MIL-A-8625 Sulfuric Anodize

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Types of Anodic coatings AOTCO Offers:

Type II   – Sulfuric acid anodizing, conventional coatings produced from sulfuric acid bath

Purpose: Intended to improve surface corrosion protection under severe service conditions or as a base for paint systems.

Type IIB  – Thin sulfuric acid anodizing, for use as non-chromate alternative for Type I and IB coatings

Purpose: Non-chromate alternative to Type I and IB coatings, where corrosion resistance, paint adhesion, and fatigue resistance are all required.

Type III  – Hard Anodic Coatings

Purpose: Provide wear and abrasion resistant surfaces with improved corrosion protection due to greater thickness and weight than the conventional anodic coatings. Sealing of Type III coatings is not recommended unless corrosion resistance is also a factor. Wear resistance is reduced by sealing. Fatigue strength can be reduced by Type III.

Type III is typically utilized in valves, sliding parts, hinge mechanisms, cams, gears, swivel joints, pistons, rocket nozzles, insulation plates, blast shields, etc.

Types of Anodic coatings AOTCO does NOT offer:

Type I – Chromic acid anodizing, conventional coatings produced from chromic acid bath

Type IB – Chromic acid anodizing, low voltage process

Type IC – Non-chromic acid anodizing, for use as non-chromate alternative for Type I and IB coatings


Class 1 – Non-dyed

Class 2 – Dyed

Acquisition requirements.  Acquisition documents should specify the following:

  1. Assembly anodizing – If an assembly is required, customer MUST provide AOTCO with this information. Due to electrolyte entrapment, assembly anodize must be pre-approved by AOTCO on a PART TO PART basis.
  2. Type of anodic coating, title, number and date of this specification.
  3. Class of anodic coating
  4. Special process operating conditions, if applicable
  5. Special cleaning and fabrication requirements
  6. Color and uniformity of Class 2 coatings, if applicable
  7. Degree of non-uniformity of dyed casting alloys
  8. Type III coating thickness, if applicable
  9. Coating weight for thickness, (for Type III) if substituted
  10. Special sealing requirements
  11. When applicable, the allowable difference in anodic coating appearance resulting from inherent base metal differences
  12. Provide the specific location of contact marks if important to the function of the part.
  13. Acceptance criteria for quality conformance inspections
  14. If paint adhesion testing is required for quality conformance testing and the required paint system

Unless otherwise specified in the contract, PO, or applicable drawing, the nominal thickness of the coating shall be 0.002 inches (2 mils).

Minimum thickness (typical) in inch of anodic coatings:

Alloy Designation Type II Thickness of Coating
Inches Mils
1100 0.000093 0.093
2024-T4 0.000125 0.125
3003 0.000103 0.103
5052 0.000098 0.098
6061-T6 0.000099 0.099
Alclad 2014-T6
Alclad 7075-T6
295-T6 0.000107 0.107
356-T6 0.000102 0.102
514 0.000086 0.086


Thickness ranges of anodic coatings on aluminum and aluminum alloys:

Coating Type Thickness Range
Inches Mil
I, IB, IC, and IIB 0.00002 to 0.00007 .02-.07
II 0.00007 to 0.0010 .07-0.1
III 0.0005 to 0.0045 0.5-4.5


Coating weight relative to type:

Coating Type Coating Weight (mg/ft2)
I and IB 200 min
IC 200 min – 700 max
II 1000 min
IIB 200 min – 1000 max


Additional relationships to consider when designing part coating:

  1. Fatigues properties of aluminum alloys can be severely reduced by anodic coatings. The amount of reduction varies with the process. As a rule of thumb, the thicker the coating the greater the fatigue will be.
  2. Hardcoat of 2 mils or more is extremely difficult to obtain on high silicon dye castings such as 360, 380, and 383.
  3. Abrasion resistance in Type III hardcoat will decrease as the thickness of coating approaches 3 mils. In general, abrasion resistance does NOT increase with coating thickness.