Anodizing Aluminum Parts in China: Types I, II, and III

Anodizing is an electrochemical process that converts the outer surface of an aluminum part into a hard, corrosion-resistant layer of aluminum oxide. Unlike paint or powder coating, the layer grows out of the metal itself, so it cannot peel or flake off. For buyers sourcing aluminum parts in China, the practical question is rarely whether to anodize — it is which type to specify. The three types are defined by the U.S. military specification MIL-A-8625: Type I uses chromic acid and produces the thinnest films (roughly 0.5 to 3 micrometers); Type II uses sulfuric acid and is the everyday decorative and protective finish (1.8 to 25 µm); Type III is hard anodizing, a thick, wear-resistant coating that can reach 150 µm.

Getting this choice right happens before you request quotes, not after parts arrive. A sourcing partner who can verify an anodizer's bath controls, racking, and testing saves more than any per-part price gap. Below: how the three types differ, which alloys suit them, what to put on your drawings, what drives cost, and which defects to check at inspection.

How does anodizing actually work?

The aluminum part becomes the anode in an electrolytic cell. The part is racked on metal contacts, immersed in an acid electrolyte bath, and direct current is applied. Oxygen released at the anode surface converts the outer layer of aluminum into aluminum oxide, a ceramic-like material. The coating grows partly into the metal and partly outward, which is why dimensional growth is modest compared with plating.

The pore structure of the oxide matters most for buyers. In Type II sulfuric anodizing, the coating forms as a porous layer that absorbs dyes readily, then gets sealed shut. In Type III hard anodizing, lower bath temperatures and higher current densities produce a much denser, less porous layer that is far harder but absorbs dye poorly.

A typical production sequence at a Chinese anodizing shop runs: alkaline cleaning and degreasing, caustic etching (which sets the matte or satin finish), desmutting to remove alloying-element residues, the anodizing bath itself, dyeing if color is required, and sealing. Skipped or rushed cleaning is where most cosmetic failures begin. Parts carry small uncoated contact points — rack marks — wherever the jig touched the metal. Every anodized part has them somewhere, so your drawing should note where rack marks are acceptable.

What are the three anodizing types, and how do they differ?

The Type I / II / III classification comes from MIL-A-8625, which also defines Class 1 (undyed) and Class 2 (dyed, with a specified color). Export-oriented Chinese anodizers routinely work to these terms, so citing them on a drawing removes ambiguity.

Type I (chromic acid) · Type II (sulfuric acid) · Type III (hard coat)

• Electrolyte: Chromic acid · Sulfuric acid, typically 15–20% · Sulfuric acid, refrigerated bath near the freezing point of water
• Coating thickness: 0.5–3 µm (0.00002"–0.0001") · 1.8–25 µm (0.00007"–0.001") · 13–150 µm (0.0005"–0.006"); 25–75 µm is the common working range
• Hardness: Soft, ductile film · HV 200–300 · HV 400–600
• Dyeability: Harder to dye, often undyed or used as paint base · Excellent dye absorption, full color range · Limited: natural bronze and gray tones to black
• Key property: Minimal effect on fatigue strength and dimensions · Best balance of cost, appearance, and corrosion resistance · 10–20 times the wear resistance of bare aluminum
• Typical uses: Aerospace precision components · Consumer electronics, architectural trim, cookware, lighting · Hydraulic cylinders, pistons, firearm components, sliding machinery parts

Type I: chromic acid anodizing

Type I is the oldest process, the Bengough-Stuart process. The chromic acid bath produces a thin, relatively soft film that changes dimensions almost not at all and has the least effect on the fatigue strength of the part, which is why aerospace has long used it for precision components and as a pretreatment before bonding or painting.

Two caveats. Chromic acid contains hexavalent chromium, so environmental rules have restricted its use and fewer shops offer Type I than in the past — confirm the capability before assuming any supplier has it. The thin film is also hard to dye and rarely used decoratively. For most buyers sourcing from China, Type I is the least likely type you will specify.

Type II: sulfuric acid anodizing

Type II is the standard. It is what a Chinese supplier almost certainly means when they say "anodized" without qualification. Decorative finishes typically run 10–15 µm, while protective specifications go up to the full 25 µm. The porous oxide accepts organic dyes in virtually any color, from clear and gold through black, red, and blue. Sealing then locks the color in and improves corrosion resistance.

If you only remember one thing about Type II: it is a cosmetic and corrosion-protection finish, not a wear finish. Sliding or rubbing contact will wear through it, and buyers who need abrasion resistance should be looking at Type III.

Type III: hard anodizing

Type III uses the same sulfuric chemistry as Type II but runs the bath near the freezing point of water at higher current densities. That combination builds a thick, dense oxide layer — commonly 25 to 75 µm, and up to 150 µm where specified — with hardness in the HV 400–600 range, a process described in detail in this hard-coat anodizing guide. Published engineering references describe hard-anodized surfaces surviving more than 10,000 abrasion test cycles, versus roughly 500 to 1,000 for ordinary Type II finishes — figures you can see laid out side by side in this Type II vs Type III anodizing guide.

