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Nobody anodizes a fastener to make it stronger. Anodizing does not add load capacity. It buys four things in exchange for a measurable thickness penalty: corrosion resistance, surface hardness and wear resistance, a uniform appearance with color options, and electrical insulation where that is wanted. The cost is dimension — the film physically grows into and out of the part, which means threads change size and tight tolerances get messy.
| Effect | What Changes | Why It Matters |
|---|---|---|
| Corrosion resistance | Surface sealed against moisture and salts | Aluminum oxide is far more inert than bare aluminum |
| Surface hardness | Film hardness typically 200–500 HV, hard anodize higher | Resists scratching and thread wear in repeated assembly |
| Wear resistance | Abrasion resistance far above bare aluminum | Sliding surfaces, repeated torque, and thread engagement |
| Appearance | Uniform finish, dyeable to most colors | Product identity, corrosion inspection, branding |
| Electrical behavior | Film is a dielectric insulator | Good where insulation is wanted, a trap for grounding |
Two layers form at once. A thin, dense barrier layer sits against the metal, and on top of it grows a thick, porous layer. The porous layer is the useful one: it is thick enough to protect and to hold dye, and it can be sealed afterward by hot water or steam, which hydrates the oxide and closes the pores.
The reaction consumes aluminum. The film grows roughly half inward, into the part, and half outward, above the original surface. That 50/50 split is the single most important practical fact in this guide, and it is the reason fastener people argue about tolerances. A 20 µm anodize film does not add 20 µm to the part — it adds roughly 10 µm outside and removes roughly 10 µm of metal underneath, changing dimensions in both directions.
| Type | Common Name | Typical Film Thickness | Fastener Use |
|---|---|---|---|
| Type I | Chromic acid anodize | 1.5–5 µm | Fatigue-critical and aerospace parts; thin, low dimensional impact |
| Type II | Sulfuric acid anodize | 5–25 µm | The general-purpose finish; corrosion, color, appearance |
| Type III | Hard anodize | 25–150 µm | Wear, abrasion, and heavy-duty service |
Thin film, good corrosion resistance, minimal dimensional change, and the least damage to fatigue strength. Type I was the aerospace standard for decades, and it still appears where fatigue matters — aircraft control fittings, thin-wall parts. The problem is environmental: chromic acid baths use hexavalent chromium, which is restricted under RoHS and REACH and increasingly hard to justify. Aerospace has largely moved to tartaric-sulphuric acid (TAS) anodizing as the drop-in replacement — same thin-film philosophy, no Cr⁶⁺. Boeing and Airbus specifications (BAC 5632, AIPS 02-04-014) now dominate that space.
The workhorse. Five to twenty-five microns, dyed or natural, sealed or not, used on everything from electronics enclosures to automotive trim to threaded fasteners. It gives the classic anodized look, takes dye well, and has good corrosion resistance when sealed. The film is not particularly hard — do not confuse Type II with hard anodize — but it resists handling scratches far better than bare aluminum.
The heavy one. Hard anodize grows 25 to 150 µm of exceptionally dense oxide, with hardness that can exceed 400 HV and wear resistance approaching that of hard chrome plating on steel. Fasteners specified in Type III are typically in high-wear applications: repeated assembly-disassembly, sliding mechanisms, thread forms that take abuse, and parts that run against other metal surfaces.
Type III has two costs. The dimensional change is large — a 50 µm coating shifts the thread profile by up to 50 µm in the worst direction — and it reduces the fatigue life of the part more than Type I or thin Type II, because the thick brittle film cracks under cyclic strain and those cracks propagate into the substrate. Hard anodize is a wear finish, not a fatigue-friendly one.
The 50/50 growth split changes thread dimensions. On an external thread (bolt, screw), the film grows outward and the pitch diameter grows. On an internal thread (nut), the film grows inward and the pitch diameter shrinks. A 20 µm Type II film on a bolt therefore adds roughly 10 µm to the pitch diameter; a 50 µm hard anodize adds roughly 25 µm. On an M6 thread with a standard 6H/6g fit, that is enough to turn a smooth assembly into a forced fit, and forced fits on aluminum threads strip.
