Passivating potential

Passivation is manifested in a polarization curve (figure C2.8.4) dashed line) by a dramatic decrease in current at a particular onset potential (the passivation potential, density, is lowered by several orders of magnitude. 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

FIG. 28-9 Typical electrochemical polarization curve for an active/passive alloy (with cathodic trace) showing active, passive, and transpassive regions and other important features. (NOTE Epp = primary passive potential, Ecaa- — freely corroding potential). 

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. 

Anodic Protection On the reverse anodic scan there will be a low current region (LCB) in the passive range. The passive potential range of the LCB is generally much narrower than the passive region seen on a forward slow scan. In anodic protection (AP) work the midpoint of the LCB potential is the preferred design range. This factor was verified for sulfuric acid in our laboratory and field studies. 

Initially, the curve conforms to the Tafel equation and curve AB which is referred to as the active region, corresponds with the reaction Fe- Fe (aq). At B there is a departure from linearity that b omes more pronounced ns the potential is increased, and at a potential C the current decreases to a very small value. The current density and potential at which the transition occurs are referred to as the critical current density, and the passivation potential Fpp, respectively. In this connection it should be noted that whereas is determined from the active to passive transition, the Flade potential Ef is determined from the passive to active transition... 

In the case of the stainless steels, or other readily passivated metals, the rapid reduction of dissolved oxygen on the freely exposed surface will be sufficient to exceed the critical current density so that the metal will become passive with a potential greater than whereas the metal within the crevice will be active with a potential less than. The passivation of the freely exposed surface will be facilitated by the rise in pH resulting from oxygen reduction, whilst passivation within the crevice will be impeded by the high concentration of Cl ions (which increases the critical current density for passivation) and by the H ions (which increases the passivation potential E, see Section 1.4). 

Perfect passivation. The potential-pH region between the passivation potential pp and the protection potential p, in which pits are not... 

Fig. 1.56(a) E-i curves and experimental potential-pH diagram for Armco iron in chloride-free solutions of different pHs (A is the unpolarised potential and P the passivation potential) and (b) E—i curves and experimental potential-pH diagram for Armco iron in solutions of different pHs containing 10 mol dm of chloride ion (r is the rupture potential and p the protection potential). (After Pourbaix )... 

The significance of the Flade potential Ef, passivation potential pp, critical current density /pn, passive current density, etc. have been considered in some detail in Sections 1.4 and 1.5 and will not therefore be considered in the present section. It is sufficient to note that in order to produce passivation (a) the critical current density must be exceeded and b) the potential must then be maintained in the passive region and not allowed to fall into the active region or rise into the transpassive region. It follows that although a high current density may be required to cause passivation ) only a small current density is required to maintain it, and that in the passive region the corrosion rate corresponds to the passive current density (/p, ). 

Table 10.32 Effect on critical current density and passivation potential on alloying nickel with chromium in In and IOn H2SO4 both containing 0-5N K2SO4 (after Myers, Beck and Fontana")...

Nickel (%) Critical current density ( crii.. Am ) Passivation potential [Pg.263]

Alloy Acid concentration Temp. ( C) Critical current density ( crit.. Am-2) Current density to maintain passivity (ip a.< Corrosion rate (mm y ) Unprotected protected Passive potential range (V)... 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

The critical current and primary passivation potential will not appear on an anodic polarisation curve when the steady-state potential already is higher than In such a case the potentiostat is unable to provide direct data for constructing the full polarisation curve. If that portion of the curve below the steady-state potential is desired, then the potential has to be held constant at several points in this range and corrosion currents calculated from corrosion rates as determined from solution analyses and/or weight losses. 

The fact that scanning speed can affect polarisation behaviour has already been mentioned. In the case of stainless steel a plot of critical potential E, vs. rate shows how becomes more positive with potential change rate (Fig. 19.43) . When a specimen was held at a fixed passive potential while aggressive ions (Cl ) were added to determine the concentration required... 

Flade Potential the potential at which a metal which is passive becomes active (see Passivation Potential). 

See also Corrosion Potential, Electrode Potential, Equilibrium Potential, Flade Potential, Open-circuit Potential, Passivation Potential, Protection Potential, Redox Potential.)... 

Let us mention some examples, that is, the passivation potential at which a metal surface suddenly changes from an active to a passive state, and the activation potential at which a metal surface that is passivated resumes active dissolution. In these cases, a drastic change in the corrosion rate is observed before and after the characteristic value of electrode potential. We can see such phenomena in thermodynamic phase transitions, e.g., from solid to liquid, from ferromagnetism to paramagnetism, and vice versa.3 All these phenomena are characterized by certain values... 

Figure 3. Current vs. potential curve for iron dissolution in phosphoric acid solution at pH 1,85. Ep, Flade potential Ep, passivation potential Epii- critical pitting potential EiP, transpassivation potential. Solid and broken lines correspond to the cases without and with CF ions, respectively.

Once a passive film is formed on a metal surface, as long as the electrode potential remains in the passive potential region, the surface is stable, i.e., scarcely dissolved. However, if there are film-destructive anions like chloride ions in solution, the passive film is locally broken, so that local dissolution of the metal substrate proceeds at the same place. 

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

From these treatments, it can be said that there is a potential region from the passivation potential to the lowest film-breakdown potential within which the passive film is stable against electrocapillary breakdown. At the potential beyond the critical pitting potential, not only passive film... 

As mentioned in Section II.3, in the presence of film-destructive anions such as chloride ions, beyond the critical pitting potential Epiti pitting dissolution proceeds, creating semispherical pits (polishing-state pits), which are different in shape from the irregular pits that develop at the active region that is less noble than the activation potential Ea, where the corrosive reaction moves from the passive state to the active state (usually the activation potential Ea is different and less noble than the passivation potential Ep). 

In this case, if the electrode potential inside the pit remains in the passive potential region, repassivation occurs immediately. 

Passivation currents, fluctuations in, 293 Passivation potential, and thermodynamic phase formation, 218 Passive film... 

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