Distribucon Especies Cr 4

7/25/2019 Distribucon Especies Cr 4

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Ccwrosion Science, Vol.39, No. 1,pp. 3-57, 1997

Copyright 0 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved

001&938X/97 %17.00+0.00

PII: s~l~9~x( ~lol-l

REVISED POURBAIX DIALOGS FOR CHROMIUM AT

25300C

B. BEVERSKOG and I. PUIGDOMENECH

Stud&k Material AB, S-61 1 82 Nykoping, Sweden

Abstract-The Pourbaix diagrams (potential-pH diagrams) for chromium at 25-300C have been revised. The

diagrams were calculated for two concentrations: 10- and IO-* molal. Extrapolation of thermochemical data to

elevated temperatures has been performed with the revised model of Helgeson-~rkh~-Flowers, which also

allows uncharged aqueous complexes, such as Cr(OH)X(aq), to be handled. The calculations show that the

hydroxides Cr(OH)a, Cr(OH)s, and the oxides CrOz and Cr03 are not stable at the investigated temperature and

concentration interval. CraOs is the only stable solid chromium compound in aqueous solution in the environment

studied, with the exception of very diluted solutions such as lo-smolal at T> 150C where no solid chromium

compound is stable. The dichromate ion does not predominate anywhere within the calculated concentration range.

Copyright @ 1996 Elsevier Science Ltd

Keywords A. chromium, C. Pourbaix diagram.

INTRODUCTION

Chromium is a metallic element well known for its good corrosion properties. The reason

for this is not due to the element itself, which is a very base metal, but to one of its solid

reaction products, Cr203. The element is protected in oxidising conditions by the formation

of Cr203, which acts as a barrier between the metal and the environment. This oxide, which

is a p-type semiconductor, grows by diffusion of cations from the metal to the oxide/solution

interface, the transport path being cation vacancies. The diffusion coefficients are very low,

which means low growth rates of the oxide. The good corrosion behaviour of chromium is

the reason to alloy metals with chromium thereby making corrosion resistant alloys. The use

of these corrosion resistant alloys containing chromium, such as stainless steels, has

increased continuously in recent years. Knowledge of the limits for the good corrosion

resistance is therefore quite important. One way to predict the corrosion resistance of

chromium is to consider the the~odynamics (equilibrium relations) for the system.

Chemical and electrochemical equilibria are elegantly summarised in a Pourbaix

diagram, which is a potential-pH diagram, The diagram is a map of the multidimensional

thermochemical space and predicts areas of immunity (no corrosion, by definition),

passivity (a solid reaction product) and corrosion (a dissolved reaction product), The

Pourbaix diagram for chromium has been studied by many authors-2 and these diagrams

Manuscript received 27 February 1996.

43


2/15

44

B. Beverskog and I. Puigdomenech

are particularly useful for studying the corrosion behaviour of many corrosion resistant

alloys, including stainless steel, as chromium plays an important part in their corrosion

resistance. Diagrams for elevated temperatures have also been reported by several

workers 2,6&i i,t4,1%17,2s h h

w ic in general have uncritically used the species that Pourbaix

used in his thesis in 1945. Since the work of Pourbaix, new findings have taken place which

makes it relevant to once again revise the Pourbaix diagram for chromium. Concentrations

of dissolved chromium species below 10M6

have only been reported in one work,s but

showing only the stability area of Cr203 at 25C.

The aim of this work is threefold. First, a partly new set of aqueous species, which has

not been used in earlier Pourbaix diagrams, has been used in the present calculations.

Secondly, a new method (in the context of Pourbaix diagrams) to extrapolate

the~o~hemical data from 25C to elevated temperatures has been used. Previous studies

had in general used the conventional correspondence principle of Criss and Cobble. The

third aim of this work is to calculate Pourbaix diagrams also for lo-* molal concentration

of dissolved species at elevated temperatures.

