If you are designing a sheet metal enclosure in aluminum and have typed "anodizing" into a Google search, you have probably discovered that the term covers at least six different surface treatments — each with its own chemistry, its own spec sheet, and its own cost band. This guide sorts them out. We cover the six anodizing families used on real OEM enclosure RFQs in 2026, lay out a side-by-side comparison, walk through the standards (AAMA 611, MIL-A-8625, ISO 10074), and finish with the 7 DFM rules that determine whether an anodized enclosure survives a salt-spray test.
Why Anodizing — And Why Only Aluminum
Anodizing is an electrochemical conversion process that grows a controlled aluminum-oxide layer on the surface of an aluminum alloy. Unlike powder coating or liquid painting, anodizing converts the surface rather than depositing a film on it. The anodic layer is integral to the substrate — it cannot delaminate, peel, or chip — and it provides both corrosion resistance and a paintable surface for organic dyes. Anodizing works on aluminum (and titanium, magnesium, and some niobium / zirconium alloys) but not on steel or stainless steel. If your enclosure is steel, anodizing is not on the menu — see our powder vs liquid painting guide for that case.
The 6 Anodizing Families
The MIL-A-8625 standard (the U.S. military anodizing specification) defines the four families most OEM programs will encounter; we add two more commonly seen in commercial European and Asian specs.
Type I — Chromic acid anodizing
Type I uses chromic acid (CrO₃) as the electrolyte. It produces a thin (0.5–2 µm) coating with excellent fatigue performance and minimal dimensional change — the historical choice for aerospace structural parts where fatigue life matters more than corrosion. Restricted under REACH (chromium VI is a Category 1 carcinogen), so most commercial programs have phased it out. Still legal under license for defense and aerospace. AAMA 611 classifies this as Class I.
Type II — Sulfuric acid anodizing (the workhorse)
Type II uses sulfuric acid (H₂SO₄) at 15–20% concentration, 18–22 °C, with a current density of 1.0–1.5 A/dm². It produces a 5–25 µm coating, depending on time and current. The workhorse for architectural aluminum, consumer electronics, indoor enclosures, decorative panels, and a long list of OEM products. Accepts organic dyes for color (black, bronze, gold, red, blue, custom). AAMA 611 Class II.
Type II-B — Sulfuric acid with integral color
Type II-B is a sub-class of Type II where the color is produced by the anodizing bath chemistry itself — typically by adding organic acids or metal salts (e.g., ferrocyanide for "architectural bronze"). The color penetrates the full depth of the anodic layer rather than sitting in surface pores, so it cannot scratch off. Used on architectural storefronts, premium consumer electronics, and decorative enclosures.
Type III — Hard anodizing
Type III (also called hardcoat) lowers the bath temperature to 0–5 °C and increases the current density to 2.5–3.5 A/dm². It produces a 25–150 µm coating with Rockwell hardness up to 60–70 HRC (compared to ~15 HRC for Type II). Used on wear surfaces, hydraulic cylinders, gear housings, military hardware, and any aluminum part that sees abrasive or sliding contact. Not compatible with all aluminum alloys — 2xxx and 7xxx series can crack or spall under hardcoat.
Phosphoric acid anodizing (PAA)
Phosphoric acid anodizing (Boeing's BAC 5884, ASTM D3933) produces a porous coating optimized for adhesive bonding — the Boeing structural-bonding standard for aircraft primary structure. Also used in some consumer electronics as a primer before organic painting. Commercial enclosure use is limited.
Boric-sulfuric acid anodizing (BSAA)
BSAA is the REACH-compliant replacement for Type I chromic anodizing. It produces a thin (0.5–2 µm) coating with similar fatigue performance to Type I but without chromium VI. Specified by Boeing, Airbus, and the U.S. military as a Type I substitute. Commercial enclosure use is rare.
