The Reason For Titanium Surface Oxide Film Generation

The Reason for Titanium Surface Oxide Film Generation

The underlying reason for the very existence of metallic titanium in most common environments is that it rapidly and spontaneously forms an oxide film with exceptional stability and properties that allow it to effectively isolate the metal from the environment.
Hence, titanium's corrosion resistance and ability to resist hydrogen absorption are dependent on the properties of this protective film.

The composition of thin titanium oxide films on titanium in an aqueous solution remains the subject of some controversy; however, it is clear that the main constituent of the films is Ti02. In addition, it seems likely that within the Ti02 matrix there exist some reduced metal cations and non-stoichiometry.

The formation of a passive film happens spontaneously upon exposure of titanium to oxidants such as air or water. Even in deaerated solution, a pH 5 is required if spontaneous passivation is to be avoided. Even under these acidic conditions. anodic film formation passivates the metal if potentials positive of about -0.3 V an applied.

It is generally accepted that anodic oxide film growth on titanium occurs by a field-assisted ion transport mechanism.Imagine a freshly exposed titanium surface reacting with a source of oxygen (H20. 02, etc.) in direct contact with the metal surface. As the first thin continuous layer of oxide is formed, it immediately becomes a physical Wet separating the two reactants( the source of Ti is found on the metal side of the oxide layer, the source of oxygen on the other). This, of course, inhibits further oxidation of the metal. In order for oxidation to continue, that is,  for the oxide film to thicken, the two components of the film must come together. This requires that one or both be transported through the existing surface oxide.

This process is further complicated by interfacial reactions at the metal/oxide, oxide/solution, or oxide/gas interfaces that convert the reactant sources (Ti metal, H20, Oz, etc.) into species suitable for transport through the oxide. In addition, whatever film thickening does occur serves to enhance the barrier to further oxidation. Thus. oxide film growth is largely controlled by the solid-state properties of the film itself.