Thermodynamics of High-Temperature Aqueous Systems - ACS

34 Thermodynamics of High-Temperature Aqueous Systems What the Electricity Generating Industry Needs to Know D. J. TURNER Downloaded by EAST CAROLINA UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch034

Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, England

In the conversion of fossil and nuclear energy to electricity, the value of high temperature solution phase thermodynamics in improving plant reliability has been far less obvious than that of classical thermodynamics in predicting Carnot cycle efficiency. Experimental studies under conditions appropriate to modern boiler plant are difficult and with little pressure from designers for such studies this area of thermodynamic study has been seriously neglected until the last decade or two. A recent editorial in the journal "Corrosion" (1) referred to the lack of thermodynamic data in high temperature water as "appalling". In the author's opinion this is no exaggeration, since it is unlikely that methods of treating boiler water or of predicting the long term corrosion behaviour of boiler plant will ever be much better than empirical until a much better under standing of the solution phase chemistry is available. In the present circumstances it is hardly surprising that laboratory corrosion tests have frequently provided an inadequate basis for designing more reliable steam generators (2). Water of v a r i o u s degrees of p u r i t y i s the normal heat t r a n s f e r f l u i d employed and a number of important problems w i t h modern b o i l e r water c i r c u i t s are markedly i n f l u e n c e d by s o l u t i o n composition. Most problems a r i s e where s o l u t i o n s can concentrate and the compositions of such s o l u t i o n s can only be obtained by c a l c u l a t i o n from thermodynamic data. This paper concentrates on the k i n d of aqueous phase data which are c u r r e n t l y most needed. Many of the needs overlap w i t h those of geochemical i n t e r e s t . However, since Barnes (3) has r e c e n t l y reviewed the l a t t e r f i e l d , s p e c i f i c a l l y geochemical needs w i l l not be discussed. "High temperature" i n t h i s paper i s g e n e r a l l y taken to mean w i t h i n about 100°C of the c r i t i c a l p o i n t of water (374 C ) , though some important problems which occur at lower temperatures are a l s o considered. The solvent p r o p e r t i e s of water change c o n s i d e r a b l y between 25 C and the c r i t i c a l p o i n t w i t h the r e s u l t that q u a l i t a t i v e conclusions based on room temperature experience can be t o t a l l y 0-8412-0569-8/80/47-133-653$06.75/0 © 1980 American Chemical Society

In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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Downloaded by EAST CAROLINA UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch034

m i s l e a d i n g when a p p l i e d to b o i l e r c o n d i t i o n s . Though there are at present too few r e l i a b l e h i g h temperature data to a l l o w many q u a n t i t a t i v e p r e d i c t i o n s , current knowledge i s s u f f i c i e n t to a l l o w q u i t e important q u a l i t a t i v e conclusions to be drawn. An attempt i s made here to d e f i n e the main types of data needed. A v a i l a b l e data and e s t i m a t i o n procedures are considered as are some important experimental and more fundamental problems which complicate c e r t a i n types of study. F i n a l l y , some examples are given of attempts to apply thermodynamic arguments to a v a r i e t y of power s t a t i o n problems. F i r s t , however, an i n d i c a t i o n i s given of c i r c u i t c o n d i t i o n s and the more economically important problems i n which high temperature aqueous s o l u t i o n s p l a y a significant role. 1.Power S t a t i o n Water C i r c u i t s F i g . 1 represents a not q u i t e t y p i c a l h i g h pressure b o i l e r water c i r c u i t operating w i t h the water i n the drum at about 350 C and at a pressure of about 165 bar. Most modern p l a n t (except f o r water r e a c t o r s which operate nearer 300 C) employs roughly these c o n d i t i o n s though w i t h s u p e r c r i t i c a l u n i t s (pressure above the c r i t i c a l pressure) the f l u i d can be considered l i q u i d - l i k e w e l l above the c r i t i c a l temperature. Since there i s no phase change i n a s u p e r c r i t i c a l u n i t , no drum i s provided and the b o i l e r i s a "once-through" u n i t . S u b - c r i t i c a l once-through b o i l e r s are a l s o used ( f o r example i n Advanced Gas Reactors, AGR) and here the water i s simply allowed to evaporate to dryness i n the b o i l e r tubes. To d r i v e one 500 MW t u r b i n e t y p i c a l l y r e q u i r e s b o i l i n g 1.5 m i l l i o n kg of water an hour; the b o i l e r c a p a c i t y would be about 0.5 m i l l i o n kg. The heat r e q u i r e d can be s u p p l i e d by burning f o s s i l f u e l , by gas heated i n a r e a c t o r core (e.g. AGR and Magnox), by l i q u i d metal (e.g. f a s t breeder r e a c t o r s ) or by water heated i n a r e a c t o r ( P r e s s u r i z e d Water Reactor, PWR). The steam generator of a PWR i s constructed q u i t e d i f f e r e n t l y from the other types i n that the h e a t i n g f l u i d (primary c i r c u i t water) r a t h e r than the b o i l i n g f l u i d i s i n s i d e the t u b i n g . I n a B o i l i n g Water Reactor (BWR), the b o i l e r i t s e l f i s i n the r e a c t o r core w i t h the f u e l cans i n s i d e the b o i l e r tubes. I n a l l u n i t s except the water r e a c t o r s , the steam i s c o n s i d e r a b l y superheated before passing to the t u r b i n e and condenser which i t w i l l leave as l i q u i d water under a vacuum at t y p i c a l l y 30 C and 40 mbar. Few high pressure drum b o i l e r s are equipped w i t h the condensate p o l i s h i n g p l a n t i l l u s t r a t e d i n F i g . 1 but once-through b o i l e r s (having no drum to a l l o w the accumulation of i m p u r i t i e s i n the water phase) normally are. The water i s pumped from the condenser back through feed-heaters and i n t o the b o i l e r again. The m a t e r i a l s of c o n s t r u c t i o n vary c o n s i d e r a b l y w i t h d i f f e r e n t b o i l e r designs. Conventional drum b o i l e r s g e n e r a l l y use carbon s t e e l b o i l e r tubes and s t a i n l e s s s t e e l superheater tubes.



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Figure 1.

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water circuit for a high-pressure boiler (( ) bled steam lines; (— · —) cooling water)

) main

circuit;



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Nuclear s t a t i o n s tend to employ f e r r i t i c chrome s t e e l s , s t a i n l e s s s t e e l s and n i c k e l based a l l o y s i n t h e i r b o i l e r s . Turbines contain a v a r i e t y of s t e e l s w h i l e condensers are u s u a l l y of brass o r , i n c r e a s i n g l y , of t i t a n i u m . Low pressure feed heaters have t r a d i t i o n a l l y been made of brass a l s o , but i n c r e a s i n g l y s t e e l s are used. Chemical treatment i s p r i m a r i l y aimed a t minimizing the c o r r o s i o n r a t e of the c i r c u i t c o n s t r u c t i o n m a t e r i a l s . The exact methods chosen w i l l depend on the design of the c i r c u i t as w e l l as on the nature of these m a t e r i a l s . The usual approach i s to maint a i n low oxygen l e v e l s and keep the water s l i g h t l y a l k a l i n e . I n a drum b o i l e r a v a r i e t y of s o l i d a l k a l i s can be used but the most common ones are d i l u t e c a u s t i c soda or mixtures of Na3P04 and Na2HP04« With once-through b o i l e r s ammonia i s normally used i n s t e a d and i t i s sometimes a l s o used i n drum b o i l e r s and PWR secondary c i r c u i t s . Hydrazine i s f r e q u e n t l y dosed i n t o the feed t r a i n t o c o n t r o l O2 l e v e l s . However, an a l t e r n a t i v e to the low oxygen a l k a l i n e s o l u t i o n regime i s to r e l y on the p r o t e c t i v e haematite f i l m which i s formed i n very high p u r i t y water i n the presence of c o n t r o l l e d q u a n t i t i e s of oxygen. This approach i s favoured i n some once-through p l a n t ( p a r t i c u l a r l y i n Germany) f o l l o w i n g the work of F r e i e r (4,5) and a s i m i l a r chemistry i s used i n most BWR p l a n t where the continuous r a d i o l y t i c production of oxygen makes i t i m p r a c t i c a l t o maintain low oxygen l e v e l s . A somewhat more d e t a i l e d summary of methods of b o i l e r - w a t e r t r e a t ment, i n c l u d i n g the a p p l i c a t i o n of c h e l a t i n g agents on-load, has been given r e c e n t l y (6). The most serious current problems where knowledge of s o l u t i o n compositions are required are of three main types: b o i l e r i n t e g r i t y , t u r b i n e i n t e g r i t y and out-of-core r a d i o a c t i v i t y . The f i n a n c i a l costs of the f i r s t two problems a r i s e mainly when f a i l u r e s i n modern, e f f i c i e n t plant r e q u i r e t h e i r replacement by p l a n t which i s considerably more expensive t o run. The t h i r d problem manifests i t s e l f i n the need t o share any e x t e n s i v e maintenance work i n h i g h l y r a d i o a c t i v e areas among hundreds of men so that none exceeds h i s permitted r a d i a t i o n dose. P l a t e 1 i l l u s t r a t e s what can be the consequences of a s t r e s s c o r r o s i o n f a i l u r e i n a t u r b i n e and P l a t e 2 the consequences of two forms of b o i l e r tube c o r r o s i o n : tube t h i n n i n g and hydrogen embrittlement. The economic consequences of such problems and c e r t a i n other areas where information on high temperature s o l u t i o n s i s needed have been discussed elsewhere (6). I l l u s t r a t i o n s of what can and cannot be done (on the b a s i s of c u r r e n t l y a v a i l a b l e thermodynamic data) towards understanding and s o l v i n g a v a r i e t y of water c i r c u i t problems are b r i e f l y discussed i n Section 3. 2.Thermodynamic Considerations Types of E q u i l i b r i a . I n a l l the problems discussed i t i s the



