The corrosion resistance of three special steels Improved by thermo-reactive diffusion (TRD) was investigated Samples prepared from the test materials were treated at graphite crucible bath where borax boric acid and ferro-silicon were mixed The coating process was done at 1220K for 6 h The corrosion resistance of the bonded samples was evaluated by the Tafel polarisation and electrochemical impedance spectroscopy (EIS) The results show that the boronizing process can Improve the corrosion resistance of the steels Also coating porosity with exposure time has been determined using the corrosion potential difference between substrate and coating and polarisation resistance R-p which is obtained through EIS modelling using equivalent circuits (C) 2010 Elsevier B V All rights reserved