Repassivation cracking

It is assumed that the slip dissolution mechanism [40] adequately describes the crack-tip process. The controlling variables are the stress intensity factor (from mechanical loading) and the crack-tip electrode potential (from electrochemical loading). The crack-tip repassivation process is important, because the kinetics of repassivation determine the fraction of the crack-tip area that remains bare over a slip-dissolution-repassivation cycle. The temperature dependence of the crack-tip process is brought into play through a temperature-dependent crack-tip strain... 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

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. 

Representative environments for which SCC has been reported in carbon steels are included in Table 7.7. The sensitivity of these steels to changes in composition and environment are illustrated by the effects of potential in Fig. 7.78 to 7.80 and by the slow strain-rate data of Fig. 7.82 and 7.83. These data support the conclusion that environment cracking is related to the susceptibility of the passive films to crack under stress, to the subsequent crack growth due to anodic dissolution and/or hydrogen embrittlement during the period of exposure of the alloy substrate, and to rates of repassivation of the exposed areas. Actual crack-front growth mechanisms are discussed in some detail in a later section. 

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.

Reference to Figure 8, together with the concept of repassivation, indicates during the propagation of cracks the current should consist of a number of successive transients. One is shown in Figure 11, which might be taken to correspond to the sequence of events... 

Generally, if the crack width is modest (e. g. it is below 0.3-0.5 mm), after the initiation of corrosion on the steel surface, the corrosion rate is low. Chemical processes in the cement paste and formation of corrosion products may seal the crack near the reinforcement and allow the protective oxide film to form again. For car-bonation-induced corrosion, repassivation can take place when the migration of alkalinity from the surrounding concrete brings the pH of the pore solution in contact with the corrosion products to values above 11.5. Repassivation may have trouble taking place or may not take place at all in the following situations ... 

Repassivation with alkaline concrete or mortar. This method is based on the appH-cation of a sufficiently thick (> 20 mm) cement-based layer of concrete or mortar over the surface of carbonated concrete. Only cracked or delaminated concrete has to be removed, while mechanically sound concrete, even if it is carbonated up to the reinforcement and thus in contact with corroding steel, will not be removed. The method relies on the diffusion of hydroxyl ions (OH ) from the new external alkaline layer towards the carbonated concrete substrate. In wet environments or in the presence of wetting-drying cycles (i. e. the worst conditions for corrosion), this can lead to realkalization of the carbonated concrete in time and thus to repassivation of the reinforcement. This method is mainly apphed in Germany and has proved to be effective in repassivating the reinforcement, usually within a few months, if the carbonation depth is not high [5]. This method should not be used if carbonation has penetrated behind the reinforcement more than 20 mm. 

Logan [42] suggested a film rupture or sHp-step dissolution model for SCC mechanism where the crack is initiated by locahzed anodic dissolution. This mechanism postulates that plastic strain in the metal at the crack tip causes a fracture of the oxide barrier film. The film rupture model (FRM) assumes a repetitive, cycling process of film rupture, dissolution of the underlying metal, and repassivation. 

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... 

T. Nakayama, M. Takano, Application of a slip dissolution-repassivation model for stress corrosion cracking of AISI 304 stainless steel in a boiling 42% MgCb solution. Corrosion 42 (1986) 10-15. 

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