Passivation criterion

Chapter 4 presents the fundamentals of passivity the film and adsorption theories of passivity criterion for passivation methods for spontaneous passivation factors affecting passivation, such as the effect of solution velocity and acid concentration alloy evaluation anodic protection systems and design requirements. A fuU discussion on stainless steel composition and crystalline structure, oxidizer concentration, and alloy evaluation is included. The chapter also considers anodic protection to establish a basis for anodic... 

Since the object to be protected represents a cell consisting of active and passive steel, considerable IR errors in the cell current must be expected in measuring the off potential. The considerations in Section 3.3.1 with reference to Eqs. (3-27) and (3-28) are relevant here. Since upon switching off the protection current, 7, the nearby cathodes lead to anodic polarization of a region at risk from corrosion, the cell currents and 7, have opposite signs. It follows from Eqs. (3-27) and (3-28) that the 77 -free potential must be more negative than the off potential. Therefore, there is greater certainty of the potential criterion in Eq. (2-39). 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

One possible classification is based on the type of physico-chemical phenomena that may occur in the sensor. Based on this criterion, there are passive flow-through sensors, which posses no reactive microzone and are... 

The albumin quotient is the most precise, routinely used criterion for assessment of the function of the blood-CSF barrier because albumin in cerebrospinal fluid originates exclusively from serum. Its parallel determination during the monitoring of any CSF protein is necessary because this is the only way to differentiate its increased concentration in cerebrospinal fluid due to passive penetration of the respective serum protein from a more specific increase in the concentration of the monitored protein. It is based on its intrathecal synthesis or on a specific transport mechanism for the given protein across the blood-CSF barrier. Unfortunately, some clinicians disregard this recommendation, and this elementary fact is not sufficiently emphasized in publications on cerebrospinal fluid (A22). 

The use of the Coanda effect is based on the desire to have a second passive momentum to speed up mixing in addition to diffusion [55, 163], The second momentum is based on so-called transverse dispersion produced by passive structures, which is in analogy with the Taylor convective radial dispersion ( Taylor dispersion ) (see Figure 1.180 and [163] for further details). It was further desired to have a flat ( in-plane ) structure and not a 3-D structure, since only the first type can be easily integrated into a pTAS system, typically also being flat A further design criterion was to have a micro mixer with improved dispersion and velocity profiles. 

We begin by showing how active transport can directly affect membrane potentials. We then compare the temperature dependencies of metabolic reactions with those for diffusion processes across a barrier to show that a marked enhancement of solute influx caused by increasing the temperature does not necessarily indicate that active transport is taking place. Next we will consider a more reliable criterion for deciding whether fluxes are passive or not — namely, the Ussing-Teorell, or flux ratio, equation. We will then examine a specific case in which active transport is involved, calculate the energy required, and finally speculate on why K+ and Cl- are actively transported into plant cells and Na+ is actively transported out. 

The electrochemical behaviour of the ribbed-functionalized iron(II) [65, 68] and ruthenium(II) [78] clathrochelates with alkylamine, thioaryl, thioalkyl, phenoxyl and crown ether substituents in a-dioximate fragments was characterized by E1/2 values for Fe3+/2+ and Ru3+/2+ couples (Table 37). The Tomes criterion values of most complexes exhibited reversible or quasi-reversible anodic processes. Moreover, the quasi-reversible oxidation processes are accompanied by the formation of insoluble products followed by passivation of the working electrode. The Ev2 values depend on the electron-donating properties of the substituents in the ribbed fragments. The correlations of E1/2 values for Ru3" 2+ and Fe + 2+ couples with these substituents Hammet s Opara constants were observed in Refs. 65, 68, and 78. These correlations are rather qualitative, but they enable one to conclude that ruthenium complexes are less sensitive to the change of substituents in dioximate fragments. There was no correlation between the Em values and the inductive Taft s (cr,) constants for substituents in dioximate fragments. 

In this study we applied these diagnostic criterion to evaluate whether the oxide films on tungsten are anion conducting in oxidizer solutions. As expected from the potentiodynamic polarization experiments, which show a constant current passivation regime, the steady state current density was found to be independent of potential in passive region using potentiostatic measurements. 

