Supercritical Fluid Science and Technology - American Chemical

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Chapter 18

Thermodynamic Analysis of Corrosion of Iron Alloys in Supercritical Water 1

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Shaoping Huang , KirkDaehling ,Thomas E.Carleson ,PatTaylor ,Chien Wai , and Alan Propp 1

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Downloaded by UNIV OF PITTSBURGH on March 12, 2016 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch018

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University of Idaho, Moscow, ID 83843 EG&G, Idaho, Inc., Idaho Falls, ID 83415

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe O ) is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 2

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High temperature and high pressure processing o f materials often involves the use of s u p e r c r i t i c a l f l u i d s . Corrosion studies are quite e s s e n t i a l for evaluation of the equipment i n s u p e r c r i t i c a l f l u i d operations. Previous electrochemical measurements for a l l o y s i n s u p e r c r i t i c a l f l u i d s are rare (1-3). The reported measurements (3) show that passivation of i r o n alloys i s d i f f e r e n t at s u p e r c r i t i c a l conditions compared to ambient conditions. The study of the electrochemistry o f iron alloys can lead to control of corrosion of equipment u t i l i z i n g the a l l o y s . Thermodynamic analysis provides the information about stable species, i . e . corrosion products under given temperatures and pressures. Thermodynamic property data of chemical species at high temperatures and high pressures are rare. Fortunately, extrapolation of thermodynamic properties into elevated conditions provides a way to conduct the thermodynamic analysis semiq u a n t i t a t i v e l y or q u a l i t a t i v e l y . A d e t a i l e d review of the extrapolation methods i s a v a i l a b l e (4). The pH-potential diagram or Pourbaix diagram, the graphical presentation o f a stable species within a pH-potential region, i s a valuable tool for analyzing the electrochemical e q u i l i b r i a i n aqueous solutions. With a diagram, the reaction products can be determined under given conditions. I f a redox reaction product of a 0097^156/89/Ό406-0276$06.00Λ) o 1989 American Chemical Society

Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Downloaded by UNIV OF PITTSBURGH on March 12, 2016 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch018

18.

277 Thermodynamic Analysis of iron Alloy Corrosion

HUANG ET AL.

metal i s a soluble i o n i c species, i t s s o l i d oxide w i l l not be expected as a stable species, therefore p a s s i v a t i o n o f t h i s metal i n such conditions i s impossible. The diagrams f o r various metals and t h e i r oxides a t room temperature are a v a i l a b l e (j>). The equilibrium c a l c u l a t i o n and the graphical presentation has been developed into a FORTRAN IV (G l e v e l ) 360/91 IBM computer program (6). The program i s capable o f c a l c u l a t i n g and p l o t t i n g pH-potential diagrams f o r systems consisting o f two elements, one metal and one nonmetal, i n the presence o f water a t 25°G and a t s p e c i f i c input a c t i v i t i e s o f the species. The diagram i s i n the form o f an array o f symbols representing the f i e l d s o f stable species. For thermodynamic analysis o f the corrosion o f i r o n a l l o y s i n s u p e r c r i t i c a l water, the above computer program was modified based upon standard thermodynamic property extrapolation methods. THEORETICAL AND EMPIRICAL APPROACH General P r i n c i p l e s . For a given chemical r e a c t i o n a t equilibrium, a stoichiometric balance i s : n M ( I ) + η Μ ( Ι Ϊ ) + n ^ 4- t^H* + n ^ O + n L - 0 x

χχ

(1)

L

where: L represents the nonmetal species with a charge 1; M(I) and M(II) represent species containing the given metal element M i n d i f f e r e n t oxidation states, I and I I ; and , b, ...h are the stoichiometric c o e f f i c i e n t s o f the corresponding species. I f nitrogen i s assumed to be an i n e r t gas, t h i s equation describes most systems consisting o f a i r and water. At constant temperature and pressure, the equilibrium condition i s , a

n

AG - AG* + RT l n ( a i

n

a

x

n

n

f

^

a ^ ) - 0

(2)

where, R i s the i d e a l gas constant, Τ i s the absolute temperature, f i s the fugacity (approximately equal to the p a r t i a l pressure) of oxygen above the solution, a± i s the a c t i v i t y o f species considered, and AG* i s the standard Gibbs free energy change f o r the reaction, which i s given by the following equation, 0

AG* - n ^ * !

+ η /ι· χ 1

χ χ

+ n^o + η μ

β

Η

Η

+ τ^μ\ + n M L

e

(3)

L

where μ*ι i s the standard chemical p o t e n t i a l o f corresponding species a t the given temperature and pressure. The method f o r extrapolation o f chemical p o t e n t i a l s to elevated temperatures and pressures i s based on the Gibbs-Duhem equation. The temperature and pressure dependency o f the chemical p o t e n t i a l of a species i can be expressed as (2» pp.144), e

e

/H