Guide
Anodizing vs powder coating vs zinc plating: which finish should you specify?
Anodizing is an electrochemical process that grows an oxide layer out of the aluminum surface itself. Powder coating is a baked-on polymer film that sits on top of the part, and zinc plating is an electroplated layer of zinc, usually on steel, that corrodes first so the steel underneath does not. The three finishes answer different questions: which base metal you have, how much the finish may move your dimensions, and what the part has to survive in service.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers and sourcing managers who have to write the finish note on a drawing for a cast, turned, milled or stamped metal part. Every thickness figure on this page is taken from the governing standard itself: MIL-A-8625F for anodic coatings on aluminum, ASTM B633 for electroplated zinc on iron and steel, and the EU RoHS Directive for hexavalent chromium. Corrosion performance is described in words, not numbers, because it depends on the part, the pretreatment and the test. For parts quoted here, the salt-spray test standard and duration are confirmed in the quote.
On this page
What is the difference between anodizing, powder coating and zinc plating?
Anodizing converts the aluminum surface into a hard oxide, powder coating adds a layer of cured paint on top of the part, and zinc plating adds a thin sacrificial metal layer to steel. Only anodizing is part of the base metal; the other two are layers that sit on it.
In anodizing the part is the anode in an acid bath, usually sulfuric acid, and current turns the surface aluminum into aluminum oxide. MIL-A-8625F, the US specification most drawings still cite, splits the process into types: Type II is conventional sulfuric-acid anodizing, often dyed, and Type III is the heavy, dense “hard coat” used for wear. Class 1 is undyed and Class 2 is dyed. Because the oxide grows both into and out of the surface, the part changes size by less than the full film thickness.
Powder coating is a dry paint. Charged powder is sprayed onto a grounded, pretreated part and then baked, so it melts and cures into a continuous film. It gives the widest choice of color and gloss and a good barrier against moisture while the film is intact, but it is the thickest of the three finishes and it hides fine detail and sharp edges.
Zinc plating is electroplating: the steel part is the cathode, and zinc builds up on it from solution. Zinc is more reactive than steel, so where the coating is scratched the zinc corrodes first and protects the exposed steel. A conversion topcoat, the chromate or passivate, is almost always applied over the fresh zinc to slow white corrosion of the zinc itself and to set the color.
How thick is each coating and how does it affect tolerances?
Zinc plating and Type II anodizing are thin, hard anodizing is several times thicker, and powder coating is typically the thickest of all. Anodizing moves each surface by about half its thickness, while plating and powder add their full thickness.
Per MIL-A-8625F Table IV, Type II anodic coatings range from 0.00007 to 0.0010 in (about 1.8 to 25 µm) and Type III from 0.0005 to 0.0045 in (about 13 to 114 µm). Type II has no single mandated thickness in that specification; it is controlled by coating weight, so if your part needs a minimum film, write it on the drawing. For Type III, the nominal thickness is 0.002 in (about 51 µm) unless the drawing says otherwise, held to plus or minus 20 percent, per MIL-A-8625F paragraph 3.7.2.1.
The dimensional rule is in the same specification. Per MIL-A-8625F paragraph 6.10.1, each coated surface grows by about one half of the coating thickness, so a 0.002 in (51 µm) hard coat adds roughly 0.001 in (25 µm) per surface and about 0.002 in (51 µm) on a diameter. The specification recommends putting both a machining dimension and a coated dimension on the drawing. Threads are more sensitive: per MIL-A-8625F paragraph 6.10.2, the pitch diameter of a 60-degree thread grows by four times the surface growth.
Per ASTM B633, electroplated zinc comes in four thickness classes tied to service conditions: Fe/Zn 5 for SC1 (mild), Fe/Zn 8 for SC2 (moderate), Fe/Zn 12 for SC3 (severe) and Fe/Zn 25 for SC4 (very severe), where the number is the minimum zinc thickness in micrometers. Plating sits entirely on top of the steel, so per ASTM B633 an Fe/Zn 8 coating adds at least 8 µm per surface and 16 µm on a diameter. On threads and press fits, check that the thread class or the fit leaves room for it, or mask the feature.
