Presents the Math

Original article © by A. Daniel Klarmann, 2007

How much Oxide makes a color
and how much Electricity does it take?

To start with, I'll assume you've read my pages on anodizing titanium and the basic physics of the color. Now, you want to know if you have the power.

Assumptions for this approximate number:

• optically smooth metal surface
• incident and reflected light both normal to surface
• color produced purely by reflective/refractive reinforcement
• oxide thickness is uniformly half of wavelength in oxide at refractive index
• Pure titanium and pure titanium dioxide, no inclusions
• Benign electrolyte, nothing to bond directly to titanium (like nitrogen, chlorine, etc)
• No metal migration to cathode
• No sharp corners or burs to bleed current
• 1 oxygen atom/ion (O^--) migrates per 2 electrons injected (100% efficient = each electron strips off one hydrogen)

Steps to calculate the volume of oxide under these ideal conditions and the amount of electricity needed:

• oxideVolume = area * (wavelength / 2) / refractiveIndex
  The refractive index is needed because light is slower in solids than in a vacuum
  Divide by 2 because the distance traveled inand out is twice the thickness therefore the wavelength is that much shorter at the same photon energy.
• oxideMass = oxideVolume * specGrav
• oxideMolecules = oxideMass / moleWt * Avagadro
  Units: g / (g/mole) = moles, * molecules/mole = molecules
• total amp-seconds = 4 * oxideMolecules / ElectronsPerCoulomb
  Times 4 because have 2 oxygen atoms per molecule and 2 free electrons per oxygen atom.

Example:

Let's turn a 1 square centimeter flat piece of titanium yellow:
Ignoring edges, that gives us 2 cm²
oxideVolume = area * (wavelength / 2) / refractiveIndex
= 2 * (6 x 10^-5 / 2) / 2.7
= 2.22 x 10^-5 = 0.0000222 cc

oxideMass = oxideVolume * specGrav
= 2.22 x 10^-5 cc * (4.2 g/cc)
= 9.32 x 10^-5 = 0.0000932 g

oxideMolecules = oxideMass / moleWt * Avagadro
= 9.32 x 10^-5 g) / (79.87 g/mole) * 6.02 x 10²³
= 7.0 x 10¹⁷ = 700,000,000,000,000,000 molecules

amp-seconds = 4 * oxideMolecules / ElectronsPerCoulomb
= 4 * (4.5 x 10²¹) / (6.24 x 10¹⁸)
= .45 amp-seconds or 100 ma for 4.5 seconds

BUT in the real world, you probably need several times that much for reasons alluded to in the Assumptions section above.
Also, the current starts high, and drops exponentially fast as the oxide builds up.

Check out my anodizing page for more detailed instructions on coloring titanium with an anodizer.
Check out my physics page for how the color actually works.
Read my titanium details page for information about the history, chemistry, and so forth of element 22