Type II vs Type III Anodize: How to Choose and Call It Out
Choose Type II sulfuric anodize for corrosion protection, paint adhesion or color with almost no dimensional change. Choose Type III hard anodize when a surface must resist wear or provide tested electrical standoff, and accept about 0.001 inch growth per surface at the 0.002 inch default, lower fatigue strength, darker colors and higher cost.
Updated
The short answer, side by side
Type II is a thin sulfuric acid coating bought for corrosion protection, paint adhesion and color. Type III is a thick, dense sulfuric acid coating bought for wear and abrasion resistance, and it costs you dimension, fatigue margin and color choice. The table separates what the specification requires from what is typical, because several properties engineers ask about (hardness, dielectric strength) have no MIL-PRF-8625 requirement at all.
| Property | Type II (sulfuric) | Type III (hard coat) | Basis |
|---|---|---|---|
| Acceptance basis | Coating weight, 1,000 mg/ft2 minimum | Thickness, 0.002 in nominal unless specified | MIL-A-8625F requirement |
| Thickness range | 0.00007 to 0.0010 in | 0.0005 to 0.0045 in | MIL-A-8625F Table IV, typical |
| Thickness tolerance | None stated (weight governs) | Plus or minus 20 percent to 0.002 in; plus or minus 0.0004 in above | MIL-A-8625F requirement |
| Growth per surface | About half the thickness | About half the thickness | MIL-A-8625F design note; AAC guide gives 50/50 for Type III |
| Sealing default | Sealed | Unsealed when wear is the purpose | MIL-A-8625F requirement |
| Wear test | None | Taber wear index, unsealed coating | MIL-A-8625F requirement |
| Hardness | No requirement | No requirement; Rockwell C 50 to 60 (520 to 700 HV) possible under some conditions | Products Finishing, December 2015, typical |
| Dielectric strength | No requirement | No requirement; specify a breakdown test if you rely on it | AAC hard coat guideline |
| Bath temperature | 68 to 72 °F | Low temperature, glycol cooled | Products Finishing, February 2011, typical |
| Voltage and current density | 18 to 24 V, 12 to 24 A/ft2 | Up to 75 V, 24 to 40 A/ft2 | Products Finishing, February 2011, typical |
| Color | Preferred base for dyes, wide range | Natural light tan to black; dyes limited to darker colors | MIL-A-8625F 3.12; AAC reference guide |
| Fatigue | Reduces fatigue strength | Reduces it more; thicker coating, greater reduction | MIL-A-8625F 6.10.7 |
| Related AMS specs | AMS 2471 undyed, AMS 2472 dyed | AMS 2469; AMS 2482 PTFE | SAE Mobilus listings |
The spec text, coating weights and seal chemistries are explained on our MIL-PRF-8625 anodizing reference. This page is about the decision between the two types, which that page does not cover.
Thickness, growth and dimensional planning
The two types differ by roughly an order of magnitude in thickness, so they need different drawing practice. A Type II part can usually be machined to finished size; a Type III part needs a pre-coat dimension.
MIL-A-8625F's design notes state that each coated surface grows by about one half of the applied coating thickness, and that both a machining dimension and a coated dimension should go on the drawing. The Aluminum Anodizers Council (AAC) reference guide describes Type III the same way: 50 percent penetration and 50 percent buildup per surface. That is the source of the "half in, half out" rule, and it is published for Type III; the spec states the half growth rule generally.
| Case | Coating | Growth per surface | Change in a diameter | Change in 60° thread pitch diameter |
|---|---|---|---|---|
| Type II at minimum weight on 6061-T6 | 0.000103 in (Table V typical) | About 0.00005 in | About 0.0001 in | About 0.0002 in |
| Type II at top of typical range | 0.0010 in | About 0.0005 in | About 0.0010 in | About 0.0020 in |
| Type III at default | 0.0020 in (0.0016 to 0.0024 at plus or minus 20 percent) | About 0.0010 in (0.0008 to 0.0012) | About 0.0020 in | About 0.0040 in |
The thread column uses the spec's rule that 60 degree thread pitch diameter grows by four times the per-surface growth. The arithmetic is ours; check it against your own tolerance stack. Three practical consequences follow.
