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Chromic Acid Anodizing Type I

Anodizing & conversion coatings · Electrolytic bath

Type I and low-voltage Type IB chromic-acid anodizing form thin, corrosion-resistant oxide on aluminum with less fatigue penalty than thick sulfuric anodize.

CNC machiningSheet metalPremium cost $$$Insulating
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Chromic Acid Anodizing Type I on a machined part

What is Type I anodize?

Aerospace drawings use MIL-A-8625 Type I or IB where trapped electrolyte, bonded structures, or fatigue sensitivity matter.

How it works

Aluminum is made anodic in a chromic-acid electrolyte while voltage is raised through a controlled cycle. The resulting oxide is thinner and more flexible than conventional sulfuric anodize, commonly about 0.5–7.5 µm depending on Type I or IB process and drawing requirement. Approximately half of the oxide grows outward. The porous film is normally sealed unless bonding requirements state otherwise. Chromic acid contains hexavalent chromium, so use is increasingly restricted despite its aerospace process tolerance.

Why specify it

  • Thin 0.5–7.5 µm oxide limits dimensional change
  • Lower fatigue debit than thick sulfuric or hard anodize
  • Good corrosion protection after sealing
  • Traditional aerospace choice for complex and bonded structures

Where it falls short

  • Hexavalent chromium creates major regulatory burden
  • Few decorative color options
  • Lower wear resistance than Type II and Type III
  • Not suitable for electrical contact surfaces

Type I anodize at a glance

Type I anodize is strongest on corrosion protection (71 of 100), at a premium cost.

Corrosion protectionVery good71 / 100
Surface hardnessFair49 / 100
Cosmetic qualityLow28 / 100
LubricityLow18 / 100
Relative costPremium48 / 100
CorrosionHardnessCosmeticLubricityEconomy

Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.

Type I anodize key numbers

Typical values from the governing specifications and shop practice; bars are this finish against the highest value among anodizing & conversion coatings finishes.

Thickness0.5–7.5 µm0.02–0.3 mil
Hardness150–300 HV
Salt spray336 h
Max service temp.80 °C176 °F
Thickness
Approximately 0.5–7.5 µm depending on Type I or low-voltage Type IB process and drawing requirement.
Hardness
150–300 HV — Typical thin chromic anodic film; not normally specified by hardness.
Corrosion (salt spray)
336 h per ASTM B117, sealed, per MIL-A-8625 — Qualification performance is measured on standard specimens.
Electrical
Insulating
Colours
  • natural gray
Appearance
Thin translucent gray to slightly iridescent film.

Where Type I anodize sits among all finishes

Surface hardness (Vickers, log scale) against corrosion protection (salt-spray hours, log scale) for every coating FabDigit offers, one cloud per finish group. Type I anodize is highlighted; hover a dot for its name or open the full map.

