Passivity breakdown mechanism

Analysis of passivation transients on an initially active surface either by applying a steep potential jump into the passive range or by creating fresh surfaces at constant applied potential by nonelectrochemical depassivation (chemical passivity breakdown mechanical scratching, ultrasonic waves, etc. radiative laser beam impact [112,113]). These techniques have proved to be of outstanding importance for the investigation of the mechanism of localized corrosion associated with passivity breakdown [114,115]. 

It is not yet completely clear why passivity breakdown occurs with anions like chloride ions. However, some models for the mechanism have been proposed. Therefore, after briefly describing such models, we will examine the electrocapillary model from the viewpoint of nonequilibrium fluctuation. 

A basic requirement for electrochemical pore formation is passivation of the pore walls and passivity breakdown at the pore tips. Any model of the pore formation process in silicon electrodes has to explain this difference between pore tip and pore wall conditions. Three different mechanisms have been proposed to explain the remarkable stability of the silicon pore walls against dissolution in HF, as shown in Fig. 6.1. 

It was observed for chloride-breakdown of the passive film on metallic iron in neutral borate solution that the amount of chloride ions required for initiating the local passivity breakdown is dependent on the film thickness, film defects, and electric field in the film as well as on the solution pH [41,42]. It was also observed that at the initial stage of the passivity breakdown the passive film locally dissolves and becomes thinner around the breakdown embryo before the underlying metal begins to dissolve in pitting at the passivity breakdown site [42,43]. From these observations, it is likely that the passivity breakdown is not a mechanical rupture of the passive film but a localized mode of dissolution of the passive film accelerated by the adsorption of aggressive anions on the film. 

Some SRB may cause loealized corrosion on stainless steels, nickel alloys, aluminium, zinc and copper alloys. Mechanisms of sulphur-assisted corrosion, with emphasis on Fe- and Ni-based materials, have recently been reviewed by Marcus [6.17]. The review includes the fundamentals of enhanced dissolution, retarding or blocking of passivation, and passivity breakdown. 

Localized corrosion of metals and alloys occurs in aggressive media (e.g., containing chloride) as a consequence of the passivity breakdown, with major impact in practical applications and on the economy. This form of corrosion is particularly insidious since a component, otherwise well protected by a well-adherent, ultrathin oxide or oxyhydroxide barrier layer (i.e., the passive film), can be perforated locally in a short time with no appreciable forewarning. Extensive studies have been conducted over the last five decades to understand localized corrosion by pitting [1-10], but the detailed mechanisms accounting for the local occurrence of passivity breakdown remain to be elucidated and combined with kinetics laws to allow reliable prediction. 

The authors did not specifically discuss the breakdown mechanism of the passive film from the point of view of the existing models of passivity breakdown in their report. However, they emphasize oxide thinning based on the results that Cu atoms are displaced from the passive film into the first layers in the aqueous media and that their charge is consistent... 

The penetration of chlorine atoms into the passive films is suggested by close examination of the relaxed structures in Figures 7.10 and 7.11. It seems to occur independently of the O-enriched or O-deficient nature of the films and of the implemented defect site. However, this aspect was not addressed by the authors in their study and thus cannot be further discussed. Detailed studies relevant for testing the penetration-induced voiding mechanism of passivity breakdown would require implementing O vacancies as point defects not only at the surface but also in the bulk of the passive films of appropriate crystalline structure. Implementation of field-assisted transport in the passive film and at its interfaces would also be required. 

Figure 3-24, Breakdown mechanism of the passive film induced by the enrichment of sulfur at the metal-passive film interface.

Under certain special environmental conditions, the passive films, which were described earlier in this Chapter, are susceptible to localized breakdown. Passivity breakdown may result in accelerated local dissolution (localized corrosion) of the metal or alloy. There are two (related) major forms of localized corrosion following passivity breakdown localized corrosion initiated on an open surface is called pitting corrosion, and localized corrosion initiated at an occluded site is called crevice corrosion. In the presence of mechanical stress, localized dissolution may promote the initiation of cracks. 

Film breaking it has been suggested that the passive film is continuously subjected to breakdown and repair (Vetter and Strehblow, 1970 Sato, 1971 Sato et al., 1971). The local breakdown events would be caused by mechanical stresses at defect sites or by electrostriction effects. In the absence of aggressive ions such as chloride, rapid repassivation takes place, whereas the presence of chloride could prevent repassivation of locally depassivated surfaces and thus cause pitting. This view of pitting considers that passivity breakdown itself is not caused by chloride, but is inherent to the nature of passive films. In this mechanism, adsorption on the passive film surface is not an important factor, but chloride adsorption on the metal surface remains a necessary step in the process of repassivation inhibition (and salt film formation). 

Specific anion dependence is expected to occur in the passive andtianspassive domains, and dissolution in the active range can be made to deviate from the hydroxo-ligand mechanism [87] only by anions able to replace OH, essentially SH" [88] and the halide ions. In the case of iron, due to the well-known passivity breakdown and subsequent localized corrosion by halide ions and particularly Cl , chloride effects have been investigated extensively. Complexing anions such as acetate have also been considered to a lesser extent. 

The role of sulfide inclusions in corrosion has been recognized in early woiks. The fact that sulfur-containing species are detrimental to the lesistanee of metals and alloys to localized corrosion has been established for a long time but the meehanisms have remained unclear until recently. In the area of passivity breakdown, where substantial research effort has been expended for several years, the effects of chloride ions have been investigated much more than the effects of sulfur. The aim of this chapter is to review the fundamental aspects of the mechanisms of S-induced corrosion, with special emphasis on the role played by adsorbed (or chemisorbed) sulfur. 

In the preceding parts of this chapter we have seen that S alone can enhance the dissolution, block or retard the growth of the passive film, and cause passivity breakdown and pitting by enrichment at the metal-film interface. All these effects can evidently also take place in the presence of Ch. A major difference between the mechanisms of action of Cl and S is that S does not seem to interact directly with the oxide surface as strongly as Ch. 

The detailed mechanism of passivity breakdown inside a crevice is not clearly established and different possibilities are still discussed. Indeed, it is quite possible that different mechanisms may be involved depending on the material, bulk environment, crevice geometry, and, possibly, surface condition of the metal. Classical Mechanism General Breakdown of Passivity in Low pH, High Chloride Solutions... 

It is assumed that passivity breakdown in a crevice is due to pH drop but, as discussed, the actual mechanisms may be diflferent. 

As the basic mechanisms of crevice corrosion have been understood to some extent, many models have been developed either to describe the steady state of an actively corroding crevice or to calculate the environment evolution and predict the passivity breakdown. These models are able to reproduce some major features of crevice corrosion, particularly geometric effects, but they all suffer from the lack of knowledge of the complex concentrated solutions that are formed in the crevice. 

Corrosion is of tremendous economic importance. In the United States, for example, a total damage of more than 300 billion is caused every year by corrosion. Among the important corrosive reactions are all kinds of oxidation reactions, anodic dissolution, passivation, breakdown of passivation, and absorption of hydrogen, usually the first step of hydrogen embrittlement. All these reactions take place in the outermost layers of the material, but many of them are not well understood on an atomic or molecular level. The surface science approach to corrosion tries to clarify the mechanisms of corrosion reactions on that level and to link this understanding to the macroscopic manifestations of corrosion. PES as a highly surface sensitive technique became an important tool in this area of research. 

---