Passivity of Tb_0·3Dy_0·7Fe_1·92in alkaline solution and its breakdown by chloride ions

Abstract

The passivation and pitting corrosion behaviour of stoichiometric Laves phase magnetostrictive compound Tb 0?3 Dy 0?7 Fe 1?92 , was investigated in 0?02M NaOH solution at different chloride ion concentrations (0?001-0?1M) by cyclic potentiodynamic polarisation. A systematic decrease in breakdown potential and corresponding increase in anodic current densities was observed with increasing chloride concentration. Potentiostatic currents were measured at several applied potentials in the passive range. The incubation period of pitting decreased with increasing chloride concentration. Microscopic examination of corroded surfaces after cyclic polarisation scans revealed that the attack by chloride ions was localised along arrays of active rare earth rich phase. At higher chloride levels, these phases were undermined. The number of pits and pit size increased with increasing chloride concentration. Electrochemical impedance spectroscopy conducted at open circuit potential and 50 mV above breakdown potential confirmed increasing corrosion rate with increasing chloride concentration. It has been concluded that the significant mechanistic step in the pitting corrosion of Tb 0?3 Dy 0?7 Fe 1?92 is adsorption of metal-chloride complex species.