Three practices keep this under control:
1. Anodize before threading. Cut the thread after anodizing, so the finished thread is bare aluminum and the tolerance is exactly what the drawing says. This is the cleanest solution and it is standard for precision threaded parts. The bare thread loses the anodize protection, so the corrosion engineer has to accept a compromise — usually acceptable because the exposed thread is a small area and can get a light post-treatment.
2. Machine the thread to the low side of the tolerance. If the thread must be anodized as finished, cut it undersized by the expected film growth. This works when the film thickness is repeatable, which is not always true across suppliers.
3. Post-machine or chase the thread after anodizing. Chase the anodized thread with a tap or die to restore fit. This removes the film on the flanks, again exposing metal, but keeps the rest of the part protected.
1. The fastener must conduct electricity. Anodize film is a dielectric. A sealed Type II film can withstand hundreds of volts per 10 µm, and hard anodize is rated for high voltage outright. For grounding screws, bonding clamps, bus-bar hardware, or any current-carrying joint, an anodized fastener is an insulator and a fire risk in a current path. If the aluminum fastener needs corrosion protection but must stay conductive, the correct finish is a chemical conversion coating — chromate conversion or a modern Cr⁶⁺-free alternative — not anodizing. The conductivity logic parallels what brass fasteners do for copper-alloy systems, covered in the .
2. The joint depends on precision fit. Slip-fit pins, press-fit studs, and close-tolerance assemblies do not survive a 20–50 µm film that nobody accounted for. Either machine after anodize or skip the anodize.
3. Fatigue is the life-limiting mode. Thick films — hard anodize especially — reduce fatigue life. For a dynamically loaded aluminum fastener, a thin Type I/TAS film or chemical conversion is the safer choice. This is a known aerospace trade-off, not a rumor.
4. The part sees repeated localized impact or point loading. Anodize film is hard but brittle. A sharp impact that would dent bare aluminum will chip the oxide film, and a chipped film can trap moisture against the substrate. For impact-prone service, a thicker, sealed Type III film is the better bet — or a rethink of the material choice entirely.
Salt spray testing shows up on nearly every anodize specification, and the results get read too literally. The common frames are ISO 9227 or ASTM B117 neutral salt spray. Practical expectations for aluminum fasteners:
| Condition | Typical ISO 9227 / B117 Result | Read It As |
|---|---|---|
| Bare 6061 | Pitting within 24–72 h | Bare aluminum is not a corrosion finish |
| Type II, sealed | 336+ h before visible pitting | Sealing quality is the variable that matters |
| Type II, unsealed | 96–200 h | Unsealed film absorbs salt and moisture |
| Type III hard anodize | 500+ h, minimal attack | Wear and corrosion in one finish |
| Type I / TAS thin film | 168–336 h | Thin but effective when sealed |
The honest read: the seal is more important than the film thickness for corrosion. A well-sealed 10 µm Type II film beats a poorly sealed 25 µm film in salt spray every time. The standard acceptance test for seal quality is the dye spot test (ISO 2143) or the phosphoric-chromic acid weight loss test (ISO 3210) — both check whether the pores are actually closed. Buyers who specify “anodized per ISO 7599, sealed” without a seal-quality test are trusting a word that should be a number.
Bare aluminum is anodic to stainless steel, titanium, brass, and most other fastener metals. Anodizing changes the galvanic picture because the oxide film is an insulator: a properly sealed anodized fastener does not complete an electrical couple with the mating metal, so the classic “aluminum corrodes at the stainless bolt” mechanism is suppressed.
But there is a trap. The film protects only where it is intact. At a scratch, a thread flank, or a bearing surface worn by torque, the exposed aluminum becomes a small anode connected to a large cathodic surface — and the corrosion concentrates exactly where the film is broken. A damaged anodized fastener coupled to stainless can corrode faster than a bare one, because the film directs all the attack to the defect. The field lesson: anodized aluminum fasteners in wet contact with stainless or titanium hardware must be assembled without scratching the film, and the joint should be checked for coating damage after assembly. The broader galvanic framework for aluminum is covered in the
Salt spray testing shows up on nearly every anodize specification, and the results get read too literally. The common frames are ISO 9227 or ASTM B117 neutral salt spray. Practical expectations for aluminum fasteners:
| Condition | Typical ISO 9227 / B117 Result | Read It As |
|---|---|---|
| Bare 6061 | Pitting within 24–72 h | Bare aluminum is not a corrosion finish |
| Type II, sealed | 336+ h before visible pitting | Sealing quality is the variable that matters |
| Type II, unsealed | 96–200 h | Unsealed film absorbs salt and moisture |
| Type III hard anodize | 500+ h, minimal attack | Wear and corrosion in one finish |
| Type I / TAS thin film | 168–336 h | Thin but effective when sealed |
The honest read: the seal is more important than the film thickness for corrosion. A well-sealed 10 µm Type II film beats a poorly sealed 25 µm film in salt spray every time. The standard acceptance test for seal quality is the dye spot test (ISO 2143) or the phosphoric-chromic acid weight loss test (ISO 3210) — both check whether the pores are actually closed. Buyers who specify “anodized per ISO 7599, sealed” without a seal-quality test are trusting a word that should be a number.
