Kinetic Stability Diagrams
Digby D. Macdonald Center for Electrochemical experience and Technology Department of Materials Science and Engineering Pennsylvania convey University 201 Steidle Bldg University Park, PA 16802 For more than forty years, the conditions for the existence of the motionless soil have been described in terms of symmetry thermodynamics in the form of Pourbaix diagrams. These diagrams plot potential versus pH relationships for various equilibrium processes (e.g., Fe/Fe3O4, Fe/Fe2+, Fe3O4/Fe2+ to define regions of stability or predominance. The ploughshargon that Pourbaix made to electrochemistry and eroding science in developing potential-pH diagrams is sincerely recognized as one of the most important in the annals of these subjects. However, Pourbaix diagrams provide an equilibrium view of passivity, whereas passive films are non-equilibrium structures, whose existence depends upon an appropriate relationship between the rate of governing body and the rate of destruction. Recently, the author has developed, using Phase Space outline (PSA), a kinetic description of passivity that overcomes many of the limitations of the equilibrium Pourbaix diagrams.
The principal limitation is the visition of large stability field for ionic species at low to moderate pH, predicting significant corrosion for many metals and alloys that exhibit high stability in charge highly acidic systems. For example, iron is known to be passive in concentrated nitric acid, but not in dilute nitric acid, and chromium containing alloys (e.g., stainless nerves) are effect to be passive to very low pH, provided that the potential is sufficiently positive. This, of course, corresponds to anodic protection, which is commonly used for protecting stainless steel tanks for storing concentrated sulfuric acid. The Pourbaix diagrams do not directly predict these relationships, unless they are extended to include metastable phases. The stability of...If you compulsion to get a full essay, order it on our website: Orderessay
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