Anodizing

Electrochemical conversion coating for aluminum. The oxide layer grows from the base metal itself -- it is not a deposited coating. Most widely used surface treatment for CNC aluminum parts. This guide covers what you actually need to specify, what it costs, and what goes wrong on the production floor.

Which Anodizing Type Do You Need?

Most parts need Type II. Only specify Type III when you have a real wear or hardness requirement -- it costs more, adds more thickness, and limits your color options. Use this table to decide.

If Your Part Needs...	Type	Typical Thickness	Cost Factor
Corrosion resistance, cosmetic appearance, brand color	Type II	0.2–1.0 mil (5–25 μm)	1x (baseline)
Electrical insulation	Type II	0.5–1.0 mil	1x
Light wear resistance (hand tools, enclosures)	Type II	0.5–1.0 mil	1x
Sliding or mating surfaces with friction	Type III (hard coat)	1.0–2.0 mil (25–50 μm)	1.5–2x
Abrasion resistance (moving parts, guide rails)	Type III	1.5–2.0 mil	1.5–2x
Maximum hardness on aluminum	Type III	2.0 mil+ (up to 75 μm)	2x+
Food-contact or medical (non-toxic surface)	Type II, unsealed or hot-water sealed	0.5 mil	1x

Rule of Thumb If you cannot articulate a specific wear or hardness requirement, go with Type II. Type III is harder to machine around (more dimensional change) and limits you to dark colors. Specifying it "just in case" adds cost with no real benefit.

Type II vs Type III at a Glance

Property	Type II (Regular / Sulfuric)	Type III (Hard Coat)
Process	Sulfuric acid bath, ~15–20 VDC, room temperature	Sulfuric acid bath, ~25–60 VDC, near-freezing (0–5 °C)
Coating Thickness	0.2–1.0 mil (5–25 μm) per surface	1.0–2.0 mil (25–50 μm) per surface
Hardness (HV)	250–350	400–600
Color Options	Clear, black, blue, red, gold, green, orange, purple, custom	Dark grey, brownish-black, bronze only
Wear Resistance	Moderate -- scratches under repeated contact	Excellent -- handles sliding and abrasive contact
Corrosion Resistance	Good (salt spray 336+ hrs at 25 μm)	Very good, but micro-cracks at thick coatings can reduce it
Dielectric Strength	200–400 V/mil	400–800 V/mil
Dimensional Change	+0.5 mil per surface (50/50 growth in/out)	+1.0–2.0 mil per surface
Cost Factor	1x baseline	1.5x–2x (thicker = more)
Lead Time Impact	+2 working days	+3–5 working days
Best For	Appearance parts, enclosures, brackets, housings	Wear surfaces, sliding components, guide pins, valve bodies

Color Availability

Colors are absorbed into the porous oxide layer before sealing. The result depends on alloy, coating thickness, and dye bath chemistry. Not every shop carries every color -- uncommon colors may require batch minimums or extended lead time.

Color	Availability	Cost Premium	Best Result On	Notes
Clear / Natural	Standard everywhere	None	6061, 5052	Slight champagne tint on most alloys. The "default" anodize.
Black	Standard everywhere	None	All alloys	Most popular color. Most consistent across alloys and batches.
Blue	Standard	None to slight	6061, 5052	Vivid on 6061. Can appear dull or greyish on 7075/2024.
Red	Standard	None to slight	6061	May shift pinkish on high-copper alloys. Requires adequate thickness (≥0.5 mil).
Gold	Standard	None to slight	6061	Usually inorganic dye -- very UV-stable. Good for outdoor parts.
Green	Standard	None to slight	6061	Range from olive to bright green depending on dye supplier.
Orange	May require specialty shop	10–20%	6061	Less common. Verify shop capability before specifying.
Purple / Violet	May require specialty shop	10–20%	6061	Organic dye, can fade under UV. Not ideal for outdoor use.
Custom / Pantone Match	Limited shops	30–50%+ setup	6061	Requires lab dip and approval. Batch minimums common. Allow extra lead time.

Type III Color Limitation Hard coat (Type III) is limited to dark grey, dark brown, and black. The thick, dense oxide layer does not absorb dye well. If you need a bright color and hard wear resistance, consider Type II with a thicker coating, or a two-step process (anodize + dry film lube).