Bare aluminum is anodic to stainless steel, titanium, brass, and most other fastener metals. Anodizing changes the galvanic picture because the oxide film is an insulator: a properly sealed anodized fastener does not complete an electrical couple with the mating metal, so the classic “aluminum corrodes at the stainless bolt” mechanism is suppressed.
But there is a trap. The film protects only where it is intact. At a scratch, a thread flank, or a bearing surface worn by torque, the exposed aluminum becomes a small anode connected to a large cathodic surface — and the corrosion concentrates exactly where the film is broken. A damaged anodized fastener coupled to stainless can corrode faster than a bare one, because the film directs all the attack to the defect. The field lesson: anodized aluminum fasteners in wet contact with stainless or titanium hardware must be assembled without scratching the film, and the joint should be checked for coating damage after assembly. The broader galvanic framework for aluminum is covered in the
| Finish | Typical Thickness | Corrosion | Wear | Conductivity | Cost | Typical Use |
|---|---|---|---|---|---|---|
| Bare aluminum | — | Poor | Poor | Yes | Lowest | Non-corrosive indoor service |
| Chemical conversion (chromate/Cr⁶⁺-free) | 0.2–1 µm | Good | Poor | Yes | Low | Paint prep, conductive corrosion protection |
| Type II anodize | 5–25 µm | Excellent (sealed) | Fair | No | Moderate | Appearance + corrosion |
| Type III hard anodize | 25–150 µm | Excellent | Excellent | No | Higher | Wear + corrosion |
| Powder coat / paint | 60–120 µm | Good | Fair | No | Moderate | Thick cosmetic protection |
| Electroplated nickel | 5–25 µm | Good | Good | Yes* | Higher | Decorative, conductive when specified |
The decision usually comes down to two forks. First: does the fastener have to conduct? If yes, conversion coating or bare, and anodizing is out. Second: is the enemy corrosion or wear? Corrosion — Type II sealed. Wear — Type III. Both — Type III with thickness control.
Titanium sits outside this comparison entirely, because titanium does not anodize in the fastener sense — its oxide is a thin self-passivating layer, and the material gets its corrosion resistance from chemistry rather than coating, as covered in the . And on the strength side, no anodized aluminum fastener competes with alloy steel in a load-bearing joint — the strength case is in the .
One comparison worth making explicitly: anodized aluminum versus the plated steel fasteners it often replaces. An anodized aluminum screw is lighter, non-magnetic, and corrosion-stable without sacrificial coatings, but it is weaker than any steel bolt and it costs more per part than a zinc-plated steel screw. Where weight and corrosion matter more than strength, the swap is rational; where the joint carries load, it is not. The strength-side comparison is covered in the and the plating-side comparison in the
Dyeing (organic dyes). The porous film absorbs dye after anodizing, before sealing. Dyeing gives the widest color range — red, blue, green, purple — and is cheap. The weakness is lightfastness: organic dyes fade outdoors, some in months. Dyed anodize is an indoor or low-UV finish.
Electrolytic coloring. Metal salts are deposited in the pores by a second AC electrolysis step. The color range is limited — bronze, black, champagne, and architectural tones — but the color is lightfast because it is metallic, not organic. European architectural anodizing runs almost entirely on electrolytic coloring, under the QUALANOD quality scheme.
For fasteners, the practical split is: if the part lives outdoors, specify electrolytic coloring; if it is indoor cosmetic hardware, dye is fine and cheaper. Black is the most common fastener color in both routes, and black electrolytic coloring is the workhorse for outdoor black hardware.
