title = "Anodizing of Aluminum: Hard-Coat and Decorative"

content = """
Anodizing is an electrochemical process that converts the surface of aluminum into a thick, porous aluminum oxide (Al2O3) layer, providing improved corrosion resistance, wear resistance, electrical insulation, and aesthetic options. Unlike electroplating which deposits metal onto the surface, anodizing converts the aluminum surface itself—the oxide grows both outward and inward relative to the original surface, with approximately 50% of the anodic oxide thickness developing into the part.

Decorative (Type II) anodizing (per MIL-A-8625 / MIL-PRF-8625F) uses sulfuric acid electrolyte at ambient temperature (65–75°F / 18–24°C) to produce a clear or tinted oxide layer typically 0.0002–0.001 inch (5–25 µm) thick. The porous anodic oxide readily absorbs dye before sealing, producing a wide range of permanent colors. After dyeing, the pores are sealed (in hot deionized water or nickel acetate solution) to close the pore structure, locking in color and improving corrosion resistance. Decorative anodizing provides good corrosion resistance (168–336 hours in salt spray), moderate wear resistance, and excellent adhesion for primers and adhesives.

Hard-coat (Type III) anodizing produces a much thicker oxide layer (0.001–0.002 inch / 25–50 µm typical, up to 0.004 inch for special applications) using sulfuric acid electrolyte at very low temperatures (28–32°F / -2 to 0°C) with higher current density. The cold bath suppresses the dissolution of the growing oxide, allowing thicker, harder coatings. Hard-coat anodize hardness is 60–70 HRC equivalent (400–600 HV), providing excellent abrasion resistance for wear surfaces, hydraulic components, and equipment subject to sliding contact. Hard-coat anodize is typically not dyed, or dyed black; it is gray to dark gray in color. The increased thickness consumes aluminum from the surface—dimension build-up per side is approximately 50% of the total oxide thickness.

Chromic acid anodize (Type I) uses chromic acid electrolyte and produces very thin coatings (0.0001 inch) with the lowest hydrogen embrittlement risk; it is used for high-strength aluminum aerospace parts where minimum dimensional change and ductility retention are critical.

Pre-anodize cleaning (alkaline etch or bright dip) determines final appearance; proper cleaning is essential for consistent, defect-free anodize.
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version = "0.0.1"
