Electrochemical approach for the definition of the Pedeferri’s diagrams of stainless steels

The Pedeferri’s diagram of stainless steels aims to define corrosion and passivity (perfect and imperfect conditions) as a function of the imposed potential and chloride content. The conditions can be obtain by determination of pitting and repassivation potentials by means of proper experimental methods. In this work, initiation of localized corrosion was evaluated by localized corrosion potential measurement by means of anodic potentiodynamic polarization test. Two kinds of sample holder were used in order to compare the effect of sample holder geometry on the occurrence of localized corrosion. One of them was according to the ASTM standard and the other one has a different geometry. Also the effect of sample orientation (horizontal or vertical) and surface area was investigated as well. The results showed that localized corrosion initiation depends on the sample holder configuration and sample geometry because of the presence of crevice zones. Moreover, for specimens of 2 cm2 and 4 cm2 the dominant mechanism of localized corrosion was pitting, however, for 1 cm2 the dominant mechanism was crevice corrosion due to the higher perimeter to surface area ratio. Finally, the sample direction changes did not show considerable differences in terms of localized corrosion potential.