Initiation chloride-induced corrosion

As mentioned above, the service life for chloride-induced corrosion is traditionally assumed equal to the initiation time and the period of propagation is not... 

Design Equation for Chloride-induced Corrosion Initiation... 

Table 12.5 Initiation time for chloride-induced corrosion estimated for different concrete cover thicknesses, utilising apparent diffusion coefficients of chlorides (D,pp) evaluated on specimens submerged in the North Sea for 16 y (concrete of 420 kg/m of Portland cement, OPC, or blast furnace slag cement with 70% GGBS and identical curing procedures) [18]...

Alkanolamine-hased inhibitors have been tested in similar conditions. For ongoing chloride-induced corrosion with a chloride level of about 1-2 % by mass of cement, in mortar specimens no reduction in corrosion rate was found (Figure 13.6) except at low chloride concentrations. This is confirmed by two other studies [1,11,14] pre-corroded rebars in mortar (w/c 0.75, cover thickness 25 mm) did not show any detectable effect on the corrosion rate of embedded steel once active corrosion had been initiated, despite the fact that the specimens had low cover and porous mortar [14]. It seems that for penetrating or migrating inhibitors the favourable effects found in solution do not occur when applied to hardened mortar or concrete laboratory specimens with ongoing steel corrosion. It is thus necessary to look for information regarding the transport of inhibitor blends in mortar or concrete. 

C. M. Hansson, Corrosion inhibitors in concrete. Part III Effect on time to chloride-induced corrosion initiation and subsequent corrosion rates of steel in mortar . Cement and Concrete Research, 2001, 31, 713. 

Tests described above show that polymer-modified cementitious coatings may have good initial carbonation resistance, which may be lost due to weathering [7] and good chloride-penetration resistance [8], but no effect on chloride-induced corrosion rate [13]. 

The beneficial action of surface treatments generally hes in the fact that they prolong the period of initiation of corrosion. Once corrosion has begun, only those treatments that effectively obstruct the penetration of water, both hquid and vapour, will reduce the corrosion rate. This effect may be significant, in particular, if corrosion is due to carbonation [27]. Chloride-induced corrosion processes attract moisture so strongly that in general, surface treatments cannot stop it [13]. 

Galvanized reinforcement, i.e. zinc coatings formed by dipping clean rebars in a bath of molten zinc, can protect steel in concrete from corrosion attack. However, the performance reported in the literature is contradictory (Bentur et al., 1997). Galvanized rebars remain passive in carbonated concrete and the corrosion rate is much lower than with black steel. In situations where chloride induced corrosion prevails, a delay in the initiation of corrosion can be expected, but at high chloride concentrations depassivation cannot be avoided completely. 

Initially, reinforcement steel in concrete is protected by the high alkalinity of the concrete and no corrosion occurs (Val Melchers 1997). However, in case of chloride-induced corrosion, the diffusion of chlorides through the protective concrete cover results in the depassivation of the concrete. As soon as a critical chloride concentration is reached, the corrosion process starts. Models for the depassivation of concrete can be found in Stewart Rosowsky (1998) and Vu Stewart (2000). Generally, the length of the initiation period depends on the concrete cover, the diffusion coefficient, the critical chloride content, the initial chloride concentration in the concrete and the chloride concentration at the surface of the concrete element. 

For steel embedded in concrete, it was observed that current densities up to 50 A/m applied for 5 months to passive steel in concrete with up to 0.4% chlorides did not lead to corrosion initiation [5]. Since steel in reinforced-concrete structures is not coated, it is not actually possible to reach such high current densities. It can be assumed, therefore, that interference from AC current cannot induce corrosion on passive steel in concrete. 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

Chloride-induced maorooell corrosion initiates on top rebar... 

Provided that the concrete is not water-saturated, it may be reasonable to assume that the initiation phase is considerably longer than the propagation period and that the end of the initiation period alone is a useful indicator of service fife. Clifton and Pommersheim have reviewed simple models based on this approach. For chloride-induced rebar corrosion, one of these is the use of Fick s second law of diffusion and the concept of a critical chloride concentration. Limitations and simplifying assumptions of this approach have been discussed in previous sections. Actual chloride concentration profiles can be measured on structures, to estimate parameters such as the diffusion coefficient used in the model. For carbonation, it has been proposed that the depth of carbonation is proportional to the square root of the exposure time. Again, the measurement of actual carbonation depth with time can be used to estimate a proportionality constant for a specific structure. 

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... 

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