Transpassive potential range

Fig. 1. Polarization curve of metals with active, passive and (a) transpassive potential range including oxygen evolution (b) passive potential range going directly to oxygen evolution (c) continuing passivity for valve metals to very positive potentials. Pitting between critical pitting lim and inhibition potential fsj in the presence of aggressive anions and inhibitors.

Ni is a frequent component for alloys as e.g. for stainless steels. Polarization curves of Fe53Ni and FelONi still show features known for pure Ni (Fig. 5). The current increase and the peaks in the transpassive range are suppressed to a large extent in acidic and alkaline solutions due to the influence of Fe [15, 48], Angular resolved XPS measurements indicate a bilayer structure of the passive film with an outer hydroxide and an inner oxide part. Circa 1 nm hydroxide is found with no change with the electrode potential. The oxide part increases linearly with the potential up to 5 nm and levels off to a constant value for the transpassive potential range at 0.70 V in 1 M NaOH and at 1.40 V for pH 2.9 [15, 48], At 0.70 V in 1 M NaOH one observes... 

Fig. 1 Schematic current density potential curve of metals with active, passive, and transpassive potential range and the critical potentials Ep and E restricting the pitting range. Valve metals with insulating passive layers showing neither transpassive metal dissolution nor oxygen evolution.

In acidic electrolytes like 0.5 M H2SO4, the inner and outer sublayers of the passive film consist of pure Cr203 and Cr(OH)3, respectively. In the transpassive potential range, up to 10% of Cr(VI) is found in the outer hydroxide part of the film. Thus the excellent corrosion... 

To determine the potential-mdependent CPT by the potentiostatic method, it is necessary to select a potential placed between the pitting potential and the transpassive potential for the relevant stainless steel (Fig. 13). A suitable choice of potential is 700 mV SCE, and in order to obtain compatibility for a range of stainless steels, the polarization was always... 

Fig. 27a. Potentiodynamic polarization Curve of Fe5Cr in 0.5 M H2SO4 with potential ranges of hydrogen evolution, active dissolution (Cr2+), passivity (Cr3+), transpassivity (C Cb2-), and oxygen evolution [69].

Passivation potential — Figure 1. Polarization curves of three metals in 0.5 M H2SO4 with active dissolution, a passive potential range, and transpassive dissolution and/or oxygen evolution at positive potentials Ep(Cr) = -0.2 V, -Ep(Fe) FP(Ni) = 0.6 V [i]... 

Measurements showed that, even in the chloride-free Na2S04 solution, MnS inclusions are dissolved [7]. Since the inclusions are dissolved rather slowly and no stable pitting occurs, the dissolution processes can be assigned to single inclusions quite well. Measurements at sites with inclusions are shown in Fig. 10. Two local potentiodynamic polarization curves of the steel DIN 1.4301 (0.017% S) were measured. In the case of an active 10 pm x 5 pm inclusion, the electrochemical current shows an abrupt increase over a limited potential range (shaded areas). To avoid the dissolution of an inactive 3 pm x 3 pm inclusion in the transpassive range, the measurement was stopped at 1000 mV. Subsequent optical microscopy studies of the same area revealed that the inclusion had been dissolved during the experiment. The SEM pictures indicate the nearly complete dissolution of the oval, active inclusion (Fig. 10, top left), whereas the inactive inclusion of a rounded shape (Fig. 10, bottom) did not dissolve at all. 

Fig. 4 Eiectrochemicai frameworks for the intergranuiar corrosion of aiioys that exhibit uniform passivity prior to sensitization in the environment given. Case (a) different primary passive potentiais and active dissoiution regions for the grain boundary and grain matrix such as is observed for Fe-Cr and Fe-Ni-Cr aiioys. ICC occurs over the potential range at which the matrix is passive while the grain boundary is active. Case (b) different critical potential for grain boundary and matrix. The critical potentials have been shown to be associated with pitting, repassivation, and/or transpassive dissolution.

The potential range above the ,j, value corresponds to the transpassive state. In the transpassive state, different reactions can take place. For example on the surface of iron and carbon steel in sulfuric acid, oxygen is liberated according to Reaction (8-2), while in the case of chromium or chromium-nickel steel, a transition occurs from the insoluble Cr203 oxide into the soluble dichromate... 

---