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Plaie 2.

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f r e e energy p r o p e r t i e s which are of concern, though e n t h a l p i e s , entropies and (molar) volumes may, of course, be used t o o b t a i n them. With s t r o n g e l e c t r o l y t e s , concern i s p r i m a r i l y w i t h how f r e e energies change w i t h c o n c e n t r a t i o n and i t w i l l be seen that knowledge i n t h i s f i e l d i s f a r more advanced and i n most respects l e s s troublesome than w i t h standard f r e e energies of r e a c t i o n where much l a r g e r u n c e r t a i n t i e s can be introduced. A c c o r d i n g l y most a t t e n t i o n w i l l be devoted t o f r e e energies and e q u i l i b r i u m constants. C l e a r l y h y d r o l y s i s constants, s o l u b i l i t i e s , steamwater p a r t i t i o n c o e f f i c i e n t s and c h e l a t i o n e q u i l i b r i a are i n v o l v e d i n the problems described as are redox e q u i l i b r i a and the formation of o r d i n a r y (monodentate) complexes and i o n p a i r s . D i f f i c u l t i e s at High Temperature. A number of d i f f i c u l t i e s combine t o e x p l a i n the s i n g u l a r l a c k of c e r t a i n kinds of thermo dynamic data i n high temperature s o l u t i o n . E x p e r i m e n t a l l y these s t a r t w i t h the need to c o n t a i n such a good s o l v e n t and c o r r o s i v e l i q u i d as water simultaneously at high temperature and pressure. Few metals ( f o r containment), i n s u l a t i n g m a t e r i a l s ( f o r e l e c t r o - c h e m i c a l measurements) o r windows ( f o r o p t i c a l measurements) are i n e r t i n more than a small range of s o l u t i o n s . Furthermore many r e a c t i o n s which can be ignored f o r k i n e t i c reasons at 25°C are l i k e l y to proceed f a s t at 300 C. Thus i t i s d o u b t f u l whether one could study the chemistry of F e l l l i n the presence of hydrogen or of F e l l u s i n g a standard p e r c h l o r a t e supporting e l e c t r o l y t e . Nevertheless, f o l l o w i n g the p i o n e e r i n g work of P r o f . E.U. Franck, experimental data of v a r i o u s kinds have been obtained a t temperatures and pressures c o n s i d e r a b l y higher than those appropriate to b o i l e r water. Without t h i s lead i t i s doubt f u l whether more than a f r a c t i o n of the data now a v a i l a b l e would have been obtained. Systems i n v o l v i n g aqueous t r a n s i t i o n metal ions are i n e v i t a b l y somewhat complicated because of t h e i r r e l a t i v e l y high charge, t h e i r tendency t o form c o o r d i n a t i o n complexes and t h e i r tendency to e x i s t i n more than one o x i d a t i o n s t a t e . At high temperatures, the d i e l e c t r i c constant of water i s very much lower than at 25 C so that the a b i l i t y of water to s t a b i l i z e h i g h l y charged species i s g r e a t l y reduced and r e a c t i o n s which lead t o a r e d u c t i o n i n t o t a l charge are correspondingly favoured. These i n c l u d e i o n a s s o c i a t i o n , complex formation w i t h anions, h y d r o l y s i s and r e d u c t i o n . This e f f e c t makes i t d i f f i c u l t or impossible t o o b t a i n meaningful r e a c t i o n f r e e energies from techniques which r e q u i r e pH b u f f e r s or supporting e l e c t r o l y t e s . A s l i g h t l y d i f f e r e n t aspect of the same problem makes i t e x c e p t i o n a l l y d i f f i c u l t t o estimate c e r t a i n types of e q u i l i b r i u m constant at high temperatures from data at low temperatures. The w e l l known r e l a t i o n s h i p s between e q u i l i b r i u m constant, K, AG , ΔΗ and AS may conveniently be w r i t t e n


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AS° +

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which, i f ΔΗ° and AS° are reasonably independent of temperature p r e d i c t s a l i n e a r p l o t of l n Κ v s . 1/T. With gases and s o l i d s , ACp i s u s u a l l y r e l a t i v e l y s m a l l and such p l o t s approximate w e l l to s t r a i g h t l i n e s over wide temperature ranges. Even when there i s some c u r v a t u r e , ACp can o f t e n be estimated w i t h i n reasonably w e l l defined l i m i t s . Log Κ f o r any endothermic r e a c t i o n ( p o s i t i v e AH ) which produces ions w i l l i n e v i t a b l y go through a maximum as a f u n c t i o n of r e c i p r o c a l temperature because, above some temperature, the decreasing s o l v a t i n g power of water must make the r e a c t i o n exothermic. The i o n i c d i s s o c i a t i o n of water i s an example as seen i n F i g . 2. An a l t e r n a t i v e way of saying t h i s , of course, i s that the r e a c t i o n has a large n e g a t i v e AC2. E s t i m a t i o n i s d i f f i c u l t because, even where i t i s known at 25°C, there i s no reason t o ο —ο b e l i e v e that AC i s independent of temperature. Cp f o r NaCl decreases from -92 J K' mol"* a t 25°C t o -836 J Κ m o l " a t 300 C ( 7 ) , and C r i s s (8) has suggested why such large negative values could be expected at h i g h temperature. An even more serious problem can a r i s e when d i s s o l v e d species expected to predominate at h i g h temperatures are undetectable at 25°C or are only present a t c o n c e n t r a t i o n s which are too low f o r them to be adequately c h a r a c t e r i z e d thermodynamically. Examples are c e r t a i n t r a n s i t i o n metal chloro-complexes (9,10) and mixed complexes of such metals w i t h hydroxide and another l i g a n d (11,12). Thus i t seems that c h l o r i d e complexing so a l t e r s the aqueous chemistry of copper and g o l d that supposedly i n e r t gold components i n autoclaves are r e v e r s i b l y o x i d i z e d by C u l l (10) and i t i s l i k e l y that mixed oxine and hydroxy complexes of F e l l c o n t r i b u t e c o n s i d e r a b l y t o the gross under-estimation (by a f a c t o r of up t o 10^) of magnetite s o l u b i l i t y i n oxine (12,14). These s o r t of problems make i t d i f f i c u l t t o o b t a i n r e l i a b l e high temperature data on the aqueous chemistry of t r a n s i t i o n metal i o n s . U n f o r t u n a t e l y the necessary t i m e s c a l e s f o r even the simpler experimental s t u d i e s are f r e q u e n t l y too long f o r a Ph.D. student to make reasonable progress i n 3 years from s c r a t c h or f o r i n d u s t r i a l researchers to make much r e p o r t a b l e progress before the patience of those s u p p o r t i n g the work i s exhausted. R e s u l t s can be r e p o r t e d f a r more r a p i d l y from, f o r example, c o r r o s i o n experiments and s i n c e c o r r o s i o n t h e o r i e s are i n general of so l i t t l e p r e d i c t i v e v a l u e , each r e l e v a n t a l l o y / e l e c t r o l y t e com b i n a t i o n needs i t s own study. I n such circumstances i t i s h a r d l y s u r p r i s i n g that thermodynamic s t u d i e s have been (with a few notable exceptions) r e l a t i v e l y p o o r l y supported, w h i l e c o r r o s i o n data con t i n u e t o be amassed without any r e l i a b l e thermodynamic framework w i t h i n which to understand them. p

