Passivation curves

At any state of pitting, the surface is a composite of active and passive areas. The anodic polarization curve for this composite surface is then the sum, at each potential, of the current densities of the passive and active curves weighted by their areas. The dashed curves, P, P2q2> P3q3, represent the positions of the active curve (initially ABHG) for active surface areas of 0.01, 0.1, and 1.0% ofthe total area. The polarization curve for the composite surface at any potential is obtained by adding the shifted curve to the passive curve. These composite-surface... 

Fig. 7.33 (a) Schematic polarization curve for iron showing passivity (curve A), active corrosion (curve B), and for oxygen reduction (curve C). (b) Effective polarization curve (curve E) when pitting has activated 1 % of the surface (Details can be found in text.)... 

When a maximum current value /max is reached, passivation of the electrode starts due to the formation of a PbS04 layer (curve A, Fig. 7.2). The presence of organic expander in the solution leads to a multifold increase in the quantity of electricity required for electrode passivation (curve B) and shifts the passivation potential at imax to more positive values. A shift of the Pb dissolution processes to more positive potentials is suggested by the appearance of an additional overvoltage in the Pb dissolution processes. These phenomena have been related... 

Figure 9 shows a typical voltammogram, starting anodic, and showing a classical passivation curve. The anodic dissolution of the Mo electrode was further studied by chronopotentiometry at various current densities. 

An experiment in which all other relevant variables are held constant is an eflective approach when it can he achieved. However, aU relevant factors may not be known. Furthermore, circumstances often preclude the ability to hold some variables constant. For example, there may be unforeseen factors that affect the course of a corrosion experiment, as in the poleirization curves in Fig. 7 for nominally identical samples of copper (UNS C10920). Most of the curves indicate the copper will actively corrode in the solution. However, one curve indicates the copper will passivate over a narrow range of potentials. Had a structure or device been built based on the passive curve it would have failed unexpectedly. Fortunately, enough replicate curves were measured to show that the passive curve was the exception [42]. 

Fig. l.lS(top) Equilibrium potemial-pH diagram for the Fe-HjO system showing the zones of stability of cations, anions and solid hydroxides (after Deltombe and Pourbaix ) and (bottom) simplified version showing zones of corrosion, immunity and passivity (curve / is the HjO/Hj equilibrium at Phj= 1 and cur s the Oj/HjO equilibrium at Poj = )... 

The functions of FactiveO ) and FpossiveO ) are calculated using the normalized active and passive curves, respectively. The normalized curves and the parameters can be found at the webpage of the ISC (International Society of Biomechanics [9]). The curves are provided as a list of control points, and the curves are made by interpolating the given control points. 

Figure P.l Passivation curve (Ia = anodic protection current)...

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

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 polarization curves the protectiveness of a passive film in a certain environment can be estimated from the passive current density in figure C2.8.4 which reflects the layer s resistance to ion transport tlirough the film, and chemical dissolution of the film. It is clear that a variety of factors can influence ion transport tlirough the film, such as the film s chemical composition, stmcture, number of grain boundaries and the extent of flaws and pores. The protectiveness and stability of passive films has, for instance, been based on percolation arguments [67, 681, stmctural arguments [69], ion/defect mobility [56, 57] and charge distribution [70, 71]. 

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. 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

These three passive systems are important in the technique of anodic protection (see Chapter 21). The kinetics of the cathodic partial reaction and therefore curves of type I, II or III depend on the material and the particular medium. Case III can be achieved by alloying additions of cathodically acting elements such as Pt, Pd, Ag, and Cu. In principle, this is a case of galvanic anodic protection by cathodic constituents of the microstructure [50]. 

Fig. 21-6 The dependence of the passivation process on the shape of the cathodic partial current potential curve (a) Anodic partial current potential curve, (b) cathodic partial current-potential curve without local cathode rest potential (c) cathodic partial current potential curve with local cathode rest potential I7j p.

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. 

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... 

Whereas lowering the potential results in a decrease in, the converse applies when the potential is raised. However, this increase in activity is again limited by the formation of a solid phase. Thus curve e of Fig. 1.15 (top) gives the equilibrium between Fe(OH)3 and Fe at any predetermined activity of the latter in the range 10 — 10". At flpe2+ = 10 g-ion/l, E= [ 1-06-t-(-6 X 0-059)] - 0-177pH which defines the boundary between corrosion and passivity at high potentials (equation 1.19). 

Although the zones of corrosion, immunity and passivity are clearly of fundamental importance in corrosion science it must be emphasised again that they have serious limitations in the solution of practical problems, and can lead to unfortunate misconceptions unless they are interpreted with caution. Nevertheless, Pourbaix and his co-workers, and others, have shown that these diagrams used in conjunction with E-i curves for the systems under consideration can provide diagrams that are of direct practical use to the corrosion engineer. It is therefore relevant to consider the advantages and limitations of the equilibrium potential-pH diagrams. 

The fact that oxides can exist as metastable phases is illustrated by the Ni-HjO diagram (Fig. 1.18) in which the curves for the various oxides of nickel have been extrapolated into the acid region of Ni stability, and this diagram emphasises the fact that nickel can be passivated outside the region of thermodynamic stability of the oxides". 

It is now appropriate to, consider the kinetics of the, hnodic reaction with particular refer ce to the phenomenon of passivity, but since the ml h anism is dealt with in detail in Seetion 1.5 this discussion will, place the emphasis on the anodic i curves. 

The typical features of a metal/solution system that exhibits an active to passive transition is shown in Fig. 1.33, which represents diagrammatically the potentiostatically determined anodic / curve for iron in HjS04. ... 

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... 

Fig. 1.34 Corrosion and passivation of Fe-18Cr-SNi stainl s steel. Potentiosiatic anodic curve JKLM, hydrogen evolution reaction, curve Hl low concentration of dissolved oxygen, curve t> FG, high concentration of dissolved oxygen, curve AflC (Section 3...

---