- Tolerance budget. At the default Type III thickness, the coating tolerance alone moves a diameter by up to about 0.0008 in. Close fits either get masked, lapped after coating, or are designed around the coated dimension.
- Order of operations for Type III. The spec notes say Type III thickness can be held as tightly as plus or minus 0.0001 in with specialized controls, and that Type III can be ground or lapped to remove surplus coating at assembly. Type II and thinner coatings are too thin and brittle for that.
- Rework. Because half of a hard coat penetrates the base metal, stripping and recoating a Type III surface consumes metal that cannot be put back. The spec advises against hard coat on surfaces that would normally be restored to size during rework.
Edge radii and hole coverage rules apply to both types but bite harder on Type III; they are listed on the anodizing reference.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Hardness and wear: what Type III actually buys
Type III is the only type with a wear requirement. MIL-A-8625F applies a Taber abrasion test to unsealed Type III coatings and has no wear test for Type II, so if a surface slides, rubs or sees abrasive media, Type II gives you no contractual wear property at all.
Hardness is not a MIL-PRF-8625 acceptance criterion for either type. A Products Finishing answer (December 2015) says there is no "typical" hardness for Type III, that it depends on alloy and anodizing conditions, and that Rockwell C 50 to 60 (520 to 700 HV) can be produced under certain conditions. The AAC hard coat guideline adds that hardness results cannot be used to prove wear resistance. If a vendor quotes a hardness number, treat it as information, not acceptance.
Two spec notes change how engineers should order Type III for wear:
- Thicker is not more wear resistant. MIL-A-8625F note 6.17 says abrasion resistance decreases as Type III thickness approaches 0.003 in and in general does not increase with thickness. Ordering 0.003 or 0.004 in "for extra wear life" can do the opposite; extra thickness is for dimensional build or insulation, not wear.
- Sealing softens it. The spec and the AAC guide both say sealing reduces wear resistance. Leave wear surfaces unsealed unless corrosion is also a requirement.
For sliding contact, consider Type III with a PTFE treatment to AMS 2482, or a bonded solid film lubricant over the hard coat; the options are covered on our dry film lubricants page.
Corrosion and sealing trade-offs
For corrosion alone, a sealed Type II coating is the normal answer, and it is the default. Types II and IIB are fully sealed unless the order says otherwise, while Type III is left unsealed when wear is the purpose and must be ordered sealed if it also needs corrosion protection.
- Type II, sealed: the spec's 336 hour salt spray test applies. Dyed Type II parts can get a duplex nickel acetate and dichromate seal for better corrosion resistance, at the cost of a slight color shift.
- Type III, unsealed: maximum wear; Amendment 1 to MIL-A-8625F requires the corrosion test on Type III only when sealing is specified. The AAC guide recommends a 5 percent dichromate seal when corrosion resistance matters and warns that some softening results.
- Type III, sealed: corrosion protection plus a reduced wear property. If one part has both wear faces and exposed faces, discuss with the processor whether the drawing can accept one seal condition for the whole part, because selective sealing is not a standard callout.
Dichromate seals contain hexavalent chromium, which matters on DoD contracts carrying DFARS 252.223-7008. Nickel acetate or hot water seals avoid that question. How the clause works is explained on our chem film page; if the part only needs corrosion protection and electrical conductivity, chem film may be a better fit than either anodize type.
Dielectric properties
Hard coat is often chosen as an electrical insulator, but MIL-PRF-8625 sets no dielectric requirement for any type, so insulation has to be specified and tested separately. The AAC hard coat guideline defines a "dielectric" application type for which an electrical breakdown test applies, measured to ISO 2376 or EN 12373-17.