1

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r e s u l t s up to 1973 were presented at a recent conference by most of the major c o n t r i b u t o r s t o the f i e l d (15-20). That of Franck (15) covers v a r i o u s p r o p e r t i e s of water and s o l u t i o n s up to 1000 C and 100 kbar. The s e l f - d i s s o c i a t i o n constant of water, Ky i s reported over t h i s range but due to the large c o m p r e s s i b i l i t y of water near the c r i t i c a l p o i n t , i t i s not easy to e x t r a p o l a t e Ky to the lower d e n s i t i e s a p p r o p r i a t e to SVP ( s a t u r a t e d vapour pressure) c o n d i t i o n s above 300 C. M a r s h a l l ' s e x t e n s i v e review (16) concentrates mainly on conductance and s o l u b i l i t y s t u d i e s of simple ( n o n - t r a n s i t i o n metal) e l e c t r o l y t e s and the a p p l i c a t i o n of extended Debye-Huckel equations i n d e s c r i b i n g the i o n i c s t r e n g t h dependence of e q u i l i brium constants. The conductance s t u d i e s covered c o n d i t i o n s t o 4 kbar and 800°C w h i l e the s o l u b i l i t y s t u d i e s were mostly at SVP up to 350 C. In the l a t t e r s t u d i e s above 300°C d e v i a t i o n s from Debye-Huckel behaviour were found. This i s not s u r p r i s i n g s i n c e the Debye-Huckel theory t r e a t s the s o l v e n t as incompressible and, as seen i n F i g . 3, water r a p i d l y becomes more compressible above 300 C. U n t i l a theory which accounts f o r électrostriction i n a compressible f l u i d becomes a v a i l a b l e , e x t r a p o l a t i o n to i n f i n i t e d i l u t i o n at temperatures much above 300 C must be considered untrustworthy. Since water becomes i n f i n i t e l y compressible at the c r i t i c a l p o i n t , the standard entropy of an i o n becomes i n f i n i t e l y n e g a t i v e , so that the concept of a standard i o n i c f r e e energy becomes meaningless. The work described by M a r s h a l l (16) , together w i t h the vapour pressure s t u d i e s on 1:1 and 1:2 e l e c t r o l y t e s up to 300 C reported by Lindsay and L i u (17) and recent t h e o r e t i c a l work by S i l v e s t e r and P i t z e r (21) and by Helgeson and Kirkham (22) provide a good understanding of the behaviour of simple e l e c t r o l y t e s over wide ranges of temperature and c o n c e n t r a t i o n . However, as j u s t seen, the behaviour under SVP c o n d i t i o n s above 300 C becomes d e c r e a s i n g l y w e l l d e f i n e d towards the c r i t i c a l p o i n t . The review of Martynova (18) covers s o l u b i l i t i e s of a v a r i e t y of s a l t s and oxides up to 10 kbar and 700 C and a l s o a v a i l a b l e steam-water d i s t r i b u t i o n c o e f f i c i e n t s . That of L i e t z k e (19) reviews measurements of standard e l e c t r o d e p o t e n t i a l s and i o n i c a c t i v i t y c o e f f i c i e n t s u s i n g Harned c e l l s up to 175-200 C The review of Mesmer, Sweeton, H i t c h and Baes (20) covers a range of p r o t o l y t i c d i s s o c i a t i o n r e a c t i o n s up to 3 0 0 ^ at SVP. Apart from the work on Fe304 s o l u b i l i t y by Sweeton and Baes (23) , the only references to h y d r o l y s i s and complexing r e a c t i o n s by t r a n s i t i o n metals above 100 C were to aluminium h y d r o l y s i s (20) and n i c k e l h y d r o l y s i s (24) both to 150 C N i k o l a e v a (24) was one of s e v e r a l at the conference who discussed the problems a r i s i n g when h y d r o l y s i s and complexing occur simultaneously. There appear t o be no experimental s t u d i e s of s o l u t i o n phase redox e q u i l i b r i a above 100°C. In view of the f i n d i n g s of Lindsay and L i u (17) that at 300 C MgCl2 behaves l i k e a 1:1 e l e c t r o l y t e , M g C l + C I " , i t seems +



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Figure 2. Temperature dependence of molal ion product of water: (a) Fisher and Barnes (36); (b) Sweeton et al. (35); (c) Sirota and Shviraev (31); (d) Correspond ence Principle estimate of Lewis (9\) ρ 1.0

1

I

βχ10 1 bar"

ε

1800

0.9 - 9 0 0.8

>^

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0.7 - 7 ( \

1400

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I

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ι 50

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ι ι ι ι ι I 0 100 150 200 250 300 350 4 0 0 TEMR C e

Figure 3.

Density, dielectric constant, and compressibility of water


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663

High-Temperature Aqueous Systems +

u n l i k e l y that the r e a d i l y p o l a r i z e d F e ^ i o n remains uncomplexed at 300 C i n CI - c o n t a i n i n g media. Sweeton and Baes (23) i n t e r preted the s o l u b i l i t y of Fe3Û4 i n HCI without i n v o k i n g C l ~ complexing. Thus i t seems u n l i k e l y that r e l i a b l e thermodynamic data on the aqueous F e ^ i o n can be obtained from t h e i r data at 300 C without independent evidence concerning the extent of C l ~ complexing. The i n t e r p r e t a t i o n w i t h regard to a n i o n i c species seems to be unambiguous, however. E x a c t l y the same problem a r i s e s w i t h the recent s t u d i e s of NiO s o l u b i l i t y by Tremaine and Leblanc (25) and again the thermodynamic data on the aqueous a n i o n i c species at 300 C are l i k e l y to be more r e l i a b l e than on the N i ^ i o n . There i s good s p e c t r o scopic evidence f o r complex formation i n c h l o r i d e s of n i c k e l ( I I ) , (26) c o b a l t ( I I ) (27), and copper ( I I ) (28)_at 300°C and above. Most of the work was done at r a t h e r high CI concentrations but q u a l i t a t i v e l y the e f f e c t s of d i e l e c t r i c constant and c o n c e n t r a t i o n are as expected. A noteworthy f e a t u r e (which e s t i m a t i o n procedures w i l l have to a l l o w f o r ) i s the change from 6 to 4 c o o r d i n a t i o n at the lower pressures (150-300 bar) and the higher CI c o n c e n t r a t i o n s . This change appears to take place w i t h only 2 or 3 CI ions coordinated to the metal (at l e a s t i n the case of Ni(II)). Since 1973, progress has been made i n a l l the f i e l d s reviewed and a more complete review of F r a n c k s work has appeared (29). For the present purposes i t i s necessary to concentrate on the two areas which are l e a s t w e l l s t u d i e d : simple e l e c t r o l y t e s between 300 C and the c r i t i c a l p o i n t and the chemistry of aqueous t r a n s i t i o n metal c a t i o n s . A number of s t u d i e s which do not f a l l i n t o these c a t e g o r i e s must, however, be mentioned because of t h e i r d i r e c t relevance to b o i l e r water chemistry. These are s t u d i e s of sodium phosphate phase e q u i l i b r i a (30), ammonia d i s s o c i a t i o n (31) and i s o p i e s t i c s t u d i e s of c a l c i u m and magnesium c h l o r i d e s (32). Two s t u d i e s of the s e l f - d i s s o c i a t i o n of D~0 have a l s o appeared (33,34). The Ky r e s u l t s of Sweeton, Mesmer and Baes (35) p l o t t e d i n F i g . 2 were reported i n 1974 and although they only extend to 300 C they may w e l l be more accurate above t h i s temperature than the experimental r e s u l t s of F i s h e r and Barnes(36), s i n c e , as mentioned, e a r l i e r , the Debye-Huckel theory may not give r e l i a b l e e x t r a p o l a t i o n s to i n f i n i t e d i l u t i o n at temperatures where water i s h i g h l y compressible. While t h e i r work (35) i n v o l v e s e x t r a p o l a t i o n to i n f i n i t e d i l u t i o n as w e l l as to higher temperatures i t i s very encouraging to note that t h e i r ACp at 300°C (-960 J K" mol" ) i s of the magnitude expected on the b a s i s of the NaCl s t u d i e s r e f e r r e d to i n S e c t i o n 2. The conductance r e s u l t s of S i r o t a and S h v i r i a e v (37) above 300 C a l s o seem more c o n s i s t e n t w i t h the r e s u l t s of Sweeton, Mesmer and Baes (35), than w i t h those of F i s h e r and Barnes (36). M a r s h a l l and F r a n c k s recent r e p r e s e n t a t i o n of data up to 1000°C and 10,000 bars (38) p r e d i c t s high temperature SVP r e s u l t s somewhat lower than those of S i r o t a and S h v i r i a e v (37). +


