Passivation and repassivation

J.R. Galvele, Present State of Understanding of the Breakdown of Passivity and Repassivation, Passivity of Metals, R.P. Frankenthal and J. Kruger, Ed., The Electrochemical Society, 1978, p 285-327... 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

The behavior of iron in nitric acid underscores the importance of recognizing the nature of passivity. Iron is resistant to corrosion in nitric acid at concentrations around 70%. Once passivated under these conditions, it can also exhibit low rates of corrosion as the nitric acid is diluted. However, if this passive film is disturbed, rapid corrosion will begin and repassivation is not possible until the nitric acid concentration is raised to a sufficient level. 

At high ultrasound intensities the time of passivation was affected significantly and repassivation was hindered completely. The researchers also found that the... 

Methods based on the principle of repassivation of rebars must insure that the reinforcement becomes passive and further depassivation is prevented during the remaining service hfe of the structure. For structures suffering carbonation-in-duced corrosion, this means that alkaline conditions have to be restored around the steel rebars. Therefore, the reinforcement will be protected even if the moisture content of the concrete should be high. 

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... 

In the absence of suspended particles, the corrosion rate of passive metals such as stainless steel or titanium in neutral media is not markedly affected by hydrodynamic conditions (Table 10.26). However, when exposed to slurries, these metals are subject to erosion corrosion because the suspended particles that impinge on the surface damage the passive film. As a consequence an anodic partial current flows which serves for film repair and repassivation of damaged areas. In the presence of aggressive anions such as chloride, passive film damage can lead to metal dissolution by pitting [23]. 

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