Breakdown voltage scales mainly with oxide thickness, which is why Type III is the type used for insulation. For a rough figure, the anodizer SAF cites the CRC Handbook of Chemistry and Physics value for alumina, 40 to 160 V/mil, and says an anodic coating should be about the same. That is a material reference range, not a coating acceptance value. At that range, a 0.0001 in Type II coating has little useful standoff and a 0.002 in hard coat is in the hundreds of volts, before any allowance for defects.
- MIL-A-8625F notes that anodic coatings develop voids at sharp edges and corners, and all anodic coatings are brittle and can crack under force fits. Edges, fastener bearing faces and press-fit bores are where insulation fails first.
- If isolation is a safety or EMI requirement, put a breakdown voltage, a test method and a sample plan on the drawing, and consider a design that does not rely on the oxide alone.
- Conversely, anodize is not conductive. Grounding and bonding points must be masked, or the surface must get chem film Class 3 instead.
Fatigue effects
Both types reduce fatigue strength, and Type III reduces it more. MIL-A-8625F note 6.10.7 states that the fatigue properties of aluminum alloys can be severely reduced by anodic coatings, that the reduction varies with the process, and that as a general rule the thicker the coating, the greater the reduction. Note 6.1.3 repeats that hard coatings may reduce fatigue strength on cyclically loaded parts.
The spec itself shows how seriously that is taken: thin chromic acid Types I and IB are used on fatigue critical parts because of their thinness, and the Type IC and IIB coating weight maximums exist for fatigue. No published spec value tells you the knockdown for your alloy, coating and stress ratio; that comes from test data held by the design authority.
- On a cyclically loaded part, call Type III only on the faces that need wear resistance and mask the rest, especially fillets, holes and other stress concentrations.
- Changing an existing Type II callout to Type III, or adding hard coat to a previously bare part, is a stress analysis change. Get the stress engineer's acceptance in writing.
- Alloy copper content also matters: alloys with 2 percent or more copper (all 2xxx, 7050 and 7178, per the spec notes) get a looser Type III wear limit, and the spec notes that 2 mils or more of hard coat is extremely difficult to obtain on 360, 380 and 383 die castings. Alloy behavior is covered on our aluminum alloys page.
Dye and color
If appearance or color coding matters, Type II is the right base. MIL-A-8625F says items to be dyed should preferably be coated by the Type II process, and Type II produces the widest color range.
Type III has its own color whether you dye it or not. The AAC reference guide gives the natural color of hard coat as light tan to black depending on alloy and thickness, and says it can be dyed in darker colors depending on thickness. In practice that means black is the common dyed hard coat color, and bright reds, blues or golds should be expected on Type II rather than Type III.
- For either type, color and allowable variation must be on the drawing. The spec only requires uniformity on wrought parts; castings may bleed or show uneven color.
- Light fastness testing applies only to dyed coatings and only when ordered, with a default limit of Delta E 3 after exposure.
- An undyed Type III part from a high copper alloy will look different from the same callout on 6061. If two parts must match, put both alloys and a color sample on the order.
Cost and availability drivers
Type III almost always costs more than Type II for the same part, and the reasons are process time, equipment, masking and inspection rather than chemistry. No published price applies to your part, so treat the following as drivers to discuss with the processor.
- Tank time and power. Products Finishing gives a common estimate: anodizing time in minutes equals mils of coating times 720 divided by current density in A/ft2. By that formula, 2 mils of hard coat at 24 to 40 A/ft2 takes roughly 36 to 60 minutes, against roughly 15 to 30 minutes for 0.5 mil of Type II at 12 to 24 A/ft2. Hard coat also needs higher voltage rectifiers and refrigerated, glycol cooled tanks.
- Fewer lines. Because hard coat needs cold tanks and high voltage rectifiers, not every line that runs Type II can run Type III, and the pool narrows again when a prime flows down approved sources. Ask about capacity early when lead time matters.
