Titanium metal anodizing creates an oxide layer on titanium with direct current

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  What titanium metal anodizing creates at the surface
• 2
  How voltage control sets thickness and color
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  Why Ti-6Al-4V and CP titanium behave differently
• 4
  Where anodizing helps and where it does not
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  Process control points that decide the result
• 6
  Anodizing titanium compared to other titanium finishes

Drop a titanium part into a weak acid bath and pass direct current through it, and oxygen forms at the anode. That oxygen does not sit on top of the metal. It bonds with titanium atoms already at the surface, so the oxide film grows inward and outward at the same time. The result is a film that is part of the part, not a layer painted onto it.

The electrolyte is usually a dilute acid or a neutral salt solution. Sulfuric, phosphoric, chromic and oxalic baths all work, and each one shifts the final color slightly at the same voltage. Concentration matters less than people expect. Current density and the voltage ceiling do most of the work.

Because the film is grown from the substrate, it cannot chip or peel the way a plating can. That is the main reason aerospace and medical engineers specify it. A scratched oxide film on titanium is a cosmetic issue, not a corrosion path, as long as the scratch does not reach bare metal.

The film is also an electrical insulator. Once it reaches a few hundred nanometers, the bath current nearly stops. That self-limiting behavior is what makes voltage, not time, the control knob on the rectifier.

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  Grown, not depositedOxygen bonds with surface titanium atoms; no adhesion layer exists to fail.
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  Self-limitingFilm growth slows sharply once the set voltage is reached.
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  Substrate-dependentAlloy chemistry changes the color produced at a given voltage.

Thickness tracks voltage in a nearly linear way at the start. Roughly 1.5 to 2.5 nm of oxide grows per volt, depending on the bath and the alloy. Set 20 V and you get a thin interference film. Set 80 V and you get a much thicker one. Time only decides whether you reach the set point, not how thick the film ends up.

Color comes from light interference, not from dye. White light hits the film, part of it reflects off the outer surface, part off the metal interface, and the two reflected waves cancel or reinforce at different wavelengths. That is why the same bath gives bronze at 15 V, purple around 25 V, blue near 35 V, and green-gold past 60 V. Change the alloy and the whole ladder shifts.

Practical shops work in bands rather than single values. A 5 V step between parts is often visible to the eye, so if a customer wants a matched set, run the whole batch in one bath at one voltage. Splitting a batch across two rectifiers is the most common cause of color mismatch.

High voltage is not free. Above roughly 100 V, bath heating and sparking become a real risk, and the film can turn powdery. For most decorative and functional work, 10 to 80 V covers everything you need.

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  15 VBronze to light gold, thin film, minimal dimensional change.
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  25–35 VPurple to deep blue, the most requested decorative range.
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  60–80 VGreen-gold to gray, thicker film, better wear resistance.

Commercially pure grades such as TA1 and TA2 anodize predictably. The color ladder is consistent, and shops can quote a voltage without testing. CP titanium is the easy case.

Ti-6Al-4V (TC4, Grade 5) is another story. Aluminum and vanadium in the alloy change how the oxide forms across alpha and beta phases. The same 30 V that gives a clean blue on CP titanium may give a duller, slightly mottled blue on Ti-6Al-4V. It is still a usable finish, just not identical between alloys.

That difference matters for assemblies. If a bracket is Ti-6Al-4V and a cover plate is CP titanium, and both are anodized at 30 V, they will not match. Either anodize them in the same alloy, or accept the difference, or pick a non-color finish such as bead blasting.

For medical and dental work, the alloy also affects biocompatibility claims. The oxide film itself is stable and inert, but the surface chemistry before anodizing matters. Parts must be clean, free of embedded iron from tooling, and passivated if the spec calls for it.

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  CP gradesConsistent color ladder, easiest to spec by voltage alone.
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  Ti-6Al-4VSlightly muted color, phase-dependent variation across the surface.
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  Mixed assembliesDifferent alloys at the same voltage will not match.

Anodizing is a surface treatment, not a repair. It will not hide machining marks. In fact, a thin interference film makes them easier to see, because light reflects off both the film and the metal beneath. If the part needs a uniform look, bead blast or polish before anodizing, not after.

The film is thin. A 30 V blue film is typically 50 to 80 nm, which is far below any tolerance that matters on a machined feature. On a part held to ±0.005 mm, anodizing will not push it out of spec. That is one of the finish's best properties for precision work.

Wear resistance is modest. Anodized titanium is not a hardcoat like Type III on aluminum. It resists light abrasion and finger oils, and it holds up in cleanroom handling, but it will wear through on sliding contact against steel. If the application involves sliding wear, look at a different approach.

Corrosion resistance is good but not unlimited. The natural oxide on titanium already protects it in most environments. Anodizing thickens that layer and adds color, but it does not turn titanium into a material for hot concentrated acids or fluoride-bearing media.

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  Good fitColor coding, cleanroom parts, wear plates with light contact.
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  Poor fitSliding wear, hiding tool marks, hot acid service.
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  Dimensional impactNegligible at typical voltages; no re-machining needed after.

Cleaning comes first. Any oil, coolant residue or embedded iron from machining will show up as a blotch or a color shift. Titanium parts should be degreased, rinsed in deionized water, and handled with clean gloves before they go into the bath. Bare fingers leave enough residue to change the finish.

The cathode matters too. A titanium or stainless cathode with enough surface area keeps current density even across the part. Small cathodes force higher local current, which can cause edge effects and uneven color on complex geometry.

Agitation and temperature control keep the bath stable. A bath that drifts a few degrees Celsius over a long run will produce a visible color drift across the batch. For color-critical work, run the bath at a fixed temperature and check it between parts.

Racking is the quiet failure point. Parts must be firmly connected so current flows, but the contact point itself will not anodize and will show a small bare spot. Design the rack contact on a non-cosmetic face, or plan a touch-up step.

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  CleanlinessDegrease, DI rinse, glove handling before the bath.
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  Cathode areaLarge enough to keep current density even.
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  Bath stabilityFixed temperature and agitation for color-critical runs.
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  Rack contactPlace on a non-cosmetic face to hide the bare spot.