Applicable Substrates for Arc Anodizing: Metals Suitable for Surface Enhancement
Aluminum, magnesium and titanium alloys are the principal industrial substrate families for arc anodizing, usually called micro-arc oxidation (MAO) or plasma electrolytic oxidation (PEO), because they can form dielectric oxides under suitable anodic conditions. That makes them candidates, not automatic approvals. Exact alloy chemistry, product form, heat treatment, casting defects, machining, geometry, electrolyte, electrical regime, post-treatment and required function determine whether the finished component is suitable. Zinc, copper and ferrous alloys normally require different established surface-treatment routes unless a specifically defined process has been independently qualified.
Begin with the process definition
"Arc anodizing" is not a universal Type designation. In this article it means discharge-assisted electrolytic oxidation in the MAO/PEO family. It is distinct from conventional Type III sulfuric hard anodizing unless a controlled customer specification explicitly establishes an authorized relationship. A buyer should ask the processor to identify the process family, substrate scope, final coating stack and performance evidence before discussing compatibility.
Substrate compatibility is also property-specific. A material may support a wear-focused ceramic layer but fail dielectric continuity at pores. Another may accept a sealed corrosion system yet produce an unacceptable color or roughness. The correct question is not "Can this metal be PEO coated?" It is "Can this alloy, surface and geometry repeatedly meet the defined finished-part requirement using an approved process?"
Why valve-metal behavior matters
Aluminum, magnesium and titanium are commonly described as valve metals because an anodically formed oxide can restrict current under suitable conditions. During MAO/PEO, dielectric breakdown and localized discharges contribute to further oxide formation. The process can incorporate species from the electrolyte and produce a layered, porous ceramic structure. Each metal family forms different oxides and requires a different qualified route.
Electrical conductivity alone does not determine suitability. A highly conductive material is not rejected merely because it carries current well, and a moderate conductor is not automatically compatible. Oxide stability, dissolution, breakdown behavior, substrate phases, thermal response, electrolyte interaction and fixture conditions act together. Avoid screening charts that reduce a complex electrochemical system to one bulk property.
Aluminum is widely used, not universally best
Aluminum offers a broad supply base, low density, cast and wrought product forms, established machining routes and extensive MAO/PEO process development. Its oxide chemistry supports useful ceramic systems for wear, electrical isolation, corrosion stacks and topcoat foundations. Those commercial and manufacturing factors explain prevalence more accurately than a claim that every aluminum alloy produces the best coating.
Wrought aluminum can provide a comparatively homogeneous starting surface, but alloy, temper, grain structure, inclusions and machining still influence the result. Pressure-die-cast alloys include silicon-rich and intermetallic phases plus surface defects that can change discharge distribution. Qualification must keep product form distinct. A flat wrought coupon cannot release a complex aluminum die casting.
Aluminum casting alloys require local evidence
AlSi10Mg, A380, ADC12 and other casting alloys can be considered for a qualified MAO/PEO route, but their coating response should not be inferred from silicon content alone. Silicon morphology, copper- and iron-bearing phases, solidification pattern, porosity, release residues and the processor's electrolyte/electrical strategy all influence architecture. Higher silicon does not automatically create a harder or more heat-resistant coating.
A machined area removes the rapidly solidified casting skin and may expose pores or a different phase distribution. Measure and section both as-cast and machined populations when both are functional. Include gates, flow junctions, thick transitions, edges and late-fill regions in trials. A good cross-section from an easy flat face can miss the actual weak location.
Do not select A360 by a false low-silicon rule
A360 may be evaluated because its full property balance can suit some die-casting applications, but it should not be described as universally low in silicon or automatically preferable for PEO. Nominal chemistry comparisons must use the controlling material specifications. Copper, iron, magnesium, silicon morphology, casting quality and surface route can matter alongside total silicon.
A380 and ADC12 or A383-family alloys remain commercially useful substrates when the casting and coating system meet project tests. Changing alloy only to improve coating can alter fill, leakage, strength, thermal behavior, tool life, supply and cost. Compare complete manufacturing outcomes, not a single finishing preference.
Magnesium requires a magnesium-specific system
Magnesium alloys are established candidates for MAO/PEO, often because the ceramic system can improve a reactive lightweight surface. Their oxide chemistry and corrosion behavior differ from aluminum. Electrolyte, waveform, cleaning, substrate impurities, coating defects and sealing or topcoating need qualification for the selected grade. An aluminum recipe should not be transferred by simply changing the voltage.
Corrosion protection may depend strongly on connected porosity and post-treatment. Edges, fastener interfaces, galvanic couples and damage can dominate component behavior. If the application involves fatigue or impact, evaluate whether the coating architecture and surface preparation affect crack initiation. "Lightweight plus hard ceramic" is a product concept, not performance evidence.
