Passivity current density potential curve

According to the current density-potential curves in Figs. 2-18 and 21-11, carbon steels can be passivated in caustic soda [27-32]. In the active range of the... 

Figure 11.57. Corrosion current density-potential curves of various metals, both original and passivated [106],... 

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.

Fig. 2-13 Anodic current-density-potential curve for Fe in 0.5 M HjSQj at 25 C in the passive range (100 /tA cm" — 1 mm a" ) activation potential = 0.8 V ...

The general electrochemical corrosion behaviour and pitting corrosion behaviour of a stable passive or passivatable metal can be assessed in terms of the current-potential curve (or current density-potential curve). Figure 4 schematically presents such a curve for a passive metal corroded in acids, e.g. chromium in sulphuric acid. Corrosion current I per unit of surface area (current density in A/cm ) is plotted against the potential U. 

Figure 1-41. Current density potential curve of metals with active, passive, and transpassive behavior.

Figure 20 (a) Typical current density potential curve of a passive metal (b) hemispherical pit with potentials inside and outside the pit according to the rejected theory frequently used for small pits. (From Ref. 31.)... 

Because the metal dissolution is an anodic process, for example, Fe(s) Fe +(aq) + 2e , the current of the process is assumed to be positive. When potential increases from Mez+zMe lo f (passivation or Flade potential), the current is increasing exponentially due to the electron transfer reaction, for example, Fe(s) -> Fe +(aq) + 2e", and can be described using Tafel s equation. At a E the formation of an oxide layer (passive film) starts. When the metal surface is covered by a metal oxide passive film (an insulator or a semiconductor), the resistivity is sharply increasing, and the current density drops down to the rest current density, 7r. This low current corresponds to a slow growth of the oxide layer, and possible dissolution of the metal oxide into solution. In the region of transpassivation, another electrochemical reaction can take place, for example, H20(l) (l/2)02(g) + 2H+(aq) + 2e, or the passive film can be broken down due to a chemical interaction with environment and mechanical instability. Clearly, a three-electrode cell and a potentiostat should be used to obtain the current density-potential curve shown in Figure 9.3. 

In addition, the reactions occurring at the impressed current cathode should be heeded. As an example. Fig. 21-7 shows the electrochemical behavior of a stainless steel in flowing 98% H2SO4 at various temperatures. The passivating current density and the protection current requirement increase with increased temperature, while the passive range narrows. Preliminary assessments for a potential-controlled installation can be deduced from such curves. 

Such cathodic loop behavior is often observed on the reverse scans of polarization curves in which pitting does not occur as shown in Fig. 10 (9). During the initial anodic scan, the oxide is thickening and the anodic line is moving to the left. Thus, upon the return scan, the unchanged cathodic line now intersects the anodic line at several places, leading to the appearance of cathodic loops. Cathodic loops do not pose fundamental problems they merely conceal the passive current density at potentials near the active-passive transition. 

In near-neutral dilute chloride solutions, concentrations of chromate, less than those suggested by Kaesche, have been observed to increase the pitting potential. Figure 6a shows anodic polarization curves from high purity A1 wire loop electrodes in deaerated 1.0 mM chloride solutions (25). Additions of 25 to 50 pM of sodium chromate were shown to elevate the pitting potential by hundreds of millivolts and reduce the passive current density by about a factor of 2. 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

These factors can be discussed with reference to the polarization curves for the initial and changing conditions within the occluded region. The combined effects of a potential drop into the pit and the effect of the lowered pH, which raises Epp and increases icrit, are also analyzed by reference to Fig. 7.6 (Ref 20). As previously assumed, the solid anodic curve is taken as representative of a stainless steel in an environment of pH = 1. The dashed extension again represents the anodic polarization behavior in the absence of a passive film. At a potential, Ecorr (or Epot if the potential is maintained potentiostatically), the passive current density would be iCOrr,pass and the active corrosion current density would be iCorr,act- Assume that a small flaw through the passive film is associated with an (IR), drop that lowers the potential in the bottom of the flaw to E,. Since this potential is higher than the passivating potential, Epp, this flaw should immediately repassivate and not propagate. 

E4.1. Calctilate and construct, using mixed potential theory, the critical passivation current density (a) potentiostatic and (b) galvanostatic polarization curve for anodic dissolution for active-passive metal that has the following electrochemical parameters Ecorr= 0-5 V vs. SCE, Epp= — 0.4 V vs. SCE, 4orr= 10 A/cm, K = 0.05,. , = 10 A/cm andEtr= + l-0V vs. SCE. 

E4.3. Construct an anodic polarization curve and calculate the critical passivation current density of an active-passive alloy using mixed potential theory with the following electrochemical parameters Econ = 0-b5V vs. SCE /corr= 10 " A/cm fca = 0.1, Epp= —0.3 V vs. SCE and /pass = 10 A/cm, Etr = + 0.9 V. 

Using the given parameters, the anodic polarization curve was constructed (Fig. E4.3) and the critical passivation current density was calculated using mixed potential theory. 

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