Initiation and inhibition of pitting corrosion of Incoloy 800 and 316 stainless steels

Cyclic voltammograms and potentiodynamic anodic polarization curves were obtained for Incoloy 800 and 316 stainless steels in varying concentrations of NaCl. The integrated charge amounts, q(a), in presence of Cl(-)ions was taken as a measure of the extent of pitting corrosion while the difference in values between pitting potential and protection potential (E(pit) - E(pp) was taken as a measure of pit repassivation ability. The pitting potential varies with the logarithm of Cl(-)ion concentrations according to sigmoidal - S - shaped curves which were explained on the basis of formation of passivitable, limiting active and continuously propagated pits depending on the concentration of Cl(-)ions. 316 stainless steel was found to have higher resistance to pitting corrosion than Incoloy 800. This was attributed to the presence of Mo in the former, addition of CrO42-, NO2-, VO3- and SiO42- to Cl- medium causes inhibition of pitting corrosion of bath alloys. The percentage inhibition efficiency (PIE) calculated from the change of the anodic charge (q(a)) in the absence and presence of inhibitors decreases in the order: CrO42- > NO2- > SiO42- > VO3-.