THERMAL OXIDATION
As stated earlier, titanium relies on the presence of an inert surface oxide film for its corrosion resistance. The film on a freshly pickled tube is about 20 angstroms thick (8 x 10^-8 in.). The slow growth of this natural oxide film over time in a neutral or oxidizing medium generally results in increasing corrosion resistance of the tube metal; therefore, any surface treatment which will thicken and toughen the oxide film will improve the metal's corrosion resistance.

This is the basis for anodizing which increases the film by means of an impressed anodic current. For many years, it has been customary to specify anodizing for titanium tubes operating in a severe environment, especially where hydrogen uptake was of concern.

However, recent work, both here and abroad has shown that a rutile TiO2 film formed by a thermal oxide treatment is superior to the anatase TiO2 film formed by anodizing.^(1)

It has been observed that anodized films can be removed easily by a simple acid pickle, whereas thermally produced films are much more resistant to an acid pickled bath and generally must be sandblasted or processed in a caustic descaling bath to effect removal.

Mill produced thermal oxide films often give the tube a blue- colored appearance. This is a result of diffraction of light through the protective oxide film. A typically dark blue tube has a film thickness of between 1000-2000 angstroms (4-8 x 10^-6 in.).

Work performed in TIMET's research facilities has demonstrated that thermally oxidized tubes have the following advantages over pickled and/or anodized tubes.

1. Improved corrosion resistance in reducing acids (see Table 3).

2. Improved resistance to crevice corrosion attack.

3. Improved resistance to hydrogen absorption (see Table 4).

It should be noted that thermal oxidation offers the useful advantages described under 1 and 2 only when marginally corrosive conditions are observed with commercially pure titanium. Naturally, the selection of more resistant titanium alloys TIMETAL 50A Pd or Code-12 (i.e., grades 7 or 12) offers a more conservative reliable and long-term solution to corrosion under more aggressive conditions.

The effect of current density of hydrogen uptake has been evaluated for specific thermal oxide surfaces (Table 5). Even though hydrogen uptake efficiency generally increases with decreasing current density, the relative reduction of hydrogen uptake offered by thermal oxidation as compared to the pickled condition appears to be greater at lower current densities. This suggests that thermally oxidized titanium could exhibit significantly improved resistance to hydrogen uptake in galvanic coupling situations where titanium is the cathode.