Repassivation repair

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. 

In the example of the drop in cooling water pH, cleaning is probably unnecessary, but the entire system should be repassivated. Whereas following out of service repairs to a heat exchanger, the exchanger should be cleaned, either mechanically or with an appropriate chemical cleaner, and then properly passivated before being put back on-line. 

The initiation of a pit is assoeiated with the breakdown of the protective film on the metal surface. In cases where pit depths increase rapidly, the environment is usually such that no repair or repassivation of the protective layer can be accomplished. In situations where many shallow pits form, the environment is usually one where repassivation of the damaged film can be made, but initiation of new sites occurs on a regular basis. 

Figure 8. Schematic of the sequence of events occurring at the tip of a propagating stress-corrosion crack. The film is fractured (B) and immediately starts to repair (C) while dissolution is occurring. Complete repassivation occurs at D by which time the crack has extended.

Figure 18.4 shows the methods used for repair of carbonated structures, which are based on repassivation of steel, limitation of moisture content of concrete and coating of the reinforcement. 

Repassivation with alkaline mortar or concrete. Repassivation of steel can be obtained by replacing the chloride-contaminated concrete with chloride-free and alkaline mortar or concrete. Because of the mechanism of chloride-induced corrosion, it is not sufficient to repair the concrete in the area where the reinforcement is de-passivated. The concrete must be removed in all areas where the chloride threshold has reached the depth of the reinforcement or is expected to reach it during the design Hfe of the repair. In fact, the concrete that surrounds the zones of corrosion usually has a chloride content higher than the chloride threshold, even though the steel remains passive because it is protected by the corroding site. In fact, a macrocell forms (Figure 18.6a) that provides cathodic polarization to adjacent steel and... 

Because of the high penetration rate of pitting corrosion and the uncertainty due to structural consequences of locaHzed attack, the methods aimed at repassivation of steel should be preferred for chloride-contaminated structures. Only if the chloride content in concrete is low and the penetration of chloride is Hmited in extent, can other repair techniques be taken into consideration. 

Non-alkaline (polymer) mortars have been used for repairs however, they do not promote repassivation and they are only based on physical effects. Epoj mortors applied as patch repairs have in the past been unsuc-... 

Measurement of the electrochemical current noise is aimed at correlating the observed current fluctuations with breakdown and repair events that might lead to the formation of stable growing pits [53, 54], In view of this mechanistic interpretation, the application of statistical methods to the occurrence of current spikes and the observed probability of pit formation lead to a stochastic model for pit nucleation. The evaluation of current spikes in the time and frequency domain yields parameters such as the intensity of the stochastic process X and the repassivation rate r [53]. They depend on parameters such as the potential, state of the passive layer, and concentration of aggressive anions. 

Furthermore, the repassivation kinetics of pits are closely related to the transport of locally accumulated halides to the bulk electrolyte [19, 29]. These details are explained best by the complexing properties of the halides. The accumulation of corrosion products and consequently of halides prevents film repair and the repassivation of corrosion pits. Any oxide formation will stop immediately because of the large local concentration of complexing halides. 

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]. 

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). 

As described in detail in the RILEM 124 SRC recommendation (RILEM, 1994), the basic repair methods for carbonation induced corrosion consist in repassivation of the steel by a mortar layer or by local repair. The procedure for patch repair is described in the published guidelines (e.g. Bentur et al., 1997). Limitation of the concrete moisture content is another repair principle, its aim is dry out the concrete, e.g. when additional facade elements are mounted to reduce energy consumption. When carbonated concrete is not completely removed in the vicinity of the reinforcement, corrosion occurs after repair when sufficient moisture is present (Schiessl and Breit, 1996). 

The ways in which inhibitive anions affect the corrosion of zinc are mainly similar to those described above for iron. In inhibition by chromate, localized uptake of chromium has been shown to occur at low chromate concentrations and in the presence of chloride ions. Inhibitive anions also promote the passivation of zinc (e.g., passivation is much easier in solutions of the inhibitive anion, borate, than in solutions of the noninhibitive anions, carbonate and bicarbonate). A critical inhibition potential, analogous to that on iron, has been observed for zinc in borate solutions. Thus inhibitive anions promote repair of the oxide film on zinc by repassivation with zinc oxide. 

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