Chloride Penetration

Certain anions, especially chloride, penetrate the protective films, which are naturally present on some metals (e.g. aluminum and its alloys and stainless steels). This process is an initiator for corrosion, especially for localized corrosion. 

In typical cause-and-effect mode, where chlorides penetrate the deposit or where a localized overconcentration of hydroxyl ions occurs, the magnetite film is disrupted and particular forms of very damaging corrosion occurs. In addition, where localized heat flux exceeds design limits within a boiler and may be accompanied by departure from nucleate boiling (DNB) conditions, overheating and metal failure may also occur. 

The depth profiling technique used on samples with a barrier film before and after the addition of chloride to the buffering borate electrolyte showed no indication of either chloride penetration or significant reduction of the average oxide layer thickness.123 This, of course, does not rule out the possibility of the formation, by any of the mechanisms suggested above, of pinholes with radii much smaller than that of the ion-gun beam, through which the entire active dissolution could take place, or the possibility that the beam missed pits formed sporadically across the surface. If pinholes which are not visible were formed, the dissolution should proceed in them with extremely high true current densities. 

These include water-based materials such as amine, esters and alkylsilanes. Water-based amines and esters have no significant effect on slump and rate of hardening of concrete. It is claimed they delay corrosion of steel both in cracked and uncracked members. Effectiveness of the admixture has been attributed to a dual mechanism of corrosion inhibition, viz. prevention of chloride and moisture ingress and formation of a protective film on the surface of the steel. Ester molecules derived from the admixture are said to line the pores of the concrete thus increasing the resistance to moisture and chloride penetration, while the film-forming characteristics of the amine promote adsorption of the molecules on the steel surface, providing a protective coat which keeps moisture and chlorides out. 

Salicylic acid (SA) Male skin (55-72) Scrotum, abdomen SA in dilute ferric chloride Penetration of SA Penetration time scrotum 15 min vs. abdomen 2hr -t ... 

For the 25 freeze-thaw cycles, it was found that plastic fillers did not change the chloride penetration profile of concrete. More cycles are needed to confirm this result and to explore effects with a larger number of cycles. 

R. B. Polder, Simulated de-icing salt exposure of blended cement concrete - chloride penetration , Proc. 2nd International RILEM Workshop Testing and Modelling the Chloride Ingress into Concrete, C. Andrade,... 

Another application of Tick s second law of diffusion is the analysis of chloride-penetration profiles in cores taken from structures that were actually exposed to chloride penetration from the outer surface. A profile of the chloride content is determined experimentally as a function of depth (Section 16.3.2). The values of Cj and D are then determined mathematically, by fitting Eq. (4) to the experimental data [19]. An example is given in Figure 2.6. The diffusion coefficient D (which may typically vary from 10 to 10 m /s as a function of the concrete s characteristics) can be used as the main parameter that describes the rate of chloride penetration. Together with the fitted surface content and using Eq. (4) the chloride penetration can be extrapolated to longer times. The limits of this approach will be discussed in Chapter 6. 

Ion migration of concrete can be tested as a measure of its resistance to chloride penetration using migration tests, of which various versions exist. Some of these tests measure non-steady-state migration, expressed as the depth of penetration of chloride ions into a specimen in an electrical field [34, 35]. Other methods apply an electrical field across a specimen until steady-state flow of chloride ions is detected in the downstream cell [36]. Because of their specific nature, a detailed description of these methods will be omitted. 

The resistivity of concrete is an important parameter used to describe, for example, the degree of water saturation, the resistance to chloride penetration or the corrosion rate. The resistivity of concrete may have values from a few tens to many thousands of n m (Table 2.3) as a function of the water content in the concrete (relative humidity), the type of cement used (Portland or blended cements), the iv/c, the presence of chloride ions or whether the concrete is carbonated or not At early ages, the resistivity of concrete is low and considerable increases occur due to hydration of the cement AU of these factors can be rationalised on the basis of ion migration in the porous and tortuous concrete microstructure a high relative humidity increases the amount of water-filled pores (decrease of resistivity), the iv/c ratio and type of cement determine the pore volume and pore-size distribution (less but more coarse pores with pure Portland cement more but finer pores and less interconnectivity of pores with blast furnace slag or fly ash) chloride ions increase the conductivity of the pore solution and carbonation decreases it. An increased resistivity is accompanied by a reduced corrosion rate [38]. Table 2.4 shows resistivities determined for mature concrete in various chmates [39-41]. 

Nordtest Standard NT BUILD 443, Accelerated Chloride Penetration,... 

Proc. RILEM. Int. workshop on Chloride Penetration into Concrete, St-Remy-les-Chevreuses,... 

