Coloring Anodized Metal

Part of Electrochemistry

How to dye anodized aluminum in permanent colors, and how to produce interference colors on titanium through voltage control.

Why This Matters

Color coding is a powerful tool for organization, safety marking, and identification in any industrial or infrastructure context. Anodized aluminum can be permanently colored in a wide range of hues by absorbing dye into the porous oxide structure before sealing. Unlike paint, which flakes and chips, anodized color is integrated into the metal surface and resists abrasion, chemicals, and UV far better.

Titanium anodizing produces brilliant interference colors — blue, purple, gold, green, pink — through the thin-film optical effects of the oxide layer. These colors require no dye at all: the color comes from the thickness of the transparent oxide layer, which is controlled precisely by voltage during anodizing. Each voltage produces a specific color.

These capabilities matter practically for part identification systems, safety equipment marking (color-coded piping, tool handles, circuit labels), and instrumentation markings that must survive harsh environments.

Dyeing Anodized Aluminum

The Mechanism

After anodizing, the aluminum oxide layer contains millions of cylindrical pores approximately 10–20 nm in diameter, perpendicular to the surface. These pores are accessible from the surface and can absorb dye molecules by capillary action.

After dyeing, hot water sealing converts the oxide to boehmite (Al₂O₃·H₂O), which closes the pores and locks the dye permanently inside the oxide matrix. The dye cannot be removed by abrasion because it is inside the surface, not on top of it.

Compatible Dyes

Dye Type	Availability	Color Range	UV Resistance
Proprietary anodizing dyes (Clariant, Sandoz, etc.)	Industrial supply	Full spectrum	Excellent
Reactive fabric dyes (Procion MX)	Fabric/craft supply	Good range	Good
Acid dyes (wool dyes)	Craft supply	Wide range	Good
Direct/fiber reactive dyes	Craft supply	Moderate range	Fair
Rit synthetic fabric dye	Supermarket	Limited range	Fair
Natural dyes (tea, walnut, onion skin)	Foraged	Brown, yellow, tan	Poor

Avoid: Disperse dyes (designed for polyester — don’t work in oxide pores), vat dyes (require reducing chemistry incompatible with aluminum).

Dye Process

Critical prerequisite: Aluminum must be freshly anodized. After sealing, no dye can be absorbed. Work quickly — exposed porous oxide begins to seal if left in air for more than a few hours.

1. Prepare dye bath:

   • Dissolve dye in hot distilled water: 5–10 g/L for deep color, 1–3 g/L for pastel.
   • Temperature: 50–65°C. Higher temperature = faster absorption, slightly lighter final color.
   • pH: 5–6 for most dyes (add acetic acid to lower pH if needed). Check dye manufacturer recommendation.

2. Immerse part:

   • Part must be clean — do not touch with bare hands after anodizing.
   • Immerse completely, ensure no air pockets.
   • Agitate gently.

3. Monitor color:

   • The wet color is significantly lighter than the final sealed color — account for this.
   • For black: immerse 15–30 minutes in concentrated black dye.
   • For pastels: 5–10 minutes in dilute bath.
   • The color intensifies with time and temperature.

4. Rinse: Lightly rinse with room-temperature distilled water. Do not use hot water — it will begin sealing.

5. Seal immediately: Hot distilled water, 95–100°C, 20–30 minutes. Color is now permanent.

Two-Color Anodizing

To create two-color parts (e.g., identification markings):

1. Anodize the whole part.
2. Dye in color 1, seal.
3. Strip one area’s oxide with 15% NaOH (reanodize targets).
4. Reanodize the stripped area.
5. Dye in color 2, seal.

Or mask areas with vinyl tape or wax before anodizing to block the oxide formation.

Interference Colors on Titanium

The Physics of Interference Color

Titanium’s oxide (TiO₂) is transparent and grows to a thickness precisely controlled by the anodizing voltage — approximately 2 nm per volt. When light reflects from the top surface and from the metal-oxide interface, the two reflections interfere constructively or destructively at different wavelengths, producing color.

This is the same physics that causes soap bubble colors and oil slick colors on water. No dye is involved — the color comes entirely from oxide thickness.

Voltage-Color Relationship

Voltage	Oxide Thickness	Perceived Color
5 V	~10 nm	Golden yellow
10 V	~20 nm	Warm gold
15 V	~30 nm	Purple/violet
20 V	~40 nm	Blue
25 V	~50 nm	Light blue
30 V	~60 nm	Green
40 V	~80 nm	Yellow-green
50 V	~100 nm	Pink/magenta
80 V	~160 nm	Second-order colors

These values are approximate — exact colors depend on alloy and surface finish.

Titanium Anodizing Process

Electrolyte: 1–5% sulfuric acid, or TSP (trisodium phosphate), or ammonium sulfate. Titanium anodizing is less demanding than aluminum — a wide range of electrolytes work.

Process:

1. Clean titanium surface — degrease, light etch in HF/HNO₃ if needed (wear extreme PPE — HF is highly dangerous), rinse.
2. Connect to anode terminal.
3. Immerse in electrolyte.
4. Apply voltage slowly to target value.
5. Hold for 1–2 minutes — color appears within seconds.
6. Remove and rinse.

No sealing required — titanium oxide does not have the same pore structure as aluminum oxide and does not need to be sealed.

Masking and Gradients

Different areas of a titanium part can be colored different colors by masking with vinyl tape and anodizing in steps from lowest to highest voltage. The already-anodized color is not changed by lower voltages, only by higher voltages.

A gradient from gold to blue to purple can be created by partially immersing the part and varying voltage during processing, or by sequential immersion at different depths with different voltages.