- Masking. Type III callouts usually come with masked threads, bores, grounding points and fatigue sensitive features. Masking is hand labor and can be a large share of the cost.
- Inspection. Type III is accepted on thickness by eddy current, with cross sections in a dispute, and on a Taber wear test; Type II is accepted on coating weight panels. Extra tests (breakdown voltage, light fastness) add cost only if you order them, so order only what the function needs.
- Rework risk. A hard coat that misses thickness cannot simply be stripped and redone on a close tolerance part, because stripping removes base metal.
Our special process cost page covers lot charges, minimums and how routing affects price.
Choose Type II when, choose Type III when
| Choose Type II when | Choose Type III when |
|---|---|
| The need is corrosion protection or a paint or adhesive base | A surface slides, rubs, or sees abrasive or erosive wear |
| The part needs a specific color or color coding | The surface needs electrical standoff you will verify by test |
| Tolerances cannot absorb 0.002 in of diametral change | You can machine to a pre-coat dimension or lap after coating |
| The part is fatigue sensitive and only needs environmental protection | The wear face can be masked away from fatigue critical features |
| The alloy is a die casting or high copper alloy where hard coat is limited | The alloy is suited to hard coat, such as 6061 or 7075 |
| Cost and supplier availability dominate | Function justifies the higher process cost and smaller supplier pool |
A common pattern on flight and ground hardware is Type II on the whole part with Type III only where function demands it, which requires two masking steps and two callouts. If that is the plan, say so on the drawing so the processor quotes both operations. The basic choice among anodize, chem film and passivation is on our surface finishing overview.
Callout examples for the drawing
These are example formats, not templates to copy without engineering review. Confirm the current revision of each document on ASSIST or SAE before release.
- Type II, black: ANODIZE PER MIL-PRF-8625F (2), TYPE II, CLASS 2, COLOR BLACK, SEALED (NICKEL ACETATE). DIMENSIONS APPLY AFTER ANODIZE. MASK AREAS MARKED "A".
- Type II, clear, AMS: ANODIZE PER AMS 2471. DIMENSIONS APPLY AFTER ANODIZE.
- Type III, wear face: HARD ANODIZE PER MIL-PRF-8625F (2), TYPE III, CLASS 1, 0.0020 PLUS OR MINUS 0.0004 IN THICK, UNSEALED, SURFACES "B" ONLY. MACHINE SURFACES "B" TO PRE-COAT DIMENSION SHOWN. MASK ALL THREADS.
- Type III, insulating: HARD ANODIZE PER MIL-PRF-8625F (2), TYPE III, CLASS 1, 0.0020 IN MIN THICK, SEALED. DIELECTRIC BREAKDOWN TEST PER ISO 2376, MINIMUM VOLTAGE AS SPECIFIED BY ENGINEERING. MASK GROUND PADS "C".
- Type III, AMS: HARD ANODIZE PER AMS 2469, thickness, seal and masking as above.
What to put on the PO
- Spec and revision for each type on the part, and whether a prime spec or approved source list overrides it
- Alloy and temper for every part in the lot, so the processor does not run mixed alloys in one load
- Type III thickness and tolerance, and whether drawing dimensions are before or after coating
- Seal condition per type, and any restriction on hexavalent chromium seals under DFARS 252.223-7008
- Color, color sample and allowable variation for dyed work
- Masked areas, referenced to drawing views, including threads, bores and grounding points
- Tests beyond the spec defaults: breakdown voltage, light fastness, extra thickness readings
- Nadcap chemical processing or customer approved source requirement
- A certificate per lot naming spec, revision, type, class, seal, thickness or coating weight results and the lot number
How anodize fits in a routing with heat treat, NDT and paint is on our special processes page. For the plating side of the coatings decision, see cadmium vs zinc-nickel plating, and the full coatings library.