02550751000255075100Corrosion protection (0–100)Surface hardness (HV, log scale, 0–100)Plating — 19 finishesHard coatings — 17 finishesAnodize & conversion — 8 finishesPaint & powder — 7 finisheshard, low corrosion protectionhard and protectiveprotective, softsoft, little protectionType II anodize — corrosion 71, hardness 54BSA anodize — corrosion 71, hardness 52Hard anodize — corrosion 71, hardness 61PTFE hard anodize — corrosion 71, hardness 61Nitriding — corrosion 55, hardness 73PEO — corrosion 70, hardness 74Sherardizing — corrosion 83, hardness 56Brass plate — corrosion 35, hardness 44Bronze plate — corrosion 40, hardness 49Cadmium plate — corrosion 70, hardness 26Copper plate — corrosion 32, hardness 39Decorative chrome — corrosion 64, hardness 71Electroless copper — corrosion 20, hardness 41Electroless nickel — corrosion 70, hardness 68Nickel plate — corrosion 51, hardness 55Gold plate — corrosion 68, hardness 39Hard chrome — corrosion 40, hardness 73Hot-dip zinc — corrosion 90, hardness 42Nickel-PTFE — corrosion 72, hardness 57Silver plate — corrosion 55, hardness 39Solder plate — corrosion 42, hardness 9Tin plate — corrosion 34, hardness 0Zinc plate — corrosion 60, hardness 30Zinc-nickel — corrosion 86, hardness 60Cerakote — corrosion 83, hardness 27E-coat — corrosion 83, hardness 15Epoxy coat — corrosion 83, hardness 15Powder coat — corrosion 83, hardness 14Wet paint — corrosion 59, hardness 10AlCrN PVD — corrosion 58, hardness 93AlTiN PVD — corrosion 52, hardness 94CrN PVD — corrosion 62, hardness 83DLC — corrosion 55, hardness 93MoS2 PVD — corrosion 20, hardness 26TiAlN PVD — corrosion 50, hardness 93TiCN PVD — corrosion 40, hardness 92TiB2 PVD — corrosion 40, hardness 94TiN PVD — corrosion 45, hardness 87WC/C PVD — corrosion 48, hardness 77ZrN PVD — corrosion 52, hardness 90CVD alumina — corrosion 70, hardness 85CVD TiCN — corrosion 38, hardness 92CVD TiN — corrosion 45, hardness 87Multilayer CVD — corrosion 62, hardness 88HVOF coating — corrosion 78, hardness 77Plasma spray — corrosion 60, hardness 68Plastic chrome — corrosion 72, hardness 71Electroless plastic — corrosion 58, hardness 39Clear coat — corrosion 20, hardness 12Soft-touch — corrosion 20, hardness 5PLATINGHARD COATINGSANODIZE & CONVERSIONPAINT & POWDERBSA anodizeType II anodizePowder coatZinc plateZinc-nickelHard chromeDLCTiN PVDTin plateCadmium plateE-coatCerakoteWet paintHot-dip zincNitridingHVOF coatingType I anodize — corrosion 71, hardness 49Type I anodize
02550751000255075100Corrosion protection (0–100)Surface hardness (HV, log scale, 0–100)Plating — 19 finishesHard coatings — 17 finishesAnodize & conversion — 8 finishesPaint & powder — 7 finishesType II anodize — corrosion 71, hardness 54BSA anodize — corrosion 71, hardness 52Hard anodize — corrosion 71, hardness 61PTFE hard anodize — corrosion 71, hardness 61Nitriding — corrosion 55, hardness 73PEO — corrosion 70, hardness 74Sherardizing — corrosion 83, hardness 56Brass plate — corrosion 35, hardness 44Bronze plate — corrosion 40, hardness 49Cadmium plate — corrosion 70, hardness 26Copper plate — corrosion 32, hardness 39Decorative chrome — corrosion 64, hardness 71Electroless copper — corrosion 20, hardness 41Electroless nickel — corrosion 70, hardness 68Nickel plate — corrosion 51, hardness 55Gold plate — corrosion 68, hardness 39Hard chrome — corrosion 40, hardness 73Hot-dip zinc — corrosion 90, hardness 42Nickel-PTFE — corrosion 72, hardness 57Silver plate — corrosion 55, hardness 39Solder plate — corrosion 42, hardness 9Tin plate — corrosion 34, hardness 0Zinc plate — corrosion 60, hardness 30Zinc-nickel — corrosion 86, hardness 60Cerakote — corrosion 83, hardness 27E-coat — corrosion 83, hardness 15Epoxy coat — corrosion 83, hardness 15Powder coat — corrosion 83, hardness 14Wet paint — corrosion 59, hardness 10AlCrN PVD — corrosion 58, hardness 93AlTiN PVD — corrosion 52, hardness 94CrN PVD — corrosion 62, hardness 83DLC — corrosion 55, hardness 93MoS2 PVD — corrosion 20, hardness 26TiAlN PVD — corrosion 50, hardness 93TiCN PVD — corrosion 40, hardness 92TiB2 PVD — corrosion 40, hardness 94TiN PVD — corrosion 45, hardness 87WC/C PVD — corrosion 48, hardness 77ZrN PVD — corrosion 52, hardness 90CVD alumina — corrosion 70, hardness 85CVD TiCN — corrosion 38, hardness 92CVD TiN — corrosion 45, hardness 87Multilayer CVD — corrosion 62, hardness 88HVOF coating — corrosion 78, hardness 77Plasma spray — corrosion 60, hardness 68Plastic chrome — corrosion 72, hardness 71Electroless plastic — corrosion 58, hardness 39Clear coat — corrosion 20, hardness 12Soft-touch — corrosion 20, hardness 5PLATINGHARD COATINGSANODIZE & CONVERSIONPAINT & POWDERBSA anodizePowder coatZinc plateType I anodize — corrosion 71, hardness 49Type I anodize
Anodize & conversionPlatingPaint & powderHard coatings

Specifications for Type I anodize

The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard, and US Government documents are free to download there.

StandardType / ClassThicknessRequirement / testStatus
MIL-A-8625
Anodic Coatings for Aluminum and Aluminum Alloys · DoD
Type I Class 10.5–7.6 µmChromic-acid anodize, non-dyed. · At least 200 mg/ft² unsealed (0.0215 g/dm²). · Coating weight; sealed corrosion test, ASTM B117, 336 h.Supersedednow MIL-PRF-8625
Type IB Class 10.5–7.6 µmLow-voltage chromic-acid anodize, non-dyed. · At least 200 mg/ft² unsealed (0.0215 g/dm²). · Coating weight; sealed corrosion test, ASTM B117, 336 h.

Test and related methods: ASTM B117 (Standard Practice for Operating Salt Spray (Fog) Apparatus).

Drawing call-out

CHROMIC ACID ANODIZE PER MIL-A-8625 TYPE IB CLASS 1, SEALED; MASK ELECTRICAL BONDING SURFACES

Pattern: ANODIZE PER MIL-A-8625 TYPE <I|IB|IC|II|IIB|III>, CLASS <1|2>, [COLOR], [THICKNESS FOR TYPE III], [SEAL], [MASKING]

Which materials take Type I anodize?

Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.

Material familyVerdictNotes
Aluminum
YesUsed principally on wrought aerospace aluminum.
  • Alloy-specific process qualification is required
  • Fatigue-sensitive parts still require engineering review
DMLS / SLM
With careOnly aluminum DMLS/SLM parts can be processed; steel and titanium builds cannot.
Brass
NoBrass is not anodized by this aluminum process.
Copper Alloy
NoCopper alloys are not anodized by this aluminum process.
FDM
NoFused-filament polymers are incompatible with this process.
Magnesium Alloy
NoMagnesium requires a magnesium-specific anodic or conversion process.
MJF
NoPolymer powder-bed prints are incompatible with this process.
Polymer
NoOrganic polymers cannot take this inorganic metal-finishing process.
SLA
NoPhotopolymer prints are incompatible with this process.
SLS
NoPolymer powder-bed prints are incompatible with this process.
Stainless Steel
NoStainless steel is not anodized by MIL-A-8625 processes.
Steel
NoThis is an aluminum anodic process; steel does not form this oxide.
Titanium
NoTitanium uses a different anodizing chemistry.
Zinc Alloy
NoZinc alloys are not anodized by this aluminum process.

How Type I anodize is applied, and what it does to the part

  1. 1Clean
  2. 2Etch / deoxidize
  3. 3Rack and mask
  4. 4Chromic-acid anodize
  5. 5Rinse
  6. 6Seal if specified
  7. 7Inspect
DimensionsAllow approximately 0.25–3.75 µm outward growth per treated surface.
Build-up per side0.3–3.8 µm — Approximately half of the anodic oxide projects outward.
MaskingUsually needed. Mask electrical contacts, tight fits, threads, and bonding areas when unsealed oxide or bare aluminum is required.
ElectricalInsulating
Temperature limit80 °C (176 °F) continuous
AppearanceThin translucent gray to slightly iridescent film.
Added lead time5–10 working days on top of the bare part

Design rules for Type I anodize

What to decide on the drawing before the part goes to the finisher.

  1. State Type I or Type IB explicitly.The voltage cycle and resulting film differ.
  2. Confirm hexavalent-chromium authorization.Many programs restrict chromic-acid processing.
  3. Define sealed or unsealed condition.Bond adhesion and corrosion performance differ.
  4. Provide rack-contact and drainage locations.Anodizing requires current contact and complete rinsing.

Type I anodize versus the alternatives

Finishes engineers weigh against Type I anodize, and when each wins.

FinishCorrosionHardnessCosmeticCostThicknessChoose it when
Type I anodize
this page
714928$$$0.5–7.5 µm—
Boric-Sulfuric Acid Anodizing Type IC
Type I anodize vs Boric-Sulfuric Acid Anodizing Type IC →
715230$$$3–8 µmwhen a non-chromic aerospace thin anodize is approved
Anodizing Type II
Type I anodize vs Anodizing Type II →
715488$$5–25 µmwhen decorative color and broader availability matter
CorrosionHardnessCosmeticLubricityEconomy
Type I anodizeBoric-Sulfuric Acid Anodizing Type IC

The radar overlays Type I anodize with Boric-Sulfuric Acid Anodizing Type IC, its most common alternative — chosen when a non-chromic aerospace thin anodize is approved. Scores in the table are 0–100 across every finish in the catalog.

Cost, lead time and compliance

Relative cost

Chromic Acid Anodizing Type I is a premium-cost finish ($$$): more expensive than about 55 % of the finishes FabDigit quotes.

Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates. The exact figure appears on the instant quote once the part is uploaded.

Lead time and compliance

Added lead time5–10 working days
RoHSConditional
NotesChromic acid contains hexavalent chromium; use requires regulatory and customer approval.

What Type I anodize is used for

  • Aerospace sheet structures
  • Bonded aluminum assemblies
  • Fatigue-sensitive brackets
  • Complex parts with difficult rinsing geometry

Type I anodize — frequently asked

What is Type I anodizing?

Type I is chromic-acid anodizing under MIL-A-8625. It typically forms a thin 0.5–7.5 µm oxide for aerospace corrosion protection with limited dimensional and fatigue effects.

What is Type IB anodizing?

Type IB is a low-voltage chromic-acid variant. It remains a hexavalent-chromium process and must be called out separately from Type I.

Type I vs Type II anodize: what is the difference?

Type I uses chromic acid and is thinner, commonly 0.5–7.5 µm. Type II uses sulfuric acid, commonly 5–25 µm, and is better suited to dyed decorative finishes.

Does chromic acid anodizing add thickness?

Yes, approximately half the oxide grows outward. A 6 µm film adds roughly 3 µm per treated surface.

Is Type I anodizing RoHS compliant?

The process uses hexavalent chromium and normally cannot be treated as a straightforward RoHS-compliant choice. Program-specific exemptions and supply-chain declarations require review.