Effect of alloying elements on the contact resistance and the passivation behavior of stainless steels

The effects of alloying elements (Cr, Mo) on the passivation and the transpassive transition behaviour of various commercial stainless steels, which are candidate materials for the bipolar plates in polymer electrolyte membrane fuel cell, were investigated via a contact electric resistance (CER) technique with graphite counter part. The contact resistance, too high for a semi-conductor film with high donor density, is not the inherent ohmic resistance of the passive film. The CER potentials decrease linearly with the pitting resistance equivalent number (PREN). The passive films of stainless steels with high PREN are more readily removed compared to those with low PREN in terms of the charge necessary to remove the passive films. The change in transpassive transition behaviour of the stainless steels from that of iron to that of chromium went on steeply with an increase in the PREN. (C) 2002 Published by Elsevier Science Ltd.