Why does changing the voltage of titanium anodizing change the color of the coating?

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I watched a video that fascinated me. The uploader was anodizing a titanium knife clip using 9V batteries and anodizing solution. He used the same solution throughout the whole video, but each time he connected another 9V battery to change the color of the coating.

Adding just one battery changed the color drastically. It went from purple to bright blue to green to gold.

How does simply changing the input voltage so drastically change what color the anodized coating is? And why does it seem to follow the visible spectrum?

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6 Answers

Anonymous 0 Comments

This is the video: https://youtu.be/9Eut-Bu4Mpw

Anonymous 0 Comments

This is the video: https://youtu.be/9Eut-Bu4Mpw

Anonymous 0 Comments

As a prerequisite, anodizing is the process of electrically forming/thickening a layer of oxide on a metal. This is similar to induced rusting and has a variety of purposes in metals like aluminum and titanium, including providing a natural protective coat, resistant to corrosion.

The color from titanium anodizing comes not from dyes but from optical interference, similar to a prism. This interference and the apparent color depends on the thickness of the oxide layer formed by anodization; and the thickness is controlled by a combination of the voltage applied and the duration of the procedure. Voltage is usually the primary control for titanium because it’s a more reliable and precise control than duration.

Anonymous 0 Comments

The colour is determined by the thickness of the oxide layer on the surface of the metal. Basically the layer thickness is on the same size scale as a single wavelength of visible light waves, causing some neat reflections to happen.

Changing the voltage changes the oxide thickness by a corresponding amount, which in turn changes the colour. So as the oxide layer thickness progressively changes (within the range of thicknesses corresponding to the wavelengths of visible light), the colour also progresses in order through the visible spectrum.

Anonymous 0 Comments

As a prerequisite, anodizing is the process of electrically forming/thickening a layer of oxide on a metal. This is similar to induced rusting and has a variety of purposes in metals like aluminum and titanium, including providing a natural protective coat, resistant to corrosion.

The color from titanium anodizing comes not from dyes but from optical interference, similar to a prism. This interference and the apparent color depends on the thickness of the oxide layer formed by anodization; and the thickness is controlled by a combination of the voltage applied and the duration of the procedure. Voltage is usually the primary control for titanium because it’s a more reliable and precise control than duration.

Anonymous 0 Comments

The colour is determined by the thickness of the oxide layer on the surface of the metal. Basically the layer thickness is on the same size scale as a single wavelength of visible light waves, causing some neat reflections to happen.

Changing the voltage changes the oxide thickness by a corresponding amount, which in turn changes the colour. So as the oxide layer thickness progressively changes (within the range of thicknesses corresponding to the wavelengths of visible light), the colour also progresses in order through the visible spectrum.