Guide
Can you anodize die-cast aluminum such as A380?
Anodizing die-cast aluminum is the same electrochemical process used on bar and plate: the casting is made the anode in an acid bath and its surface is converted into aluminum oxide. On Al-Si-Cu die-casting grades such as A380 a film does form, but the film on these high-silicon, copper-bearing alloys is darker, grayer and less even than on wrought aluminum, and it protects less. ADC12, the common JIS grade in the same family, is not a good anodizing alloy either.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers and sourcing managers who have a die-cast housing, cover or bracket on the drawing and a finish note that says “anodize”. It sets out what the governing military specification and the die-casting trade association actually say about anodizing castings, why the alloy chemistry decides the result, which finishes usually serve a die casting better, and when the honest answer is to change the alloy or the process instead of the finish.
The short version: if the anodize is there for looks, a die-cast Al-Si-Cu alloy will disappoint. If it is there for corrosion protection or paint adhesion, a conversion coating or an organic coating normally does the job better. If the part truly needs a clear, uniform or hard anodic film, the part probably needs to be machined from a wrought alloy.
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Can high-silicon die castings such as A380 be anodized at all?
A conventional sulfuric-acid (Type II) anodic film does form on A380-type alloys, but buyers should expect a gray to dark-gray, somewhat mottled surface rather than the clean silver or bright dyed colors seen on wrought parts.
Nothing in the chemistry stops the aluminum matrix of a die casting from oxidizing. What changes is everything else in the metal. ADC12, A380 and 383 are aluminum-silicon-copper alloys, and the silicon and copper-rich phases do not convert into clear aluminum oxide the way the surrounding aluminum does. They stay in or under the film and show through it as a gray cast, and they interrupt the film so it is thinner and less continuous over them.
The die-casting skin adds a second problem. High-pressure die castings carry gas and shrinkage porosity, and the surface layer can hold release agent and oxides from the die. Acid and dye get into pores during processing and bleed back out afterward as stains or streaks. That is why a finisher asked to anodize a high-silicon die casting will rarely promise a color, a uniformity level or a salt-spray result on it.
What does MIL-A-8625 say about anodizing castings?
MIL-A-8625F allows castings to be anodized but lowers the bar for them: dyed casting alloys may show dye bleed-out and a lack of color uniformity, and chromic-acid and hard-coat types are restricted on high-silicon and high-copper alloys.
MIL-A-8625F is the US Department of Defense specification for anodic coatings on aluminum (MIL-A-8625F, 1993), and commercial drawings cite it far beyond defense work. Four of its paragraphs speak directly to die castings:
- Type I (chromic acid). Unless the contract says otherwise, Type I is not applied to alloys with nominal copper above 5.0 percent, nominal silicon above 7.0 percent, or total nominal alloying elements above 7.5 percent (paragraph 3.4.1). An Al-Si-Cu die-casting alloy exceeds the total limit on silicon and copper alone.
- Type III (hard coat). Unless otherwise specified, hard coat is not applied to alloys with nominal copper above 5 percent or nominal silicon above 8.0 percent; higher-silicon alloys may be hard anodized only with the buyer’s approval (paragraph 3.4.3). A guidance note adds that a hard coat of 2 mils or more is extremely difficult to obtain on high-silicon die castings such as 360, 380 and 383, per MIL-A-8625F paragraph 6.16.
- Class 2 (dyed). The color on wrought alloys must be uniform, but cast alloys may show dye bleed-out or a lack of color uniformity tied to the porosity of the casting, and the acceptable degree of non-uniformity is set by the buyer (paragraphs 3.6 and 3.6.1.1).
- Workmanship. Appearance differences caused by the base metal, including parts with both cast and machined surfaces, are not grounds for rejection unless the contract says so (paragraph 3.13).
Put together, the specification tells a buyer two things. First, a die casting can meet MIL-A-8625 Type II and still look uneven, so the drawing has to say what appearance is acceptable. Second, a hard coat callout on an A380 or ADC12 part is a flag that needs an engineering conversation before quoting, not a routine finish. The silicon threshold matters here: the Aluminum Association registers A380.0 die castings at 7.5 to 9.5 percent silicon and 383.0 at 9.5 to 11.5 percent (Aluminum Association Pink Sheets, 2025), so 383.0 sits wholly above the Type III default limit and A380.0 straddles it. ADC12 is the higher-silicon JIS grade in the same family; confirm the lot chemistry against JIS H 5302 rather than assuming which side of the line it falls.
