Passivity physical theories

Physical Theories.—Several theories have been put forward according to which the phenomena of passivity are due to a physical change in the superficial layers of the iron. What the precise nature of that change may be remains a matter of controversy. 

The third approach is called the thermodynamic theory of passive systems. It is based on the following postulates (1) The introduction of the notion of entropy is avoided for nonequilibrium states and the principle of local state is not assumed, (2) The inequality is replaced by an inequality expressing the fundamental property of passivity. This inequality follows from the second law of thermodynamics and the condition of thermodynamic stability. Further the inequality is known to have sense only for states of equilibrium, (3) The temperature is assumed to exist for non-equilibrium states, (4) As a consequence of the fundamental inequality the class of processes under consideration is limited to processes in which deviations from the equilibrium conditions are small. This enables full linearization of the constitutive equations. An important feature of this approach is the clear physical interpretation of all the quantities introduced. 

Figure 39. Current-time variation in nickel pitting dissolution in NaCl solution.89,91 1, double-layer charging current 2, fluctuation-diffusion current 3, minimum dissolution current 4, pit-growth current (Reprinted from M. Asanuma andR. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film, J. Chem. Phys. 106, 9938, 1997, Fig. 2. Copyright 1997, American Institute of Physics.)...

Fleischer, R.L., Theory of Passive Measurement of Radon Daughter and Working Levels by the Nuclear Track Technique, Health Physics 47 263-270 (1984). 

Ellipsometry is a branch of specular reflection spectroscopy which is particularly useful for characterising thin films on surfaces in situ, e.g. in the presence of supernatant liquids. It is applicable to many problems in corrosion and passivation, lubrication, the physics and chemistry of new materials, and biological chemistry. The principles behind it have been known for a long time, but it remains a rather under-used technique. This situation is now changing because of a number of factors, such as the advent of cheap computing power, and the impetus, brought about by the great economic importance of the semiconductor industry, to produce more powerful theories to interpret the results. 

The general discussion and controversy as to whether the microfibrils are formed by apposition or deposition of cellulosic materials also applies to the plant cell-wall but, here, the question assumes much greater significance, especially with respect to the architecture of the cell wall and the precise orientation of the microfibrils within its layers and lamellae. How these structures are formed and to what extent the processes involved are carried out and controlled by the living cell, or by inanimate, physical forces, pose major questions that have been extensively investigated and discussed. Various theories for the passive and active orientation of the microfibrils in growing-plant cell-walls have been reviewed in several botanical ar-ticles with excellent discussions, and will only briefly be mentioned here as background. 

Adherents to this theory have different opinions on the potential at which the film forms. Its thickness, the mechanism of formation, and, most Important, the "cause of passivity. In the earlier theories It was postulated that the passivation follows the formation of a "primary layer" of small conductivity, x lth porous character, which Is sometimes due to precipitation of metal salt on and near the electrode.(32) In the pores the current Increases, and by polarization at an "Umschlagspotentlal" (Vj, = V, Figure 1) an actual passive layer is formed. Thus the essential concept of the passivation process Is connected with the change of the properties (chemical or physical) of the primary film at a certain potential. The passive film Is free from pores and presents a barrier between the metal and the environment. It is electronically conductive and slowly corrodes In solution.(6,8,24,37)... 

The most widely used NDA instruments rely on the detection of nuclear radiation such as gamma rays and/or neutrons. Physical measurement techniques are also used with available instruments that measure heat, weight, liquid volume, thickness, and light emission/absorption. These physical techniques may be applied by themselves, or they may be used in combination with other nuclear measurements to provide quantitative measurements of the nuclear material. The general reference on the theory and application of passive NDA (PANDA) is given by reference (Reilly et al. 1991) and its addendum (Reilly et al. 2007). 

Let us consider again a sorption system consisting on a sorbent-sorbate phase and a sorptive gas located between the plates or cylinders of a capacitor, Fig. 6.8. This system is an electric network which for small applied voltages (U(t)) can be interpreted as a Linear Passive System (LPS). That is a stimulus (U(t)) applied to the system creates a response, the electric current I(t), which is linearly related to U(t). However it may exhibit a phase shift and also lead to energy dissipation, i. e. Ohmian heat which, as a consequence of the Second Law of Thermodynamics at finite ambient temperature, never can completely be reverted again to electric energy. Linear Passive Systems can be found quite frequently in Physics. A mathematical theory of such systems has been developed by H. Kdnig and J. Meixner in the 1960 s, [6.27] and later on extended and applied to various stochastic processes, i. e. statistical physics by J. U. Keller, [6.28]. 

From the point of view of non-equilibrium statistical physics, understanding the behavior of such active membranes is challenging. Recent theoretical results [8,9] have shown that the shape fluctuations of active membranes should differ both qualitatively and quantitatively from those of passive membranes. The presence of active proteins imbedded in the membrane is expected to induce a magnification of shape fluctuations due to a modification of the fluctuation spectrum. In search of the simplest model to test the theory [8,9], the shape fluctuations of giant lipid vesicles have been studied with the active protein, bacteriorhodopsin (BR) incorporated inside the lipid bilayer. 

Bagratashvili, V. N., Kuzmin, M. V., Letokhov, V. S., and Stuchebrukhov, A. A. (1985a). Theory of multiple-photon IR excitation of polyatomic molecules in the model of active and passive modes of vibrational reservoir. Chemical Physics, 97, 13-29. 

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