“The threads will not go in.” The classic tolerance failure from Section 4. The film grew the pitch diameter, the assembler forced it, and the thread stripped or the oxide chipped. Fix: anodize-before-thread, undersize the thread, or chase after coating. Also torque discipline — anodized threads gall less than bare aluminum, but they are still aluminum, not steel.
“It came out of the box scratched with white marks.” White marks on dyed anodize are the oxide film fractured. Causes: handling damage before sealing is complete, soft Type II film on a wear part, or dye that was never sealed properly. Fix: match the type to the wear demand — Type III for parts that get handled — and require sealed film with a seal-quality test.
“The colors do not match between orders.” Covered in Section 8. Fix: one production run per assembly, electrolytic coloring for long-term reproducibility, and a signed color standard.
“It still corroded in the salt fog.”
| Topic | North America | Europe |
|---|---|---|
| Primary specs | MIL-A-8625 (Type I/II/III), ASTM B580 | ISO 7599 (general), ISO 10074 (hard anodize) |
| Thin-film aerospace route | MIL-A-8625 Type I chromic | TAS (tartaric-sulphuric), REACH-driven |
| Architectural quality scheme | AAMA 611 | QUALANOD |
| Coloring | Dye and electrolytic both common | Electrolytic dominant for outdoor |
| Film thickness units | µm or mil | µm |
| Cr⁶⁺ pressure | Moderate (RoHS applies to products) | High (REACH authorization burden) |
If yes — grounding, bonding, current path — anodizing is off the table. Use chemical conversion coating or bare aluminum with appropriate galvanic management. The conductivity requirement overrides everything else.
| Requirement | Finish |
|---|---|
| Corrosion in wet service | Type II, sealed, 10–25 µm |
| Wear, repeated assembly, abrasion | Type III, 25–50 µm, hardness tested |
| Appearance, color match, brand | Type II dyed or electrolytic colored |
| Fatigue-critical dynamic load | Type I / TAS thin film, or conversion coating |
| Conductive corrosion protection | Chemical conversion coating |
Check pitch and film thickness against the fit class. If the thread is fine-pitch (≤ 1.25 mm) or the film exceeds ~15 µm, plan for anodize-before-thread or post-coating chase. Verify with a functional thread gauge on coated parts.
A correct anodize spec names the standard, the type, the thickness range, the sealing requirement, the seal-quality test, the color, and the batch-control rule. “Anodized” alone is not a specification — it is a conversation starter that leaves the supplier in charge of your tolerance and your corrosion life.
It grows a hard aluminum oxide film on the surface, improving corrosion resistance, wear resistance, and appearance. The film is part of the metal and cannot peel or flake.
Yes, when the film is sealed. Sealed Type II film typically passes 336+ hours of neutral salt spray; unsealed film performs far worse.
Yes. The film grows about half inward and half outward, changing pitch diameters on threads. Fine-pitch threads and thick films need tolerance planning.
No. The oxide film is a dielectric insulator. Use chemical conversion coating instead for grounding and current-carrying joints.
Type III anodizing, typically 25–150 µm thick, with hardness above 400 HV. It resists wear and abrasion but reduces fatigue life.
A sealed, intact film insulates the couple and suppresses galvanic attack. But scratched film concentrates corrosion at the defect, so assembly must avoid film damage.
Electrolytic coloring — bronze, black, champagne — because metallic color is lightfast. Organic dyes fade outdoors and suit indoor parts only.
Thin films have minimal effect. Thick hard anodize can reduce fatigue life because the brittle film cracks under cyclic strain.
Eddy-current instruments per ASTM B244 measure film thickness on aluminum non-destructively; cross-section microscopy and weight-loss methods are the lab alternatives.
International Organization for Standardization. “ISO 10074:2019 — Anodizing of aluminium and its alloys — Specification for hard anodic oxidation coatings.”
ASTM International. “ASTM B580-79(2020) — Standard Specification for Anodic Oxide Coatings on Aluminum.”
ASTM International. “ASTM B244-22 — Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum.”
Wikipedia. “MIL-A-8625.”
QUALANOD. “Quality Label for Anodizing of Aluminium.”
MatWeb Material Property Data. “Aluminum and Anodized Coating Property Data.”