Batch Color Matching If you order parts in multiple batches, color can shift between runs. This is normal -- anodize color depends on bath chemistry, temperature, and time, which drift over time. For critical color consistency, consolidate all parts into a single batch.

Dimensional Impact

Anodizing is a conversion process -- the oxide layer grows both outward and inward from the original surface. Approximately half the coating thickness is added above the original surface, and half consumes the base metal. This matters for every feature with a tolerance tighter than ±0.005 in.

Anodize Type	Thickness	Growth Per Surface	Total on a Diameter (both sides)	Impact on Thread Pitch Diameter
Type II (thin)	0.2 mil (5 μm)	+0.1 mil	+0.2 mil (5 μm)	Negligible
Type II (standard)	0.5 mil (12 μm)	+0.25 mil	+0.5 mil (12 μm)	May require class 2B to 3B adjustment
Type II (thick)	1.0 mil (25 μm)	+0.5 mil	+1.0 mil (25 μm)	Significant -- size threads before anodize
Type III (standard)	1.5 mil (38 μm)	+0.75 mil	+1.5 mil (38 μm)	Critical -- must pre-size or tap after
Type III (thick)	2.0 mil (50 μm)	+1.0 mil	+2.0 mil (50 μm)	Critical -- plan for post-anodize reaming

Feature Type	Risk Level	Recommendation
Press-fit bores (H7/p6)	High	Mask the bore, or machine to final size after anodize. Anodize buildup will change the interference fit.
Threaded holes (tapped)	Medium	Standard Type II is usually fine. For Type III, oversize the tap drill or chase threads after coating.
Shaft diameters in bearings	High	Mask bearing journals or grind after anodize. Even 0.5 mil buildup changes clearance fits.
Sealing surfaces (O-ring grooves)	Medium	Anodize in the groove is fine for static seals. For dynamic seals, consider masking.
Sharp edges (90° corners)	Medium	Coating thins at sharp edges. Break all edges with 0.005–0.015 in chamfer or radius.
Blind holes (deep)	Medium	Solution pools in deep blind holes, causing thicker coating or burning at the bottom. Drain holes help.

Masking Masking (using plugs, caps, or chemically resistant tape) prevents anodize from forming on specific surfaces. Masking adds $2–8 per part depending on complexity and number of masked features. For critical bores or bearing surfaces, this cost is usually worth it compared to re-machining after anodize.

Material Requirements

Not all aluminum anodizes well. Alloy selection is the single biggest factor in anodize quality. Copper content is the main problem -- anything above ~1% copper causes dark spots, streaky color, and poor hard coat formation.

Alloy	Copper Content	Type II Result	Type III Result	Color Quality	Notes
6061-T6	0.15–0.40%	Excellent	Excellent	Best	The go-to alloy for anodized parts. Clean colors, consistent finish, good hard coat. Use this unless you have a specific reason not to.
6082-T6	0.10%	Very good	Good	Very good	European equivalent of 6061. Slightly higher Mn content. Performs nearly identically.
5052-H32	0.10%	Good	Fair	Good	Good colors, acceptable Type II. Hard coat is softer due to lower alloying content.
5083-H111	0.10%	Good	Fair	Good	Marine-grade alloy. Good Type II. Not ideal for hard coat.
7075-T6	1.2–2.0%	Fair	Poor	Poor	Copper causes dark smudges and non-uniform color. Hard coat is brittle and can flake. Only use if you need 7075's strength and can accept the cosmetic compromise.
2024-T4	3.8–4.9%	Poor	Not recommended	Very poor	Very high copper. Grey-brown discoloration, streaky, blotchy. Avoid for cosmetic parts.
2014-T6	3.9–5.0%	Poor	Not recommended	Very poor	Same copper problem as 2024. Use only when strength requirement demands it.
Cast Aluminum (A356, ADC12)	Varies	Variable	Not recommended	Variable	High silicon content causes grey mottling and poor dye uptake. Surface porosity traps chemicals. Type II works for non-cosmetic parts. Hard coat is unreliable.

