Titanium corrosion
In this article I want to show you the power of electrochemical impedance. Very easy. In this application I was asked to find Why titanium could tolerate 100 degrees Celsius in 1 M H2SO4. This is while processing nickel ores.
Figure 1 presents the response of a titanium electrode in sulfuric acid and 100 degrees C. As time progresses the semicircle is getting smaller. This means the corrosion rate (CR) on titanium is increasing. However, this increase was very small and the CR was very steady at all times. Then, after almost 3 hours the single semicircle transformed into this,
This is the impedance response in the presence of a failed coating! This was accompanied by a very high CR. The next figure shows the CR results.
If this is true, this technique is very informative about things happening at the interface! therefore, I rushed to make scanning electron microscope (SEM) image of the electrode surface, i.e.,
The figure to the right presents the naked zones of titanium metal. The darker zones are covered by titanium dioxide, TiO2. This is the failing coating. It peeled from the surface. It looks like a failed plastic liner! amazing!
All this is OK. But why this did not occurred in the presence of nickel ions in the sulfuric acid solution at 100 C????
Nickel ions get reduced in the presence of Ti(+3) ions at 100 degrees Celsius,
Ni(+2) + Ti (+3) ---> Ni + Ti (+4)
and this reaction favor the production of Ti(+4). In turns, this supports the formation of the TiO2 coating. Hence, nickel ions protect the titanium dioxide coating.
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