Chloride critical values

Figure 2. Model of the corrosion process of steel in concrete (1) - in the presence of chloride (critical value pH=9), (2) - no chlorides (phrll) pH > 13.5 metastate state.

The dangers linked to unstable substances that are insoluble compared with those that are soluble are also concentration effects. This is the case for many polyperoxides. Even those that are soluble can detonate if their concentration exceeds a critical value eg 15% for the peroxide of vinylidene chloride. 

A significant contribution of rheo-NMR has been to show that the uniform shear-rate assumption may be violated in the case of certain classes of fluids in which pathological flow properties are exhibited. Figure 2.8.10 shows shear-rate maps [26] obtained for the wormlike surfactant system, cetylpyridinium chloride-sodium salicylate in water. While the velocity gradients show no deviation from uniformity at very low shear rates, above a certain critical value yc a dramatic variation in the rate-of-strain across the 7° cone gap is found. In particular a very high shear-rate band is found to exist at the mid-gap. 

These critical values differ from solvent to solvent. With the solvents most fully investigated, sulphuric acid for poly-(p-phenylene terephthalamide) and dimethyl acetamide/lithium chloride for poly (p-benzamide), there are also critical values of solvent composition the sulphuric acid must exceed a critical strength, the lithium chloride in the dimethyl acetamide must exceed a critical concentration. The critical values are, of course, interdependent rather than absolute. Diagrams that display some of the critical values for the two systems cited have been published in patents10,1 l Figures 3 and 4 illustrate the type of information available. 

Combustion, Flames and Explosion in Gases , 2nd Editn, Academic Press, NY (1961), 322—23 (Dust quenching occurs at a critical value of the surface area of the dust per unit vol of the suspension, and depends on the nature of the salt. Better results are obtained with salts having a mp under 200°. Alkali halides are better than carbonates, potassium better than sodium, fluoride better than iodide and better than chloride. If the dust concentration is high enough, even detonation waves can be extinguished)... 

Effect of Sodium Chloride Concentration. Figure b compares interfacial tensions of several different surfactant concentrations verses n-undecane in the presence of 0.1 M sodium chloride with values obtained without salt. Salt reduces the interfacial tension at all surfactant concentrations. Aqueous potassium oleate has a critical micelle concentration of 0.001 M (13). It could be inferred from Figure b that 0.001 M sodium oleate with no added salt is below the cmc, because of the high interfacial tension. If so, the much lower interfacial tension in the presence of 0.1 M sodium chloride stems from reduction of the cmc expected in the presence of added salt (lb). 

Fig. 7-6. Variation of particle size with relative humidity (Kohler diagram) for sodium chloride particles having different dry radii. Deliquescence occurs at 75% r.h. Note the hysteresis effect when the humidity is raised or lowered beyond the critical value. Curves for relative humidities greater than 75% were calculated. The scale above 100% is expanded. The experimental points refer to observations of Tang et al. (1977) on submicrometer-sized monodis-perse sodium chloride particles.

The determination of Ge by means of atomic absorption spectrometry (AAS) during the late 1960s was replaced by introduction of the hydride technique with sodium tetrahydroborate as a means of reduction. The detection limits indicated are 2xl0 g and 0.01 pg mL Abbasi et al. (2001) showed that there is a possibility of losing Ge in the presence of chloride when acid digestion procedures are carried out in open vessels. This is more pronounced if the Ge concentration is very low. In fact, 100 pg g can be considered as a critical value above which both open and closed vessels digestion methods can be used without any significant loss. 

In practice, since the total chloride content can be measured much easier than the free chloride concentration (Section 16.3.2), the chloride threshold is expressed as a critical total chloride content. The critical value is usually given as a percentage of chlorides with respect to the mass of cement, since the amount of chlorides that can be tolerated increases as the cement content in the concrete increases. 