Titanium needs function-specific qualification
Titanium and titanium alloys can support plasma-electrolytic oxide development, but applications vary widely. A porous surface intended to support bonding or biological response is not the same as a dense dielectric or wear system. Electrolyte-incorporated species, phase composition, roughness, cleanliness and post-treatment may be central acceptance characteristics.
Do not claim medical biocompatibility from substrate name or coating morphology. Medical use can require material characterization, contamination control, biological evaluation and process validation beyond MAO feasibility. Aerospace or electrical use likewise needs its exact customer specification and approved processor scope. Titanium compatibility begins the review; it does not complete it.
Zinc and copper need a proof-first answer
Conventional industrial MAO/PEO is centered on valve-metal substrates. Zinc and copper alloys do not normally enter the same established process window as aluminum, magnesium or titanium. Literature may describe plasma-electrolytic treatments, conversion layers or experimental routes on unusual substrates, but those labels do not establish a purchasable MAO coating with the required function.
For a zinc die casting, mature options can include conversion treatment, plating, paint or powder coating, depending on alloy, appearance and environment. Copper alloys may use plating, passivation, patination, clear organic protection or other specified finishes. If a supplier proposes "arc anodizing" for either family, require its process definition, substrate-specific data, final stack, dimensional effects and independent qualification before redesigning around it.
Steel and ferrous alloys usually follow other routes
Steel and stainless steel are not ordinary MAO substrates in the same sense as aluminum, magnesium and titanium. Oxidation, nitriding, plating, thermal spray, physical vapor deposition, paint and conversion routes serve different ferrous applications. Select among them by wear, corrosion, temperature, dimensions and substrate heat-treatment limits.
Hybrid research routes or coatings applied over an intermediate layer may be called plasma electrolytic oxidation. That is a multi-layer system and should be specified as such. The buyer must know which layer supplies adhesion, load support and corrosion resistance. Marketing similarity to aluminum PEO is not a substitute for a process drawing and evidence.
Substrate suitability matrix
| Substrate family | Initial MAO/PEO position | Main qualification risks | Buyer evidence |
|---|---|---|---|
| Wrought aluminum | Common candidate | Alloy/temper variation, machining, roughness and local defects | Material record, production surface and function-specific tests |
| Cast aluminum | Candidate with casting-specific qualification | Silicon/intermetallic phases, pores, skin and machined transitions | Location map, cross-sections and finished-part tests |
| Magnesium alloy | Common candidate with dedicated process | Reactive substrate, connected porosity, galvanic interfaces and sealing | Grade-specific final-system corrosion/wear evidence |
| Titanium alloy | Candidate for defined functional surfaces | Architecture, incorporated species, cleanliness and application approval | Chemical/phase and functional evidence tied to intended use |
| Zinc or copper alloy | Not a default MAO route | Undefined process, unstable/nonprotective layer or experimental transfer | Proof of process definition and substrate-specific qualification, or choose another finish |
| Steel/ferrous alloy | Use established alternative by default | Mislabelled hybrid system and unsuitable oxide-growth assumptions | Complete stack specification and comparative finish selection |
Product form can change the answer
Forged, rolled, extruded, machined, cast and additively manufactured material with the same nominal alloy can present different microstructures and defects. Heat treatment, residual stress, grain flow, inclusions and surface contamination change what reaches the electrolyte. Keep product form in the coating qualification record. A transfer between forms needs a technical justification and targeted verification.
Additively manufactured surfaces can carry attached particles, contour porosity and anisotropic microstructure. Cast surfaces can contain laps and trapped gas. Machining can expose cleaner metal or open defects. Blasting changes roughness and can embed media. The approved starting condition should be described well enough to reproduce it, not merely called "clean metal."
Geometry, size and assembly materials matter
Deep recesses, blind holes, narrow gaps, thin edges and large area ratios can complicate current distribution, gas release, cooling, racking and rinsing. A small flat coupon proves material response under limited geometry; it does not establish uniformity on a housing. Map high- and low-response locations during the trial and define any excluded surfaces.
Mixed-metal assemblies should generally be reviewed before coating. Dissimilar materials may require disassembly, selective masking or different finish routes. Fasteners and inserts can create electrical contact, chemical compatibility and galvanic issues. Specify whether inserts are installed before or after MAO, and validate the final assembly rather than assuming the oxide eliminates every couple.
Pretreatment is substrate-specific
Degreasing, cleaning, deoxidizing, etching or mechanical preparation must remove contamination without creating an unsuitable surface. Release agent, coolant, polishing compound, corrosion inhibitor and fingerprints can disturb discharge or adhesion of later layers. Excessive chemical attack may open casting porosity. An approved pretreatment should follow the exact alloy and product form.
Cleaning acceptance may require water-break observation, residue analysis or another project-specific control, but no single check proves the surface is ready for every PEO system. Control time between preparation and coating, packaging and handling. If parts cross supplier boundaries, define the incoming surface condition and responsibility for re-cleaning.
Coating function decides whether a candidate is suitable
Wear
Define counterface, contact pressure, motion, lubrication, roughness and failure. A hard phase on an unsupported or porous substrate may fracture. Test both coating and mating-part wear.