CHOICE OF CHROMIUM SPECIES

Chromium has the electron configuration [Ar]3d54s. The relatively low energy in the s-

and d-levels cause chromium compounds to have the oxidation numbers O-VI. The

oxidation numbers that are most common and stable in water solutions are II, III and VI.

The oxidation numbers IV and V exist as intermediates in redox reactions. However, these

products are unstable with respect to disproportionation to III and VI or react with the

solvent. The most important and most stable oxidation number in water solution is III and

Cr(I1) is a strong reducing agent, which is not particularly stable in aqueous solutions, not

even at low potentials. Cr(I1) is easily oxidised by oxygen to Cr(II1). All Cr(V1) compounds

are oxycompounds in aqueous solutions and very strong oxidation agents in acidic

solutions.

Table 1 shows 40 chromium species that have been considered in this work for the

aqueous system of chromium. Six solids and 10 aqueous chromium complexes have been

included in the calculations, Cr20s, CrOz and CrOs are the only oxides of importance.22

CrOz has been included although it is unstable in aqueous solution because it has been

claimed to exist at elevated temperatures. Nothing is known about the hydrolysis reactions

of Cr* + .23

Thirteen solids, one gaseous and 10 aqueous complexes have been excluded

since they are not stable in equilibrium with aqueous solutions. The reasons for the

exclusions are the following: thermodynamic data for the chromium hydride system have

been reported in the literature.24-26 However, no chromium hydride is included in our

calculations owing to the kinetic effects involved in this system.25 Of the oxides, GO(s) is

not stable in aqueous solution and Cr304 is a powerful reducing agent. The tetra and penta

oxides are also unstable in aqueous solutions. Hydrous Cr203 (Cr203 H20) is commonly

called chromium(II1) hydroxide, although its water content is variable. This solid is less

stable than the oxide and Cr(OH)3. The solid Cr(OH)3*nH20 is not considered because its

stability is expected to be lower than that of the unhydrous hydroxide. Only the enthalpies

are available for the hydrated forms of Cr203. CrOOH(s) has been excluded from this

study,because we believe, as discussed in the Results and Discussion, that this solid is not


3/15

Pourbaix diagrams for Cr

45

Table 1.

Considered species in the aqueous system of chromium

Condition Oxidation number

Included

Excluded

crystalline

solid

crystalline

solid

9,

gaseous

crystalline

3,

7.

9.

,,

,,

3,

3.

solid

crystalline

3,

dissolved

>,

7,

7,

7,

,,

,,

,.

7,

1,

1.

z

0

I

II

1

31

II/III

III

III/IV

IV

.

VI

VIII

X

II

III

1

V

VI

1

Cr

CrW-02

Cr203

WOW3

CrOz

CrOs

Cr*+

Cr3 +

CrOH+

Cr(OH):

Cr(OHMaq)

Cr(OH); (?)

H2Cr04

HCrOi

CrOi-

Cr@

6+10=16

CrH

cro

CrH2

CrH2

cr304

Cr203. H20

CrzOs. 2 Ha0

Cr203. 3 Hz0

Cr OH)s n Hz0

CrOOH

Cr2H7

Cr(OH)4

Cr04

Cr05

CrHAaq)

CrO:-

Cr(OH)i-

Crz(OH)i

Cr@H),

CrO:-

Cr04 +

CrOz+

CrsOf,

Cr40&

14+ lo=24

thermodynamically stable with respect to the oxide. The existence of a hydrated Cr(IV)

oxide (Cr(OH)4) has not been verified.22,23,39

CrOi-(aq) and Cr(OH)z-(aq), if they form at

all, would only be stable in unusually high alkaline media and are therefore not considered

here. The polynuclear hydrolysis products of chromium(II1) have not been included as

they are stable only in concentrated solutions.23

The dioxide and the tetra hydroxide as

well as the ion CrOi-(aq) have the oxidation numbers I, IV and V, respectively, and are

thereby not stable in water solutions as they disproportionate.** The six valent ions Cr04+

and CrO:+(aq) are not consistent with modern electrostatic theory of chromium

hydrolysis.