Powder coating has no single governing thickness standard. The dry film thickness a given powder needs is set by its manufacturer, and it is typically several times the thickness of a Type II anodize or a zinc plate. Take the range from the powder data sheet, write it on the drawing, and mask threads, bores and datum faces that cannot take the build-up.
| Property | Anodize | Powder coat | Zinc plate |
|---|---|---|---|
| Base metals | Aluminum and aluminum alloys (MIL-A-8625F scope) | Aluminum, zinc and steel parts that tolerate the cure oven | Iron and steel (ASTM B633 scope) |
| Callout standard | MIL-A-8625F Type II or Type III, Class 1 or 2 | No single standard; name the powder product, color and film thickness | ASTM B633, service condition and type |
| Thickness | Type II 0.00007–0.0010 in; Type III 0.0005–0.0045 in, nominal 0.002 in (MIL-A-8625F) | Typically the thickest of the three; set by the powder data sheet | Fe/Zn 5, 8, 12 or 25 µm minimum (ASTM B633) |
| Effect on dimensions | About half the thickness per surface (MIL-A-8625F 6.10.1) | Full film thickness per surface | Full coating thickness per surface |
| Wear | Type III is hard and wear resistant; Type II is moderate | Resists handling scratches; softer than hard anodize | Low; zinc is soft |
| Corrosion | Good on wrought aluminum when sealed; weaker on high-silicon cast alloys | Good barrier while intact; can undercut at chips | Sacrificial: zinc corrodes first; improves with thicker class and passivate |
| Color | Clear or dyed; cast alloys come out darker and less uniform | Widest range of colors and gloss levels | Clear-blue, yellow or black passivate |
| Conductivity | Oxide is insulating; mask contact faces | Insulating; mask ground points | Conductive |
| Masking | Threads, bores and contact faces as called out | Threads, bores, ground contacts and datum faces | Tight threads and press fits, or allow for build-up |
| Cr(VI) risk | MIL-A-8625F lists a dichromate seal among its options; name another seal if RoHS applies | Pretreatment chemistry; ask for a chromium-free pretreatment | Older chromate topcoats; ASTM B633 adds Types V and VI for passivates that replace Cr(VI) |
Which finish works on which base metal?
Among these three finishes, anodizing applies to aluminum, zinc plating to steel and iron, and powder coating to aluminum, zinc and steel alike. The base metal usually decides the shortlist before appearance does.
MIL-A-8625F is written for aluminum and aluminum alloys. Titanium and magnesium can also be anodized, but under their own specifications, and steel and zinc are not anodized in this sense at all. Within aluminum, the alloy matters. Per MIL-A-8625F paragraph 3.4.3, Type III coatings are not applied to alloys with a nominal silicon content above 8.0 percent unless the buyer approves it. That limit catches most high-pressure die-casting alloys.
ADC12 is not a good anodizing alloy, and neither is A380: on these high-silicon, copper-bearing die-casting alloys the film comes out darker, grayer and less uniform than on wrought aluminum. The North American Die Casting Association rates 380 and 383 at 3 of 5 for anodized appearance, where 1 is best (NADCA, Table 4.3). As typical industry guidance, anodizing is kept for wrought or machined aluminum, and die castings are powder coated or plated instead. The ADC12 vs A380 guide covers the alloy side of that choice.
Steel parts, such as stamped brackets or turned shafts and bushings, are the natural home of zinc plating, and ASTM B633 covers electroplated zinc on iron and steel only. Zinc die castings are a separate case: they are usually plated or powder coated, and because B633 does not cover a zinc base metal, the finish on a zinc casting needs its own specification on the drawing. Powder coating works across all three metals, as long as the part and any inserts survive the cure temperature and the pretreatment suits the base metal. The die casting page lists the two alloys sourced from partner foundries; the quote names the production site and origin.
What is trivalent chromate passivation and why does RoHS matter?
Trivalent chromate passivation is a thin conversion layer applied over fresh zinc plating that uses chromium in its trivalent form, Cr(III), instead of hexavalent chromium, Cr(VI). The EU RoHS Directive restricts hexavalent chromium in electrical and electronic equipment, so the passivate chemistry decides whether a plated part can go into those products.
