Passivation range

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

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. 

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. 

External magnetism due to casing or steel in the well vicinity is used in passive ranging tools for blowout well detection from a relief well. 

Another indication of the influence of precipitated phases on anodic behaviour may be seen in the curve for Alloy C in Fig. 4.28, where the small peak in the middle of the passive range is probably attributable to anodic dissolution of an intermetallic phase (n) and MjC carbide . ... 

Fig. 4.32 Anodic behaviour of Ni-Al alloys in 0-5 m H2SO4, de-aerated with H2, at 22°C the potential was increased by 0-01 or 0-02 V every 3 min in the active range and by 0-04 V in the passive range (after Crow, era/. )...

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

The passive range of typical stainless steels conveniently spans most of the Eh stability field of neutral water. This can be appreciated by examination of the E° or Eh values for reactions 16.20 to 16.25, with the caveat that these refer to pure iron and chromium metals rather than to stainless steels and that the conditions are standard ones rather than, for example, the very low [Cr3+] in equilibrium with the FeCr2C>4 film. The formation... 

Lorentz curves with a tail function for the asymmetry of the XPS signals for these transition elements. These sets were kept constant in position, shape and size relative to each other. The XPS spectrum of an actual specimen was described with a least-square fit by variation of the size of the appropriate standard sets [36]. Various standards were prepared and measured for a subsequent data analysis of actual specimens. Only some few examples are mentioned here. Pure metal standards are Ar-sputter cleaned specimens. Fe(III) oxide corresponds to a thick passive layer formed at the positive end of the passive range. For Fe(II), a passive layer formed on Fe5Cr is reduced in 1 M NaOH at ca. = -1.0 V (SHE) [12]. For NiO, oxide grown at 1000 °C on pure Ni in air was used as a standard. For Ni(III)oxyhydroxide, NiOOH was deposited by oxidation of Ni2+ from weakly alkaline solution or formed... 

The polarization curve of Ni in 0.5 M H2SO4 shows a similar behavior to Cr with a clear separation of the anodic peaks of active dissolution, the passive range of 0.5... 

Both reactive metal components are oxidized at the metal/oxide interface. However, in the passive range Fe(III) ions are dissolved preferentially with a slow, but still larger, rate by at least one order of magnitude. This situation leads to an accumulation of Cr(III) within the passive layer. XPS studies yield a Cr content of >70 at. % [69-72], In the active/passive transition range, Cr is accumulated to 90% and it reaches a plateau of 80% in the passive range. Finally, it decreases for E > 1.0 V in the transpassive range (Fig. 27b). 

Under -> open-circuit conditions a possible passivation depends seriously on the environment, i.e., the pH of the solution and the potential of the redox system which is present within the electrolyte and its kinetics. For electrochemical studies redox systems are replaced by a -> potentiostat. Thus one may study the passivating properties of the metal independently of the thermodynamic or kinetic properties of the redox system. However, if a metal is passivated in a solution at open-circuit conditions the cathodic current density of the redox system has to exceed the maximum anodic dissolution current density of the metal to shift the electrode potential into the passive range (Fig. 1 of the next entry (- passivation potential)). In the case of iron, concentrated nitric acid will passivate the metal surface whereas diluted nitric acid does not passivate. However, diluted nitric acid may sustain passivity if the metal has been passivated before by other means. Thus redox systems may induce or only maintain passivity depending on their electrode potential and the kinetics of their reduction. In consequence, it depends on the characteristics of metal disso-... 

Passivation potential — A metal turns passive if the electrode potential is shifted above the passivation potential Ep into the passive range of the -> polarization curve (Fig. 1). This critical potential depends on the thermodynamic properties of the metal. In many cases it equals the value deduced from the thermodynamic data for the formation of an oxide layer of the metal in aqueous electrolytes according to Eq. (1). This reaction is - pH dependent by -0.059 V/pH. In some cases it corresponds to the oxidation of a lower valent to a higher valent oxide (Eq. (2)). For iron the passivation potential in acidic electrolytes has been explained by Eq. (3). 

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