Contribution to the discussion on Localized Corrosion of Stainless Steels in natural sea water

Electrochemical hysteresis methods are employed to develop experimental potential-pH diagrams for some commercial austenitic and ferritic stainless steels in 3% sodium chloride solutions at room temperature. The results were compared with the electrochemical and the gravimetric behaviour of the single stainless steels in natural sea water. The comparison suggested some considerations about the mechanism of initiation and growth of pitting and crevice corrosion prevailing in natural environment. The higher probability of pits observed in field exposure was correlated to the deposit of a microbiological slime on metal surface that ennobles the free corrosion potentials of passive surfaces up to +400 ÷ +450 m V SCE. The crevice corrosion initiation was in turn attributed to a moderate local acidification originating in conditions of reduced diffusion under the action of passivity current that makes possible pit nucleation which is otherwise unlikely to occur. The pitting growth speeds up the acidification process in the interstice until it causes general corrosion of the walls.