Pedeferri diagrams of AISI 304 and AISI 316 stianless steels : effect of chlorides on pitting and repassivation potentials

Stainless steels are widely adopted in corrosive environments and their resistance to local-ized corrosion is one of the most studied and investigated property. They may suffer in fact of localized corrosion attack, identified as pitting and crevice. Materials engineers need practical tools to select materials for specific corrosive environments as a function of chlo-rides contents, temperature and other parameters. Potentiodynamic polarization behavior of AISI 304 and AISI 316 stainless steels in NaCl solution was investigated in order to evaluate the variation of pitting potential, EPIT, and repassivation or protection potential, EPRO, by varying the chlorides content of the solution. The aim of this work is to contribute in setting up potential-chlorides diagram (also called Pedeferri diagram) for both materials. This work involved a wide literature survey and ex-perimental work to enrich the diagrams with the obtained data, also providing a comparison between them. The importance of Pedeferri diagram stays in its capability of predict pitting and protection potentials ranges where pitting corrosion and repassivation occur for a certain potential and for certain environmental parameters. The data deviation of EPIT and EPRO was studied, along with that of corrosion potential, ECORR. Metastable pitting phenomenon, i.e. repassivation of pits with dimensions smaller than the critical one for stable pit growth, was found and its behavior against chlorides concentration increases on both materials was stud-ied too. In line with the theoretical explanation, the results showed that the mean values and the data deviation of the three mentioned potentials was decreasing with increasing chloride con-tents. Also metapitting potential values were decreasing with almost the same trend of EPIT and EPRO. Moreover, AISI 316 proved to have higher pitting and protection potentials than AISI 304 because of its higher PREN, so the resulting Pedeferri diagram was shifted to-wards nobler potential values.