Pitting corrosion inhibition potentials

Nitrate ions have a special influence by inhibiting pitting corrosion in neutral and acid waters atU> [Eq. (2-50)] [48,52], corresponds to a second pitting potential and is designated the inhibition potential. The system belongs to group IV, with pitting corrosion at U U... 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

A passive metal, which is subject to pitting corrosion beyond the pitting potential, Epit, in the presence of chloride ions, will be inhibited from pitting corrosion if an n-type oxide makes the electrode potential of the metal less positive than Epit. Furthermore, for a corroding metal in the active... 

Mishra and Balasubramanian studied the corrosion behavior of pnre A1 that had been exposed to 3.5% NaCl solution containing different concentrations of LaClj or CeClj. Polarization resistance and EIS data showed that LaClj additions provided better corrosion protection than CeClj with the maximum corrosion inhibition being observed at 1000 ppm. The REMs acted as cathodic inhibitors and caused only minor changes in the pitting potential. SEM observations revealed REM oxides/hydroxides precipitates with different morphologies on the smface ofAl. 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

Overall, these results indicate that chromates inhibit corrosion by elevating the pitting potential on aluminum with respect to the corrosion potential, which decreases the probability for the formation of stable pits. In general a chromate chloride concentration ratio in excess of 0.1 is necessary to observe significant anodic inhibition. 

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