Depassivation breakdown

Studies of the structure of passive layers are eventually of technological value only if they can substantially delay the breakdown of that passive layer which is so important to the stability of the metal it protects. As far as the all-important iron and its alloys are concerned, the polymeric oxide model, with the part played by water in putting together the polymer elements, seems to be the most consistent with the facts. In considering its breakdown, one generally discusses this in terms of the effects of Cl" adsorption, but there are other ions (T, Br, SO ) that also cause depassivation. 

Why is it assumed that an amorphous film will be protecting, i.e., passive It is because depassivation—the breakdown of the film—is associated with die easy passage (by means of electrodiffusion) of Fe from die metal through the film to die solution. This is solid-state ionic conduction and depends on the presence of vacancies in a crystalline lattice. An amorphous lattice has no regular vacancies as does a solid crystal, and hence the rate of electrodiffusion of Fe through it is greatly diminished, as is the breakdown of die passive layer. 

At the transpassivation potential (Figure 6.3), the properties of the passive film change and one observes a renewed increase in the rate of dissolution. This behavior is referred to as anodic depassivation. It may be the result of film oxidation at high anodic potentials or of film breakdown favored by the presence of certain anions. Generally speaking, in the transpassive potential region, one observes three types of metal dissolution behavior ... 

A number of authors have attributed anodic depassivation in presence of aggressive anions to the presence of pre-existing defects in the passive film, such as pores or cracks formed during its growth (Figure 6.40(b)) or imperfections related to the substrate structure. At such defects the electrolyte may enter into direct contact with the metal surface, leading locally to rapid dissolution and formation of a pit. Experience shows that high purity metals with a well defined surface stmcture pit less readily. Passive films formed on such substrates contain fewer imperfections, and are therefore less prone to breakdown and pit initiation. 

Whatever the origin of defects in a passive film, a local loss of passivity can only occur when the exposed metal surface does not immediately repassivatc. Indeed, it has been observed that already well below the critical pitting potential depassivation and repassivation events may occur. These can be seen particularly well when working with electrodes of small surface area (microeleetrodes), because they contain relatively few defects that lead to breakdown events. Individual events therefore can be studied more easily. The results of Figure 6.41 illustrate the described behavior. It presents potentiostatic transients observed in the passive potential region on an iron-chromium alloy in NaCl using a microelectrode [40]. Each individual current peak represents a... 

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). 

Analysis of passivation transients on an initially active surface either by applying a steep potential jump into the passive range or by creating fresh surfaces at constant applied potential by nonelectrochemical depassivation (chemical passivity breakdown mechanical scratching, ultrasonic waves, etc. radiative laser beam impact [112,113]). These techniques have proved to be of outstanding importance for the investigation of the mechanism of localized corrosion associated with passivity breakdown [114,115]. 

Active dissolution as a transient regime in the initial stages of metal passivity Dissolution of locally depassivated metals following passivity breakdown (e.g., pits, crevices, grain boimdaries)... 

Besides their possible role in pit initiation (see earlier), the dissolution of the MnS inclusions can generate thiosulfates [47, 67] or more likely sulfide [51, 52] ions, which are known to favor passivity breakdown. Indeed, Crolet et al. [54] observed higher depassivation pH on stainless steels with high sulfur content. According to Brossia and Kelly, the initiation involves a critical [Cl ]/[HS ] ratio. This efiect of MnS is unlikely in modem stainless alloys with very low sulfur contents. 

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