The restriction sits in Annex II of Directive 2011/65/EU, as amended by Delegated Directive (EU) 2015/863, which sets a maximum concentration of 0.1% by weight of hexavalent chromium in homogeneous materials (EUR-Lex, (EU) 2015/863). A zinc coating and its passivate on a bracket inside a piece of electrical equipment count as a homogeneous material in their own right, which is why the topcoat matters and not just the steel.
Traditional yellow and olive chromates were hexavalent. ASTM B633 was revised to address RoHS and now includes Types V and VI, which permit passivate treatments that replace hexavalent chromium (ASTM B633-23, scope). Trivalent passivates come in clear-blue, yellow and black. The same question applies to anodizing: MIL-A-8625F lists a sodium dichromate seal among its sealing options, and dichromate is hexavalent, so name a different seal when RoHS applies.
This page describes the directive; it does not assess any product against it. Whether a finished part meets RoHS is a question about that part’s materials, so ask the finisher for a written declaration covering the plating, passivate and seal, not a general statement. The surface finishing page shows the trivalent chromate among the finishes our partner plants apply.
Which finish should I specify on my drawing?
Specify the finish the base metal and the function call for, then write one note that names the standard, the type or class, the thickness and the masked features. A note like that leaves the finisher nothing to guess.
The examples below are generic callouts to adapt, not the defaults of any supplier. Check each against the current revision of the standard you cite; MIL-A-8625F is now maintained as a performance specification, MIL-PRF-8625.
- ANODIZE PER MIL-A-8625F, TYPE II, CLASS 2, BLACK. MASK ALL THREADS. For machined aluminum housings where color and moderate protection matter.
- HARD ANODIZE PER MIL-A-8625F, TYPE III, CLASS 1, 0.002 THICK. DIMENSIONS APPLY AFTER COATING. For wear surfaces on wrought aluminum; say whether dimensions are before or after coating.
- ZINC PLATE PER ASTM B633, SC 2 (FE/ZN 8), CLEAR TRIVALENT PASSIVATE, NO HEXAVALENT CHROMIUM. For steel brackets and turned parts; add the B633 type from the edition you cite.
- POWDER COAT, [POWDER PRODUCT], BLACK, [GLOSS], FILM THICKNESS PER POWDER DATA SHEET. MASK BORE A AND GROUND POINT B. For die castings and parts where color matters most.
Three habits prevent most finish disputes. Mark which faces are cosmetic and which are functional, so appearance standards are not applied to hidden faces. Mark every feature that must stay bare, such as threads, ground contacts and press-fit bores. If the part must pass a corrosion test, name the test standard and duration on the drawing rather than a finish name alone. The RFQ preparation guide covers the rest of the package, and you can send a drawing for a quote with the finish note as written.
Frequently asked
Can steel or zinc parts be anodized?
Not in the sense of MIL-A-8625F, which covers aluminum and aluminum alloys. Steel parts are usually zinc plated or powder coated, and zinc die castings are usually plated or powder coated.
Is powder coating better than anodizing for corrosion?
It depends on the part. Powder coat is a strong barrier while it is intact but can undercut where it chips; anodizing cannot chip off because it is grown from the metal, but on high-silicon cast alloys it protects less well. If corrosion matters, name the test on the drawing.
Can hardened steel parts be zinc plated?
With care. Electroplating can introduce hydrogen, and ASTM B633 says steels with a tensile strength above 1700 MPa should not be zinc electroplated to that specification (ASTM B633-23, scope). For fasteners, B633 points buyers to ASTM F1941/F1941M.
Why does anodized die-cast aluminum look darker than machined bar stock?
Silicon and copper in die-casting alloys do not form a clear oxide, so the film comes out gray and uneven. The ADC12 vs A380 guide gives the published ratings.
Is a trivalent passivate automatically RoHS compliant?
It avoids hexavalent chromium by design, but RoHS compliance is judged on the finished part’s homogeneous materials. Ask for a written declaration for your part.
Sources
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 10 September 1993, with Amendment 1 of 15 September 2003 (paragraphs 3.4.3, 3.7.2.1, 6.10.1, 6.10.2 and Table IV)
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023
- Directive 2011/65/EU on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS), European Parliament and Council, 2011
- Commission Delegated Directive (EU) 2015/863 amending Annex II to Directive 2011/65/EU, European Commission, 2015
- Aluminum Alloys, including Table 4.3, NADCA Design, North American Die Casting Association, accessed September 2026