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A Not-so Noble Metal

Titanium isn't corrosion resistant after all...

Few metals are actually resistant to corrosion when exposed to oxygen. But a limited group known as  "Noble Metals", including gold, silver, and platinum, claim the elevated stature of invulnerability to oxidation. Although titanium is often noted for its corrosion resistance, Ti alone cannot claim any level of nobility as it will readily react with oxygen to form a thin layer of titanium dioxide (TiO2) on its surface. But not all is lost in terms of corrosion resistance. It is actually the very slow growth of this 1-20nm thick (depending on exposure time with oxygen) oxide layer that creates a protective barrier against continued oxidation of the underlying Ti metal. It is this thin oxide layer that actually provides the corrosion resistance often associated with titanium and its alloys.

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Ti Anodizing in a Nutshell

Using electricity to manipulate the thickness of the Ti oxide layer

Titanium anodizing is the process of using an electrolytic cell to build a specific thickness of TiO2 on the surface of titanium parts, perferably those in the form of bike frames, components, or bottle cagesIf you're not familiar with the components of an electrolytic cell and how it works then buckle-up buttercup, and check out the fairly awesome graphic I made below. If you are familiar...

In general, the anodizing process involves connecting a titanium workpiece/part to the positive terminal of a DC power supply, and submerging the part in an electrolyte solution. You've now got the anode half of the cell ready party.  A second conductive piece of metal, the cathode, is connected to the negative terminal of the power supply and is also submerged in the electrolyte solution (avoiding contact with the anode!). Along with H2O, the electrolyte solution is made up of disociated ions that increase conductivtiy beyond that of just plain water. The electrolyte is key to ensuring no voltage drop does occurs between the anode and cathode.

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When activated, the power supply generates a specified voltage difference between the anode and cathode that forces electrons to boogie and flow. The voltage differential begins to pull electrons from the Ti anode while also returning electrons to the electrolyte solution at the cathode. The positive charge of Ti ions on the surface of the anode results in the electrolysis of water molecules in the electrolyte solution near the the electrolyte/oxide interface. Electrolysis splits the H2O molecules into H+ cations and both O2− and OH− anions. Due to the negative ionic charge of oygen, the available O2- anions diffuse through the existing oxide layer to react with the postively charged Ti cations. The Ti oxide formation rate is governed by the diffusion and reaction rates of the oxygen anions within the oxide layer. Newly formed TiO2 pushes up on the existing oxide and gradually grows outward from the Ti surface.

 

The overall reaction can be summarized with the following:

Ti+2H2O→TiO2 +4H+ +4e−

Note the lack of electrolyte ions in the reaction. The oxidation and reduction reactions taking place at the anode and cathode, respectively, do not involve any dissociated ions from the electrolyte. However, the electrolyte is still a critical component of anodizing. This is because water alone does not provide a high level of conductivity. To increase conductivity and prevent a drop in voltage between the anode and cathode, an electrolyte is used.

The Color of Ti

How color interference creates a rainbow on the surface of titanium...

White light consists of a range of different wavelengths (380-700 nm) that make up the visible color spectrum, which can be detected by the human eye. This is best demonstrated by the dispersion of light by a prism, which seperates the wavelengths into a rainbow of colors (remember that science class?). In most cases, our eyes detect the color of an object due to the presence of pigments or dyes that reflect (typically at the molecular level) a particular wavelength of the visible spectrum while absorbing the others. The corresponding color of that particular reflected wavelength is then interpreted by our eyes as the corresponding color. As an example, the color you see when looking at anodized aluminum parts works in this way as it requires a dye after anodizing to achieve the desired color. But titanium is different...

 

Rather than reflecting a specific color wavelength, color interference produces color by reinforcing (constructive interference) certain wavelengths of the visible color spectrum, while cancelling out (destructive interference) other color wavelengths. This is probably easier to visualize, so take a look below. When a piece of anodized titanium is exposed to light, the clear oxide layer allows a portion of light to pass through it to be reflected off the surface of the underlying Ti metal below the oxide layer. But some of the light doesn't make it through the transparent TiO2, and is insted reflected off the surface of the oxide layer. This is actually very similar to how light reflects of the surface of a body of water, but a certain magic happens because the Ti oxide layer is so thin (20-200 nm). The thin layer causes the light reflected off the surface of the oxide layer to interfere with light being reflected off the surface of Ti metal. Depending on the thickness of the oxide layer, certain wavelengths will be reinforced while others are canceled out. The reinforced wavelengths are what reach our eyes and therefore what we interpret the color of the titanium to be.

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