2000-Series Alloys and Hard Coat Do not specify Type III hard coat on 2024 or 2014. The high copper content prevents proper oxide formation -- you get a soft, patchy coating that flakes off. If you need both high strength and wear resistance, consider 7075 with Type II (accepting the cosmetic trade-off) or switch to 6061 if the strength allows it.

Cast Aluminum Warning Die-cast and sand-cast parts anodize poorly. Silicon (5–12% in most casting alloys) does not anodize and appears as dark spots or a mottled grey surface. For cosmetic cast parts, consider powder coating or e-coating instead.

Anodizing Process Steps

Understanding the process helps you understand why certain things go wrong. Each step must be controlled -- skipping or rushing any step shows up as a defect in the final part.

Step	Process	What Happens	What Can Go Wrong
1. Cleaning	Alkaline degreaser, 140–180 °F	Removes cutting oil, fingerprint oils, and shop contamination from the machined surface.	Insufficient cleaning leaves oily spots that repel anodize, causing bare patches.
2. Etching	Sodium hydroxide (NaOH), 120–140 °F, 3–10 min	Dissolves the natural aluminum oxide layer and removes ~0.1–0.3 mil of surface material. Creates a uniform matte finish.	Over-etching removes too much material and rounds edges. Under-etching leaves shiny streaks.
3. Desmutting	Nitric acid (HNO₃) or non-chromate desmut, room temp	Removes the "smut" (insoluble alloying elements left after etching). Critical for copper-bearing alloys.	Incomplete desmutting on 7075/2024 leaves dark particulate that shows through the anodize.
4. Anodizing	Sulfuric acid bath (H₂SO₄), Type II: 15–20 VDC; Type III: 25–60 VDC	Electric current grows the aluminum oxide layer from the surface. Thickness is controlled by time and current density.	Burning (too much current), thin spots (low current density in recesses), pitting (contamination).
5. Dyeing	Organic or inorganic dye bath, 100–140 °F, 5–30 min	Dye molecules enter the porous oxide layer. Thicker coating = more dye absorption = deeper color.	Uneven dye from inconsistent coating thickness. Over-dyeing causes muddy colors.
6. Sealing	Hot water (deionized, 200 °F+) or nickel acetate, 10–30 min	Hydrates the oxide, swelling it shut. Locks in dye and maximizes corrosion resistance.	Under-sealing leaves pores open -- dye leaches out, corrosion resistance drops. Over-sealing causes surface powdering.

Common Failures

These are the defects that show up on the production floor most often. Knowing the root cause helps you prevent them at the design stage rather than discovering them at final inspection.

Failure	Appearance	Root Cause	Prevention
Burning	White powdery spots, rough texture, sometimes visible craters	Excessive current density at sharp corners, edges, or protruding features. The oxide breaks down locally and powders away.	Break all edges (chamfer or radius). Increase anodize rack contact area. Reduce current density at start.
Pitting	Small dark pinholes or craters scattered across the surface	Chloride or heavy metal contamination in the anodize bath. Can also come from incomplete cleaning leaving corrosive residue.	Shop must maintain bath chemistry. Use deionized water for rinses. For your part: ensure thorough cleaning of tapped holes and crevices.
Uneven Color / Streaking	Bands or streaks of lighter/darker shade. Dye seems to "wash off" in areas.	Uneven coating thickness from inconsistent current distribution. Common on large parts, deep pockets, or parts with thick and thin sections.	Ensure good racking contact points. Avoid large flat surfaces without features. Consider jigs for consistent current flow.
Blotchy / Smudgy Appearance (7075, 2024)	Dark patches, cloudy areas, non-uniform base color even before dye	Copper-rich intermetallic particles in high-copper alloys do not anodize uniformly.	Switch to 6061 for cosmetic parts. If 7075 is required, accept the appearance limitation or use a darker color (black hides it best).
Soft / Powdery Coating	Coating wipes off easily with finger or cloth. Matte, chalky feel.	Over-sealing (too hot or too long in the seal bath). Also caused by excessive bath temperature during anodizing.	Control seal bath temperature strictly. For Type III, bath temperature must stay below 5 °C.
Dimensional Non-Conformance	Bores too small, shafts too large, threads don't gauge	Designer did not account for anodize buildup. Worse with Type III or thick Type II.	Calculate growth into tolerances from the start. Mask critical features. Or machine critical features after anodize.
Peeling / Flaking	Oxide layer lifting off in sheets or patches	Very rare on properly processed parts. Usually caused by excessive coating thickness on high-copper alloys, or contamination that prevents oxide bonding.	Stay within standard thickness ranges. Avoid Type III on 2000-series alloys. Ensure parts are clean before anodize.

