Ti/TiO2/Ni2+ interface: unexpected protection of Ti by Ni2+ cations in hot sulphuric acid

The processing of nickel ores involves: high temperatures (>100 degrees C), high [H2SO4] and high [Cl-]. Despite the severity of this service, pure titanium can be used because it presents very low corrosion rates. Looking for the fundamental causes of this behaviour, it was found that the potential of the couple Ti4+/Ti3+ shifted to more negative potentials than the couple Ni2+/Ni in these conditions. Thus, Ni2+ was reduced to Ni metal, oxidising Ti3+ to Ti4+. This increased the concentration of Ti4+ which was critical for the formation of the protective TiO2 layer. In turn, the TiO2 passivated the active titanium dissolution. Also, the deposited Ni catalysed the cathodic kinetics, producing a mixed potential in the passive region for titanium. This process involves Ti/TiO2/Ni2+ and Ti/Ni2+ interface systems, and Ti/Ni2+ behaviour is somewhat like a titanium-nickel alloy.