Anodizing Aluminum: Grades, Type II vs Type III, and What to Specify

Anodizing converts the surface aluminum into Al2O3, and the alloying elements left behind dictate the result. Pure-ish wrought grades anodize clear and bright; high-copper grades fight you. 6061-T6, with roughly 1% magnesium and silicon, is the workhorse: it takes both Type II sulfuric and Type III hardcoat cleanly, dyes to consistent black and color shades, and produces a slightly gray-tinted clear that customers recognize. 5052, with magnesium as the main alloying element and almost no copper, anodizes even brighter and is the go-to for decorative and architectural work. The trouble starts with copper-bearing alloys. 2024 (≈4.4% Cu) and 7075 (5-6% Zn plus copper) anodize darker, yellower, and less uniformly because the copper-rich intermetallics don't convert to clean oxide. 7075-T73 will hardcoat for wear and corrosion service, but expect a bronze-to-charcoal natural color rather than the gray of 6061, and tighter process control to avoid burning at the part edges. If a customer hands you a 2024 part and demands a bright clear cosmetic finish, that is a conversation, not a setup.

Type II (conventional sulfuric, per MIL-A-8625 / now MIL-PRF-8625) builds 0.0002-0.001 in (5-25 microns) of oxide, takes dye well, and is your default for cosmetic and mild corrosion protection. Type III hardcoat runs a colder bath and higher voltage to grow a denser layer typically 0.001-0.004 in (25-100 microns), with surface hardness in the 60-70 Rc-equivalent range and far better abrasion and dielectric performance. The dimensional catch trips up most first-time buyers: anodize grows roughly half into the part and half out, so a 0.002 in Type III coating adds about 0.001 in per surface, closing a bore by about 0.002 in on diameter. On precision bores and pins, machine to account for buildup or mask and post-machine. Type III also rounds sharp corners and can crack at edges below about 0.020 in radius, so break your edges before the part goes to the tank. For sealed corrosion performance, hot DI water or nickel-acetate seal closes the pores; for max wear, many shops leave hardcoat unsealed and accept slightly lower salt-spray numbers.

Anodizing is cheap per part but priced by rack space, surface area, and bath time, so geometry and volume swing the number more than alloy does. Type II clear runs roughly $0.50-3 per part in moderate quantity; black and color dye adds a dollar or two; Type III hardcoat typically lands 2-4x the Type II price because of the chilled bath and longer dwell. Masking, plugging threaded holes, and selective anodize (some surfaces conductive for grounding) all add hand labor that dominates small-lot cost. Standard lead time at a captive anodize line is 3-5 business days; rush is 1-2 days at a premium. The real schedule killers are NADCAP and aerospace traceability requirements, where every bath gets logged and the part may sit for a sample-coupon hardness or coating-weight test. Budget an extra few days for AS9100 jobs and confirm whether your customer requires Type III thickness verified by eddy-current or by destructive cross-section.

Most anodize rejects trace back to the metal, not the tank. Smut and inclusions from re-melted scrap stock show as dark streaks; weld zones anodize a different shade than parent metal because filler chemistry differs, so welded assemblies almost never anodize cosmetically uniform. Handling marks and fingerprints etch in permanently after the caustic pre-clean, so once a part is racked it shouldn't be touched bare-handed. Galvanic burning shows up on thin sections and sharp points where current density spikes, and it's worst on copper-heavy 2024 and 7075. Color mismatch between lots is the most common cosmetic complaint: different mill heats, different tempers, and even different machining (as-machined vs bead-blasted) take dye differently, so for cosmetic parts specify one finish prep, ideally a uniform bead blast, before anodize to mask the variation.