Pietro Pedeferri: l'ossidazione anodica tra scienza e arte

The paper is oriented to the study of chlorides corrosion of carbon steel in simulating alkaline concrete pore solutions.Cyclic voltammetries (CV) were performed in order to evaluate the influence of pH, in the range of 12.6-13.8, on theinhibiting properties of nitrite and anions of two organic acid, lactate and aspartate, on localized corrosion initiation.The results confirm that the corrosion resistance of steel in solutions with the same pH of concrete is strictly dependentupon the chlorides-alkalinity ratio (Figures 6, 7). The inhibitors showed an additive contribute to alkalinity withdifferent weight for the three substances considered during testing.The CV tests were performed in aerated solutions at room temperature on disk specimens machined from a carbonsteel bar, with an exposed surface of 0.67 cm2. Disk electrode (RDE) rotating at 2500 rpm was employed for reducingthe effect of diffusion. The test was performed after conditioning of the working electrode at -1 V vs SCE for 60 s, reequilibrationfor 15 s and two polarization cycles between -1.7 and + 1 V vs SCE at 50 mV / s scan rate.Before test, the specimen was passivated in the test solution for 72 hours to grant the formation of a stable protectivelayer on the steel surface without chloride addition. The inhibitors were added into the passivation solution beforethe immersion of specimen, at different concentrations, 0.01, 0.1 and 1 M for sodium nitrite, 1 M for sodium lactateand 0.1 M for sodium aspartate. Two characteristic potentials related to pitting initiation susceptibility, E1,1st and E1,2nd,were derived by the first and the second CV cycle, respectively (Figures 1, 3, 4, 5). The results confirm that the criticalthreshold for localized corrosion initiation can be expressed as a function of a critical [Cl-] / [OH-] ratio; the probabilityof pitting initiation is low in the case of [Cl-] / [OH-] ratio lower than 0.6 (eqs. 1-4). The nitrite addition causes anincrement in the critical [Cl-] / [OH-] ratio (Figure 8-10) that can be quantified by introducing an efficiency parameterf* (eq. 9; Figure 13). This parameter defines the effectiveness of the inhibitor compared to alkalinity. It is about onefor nitrite ions. Thus, nitrite ions show an inhibition of corrosion initiation similar to that of hydroxyl ions. For aspartateions (Figure 11), f* depends upon pH (Figures 14, 15); above 12.6, the higher the pH, the higher the f* was noticed. AtpH equal to 13.8, the aspartate showed a f* value higher than nitrites at the same concentration.Sodium lactate showed an almost nil f* parameter, denoting a negligible inhibition on pit initiation (Figure 12). Thus,the positive effect observed during previous long-term tests on concrete specimens should be ascribed to an effect onthe propagation phase rather than initiation of the localized corrosion