Titanium Anodizing
Difficulty: Easy–Medium | Time: 30–60 min | Visual Impact: Very High
Background
Titanium anodizing produces some of the most vivid colors achievable in a home lab — and they require no dyes or pigments whatsoever. The colors come entirely from physics: thin-film interference.
When titanium is connected to the positive terminal of a power supply in an electrolyte, oxygen from the water is driven into the titanium surface, building up a layer of titanium dioxide (TiO₂). The thickness of this layer is controlled precisely by the applied voltage — roughly 2 nm of oxide per volt. White light reflecting off the outer oxide surface and the metal beneath interfere with each other constructively or destructively depending on the wavelength, producing a pure spectral color. The same physics explains the rainbow in a soap bubble or an oil slick, but here the thickness — and therefore the color — is electrically programmable.
The color sequence follows the visible spectrum as voltage increases, then cycles again in a second, paler series. A well-anodized piece shows the exact color corresponding to its voltage: there are no intermediate shades unless you deliberately create a gradient.
The process was originally developed for aerospace and industrial applications — titanium implants, aircraft components, jewelry — but the chemistry is accessible enough for a kitchen table.
Colour Reference
| Voltage | Colour | Oxide thickness (approx.) |
|---|---|---|
| 4 V | Champagne / pale gold | ~8 nm |
| 8 V | Golden yellow | ~16 nm |
| 12 V | Purple / dark violet | ~24 nm |
| 16 V | Blue-violet | ~32 nm |
| 20 V | Deep blue | ~40 nm |
| 25 V | Cobalt / royal blue | ~50 nm |
| 30 V | Sky blue | ~60 nm |
| 35 V | Blue-green | ~70 nm |
| 40 V | Yellow-green | ~80 nm |
| 45 V | Straw / olive | ~90 nm |
| 55 V | Pink / coral | ~110 nm |
| 65 V | Turquoise / cyan | ~130 nm |
| 80 V | Pale pink / salmon | ~160 nm |
| 90 V | Pale green | ~180 nm |
| 100 V | Near-white / silver | ~200 nm |
Colors are approximate and shift with surface finish (polished surfaces give brighter, more saturated colors; matte surfaces give softer, more diffuse tones), electrolyte composition, and temperature. The sequence above is for dilute sodium carbonate electrolyte.
Materials
| What | Notes |
|---|---|
| Titanium piece | Wire, sheet, or a titanium jewelry blank |
| DC power supply, 0–100 V | See power supply notes below |
| Sodium carbonate | ~5 g for 500 mL electrolyte |
| Steel or stainless steel electrode | A steel nail or spoon works as the cathode |
| Connecting wires with alligator clips | |
| Isopropyl alcohol (≥70%) | For cleaning before anodizing |
| Water | Distilled or tap |
| Safety glasses |
Power Supply Options
- Variable DC lab supply (recommended): Allows precise voltage selection and current limiting. Set current limit to ~0.5 A.
- 9 V batteries in series: 1 battery = 9 V, 2 = 18 V, 3 = 27 V, 4 = 36 V, 5 = 45 V. Covers most of the first color cycle without specialized equipment. Connect the titanium to the positive terminal.
- Phone charger + USB boost converter: Adjustable boost converters (widely available, <$5) can output 5–35 V from a USB power source with a voltage-setting dial.
For voltages above 50 V: exercise extra caution — risk of electric shock increases. Keep hands dry, avoid touching both electrodes simultaneously, and work with the power off when adjusting connections.
Surface Preparation
The color only forms on clean metal. Any oil, fingerprint, or oxide variation will show up as a blotchy or uneven result.
- If the titanium is scratched or uneven, sand it progressively (220 → 400 → 800 → 1200 grit) to achieve the surface texture you want. A matte 400-grit finish produces soft, earthy tones; a polished finish gives jewel-like saturated colors.
- Rinse thoroughly with water.
- Wipe with isopropyl alcohol and let dry completely.
- Do not touch the surface with bare fingers after this point. Handle only by edges or by unpolished areas.
