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David C. Silverman |
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Generation of a diagram - Iron in water An internally consistent thermodynamic data base has to be available before the potential-pH diagram can be calculated and applied to practical corrosion prediction. One possible procedure for ensuring consistency might be summarized in this figure (172k).
In that paper Fe+2 and Fe+3 were used directly from published thermodynamic data
for those ions. Required thermodynamic values for other ions were estimated by
combining the tabulated value for the non-hydrolyzed iron species and the equilibrium
constant between it and the next hydrolyzed species in the sequence (e.g. between Fe+2
and FeOH+). Thermodynamic properties of each
of the subsequent hydrolyzed species of the same valence were calculated using the next
higher hydrolysis reaction. Hydrolysis constants are available in the literature.
Solids and oxides were used as tabulated in the literature. The following table
reproduces the free energy data as presented in the above publication.
Data such as those shown in the above table were run through THERMEXPERT to create the "Metal State Diagram" In addition, these diagrams have been generated after removing from consideration related species that are not at lowest free energy but which might appear on an initially calculated diagram because their formation from each other does not involve hydrogen ion or valence change. Such species are related only by addition or removal of water. The pH and potential are not variables in these reactions. In this case, examples are reactions among Fe2O3, FeOOH, Fe(OH) 3, and the dissolved Fe(OH)3(aq). These four compounds are related only through a reaction with water. All might initially appear on the same calculated diagram in some pH and potential region since they are related only through reaction with water and neither hydrogen ion or electron transfer (change in valence state) are not involved. In this case, the compound Fe2O3 was chosen over FeOOH even though the latter might have a slightly lower free energy of formation among reactions that include water and those other compounds. One might substitute one of the other compounds for consideration if the system being modeled requires it. These points are also discussed in the section Using THERMEXPERT - a step-by-step procedure. The potential-pH diagram which is most familiar to people is the overlay of these two diagrams. Such an overlay is shown in this figure As discussed in the publication D. C. Silverman, "Presence of Solid Fe(OH)2 in EMF-pH Diagram for Iron", Corrosion, Vol. 38, p. 453 (1982) 1 (172k),
this combined diagram
differs from that originally published in (M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE International,
Houston, TX, 1974). The most important differences
are a region of stability of solid Fe(OH)2 that lies between the iron and magnetite
regions of stability and a region of stability for Fe(OH)4- at high pH and somewhat
elevated potentials. Solid Fe(OH)2 would be expected to impart only imperfect
corrosion resistance. Indeed, as shown in that older reference, corrosion
of iron has been observed in this pH and potential region. Dissolved Fe(OH)4-
is a corrosion product suggesting that dissolved metal is the most stable state in
this high pH region. Corrosion in this pH region and somewhat elevated potential region has been
observed in strongly caustic processes. The diagram also shows the complete spectrum
of hydrolysis products.
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Previous Page: Background on the Potential/pH (Pourbaix) Diagram Next Page: Case Study-Titanium/chloride under acidic conditions 1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above. | ||||||||||||||||||||||||||||||||||||||
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David C. Silverman, Ph.D. - Primary Consultant |
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