The anionic hydroxo chromium(II1) complex in alkaline solutions is denoted as


4/15

46

B. Beverskog and 1. Puigdomenech

Table 2.

Thermodynamic data at 25C for the system chromium-water. In general more digits than required by

the expected uncertainty are given in order to retain the values for the changes in the reactions among individual

species

Species

Arc0

(kJ mol-)

S

(J K-t mol-)

c(T)/(J K- mol-)

=a+bT+cT-=

a*

bx

I O3

cxw6

Cr(cr)

Cr(OHMs)

Cr203W

Cr(OHX(s)

CrOa(cr)

CQfcr)

0

- 585.57

- 1053.09

-873.17

- 548

-510.04

23.6 21.76

8.98

-0.096:

El. 116.064

8.648

-2.874

81.2 119.37

9.20

- 1.565

105 127.612

41.639

-4.217

54 94.6

17.2

73.2

71.76

87.86

- 1.674

Cr2 + -174

-100

-11

Cr+

-215 -293

-30

CrOH*

-431.8 -151 160

Cr(OH)z +

-633.19

-92 340

Cr(OH)s(aq)

-834.13 -38 480

chromite aniod - 1005.89 -83.8

410

CrO:-

HCrO;

H$XUaq)

Cr_@-

- 727.75

-765.14

- 764.00

- 1302.23

50.21

-251

195.2

-50

311

84

299.5 - 175

+For aqueous species a corresponds to the standard partial molar heat capacity at 25C and the revised

Helgeson-Kirkham-Flowers model has been used to obtain its temperature dependence.

X

C$Cr(cr), T)/(J K- mol-) = o + bT + CT-~ +

d T ,

with d =

2.26 x IO-.

8 Data obtained using stoichiometry Cr(OH);, see text,

Cr(OH); and not the traditional notation of CrO, generally used in Pourbaix diagrams.

The difference is two water molecules. This is also in agreement with the nomenclature used

by Baes and Mesmer.23

THERMOCHEMICAL DATA

A critical review of published thermodynamic data has been performed for the solids

and aqueous species described in the previous section, and best values have been selected as

described below. Data is usually available only for a reference temperature of 25C in the

form of standard molar Gibbs free energy of formation from the elements (A@) and

standard molar entropy (So). Equations for the temperature dependency of the standard

molar heat capacity (Cj) are usually available for solid and gaseous compounds. For

aqueous species, the standard p rti l molar properties are usually available. Extrapolation

of the~odynamic data to other temperatures is performed with the methodology described

elsewhere.* For aqueous species T-extrapolations have been based on the electrostatic

model of Helgeson et uZ.28--31

nd our methodology requires a value of Cj at 25C.


5/15

Pourbaix diagrams for Cr 47

The data selected for the calculations performed in this work are summarised in Table 2.

The selection criteria behind the values in this table are discussed below.

SOLIDS

The data for Cr(cr), CrzOa(cr) and CrOs(cr) are those in Kubaschewski et

a L 3

For

Cr02(s), the values listed in

33Y34ave been used. The parameters for the T-equation of

C,O(CrOz) are those estimated by35.

For the Cr(I1) hydroxide, the value of AfG has been derived from the only available

solubility equilibrium constant,36Y37

while for Cr(OH)3(s) the upper limit for the solubility

constant proposed in the careful study of Rai et a l . 3 8 has been used. Of the estimates for

s(Cr(OH)3(s))34,37,39

that in the most recent review by Niki34 is adopted, while for

Cr(OH)*(s) the only estimate for s available39 is accepted. The approximation has been

made here that the parameters for the q(T) equations for Cr(OH)*(s) and Cr(OH)3(s) are

equal to those of their iron(I1) and (III) analogues as reported in 4o because there are no

literature values available. This corresponds to q(25C) of 86.3 and 92.6 J K- mol- for

Cr(OH)*(s) and Cr(OH)3(s), respectively.