In case a, future penetration of chlorides has to be evaluated in order to assess if the chloride threshold will be reached at the surface of the reinforcement before time tf. If this is expected, the concrete with a chloride content higher than the critical value has to be removed (and replaced with a chloride-free material that prevents further penetration of chloride). Equation (1), Chapter 6, may be used to evaluate the future penetration of chlorides. By fitting the present chloride profile, the surface content and the apparent diffusion coefficient Dj j, may be calculated at time Since these parameters are evaluated on the actual structure and usually after a long time of service, it is often reasonable to assume that they will not change significantly in the future (unless the conditions of exposure of the structure will change). Equation (1), Chapter 6, can then be used to plot the expected chloride profile at time tf. 

Side effects. During chloride extraction, hydroxyl ions are formed around the reinforcing steel, locally increasing the pH and sodium and potassium ions are enriched around the steel. These changes might stimulate aUcah-silica reaction (ASR, Section 3.4). In the framework of COST 521, the possibility of ASR was checked as a side-effect of chloride extraction [28,36,80,81]. The aggregates studied were reactive and the alkali content of the cement was just below the critical values. The results obtained with non-carbonated concrete showed that, under the worst conditions, chloride extraction induced concrete expansion, but no cracking was observed. 

The splash/tidal zone of bridges and walls represent cylindrical columns immersed in water. These structures involve entry in two dimensions. Chloride ions enter concrete by adsorption at the surface, which is given by an empirical equation. The effective chloride diffusion coefficient is derived from concrete permeability, water/cement ratio, and concrete resistivity. When concentration reaches a critical value in the vicinity of steel, corrosion begins. As shown in Fig. 12.6, a boundary layer exists adjacent to the concrete... 

Pitting corrosion (sometimes only called pitting) occurs on more or less passivated metals and alloys in environments containing chloride, bromide, iodide or perchlorate ions when the electrode potential exceeds a critical value, the pitting potential (Figure 7.27), which depends on various conditions. The pitting potential is not a thermodynamically defined potential and depends for one thing upon the rate of potential increase when the polarization curve is recorded. 

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

Recent studies on electrical potential oscillations across a liquid membrane consisting of an oil layer, 90% oleic acid and 10% 1-propanol, containing tetraphenyl phosphonium chloride (TPPC), between aqueous solutions of 0.5 M NaCl and KCl in tow different compartments have been reported by Yoshikawa and Matsubara [52]. TPPC is a cationic lipophilic salt, which can act as a surfactant. On exposure to amine vapour, initially upward deflection occurs, indicating that the KCl solution becomes negative with respect to NaCl solution. Electric potential oscillations did not occur when the concentration of ammonia was below a critical value. 

Crystals were grown on glass substrates by evaporating a dilute sodium chloride solution. The tip of the AFM was scanned across the surface to locate the crystals and to measure their size from the flat square top surface. The crystals were cubes up to 50 nm in size. Then, the contact force was raised to a high level and the tip was used to break the crystals from the surface. Humidity was found to be very important and was controlled to high precision in the microscope. Once the probe touched a crystal, the lateral force rose to a critical value, then fell as the cube sheared across the surface. The fracture stresses were calculated from the peak force and the crystal interface area, and plotted as a function of crystal size in Fig. 13.11(a). Two striking observations were drawn from this curve in the first place, the stress was much larger than expected for a soft material like NaCI,... 

Figures 8.4 and 8.5 indicate the critical potentials noble to which S.C.C. of 18-8 stainless steel initiates when exposed to magnesium chloride solution boiling at 130°C with and without inhibiting anion additions [27]. Anodic polarization induces shorter cracking times the more noble the controlled potential cathodic polarization, on the other hand, extends the observed cracking times. Below the critical value of -0.145 V (S.H.E.), the alloy becomes essentially immune (Fig. 8.4). Addition of various salts, such as sodium acetate, to the magnesium chloride solution shifts the critical potential to more noble values. When the amount of...

In Fig. 7, we show examples of simulated radial density profiles for solvent centers around a single dilute chloride ion at the indicated supercritical and ambient water (AW) conditions[49]. Many other solutes have been studied.[10] In Fig. 7, the density and temperature (p, T. ) of SCW is indicated in units reduced by the critical values for... 

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