Abstract
The dissolution behavior of Co, Mo, Ni-Cr-Co alloys and Ni-Cr-Mo alloys has been studied in 3 mol/dm3 NaNO3 solution at 30°C. The polarization curves and the dissolution current efficiency were measured in a flow cell. Co and Mo were always active at the potentials and current densities where Ni-Cr alloys were passive and secondary passive. These components promoted the transpassive and secondary transpassive dissolution of the alloys, dependent on the Cr level. The additions of more than 40%Co or more than 10%Mo to the about 10% Cr alloys resulted in the disappearance of the transition from the transpassive to secondary passive state. The current density establishing the onset of the secondary transpassive dissolution decreased with increasing the Co or Mo content. For the alloys containing more than 20%Cr, the effects of Co and Mo had only a secondary importance on activation of the secondary passivity. The effect of Mo on promotion of the transpassive dissolution was discussed briefly by XPS study of the surface film.