AQUEOUS SPECIES

The thermodynamic properties of H,O(l) at 25C recommended by CODATA4 have

been used in this work. The temperature dependence of these properties has been calculated

with the model of Saul and Wagner.42

The dielectric constant of water (which is needed for

the revised Helgeson-Kirkham-Flowers model that describes the temperature dependency

of the thermodynamic properties of aqueous solutes) has been obtained with the equations

given by Archer and Wang.43

The data for Cr*+ and Cr3+

are those selected in the review by Niki.34 The value of

AfZ (Cr3+) thus selected is that obtained by Dellien and Heplera (Af@ = -251 kJ mol-)

and agrees also with the values obtained by Vasilev et

a 1 . 4 5

Because Cr3+ is a key species in

the process of deriving the other thermodynamic data, it is unfortunate that the

uncertainties in AfG and s for Cr3+ in Table 2 are quite large. For example, ,S(Cr3+)

data (mostly estimates) in the range - 37@-- 270 J K- mol- have been published in

the literature.57Y33,34,45-47Values of q(Cr3+) w -30 J K- mol- and C?(C$+) x - 11

J K-t mol- have been derived from the work of Spitzer et

a l . 4 8 -5 0

P

Cr(II1) solutions in the presence of excess hydroxide contain the chromite ion, which

appears to be a polynuclear complex as it does not pass through semipermeable

membranes.5

A spectroscopic and chromatographic study has confirmed the

polynuclear nature of this complex (or complexes), and has established that no Cr(OH&

ions are formed as the result of Cr(II1) hydrolysis in aqueous solutions. The exact

stoichiometry of the chromite ion is therefore still unknown, but for the purpose of drawing

the Pourbaix diagrams, the formula Cr(OH); is used in this work, and data for this

hypothetical stoichiometry are given in Table 2. It must be pointed out, however, that this

assumption results necessarily in erroneous T-extrapolations of the thermodynamic data for


6/15


7/15


The thermodynamic calculations have been summarised in two types of equilibrium

diagrams: Pourbaix diagrams and predominance diagrams for dissolved species. Both types

of diagrams are predominance diagrams, but the former contains solid as well as dissolved

metallic species (and sometimes also gaseous species), while the latter only contains

dissolved species. Stability areas for solids in a Pourbaix diagram must be calculated with a l l

the dissolved species included. Omitted metallic species often result in diagrams with

misleading information, and it is therefore of vital importance that Pourbaix diagrams are

based on the species that represent todays knowledge of the chemical system in question.

Predominance diagrams for dissolved species are useful because they contain only dissolved

species, and they reveal the most stable aqueous species below the area hidden by a stable

solid phase. This type of diagram contributes additional information (as compared to the

corresponding Pourbaix diagrams) only when the metal in question has more than one

oxidation states. Otherwise the predominance for most of the aqueous species is already

predicted by a Pourbaix diagram, because the predominance areas of dissolved species when

the metal has only one oxidation state are pH-dependent and not dependent on the

potential. It would seem that predominance diagrams for dissolved species are independent

of the concentration level at which they are constructed. However, this might not be true as

concentrated solutions can contain polynuclear metallic aqueous ions, which in general do

not exist in diluted solutions. Therefore, predominances diagram for dissolved species

calculated for concentrated solutions often deviate from those for diluted solutions. In the

case of very diluted solutions such as 10m6 nd lo- molal, the predominant diagrams for

dissolved species are the same. Pourbaix diagrams and predominance diagrams for

dissolved species are often superimposed to show the predominating dissolved species in

each part of the solid stability areas in a Pourbaix diagram. However, this can make the

diagram unclear and difficult to read, and it is avoided in this work by separating the

Pourbaix diagrams and the pr~ominan~ diagrams for dissolved species.

EXPERIMENTAL RESULTS AND DISCUSSION

There is unfortunately no experimental data in the literature on the stability of Cr(II1)

hydrolysis complexes at temperatures above 25C and the Pourbaix diagrams reported here

at T > 25C are therefore of a tentative nature. This is further increased by the uncertainty in

the the~odynamic data for the C?+ and Cr3+

ions, and by the unknown stoichiometric

composition of the species occurring in alkaline solutions of Cr(II1). Nevertheless, the

diagrams presented here correspond to the best knowledge available today on the chemistry

of chromium.