Get Type II or Type III anodize sourced to the callout
Send the drawing, alloy and finish notes through the special process request form. If a part carries both Type II and Type III, we source processors who can run both with the masking it needs, flow down the type, class, thickness, seal and any dielectric or wear test, and check each cert against the drawing before parts ship. A person replies within one business day. We sign a mutual NDA before files, and controlled drawings follow our controlled programs process.
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Questions
Is Type III anodize better than Type II for corrosion?
Not by default. Type III is left unsealed when wear is the purpose, and Amendment 1 to MIL-A-8625F requires the Type III salt spray test only when sealing is specified. Type II is sealed by default and tested to 336 hours. Type III can be ordered sealed for corrosion, but sealing reduces its wear resistance. If the part only needs corrosion protection, a sealed Type II coating is usually the right and cheaper callout.
How much does hard anodize change part dimensions?
About half the coating thickness per surface. At the 0.002 inch default, each surface grows about 0.001 inch, a diameter about 0.002 inch, and a 60 degree thread pitch diameter about 0.004 inch. With the plus or minus 20 percent thickness tolerance, growth per surface ranges from about 0.0008 to 0.0012 inch. Put both a pre-coat machining dimension and a coated dimension on the drawing, as MIL-A-8625F recommends.
Should I order thicker hard coat for more wear life?
No. MIL-A-8625F note 6.17 says Type III abrasion resistance decreases as thickness approaches 0.003 inch and in general does not increase with thickness. Extra thickness is useful for dimensional build or electrical standoff, not wear. For longer wear life, leave the coating unsealed, choose an alloy suited to hard coat such as 6061 or 7075, and consider a PTFE treatment or dry film lubricant on sliding faces.
What is the dielectric strength of hard anodize?
MIL-PRF-8625 sets no dielectric requirement, so there is no spec value. Breakdown depends mostly on thickness, plus sealing, alloy and defects. The anodizer SAF cites an alumina range of 40 to 160 volts per mil from the CRC handbook as a rough guide. If insulation matters, specify a minimum breakdown voltage and test method, such as ISO 2376, as the AAC hard coat guideline does.
Can Type III hard anodize be dyed red or blue?
Usually not well. The Aluminum Anodizers Council says hard coat ranges naturally from light tan to black depending on alloy and thickness and can be dyed in darker colors. Black is the common dyed hard coat. MIL-A-8625F says dyed items are preferably coated by the Type II process, so bright colors belong on Type II. Put the color and allowable variation on the drawing either way.
Does hard anodize reduce fatigue strength?
Yes, more than Type II. MIL-A-8625F says anodic coatings can severely reduce aluminum fatigue properties and that thicker coatings generally cause greater reductions; it also warns that hard coatings may reduce fatigue strength on cyclically loaded parts. Mask hard coat away from fillets, holes and other stress concentrations, and treat any change from Type II to Type III as a stress analysis change.
Related
Sources
- DLA ASSIST copy via ibiblio: MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys (10 September 1993), including design notes 6.10 to 6.17
- DLA ASSIST copy via ibiblio: MIL-A-8625F Amendment 1 (15 September 2003)
- Finishing and Coating (March 2021): Rollout of MIL-PRF-8625 Rev F and implications for anodizers
- Aluminum Anodizers Council: Anodizing Reference Guide
- Aluminum Anodizers Council: Application guideline for selection of hard anodic oxide (hardcoat) finishes
- Products Finishing (December 2015): Typical hardness of Type III hardcoat anodized coatings
- Products Finishing (February 2011): Aluminum Anodizing, process conditions and time estimate
- SAF: Is the anodic coating a good electrical insulator
- SAE Mobilus: AMS2469 hard anodic coating treatment (revision history)
- SAE Mobilus: AMS2471 sulfuric acid anodize, undyed (revision history)
- SAE Mobilus: AMS2482F hard anodic coating, PTFE impregnated or codeposited
- Acquisition.gov: DFARS 252.223-7008 Prohibition of Hexavalent Chromium (January 2023)
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