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Pitting film breakdown

Zahavi, J. and Metzger, M., On the Rdle of Grain Boundaries in Film Breakdown and Pitting of Aluminium , J. Electrochem. Soc., 119, 218c (1972)... [Pg.207]

It is a valve metal and when made anodic in a chloride-containing solution it forms an anodic oxide film of TiOj (rutile form), that thickens with an increase in voltage up to 8-12 V, when localised film breakdown occurs with subsequent pitting. The TiOj film has a high electrical resistivity, and this coupled with the fact that breakdown can occur at the e.m.f. s produced by the transformer rectifiers used in cathodic protection makes it unsuitable for use as an anode material. Nevertheless, it forms a most valuable substrate for platinum, which may be applied to titanium in the form of a thin coating. The composite anode is characterised by the fact that the titanium exposed at discontinuities is protected by the anodically formed dielectric Ti02 film. Platinised titanium therefore provides an economical method of utilising the inertness and electronic conductivity of platinum on a relatively inexpensive, yet inert substrate. [Pg.165]

Potential-time relationships have been widely used for studying film formation and film breakdown, as indicated by an increase or decrease in the corrosion potential, respectively. May studied the corrosion of 70/30 brass and aluminium brass in sea-water and showed how scratching the surface resulted in a sudden fall in potential to a more negative value followed by a rapid rise due to re-formation of the film conversely, the pitting of stainless steel in chemical plant may be detected by a sudden decrease in potential... [Pg.1011]

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. [Pg.218]

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. [Pg.233]

From these treatments, it can be said that there is a potential region from the passivation potential to the lowest film-breakdown potential within which the passive film is stable against electrocapillary breakdown. At the potential beyond the critical pitting potential, not only passive film... [Pg.242]

Flade potential, 247 Flame-annealed gold surfaces and the work of Kolb, 81 Flat band potential, 483 Fluctuations asymmetrical and unstable systems, 255 controlling progress in pitting, 299 in pitting dissolution, 251 and corrosion processes, 217 during dissolution, 252 at electrodes, theory, 281 during film breakdown, 233 and mathematical expressions thereof, 276... [Pg.631]

An important correlation between the pitting potential and the pzc results and is shown in Fig. 12.57. It is seen that the pitting potential (an arbiter of the ease of corrosion) is directly related to the potential of zero charge of the alloy electrodes with their oxide-covered surfaces. The considerable shift of pzc ( 0.8 V) with the alloys correlates excellently with the shift of potential at which the current through the film curves up sharply, i.e., at which the film breakdown has led to pitting and thus easier transport of ions (and charge) through the film. [Pg.199]

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. [Pg.312]

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... [Pg.216]

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... [Pg.368]

The bursts of current result from metastable pits, where breakdown of the film, followed by rapid repassivation, occurs. As the potential is increased, breakdown is more likely and repassivation is slower, until the potential is reached, beyond which breakdown is the predominant process. [Pg.282]

FIGURE 22.26 Schematic polarization curves for anodic metal dissolution, passivation, passivity breakdown, pitting corrosion, and transpassivation Eb = film-breakdown potential and Ep]l — pitting potential. [Pg.564]

Corrosion at Inhomogeneous Films Breakdown, Pitting and Localized Attack... [Pg.261]

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 ... [Pg.296]

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... [Pg.415]


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See also in sourсe #XX -- [ Pg.159 ]




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