Passive film rupture

Fig- 7.73 Schematic representation of (a) passive film, (b) passive film rupture by stress-induced slip resulting in exposure of bare substrate, (c) crack initiation by anodic dissolution initiating crevice corrosion conditions before repassivation of exposed substrate, and (d) repassivation of exposed substrate before crack initiation. 

From Sasaki, K. and Burstein, G.T. (2000) Observation of a threshold impact energy required to cause passive film rupture during slurry erosion of stainless steel. Philosophical Magazine Letters, 80(7), 489-493. 

Fig. 13.13 Schematic of the passive film rupture with activation of the anodic reaction...

It has been said in the previous paragraph, that materials that were considered practically immune to SCC were found to be sensitive to it when affected by a crack. The existence of a crack is, therefore, fundamental for SCC to develop. Local chemistry that exists at the tip of a notch may be completely different from the general chemistry of the environment in which the material is operating and this can trigger a SCC that was never supposed to start. The effect of such a notch or crevice on corrosion has been studied in Sect. 13.7. It was shown how the passivizing film rupture at notch tip would lead to a slip dissolution process and, therefore, to corrosion. This initial notch or crack may not exist in the material, yet it can easily form as well. Fatigue may be the cause of crack initiation, but also a static stress can do it. In these cases we have what is referred to as pitting. 

Sulphuric acid is frequently made, stored and conveyed in lead. The corrosion resistance is excellent (see Figure 4.15) provided that the sulphate film is not broken in non-passivating conditions. Rupture of the film may be caused by erosion by high velocity liquids and gases containing acid spray. 

From the conceptual diagram in Fig. 15, it is obvious that if the radius of the nucleus exceeds the critical radius r the nucleus will grow into a macroscopically ruptured small pore. The passive film is more or less defective and the size of the defect will fluctuate from moment to moment. It is therefore reasonable to assume a certain probability that pore nuclei larger than the critical radius are formed in the film. 

In the potential region where nonequilibrium fluctuations are kept stable, subsequent pitting dissolution of the metal is kept to a minimum. In this case, the passive metal apparently can be treated as an ideally polarized electrode. Then, the passive film is thought to repeat more or less stochastically, rupturing and repairing all over the surface. So it can be assumed that the passive film itself (at least at the initial stage of dissolution) behaves just like an adsorption film dynamically formed by adsorbants. This assumption allows us to employ the usual double-layer theory including a diffuse layer and a Helmholtz layer. 

In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102... 

Even if the surface is not perfectly smooth, the initial event that must occur in the development of a nucleus is passivity breakdown, in which the protective oxide layer is ruptured to expose the underlying metal to the aqueous environment. The most highly developed theory for this process is the point defect model (PDM) [59-65]. This model postulates that the generation of cation vacancies at the film/solution interface, and their subsequent transport across the barrier layer of the passive film, is the fundamental process fiiat leads to passivity breakdown. Once a vacancy arrives at the metal/film interface, it may be annihilated by reaction (i) in Fig. 31 ... 

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. 

Crack-tip growth mechanisms have been proposed that do not involve dislocation movement explicitly, but rather, in response to the stress field at the crack tip, interstitial atoms diffuse to the region of the stress field to reduce the stress substitutional atoms also will diffuse to the tip if the local stress is thereby reduced. Crack-tip growth would be increased if this local change in alloy composition enhances dissolution during slip displacement or alters the passive film such that it is more easily ruptured by dislocations emerging to the surface. That is, there is continuously produced at the crack tip a film that is more easily ruptured than the more stable passive film on the sides of the crack (Ref 158). 

Using radiotracer studies, two classes of bound water M-H2O and M-OH (aquo and olation groups) and MO or M-OOH with 0x0 and olation bridges were identified in the passive film [84—86]. The nature of the water was found to be critical to the dynamic rupture and self-repair of the film. 

The LSP mechanism proposes that SCC results from the effect of the structure ahead of the crack tip [61]. This mechanism assumes that a galvanic corrosion between active sites (weakened passive site) and surroimding passive surfaces produces large anodic currents at the rupture site. Repassivation of the active sites is prevented by the presence of weakened passive films on the surface. It has been su ested that the weakened passive film... 

Both the protective-film-rupture theory and the high-index-plane theory may be valid for one and the same system, depending on conditions. This has been demonstrated by the behavior of an iron wire strained in an aerated nitrate solution.When straining was done in the active region of potentials, the observed increase of anodic current was of the magnitude expected on the basis of the high-index-plane theory (within a power of 10). However, in the passive region, a 1500-2000-fold increase was observed which can be explained only in terms of the film-rupture mechanism. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

The scratch repassivation technique discussed above may be more useful for studying SCC than pitting [107]. This is because natural pitting is often initiated at defect sites in the passive film that are electronic or chemical in nature. Such sites are not necessarily probed by a mech inical scratch. Scratching can simulate SCC where mechanical film rupture processes are operative and anodic dissolution controls the SCC process. In general, once the passive film is mechanically disturbed at some potential, an electrochemical current can be measured that will decay back to a low level when repassivation has occurred. A measure of the crack growth rate, V, (cm/s) can be obtained from these data... 

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