Critical anode current densities

Higher currents polarize anodes current density is critical. 

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

Critical Anode Current Density anodic current density that must be exceeded in order to produce an active to passive transition (for a given metal it varies with the nature of the solution, temperature, velocity, etc.). 

In acidic electrolytes with fluoride, silicon is stable at OCP, while electrochemical dissolution takes place for anodic potentials. For anodic current densities below the critical current density JPS PS is formed and the electrolyte-electrode interface is found to be Si-H covered. Species active in the dissolution process are HF, (HF)2 and HF2. A dissolution reaction proposed for this regime is ... 

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. 

Current densities cited are average based on racked work, not Hull cell derived, k Higher currents polarize anodes current density is critical. 

Figure 2 Typical anodic dissolution behavior of an active-passive metal. ZJpp = primary passivation potential, iait = critical anodic current density, and ipass = passive current density. (After Ref. 71.)...

The critical passivation current density is calculated by subtituting the anodic current density, 4, with and the anodic potential, E, with Epp in Tafel equation ... 

The effect of acid concentration on polarization of active-passive metals is shown in Fig. 4.8. Higher hydrogen ion concentration increases the critical anodic current density and decreases the passive potential range. Severe corrosion conditions present at higher acidity also increase current densities and corrosion rates at all potentials. Figure 4.9 presents the data for iron passivation in phosphoric acid/phosphate buffer solutions of... 

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. 

The function of alloyed copper is the same as that of alloyed palladium in titanium mentioned in Section 6.4, namely, to accelerate the cathodic reaction or O2 reduction) to the point where the anodic current density reaches or exceeds the critical value for anodic passivation. 

Passivity is normally manifest as a sharp drop in the anodic current density to a constant value at a critical potential that is commonly referred to as the Hade potential (Figure 4.4.27). For many metals and alloys, the current density drops by three or more orders in magnitude leading to a corrosion rate in the passive state... 

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