Which metals are most compatible with arc anodizing?
Aluminum, magnesium and titanium alloys are the metal families most commonly compatible with industrial arc anodizing, MAO or PEO because they can develop dielectric oxides under suitable anodic conditions. Compatibility is not ranked universally: each family needs its own electrolyte and electrical regime, and each alloy/product form needs qualification. Aluminum is the broadest commercial starting point; magnesium and titanium are strong candidates for different functions. Zinc, copper and steel normally use other surface-treatment routes unless a specially defined process has substrate-specific evidence.
Compatibility begins at the metal family, not the finish catalog
MAO/PEO relies on oxide growth and dielectric breakdown on a suitable substrate. A metal family that supports this behavior can enter process development, but the component is not approved until coating architecture and function are demonstrated. Conductivity, melting point or native oxide alone cannot predict the result. Alloy phases, surface dissolution, fixture, electrolyte and post-treatment interact.
The term arc anodizing should be defined in the quote as MAO/PEO. It should not silently mean conventional Type III hard anodizing. Ask the processor which metal families and exact alloy/product forms are currently qualified. A laboratory report on one wrought aluminum coupon does not cover cast magnesium, titanium or even a different aluminum casting.
Aluminum offers the widest manufacturing choices
Aluminum is common because it combines low density, casting, extrusion, forging and machining options with extensive PEO development. Wrought alloys can offer a comparatively homogeneous surface. Cast aluminum can enable integrated ribs, bosses and housings but introduces silicon-rich phases, intermetallics and porosity. Those differences create separate qualification populations.
For die castings, do not declare A360 automatically best or A380 automatically unsuitable. Compare controlling chemistry, casting integrity, as-cast and machined surfaces, geometry and the qualified process. Select the alloy for the complete part, then prove the finish.
Magnesium can benefit from a dedicated ceramic system
Magnesium alloys are established MAO/PEO candidates, particularly when a reactive lightweight substrate needs a wear, corrosion or topcoat-supporting surface. Their coating chemistry and substrate corrosion behavior differ from aluminum. A magnesium-specific pretreatment, electrolyte, electrical regime and final sealing or topcoat may be required.
Compatibility must include galvanic interfaces, edges, coating damage and substrate impurities. A ceramic-looking layer is not enough. Qualify the final system using the intended grade, heat treatment, casting or wrought route and component exposure. If fatigue or impact matters, check the finished surface under representative loading.
Titanium compatibility depends on the intended architecture
Titanium MAO/PEO can be developed for dielectric, wear, bonding, surface-chemistry or other specialized functions. A porous titanium oxide intended for bonding or biological research is not automatically suitable as a corrosion or high-voltage barrier. Composition, phases, roughness, cleanliness and incorporated electrolyte species may all become acceptance characteristics.
Medical, aerospace and electrical labels add no evidence by themselves. The governing customer specification, processor approval and application tests decide. Treat titanium as a capable substrate family with a function-specific qualification route, not a premium substitute for aluminum.
Family-level screening table
| Metal family | Screening position | Main risk | Approval evidence |
|---|---|---|---|
| Aluminum | Broad commercial candidate | Alloy/product-form variation, casting phases and porosity | Production-surface maps and required functional tests |
| Magnesium | Established candidate with dedicated route | Reactive substrate, connected defects and galvanic interfaces | Grade-specific final-system evidence |
| Titanium | Candidate for defined specialized functions | Architecture and incorporated chemistry may dominate suitability | Application-specific chemical and functional qualification |
| Zinc/copper | Not a default industrial MAO route | Undefined or experimental process transfer | Substrate-specific process proof or select another finish |
| Steel | Use other established routes by default | Mislabelled hybrid stack | Complete alternative coating specification |
How procurement should compare candidates
Define the function first: wear contact, corrosion environment, dielectric location, thermal role, bonding or topcoat foundation. Provide exact alloy, product form, heat treatment, surface condition, geometry, masked areas and post-treatment. Run trials on production-intent parts and measure high-risk locations. A material is compatible only when the final route repeatedly meets those requirements.
Ask for change triggers covering alloy source, heat treatment, casting process, machining depth, cleaning, fixture, electrolyte family, waveform family and post-treatment. If a supplier proposes zinc or copper MAO, request a clear process definition and independent test evidence before treating it as equivalent to valve-metal PEO. The shortlist is simple; approval is an evidence package.
Use a two-stage compatibility trial
First run a material-response screen that includes the exact alloy and production surface, not only a polished laboratory coupon. The screen should reveal unstable discharge, gross burning, severe nonuniformity and obvious interaction with pores or phases. It can eliminate an impractical candidate, but it should not release production because simple samples underrepresent edges, holes, rack contacts and mixed surface states.
The second stage uses production-intent components and the complete seal, impregnation, polish or topcoat. Locate sections and functional tests according to the drawing risk. For wear, include the actual counterface and motion. For corrosion, include edges, joints and coating damage relevant to service. For dielectric use, test specified electrodes and zones rather than converting average thickness into an electrical claim.
Compatibility must include inspection and supply
A metal/process combination is commercially compatible only if its decisive characteristics can be inspected with defined methods and sampling. Rough ceramic surfaces can complicate thickness readings; connected pores may not be represented by one polished section; rack areas may need explicit exclusion or separate criteria. Agree on calibration standards, specimen preparation and failure ratings before qualification data are generated.
Also confirm that the processor can handle the component area, mass, geometry, fixtures and downstream operations. Capacity for small aluminum coupons does not establish a magnesium housing route, and a titanium research treatment does not establish controlled production. Buyers should approve the named substrate population and supply chain, then require notification when a material, prefinish or coating-system change crosses that boundary.
