PASSIVATION THRESHOLDS IN IRON-CHROMIUM ALLOYS PREPARED BY ION-BEAM SPUTTERING

The threshold contents of chromium required for chromium-like passivity of Fe-Cr alloys have been determined using sputtered thin films with lateral concentration gradients. Clear thresholds are found for Fe-Cr alloys at 10 and 17 at.% for passivation, and 17 and 13 at.% for reactivation, in sulphuric acid solution. The former two values coincide with the critical compositions for 1st-2nd-3rd nearest-neighbour (p(c){1,2,3}) and 1st-2nd nearest neighbour (p(c){1,2}) site percolation for the b.c.c. lattice, respectively. The reactivation threshold was found to change from p(c){1,2} to 13 at.% when the passivation potential was altered from -250 mV(SCE) to a high value, such as 500 mV(SCE).