Passive film breakdown

As mentioned, corrosion is complexly affected by the material itself and the environment, producing various kinds of surface films, e.g., oxide or hydroxide film. In the above reactions, both active sites for anodic and cathodic reactions are uniformly distributed over the metal surface, so that corrosion proceeds homogeneously on the surface. On the other hand, if those reaction sites are localized at particular places, metal dissolution does not take place uniformly, but develops only at specialized places. This is called local corrosion, pitting corrosion through passive-film breakdown on a metal surface is a typical example. 

Figure 13 shows the relationship between the time interval At of passive film breakdown of stainless steel with chloride ions and the logarithms of cumulative probability P(Af) for breakdown at time intervals longer than At. From these results, it is clear that the logarithm of the probability is almost proportional to the time interval, and therefore the cumulative probability for film breakdown follows Poisson s distribution, i.e., the following equation is obtained,... 

The rate of the passive film breakdown at Al-Ta is one-tenth that of Al at the same field strength within the oxide. But that is not what would be expected. The concentration of the alloying element is too small (1 place in 20) to retard the... 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

One approach to using the AFM to study localized corrosion is to press hard vnth the tip or scratch the surface to stimulate passive film breakdown. Scratching with large tips has been used with success to study the repassivation process [120, 121]. By scratching in a controlled fashion over a small area with an AFM tip, it is possible to study the conditions under which the freshly bared surface will repassivate or propagate into localized corrosion [120, 121]. The effects of potential and environment, including inhibitors can be probed. 

The critical current density and passivation potentials are important characteristics that control metal passivation properties. In the transpassive region, E, the current starts to increase due to oxygen evolution or passive film breakdown. In Fig. 1.3, the passive region is more anodic than the active region. This property of the active-passive metal or aUoy is not observed in the case of normal metals and is only used to define passivity. 

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 ... 

Zirconium and zircaloy-4 in 1 M NaCl, 1 M KBr, and 1 M aqueous KI solutions were found susceptible to SCC only above the pitting potential (zone 1) [168]. Zirconium alloy SCC in aqueous halide solutions occurs as a result of electrochemical passive film breakdown followed by intergranular attack due to anodic dissolution (dealloying assisted by stress). The final step was a fast transgranular propagation. A surface-mobility SCC mechanism was suggested to explain experimental results. Figure 9.47 shows... 

INTERACTION WITH AGGRESSIVE SPECIES AND IMPLICATIONS FOR PASSIVE FILM BREAKDOWN... 

Figure 6.43 Rotating ring-disk experiment of passive film breakdown on Fe after injection of chloride ions. Parameters observed to vary as a function of time the current density of the disk the reduction current density of Fe " on the ring and the current density of formation of Fe on the disk. Electrolyte is borate solution of pH 7.3. Concentration of NaCl added at time t = 0 is O.Olmol 1 Dashed line shows the time to reach a uniform NaCl concentration [41].

Hashimoto, M. Miyajima, S. Murata, T. A stochastic analysis of potential fluctuation during passive film breakdown and repair on iron, Corros. Sci. 33 (1992) 885. 

Bertocci, U. and Kruger, J. (1980) Studies of passive film breakdown by detection and analysis of electrochemical noise. Surface Science, 101, 608-618. 

Easier diffusion of B produces features such as a smoother dissolution front, internal vacancy clusters (polyatomic voids), and islands of A-type atoms hindered from dissolution. Qualitatively similar conclusions are drawn on 3D lattices except for the specific generation of pores with easier diffusion of B atoms as predicted [201 ] by a nonstochastic approach. This tendency to generate a tunneling attack at the cost of only surface diffusion could be considered as a likely explanation of pit nucleation at the atomic level, with no need for the concept of passive film breakdown. 

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