R. B. Polder, Simulated de-icing salt exposure of blended cement concrete - chloride penetration , Proc. 

Limiting C3A may, on the other hand, have an adverse effect on the chloride-penetration resistance of concrete. The use of cement with low CjA (such as ASTM type V) in many marine structures, in particular in The Middle East built in the 1970s and 1980s, has caused many of them to suffer from extensive reinforcement corrosion. Here it was wrongly assumed that a better resistance against sulfate attack would also mean a better resistance to all adverse effects of seawater [26]. 

The service life of reinforced-concrete structures can be divided in two distinct phases (Figure 4.1). The first phase is the initiation of corrosion, in which the reinforcement is passive but phenomena that can lead to loss of passivity, e.g. carbonation or chloride penetration in the concrete cover, take place. The second phase is propagation of corrosion that begins when the steel is depassivated and finishes when a limiting state is reached beyond which consequences of corrosion cannot be further tolerated [6, 7]. 

The apparent diffusion coefficient, obtained from real structures or laboratory tests, is often also used as the parameter that describes the resistance of concrete to chloride penetration, e. g. when performances of different materials are compared. The lower D,pp is, the higher the resistance to chloride penetration is. It should, however, be observed that, while the diffusion coefficient obtained from pure (steady-state) diffusion tests can be considered as a property of the concrete, the apparent diffusion coefficient obtained from real stractures also depends on... 

Figure 6.8 Different zones of a marine structure, in relation to chloride penetration and corrosion of reinforcement...

Figure 6.9 Example of chloride-penetration contours in a marine structure as a function of the height above the seawater (Portland cement, wjc = 0.5, C3A = 10% after NOrnberger [30])...

L. Bertolini, M. Gastaldi, M. Carsana, M. Berra, "Gomparison of resistance to chloride penetration of concretes and mortars for repair , 3 Rilem Workshop Testing and Modelling Chloride ingress into Concrete,... 

J. M. Frederiksen (Ed.), HETEK -Chloride Penetration into Concrete. State of the Art. Transport Processes, Corrosion Initiation, Test Methods and Prediction Models, The Road Directorate, Report No. 53, Copenhagen, 1996. 

R. N. Swamy, H. Hamada, J. C. Laiw, A critical evaluation of chloride penetration into concrete in marine environment , Proc. Int. Conf on Corrosion and Corrosion Protection of Steel in Concrete, R. N. Swamy (Ed.), Sheffield Academic Press, 24-29 July 1994. 

The most frequent type of macrocell in reinforced-concrete structures exposed to the atmosphere is that established between more superficial rebars that have been depassivated by carbonation or chloride penetration, and internal passive rebars. Another example may be walls where chloride penetrates from one side and oxygen penetrates from the other side, which may occur in hoUow structures Hke tunnels and offshore platform legs or with ground retaining walls. 

Depassivation of rebars due to carbonation or chloride penetration often does not extend to the whole surface of the reinforcement but, for instance, it is limited to the outer layer of rebars, or to parts where the concrete cover has a lower thickness... 

In this regard, therefore, information wiU be restricted to explaining the generally accepted approach of design for durabihty based on models for carbonation and chloride penetration. In addition, a recently proposed method for quantified service life design is illustrated. Measures against chemical and physical attack of concrete have been addressed in Chapter 3. 

Evaluation of the Service Life with Respect to Chloride Penetration... 

The degradation models for corrosion initiation include carbonation and chloride penetration as a function of time and variables related to concrete composition, external conditions and so on. A corrosion propagation model is also included. 

In the design process, the DuraCrete approach works as follows. The chloride surface content and the intended cover depth for the structure are identified. As a first step, concrete compositions are selected for appropriateness using input values for chloride-penetration resistance from a database. 

The next step is to test one or more of the selected concrete compositions using a compliance test that determines the chloride-penetration resistance. DuraCrete has adopted the rapid chloride migration (RCM) test for this purpose [21,25], which is a short-term test based on acceleration of chloride penetration by an electric field. 

The resistance of the selected concrete mix against chloride penetration is determined using this test and if necessary, the composition is modified until satisfactory values are obtained. At this stage, increasing the cover depth may also be possible. 

In the execution stage, quahty control is applied on site using simple methods with known correlation to the RCM test. For quality control with regard to chloride-penetration resistance, DuraCrete has adopted an electrical resistivity test, the two-electrode method (TEM), to be carried out on specimens kept in a fog room by a test described in Chapter 2. The correlation between the RCM and TEM values is based on theoretical and empirical work mentioned in Section 2.6 [26,27] and was documented for a wide range of concretes [28]. 

J (t) = design value of the chloride-penetration resistance at any point in time (y/mm t = time (y),... 

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