What Is Anodizing?
Anodizing is an electrochemical process that thickens the natural oxide layer on a metal — most commonly aluminum — by making the part the anode in an electrolytic bath. The result is a harder, more corrosion-resistant surface that can be dyed before sealing.
How Anodizing Works
The part to be anodized is immersed in an acid electrolyte and connected as the anode of a DC circuit. Current passed through the bath drives oxygen ions to the part's surface, converting the base metal into a layer of its own oxide. Because the coating grows out of the metal itself rather than being deposited onto it, it is mechanically bonded to the part — it cannot flake or peel the way a plated or painted coating can.
The 12-Step Process
Racking
Parts are fixtured to conduct current evenly and survive the electrolyte bath.
Degreasing
Alkaline cleaning removes oils and shop residue before any chemical treatment.
Rinse
Cascade rinse removes cleaning residue before etching.
Etching
Caustic etch removes a controlled layer of base metal for a uniform surface.
Rinse
Second cascade rinse stage clears etch residue.
Desmutting
Acid desmut removes smut left by etching, in preparation for anodizing.
Rinse
Final rinse before the part enters the anodizing bath.
Anodizing
DC current in an acid electrolyte grows the oxide layer to the specified thickness.
Rinse
Removes residual electrolyte before coloring.
Coloring
Electrolytic or dye coloring fills the porous oxide layer, where specified.
Sealing
Hot or cold sealing closes the pores, locking in color and corrosion resistance.
Final Rinse & Dry
Final rinse and dry before inspection and packing.
Type I, II and III Anodizing
MIL-A-8625 defines three anodizing types by electrolyte and process conditions. The right type depends on the base metal, required coating thickness, and whether the part needs to be dyed.
| Type I — Chromic Acid | Type II — Sulfuric Acid | Type III — Hard Anodizing | |
|---|---|---|---|
| Electrolyte | Chromic acid | Sulfuric acid | Sulfuric acid, low temperature |
| Typical thickness | 2–10 µm | 5–25 µm | 25–100 µm |
| Common use | Aerospace, fatigue-sensitive parts | Architectural, decorative, general | Wear surfaces, aerospace, MRO |
| Dyeable | Limited | Yes | Limited — dark shades only |
| Reference standard | MIL-A-8625 Type I | MIL-A-8625 Type II | MIL-A-8625 Type III |
Anodizing vs Electroplating
Anodizing converts the base metal's own surface into an oxide layer, so the coating is integral to the part. Electroplating deposits a separate metal layer — zinc, nickel, chrome, copper — onto the surface. Anodizing works only on certain metals like aluminum and titanium; electroplating works on most conductive substrates.
FAQ
Anodizing is an electrochemical process that thickens the natural oxide layer on a metal — most commonly aluminum — by making the part the anode in an electrolytic bath. The resulting oxide layer is harder, more corrosion-resistant, and can be dyed before sealing.
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