+

1

1

1

1


664

THERMODYNAMICS

OF


APPLICATIONS

M a r s h a l l has extended h i s high temperature s o l u b i l i t y s t u d i e s (39,40,41) and has begun some work on l i q u i d - v a p o u r c r i t i c a l temperatures of s o l u t i o n s (42,43) which should prove v a l u a b l e . Some of M a r s h a l l ' s higher temperature r e s u l t s (>300 C) have been questioned (44) and there do seem to be unexplained d i f f e r e n c e s between s t u d i e s i n s t a i n l e s s s t e e l and t i t a n i u m v e s s e l s (45). The problem of measuring the thermodynamic p r o p e r t i e s of aqueous t r a n s i t i o n metal ions above 100 C has a l s o received_some a t t e n t i o n w i t h s t u d i e s on ¥e* complexing w i t h CI (46) , Br (47) and SO^" (48) up to 150°C and the formation of a n i o n i c hydroxy complexes of P b up to 300°C (49). P r e l i m i n a r y heats of s o l u t i o n of C0CI2 and CuCl2 have been measured up to 300 C by Cobble and Murray (50). H y d r o l y s i s was suppressed by HCI a d d i t i o n so that when the work i s completed and when the extent of CI complexing (and C u r e d u c t i o n ) can be allowed f o r the data w i l l prove extremely v a l u a b l e . P r e l i m i n a r y c o n c e n t r a t i o n c e l l s t u d i e s on the CI complexing of C d and N i ^ up to 170 C (51) support the conclusions given e a r l i e r that such complexing w i t h f i r s t row t r a n s i t i o n metal ions i s l i k e l y to be s i g n i f i c a n t by 300 C. The s o l u b i l i t y of Ye^O^ has been studied i n the temperature range 500-600°C by Chou and Eugster (52) using the HCI f u g a c i t y b u f f e r developed e a r l i e r (53). Under the c o n d i t i o n s used both HCI and the s o l u b l e i r o n species FeCl2 are completely a s s o c i a t e d . C l e a r l y the d e r i v e d thermodynamic data are a l s o of p o t e n t i a l value but more work on Cl~-complexing i s needed before they can throw l i g h t on the aqueous chemistry of F e under h i g h pressure b o i l e r conditions. Many of the 25 C o x i d a t i o n p o t e n t i a l estimates of Latimer (54) were obtained simply from a knowledge of what r e a c t i o n s proceed and what do not. Hence p r e p a r a t i v e and decomposition experiments i n simple autoclaves are a l s o of considerable v a l u e provided that f u l l experimental d e t a i l s are p u b l i s h e d . Swaddle's group has performed a number of such studies on the t r a n s i t i o n metals from which b o i l e r water c i r c u i t s are made (55,56,57) and a l s o on species of more d i r e c t relevance to l a b o r a t o r y s t u d i e s (58,59,60). Quite t r i v i a l unexpected observations i n autoclave s t u d i e s can be used to place l i m i t s on e q u i l i b r i u m constants. In complex systems, unique i n t e r p r e t a t i o n s w i l l u s u a l l y be impossible but the observations may s t i l l prove u s e f u l i f they can be supplemented by estimated data (H), 61). +


2 +

2 +

2 +

+

2 +

E s t i m a t i o n Procedures. There are b a s i c a l l y two ways which have been developed to d e a l w i t h the f a c t that heat c a p a c i t y terms are large i n r e a c t i o n s i n v o l v i n g i o n s . One i s based on e m p i r i c a l r e l a t i o n s h i p s (the entropy correspondence p r i n c i p l e ) between i o n i c entropies at d i f f e r e n t temperatures which C r i s s and Cobble (62) developed and checked to 200°C. Lewis (63) has checked a number of i t s p r e d i c t i o n s against a v a i l a b l e experimental evidence and has found the method reasonably s a t i s f a c t o r y f o r s e v e r a l


34.

TURNER

665


systems up to 250°C. He (64) and o t h e r s , f o r example Macdonald and colleagues (65,66) have used the method to estimate e q u i l i b r i u m constants to 300°C and above. I t cannot be claimed, however, that the method has been ο · · e x t e n s i v e l y checked above 250 C and i t appears that i t i n e v i t a b l y must become u n r e l i a b l e over 300 C. The average heat c a p a c i t i e s of ions c a l c u l a t e d by C r i s s and Cobble (67) show no s i g n that C values of ions are becoming i n c r e a s i n g l y negative w i t h temperature, although, as was seen i n S e c t i o n 2, the e f f e c t i s becoming considerable by 300 C. Nevertheless many e q u i l i b r i u m constants could probably be obtained to w i t h i n an order of magnitude a t 300 C i f a r e l i a b l e estimate could be made of the thermodynamic p r o p e r t i e s of any uncharged species or i o n p a i r s . U n f o r t u n a t e l y , there i s , as y e t , no r e l i a b l e method of c h a r a c t e r i s i n g such species i f (as w i l l f r e q u e n t l y be the case) they are only s t a b l e at high temperatures. With the s e l f - d i s s o c i a t i o n of water t h i s i s not, of course, a problem and, as can be seen i n F i g . 2, f o r Ky the entropy correspondence method only begins to manifest i t s underestimation of the magnitude of AC above 300 C. 2_ I t i s not a problem e i t h e r f o r the p r o t o n a t i o n constant of S ( i . e . the r e c i p r o c a l of the second d i s s o c i a t i o n constant of H2S) some estimates of which are shown i n F i g . 4. N e i t h e r Cobble's estimate (68), u s i n g the correspondence p r i n c i p l e (curve a) nor Pohl's (69) e x t r a p o l a t i o n (curve b) u s i n g an e m p i r i c a l equation due to Harned and Embree (70) i s showing any i n d i c a t i o n of the expected minimum i n K. The e x t r a p o l a t i o n used by Khodakovskii et a l (71) (curve c) i s based on the more f r e q u e n t l y used expression of Harned and Robinson (72) and a d i f f e r e n t s e l e c t i o n of low temperature data. While t h e i r r e s u l t looks more reasonable i t i s d i f f i c u l t to have much confidence i n any of the r e s u l t s even up to 200 C. The apparent f a i l u r e of the correspon dence p r i n c i p l e may a r i s e as much from the choice of low temperature data as a f a i l u r e of the r e l a t i o n s h i p i t s e l f . The disagreement between the c a l c u l a t e d standard f r e e energies of formation of aqueous F e and those deduced by Sweeton and Baes (23) has been commented on by the author (9) and by Tremaine, Van_Massow and Shierman (73). I n view of the problem at 300 C w i t h CI -complexing (discussed e a r l i e r ) i t seems u n l i k e l y to the author that the thermodynamics of d i s s o l u t i o n of magnetite i n a c i d s o l u t i o n are q u i t e as w e l l c h a r a c t e r i z e d as i s suggested by the c a l c u l a t i o n s of Tremaine et a l (73). The second method of e s t i m a t i o n which has so f a r been developed i s based on c o n s i d e r a t i o n of those AC values which were a v a i l a b l e i n 1967 when Helgeson developed i t ( 7 % ) . This method e s s e n t i a l l y separates e l e c t r o s t a t i c and n o n - e l e c t r o s t a t i c c o n t r i b u t i o n s to AS and. AC and Helgeson has compared the p r e d i c t i o n s of a number of d i f f e r e n t assumptions concerning AC° w i t h published high temperature e q u i l i b r i u m constants. He concluded that the best one to make i s that ACp i s p r o p o r t i o n a l , at each temperature, to the e l e c t r o s t a t i c c o n t r i b u t i o n , AC . This


p

p

2 +

p



APPLICATIONS


666


34.

TURNER

661


assumption, combined w i t h a simple e l e c t r o s t a t i c model, leads t o an e x p l i c i t r e l a t i o n s h i p between l o g Κ and temperature which i n c l u d e s i n i t the temperature dependence of the d i e l e c t r i c constant of water: A

ln Κ -

S

^

9

8

)

{298 " f (1 - exp [exp(b + aT) - C + (Τ - 298)/θ])}


AH°(298) RT

m

'··

U

;

Here C(= exp (b + 298a)), ω(= 1 + aC6), b and θ are constants i n an e x p r e s s i o n f o r the temperature dependence of ε, the d i e l e c t r i c constant of water: ε = ε exp Q- exp (b + aT) - Τ/θ]

... (2)

ο

Equation (1) was a s i m p l i f i c a t i o n d e r i v e d from the f o l l o w i n g more complete, but l e s s r e a d i l y usable equation: S°(298) ln Κ = — ^ {298 - Τ +^ (1 - exp Qexp(b + aT) -C + (Τ- 298)/θ])} _ ^!g98)_

+

A S ^

+

a

[

l

n

(

T

/

2

9

8

)

.

