Passivity breakdown mechanism formation

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. 

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. 

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

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). 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

It is also well known that a local breakdown passivity that leads to pitting can be treated as a random phenomenon occurring stochastically with respect to time and location on the surface of the metal.21-23,97 Reigata et al.% have recently formulated the stochastic formation mechanism of a pit... 

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]. 

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. 

The voltammetric behavior of calcium electrodes is controlled by the surface chemistry described in 1-6. In Ca(C104)2 solutions, the electrodes are strongly passivated, due to the formation of CaC. Hence, high overvoltage (>1 V versus Ca/Ca ) is required in order to drive any anodic process of calcium in the solvents/Ca(C104)2 solutions. In contrast, Ca electrodes dissolve at low overpotential in BF4 salt solutions of all of the above solvents. The lowest overpotential required to obtain a massive Ca dissolution was measured in AN/TBABF4 or AN/Ca(BF4)2 solutions (this being in line with the results in Ref. 448). As discussed in Ref. 449, the voltammetric response of Ca electrodes in these solutions reflects Ca dissolution via a breakdown and repair mechanism of the surface films. 

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 ... 

Whether and how readily passivation occurs will depend on both the metal and the solution environment. Even when passivation is observed, however, there is a further phenomenon which must sometimes be considered. This is pitting, a process which causes an increase in current in the passive region and which in its severest form causes complete breakdown of the passive film. It is commonest in media containing halide ion. The term derives from characteristic circular holes which appear in the passive film but their mechanism of formation is not well understood. Generally pitting becomes worse with increasing halide ion concentration and as the potential is made more positive (see Fig. 9.7) and it is thought likely that the pits... 

Albery et al. [39, 49] prepared poly(3-thiopheneacetic acid) and its copolymer with thiophene by electrochemical polymerization. Bartlett et al. [50] electrochemically synthesized conducting poly(3-thiophene-acetic acid) films in dry acetonitrile containing tetraethyl ammonium tetrafluoroborate. These films are redox active in acetonitrile, however, stability was reportedly poor in comparison with poly(3-methylthio-phene) and poly(methyl 3-thiopheneacetate) due to traces of water. In dry acetonitrile, the polymer can be electrochemically oxidized and reduced. Upon oxidation in water and methanol, poly(3-thiopheneacetic acid) film converted into a passive film. Based on the electrochemistry and an FT-IR study, Bartlett et al. postulate the mechanism for the electrochemical passivation shown in the Figure 4.33. In the mechanism, passivation of the polymer involves the formation of an intermediate cyclic lactone and subsequent breakdown by reaction with solvent. This process does not destroy the conductivity of the polymer so the process can continue until all the monomer units within the film are converted to a lactone form (Figure 4.33, IV). The electrochemical passivation is not observed... 

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