What types of anodizing are available? How to choose for aluminum CNC parts

Anodizing is a process in which metals such as aluminum, titanium, or magnesium are used as the anode and an electric current is applied in an electrolyte to form an oxide film on the surface. The oxide film is formed from the metal of the workpiece itself, so it bonds strongly to the base material and does not easily peel off. Common sulfuric acid anodizing produces a film thickness of about 5–25 µm, while hard anodizing can reach 25–150 µm. Electroplating, in contrast, deposits another layer of metal (such as nickel, chromium, or zinc) onto the workpiece surface, relying on adhesion. The two structures and applications are completely different. For CNC machined parts, if corrosion resistance, wear resistance, and light weight are all required, anodizing is the mainstream choice for aluminum parts. If electrical conductivity, welding, or a different metallic luster is needed, electroplating or other surface treatments should be considered. When requesting a quote, buyers should first clarify whether they need an oxide film or a plated coating to avoid rework or rejection later.

Sulfuric acid anodizing is the most common type in the industry. The film thickness is typically 5–25 µm, with moderate hardness and the ability to be dyed in various colors. It is suitable for aluminum exterior parts, 3C structural components, automotive and motorcycle interior parts, and general machinery housings. From a CNC machining perspective, sulfuric acid anodizing has a relatively small effect on tolerances; leaving 0.02–0.05 mm of allowance for the film thickness after machining is sufficient. Buyers should note that sulfuric acid anodizing is not suitable for sliding parts with high wear requirements, nor for areas that require subsequent welding, because the oxide film hinders electrical conductivity and soldering. If parts have threaded holes, mating surfaces, or plug-in ends, the drawing should indicate which areas need masking or should be excluded from anodizing to avoid dimensional interference.

Chromic acid anodizing (chromate process) produces a thinner film, approximately 2–5 µm, with a grayish-green color. It offers excellent corrosion resistance but lower hardness, and is mainly used for aerospace structural parts and components sensitive to fatigue strength, because the chromic film has minimal impact on the substrate's fatigue. Oxalic acid anodizing produces a film thickness of about 8–20 µm, with a yellowish tint and good dyeability, commonly used for electronic heat sinks, decorative parts, and applications requiring moderate wear resistance. These two processes are relatively uncommon in Taiwan's CNC supply chain; if buyers specify them, they should confirm in advance whether the surface treatment facility has the corresponding chemical tanks and wastewater treatment capabilities. From the machining side, the dimensional change from chromic and oxalic anodizing is similar to sulfuric anodizing, so tolerance allowances can follow general anodizing specifications.

Anodizing can be combined with various surface treatments, but the sequence and compatibility must be confirmed in advance. Common combinations include: painting after anodizing (for enhanced color and weather resistance), lubrication after anodizing (to reduce friction coefficient), and local grinding after anodizing (to preserve mating surface precision). However, electroplating after anodizing is not recommended because the oxide film affects coating adhesion; if conductive areas are needed, they should be protected with masking tape or wax before anodizing. For CNC machined parts requiring wear resistance, conductivity, and appearance simultaneously, the process is usually split into multiple steps or treated in zones. If buyers have multiple surface treatment requirements, they should provide a complete process flow chart during the RFQ stage to avoid rework due to compatibility issues discovered after a single process is completed.

The most common mistake is providing only a 3D drawing without specifying the material grade, coating thickness, and tolerance standards, which leads to disputes because the machining shop and the surface treatment shop each interpret the requirements differently. The second mistake is overlooking the effect of the anodic coating on threaded holes and mating surfaces; for example, a drawing may specify an M3×0.5 thread without allowing extra space for anodizing, resulting in interference with tapping or assembly. The third mistake is requesting a quote for hard anodizing as if it were standard anodizing. Hard anodizing involves longer processing time, higher chemical costs, and lower yield, so the price difference can be 2–3 times. The fourth mistake is failing to specify color and color tolerance. Anodic dyeing is a batch process, so color variation between batches is inevitable and must be clearly defined on the drawing or in the contract. If buyers prepare the five key items—material, coating thickness, tolerance, color, and inspection method—before requesting a quote, they can significantly reduce the time spent on back-and-forth clarification.