Localized film breakdown

At the area between the breakdown potential Eb and the critical pitting potential pit local film breakdown occurs, which leads to the creation of pit nuclei. However, these nuclei are immediately repassivated. Consequently, in this potential region it is concluded that breakdown and repair are continuously repeated without creating pit growth. 

As mentioned in the foregoing text, localized corrosion of metals starts with a local breakdown of passive films and localized metal dissolution then occurs at the breakdown site. The local film breakdown is caused by the adsorption of aggressive anions such as chloride ions, CP, which are hard bases. No reliable information is available about the acid-base characteristic of the passive film surface. It is however expected that the breakdown of passive films will be prevented, if some anions or molecules are firmly adsorbed expelling chloride ions from the film surface. In literature, though, we have seen almost no reliable studies on this subject. 

Numerous studies have attempted to elucidate the role of Mo in the passivity of stainless steel. It has been proposed from XPS studies that Mo forms a solid solution with CrOOH with the result tiiat Mo is inhibited from dissolving trans-passively [9]. Others have proposed that active sites are rapidly covered with molybdenum oxyhydroxide or molybdate salts, thereby inhibiting localized corrosion [10]. Yet another study proposed that molybdate is formed by oxidation of an Mo dissolution product [11]. The oxyanion is then precipitated preferentially at active sites, where repassivation follows. It has also been proposed that in an oxide lattice dominated by three-valent species of Cr and Fe, ferrous ions will be accompanied by point defects. These defects are conjectured to be canceled by the presence of four- and six-valent Mo species [1]. Hence, the more defect-free film will be less able to be penetrated by aggressive anions. A theoretical study proposed a solute vacancy interaction model in which Mo " is assumed to interact electrostatically with oppositely charged cation vacancies [ 12]. As a consequence, the cation vacancy flux is gradually reduced in the passive film from the solution side to the metal-film interface, thus hindering vacancy condensation at the metal-oxide interface, which the authors postulate acts as a precursor for localized film breakdown [12]. 

Schematic modeling of the effect of sulfides First the sulfides dissolve more or less quickly, depending on their stability and of the solution corrosivity. Then dissolved sulfur containing species redeposit on the passive film close to the sulfide inclusion, promoting local film breakdown or preventing film healing after pit nucleation.

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

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. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

Breakdown of anodic films is yet another phenomenon for which EIS is well suited. Equivalent circuit analysis has been used to analyze EIS spectra from corroding anodized surfaces. While changes in anodic films due to sealing are detected at higher frequencies, pitting is detected at lower frequencies. Film breakdown leads to substrate dissolution, and equivalent circuit models must be amended to account for the faradaic processes associated with localized corrosion. 

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... 

Fig. 30a-c. Optical micrographs of PS film squares during a "fragility test illustrating a craze initiation, b local fibril breakdown and c catastrophic fracture. Tensile stress lies along the horizontal direction (From Ref. courtesy Macromolecules (ACS))... 

In the presence of aggressive anions such as chloride ions in solution, the passive film on metals occasionally breaks down leading the underlying metal into a localized type of corrosion. In general, as shown in Figure 22.26, the chloride-breakdown of passivity occurs beyond a certain critical potential, called the film-breakdown potential, Eb. The film-breakdown is then followed either by... 

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. 

Corrosion at Inhomogeneous Films Breakdown, Pitting and Localized Attack... 

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. 

Fig. 1.20 Cleared area on a CNT electrode. Localized dielectric breakdown results in an electrical short through the film. Corona discharging bums away the CNTs in the area surrounding the short, isolating it from the rest of the electrode and allowing the device to continue operating...

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