The 6-Row Comparison
For an OEM decision, six rows matter: alloy compatibility, thickness, color capability, corrosion performance, wear performance, and cost.
| Type | Alloy compatibility | Thickness | Color capability | Salt spray (ISO 9227) | Hardness | Relative cost |
|---|---|---|---|---|---|---|
| Type I (chromic) | Most alloys incl. 2xxx | 0.5–2 µm | Limited (gold tint) | ≥ 336 h | Low | 1.4× baseline |
| Type II (sulfuric) | 1xxx–7xxx (avoid 2xxx/7xxx with high Cu) | 5–25 µm | Full organic dye range | ≥ 500 h (sealed) | Moderate | 1.0× baseline |
| Type II-B (integral color) | Same as Type II | 5–25 µm | Bronze / black / champagne | ≥ 1,000 h | Moderate | 1.2× baseline |
| Type III (hardcoat) | 5xxx / 6xxx / 7xxx (not 2xxx) | 25–150 µm | Black / dark gray (dyes limited) | ≥ 2,000 h | Very high (60–70 HRC) | 1.6× baseline |
| PAA | Most alloys | 2–6 µm | None (bonding primer) | N/A (bonding use) | Low | 1.3× baseline |
| BSAA | Same as Type I | 0.5–2 µm | Limited (gold tint) | ≥ 336 h | Low | 1.4× baseline |
Standards to Spec On
Three standards cover most OEM anodizing specs. Quote them on the drawing or in the purchase order:
- AAMA 611 — Voluntary specification for anodized architectural aluminum. Class I (≥ 18 µm), Class II (≥ 10 µm), Class III (≥ 6 µm). Used on North American architectural projects and indoor enclosures.
- MIL-A-8625 — U.S. military. Type I, II, II-B, III. The default for defense and aerospace.
- ISO 10074 — International standard for hard anodizing of aluminum. The reference for European hardcoat programs.
- Qualanod — European quality-marking scheme. Specifies process and quality controls; common on premium European architectural aluminum.
Sealing — The Hidden Half of the Process
Anodizing without sealing leaves a porous oxide layer that will absorb stains and corrode. Sealing closes the pores. There are four families:
- Hot water sealing — Immersion in near-boiling deionized water (≥ 95 °C) for 15–30 minutes. Hydrates the oxide to boehmite, swelling the pores shut. The baseline.
The other three are nickel acetate (improved corrosion resistance, environmental concerns in EU), cold sealing (nickel-fluoride at room temperature, fast cycle), and duplex sealing (nickel + polymer, premium outdoor).
7 DFM Rules for Anodized Enclosures
- All weldments before anodizing. Welds change the alloy microstructure; anodizing a weld zone produces uneven thickness and color.
- Specify alloy by temper. 6061-T6 and 5052-H32 anodize uniformly. 6063 is the architectural default. 2xxx and high-copper 7xxx alloys show brown streaks.
- Bend radius ≥ 2× thickness for Type II, ≥ 4× for Type III. The anodic layer is brittle and cracks at small radii.
- Mark masking areas on the drawing. Threads, ground pads, and bearing surfaces must be masked; the anodic layer adds 10–25 µm to the dimension.
- Tolerances must include the anodic build-up. On a Type II part, the linear dimension grows 5–25 µm. Tolerances under ± 25 µm will fail post-anodize.
- Color match runs are batch-bound. Specify a color standard (sample panel) for each batch; ΔE ≤ 1.0 is achievable within one lot, ΔE 2–3 across lots is typical.
- Sample approval before production. Always run a 3-piece pilot before a 1,000-piece production order. Color and gloss are subjective; sign off in person.
Where Anodizing Wins — And Where It Doesn't
Anodizing wins on aluminum enclosures that need: (a) long-life UV stability without paint (20+ years Florida exposure for Type II-B), (b) a "metal" aesthetic with metallic depth that paint cannot match, (c) high wear resistance (Type III hardcoat), or (d) a thin film that does not change the part dimension (Type I / BSAA at 0.5–2 µm). Anodizing loses on: tight-tolerance surfaces where build-up matters, parts with mixed metals (anodizing only works on aluminum), and high-volume decorative parts where powder coating is 50% cheaper per part. For an in-depth walkthrough of the powder vs liquid decision (which often replaces anodizing for outdoor enclosures), see our powder vs liquid painting guide.
Closing — How to Specify Anodizing on a Drawing
Three things to put on the drawing: alloy and temper (e.g., "6061-T6 per ASTM B209"), anodizing type and class (e.g., "Type II per MIL-A-8625, Class 2, sealed, color: RAL 9005"), and any masking notes (e.g., "all M5 threads masked, all ground pads masked"). Add a finishing specification note that references the standard. Send the spec to your finisher with a color sample panel for sign-off. For OEM enclosures, SanJun works with three anodizing partners in the Pearl River Delta — request our partner list via the contact form.