Titanium Anodizing
Summary
By making titanium the anode in an electrolyte, you grow a titanium-dioxide film whose thickness — and therefore its colour — is set precisely by the voltage, with no dyes at all. You’ll be able to explain why voltage controls the oxide thickness, how thin-film interference makes colour, and why polished metal looks more saturated.
History
Anodizing — growing a protective oxide by making a metal the anode of a cell — was industrialised in the early 20th century, first on aluminium to toughen and colour aircraft parts. Titanium’s version turned out to be prized less for protection than for colour: the very thin-film interference Isaac Newton1 catalogued in his “rings” in the 1600s, now tunable by a dial instead of by pressing glass.
Because the colours come from the oxide itself and not from any dye that could wear off or leach out, anodized titanium became a favourite for body-safe jewellery and for colour-coding surgical implants and instruments — a surgeon can tell one titanium part from another at a glance. Reactive-metal artists have worked with the same electric palette since the 1970s.
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 = brighter, matte = softer), electrolyte, and temperature. This sequence is for dilute sodium carbonate electrolyte.
Hazards & preparation
PPE: safety glasses; keep hands dry.
- Electricity in a liquid. Above ~50 V the shock risk is real — never touch both electrodes at once, keep hands dry, and switch off before adjusting connections.
- Sodium carbonate electrolyte is mildly alkaline; the cathode gives off a little hydrogen gas, so ventilate and keep flames away.
Disposal: the dilute carbonate electrolyte is low-hazard — flush down the drain with water. See the Safety page.
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): precise voltage and current limiting. Set current limit to ~0.5 A.
- 9 V batteries in series: 1 = 9 V, 2 = 18 V … 5 = 45 V. Covers most of the first colour cycle. Titanium to the positive terminal.
- Phone charger + USB boost converter: an adjustable boost converter can output 5–35 V from USB with a voltage dial.
For voltages above 50 V, take extra care with shock risk.
Surface Preparation
The colour only forms on clean metal; any oil or fingerprint shows as a blotch.
- If scratched or uneven, sand progressively (220 → 400 → 800 → 1200 grit). Matte 400-grit gives soft tones; a polish gives jewel-like saturation.
- Rinse with water.
- Wipe with isopropyl alcohol and dry completely.
- Do not touch the surface with bare fingers after this — handle by edges only.
Electrolyte Preparation
Dissolve 5 g sodium carbonate in 500 mL water (~1%) — conductive enough to anodize but dilute enough to keep the current low and controllable. Alternatives: baking soda (slightly less conductive) or ammonium sulfate (~10 g/L, keeps pH neutral).
Procedure
Single-colour anodizing
- Pour ~100 mL electrolyte into a glass or plastic container.
- Set the supply to the target voltage (colour table) and current limit ~0.2 A. Keep it off while connecting.
- Clip positive (red) to the titanium, negative (black) to the steel electrode, and submerge the steel — but not yet the titanium.
- Turn the supply on and slowly lower the titanium into the electrolyte. A brief pulse of current flows (under a second) and the colour appears almost instantly.
- Lift out, rinse, and inspect. Anodizing is complete when the current falls to near zero — the oxide is an insulator that self-limits at the thickness for that voltage.
Gradient anodizing
Work from high voltage to low (a higher voltage anodizes over a lower colour, not vice versa). Set the highest target voltage and dip only the tip for 1–2 s; then step the voltage down and dip a bit deeper each time, so a fresh section enters while the earlier one stays in air. The result is a spectrum gradient.
Selective masking
Apply electrical tape or nail polish to areas you want to protect, anodize, then peel to reveal the natural grey metal underneath in a pattern.