Another important criterion is to determine whether the nodular transformation in the liver is complete or incomplete. This morphological characteristic can be found both in progressive and stationary cirrhoses. In complete cirrhosis, the parenchyma is completely partitioned by connective tissue septa. The collapse of fibres (due to portocentral bridging necroses) results in the development of portocentral septa (= passive septa). Due to the spreading of the inflammation to the periportal parenchyma, septa develop and branch out from the portal fields (= active septa). Generally, they lead to capillarization of the sinusoidal walls as well. (s. fig. 35.7) Incomplete cirrhosis only displays the formation of short septa (= subsepta), so that there are areas of incomplete (partial) subdivision of the parenchyma. 

In addition to the precise knowledge of the chemistry of the passive film, another requirement is necessary if the crystallinity of the passive films is investigated it is to work with well-defined substrate surfaces. The criterion of a well-defined surface is the observation of atomic terraces. This is obtained with single-crystal surfaces. The use of... 

Does a soil-fluid-chemical system behave as an active electrochemical system or a passive electrical conductor under the influence of a DC electric field This is a fundamental question of significant implications. The evaluation criterion that can be used to differentiate the two systems of completely different nature is vested in Faraday s laws of electrolysis, as the transfer of electrons from the electrodes to the system and vice versa in an ideal electrochemical system is invariably associated with chemical reactions obeying Faraday s laws of electrolysis (Antropov, 1972). The two important fundamental laws of electrolysis can be simply expressed as follows (a) the amount of chemical deposition is proportional to the quantity of electric charges flowing through the system in an electrolytic process, and (b) the masses of different species deposited at or dissolved from electrodes by the same quantity of electric charges are directly proportional to their equivalent weights (Crow, 1979). 

A) Thermodynamics of the Active-Passive Transition If faced with the task of deciding whether a particular metal would be suitable as a fabrication material In a given environment, one likes to know a) what kinds of spontaneously occurring reactions are expected and b) what Is the magnitude of the rate of metal dissolution. The latter Is the real criterion In making the decision and Is usually evaluated through experimentation. The former provides a yes or no answer regarding the stability of the metal and falls within the scope of equilibrium thermodynamics. 

The experience on bridge decks shows that, in the cases in which the cathodic protection path runs according to 4-5 of Figure 20.4, the current required to maintain protection conditions (verified by the so-called four-hour 100 mV potential decay empirical criterion decreases with time, even after months or years from start up. This happens because the cathodic current can bring about repassivation of steel in active zones. When passivity is established on the entire surface of the steel, the current required to maintain passivity is reduced to a few mA/m (e. g. 2-5 mA/m ). If the CP path runs according to 4-6, the current density to fulfil the protection criterion remains high and does not decrease with the time, since passivity is not obtained. 

Figure 20.3c shows the effect of application of cathodic protection on carbonated concrete. The applied cathodic current density, even if it brings about only a small lowering of the steel potential, can produce enough alkalinity to restore the pH to values higher than 12 on the reinforcement surface and thus promote passivation. The effectiveness of cathodic protection in carbonated concrete was studied with specimens with alkaline concrete, carbonated concrete and carbonated concrete with 0.4% chloride by cement mass that were tested at current densities of 10, 5, and 2 mA/m (of steel surface) [45]. Carbonated concrete specimens polarised at 10 mA/m showed that, although initially protection was not achieved since the four-hour decay was slightly lower than 100 mV, after about four months of polarization, the protection criterion was fulfilled and higher values, in the range 200-300 mV of the four-hour potential decay were measured (Figure 20.6). The same results were obtained on carbonated and slightly chloride-contaminated concrete.

Restricted by the level of economic development and staff own quality, the safety concept of miners is generally we at present. It is in the dependent and passive position basically. They abide by the rules and regulations only because they are afraid of being punished. The so-called safety first and safety discipline consciousness is fuzzy. In addition, various safety regulations in some enterprises are to some extent on paper or verbal, not into a habit and behavior criterion of all employees, which is not enough to let them consciously accept and promote coal mine safety culture. 

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