Transition, thermodynamics active-passive

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

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. 

Active and Active-Passive Transition Impedance and Frequency-Resolved RRDE Measurements The assoeiation of impedanee and fiequency-resolved RRDE techniques initially introduced for iron [38,117,197] has been extensively applied to the prepassive and passivation ranges of Fe-Cr alloys with and without chloride added [174,194,195], Of course, the interpretation is more intricate than for pure metals (even with multiple dissolution valences). For kinetic reasons, Cr species are not detectable on the ring. In order to draw unambiguous conclusions, reasonable assumptions had to be made concerning simultaneous alloy dissolution at steady state (see thermodynamics and rate constant approach earlier) and dissolution valences of Cr. 

Let us mention some examples, that is, the passivation potential at which a metal surface suddenly changes from an active to a passive state, and the activation potential at which a metal surface that is passivated resumes active dissolution. In these cases, a drastic change in the corrosion rate is observed before and after the characteristic value of electrode potential. We can see such phenomena in thermodynamic phase transitions, e.g., from solid to liquid, from ferromagnetism to paramagnetism, and vice versa.3 All these phenomena are characterized by certain values... 

It is not appropriate here to consider the mechanism of passivation (see Section 1.5), but it is apparent from Fig. 1.33 that the transition from the active to the passive state must be associated with a fundamental change in the nature of the metal surface, and it is now the generally accepted view that passivity is due to the formation of a very thin solid film of metal corrosion product, usually of oxide, on the metal surface. In this connection it should be noted that metal oxides are thermodynamically unstable in acid solutions so that the oxide formed during passivation must be regarded as a metastable form of the oxide that is stable at a higher pH (Fig. 1.18). It follows that the... 

Before discussing the kinetics of crack-healing, we describe thermodynamically the experimental conditions associated with the temperature and partial oxygen pressure. Generally SiC oxidation includes active-to-passive transition, where passive oxidation is given by Eq. (1) and active oxidation is given by... 

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