What you should see
The moment the clean titanium touches the electrolyte with the power on, it flashes a pure, saturated colour — champagne, violet, or a brilliant blue depending on the dial — and then stops, because the oxide seals itself. Step the voltage and re-dip and you can paint a whole spectrum gradient onto one piece, all from physics, no pigment.
| Problem | Likely cause | Fix |
|---|---|---|
| Uneven / blotchy colour | Surface contamination (oils, fingerprints) | Re-clean with isopropyl alcohol; handle by edges |
| Colour too pale / washed out | Surface too rough | Polish more finely |
| Colour “wrapped” to the next cycle | Over-voltage | Can’t strip chemically — sand to bare metal and redo |
| No colour / current near zero | Poor connection or electrode not submerged | Check clips; make sure the steel cathode is in the electrolyte |
| Current too high (>~0.5 A) | Electrolyte too concentrated | Dilute with more water |
What Is Happening
Anodizing is an electrochemical oxidation. At the titanium anode:
\[\text{Ti} + 2\text{H}_2\text{O} \rightarrow \text{TiO}_2 + 4\text{H}^+ + 4e^-\]
Oxygen from water is built into the surface, growing the oxide atom by atom. At the steel cathode, hydrogen forms:
\[4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\]
voltage drives oxygen into the titanium to grow a TiO₂ film of a set thickness; light bouncing off its two surfaces interferes to make the colour.
The oxide grows inward into the metal, so the surface stays smooth and dimensions barely change. The colour arises from thin-film interference: light reflecting off the air–oxide and oxide–metal interfaces travels different path lengths; wavelengths whose path difference is a whole number of wavelengths reinforce (that colour is seen), while others cancel. As the film thickens with voltage, the reinforced colour cycles through the spectrum.
Questions to Explore
Why does voltage control colour? The oxide grows ~2 nm/volt. Why does a set voltage stop growth at a set thickness?
Hint / answer
The oxide is an insulator, so as it thickens it drops more of the voltage across itself and the field pushing oxygen through weakens. Growth stalls once the film is thick enough that the field can no longer drive ions through — a thickness fixed by the applied voltage. More volts, thicker film, different colour.
Why do interference colours appear with no dye? How does a few-hundred-nanometre path difference make visible colour, and why cycle with thickness?
Hint / answer
Light reflects off the top and bottom of the clear oxide; the two reflections either reinforce or cancel depending on the film thickness versus the light’s wavelength. At a given thickness one colour is reinforced and seen. As the film thickens, which wavelength fits changes, so the colour marches through the spectrum — the same effect as an oil slick.
Why does a polished surface give more saturated colour than matte? Same oxide thickness — why the difference?
Hint / answer
A smooth surface reflects light cleanly in one direction, so the interference is crisp and the colour pure. A rough matte surface scatters reflections at many angles and thicknesses, blurring the interference and washing the colour out to a softer, greyer tone.
Why is titanium especially good for this? What makes TiO₂ better than aluminum’s or steel’s oxide?
Hint / answer
TiO₂ is transparent, has a high refractive index (strong, vivid interference), grows as a tight, even, well-adhering film, and is chemically tough. That combination gives bright, stable, uniform colours that many other metal oxides — duller, rougher, or less transparent — can’t match.
How do these colours differ from pigment colours? No dye — what practical consequences?
Hint / answer
Pigment colour comes from molecules absorbing light, which can fade, bleach, or wash off. Anodized colour is structural — it’s the oxide thickness itself — so it can’t fade or wash away; it only changes if the oxide is physically scratched or ground off.
Going further
- Second-cycle colours. Above ~100 V the sequence repeats in a paler series — 110 V a soft gold, 120 V a muted purple, and so on.
- Torch colouring. Briefly heating titanium with a propane torch grows the same interference oxide (purple → blue → gold) — less controllable, but no electricity.
- Patterned art. Combine masking with several voltage steps to lay detailed multi-colour designs onto titanium sheet.
- Related electrochemistry: split water with a current in Water Electrolysis.
Footnotes
Isaac Newton — English physicist (1643–1727) who catalogued thin-film interference colours (‘Newton’s rings’).↩︎