Chromic Acid Anodizing (governed by MIL-A-8625, Type I), often referred to as Type I Anodizing, is an electrochemical process (governed by MIL-A-8625, Type I) uses a chromic acid electrolyte to grow a thin, opaque oxide layer on the surface of aluminum. The coating is typically between 0.00002” and 0.0001” thick (0.5 to 2.5 microns). It results in a distinct greyish, iridescent appearance that is less porous and more ductile than other types of anodizing.

Best Industries:

Performance Types:

Best Materials:

High-precision Chromic Acid Anodizing (Type I) for aerospace & defense. MIL-A-8625 compliant coating for fatigue resistance and corrosion protection.

Key Advantages: Fatigue Strength and Precision

Chromic Anodize is specified when the structural integrity of the part is the top priority:

Minimal Fatigue Loss: Standard sulfuric anodizing can reduce the “fatigue life” of an aluminum part (making it more likely to crack under repeated stress). Chromic anodize has a negligible effect on fatigue, making it essential for flight-critical structures.
Corrosion Resistance in Joints: Because chromic acid is non-corrosive to aluminum, any electrolyte trapped in a complex assembly (like a riveted joint or a deep blind hole) will not eat away at the metal from the inside out.
Tight Tolerances: Due to its extreme thinness, it is the best anodizing choice for high-precision components where a thicker Type II coating would interfere with assembly fit.
Excellent Paint Base: The “porous-yet-thin” structure of the oxide provides an exceptional surface for aerospace primers and topcoats to bond to.

The Role of Supplemental Coatings

While Chromic Anodize provides good corrosion resistance on its own, it is frequently treated with a “seal” to maximize its performance:

Dichromate Seal: The traditional “gold/yellow” seal that provides maximum salt-spray protection.
Hot Water/Nickel Acetate Seal: Used when a clear or grey finish is preferred while still closing the pores of the oxide layer.
Paint/Primer: In the aircraft industry, Type I is almost always followed by an epoxy primer and a polyurethane topcoat.

Specifications

Specification	Comments
MIL-A-8625F	.00002 – .0003
Type I	Conventional coating produced from chromic acid bath
Type IB	Low voltage chromic acid anodizing 22V
Class 1	Non-dyed
Class 2	Dyed

We offer 3 types of Anodized Plating. Click the buttons below to see more details on each type:

Type I - Anodize Chromic

Type II - Anodize Sulfuric

Type III - Anodize Hard

Industry Applications

Chromic Anodize is the “gold standard” for the aerospace and defense industries:

Aircraft Skins and Ribs: Used on large structural panels where weight and fatigue resistance are critical.
Precision Machined Castings: Ideal for complex castings that might have microscopic porosity, as the acid won’t damage the part if it gets trapped.
Welded Assemblies: Used for fuel tanks and hydraulic components where cleaning out every bit of electrolyte is difficult.
Firearm Components: Historically used for internal parts where a thin, protective, but non-interfering layer was required.

Finish Comparison

	Chromic (Type I)	Sulfuric (Type II)	Hardcoat (Type III)
Coating Thickness	Thinnest (0.5–2.5 µm)	Moderate (5–25 µm)	Thickest (25–100+ µm)
Fatigue Life Impact	Minimal	Moderate	Significant
Aesthetics	Opaque / Grey / Dull	Clear / Bright Colors	Dark Grey / Brown / Black
Hardness	Softest	Moderate	Highest (Ceramic-like)
Main Industry	Aerospace / Defense	Consumer / Automotive	Industrial / Wear Parts

Chromic (Type I)
Coating Thickness	Thinnest (0.5–2.5 µm)
Fatigue Life Impact	Minimal
Aesthetics	Opaque / Grey / Dull
Hardness	Softest
Main Industry	Aerospace / Defense

Sulfuric (Type II)
Coating Thickness	Moderate (5–25 µm)
Fatigue Life Impact	Moderate
Aesthetics	Clear / Bright Colors
Hardness	Moderate
Main Industry	Consumer / Automotive

Hardcoat (Type III)
Coating Thickness	Thickest (25–100+ µm)
Fatigue Life Impact	Significant
Aesthetics	Dark Grey / Brown / Black
Hardness	Highest (Ceramic-like)
Main Industry	Industrial / Wear Parts

Why Choose One Over the Other?

Chromic Anodize (Type I):
The “Aerospace” Choice

As you noted with high-precision systems, Type I is the thinnest of the three, typically adding only 0.5 to 2.5 microns of thickness. Because it is so thin, it has the least impact on the fatigue strength of the aluminum, which is critical for parts that vibrate or flex during flight. Its unique “self-healing” nature means that if any acid is trapped in a crack or seam, chromic acid won’t eat away at the part from the inside like sulfuric acid would.

Best for: Complex aerospace assemblies with rivets or seams, flight-critical fatigue-sensitive parts, and as a base for high-performance paints.

Sulfuric Anodize (Type II):
The “Aesthetic” Choice

Type II is the most common form of anodizing found in the world today. >It creates a thicker, porous oxide layer (up to 25 microns) that acts like a sponge for organic dyes. This is how you get vivid reds, blues, and blacks on aluminum parts. While it is more durable than Type I, it adds more measurable thickness, meaning you must account for “build-up” in your designs. It is the perfect middle ground between cost, protection, and beauty.

Best for: Consumer electronics, architectural trim, sporting goods, and general-purpose industrial brackets.

Hardcoat Anodize (Type III):
The “Armor” Choice

Hardcoat is the “Industrial” choice for extreme environments. It is processed at much lower temperatures with higher voltages to create a very dense, ceramic-like skin that can be up to 100 microns thick. This layer is so hard it is measured on the Rockwell C scale (often reaching 60–70 HRC). Because the coating is so thick and dense, it naturally turns a dark gray or bronze color, which makes it difficult to dye anything other than black.

Best for: Hydraulic cylinders, gears, firearms, and sliding components that face constant abrasion or salt-spray exposure.

A Note on Environmental Transition

Because Chromic Acid contains Hexavalent Chromium, it is being phased out in many non-military applications due to environmental regulations (REACH/RoHS). The industry is moving toward Boric-Sulfuric Acid Anodizing (BSAA) or Tartaric-Sulfuric Acid Anodizing (TSAA) as environmentally friendly alternatives that mimic the thinness and fatigue resistance of Type I Chromic Anodize.

Chromic Acid Anodizing