Later revisions of the anodizing specification are issued as MIL-PRF-8625. The paragraph numbers above are from revision F; check them against the revision your drawing actually names.
How do the aluminum die-casting alloys rank for anodizing?
On the North American Die Casting Association’s comparison, based on ASTM B85, the common Al-Si-Cu alloys rate middling to poor for anodized appearance and protection, and only the low-silicon aluminum-magnesium alloy 518 rates best on both.
NADCA publishes a relative rating of the eight aluminum die-casting alloys in its design guidance (NADCA, Table 4.3). The finishing columns are reproduced below. A rating of 1 is most desirable and 5 least desirable, and the ratings compare die-casting alloys only, not die castings against wrought metal.
| Alloy | Anodized appearance | Anodized protection | Electroplating | Corrosion resistance |
|---|---|---|---|---|
| 360 | 3 | 3 | 2 | 2 |
| 380 | 3 | 4 | 1 | 4 |
| 383 | 3 | 4 | 1 | 3 |
| 384 | 4 | 5 | 2 | 5 |
| 390 | 5 | 5 | 4 | 4 |
| 413 | 5 | 3 | 3 | 2 |
| 443 | 2 | 2 | 2 | 2 |
| 518 | 1 | 1 | 5 | 1 |
Two patterns stand out. The anodizing columns get worse as silicon rises: the eutectic alloy 413 and the hypereutectic 390 rate worst for appearance, while 518, which NADCA describes as based on the aluminum-magnesium system and able to be polished and anodized for a decorative finish, rates best. The electroplating column runs almost the other way. The same 380 and 383 that anodize poorly rate best for taking and holding an electroplate, which is a hint about where to look for an alternative finish.
The table also explains why anodizing is rarely the first choice even on a well-made casting. An alloy chosen for die filling and hot-crack resistance, which is why A380 and 383 dominate commercial die casting, is by the same chemistry an alloy that anodizes gray. The ADC12 vs A380 guide covers how those grades differ from each other.
Which finish should you specify on an ADC12 or A380 casting instead?
Choose by what the anodize was meant to do: powder coat or wet paint over a conversion coating for color and corrosion protection, a chemical conversion coating alone for bare-metal protection or electrical contact, and electroplating for a metallic look.
Most “anodize” notes on a casting drawing are carried over from a machined part or picked for a color. Once the job of the finish is named, the choice is usually straightforward:
- Color and appearance: powder coat or liquid paint. Both hide the gray, uneven substrate that anodizing would expose. Porosity can outgas during the cure and leave pinholes or blisters, so a pre-bake before coating is typical practice on die castings.
- Corrosion protection under paint: a chemical conversion coating. MIL-DTL-5541 Class 1A is intended both to protect unpainted aluminum and to improve adhesion of paint systems (MIL-DTL-5541F, 2006).
- Grounding or electrical contact: MIL-DTL-5541 Class 3, which the specification intends for electrical and electronic uses where lower contact resistance than Class 1A or an anodic film is required. An anodic film is an electrical insulator, which is often overlooked when a housing also has to ground.
- Bright metallic look: electroplating, where the NADCA ratings above favor 380 and 383.
Two cautions carry over from the specifications. MIL-DTL-5541F notes that on castings the conversion coating color may not be as uniform as on wrought alloys, so a chem-film part will not look perfectly even either. And the Type I and Type II split in that specification is about hexavalent chromium; for parts going into the EU, the anodizing vs powder coating vs zinc plating guide covers the RoHS side of the choice.
The surface finishing page lists the finishes our partner plants apply, and the quote names where each step is done; the die casting page covers the ADC12 and Zamak 3 part families sourced from partner foundries and machined at our lead CNC plant.
When should you change the alloy or process instead of the finish?
Change the alloy or the process when the part genuinely needs a clear, uniform, dyed or hard anodic film, because no amount of finishing skill turns an Al-Si-Cu casting into a good anodizing substrate.