1

+

2

9

8

/

T

]

+3(T-g8)

2

...

(3)

The f i r s t term represents how the e l e c t r o s t a t i c c o n t r i b u t i o n s d i f f e r from those at 2 5 ° C , A S ° (298) being the e l e c t r o s t a t i c c o n t r i b u t i o n to the r e a c t i o n ' s standard entropy a t 25 C . The l a s t two terms d e r i v e from the assumption that the n o n - e l e c t r o s t a t i c ο ο p a r t of AC , AC , can be represented by Ρ Ρ>η' AC°

ρ,η

= α

+ $T

...

(4)

2 F o l l o w i n g Helgeson (74) a term XT ( i n ( 4 ) ) i s ignored. On h i s model the c o n t r i b u t i o n of each i o n t o AS^ i s given by ... (5)

S° = -A [exp{exp(b + aT) } + Τ/θ] Ql/Θ + a exp(b + aT)] /ε

2 where A = (Ze) N/2r, Ze i s the charge on the i o n , r i t s r a d i u s and N, Avogadro's number. ε i s defined i n equation (2) and takes the v a l u e 3 0 5 . 7 . The e l e c t r o s t a t i c model i s crude and the choice of r t o be employed i s somewhat a r b i t r a r y , but Helgeson s model t o a c e r t a i n extent allows f o r t h i s by t a k i n g up u n c e r t a i n t i e s i n the e l e c t r o s t a t i c c o n t r i b u t i o n i n α and 3 . This was a q u i t e i n t e n t i o n a l f e a t u r e of the model because i t i s b e l i e v e d that much of the u n r e l i a b i l i t y of h y d r a t i o n models a r i s e s from non0

1


668

T H E R M O D Y N A M I C S OF


APPLICATIONS

e l e c t r o s t a t i c c o n t r i b u t i o n s . Helgeson used equation (3) t o curve f i t a l l s u f f i c i e n t l y r e l i a b l e experimental data and from t h i s obtained best f i t values of a, 8 and AS f o r a number of equilibria. The author has r e c e n t l y attempted to use t h i s method to estimate the e q u i l i b r i u m constant, K^, of the r e a c t i o n


3I

+ 3H 0

2

2

=

5 I ~ + I 0 + 6H

+

3

up to 300°C from experimental data at 25°C and 60°C (75). The r e a c t i o n i s , of course, a severe t e s t as i t produces 12 ions from 3 molecules of n e u t r a l s o l u t e . Equation (1) i s t o t a l l y u n s a t i s f a c t o r y s i n c e i t f a i l s to p r e d i c t the expected maximum i n the e q u i l i b r i u m constant as seen i n F i g . 5. Thus an attempt was made to use a c a l c u l a t e d AS ( v i a equation (5)) and see i f a b e t t e r estimate was p o s s i b l e u s i n g what Helgeson s r e s u l t s (74) suggested were reasonable values of α and 3. Two assumptions were t r i e d , one was that A C p i s independent of temperature - i . e . 3=0and the other made use of the o b s e r v a t i o n that i n Helgeson s Table 2 there i s an approximate r e l a t i o n s h i p between α and 3, α = -313 (±48). These estimates are a l s o shown i n F i g . 5 and i t i s c l e a r from the divergence of r e s u l t s above 100 C that the method i s too s e n s i t i v e t o the values of α and 3 to be of use at l e a s t i n t h i s case. The author b e l i e v e s (75) the correspondence p r i n c i p l e method (as used by Lewis (64) based on Cp ( I 2 ) = 65, although u n c e r t a i n due to lack of appropriate data on I 2 , provides the best estimate. Almost c e r t a i n l y f r e e energy approaches l i k e Helgeson s can be improved by b e t t e r i o n i c h y d r a t i o n models. To t h i s end a number have been q u a l i t a t i v e l y compared (76) and checked against experimental data on NaCl (77,78). More e x t e n s i v e c a l c u l a t i o n s based on one ( f i x e d hydration) model have a l s o been presented (79) and found to p r e d i c t i o n i c f r e e energies b e t t e r than the correspondence p r i n c i p l e between 150 and 275 C. At higher temperatures, however, the model i s l e s s s a t i s f a c t o r y . Much more work i s needed i n t h i s area s i n c e , i f such methods are to prove r e l i a b l e i n the d i f f i c u l t r e g i o n between 300 C and the c r i t i c a l p o i n t , the h y d r a t i o n models must be as free as p o s s i b l e from e m p i r i c a l f i t t i n g parameters. e

1

jn

1

1

3.Applications There f o l l o w some examples of attempts to apply thermodynamic arguments to a number of p l a n t problems. A t t e n t i o n i s d i r e c t e d as much to what can and cannot be done w i t h c u r r e n t l y a v a i l a b l e data as to the p r a c t i c a l s i g n i f i c a n c e of the r e s u l t s . Areas where work i s p a r t i c u l a r l y needed are s t r e s s e d . Generation of C o r r o s i v e Environments. The m a t e r i a l s of c o n s t r u c t i o n of a water c i r c u i t a r e , of course, s e l e c t e d t o be


669



TURNER

1

1000Q/K)"

Figure 5. Predictions of K : (1) Helgeson Equation 1; (2) Equation 5, β = 0; (3) entropy correspondence~C °(I (aq)) = 0; (4) entropy correspondence C °(I (aq)) == 65; (5) Equation 3,a= -313β h

p

2

p


2


670

THERMODYNAMICS

OF

AQUEOUS SYSTEMS

W I T H INDUSTRIAL

APPLICATIONS

c o r r o s i o n r e s i s t a n t i n whatever s o l u t i o n s they are expected t o see. Most s e r i o u s problems a r i s e because i m p u r i t i e s i n or a d d i t i v e s t o the water are able to concentrate, sometimes by many orders of magnitude. This can occur where water i s b o i l i n g on t h i n porous oxide l a y e r s under high heat f l u x e s and i n regions of t u r b i n e s where the steam i s nominally dry but of such a pressure that concentrated s o l u t i o n s could be i n e q u i l i b r i u m w i t h i t . Simulations of the former occurrence have shown c o n c e n t r a t i o n f a c t o r s of ΙΟ» t o be p o s s i b l e while i n theory the l a t t e r s i t u a t i o n s cotfld r e s u l t i n even higher concentrations. I n PWR steam generators, e l e c t r o l y t e s can concentrate between the tubes and t h e i r support p l a t e s . The a c c e l e r a t e d production of c o r r o s i o n product leads t o "denting" by crushing of the supported tubes. I f the i m p u r i t y l e a k i n g ( v i a the condensers) t o a b o i l e r i s sea-water, the chemistry i s f a i r l y simple and i t i s easy t o pre d i c t roughly under what c o n d i t i o n s the concentrating l i q u i d w i l l go a c i d due t o M g h y d r o l y s i s (80,81). However, without more r e l i a b l e data on t h i s h y d r o l y s i s r e a c t i o n the best that can be hoped f o r i n e s t i m a t i n g the pH i s ±0.5 pH u n i t at 300°C (80). The much more complex s i t u a t i o n which a r i s e s when the condenser leak allows i n r i v e r or lake water can be d e a l t w i t h f o r m a l l y (82,83) but the u n c e r t a i n t i e s i n the data are u s u a l l y too large t o y i e l d r e l i a b l e pH estimates. We have been a b l e , however, on occasions to use a very simple model t o help understand s p e c i f i c p l a n t problems where r i v e r water analyses were a v a i l a b l e and on one occasion t o show that a t d i f f e r e n t times the b o i l e r water had (as c o r r o s i o n evidence suggested) a l t e r n a t e d between a c i d i c and a l k a l i n e c o n d i t i o n s . The model assumes that by 350 C any normally d i s s o c i a t e d multi-charged ions w i l l be s u f f i c i e n t l y unstable that they w i l l undergo whatever appropriate h y d r o l y s i s r e a c t i o n s can reduce t h e i r charge t o u n i t y . Whether the water goes a c i d o r a l k a l i n e then simply depends on whether the t o t a l (equivalent) concentration of m u l t i p l y charged c a t i o n s exceeds or i s smaller than the c o n c e n t r a t i o n of m u l t i p l y charged anions. When a 60 MW t u r b i n e a t Hinkley A power s t a t i o n d i s i n t e g r a t e d i n 1969 from s t r e s s c o r r o s i o n c r a c k i n g of a low pressure t u r b i n e d i s c (consequences shown i n P l a t e 1) i t was considered that Na^H s o l u t i o n s were most probably i n v o l v e d (84) and i t was soon found that i f NaOH were the s o l e e l e c t r o l y t e present i t s maximum concentration (based on vapour pressure depression) was s u f f i c i e n t t o have caused the c r a c k i n g . However, i t was a l s o found that i n mixtures i t was only the f r e e NaOH which l e d t o r a p i d s t r e s s c o r r o s i o n c r a c k i n g . Considerations of a c i d gas s o l u b i l i t y and s o l u t i o n thermodynamics showed that at the C O 2 and acetate l e v e l s present i t was most u n l i k e l y that f r e e NaOH was present i n s u f f i c i e n t q u a n t i t y to be r e s p o n s i b l e f o r the H i n k l e y f a i l u r e (85). Ammonium acetate and ammonium carbonate had a l s o been found to induce s t r e s s c o r r o s i o n c r a c k i n g of the appropriate s t e e l s but 2+



