Passivity current-potential diagrams

Similar current-potential diagrams were obtained for nickel. The current-potential curve for Ni is shown in Figure 10.16. The passive layer of Ni behaves like a p-semiconductor. At 0.85 V NiO is oxidized to NijOj and is thus connected with a steady increase in corrosion rate (Figure 10.16). At higher potentials a further oxidation to Ni02 with a new decrease in current is observed. Additionally oxygen evolution starts. 

Figure 3. Mixed potential diagram illustrating controls on the kinetics of corrosion at a pitted, oxide-covered metal. The potential range is from -700 to +300 mV/NHE. Arrows (B) corrosion current at the bottom of the pit, controlled by Fe Fe + (acid) and 2H - H2 (M) corrosion current at the mouth of the pit, controlled by the partial currents for Fe -> Fe2+ (passivated) and RX RH (Pit) corrosion current for the short-circuited pit, controlled by Fe Fe + (acid) and RX - RH. The three solid curves are generated using the Tafel equation and exchange current densities and Tafel slopes from reference (9). The dashed curve was measured at 5 mV s in pH 8.4 borate buffer, using methods described in reference (9).

FIGURE 15.3 Schematic Evans diagram for the behavior of an active-passive metal. (m/m )> reversible potential of the couple o(m/m+)> exchange current density Epp, passivation potential trio critical anodic current density ip, passive current density Ef, transpassive potential. 

To study the effectiveness of coatings, the coated panels after 18 m exposure at P3 were dipped in SAEJ solution for 4,080 h followed by cyclic polarisation in same medium. Interestingly, both the polarisation diagram (Fig. 3.8) showed a tendency to passivate at higher potential and during reverse cycle Z p (zero current potential) changed to greater value. Icon was also lower (Table 3.4). 

Figure 6.10 Evans diagram showing the effect of passivation current density on corrosion behavior (a) corrosion potential in the active region (b) corrosion potential in the passive region.

From the thermodynamic point of view, some metals can be polarized in a range of potentials where the metal enters into an immune (no corrosion) or passive state (creation of resistant oxides). These states could be achieved, respectively, by cathodic and anodic electrochemical methods of protection, applying external polarization (potential) using direct current. Pourbaix diagrams provide useful... 

It has been pointed out that the passive currents for all three alloys whose polarization diagrams were shown in Figure 12 are about the same. Even the passive current for pure Cr (Fig. 1) is about the same as for the alloys. The noticeable difference in electrochemical behavior between the alloys is in the passivation current and the pitting potential. As shown earlier, the alloying elements are enriched on the... 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

Corrosion can also be suppressed by Ihe controlled application of current to the metal as an anode. This is called anodic protection. Passivity is induced and preserved hy maintaining the potential nf the alloy at. or above, a critical potential in what is called the range of passivity in a potcntiostalic diagram. Such diagrams are based on the relationship between applied anodic current density and the corresponding potential in the environment of interest. 

FIGURE 5 Typical potential sweep diagram on a zinc electrode. Current density decreases rapidly near -900 mV versus SHE (standard hydrogen electrode) as reaction products cover the electrode surface and passivate the electrode. 

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