There are three practical routes, and each trades something away:
- Machine the part from wrought aluminum. A wrought alloy such as 6061 has far less silicon and copper than any die-casting grade and anodizes clear, even and dyeable. The cost is that every feature is cut instead of cast, which suits lower volumes and simpler geometry. The CNC milling and CNC turning pages cover that route.
- Keep die casting but change the alloy. NADCA rates 518 best for anodized appearance and protection and 443 next, but the same table rates both near the bottom for die-filling capacity, and neither is among the grades most die casters run day to day. Confirm the alloy is available before designing around it.
- Keep the alloy and anodize only where it matters. If a machined bore or face needs a film and the rest of the casting does not, a masked or selective finish, or a different finish on the cast surfaces, may be simpler than fighting the whole part.
What should the drawing and RFQ say when a casting must be anodized?
Name the standard, type and class, state that the part is a die casting in a named alloy, and define acceptable appearance with approved boundary samples rather than words alone.
A useful callout reads like this: the alloy with its governing standard (for example ADC12 per JIS H 5302 or A380.0 per ASTM B85), the finish by specification, type and class (for example MIL-A-8625 Type II Class 1), which surfaces are cosmetic and which are functional, and a note that cast-surface appearance is to match approved samples. MIL-A-8625F leaves the acceptable degree of non-uniformity on dyed castings to the buyer, so the drawing has to set that limit, or there is no agreed limit to reject against.
This matters more when the casting is sourced from Asia. Die casters across the region quote in JIS grades such as ADC12, and a finish that looks acceptable to the supplier under factory lighting can read as a defect at incoming inspection on the other side of an ocean shipment. Ask for anodized sample coupons or first-article parts from the actual alloy and die before release, agree light and dark limit samples, and keep one set at each end. Write the finish requirement into the RFQ package itself, not a later email; the RFQ preparation guide has the full checklist.
If an existing die-cast part carries an anodize callout, send the drawing for a quote as it stands and say what the finish is for, so a better-suited finish can be proposed where one exists. That lets the finish question be settled before tooling rather than after the first shipment.
Frequently asked
Does a black dye hide the gray cast on a die casting?
A black dye usually hides the gray cast of the alloy better than light colors do, so black is the most forgiving dyed color on a die casting. Expect some unevenness and possible dye bleed from pores, which MIL-A-8625F permits on dyed castings to a degree the buyer sets.
Can A380 be hard anodized?
Only with care and the buyer’s approval under MIL-A-8625F, whose Type III default excludes alloys above its nominal silicon limit, and whose guidance notes that thick hard coats are extremely difficult on 360, 380 and 383. For a wear surface, consider a wrought insert or a different surface treatment.
Is chem film the same as anodizing?
No. A chemical conversion coating per MIL-DTL-5541 is a thin film formed by chemical reaction without applied current, used to protect bare aluminum, prime it for paint or keep it electrically conductive. Anodizing grows a much thicker, insulating oxide under current.
Why did my anodized casting come out with streaks?
Two causes are common. Where a cast skin meets a machined face, the two surfaces anodize differently, and MIL-A-8625F (3.13) accepts that look as an inherent base-metal difference. On dyed (Class 2) castings, dye seeping out of pores is also allowed by the spec, to a degree the buyer defines. Electrolyte left in pores or crevices is a different matter: the spec’s guidance links it to severe attack, so treat it as a rinsing and sealing problem to fix, not a cosmetic variation.
Can a zinc die casting be anodized?
Anodizing as specified in MIL-A-8625 is an aluminum process. Zamak 3 and other zinc die castings are normally plated or powder coated instead.
Sources
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 10 September 1993 (paragraphs 3.4.1, 3.4.3, 3.6, 3.6.1.1, 3.13, 6.4 and 6.16)
- MIL-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, US Department of Defense, 11 July 2006 (scope and classification, paragraph 6.1 and the color-uniformity note)
- Aluminum Alloys, including Table 4.3, NADCA Design, North American Die Casting Association, accessed September 2026
- Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot (Pink Sheets), The Aluminum Association, revised August 2025
- ASTM B85/B85M-25, Standard Specification for Aluminum-Alloy Die Castings, ASTM International, 2025
- JIS H 5302:2006, Aluminum alloy die castings (catalog record), Japanese Standards Association, 2006