34.

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671

s i m i l a r thermodynamic arguments showed that n e i t h e r e l e c t r o l y t e could i n p r a c t i c e approach the l e v e l s r e q u i r e d to cause c r a c k i n g (86,85). Subsequent attempts to improve the estimates (87) confirmed these f i n d i n g s . The e f f e c t o f a c i d gases l i k e C O 2 and a c e t i c a c i d on reducing the f r e e NaOH concentrations i n t u r b i n e s makes one wonder i f i t may be p o s s i b l e to p u r i f y b o i l e r water too much f o r the good of the t u r b i n e . I f t h i s i s so, then some of the observations r e c e n t l y made by Bussart, Curran and Gould (88) on the e f f e c t of water chemistry on modern large t u r b i n e s may not, a f t e r a l l , be " p a r a d o x i c a l " . I t was e v e n t u a l l y concluded that the most l i k e l y chemical c u l p r i t i n the Hinkley t u r b i n e s was molybdate which can be formed from the M0S2 l u b r i c a n t under the c o n d i t i o n s used during t u r b i n e assembly (89) o r leached from the s t e e l under stagnant c o n d i t i o n s i n s u f f i c i e n t q u a n t i t y to induce s t r e s s c o r r o s i o n (90). Understanding Corrosion Processes. The a p p l i c a t i o n o f Pourbaix diagrams to c o r r o s i o n problems i s w e l l known and w i l l not be considered here. Much e f f o r t has gone i n t o producing such diagrams f o r high temperature use. A recent paper (91) l i s t s 15 references to the s u b j e c t . The diagrams are p a r t i c u l a r l y u s e f u l i n i n t e r p r e t i n g c o r r o s i o n or e l e c t r o c h e m i c a l s t u d i e s conducted a t c o n t r o l l e d p o t e n t i a l . However, w i t h few exceptions (92) l i t t l e a t t e n t i o n has been given to the r o l e of s o l u t i o n phase a d d i t i v e s and i m p u r i t i e s i n i n f l u e n c i n g the composition o l the c o r r o s i o n f i l m , although q u i t e s u b t l e compositional d i f f e r e n c e s across a c o r r o s i o n f i l m have been discussed i n terms o f redox p o t e n t i a l (93,94). Since a l l chemical r e a c t i o n s w i l l be much f a s t e r above 300°C than at 25 C i t seems l i k e l y that redox b u f f e r i n g by s o l u t i o n components should be more p r e d i c t a b l e thermodynamically (once the data become a v a i l a b l e ) a t the higher temperatures. A f e a t u r e of c o r r o s i o n s t u d i e s which has been s t r e s s e d r e c e n t l y (2) i s the complete f a i l u r e of l a b o r a t o r y t e s t s on t h e i r own to p r e d i c t how r e l i a b l e operation of some nuclear steam generators can be maintained. At l e a s t a p a r t o f t h i s problem i s l i k e l y to a r i s e from d i f f e r e n t redox and/or pH c o n d i t i o n s imposed by the s o l u t i o n i n autoclave t e s t s and i n p l a n t c o n d i t i o n s and many low l e v e l contaminants could be i n v o l v e d . In view of what has been s a i d e a r l i e r concerning the r o l e of Mo(VI) i n stagnantwater i t i s c l e a r that some data, a t l e a s t on the thermodynamics of aqueous Mo s p e c i e s , should be sought at high temperatures. Some molybdate appears to be able to enter s o l u t i o n through the vapour at 250 C (61), so the contamination problem i s not n e c e s s a r i l y solved by the use of l i n e r s . Presumably other species capable of i n f l u e n c i n g redox p o t e n t i a l s and pH can a l s o contaminate s o l u t i o n s through the vapour. I t seems to the author that u n t i l some means i s a v a i l a b l e f o r e s t i m a t i n g the pH and redox p o t e n t i a l of s o l u t i o n s both i n autoclave s t u d i e s and under s p e c i f i c l o c a l p l a n t c o n d i t i o n s there w i l l always be doubt about the p r e d i c t i v e value of many c o r r o s i o n s t u d i e s c a r r i e d out i n autoclaves.



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APPLICATIONS

Out of Core R a d i o a c t i v i t y . I n water r e a c t o r s the main source of out of core a c t i v i t y a r i s e s from the c o r r o s i o n products of the out of core c i r c u i t which are t r a n s p o r t e d i n t o the core and, a f t e r neutron i r r a d i a t i o n , subsequently t r a n s p o r t e d back. Since the core of a PWR w i l l i n e v i t a b l y be h o t t e r than the steam generator, there has been i n t e r e s t , p a r t i c u l a r l y a t Atomic Energy of Canada L t d . (A.E.C.L.), i n the e f f e c t of temperature on the s o l u b i l i t y of the v a r i o u s c o r r o s i o n products present and how t h i s i n f l u e n c e s t h e i r t r a n s p o r t a t i o n round the c i r c u i t (95). The s o l u b i l i t i e s c o u l d , even w i t h p e r f e c t data, only be t i e d down t o a range of l e v e l s f o r most elements because of the presence of r a d i o l y t i c a l l y produced H 2 and O 2 a t l e v e l s which are not at e q u i l i b r i u m . The system i s f u r t h e r complicated by the presence of mixed s p i n e l s as w e l l as pure oxides amongst the c o r r o s i o n products. Despite these c o m p l i c a t i o n s , however, a combination of d e t a i l e d sampling and thermodynamic r a t i o n a l i z a t i o n (based mainly on estimated data (65, 66) i s r e s u l t i n g i n a g r e a t l y improved understanding of the processes i n v o l v e d (96) . There i s l i t t l e doubt that the experimental programme being pursued by A.E.C.L. w i l l lead t o b e t t e r understanding o f the behaviour of c o r r o s i o n products i n a l l types of p l a n t . Thermodynamic arguments have a l s o been used i n support of work on decontaminating the c i r c u i t s of BWRs (11). I t was shown that c o n v e n t i o n a l c i t r i c a c i d c l e a n i n g s o l u t i o n s could not d i s s o l v e e i t h e r Fe2Û3 or important Co-containing s p i n e l s unless q u i t e strong reducing agents were present and i t was a l s o shown that the r i s k of e l e c t r o d e p o s i t i o n on s t e e l surfaces during the decontamination i s g r e a t l y reduced under s t r o n g l y reducing c o n d i t i o n s . There was some evidence from r e s u l t s on decontamination of the W i n f r i t h SGHWR (Steam Generating Heavy Water Reactor) that ^°Co may have been e l e c t r o d e p o s i t e d e i t h e r during e a r l y decontaminations or on load; the l a t t e r seems p o s s i b l e thermodynamically though u n l i k e l y k i n e t i c a l l y under normal ( r a d i a t i o n - f r e e ) c o n d i t i o n s . The high temperature thermodynamic data were, however, considered i n s u f f i c i e n t l y r e l i a b l e t o be c e r t a i n of the s i g n i f i c a n c e of some of the p l a n t o b s e r v a t i o n s . I t i s , however, c l e a r that e l e c t r o d e p o s i t i o n of ^ C o n load (as w e l l as d u r i n g decontamination) i s u n l i k e l y under s t r o n g l y reducing c o n d i t i o n s such as those which are nominally maintained i n PWR primary c i r c u i t s . The r e l e a s e of r a d i o a c t i v e i o d i n e s from BWR c i r c u i t s , f i r s t i n t o the steam phase and then i n t o the t u r b i n e h a l l , has a l s o been considered thermodynamically (75) . A r e - a n a l y s i s of some experimental data of S t y r i k o v i c h e t a l (97) , suggested that iodates were not, as had been t e n t a t i v e l y proposed, l i k e l y t o be present. S t y r i k o v i c h ' s p r e d i c t i o n of HIO as a p r i n c i p a l species under BWR c o n d i t i o n s was confirmed, but i t was concluded t h a t h i s experiments had not measured i t s steam/water p a r t i t i o n c o e f f i c i e n t . In view of the meagre experimental evidence, however, more work on t h i s system i s d e s i r a b l e . Q



