Which metals can be treated with arc anodizing?
Aluminum, magnesium and titanium alloys are the main metal families treated by arc anodizing, meaning MAO/PEO in this article set. They can form adherent anodic oxides under suitable electrical and electrolyte conditions. That family-level answer is only the first screen. The exact alloy chemistry, product form, heat treatment, surface condition and part geometry determine whether a useful, repeatable coating can be produced. Zinc, copper and steel should not be assumed compatible with a light-metal MAO recipe; an unusual-substrate claim needs processor-specific proof.
Metal-Family Screening
| Substrate family | Why it is considered | Important variables | Qualification emphasis |
|---|---|---|---|
| Aluminum alloys | Established oxide-forming family with many wrought and cast applications | Silicon, copper and other phases; wrought versus cast structure; heat treatment; machining and porosity | Local architecture, roughness, dimensional change and function on the production alloy |
| Magnesium alloys | MAO can provide a converted surface and foundation for a protective stack | Reactive substrate, alloy phases, casting defects, edge exposure, sealer/topcoat compatibility | Corrosion path control and damage behavior of the final stack |
| Titanium alloys | Stable oxide formation can support texture, wear, optical or surface-chemistry objectives | Alloy, surface preparation, electrolyte incorporation, phase, roughness and end-use controls | Application-specific chemistry, function, cleaning and regulatory evidence |
| Zinc, copper or other proposed metals | Only a specialized process may be proposed | Oxide stability, dissolution, safety, adhesion and recipe ownership | Documented processor capability and full functional proof; do not extrapolate from aluminum |
Aluminum Is Common, Not Uniform
Aluminum covers chemically and metallurgically different materials. A wrought 6xxx alloy, a high-pressure die-cast Al-Si alloy and an additively manufactured AlSi10Mg part do not present the same surface. Silicon-rich particles, copper-bearing phases, intermetallics, segregated skin and exposed pores can alter local current response and discharge development. A process window built on wrought material is not proof for a casting with the same base-metal name.
High silicon content does not automatically produce a harder or better MAO coating. It may change color, roughness, growth behavior and phase distribution, with effects that depend on chemistry and waveform. Likewise, an A360 label is not a universal reason to prefer it to A380 or ADC12. Casting performance, mechanical requirements, supply, machining and coating response must be balanced. The correct grade is the one whose production-intent parts pass the complete requirement.
For an aluminum die casting, inspect as-cast and machined regions separately. Machining can remove a segregated skin but expose porosity. Shot blasting can change roughness and embed contamination. Impregnation, heat treatment or repair may affect preparation. These operations must be identified before coating trials and held under change control afterward.
Magnesium Needs a Final-Stack View
Magnesium is a normal MAO candidate, but its reactivity makes defect pathways important. The conversion layer may be only one element in a sealed, impregnated or topcoated system. Open porosity, a damaged edge, a threaded breakthrough or a galvanic junction can expose the substrate even when flat-panel coating measurements look acceptable. Qualification should include the actual post-treatment and representative damage states.
Cleaning and pretreatment must be suitable for the selected magnesium alloy. Long delays, contamination or excessive attack can change the starting surface. Cast magnesium also brings flow marks, pores and local phase variation. Corrosion tests should use the production alloy and relevant electrolyte or humidity exposure, followed by sectioning to locate the path. A generic salt-spray duration without a failure definition does not establish field suitability.
Titanium Needs End-Use Evidence
Titanium alloys can be MAO/PEO treated to alter oxide thickness, texture, phase, surface chemistry or optical appearance. The target determines the recipe. A wear-oriented industrial surface and a porous surface intended for later bonding are not interchangeable. Electrolyte-derived species and finishing steps may matter as much as the titanium grade.
Titanium substrate choice alone does not make a treated part suitable for medical use. Patient-contact or implant applications require separate evidence for chemistry, biological response, cleaning, sterilization, residues, wear debris and regulatory controls. Industrial MAO data cannot be promoted into that approval. The buyer should state the use context and required standards before a processor proposes a route.
What About Zinc, Copper and Steel?
Zinc and copper alloys are not routine MAO substrates under recipes designed for aluminum, magnesium or titanium. Their anodic dissolution and oxide behavior are different. Steel forms other oxide systems and is normally protected or hardened through different finishing routes. A processor may have a specialized discharge-assisted method for an unusual metal, but its name does not establish equivalence to a qualified light-metal MAO coating.
Request the exact substrate designation, process description, cross-section, adhesion or damage behavior, functional test and production history relevant to the proposed part. Also confirm electrolyte safety, dimensional effects and scale-up. If this evidence is unavailable, compare plating, conversion coating, paint, powder, thermal spray or another route rather than forcing an MAO specification.
Part-Level Qualification Plan
The arc-anodizing substrate guide provides the broader screening logic. For a purchase decision, send the mill or foundry specification, composition limits, material condition, product form, manufacturing route and drawing. Identify cast faces, machined zones, repaired areas, contact locations and surfaces that will be masked or post-finished.
Run trials on production-intent parts. Record preparation, fixture, load and arc-anodizing process conditions. Inspect cross-sections at open flats, edges, recesses and transitions. Measure final dimensions and roughness, then perform the actual wear, corrosion, electrical, bonding or cosmetic test. Compare more than one alloy only when all other test conditions are held constant.
Once approved, define which material changes require notification and requalification. Composition range, heat treatment, foundry, casting process, impregnation, surface preparation and machining sequence can all change coating response. The practical answer is therefore: aluminum, magnesium and titanium are ordinary candidates; every grade and part remains conditional until its final system passes a documented production-intent trial.
