I have performed Mott Schottky plot of duplex stainless steel in 1M NaCl solutions using FRA POTENTIAL SCAN on a AUTOLAB PGSTAT 302N. frequency used was 1000 Hz and scanned from 0.9 V to -0.5 V.But not getting desired curve it should consist of both p type and n type or there should be two slope positive and negative. Can you point out what is wrong with the setup and procedure.