Another source of uncertainty is the relative stability of the oxides and hydroxides of

Cr(II1). Deltombe et af. report a lower solubility, i.e. a higher stability, for Cr(OH)3(cr,hex)

than for Cr2Os(cr). Their data apparently originate from the work of Latimer37 who

estimated a value for S(Cr(OH)3, cr, hex). These data have later been extensively used.57Y8

Our values reflect instead the solubility of precipitated solid hydroxide and of crystalline

dichromium(II1) trioxide, with a higher stability for the oxide than for the hydroxide, which

agrees with the behaviour of other metal cations.

Lee,8 based on the data in Ref. 56, presented Pourbaix diagrams for chromium drawn

with the assumption that CrOOH(s) predominates in the temperature range 60-500C.


8/15

50


Ziemniak5 also gives calculated solubilities for CrOOH(s) based on unpublished results by

Ziemniak et al. According to Laubengayer and Macune

58 he oxyhydroxide of chromium is

a meta stable phase. The passive film on chromium consists of a bilayered structure with an

outer hydroxide layer, Cr(OH)s, and an inner oxide layer, Cr.203.59 Immediately after

immersion in an aqueous solution, the hydroxide layer has been found to be three times

thicker than the the oxide layer, and 20 h later the thicknesses of the layers were equa1.59

Nevertheless, the thickness of the hydrated layer after prolonged exposure is still unclear,

although it is reasonable to assume that the hydrated layer would be a minor part of the

~ ac

7

pHam%

7

PH~wo~~

Fig. 1. Pourbaix diagram for chromium and [Cr(aq)bt = 10m6 m at 25, 100,200 and 300C.


9/15


passive film at prolonged exposures. This view is indirectly confirmed by several studies

which show that the passive film on chromium consists of Cr203.606 CrOOH has been

found in short time experiments analysing the passive layer ex

situ

by XPS (X-ray

Photoelectron Spectroscopy).62 In si tu reflectance spectroscopy studies have been unable to

obtain any evidence of either Cr(OH)s or Cr02 in the passive film.60 The latter has been

claimed by Sukhotin et

al.

to be the main compound in the passive film,6x5 although this

has not been confirmed by other researchers. However, the same study with reflectance

spectroscopy6

was not able to exclude the existence of the oxyhydroxide in the passive

layer. In conclusion, it appears that CrOOH(cr) is possibly formed during short time

experiments and that the oxyhydroxide is not thermodynamically stable compared to

Cr203(cr), a behaviour similar to that displayed by the corresponding iron compounds.

Consequently, CrOOH(cr) has been excluded from our calculations. More experimental

studies are needed before the thermodynamic stability of the oxyhydroxide of chromium is

fully established.

Some previously published Pourbaix diagrams879Y5ncorrectly report CrO as the solid

Cr(I1) phase in neutral solutions. This apparently originates from the work of Deltombe

et

al.

where in their Table 1 the hydroxide of chromium(I1) is denoted by the formula CrO

hydr..

Wolf points out2 that chromium hydride may be formed either electrochemically, or

from gaseous hydrogen at Prr2(s)

> lo4 atm, which corresponds to EsHE < - 0.65 V in

neutral aqueous solutions at 25C and a comparison with Fig. 1 shows that the CrH(s)

would predominate over Cr(cr). Nevertheless, chromium hydrides have not been included in

our calculations due to the predominance of kinetic effects in this system, and therefore no

hydrides appear in the Pourbaix diagrams presented in this work.

