Thermodynamic Theory of “Irregular” Solutions and Kinetic Model of the Passive Alloy Dissolution

Abstract

Developing a productive multivariate theory of corrosion, as well as anticorrosion alloying, is impossible unless the interactions between the alloy components are taken into account. These interactions may most substantially affect the qualitative regularities of the dissolution in passive state [1][2][3].The quantitative theoretical description of the dissolution kinetics is based on treating the passivating oxide as a solid solution of "component molecules", an interaction between which should necessarily be taken into account at least in the close-neighbors approximation. Considering thermodynamic "entities" as oxide "molecules" of the alloy components rather than metal cations in the oxide was based in [2] on the concept of the passive metal dissolution [4][5][6][7][8][9] as the transfer of metal cations to the electrolyte together with the stoichiometrically balanced amount of the oxide oxygen atoms. In other words, it is as if M particles ( z i is the charge of the i th alloy component ion) are actually transferred. Bearing in mind this idea, we proposed [1] a thermodynamic model of the interaction between the oxide "molecules," based on the theory of regular solutions, that is, assuming that the interaction between the solution components does not affect its structure ordering, so that the location of component molecules is chaotic as in an ideal solution where interactions between molecules of different kinds are the same or simply absent.Let us note that in a conventional classification of solutions, people use an ambiguous terminology. It may seem strange that a solution with a chaotic arrangement of molecules is named "regular," while a O Z i /2 partially structured (not chaotic) one is named irregular. However, the term "regular" primarily means customary, which reflects the historically conventional idea that a solution is similar to a gas mixture, in which a "uniformly" chaotic arrangement of molecules is typical, while an "irregular" solution is unusually ordered. ADVANTAGES AND DRAWBACKS OF THE MODEL [2, 3] OF PASSIVE OXIDE FILM AS A REGULAR SOLUTION OF THE ALLOY COMPONENT OXIDE "MOLECULES"In [2], we used a thermodynamic model [1] as a basis in constructing a model of the passive alloy dissolution.For that purpose, we utilized the ideas of the transition state theory [10], which enables one to relate the chemical potential of each solution component to its value in the individual state (that is, oxide "molecules" in the own phase). (For brevity, solution means solid solution, because electrolytes lie beyond the scope of this study.) This made it possible to take into account the change in the dissolution rate of each component due to mixing, i.e., as a result of the appearance of "molecules" of other kinds close to the "molecule" of a particular kind.Model [2] enabled us to estimate partial i i and summary i dissolution rates of the alloy as functions of its composition knowing the dissolution rates of the individual passive metals constituting the alloy. For partial dissolution rates of a binary...