Anodizing
Electrochemical process that converts the aluminum surface into a durable, corrosion-resistant oxide layer. Type II for decorative color finishes and Type III (hardcoat) for wear resistance up to 70 µm thickness.
About Anodizing
Anodizing is an electrochemical conversion process that transforms the surface of aluminum into a hard, durable aluminum oxide layer. Unlike paint or plating, the anodic layer is fully integrated with the underlying aluminum substrate, meaning it will not chip or peel. The process is performed by immersing the aluminum part in an acid electrolyte bath and passing an electrical current through the solution, causing oxygen ions to bond with aluminum atoms at the surface.
Anodizing is the preferred surface treatment for aluminum parts that require corrosion resistance, wear protection, or a decorative color finish. It is widely used in aerospace, consumer electronics, medical devices, automotive components, and architectural hardware. The porous structure of the anodic layer allows it to absorb dyes before sealing, enabling a wide range of color options including black, red, blue, gold, and green.
The three main types of anodizing—Type I (chromic acid), Type II (sulfuric acid), and Type III (hardcoat)—offer different performance characteristics to match specific application requirements. Type II is the most common for decorative and general-purpose use, while Type III provides exceptional hardness and wear resistance for demanding mechanical applications. All types maintain the dimensional precision of CNC machined parts with predictable and controllable coating thickness buildup.
Finish Properties
Available Types
Type I (Chromic Acid)
Thin-film anodizing using chromic acid electrolyte. Produces the thinnest oxide layer with excellent corrosion resistance and minimal impact on fatigue strength. Commonly specified for aerospace applications where fatigue life is critical. The thin film makes it ideal for tight-tolerance assemblies.
0.5-7 µm
Type II (Sulfuric Acid)
The most widely used anodizing process, employing sulfuric acid electrolyte. Produces a moderately thick oxide layer that readily absorbs dyes for decorative color finishes. Provides good corrosion resistance and moderate wear protection. Ideal for consumer products, electronics enclosures, and architectural components.
5-25 µm
Type III (Hardcoat)
Heavy-duty anodizing performed at lower temperatures and higher current densities to produce an extremely hard, dense oxide layer. Achieves hardness comparable to case-hardened steel (up to 70 HRC). Used for high-wear applications such as pistons, hydraulic cylinders, valve bodies, and sliding mechanisms.
25-70 µm
How It Works
The anodizing process begins with thorough cleaning and etching of the aluminum part to remove surface contaminants and create a uniform base. The part is then immersed in an acid electrolyte bath—chromic acid for Type I, sulfuric acid for Type II and III—and connected as the anode in an electrical circuit. When direct current is applied, oxygen ions from the electrolyte combine with aluminum atoms at the surface, forming a dense aluminum oxide (Al₂O₃) layer that grows both inward and outward from the original surface. The resulting oxide layer is porous, which allows it to absorb dyes for coloring. After optional dyeing, the part is sealed in hot deionized water or nickel acetate solution to close the pores and lock in the color, maximizing corrosion resistance.
Design Guidelines for Anodizing
Mask threads, press-fit bores, and tight-tolerance features (±0.025 mm or tighter) before anodizing to prevent coating buildup from affecting fit.
Allow for 50% inward growth and 50% outward growth of the oxide layer when dimensioning parts—a 25 µm Type II coating adds approximately 12.5 µm per side.
Avoid mixing aluminum alloys in the same anodizing batch, as different alloys (e.g., 6061 vs. 7075) produce different color shades even with identical dye processes.
Design parts with uniform wall thickness where possible to ensure consistent current distribution and even coating thickness across the surface.
Specify surface finish requirements before anodizing—the process amplifies existing surface imperfections, so bead blasting or polishing should be done prior to anodizing.
Compatible Materials
Materials that can be finished with anodizing.
Anodizing FAQ
Type II (sulfuric acid) anodizing produces a 5-25 µm oxide layer suitable for decorative finishes and moderate corrosion protection. Type III (hardcoat) anodizing produces a 25-70 µm layer at lower bath temperatures and higher current densities, resulting in a much harder coating (up to 70 HRC) designed for high-wear mechanical applications. Type III is typically darker in natural color and offers significantly better abrasion resistance, but at higher cost and with greater dimensional buildup.
Type II anodizing supports a wide range of dye colors including black, red, blue, gold, green, orange, and purple. Black is the most popular and consistent color across alloys. Lighter colors like red and gold may vary slightly between aluminum alloys. Type III hardcoat naturally produces a dark gray to near-black finish and has limited color options due to the density of the oxide layer. Clear (undyed) anodizing is available for all types.
Anodizing is primarily used on aluminum and its alloys. Titanium can also be anodized to produce interference-based colors, but this is a different process. Steel, stainless steel, brass, and copper cannot be anodized. For these materials, alternative surface treatments such as electroplating, powder coating, or black oxide are recommended.
Yes. The anodic oxide layer grows approximately 50% into the substrate and 50% outward. For Type II at 20 µm total thickness, expect roughly 10 µm per side of dimensional growth. For Type III at 50 µm, expect about 25 µm per side. Critical dimensions such as bore diameters and mating surfaces should be masked or accounted for in the design with appropriate tolerances.
Anodized aluminum oxide is an electrical insulator. The anodic layer blocks electrical current flow at the surface, which is advantageous for EMI shielding enclosures but problematic for grounding points. If electrical conductivity is required at specific locations, those areas must be masked during anodizing or the coating must be locally removed after processing. Chem film (chromate conversion) is preferred when surface conductivity must be maintained.
Ready for Anodizing?
Upload your CAD file and specify your finish requirements for an instant quote.