Electrolyte Preparation
Dissolve 5 g of sodium carbonate in 500 mL of water. Stir until fully dissolved. This gives approximately a 1% solution — conductive enough to anodize efficiently but dilute enough to keep current low and controllable.
Alternatives: Baking soda (sodium bicarbonate, same ratio) works but is slightly less conductive. Ammonium sulfate (~10 g/L) gives similar results and keeps pH neutral.
Procedure
Single-Color Anodizing
- Pour ~100 mL of electrolyte into a glass or plastic container.
- Set your power supply to the target voltage (refer to the color table above) and current limit to ~0.2 A. Keep it off while connecting.
- Clip the positive (red) lead to the titanium piece. Clip the negative (black) lead to the steel electrode. Submerge the steel electrode in the electrolyte. Do not yet submerge the titanium.
- Turn the power supply on. Slowly lower the titanium into the electrolyte. A brief pulse of current will flow (typically under 1 second) and the color will appear almost instantly.
- Lift the titanium out, rinse with water, and inspect.
The anodizing is essentially complete when the current drops to near zero — the growing oxide layer is an insulator, and once it reaches the target thickness for the applied voltage, current stops. This usually takes under 5 seconds.
Gradient Anodizing
A single piece can show multiple colors as a smooth gradient — like the spectrum on titanium jewelry.
- Set the power supply to your highest target voltage (e.g., 65 V for turquoise).
- Submerge only the tip of the titanium piece in the electrolyte for 1–2 seconds, then lift out. The submerged portion now shows the color for that voltage.
- Reduce voltage to the next target (e.g., 45 V). Submerge slightly deeper than before, so that the previously anodized tip is in the air and a new section enters the electrolyte.
- Continue stepping down in voltage, submerging a new section each time.
Work from high voltage to low — a higher voltage will anodize over a lower-voltage color, but not vice versa. The final piece will show a gradient from the color of the lowest voltage at the deepest end to the highest voltage color at the tip.
Selective Masking
To anodize a pattern, apply strips of electrical tape or nail polish to the areas you want to protect. After anodizing, peel the tape to reveal the unanodized (natural grey) metal underneath.
What Is Happening
Anodizing is an electrochemical oxidation reaction. At the titanium anode:
\[\text{Ti} + 2\text{H}_2\text{O} \rightarrow \text{TiO}_2 + 4\text{H}^+ + 4e^-\]
Oxygen from water molecules is incorporated into the metal surface, building the oxide layer atom by atom. At the steel cathode, hydrogen is produced:
\[4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\]
The oxide layer grows inward into the metal, not outward on top of it, which is why the surface remains smooth and the dimensions of the piece barely change.
The color arises from optical thin-film interference. Light waves reflecting off the air–oxide interface and the oxide–metal interface travel different path lengths. If the path difference is a whole number of wavelengths of a particular color, those wavelengths are reinforced (constructive interference) and you see that color. Other wavelengths are cancelled out (destructive interference). As the oxide thickens, the path difference changes, and the reinforced color shifts — cycling through the spectrum.
Tips and Troubleshooting
Uneven or blotchy color — Usually caused by surface contamination (oils, fingerprints). Re-clean with isopropyl alcohol and try again. A quick re-sanding and cleaning recovers most pieces.
Color too pale or washed out — The surface may be too rough. Try polishing more finely.
Color went too far (wrapped to the next cycle) — You can’t remove the oxide chemically without damaging the surface. Sand back to bare metal and start again.
No color forms / current stays near zero — The electrolyte connection may be poor. Check that the steel electrode is actually submerged and the clips are secure.
Current too high (more than ~0.5 A) — Reduce the electrolyte concentration or add more water. High current causes uneven rapid growth.
Going Further
- Second cycle colors: Voltages above ~100 V re-enter the color sequence in a paler second cycle. 110 V gives a softer gold, 120 V a muted purple, and so on.
- Heat coloring: A propane torch can also produce interference colors on titanium (purple → blue → gold as you heat briefly). This is less controllable but requires no electricity.
- Patterned anodizing: Combine masking and multiple voltage steps to create detailed geometric designs on titanium sheet.