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New Methods of Chemical Treatment, As d e s c r i b e d elsewhere (98) c o n s i d e r a b l e success has been achieved i n t r e a t i n g low pressure b o i l e r s w i t h polyamino-carboxylic a c i d type c h e l a t i n g agents. Some years ago the author suggested that weaker, but more thermally s t a b l e , c h e l a t i n g agents such as oxine (8 hydroxy q u i n o l i n e ) might f i n d a p p l i c a t i o n i n m a i n t a i n i n g once-through b o i l e r s f r e e of d e b r i s . The main doubt (98,99) was whether such b i d e n t a t e c h e l a t i n g agents are strong enough (thermodynamically) to be of any value and e a r l y estimates of magnetite s o l u b i l i t i e s up t o 200 C (14) were not encouraging. Apart from some recent work of Alexander (100) there i s s t i l l h a r d l y any data on metal ion c h e l a t i o n above 100 C. However i t has been p o s s i b l e t o estimate roughly some r e l e v a n t h i g h temperature s t a b i l i t y constants (12) and c r u d e l y c o r r e c t them on the b a s i s of measured i r o n l e v e l s d i s s o l v e d from Fe304 by oxine (101). On t h i s b a s i s the chances of u s i n g oxine s u c c e s s f u l l y i n a once-through b o i l e r look good, c a t e c h o l may be e f f e c t i v e and d i c a r b o x y l i c a c i d s may be usable i n an adaption of a Russian method of t r e a t i n g superc r i t i c a l b o i l e r s (102) t o s u i t s u b - c r i t i c a l once-through b o i l e r s . E s t i m a t i o n i n t h i s f i e l d i s , at the moment, i n e v i t a b l y g r o s s l y approximate because of the lack of high temperature data and the l i k e l i h o o d (discussed i n S e c t i o n 2) of forming mixed hydroxy-chelate complexes at h i g h temperatures. Experimental work i n t h i s area i s p a r t i c u l a r l y needed. 4.Concluding Remarks There are many a d d i t i o n a l ways i n which thermodynamic arguments c o u l d , i n p r i n c i p l e , be used both f o r pure p r e d i c t i o n and f o r r a t i o n a l i z i n g p l a n t f i n d i n g s . I t i s a l s o necessary t o q u a n t i f y most of the r a t h e r q u a l i t a t i v e c o n c l u s i o n s discussed i n S e c t i o n 3. The l e a s t p r e d i c t a b l e systems i n v o l v e the behaviour of t r a n s i t i o n metal ions a t h i g h temperatures and u n t i l a good deal more work i s done to d i s e n t a n g l e t h e i r complexing and h y d r o l y s i s r e a c t i o n s i t i s u n l i k e l y that much progress can be made. The author very much hopes that the complexity of the t r a n s i t i o n metal systems w i l l not i n h i b i t work i n t h i s area. The l o s s to a user of thermodynamics i n my f i e l d would have been c o n s i d e r a b l e i f Sweeton and Baes had been put o f f beginning or r e p o r t i n g t h e i r (23) Fe3Û4 s o l u b i l i t y work by the f e a r t h a t on i t s own, i t might not y i e l d a complete answer t o the understanding of Fe304 s o l u b i l i t y . I n t h i s p a r t i c u l a r case the d e c i s i o n was probably not d i f f i c u l t as there may be no problem below 250-300 C. The q u e s t i o n which w o r r i e s me i s how people are to be encouraged to study systems which are o b v i o u s l y more complex, e x p e r i m e n t a l l y d i f f i c u l t and u n l i k e l y , on t h e i r own, to y i e l d r e l i a b l e thermodynamic r e a c t i o n f r e e e n e r g i e s , (e.g. Fe304 s o l u b i l i t y a t higher temperatures or s t u d i e s on mixed complexes). S i m i l a r l y , i f S t y r i k o v i c h e t a l (97) , had w o r r i e d about iodates and not given what the present author b e l i e v e s i s an q


THERMODYNAMICS

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OF

AQUEOUS

SYSTEMS

W I T H INDUSTRIAL

APPLICATIONS

i n c o r r e c t i n t e r p r e t a t i o n of the r e s u l t s , the data might s t i l l be u n a v a i l a b l e . While a more d e t a i l e d t a b u l a t i o n of the raw data would have been b e t t e r , the data themselves are v a l u a b l e i n p r o v i d i n g the only a v a i l a b l e experimental work on the behaviour of i o d i n e i n h i g h temperature water. I t i s to be hoped t h a t , i n c r e a s i n g l y , J o u r n a l s w i l l provide f a c i l i t i e s f o r authors t o t a b u l a t e t h e i r raw data. The J o u r n a l of Chemical Thermodynamics i s t o be congratulated on the amount of data they p r i n t e d i n the paper of Sweeton and Baes (23). I f the formation constant of F e C l and r e l a t e d constants can e v e n t u a l l y be measured or estimated, and i f a r e a n a l y s i s proves necessary, the data are a l l there t o use. In view of the d i f f i c u l t i e s discussed i n S e c t i o n 2 i t seems that many of the more important e q u i l i b r i a of relevance t o power s t a t i o n o p e r a t i o n w i l l not be d i r e c t l y measurable. I t i s c e r t a i n , t h e r e f o r e , that great emphasis w i l l have t o be placed on methods of e s t i m a t i n g high temperature data. I t a l s o seems c l e a r t h a t , i f these are t o be checked up t o 350 C,a v a r i e t y of experimental techniques may w e l l prove necessary t o s o r t out usable thermodynamic data from experiments which, on t h e i r own, cannot give them. A l t e r n a t i v e l y , i f e s t i m a t i o n procedures can be developed which are s u b s t a n t i a l l y f r e e from e m p i r i c a l f i t t i n g parameters, they may not r e q u i r e extensive checking.


+

This review was c a r r i e d out at the C e n t r a l E l e c t r i c i t y Research L a b o r a t o r i e s and i s p u b l i s h e d by permission of the C e n t r a l E l e c t r i c i t y Generating Board.

5.Literature Cited

1. Staehle, R.W., Corrosion, 1977, 33, 1 2. Faster, D., Kernenergie, 1979, 22, 118 3. Barnes, H.L., "High Temperature High Pressure Electrochemistry in Aqueous Solutions", N.A.C.E.-4, Houston, Texas, 1976, p.14 4. Freier, R.K., VGB-Speisewassertagung, 1969, p.11 5. Freier, R.K., VGB-Speisewassertagung, 1970, p.8 6. Turner, D.J., paper presented at symposium "Industrial Uses of Thermochemical Data", 1979, sponsored by NPL and Chemical Society, University of Surrey, Oct. 7. Liu, C. and Lindsay, W.T. Jr., J. Solution Chem., 1972, 1, 45 8. Choi, Y-S. and Criss, CM., Faraday Disc, of Chem. Soc., 1977, 64, 204


34. TURNER High-Temperature Aqueous Systems 675

9. Turner, D.J., C.E.R.L. Note, 1976, RD/L/N 165/76 10. Turner, D.J., C.E.R.L. Note, 1979, draft "Some Unexpected Reactions Between Metals and Aqueous Solutions at High Temperature; Part I. Gold and Platinum Dissolution" 11. Turner, D.J., C.E.R.L. Note, 1976, RD/L/N 213/76 12. Turner, D.J., C.E.R.L. Note, 1978, RD/L/N 11/78


