Corrosion of Sintered Chromium in HNO<SUB>3</SUB> Solution

Corrosion behaviours of 99.83%Cr, Fe-Cr alloys and Ti were investigated by measuring polarization curves and corrosion rates in the solutions of various HNO3 concentrations.Ti and the Fe-30Cr alloy were severely corroded by less than 60%HNO3 solutions above 400 K. On the other hand, Cr had much better corrosion resistance than Ti and the Fe-30Cr alloy in these conditions. It is because the corrosion potential of Cr is kept in the passive and the surface film of Cr in the passive state is very stable. However, with increasing concentration of HNO3, the corrosion potential of Cr sifted in a much more noble direction by the cathodic reaction of NO−3, and then the trans-passive dissolution reaction as Cr6+ occurred. Therefore intergranular corrosion of Cr occurs in more than 65%HNO3 solutions above 400 K. As the formation reaction of Cr3+ from Cr6+ is also included in the corrosion reaction of Cr in HNO3, the corrosion of Cr in HNO3 is accelerated by this cathodic reaction.