Titanium Anodizing Science
A Not-so Noble Metal
Titanium isn't as resistant to corrosion as you might think...
Few metals are actually resistant to corrosion when exposed to oxygen. But a limited group of "Noble Metals", including gold, silver, and platinum, are able to claim the elevated stature of oxidation invulnerability. Although it is often noted for its corrosion resistance, titanium is actually not a member of the noble class and will readily react with oxygen to form a thin layer of titanium dioxide (TiO2) on its surface. But your Litespeed salesperson wasn't lying to you about the lifetime of corrosion resistance. The very thin oxide layer (1-20 nm thick depending on exposure time with the atmosphere) that forms naturally provides a protective barrier against continued oxidation of the underlying Ti metal. It is actually this layer of TiO2 that has given titanium its reputation when it comes to withstanding the elements.
How Titanium Anodizing Works
How electricity and chemistry form TiO2
Titanium anodizing describes the process of using an electrolytic cell to build a specified thickness of oxide (TiO2) on the surface of titanium parts (perferably in the form of bike frames, components, or bottle cages). If you're not familiar with the parts of an electrolytic cell and how it works, then buckle-up buttercup, and check out the fairly awesome graphics below. If you are familiar... well...
In general, the anodizing setup involves connecting a titanium workpiece/part to the positive terminal of a DC power supply and submerging it in an electrolyte solution. You've now got the anode half of the party in place. A second conductive piece of metal, the cathode, is also submerged in the electrolyte solution (making sure to avoid contact with the anode!) and connected to the negative terminal of the power supply.
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Activate the power supply and the magic of chemistry happens. The generated current and voltage differential between the anode and cathode forces the electrons within the electrolytic cell to boogie. Moving in the direction of the current, electrons are removed from electrolyte solution at the anode, and returned at the cathode. It is this transfer of electrons that drives oxidation-reduction reactions at the anode and cathode, respectively, and utlimately leads to the formation of TiO2 on the surface of the Ti anode. The oxidation and reduction reactions that are necessary for anodizing (and electrolysis in general for all you hydrogen junkies) are possible due to the asymetric molecular structure of a water. The H2O molecule is characterized by a polar covalent bond between oxygen and hydrogen atoms. The bond is relatively weak and results in an uneven charge density across the molecule. The oxygen side carries a slightly negative charge, while the hydrogen side of the molecule is slightly positive in charge. And it is this small characteristic of H2O that makes anodizing possible. Within the electrolytic cell, the negative charge of the cathode pulls at the hydrogen side of the H2O molecules, while the oxygen side is pulled toward the positive charge of the anode.
As each free electron enters the solution at the cathode, the freebie negative charge is too much for one of the H atoms of a H2O molecule to resist. An H+ ion seperates from the H2O molecule to grab the electron. This leaves a negatively charged hydroxl ions (OH(-)) behind with a negative charge that pulls it toward the postive anode. After gobbling up a free electron, the now nuetral H atom quickly reacts with a partner H atom that has also grabbed a free electron to stablize as H2 (hyrdogen gas - ahhh thats what those bubbles on the cathode are!). This is the reduction reaction. The oxidation reaction occurs when negatively charged OH(-) ions reach the anode. Here electrons are pulled from the Ti anode resulting in Ti(4+) cations to be distributed across the surface of the Ti anode. The positive charge of the Ti(4+) cations now act to finish the dismantling of H2O molecules that started at the cathode. Each Ti(4+) ion pulls apart two of the OH(-) anions near the surface of the anode. This results in the release of two more free H(+) ions as well as two O(2-) ions that quickly react with the Ti(4+) ion to form (you guessed it) TiO2.
So what's up with the electrolyte? Along with H2O, the electrolyte solution is made up of dissociated ions that provide increased conductivtiy. The addition of these ions to the water is critical to ensure that no voltage drop occurs between the anode and cathode as water alone does not provide a high level of conductivity. But note the lack of the dissociated electrolyte ions involved in the oxidation and reduction reactions. Consequently, anodizing can be accomplished using a variety of different electrolytes such as trisodium phosphate (TSP).