Electrochemical behaviour of sintered yttria reinforced ferritic stainless steels

The electrochemical behaviour of 434L ferritic stainless steels produced by the powder metallurgy (PM) route has been studied. The effects of sintering temperature (1250°C and 1400°C) and dispersoid addition (10%Y 2 O 3 ) have been investigated by potentiodynamic polarisation and electrochemical impedance studies in 0·05M H 2 SO 4 solution. All the alloys exhibited active-passive behaviour in the electrolyte. The critical current density for passivation was lower in the case of the PM samples when compared with 430, which has been attributed to the presence of Mo in the samples. The passive current density of the PM samples correlated with the sintered porosities. All the other passivation parameters were similar for all the materials. The nature of passive film was investigated using electrochemical impedance spectroscopy. A lower capacitance (i.e. more corrosion resistant) surface film was obtained after sintering at the higher temperature. Yttria dispersoids in ferritic stainless steel decreased the corrosion resistance of the surface film after sintering at the higher temperature, whereas they did not significantly affect the surface film behaviour after lower sintering temperature. The results have been correlated to microstructural parameters of the PM materials.