The tradeoff: the dense pore structure resists dye, so hard-anodized parts come out in natural dark bronze or gray tones, or black with limited saturation. Cycle times are longer, refrigeration costs energy, and masking and racking need more care. Expect Type III to cost roughly two to three times a comparable Type II job.

Which aluminum alloys anodize best?

The alloy you choose sets the ceiling for finish quality before the part ever reaches the anodizing tank. Copper and zinc in the alloy disrupt the oxide layer and discolor it. The table below reflects widely published engineering guidance.

Alloy · Series · Type II finish · Type III finish · Color anodize?

• 6061-T6: 6xxx (Al-Mg-Si) · Excellent · Excellent · Yes, full spectrum
• 6063-T5/T6: 6xxx (Al-Mg-Si) · Excellent, cleaner cosmetic finish than 6061 · Good · Yes, full spectrum
• 5052-H32: 5xxx (Al-Mg) · Good, slightly darker clear coat · Good · Yes, limited colors
• 7075-T6: 7xxx (Al-Zn-Mg) · Acceptable · Acceptable · Limited, yellowish-gray tint
• 2024: 2xxx (Al-Cu) · Poor · Poor · Not recommended
• Cast alloys (A380, 356): Cast · Poor to acceptable · Poor · Not recommended

The practical rule most shops in Guangdong and Zhejiang will tell you: for cosmetic work, design in 6061 or 6063 from the start. 7075 gives you higher strength (roughly 500 MPa yield against about 275 MPa for 6061), but the zinc content leaves a yellow-gray cast that makes bright or consistent color dyeing unreliable. 2024's high copper content dissolves unevenly in the anodizing bath, leaving rough, non-uniform coatings. And cast alloys carry silicon and iron inclusions that show up as dark spots or pits after anodizing.

If a supplier quotes you an attractive price for "anodized 7075 in red," treat it as a signal to ask for a pre-production sample before committing. Alloy substitutions are a common source of cosmetic disputes.

What can color anodizing actually do?

With Type II, a lot. The porous oxide layer absorbs organic dyes before sealing, so black, gold, champagne, red, blue, and green are all routine at experienced Chinese anodizing shops. Two limitations are worth internalizing.

First, the base alloy color always shows through. A dark bronze on 7075 will not match the same dye on 6061, and even the same alloy from different melts can shift slightly. Buyers who need tight batch-to-batch color matching should keep a signed physical limit sample with the supplier — one approved part that defines the acceptable color range — and reference it in the purchase order.

Second, some organic dyes fade under prolonged UV exposure, so confirm the dye and sealing are rated for exterior use on outdoor parts. On 6063 extrusions, mechanical die lines or extrusion weld lines can show up as streaks after anodizing; this defect starts in the extrusion die, not the anodizing tank, and polishing or re-nitriding the die is the actual fix.

For Type III, calibrate expectations downward: the dense layer produces natural bronze, gray, or dark tones, and black is achievable but with less depth than a Type II black. Bright colors on hard anodize are not realistic.

Why sealing matters more than buyers think

Anodizing creates the oxide; sealing makes it durable. Unsealed Type II coatings have open pores that absorb dirt and moisture, and dye can bleed out. Common production methods include hot deionized water immersion and nickel acetate sealing; Type III coatings may receive a dichromate seal that also improves corrosion resistance.

A well-known verification is the dye stain test: a drop of dye applied to the surface should not stain a properly sealed coating. It is a reasonable item to include in your incoming QC plan. If a sample's black dye rubs off on a fingertip or bleeds when wiped with a damp cloth, the sealing step is the first suspect.

What thickness should you put on the drawing?

Vague drawings cause most anodizing disputes. "Anodized black" on a drawing tells the shop nothing about film thickness, dye, sealing, or where rack marks may fall. A complete callout reads something like:

Anodize per MIL-A-8625F, Type II, Class 2, 15–20 µm, black, sealed. Rack marks permitted on back face only.

Two notes on that example: specify a range rather than a single number, because baths drift, and remember the coating grows in both directions — roughly half of the total thickness builds outward per surface. A 20 µm Type II coating adds about 10 µm per surface — significant if your part has threads or press-fit bores. For tight-tolerance features, either mask them (masked areas must be defined on the drawing) or specify a thinner film on those surfaces.

Typical thickness guidance:

• Decorative interior parts: 10–15 µm Type II
• Exterior or corrosion-critical parts: 15–25 µm Type II, sealed
• Sliding or wear surfaces: 25–50 µm Type III (thicker where the wear regime demands it)
• Aerospace thin-film work: Type I per specification, verified capability

Ask the shop to state the thickness they will hold and how they measure it; eddy-current gauges are the standard nondestructive check, and any serious anodizer owns one.

What drives anodizing cost in China?

No single published price exists, because anodizing is priced as a service on your parts, not a commodity — and listings on Alibaba or Made-in-China vary widely by size, quantity, and region. The cost drivers are consistent, though, and knowing them lets you design the part and the order to land where you want.

Process type. Type III costs roughly two to three times a comparable Type II job because of longer cycle times, refrigerated baths, and higher energy use. Type II remains the economical default for anything that does not see wear.

