Depassivation temperature

Interpretation. The results of concrete resistivity measurements can be used for a quantitative or qualitative interpretation. Resistivity data measured on a structure and corrected for the temperature effect can be compared to reference data of similar concrete types (Table 2.4). Usually additional information is necessary. If, for example, a wet structure made with OPC has a mean resistivity value of 50 fl m, it means that the water-to-cement ratio and the porosity must be quite high. Consequently, the corrosion rate after depassivation will be high. 

Contact between two different metals Difference in oxygen accessibility Grain boundary precipitation Local depassivation by aggressive anions Difference in temperature... 

Laboratory corrosion tests are designed to simulate the process. Short test times and inability to capture every nuance of the process are limitations. However, laboratory corrosion testing provides important evidence of conditions that might depassivate active-passive alloys, such as increase in temperature or a threshold concentration of an aggressive species. 

Once the steel is depassivated due either to chlorides or to carbonation, corrosion can proceed in the presence of oxygen and humidity. The corrosion rate determines the time it takes to reach the final state of the structure (Fig. 8-7), but it should be borne in mind that this rate can vary considerably depending on temperature, humidity, etc. (Andrade et al., 1996 Zimmermann et al., 1997). 

Activation Time. The time from initiation of the battery to the point at which it delivers and sustains a requisite level of voltage across a specified electric load is defined as the activation time. For a spin-dependent liquid-electrolyte reserve battery, this time would include the times for ampoule opening, electrolyte distribution, clearing of electrolyte short circuits in the filling manifold, depassivation of electrodes, and elimination of any form of polarization. Activation times are usually longest at low temperatures, where increased viscosity of the electrolyte and decreased ion mobility are most significant. 

The effect of temperature on the critical hydrochloric acid concentration for the depassivation of four titanium materials in deaerated 1M sodium chloride solution is shown. For each material, the critical Gorx rrtrHlian decreases with increasing temperature. 

CF tests on smooth specimens were performed at room temperature on a 316 L austenitic stainless steel in a 0.5 N H2SO4 solution at different electrochemical potentials and for a prescribed plasfic strain amplitude of 4 x 10 (s = 10 s ). The depassivation-repassivation process occurs in a very regular way, well before any microcracks can form [11]. It is of particular interest to follow the evolution of the maximum flow stress in the corrosive solution at free potential and at imposed cathodic potential and to compare this evolution with that observed in air (Figure 12.9). It clearly appears that (a) a cyclic softening effect occurs at the free potential in comparison with the behavior in air (b) this softening effect disappears when the cathodic potential is applied (and the anodic dissolution is markedly reduced), after about 150 cycles (c) the softening effect then occurs in the same way when... 

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