Cost Drivers

Anodize pricing varies widely by region and shop, but the relative cost structure is consistent. Here is what actually moves the price.

Cost Factor	Impact	Detail
Setup / Lot Charge	High for small orders	Most shops charge a minimum lot fee ($50–200). On a small order of 10 parts, the setup dominates the per-part cost. Larger batches amortize this.
Type III vs Type II	+50–100%	Hard coat runs colder, slower, and at higher voltage. Energy cost and cycle time both increase. The thicker coating also means more material consumed.
Color Premium	0–50%	Clear and black are typically no extra charge. Standard colors (blue, red, gold) are usually included. Custom/Pantone matches add setup cost and may require a batch minimum.
Masking	$2–8 per part	Each masked feature (bore, surface, thread) takes labor to apply and remove. Complex masks (precision bores) cost more than simple tape masks.
Thickness (beyond standard)	+10–30%	Standard Type II is ~0.5 mil. Requesting 1.0 mil increases cycle time. Above 1.0 mil, some shops charge a premium due to higher reject risk.
Rush / Expedite	+25–100%	Standard lead time is 2–5 working days. Rushing disrupts batch scheduling. Some shops will not rush at all.
Part Size / Weight	Low to moderate	Larger parts require bigger racks and more tank space. Very small parts may need special racking. Extremely large parts may not fit standard tanks.
Military / Aerospace Spec	+20–50%	MIL-A-8625 or AMS 2468/2469 compliance adds documentation, testing, and process control requirements.

Common Mistakes

Mistake	Consequence	Fix
Not accounting for anodize thickness on critical dimensions	Bores too small, shafts too large, threads do not accept fasteners. Parts fail inspection.	Add anodize growth to your tolerance stack. For Type II standard, assume +0.5 mil per surface. For Type III, assume +1.5 mil. Mask critical features or machine after anodize.
Specifying Type III when Type II is sufficient	Higher cost, longer lead time, limited color options, and more dimensional impact with no functional benefit.	Specify Type II unless there is a real wear, abrasion, or hardness requirement. Document the reason for Type III on the drawing.
Using 7075 or 2024 for cosmetic anodized parts	Dark smudges, streaky color, non-uniform finish. Customer rejects parts at incoming inspection.	Use 6061 for any part where appearance matters. Reserve 7075 for structural parts where you accept the cosmetic limitation.
Sharp 90° edges with no break	Burning at edges, thin/missing coating at corners, inconsistent color at edges.	Add 0.005–0.015 in chamfer or radius to all edges. Call it out on the drawing as "BREAK ALL SHARP EDGES 0.010 MAX."
Ordering parts in multiple small batches instead of one	Visible color difference between batches. Parts from batch 1 and batch 2 look mismatched when assembled.	Consolidate all anodized parts for a project into a single batch. If reorders are inevitable, keep a spare anodize sample from the original batch for reference.
Deep blind holes with no drain path	Solution pools at the bottom of the hole, causing thick coating, burning, or chemical staining.	Design drain holes where possible. If not possible, specify that blind holes must be masked, or accept the appearance inside the hole.
Not specifying sealing method	Inconsistent corrosion resistance and color retention across suppliers. Some shops use nickel acetate (better), some use hot water (cheaper).	Specify sealing method on the drawing: "SEAL: NICKEL ACETATE PER MIL-A-8625" or "SEAL: HOT DEIONIZED WATER." For food or medical, specify "SEAL: HOT WATER, NO NICKEL."
Calling out anodize on a drawing with no thickness spec	Shop applies their default, which may be too thin for your application or too thick for your tolerances.	Always specify thickness: "TYPE II, CLASS 1, 0.0005 IN MIN" or "TYPE III, 0.002 IN MIN PER MIL-A-8625 TYPE III."

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