Approccio sperimentale per la definizione dei diagrammi Pedeferri degli acciai inossidabili

The Pedeferri diagram (potential vs. chloride concentration) is a cathodic prevention and protection diagram proposed for carbon steel reinforcement in concrete. The diagram predicts the conditions of localized corrosion (pitting and crevice corrosion), passivity (perfect and imperfect) as a function of the imposed potential and chloride content. The Pedeferri diagram can be extended to stainless steels which, as carbon steel in concrete, show a passive behaviour and can suffer localized corrosion in the presence of chlorides. The diagram depends on factors related to the metal, the electrolyte and on the experimental method used to determine it. Accordingly, it is necessary to define the proper electrochemical technique to determine localized corrosion and re-passivation potentials which define the diagram domains, since they depend on the adopted electrochemical test. Anodic potentiostatic and potentiodynamic tests were carried out on austenitic stainless steel AISI 304L in solution with different salinity. The effect of sample configuration and potential scan rate is also discussed. Potentiodynamic polarization test would seem preferable to the potentiostatic since it allows to obtain both localized corrosion and re-passivation potential and it is less time consuming. Tests confirm that potential scan rate is a critical parameter: on the one hand the high scan rate (such as defined by ASTM standard test method) reduces test time, on the other hand it can overestimate the resistance to localized corrosion