Passivity breakdown mechanism initiation

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 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. 

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. 

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

Surface films appear to play a major role in the initiation of SCC and may also contribute to hydrogen embrittlement effects. It is assumed that the main role of the surface film is to localize the damage inflicted on the material by the environment. This can be caused by mechanical breakdown of the protective film by slip step or electromechanical breakdown of the passive film.95 SCC may be related to the nature of the surface film. It has been observed that the SCC of C-Steels is related to the presence of magnetite in several low -temperature environments (around 90°C), except... 

There is also evidence that the beneficial effect of molybdenum is to interfere with pit propagation. If the mechanism is active at the initiation of localized breakdown of the passive film, then, effectively, pitting will not occur. Based on the low solubility of molybdenum chloride, Mo03, and polymolybdates in acid solutions, one mechanism proposes that molybdenum enhances the formation of salt films of these species within the pit. This can decrease the IR potential drop to the pit... 

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

Three main mechanisms for passive film breakdown and pit initiation have been suggested in the literature through penetration, adsorption, or film breaking [20—22]. These mechanisms apply to pure metal systems because they do not consider second-phase particles in the passive film matrix, which very often initiates pitting. For example, as already discussed, dissolution of MnS inclusion at the MnS/matrix is the initial pit formation step in steel [15]. In the absence of chloride ions, the protective hydrated iron passive film slowly converts into dissolved ferric ions ... 

Although the microscopic mechanism of pit initiation and oxide breakdown is still not fully understood (40, 41), the macroscopic behavior of enhanced local dissolution and diffusion of dissolved metal ions can be described using current-potential (i-E) curves (Figure 3). The solution conditions in a pit create two distinct electrochemical cells. At the bottom of the pit, the oxidation half-reaction is acidic dissolution of Fe (equation 1), which is balanced primarily by reduction of water to hydrogen gas (equation 3). The second cell is at the mouth of the pit, where the halfreactions are dissolution at a passivated iron metal surface (alkaline conditions) and reduction of water or stronger oxidants such as O2 or RX. 

In conclusion, the present discussion of proposed film breakdown and pit initiation mechanisms suggests that several phenomena are responsible for the loss of passivity and the onset of pitting when a metal is polarized to a high potential in presence of aggressive anions. Structural defects in the passive film reflecting those of the metal, anion adsorption on the film and the metal surfaces and the effect of anions on the kinetics of the electrochemical reactions governing oxide formation and metal dissolution are most critical. Practical consequences of these phenomena for pitting corrosion will be discussed in Section 7.3. 

---