13. Passell, T.O., E.P.R.I. Jnl., 1977, 2, 42 14. Bawden, R.J., C.E.R.L. Note, 1976, RD/L/N 72/76 15. Franck, E.U., "High Temperature Pressure Electrochemistry in Aqueous Solutions", N.A.C.E.-4, Houston, Texas, 1976, p.109 16. Marshall, W.L., ibid, p.117 17. Lindsay,W.T.and Liu, C., ibid, p. 139 18. Martynova, O.I., ibid, p.131 19. Lietzke, M.H., ibid, p.317 20. Mesmer, R.E., Sweeton, F.H., Hitch, B.F. and Baes, Jr.,C.F., ibid, p.365 21. Silvester, L.F. and Pitzer, K.S., J. Phys. Chem., 1977, 81, 1822 22. Helgeson, H.C. and Kirkham, D.H., Amer. J. Sci., 1976, 276, 97 23. Sweeton, F.H. and Baes, C.F., Jr., J. Chem. Thermodynamics, 1970, 2, 479 24. Nikolaeva, N.M., "High Temperature High Pressure Electro chemistry in Aqueous Solutions", N.A.CE.-4, Houston, Texas, 1976, p.146 25. Tremaine, P.R. and Leblanc, J.C., "The Solubility of Nickel Oxide and Hydrolysis of N i in Water to 573 K". In press 2+

26. Lüdemann, H.D. and Franck, E.U., Ber Bunsenges. physik. Chem., 1968, 72, 514 27. Lüdemann, H.D. and Franck, E.U., Ber Bunsenges. physik. Chem., 1967, 71, 455


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28. Scholz, Β., Lüdemann, H.D. and Franck, E.U., Ber. Bunsenges. physik. Chem., 1972, 76, 406 29. Franck, E.U., "Phase Equilibria and Fluid Properties in the Chemical Industry", ed. T.S. Storvik and S.I. Sandler, A.C.S. Symposium, Series No. 60, Chap. 5, p.99 30. Broadbent, D., Lewis, G.G. and Wetton, E.A.M., J. Chem. Soc. Dalton Trans., 1977, 464


31. Hitch, B.F. and Mesmer, R.E., J. Soln. Chem., 1976, 5, 667 32. Holmes, H.F., Baes, Jr., CF. and Mesmer, R.E., J. Chem. Thermodynamics, 1978, 10, 983 33. Shoesmith, D.W. and Lee, W., Can. J. Chem., 1976, 54, 3553 34. Mesmer, R.E. and Herting, D.L., J. Soln. Chem., 1978, 7, 901 35. Sweeton, F.H., Mesmer, R.E. and Baes Jr., CF., J. Soin. Chem., 1974, 3, 191 36. Fisher, J.R. and Barnes, H.L., J. Phys. Chem., 1972, 76, 90 37. Sirota, A.M. and Shviriaev, Yu.V., "High Temperature High Pressure Electrochemistry in Aqueous Solutions", N.A.C.E.-4, Houston, Texas, 1976, p.169 38. Marshall, W.L. and Franck, E.U., Paper presented at International Association for the Properties of Steam, Sept. 1979, Munich, West Germany 39. Marshall, W.L., J. Inorg. Nucl. Chem., 1975, 37, 2155 40. Marshall, W.L. and Slusher, R., J. Inorg. Nucl. Chem., 1975, 37, 2165 41. Marshall, W.L. and Slusher, R., J. Inorg. Nucl. Chem., 1975, 37, 2171 42. Marshall, W.L. and Jones, E.V., J. Inorg. Nucl. Chem., 1974, 36, 2313 43. Marshall, W.L. and Jones, E.V., J. Inorg. Nucl. Chem., 1974 36, 2319 44. Templeton, C.C., J. Chem. Thermodynamics, 1976, 8, 99 45. Marshall, W.L., ibid, p.100


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46. Nikolaeva, N.M. and Tsvelodub, L.D., Zh. Neorg. Khim., 1977, 22, 380 47. Nikolaeva, N.M., Zh. Neorg. Khim., 1977, 22, 2447 48. Nikolaeva, N.M. and Tsveladub, L.D., Zh. Neorg. Khim., 1975, 20, 3033


49. Tugarinov, I.Α., Ganeev, I.G. and Khodakovskii, I.L., Geokhimya, 1975, 9, 1345 50. Cobble, J.W. and Murray, Jr., R.C., Faraday Disc, of Chem. Soc., 1977, 64, 144 51. Bawden, R.J. and Turner, D.J., unpublished results 52. Chou, I.-M. and Eugster, H.P., Amer. J. Sci., 1977, 277, 1296 53. Frantz, J.D. and Eugster, H.P., Amer. J. Sci., 1973, 273, 268 54. Latimer, W.L., "Oxidation Potentials", Prentice Hall Inc., New Jersey, 1952 55. Swaddle, T.W., Lipton, J.H., Guastalla,G. and Bayliss, P., Can. J. Chem., 1971, 49, 2433 56. Kong, P-C., Swaddle, T.W. and Bayliss, P., Can. J. Chem., 1971, 49, 2442 57. Gainsford, A.R., Sisley, M.J., Swaddle, T.W. and Bayliss, P., Can. J. Chem., 1975, 53, 12 58. Henderson, M.P., Miasek, V.I. and Swaddle, T.W., Can. J. Chem., 1971, 49, 317 59. Newton, A.M. and Swaddle, T.W., Can. J. Chem., 1974, 2751 60. Fabes, L. and Swaddle, T.W., Can. J. Chem., 1975, 53, 3053 61. Turner, D.J., C.E.R.L. Note, 1979, draft "Some Unexpected Reactions Between Metals and Aqueous Solutions at High Temperature. Part II. Steam Volatilization of Components from Stainless Steel" 62. Criss, C.M. and Cobble, J.W., J. Amer. Chem. Soc., 1964, 86, 5385 63. Lewis, D., Arkiv. Kemi., 1971, 32, 385 64. Lewis, D., Aktieb. Atomenergi Report, 1971, AE-436


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65. Macdonald, D.D., Shierman, G.R. and Butler, P., Atomic Energy of Canada Ltd. Reports, 1972, AECL-4136, 4137, 4138, 4139 66. Macdonald, D.D. and Rummery, T.E., Atomic Energy of Canada Ltd. Report, 1973, AECL-4140 67. Criss, CM. and Cobble, J.W., J. Amer. Chem. Soc., 1964, 86, 5390 68. Cobble, J.W., J. Amer. Chem. Soc., 1964, 86, 5394 Downloaded by EAST CAROLINA UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch034

69. Pohl, H.A., J. Chem. and Eng. Data, 1962, 7, 295 70. Harned, H.S. and Embree, N.D., J. Amer. Chem. Soc., 1934, 56, 1050 71. Khodakovskii, I.L., Zhogina, V.V. and Ryzenko, B.N., Geokhimiya, 1965, 827 72. Harned, H.S. and Robinson, R.A., Trans. Faraday Soc., 1940, 36, 973 73. Tremaine, P.R., Von Massow, R. and Shierman, G.R., Thermochimica Acta., 1977, 19, 287 74. Helgeson, H.C, J. Phys. Chem., 1967, 71, 3121 75. Turner, D.J., "Water Chemistry of Nuclear Reactor Systems", British Nuclear Energy Society, London, 1978, p.489 76. Turner, D.J., C.E.R.L. Note, 1969, RD/L/N 25/69 77. Bawden, R.J., C.E.R.L. Note, 1977, RD/L/N 85/77 78. Turner, D.J., Faraday Disc, of Chem. Soc., 1977, 64, 231 79. Tremaine, P.R. and Goldman, S., J. Phys. Chem., 1978, 82, 2317 80. Turner, D.J., "High Temperature High Pressure Electro chemistry in Aqueous Solutions", N.A.C.E.-4, Houston, Texas, 1976, p.188 81. Bawden, R.J., C.E.R.L. Note, 1973, RD/L/N 20/73 82. Bawden, R.J., C.E.R.L. Note, 1977, RD/L/N 87/77 83. Bawden, R.J., C.E.R.L. Note, 1979, RD/L/N 100/79 84. Hearn, B. and D. de G. Jones, C.E.R.L. Report, 1971, RD/L/R 1699


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85. Bawden, R.J. and Turner, D.J., C.E.R.L. Note, 1973, RD/L/N 213/73 86. Turner, D.J., C.E.R.L. Note, 1971, RD/L/N 269/71 87. Turner, D.J., C.E.R.L. Note, 1974, RD/L/N 238/74 88. Bussert, B.W., Curran, R.M. and Gould, G.C., Trans. ASME, 1978, Paper No. 78-JPGC-Pwr-9 89. Turner, D.J., C.E.R.L. Note, 1974, RD/L/N 204/74 Downloaded by EAST CAROLINA UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch034

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Thermodynamics of High-Temperature Aqueous Systems - ACS

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