Surface preparation. A polished or bead-blasted pre-finish costs more than a simple etched matte, and the difference is entirely labor. If your design calls for a cosmetic polish, that line on the drawing costs more than the anodizing itself on small parts.

Thickness and dye. Thicker films mean longer tank time. Dyeing adds a bath cycle and a color-matching responsibility; custom colors cost more than the shop's standard black or clear.

Racking, masking, and batch size. Parts are racked by hand, and each rack position holds one part. Small parts that pack densely onto a rack anodize cheaply per piece; large or awkward parts that need dedicated jigs do not. Masking threaded holes or contact surfaces is manual labor. Larger batch sizes amortize setup, racking, and bath make-up across more parts, which is why the per-part price drops steeply with quantity.

Alloy. 6061 and 6063 anodize predictably; 7075 and cast grades raise the scrap and rework rate, and shops price that risk in.

In practice, most of the savings available to a buyer are in design and ordering: standardize on 6061/6063, keep finishes to the shop's standard colors, consolidate quantities, and specify masking precisely so the shop is not guessing.

Common defects and the QC checks that catch them

The same anodizing defects appear in Chinese shops as anywhere else; what varies is how reliably a given shop controls them — which is why a factory audit of the anodizer before you commit is worth more than a lower quote.

Anodizing burn. Bright or whitish areas surrounded by darker coating, often with little or no film in the affected spot. Causes include excessive current density, poor electrical contact at the rack, and inadequate electrolyte agitation. Burned parts are scrap.

Color differences between parts or batches. Causes range from alloy inconsistency (different melts in one batch) to drifting bath concentration, temperature, or dye conditions. The remedy on your side is the signed limit sample mentioned earlier, plus specifying that all parts in a shipment come from the same material batch.

Streaking and die lines. Longitudinal streaks on extruded profiles that become visible after anodizing usually originate in the extrusion die — worn bearings, weld lines, or lubricant residues — not in the anodizing tank. Switching anodizers will not fix them. Satin brushing before anodizing can mask mild cases.

Rack marks in the wrong place. Every part has contact marks; if you did not specify where they may fall, the shop chose for you. Put it on the drawing.

Poor sealing. Dye that rubs off, stains that appear after handling, or corrosion starting in the pores. The dye stain test catches this at the shop before parts ship.

Pits and white spots. Often traced to chloride contamination in the bath (one published defect guide puts the critical chloride level around 80 ppm in sulfuric electrolyte) or to inclusions in cast alloys. Deionized water for bath make-up and rinsing is the standard prevention.

A practical incoming-inspection routine for anodized parts: verify film thickness with an eddy-current gauge on several points per part, check color against the limit sample under consistent lighting, wipe-test a dyed part for dye bleed, confirm rack-mark locations against the drawing, and run a dye stain spot check for sealing on a sampling basis.

Frequently asked questions

What is the difference between Type II and Type III anodizing?

Type II is a sulfuric-acid anodize producing a 1.8–25 µm porous coating — excellent for color dyeing and corrosion protection at moderate cost. Type III is a hard anodize built at low bath temperatures, a 13–150 µm dense coating at HV 400–600 for wear resistance, at roughly two to three times the cost and with limited color options.

Can 6061 aluminum be anodized black?

Yes. 6061 is one of the best alloys for black anodizing: it produces a uniform porous oxide in Type II that absorbs black dye evenly, and it hard-anodizes well in Type III when wear resistance is needed. For the most consistent cosmetic black, many shops prefer 6063 extrusions.

How long does anodized aluminum last outdoors?

Properly sealed Type II coatings in the 15–25 µm range are widely used on architectural components with long service lives; some organic dyes can fade under UV, so exterior work needs dye and sealing rated for outdoor exposure. Type III coatings, being thicker and denser, offer stronger corrosion resistance for harsh environments.

Is anodizing or powder coating better for aluminum parts?

They solve different problems. Anodizing grows from the metal, cannot peel, preserves fine detail and threads, and protects in thin films. Powder coating is thicker, hides surface imperfections better, offers unlimited colors, and is usually cheaper for large parts — but it can chip and rounds off sharp edges. For precision machined parts, anodizing is usually the better fit.

How to decide: a spec rule for your RFQ

Three questions settle the choice for most parts. First, does the surface see sliding, rubbing, or abrasive contact? If yes, specify Type III hard anodize and accept the limited color range. If no, Type II is the economical answer. Second, does the part need a specific color? Then it is Type II on 6061 or 6063, with a signed limit sample held by the supplier. Third, are you in aerospace with a thin-film fatigue requirement? Then it is Type I, after confirming the shop's capability and regulatory standing.

Whichever type you choose, put the full callout on the drawing — spec, type, class, thickness range, color, sealing, and permitted rack-mark locations — and require a pre-production sample for approval. Most anodizing disputes in China sourcing trace back to a drawing that said "anodized" and nothing else.

If you are qualifying an anodizing supplier and want an independent check on their process controls, test equipment, and sample quality before you place the order, write to hi@cnally.com or reach out through the contact page.