Two general remarks can be concluded regarding the temperature and concentration

Table 3. Calculated thermodynamic stability of chromium species in the system of chromium-water

(P = predominates at 10e6 molal; p = predominates at lo- molal; d = appears in the predominance diagram for

dissolved species)

Species

25C 50C 100C

150C

200C 250C 285C

3WC

Cr(cr)

Cr(OHMcr)

Cr(OHMcr)

Cr203@d

Cfl2W

C 3W

C;L+(aq)

Cr3 + (aq)

CrOH+(aq)

Cr(OH):(aq)

Cr(OHMas)

Cr(OH);(aq)

HDWaq)

HCrWaq)

Cro2,-(aq)

CrzO:-(aq)

4

PP

Pp

PP

PP PP PP

PP

PP 4

PP PP

Ppd

W

W

Ppd

d

Ppd

Ppd

F$

Ppd

W

W

w

W

Ppd

Ppd

Pp

PP

PP

Pp

Ppd

4d

Ppd

4d

:d

Ppd

Ppd

4d


10/15


0

7

14

Mnc

c5

t

L

W

2

1

0

1

2

0

7

14

PHl aJS

0

7 0 7

PHNOOOC

PHsEos

Fig. 2. Pourbaix diagram for chromium and [Cr(aq)ltot = lo- m at 25, 100,200 and 300C.

dependence in the calculated diagrams. First, the temperature changes the size of the

different stability areas of immunity, passivity and corrosion. The immunity area (stability

of the metal itself) and the passive area (stability of solid compounds) decrease with

increasing temperature. The corrosion area (stability of dissolved species) at acidic pH

decreases, while the corrosion area at alkaline pH and the pH-independent corrosion area at

high potentials increase with temperature. The reason for this behaviour is related to the

temperature dependence of the ionic product of water. Secondly, the concentration of


11/15


53

-2

0

7

14

pHz5c

0 7 14

Pkc

0

7

PHNIO

2

w

2

1

0

-1

-2

0 7

Fig. 3. Predominance diagram for dissolved chromium species and lCr(aq)]tOt < IO@ m at 25, 100, 200 and

3OQC.

dissolved chromium species changes also the size of the different stability areas. The

immunity and passivity areas increase with increasing concentration, while the corrosion

areas decrease.

Thermodynamic calculations have been performed resulting in 24 diagrams,% but as all

these can not be shown in this paper, the results are summarised in Table 3, where P stands

for stability in the Pourbaix diagram, d stands for dissolved and mpesents appearance in


12/15

54 B.

Beverskog nd I. Puigdomenech

the predominance diagram for dissolved species. Unmarked species do not show in the

diagrams at any temperature at the activity levels used.

The Pourbaix diagrams (10m6 and IO-*m) and predominance diagrams calculated

for 25, 100, 200 and 300C are shown in Figs l-3. Pourbaix diagrams for chromium in

these figures are in fair agreement with those previously published at elevated

temperatures,

2 6 8-11 14 15 17 20

and they show a very base metal, as the immunity region

is situated below the hydrogen (H+/H$ line, Fig. 1. The dissolved species of chromium,

which represent corrosion, are Cr2+ and Cr3+

together with its four hydrolysis steps.

Chromium corrodes in acidic solutions to form Cr2+,

which is unstable and can oxidise

further to three or six valent forms. Depending on pH, temperature, and concentration,

Cr(II1) species can be either aqueous complexes or a solid compound (CrzOs) which

represents passivity. Alkaline solutions dissolve chromium to form the chromite species

(in this work represented by Cr(OH);, cf. the discussion in the section on

Thermochemical data).

Cr02 has no stability field, which was expected as it is well known that Cr(IV) is not

stable in contact with aqueous solutions. This is also in agreement with the calculations of

Silverman. I3

None of the hydroxides, the dioxide or the trioxide is the~odynamically

stable, owing to the stability of Cr203, and the calculated solubility of the hydroxides is

higher than that of Cr20s (not shown in the figures). Cr203 is not stable in solutions

containing strong oxidising agents due to the formation of soluble chromate (Cr(V1))

species (H&rod, HCrO; and CrOi-), which establish a corrosion area at all pH-values.