Corrosion
Define the final sealed, impregnated or topcoated system. Substrate defects and edges may control attack. Salt spray is a comparative method, not a service-life conversion.
Dielectric isolation
Define electrodes, voltage protocol, leakage and locations. Average thickness cannot prove continuity at pores, corners or rack contacts.
Bonding or topcoat foundation
Define surface energy, roughness, contamination, open time and the complete adhesive or coating stack. An intentionally porous architecture may be useful when controlled.
How to qualify a new substrate
Start with material certificates and the controlled manufacturing route. Screen representative samples for stable processing and gross defects, then run production-intent parts. Map architecture at geometry extremes, measure finished dimensions and test the actual function. Include post-treatment and final inspection. Retain reference parts and records from the approved trial.
If the trial fails, identify whether the cause is metal family, alloy phases, casting or additive defects, machining, contamination, fixture, process regime, electrolyte, post-treatment or an unrealistic requirement. Do not change several variables at once. A structured experiment provides transferable knowledge; a single visually good sample does not.
Separate substrate transfer from process transfer
There are two different changes hidden inside a request to coat a new metal. Substrate transfer asks whether the same process can move from one alloy, temper or product form to another. Process transfer asks whether a result produced by one processor, fixture, electrolyte and electrical system can be reproduced elsewhere. Neither transfer is automatic. A coating designation without its controlled route cannot bridge the gap.
For substrate transfer, hold the coating route and test method stable while comparing the approved material with the candidate. Use enough locations to represent casting skin, machined surfaces, edges and recesses. For process transfer, hold the substrate and finished requirement stable while each processor documents its own operating window and resulting architecture. The acceptance criteria can be common even when proprietary settings differ.
Build an approval record that survives production changes
The approval record should identify the material specification and revision, supplier and product form, heat treatment, casting cavity or additive build condition where relevant, machining state, cleaning route, processor, final coating stack and post-treatment. Attach the drawing zones, rack and mask plan, inspection locations, test reports and approved visual references. These details turn a successful trial into a reproducible manufacturing baseline.
Assign review triggers rather than stating that no change is allowed. A change in base-metal source may need certificate review and a limited verification lot; an alloy or product-form change may require full functional requalification. A new cavity, deeper machining cut or revised insert can reopen only the affected locations if the technical rationale supports that scope. Change control should match risk, not administrative convenience.
Use failures to distinguish a material limit from a correctable route
A failure at every location and across reasonable process trials may indicate that the substrate/function combination is not practical. Local failures tell a different story. Breakdown at rack points may be a fixture or acceptance-zone problem; corrosion from opened die-casting pores may require a casting or sealing change; dielectric failure at a sharp edge may point to geometry and field concentration. Cross-section the failed location before changing alloy.
The corrective action must be verified against the original requirement and against new risks. Smoothing an outer layer for a seal can alter thickness and expose connected pores. Increasing coating development to improve wear may affect dimensions or fatigue response. Adding an organic sealer can improve corrosion behavior while changing temperature, bonding or electrical performance. The final stack, not an isolated layer, is the purchased result.
Use alternatives when MAO adds no decision value
MAO/PEO is not automatically preferable to conventional anodizing, conversion coating, plating, paint, powder coating, PVD, thermal spray or another surface system. Compare the property needed, substrate, allowable temperature, dimensions, geometry, lot size, process availability, inspection and repair. A mature alternative can carry lower technical and supply risk.
For aluminum components, compare the classification and verification of arc anodizing with conventional anodizing requirements rather than treating one as a universal upgrade. The route that passes the component's tests with acceptable manufacturing risk is the right choice.
RFQ inputs for substrate screening
Provide metal family, exact alloy specification, product form, temper/heat treatment, manufacturing route, surface condition, part size and mass, model and drawing, zones to coat/mask/rack, finished dimensions, annual and lot quantity, mating materials, service environment, required function, post-treatment, test methods, sampling, documentation, packaging and change-control expectations. State whether a candidate alloy may change or is design-locked.
Ask the supplier for its qualified substrate range, excluded compositions or geometries, approved process identifier, trial plan, external laboratory scope and responsibility for failed trials. Quotations should separate substrate development from production processing when evidence does not yet exist. Do not infer capability from one metal-family example or from an unrelated coupon report.
Final substrate decision
Aluminum, magnesium and titanium are the practical starting families for MAO/PEO, but the actual decision is made at alloy, product-form and finished-component level. Aluminum's prevalence reflects manufacturing breadth as well as oxide behavior. Magnesium and titanium require their own process and application controls. Zinc, copper and steel should default to more established finish routes unless a clearly defined alternative process is qualified.
The most useful procurement outcome is not a list of "compatible metals." It is a substrate/coating approval record that states what was made, where it was measured, what function it passed and which changes reopen the decision. That evidence protects both design intent and production scale-up.
Related substrate questions
