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Boric-Sulfuric Acid Anodizing Type IC

Anodizing & conversion coatings · Electrolytic bath

Boric-sulfuric acid anodizing is a thin aerospace aluminum anodize recognized as MIL-A-8625 Type IC.

CNC machiningSheet metalPremium cost $$$Insulating
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Boric-Sulfuric Acid Anodizing Type IC on a machined part

What is BSA anodize?

It was developed as a non-chromic alternative to Type I for corrosion protection and bonded or fatigue-sensitive structures.

How it works

The aluminum workpiece is anodized in a mixed boric- and sulfuric-acid electrolyte under a controlled voltage cycle. The process creates a thin porous aluminum-oxide film, commonly about 2.5–7.5 µm, with lower dimensional and fatigue effects than conventional Type II. The film may be sealed for corrosion service or left in a qualified condition for adhesive bonding. Although the anodizing bath avoids chromic acid, downstream seal chemistry must also be checked before making a chromium-free claim.

Why specify it

  • Typical 2.5–7.5 µm thickness limits buildup
  • Avoids chromic acid in the anodizing bath
  • Lower fatigue effect than thick Type II or Type III
  • Established aerospace replacement for Type I on approved programs

Where it falls short

  • Not interchangeable with Type I without drawing approval
  • Less decorative and wear-resistant than Type II
  • Electrical contacts still require masking
  • Seal chemistry can defeat a chromium-free objective

BSA anodize at a glance

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

Corrosion protectionVery good71 / 100
Surface hardnessFair52 / 100
Cosmetic qualityLow30 / 100
LubricityLow18 / 100
Relative costPremium50 / 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.

BSA 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.

Thickness3–8 µm0.1–0.3 mil
Hardness180–350 HV
Salt spray336 h
Max service temp.80 °C176 °F
Thickness
Approximately 2.5–7.5 µm for aerospace boric-sulfuric anodizing.
Hardness
180–350 HV — Typical thin anodic oxide; hardness is not usually the acceptance criterion.
Corrosion (salt spray)
336 h per ASTM B117, sealed, per MIL-A-8625 — Applies to qualified sealed specimens.
Electrical
Insulating
Colours
  • natural gray
Appearance
Thin translucent clear-gray film with subdued iridescence.

Where BSA 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. BSA 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 54Type I anodize — corrosion 71, hardness 49Hard 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 & POWDERType I anodizeHard anodizePowder coatZinc plateZinc-nickelHard chromeDLCTiN PVDTin plateCadmium plateE-coatCerakoteWet paintHot-dip zincNitridingHVOF coatingBSA anodize — corrosion 71, hardness 52BSA 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 54Type I anodize — corrosion 71, hardness 49Hard 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 & POWDERType I anodizePowder coatZinc plateBSA anodize — corrosion 71, hardness 52BSA anodize
Anodize & conversionPlatingPaint & powderHard coatings

Specifications for BSA 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 IC Class 10.5–25.4 µmNon-chromic alternative to Type I or IB, non-dyed. · 200–700 mg/ft² unsealed (0.0215–0.0753 g/dm²); higher weight only when specified. · Coating weight; sealed corrosion test, ASTM B117, 336 h.Supersedednow MIL-PRF-8625

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

Drawing call-out

BORIC-SULFURIC ACID ANODIZE PER MIL-A-8625 TYPE IC 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 BSA anodize?

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

Material familyVerdictNotes
Aluminum
YesQualified primarily for aerospace wrought aluminum.
  • Program and alloy approval remain necessary
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 BSA anodize is applied, and what it does to the part

  1. 1Clean
  2. 2Deoxidize
  3. 3Rack and mask
  4. 4Boric-sulfuric anodize
  5. 5Rinse
  6. 6Seal or prepare for bonding
  7. 7Inspect
DimensionsAllow roughly 1.25–3.75 µm outward growth per surface.
Build-up per side1–4 µm — Approximately half of the oxide grows outward.
MaskingUsually needed. Mask electrical contacts, precision fits, and adhesive-bond areas when the approved bonding process requires it.
ElectricalInsulating
Temperature limit80 °C (176 °F) continuous
AppearanceThin translucent clear-gray film with subdued iridescence.
Added lead time5–10 working days on top of the bare part

Design rules for BSA anodize

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

  1. Confirm program approval for Type IC.A legacy Type I drawing cannot be changed by the processor.
  2. State sealed or bonding condition.The final pore treatment changes adhesion and corrosion.
  3. Audit the complete chemistry, including seal.A non-chromic bath does not guarantee a chromium-free finished process.
  4. Allow about half the oxide thickness outward.Even thin anodize affects precision diameters.

BSA anodize versus the alternatives

Finishes engineers weigh against BSA anodize, and when each wins.

FinishCorrosionHardnessCosmeticCostThicknessChoose it when
BSA anodize
this page
715230$$$3–8 µm—
Chromic Acid Anodizing Type I
BSA anodize vs Chromic Acid Anodizing Type I →
714928$$$0.5–7.5 µmwhen the legacy chromic process is mandatory
Anodizing Type II
BSA anodize vs Anodizing Type II →
715488$$5–25 µmwhen decorative color is required
CorrosionHardnessCosmeticLubricityEconomy
BSA anodizeChromic Acid Anodizing Type I

The radar overlays BSA anodize with Chromic Acid Anodizing Type I, its most common alternative — chosen when the legacy chromic process is mandatory. Scores in the table are 0–100 across every finish in the catalog.

Cost, lead time and compliance

Relative cost

Boric-Sulfuric Acid Anodizing Type IC is a premium-cost finish ($$$): more expensive than about 57 % 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
RoHSCompliant
NotesThe anodizing bath avoids hexavalent chromium; verify the seal chemistry and customer restrictions.

What BSA anodize is used for

  • Aerospace skins and brackets
  • Adhesively bonded aluminum structures
  • Fatigue-sensitive aircraft parts
  • Chromic-acid replacement programs

BSA anodize — frequently asked

What is boric-sulfuric acid anodizing?

It is MIL-A-8625 Type IC, a thin aluminum anodize developed as an alternative to chromic-acid Type I. Typical thickness is about 2.5–7.5 µm.

Is Type IC anodize chromium free?

The boric-sulfuric anodizing bath avoids chromic acid. The complete process is chromium-free only if the selected seal and all post-treatments are also chromium-free.

Type IC vs Type II anodize: which is thicker?

Type IC is typically about 2.5–7.5 µm. Type II is commonly 5–25 µm and offers more decorative dye capacity and wear resistance.

Does boric-sulfuric anodize add thickness?

Yes; approximately half grows outward. A 6 µm Type IC film adds about 3 µm per treated surface.

Can Type IC replace chromic Type I?

Only when the design authority approves the substitution. Aerospace qualification is tied to the drawing, processor, alloy, and complete sealed or bonding process.