Thanks for posting. I’m an ME with a few materials courses 40+ years ago, but I never studied anodizing. This guy does a great job of putting the process into easy to understand lay terms. What I didn’t understand is why titanium can use light interference based on the oxide thickness to create different colors, but aluminum uses the dye process.
i think it's because the the Oxides of Aluminum are either white or transparent unless doped with some impurities such as chrome. However, as he posted out the centers of the hexagonal areas are hollow and will accept a inserted dye, which will give you a whole gamut of dye colors not available with doping.
The explanation of the titanium seems to show that between the surface and the substrate of the Titanium oxide is always exactly a half-wave length of the inciding light so the thicker the coating the different the wave length that will interfere, ergo, the different the wavelength that will refract while the other colors are either absorbed or scattered. At least that was what I got out of his explanation.
I would think because of the different properties of the two metals’ oxides would produce different results.
Think of the visual differences between Aluminum oxide, Iron oxide, and Copper Oxide for instance.