The electrochemical potential for a given pH value at which there is equilibrium between

Cr203 and the aqueous chromate species is lowered with increasing temperature. Ferritic

stainless steel corrodes in aerated hot caustic solutions, and this is confirmed by our

calculations (Fig. 1). This figure also shows that the predominance of each chromium(V1)

species (HzCrOd, HCrO;, and&O-) moves towards more alkaline pH-values with

increasing temperature. The passive area of chromium is below the upper limit of the

stability field of water (i.e. the oxygen/water line (OJH,O>) at all temperatures, implying

that chromium can corrode at potentials between the passive area and the oxygen line.

Potentials above this line dissolve chromium with oxygen evolution and formation of

Cr(VI) species inde~ndently of pH. The formation of the dichromate ion (Cr20:-) is

negligible in the investigated concentration range of [Cr(aq)],,, 5 10m6molal. This result is

contradictory to earlier published Pourbaix diagrams,9p12,8 but in agreement with

experimental evidence,

23 which shows that the dinuclear Cr(V1) anion is more stable than

HCrO; at high concentrations ( > lo-* M). The trioxide is very soluble in aqueous solution

and therefore does not have a stability field in the diluted concentrations used in this work.

Our diagrams calculated at elevated temperatures differ substantially from those in Refs

8,9, ll,20 and 2 1 in that previous studies predicted a substantially larger predominance of

the chromite anion over the stability of Crz03(cr). This arises from the uncertainty in the

nature and stoichiometry of the chromite ion, which is here denoted by CrfOH), but by

CrO; in Refs 8, 9, 1 I, 20 and 21, the difference being two water molecules (see also the

discussion in the previous section). Temperature extrapolations of thermodynamic data for

this ion have been performed in this work on an entropy value based on calorimetric

results,51 while previous Pourbaix diagrams at elevated temperatures have been based on a

method to estimate s for oxyanions6 Unfortunately, no experimental data is available to

ascertain the relative stabilities of the chromite ion over Cr;?Os(cr) (or Cr(OH)3(cr)), and this

matter remains unsolved for the time being.


13/15


The Pourbaix diagrams for chromium with lo-* molal concentration, Fig. 2, are very

similar to those at 10m6 molal, Fig. 1. The passive area (CrzOs) at lo-* molal is strongly

reduced, and at T > 150C no solid chromium compound is stable at these low

concentrations, in agreement with the calculated solubility of Cr203.0723

The predominance diagrams for dissolved chromium species, Fig. 3, contain the

oxidation numbers II, III and VI. The chromium species with valency two is C?. The

oxidation number III is represented by Cr3+, CrOH2+, Cr(OH)l, Cr(OH)3(aq), and

Cr OH); .

Six valent chromium species are H2Cr04(aq), HCrOiandCrOi-. All mentioned

aqueous species predominate in the whole temperature interval, with the exception of

H2Cr04(aq) which does not appear in the diagrams at 25C.

CONCLUSIONS

The revised Pourbaix diagrams for the system chromium-water at 25-300C and

concentrations of 10v6 and lo-* molal show that:

1. Cr(OH)2, Cr(OH)3, Cr02 and CrOs are not thermodynamically stable at any

temperature.

2. Cr20s(cr) is the only chromium oxide stable at equilibrium in aqueous solution, with

the exception of lo-* molal and T > 150C where no solid chromium compound is stable.

3. The uncharged aqueous complex, Cr(OH)s(aq), predominate at the concentration of

lo-* m and appears in all the predominance diagrams for dissolved species.

4. The dichromate ion, Cr@-(aq), does not predominate either in the Pourbaix or in

the predominance diagrams at [Cr(aq)ltot I 10e6 molal.

AcknowledgementsThe authors express their gratitude to S.-O. Pettersson for his skilful computer and editorial

help. This work was financed by the Swedish Nuclear Power Utilities.

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2.

3.

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I .

8.

9.

IO.

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12.

13.

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(1974).

P. A. Brook, Corr . Sci. 12, 291 1972).

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