MIL-PRF-8625 *
Sulfuric Anodize
Specifications

* Note: MIL-A-8625 is an older version of this spec. Owing to ongoing customer demand AOTCO continues to finish to this spec as well as the current MIL-PRF-8625 spec.

AOTCO's Certifications & Approvals for this Spec

• NADCAP Accredited

• ROHS Compliant

Technical Notes

Types of Anodic Coatings Available

• Type 1 – Conventional coatings produced from chromic acid bath
  Purpose: Delivers an electrically non-conductive surface with improved scratch and corrosion resistance. Primarily used in aerospace applications and on castings where components that may undergo flexing due to heat changes or mechanical means

• Type 2 – 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 2B – Thin sulfuric acid anodizing, for use as non-chromate alternative for Type 1 and 1B coatings
  Purpose: Non-chromate alternative to Type 1 and 1B coatings, where corrosion resistance, paint adhesion, and fatigue resistance are all required

• Type 3 – 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. Reminder: Sealing of Type 3 coatings is not recommended unless corrosion resistance is also a factor, as both wear resistance and fatigue strength can be reduced by sealing

Types of Anodic Coatings NOT Offered

• Type 1B – Chromic acid anodizing, low voltage process

• Type 1C – Non-chromic acid anodizing, for use as non-chromate alternative for Type 1 and 1B coating

Classes

• Class 1 – Non-dyed
• Class 2 – Dyed

Acquisition Requirements

Acquisition documents should specify the following

• 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

• Type of anodic coating, title, number and date of this specification

• Class of anodic coating

• Special process operating conditions, if applicable

• Special cleaning and fabrication requirements

• Color and uniformity of Class 2 coatings, if applicable

• Degree of non-uniformity of dyed casting alloys

• Type 3 coating thickness, if applicable

• Coating weight for thickness, (for Type 3) if substituted

• Special sealing requirements

• When applicable, the allowable difference in anodic coating appearance resulting from inherent base metal differences

• Provide the specific location of contact marks if important to the function of the part

• Acceptance criteria for quality conformance inspections

• 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

Thickness Ranges of Anodic Coatings on Aluminum and Aluminum Alloys

Coating Weight Relative to Type

Additional Relationships to Consider When Designing Part Coating

• 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
• Hardcoat of 2 mils or more is extremely difficult to obtain on high silicon dye castings such as 360, 380, and 383
• Abrasion resistance in Type 3 hardcoat will decrease as the thickness of coating approaches 3 mils. In general, abrasion resistance does NOT increase with coating thickness