to ensure compatibility with anodizing.

• Surface Quality Control: The oxide film amplifies minute defects on the aluminum surface. Therefore, burrs, flash, and deep scratches (scratches deeper than 0.05mm will be clearly visible after oxidation) must be strictly eliminated after machining. Simultaneously, the surface of the parts must be cleaned promptly after machining to avoid residual hand sweat and cutting fluid, as these impurities will directly cause defects such as pitting and white spots on the oxidized surface. For precision parts, an additional manual polishing or sandblasting process is required to ensure uniform surface roughness, laying a good foundation for the oxidation process.
• Structural Design Optimization: To ensure uniform oxidation and satisfactory appearance, structural optimization is necessary during CNC machining. First, reserve electrode clamping positions. These clamping points should avoid visible surfaces and be selected on non-mating surfaces to prevent clamping marks from affecting the product’s appearance. Second, add auxiliary anodes. For complex cavities and deep-hole parts, auxiliary anode positions must be designed to ensure uniform electric field distribution and avoid localized excessively thin oxide films or incomplete oxidation. Third, avoid enclosed cavities. Enclosed structures prevent electrolyte drainage; drainage holes must be provided, with a recommended diameter ≥1.5mm, to ensure smooth electrolyte flow.
• Dimensional Compensation Design: The growth pattern of the oxide film is “50% outward growth and 50% inward penetration.” If no dimensional allowance is reserved during CNC machining, the dimensions of the part after oxidation will exceed the tolerance range, affecting assembly accuracy. The specific compensation standards are as follows: For ordinary oxidation (10μm film thickness), the outward growth is approximately 3–5μm, and the inner hole shrinks by approximately 3–5μm; for hard oxidation (50μm film thickness), the outward growth is approximately 25μm, and the inner hole shrinks by approximately 25μm; for precision parts with a fitting accuracy ≤0.02mm, the requirement of “secondary finishing after oxidation” must be clearly stated in the process documents to ensure that the dimensions meet the standards.

Standard process flow

After CNC machining is completed and passes inspection, the process proceeds to the standard anodizing process, which consists of seven core steps. Each step has clear specifications and inspection standards that must be strictly followed.

• Degreasing and Oil Removal: The core of this process is to remove oil, cutting fluid residue, and dust from the surface of aluminum parts. Immersion in an alkaline solution at 50–70°C can be used; ultrasonic cleaning is preferred for precision parts. Commonly used reagents include sodium hydroxide solution and specialized metal degreasing agents. The inspection standard is a continuous, unbroken water film and the absence of oil droplets or stains on the surface.
• Acid Pickling and Film Removal: The core of this process is to remove the natural oxide layer from the surface of aluminum parts, avoiding affecting the quality of the artificial oxide film. Immerse in a phosphoric acid-nitric acid mixture at room temperature for 1–3 minutes, strictly controlling the time to prevent excessive acid corrosion and the formation of pinhole defects.
• Surface Finishing (Optional Process): Based on product design requirements, select the appropriate surface treatment method to create a differentiated texture. There are three common methods: First, sandblasting, using 80-200 mesh glass beads, achieves a uniform matte finish, suitable for understated and minimalist aesthetics. Second, chemical polishing enhances surface gloss, creating a mirror-like texture, suitable for products with high aesthetic requirements. Third, mechanical polishing, for materials like 7075 that are prone to texture, removes surface grain lines and improves the overall texture.
• Anodizing (Core Process): Directly determines the thickness, hardness, and other properties of the oxide film. The electrolyte must match the process type (sulfuric acid for ordinary anodizing, a sulfuric acid-oxalic acid mixture for hard anodizing); temperature, voltage, and current density must be strictly set and monitored; the time must be adjusted according to the film thickness (15-60 minutes for ordinary anodizing, 60-180 minutes for hard anodizing).
• Dyeing (Optional): The core is to give parts various colors. Immerse in an organic dye solution at room temperature for 5-20 minutes (the longer the time, the deeper the color). Ordinary anodizing can choose multiple colors, while hard anodizing is mainly black/dark gray; colored parts require highly weather-resistant dyes to avoid fading.
• Sealing Treatment: The core of this process is to seal the porous structure of the oxide film, locking in performance and color. Boiling water sealing (deionized water 95–100°C, 20–40 minutes) is suitable for ordinary anodized and uncolored parts; chemical sealing (nickel acetate solution immersion at room temperature for 15–30 minutes) is suitable for colored parts and hard anodized parts, providing more stable results.
• Drying and Final Inspection: The core of this process is to control the final quality. Low-temperature drying at 60–80°C is used to avoid color deformation and oxide film peeling caused by high temperatures. Inspection items include film thickness, adhesion, color difference, and surface defect detection.

Process selection and adaptation

Based on the actual application scenario and the product’s usage environment and performance requirements, appropriate processes and materials are selected to achieve the best cost-effectiveness. The specific solution is as follows:

• Water-related products (swimming goggle frames, diving harnesses, etc.): Prioritize 6061 stainless steel with ordinary anodizing, 10–15μm film thickness, black/military green options, balancing aesthetics and seawater corrosion resistance.
• Precision hardware accessories: 6063 stainless steel with ordinary anodizing, 15–20μm film thickness, high cost-effectiveness, suitable for outdoor corrosion and wear resistance requirements, and suitable for mass production.
• High wear-resistant components (diving shafts, mechanical transmission parts, etc.): 7075 stainless steel with hard anodizing, 30–50μm film thickness, utilizing high hardness to improve wear resistance and service life.

Common defects and solutions

In anodizing production, defects often stem from improper parameter control, oversights in preceding processes, or reagent aging. Targeted solutions are needed to reduce scrap rates and stabilize quality.

• Color Difference and Uneven Color: The most common appearance defect, affecting product consistency. Core causes include: batch-to-batch alloy differences, electrolyte parameter fluctuations, and inconsistent dyeing times. Solutions: Use the same batch of aluminum from the same furnace; monitor electrolyte temperature, concentration, and pH in real time; standardize dyeing parameters and confirm color through pre-batch sampling.
• Pinholes and Pitting: Reduce protective performance. Causes include: incomplete degreasing, excessive acid corrosion, and impurities in the electrolyte. Solutions: Enhance ultrasonic cleaning and add a degreasing process; strictly control acid pickling parameters; regularly filter the electrolyte and replace aging reagents.
• Scorching and Erosion: A serious performance defect that causes oxide film peeling. Causes include uneven electric field distribution (poor fixture contact, improper auxiliary anode setting). Solutions: Optimize fixtures to increase contact area; adjust current density and balance the electric field; add auxiliary cathodes for complex parts.
• White spots and powdery residue: Affecting appearance and protection, often occurring during the sealing process. Causes include: incomplete neutralization cleaning, improper sealing parameters, and insufficient purity of deionized water. Solutions: Use three-stage deionized water cleaning; strictly control sealing time and temperature; ensure the resistivity of deionized water is ≥10 MΩ·cm.
• Abnormal texture in 7075 material: Occurs only in 7075 aluminum, affecting appearance. Causes include uneven grain size and alkaline etching exacerbating the texture. Solutions: Use high-quality 7075 raw material; switch to acidic etching; increase mechanical polishing to remove grain texture.