Both Type II and Type III are sulfuric acid anodizing under the MIL-A-8625 specification, and both convert the surface of aluminum into a hard, corrosion-resistant oxide. The difference is how thick and how hard. Type II is the conventional finish, roughly 5 to 25 microns thick, dyeable in almost any color, and used where appearance and moderate protection matter. Type III, or hardcoat, is grown at lower temperature and higher current to produce a much thicker layer, roughly 25 to 75 microns, reaching 60 to 70 on the Rockwell C scale, harder than many tool steels, for parts that must survive heavy wear and abrasion. Choose Type II for looks and light duty, Type III for wear resistance.<\/p>\n\n\n\n
This guide compares the two on thickness, hardness, color, and dimensional growth, the last of which trips up more designs than any other factor. Anodizing is a finishing step on our CNC machining services<\/a>, and the right alloy choice, covered in our guide to choosing CNC machining materials<\/a>, affects how a part anodizes.<\/p>\n\n\n\n Type II uses a sulfuric acid bath at around room temperature to build an oxide layer typically 5 to 25 microns thick. The layer is porous before sealing, which is exactly what allows dyeing: dye fills the pores, then sealing locks in the color. That makes Type II the finish you see on consumer electronics, enclosures, and most commercial aluminum parts, available in black, red, blue, gold, clear, and more. It offers good corrosion resistance and light wear resistance, and adds little thickness, but its surface, while harder than bare aluminum, is still relatively soft and can be scratched by harder metals.<\/p>\n\n\n\n Use Type II for cosmetic parts, color coding, enclosures, and components that need corrosion protection without heavy mechanical wear, or as a base for further coating.<\/p>\n\n\n\n Type III uses the same sulfuric chemistry but at near-freezing bath temperatures and higher current density, which grows a denser, much thicker oxide, typically 25 to 75 microns and sometimes more. The result is exceptional hardness, around 60 to 70 HRC, with outstanding abrasion, erosion, and wear resistance, plus the highest corrosion protection available for aluminum and useful dielectric and thermal-barrier properties. The trade-off on appearance: the thick, dense oxide has a natural dark gray to bronze color and cannot take bright colors. It dyes well to black, though often a matte charcoal rather than jet black, while reds and blues come out muddy.<\/p>\n\n\n\n Use Type III for pistons, guide rails, gears, robotic joints, valve components, and any aerospace, defense, or industrial part exposed to wear or harsh environments.<\/p>\n\n\n\n (For reference, Type I is a thinner chromic acid anodize used where minimal dimensional change and fatigue performance matter most.)<\/p>\n\n\n\nType II: Conventional Anodizing<\/strong><\/h2>\n\n\n\n
Type III: Hardcoat Anodizing<\/strong><\/h2>\n\n\n\n
Type II vs Type III at a Glance<\/strong><\/h2>\n\n\n\n
Property<\/td> Type II (conventional)<\/td> Type III (hardcoat)<\/td><\/tr> Standard<\/td> MIL-A-8625, sulfuric<\/td> MIL-A-8625, sulfuric, low temp<\/td><\/tr> Thickness<\/td> ~5\u201325 \u00b5m (0.0002\u20130.001 in)<\/td> ~25\u201375 \u00b5m (0.001\u20130.003 in)<\/td><\/tr> Hardness<\/td> Moderate<\/td> ~60\u201370 HRC, harder than tool steel<\/td><\/tr> Color<\/td> Any color, dyeable<\/td> Dark gray to bronze; black only<\/td><\/tr> Corrosion resistance<\/td> Good<\/td> Highest<\/td><\/tr> Best for<\/td> Cosmetic, light wear, color coding<\/td> Heavy wear, harsh environments<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n The 50\/50 Rule: Dimensional Growth<\/strong><\/h2>\n\n\n\n