Spec Basics – MIL-A-8625 Type I in Plain English
If you work with aluminum components for aerospace or military applications, you have likely seen MIL-A-8625 Type I on a print. But understanding what that specification actually means for your parts is another matter. This article breaks down Type I chromic anodizing into three simple concepts: thickness, sealing and continuity. By the end, you will know exactly what to look for on your certification sheets.
The Science Behind the Protection
Chromic anodizing creates a thin oxide layer on aluminum parts through an electrochemical process. The part gets submerged in a bath containing chromic acid and an electrical current converts the outer surface of the aluminum into aluminum oxide. This oxide layer grows from the base metal outward and inward, meaning it becomes part of the part itself rather than a coating stuck on top.
The MIL-A-8625 specification defines Type I as conventional chromic acid anodizing. The resulting film typically measures 2 to 7 micrometers thick, which is roughly 0.00008 to 0.00028 inches. To put that in perspective, this layer is thinner than a human hair. That thinness matters because the part maintains its original dimensions, threads stay in spec and press-fit holes still fit. Tight tolerances also do not get blown out.
Thickness
The thickness requirement for Type I anodizing falls between 0.00002 and 0.0007 inches. The actual thickness you need depends on the application and what the part drawing calls out and the purchase documents must specify the required coating thickness.
What to Verify on Your Certs
Look for the measured thickness value and confirm it falls within the specified range. Also note that alloy composition affects the final thickness. High-copper alloys like 2024-T3, for example, produce thinner oxide films than other alloys, but they still meet the spec requirements when processed correctly.
Sealing The Step That Locks in Protection
Freshly anodized aluminum has a porous surface. Those pores need to get sealed to close off pathways for moisture and contaminants. Sealing is what gives chromic anodizing its corrosion resistance. The difference is dramatic. Unsealed Type I coatings provide roughly 100 hours of salt spray protection. Seal the same coating using dichromate and that number jumps to 750 hours.
The sealing process also affects paint adhesion. An unsealed anodized surface acts as an excellent primer for paints and adhesives because the open pores give the paint something to grip. A sealed surface trades some of that mechanical grip for superior stand-alone corrosion resistance.
What to Verify on Your Certs
Check whether sealing was performed and what method was used. For maximum corrosion resistance, look for dichromate sealing. For paint adhesion without sealing, that should be noted as well.
Continuity
One unique property of chromic anodizing is its ability to reveal defects in the base metal. The process exposes grain boundaries, porosity and other metallurgical irregularities. If an aluminum part has hidden cracks, laps or inclusions from casting or forging, Type I anodizing will make them visible. This discovery feature is one reason the aerospace industry prefers chromic anodizing for flight-critical components.
What to Verify on Your Certs
The inspection report should note any surface irregularities found during processing. A clean part with good continuity means the base metal is sound.
Fatigue Resistance
Aluminum parts under cyclic loading need to be handled carefully. Some anodizing processes introduce surface cracks that can start fatigue failures. Type I chromic anodizing has a much smaller effect on fatigue life compared to thicker sulfuric or hard anodize coatings. That is why MIL-A-8625 specifies Type I for fatigue-critical components. When a part sees repeated stress, chromic anodizing keeps the fatigue penalty low.
What to Verify on Your Certs
If fatigue is a concern for your application, make sure the cert shows Type I or Type IB processing. These are the thin coatings approved for fatigue-sensitive parts.
How to Read Your Certs Like a Pro
When you receive certification paperwork for Type I chromic anodizing, here is what you should check:
- Spec + Type: Look for MIL-A-8625 Type I (or Type IB for low-voltage processing)
- Thickness: Confirm the measured thickness matches the drawing requirements
- Sealing: Note whether sealing was performed and what method was used
- Appearance: Chromic anodized parts have an opaque gray finish that varies by alloy
- Continuity check: Verify that the part passed inspection without showing unacceptable defects
Why This Matters for Your Parts
Chromic anodizing serves three purposes. Corrosion protection, paint adhesion and defect detection. When specified correctly, it keeps aluminum parts from corroding in marine and industrial environments. It gives paint a surface that will not peel and reveals hidden flaws before those flaws cause field failures.
RMF USA performs Type I chromic anodizing to MIL-A-8625 for customers across aerospace, defense and other demanding industries. Contact our team to discuss your next project or request a quote for your aluminum components.
