distribucon especies cr 3

7/25/2019 Distribucon Especies Cr 3

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Pourbaix diagrams for chromium in concentrated aqueous lithium bromidesolutions at 25 C

M.J. Muoz-Portero, J. Garca-Antn *, J.L. Guin, V. Prez-Herranz

Departamento de Ingeniera Qumica y Nuclear, Universidad Politcnica de Valencia, P.O. Box 22012, E-46071 Valencia, Spain

a r t i c l e i n f o

Article history:

Received 23 September 2008Accepted 19 January 2009

Available online 23 January 2009

Keywords:

A. Chromium

A. Lithium bromide

C. Activity

C. Pourbaix diagram

a b s t r a c t

Pourbaix diagrams (electrode potential-pH diagrams) for CrBrH2O system at 25 C were developed in

400, 700, 850, and 992 g/L (4.61, 8.06, 9.79, and 11.42 M) LiBr solutions, common concentrations in dif-

ferent parts of absorption devices. The diagrams were compared with the simple CrH 2O system at 25 C.

Equilibria for the CrBrH2O system at 25 C were determined for bromide ion activities of 15.61,

194.77, 650.06, and 2042.65, which corresponded to the 400, 700, 850, and 992 g/L LiBr solutions, respec-

tively. Activities of all the dissolved species containing chromium were plotted for 106, 104, 102, and

100. Comparison of the simple CrH2O system at 25 C with the diagrams for CrBrH2O system at 25 C

showed that the dominant aqueous Cr(III) species in acid solutions was Cr +3 for Br activities of 15.61,

194.77, and 650.06, whereas it was CrBr+2 for Br activity of 2042.65. Aqueous CrBr+2 formed at a Br

activity higher than 943.05. The chromium solubility range in the acid area of the diagrams extended

slightly to higher pH values with increasing Br activity and decreasing water activity, as a result of

destabilization of Cr2O3.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Aqueous solutions containing high concentrations of lithium

bromide (LiBr) are used as absorbent solutions for almost all types

of heating and refrigerating absorption systems that use natural

gas or steam as energy sources [16]. Absorption units reduce

the use of chlorofluorocarbon (CFC) refrigerants and eliminate con-

cerns about lubricants in refrigerants. Although LiBr possesses

favourable thermophysical properties, it can cause serious corro-

sion problems on metallic components in refrigeration systems

and on heat exchangers in absorption plants [722].

With the advances in refrigeration technology new double ef-

fect LiBr absorption machines have been developed. These sys-

tems exhibit a higher energetic efficiency than simple effect

machines, although they also reach higher temperatures, which

cause important corrosion problems. In recent years, a number

of new, highly alloyed materials have been developed to meet

the increasing demands placed on corrosion resistance. Develop-

ments have led to the introduction of alloys with higher chro-

mium (Cr) and molybdenum (Mo) contents, which show much

promise of improved localized corrosion resistance. Cr is one of

the main elements responsible for the formation of the passive

film in stainless steels. These alloys, such as the high-alloy

austenitic stainless steels Alloy 31 (UNS N08031) and Alloy 33

(UNS R20033), have been studied in LiBr heavy brine solutions

[13,15,20,21]. In these investigations the alloys showed greatercorrosion resistance and more effective passivation behaviour

than the baseline stainless steels.

Chromium is a metallic element well known for its good corro-

sion properties. The reason for this is not due to the element itself,

which is a very reactive metal, but to one of its solid reaction prod-

ucts, Cr2O3. The element is protected in oxidising conditions by the

formation of Cr2O3, 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[23]. The good corrosion behaviour of chromium is the rea-

son to alloy metals with chromium thereby making corrosion

resistant alloys. Knowledge of the limits for the good corrosion

resistance is therefore an important issue. One way to predict the

corrosion resistance of chromium is to consider the thermodynam-

ics (equilibrium relations) of the system.

Chemical and electrochemical equilibria are commonly summa-

rised in Pourbaix diagrams, which are electrode potential-pH dia-

grams. Pourbaix diagrams have been used extensively to predict

and rationalize corrosion-related processes. Pourbaix diagrams

predict areas of immunity (no corrosion, by definition), passivation

(a solid reaction product), and corrosion (a dissolved reaction prod-

uct) [2430]. Pourbaix diagrams for chromium are particularly

useful to study the corrosion behaviour of many corrosion resistant

alloys, including stainless steel, as chromium plays an important

role in their corrosion resistance.

0010-938X/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2009.01.004

* Corresponding author. Tel.: +34 963877632; fax: +34 963877639.

E-mail address: [email protected](J. Garca-Antn).

Corrosion Science 51 (2009) 807819

Contents lists available at ScienceDirect

Corrosion Science

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2/13

The goal of the present work was the prediction of the general

conditions of immunity, passivation, and corrosion for chromium

in concentrated aqueous LiBr solutions. Pourbaix diagrams (elec-

trode potential-pH diagrams) for CrBrH2O system at 25 C were

developed in 400, 700, 850, and 992 g/L (4.61, 8.06, 9.79, and

11.42 M) LiBr solutions, common concentrations in different parts

of absorption devices. The diagrams were compared with the sim-

ple CrH2O system at 25

C.

2. Procedure

2.1. Standard Gibbs free energies of formation

Pourbaix diagrams were constructed from standard Gibbs free

energy of formation (DGof) data at 25 C for all the species consid-

ered. Sixteen chemical species were considered for the CrH2O

system:

1. Six solid species: Cr, Cr(OH)2, Cr2O3, Cr(OH)3(s), CrO2, and CrO3.

2. Ten aqueous species: Cr+2, Cr+3, CrOH+2, Cr(OH)2+, Cr(OH)3(aq),

Cr(OH)4, CrO4

2, HCrO4, H2CrO4, and Cr2O7

2.

For the construction of the Pourbaix diagrams for the CrBr

H2O system, two additional species were considered, as well as

the sixteen species for the CrH2O system:

1. One solid species: CrBr2.

2. One aqueous species: CrBr+2.

The DGofvalues on which the equilibria were based are listed in

Table 1, together with the oxidation number, the state of the spe-

cies, and the sources of the data [23,31,32]. A critical review of

published thermodynamic data has been performed for all the spe-

cies by Beverskog and Puigdomenech [23], and the best values

have been selected. The data for solid CrBr2 and aqueous CrBr+2

are those selected by Bard et al. [31].

2.2. Reactions

Equations of different reactions were written between all spe-

cies in the CrBrH2O system. Pairs of species (A and B) were con-

sidered in each reaction together with the H+ ion, the electrical

charge (e), H2O, and the Br ion. Thus, these reaction equations

had the following general form:

aA mH nebB cH2O dBr 1where A and B were the two species containing chromium involved

in the reaction.

For the CrH2O system, sixteen species were considered and the

number of reactions was 120. Eighteen species were considered for

the CrBr

H2O system, so that the number of reactions increasedfrom 120 to 153. Reactions were divided into four types:

1. Electrochemical reactions involving H+. These electrochemical

reactions depended both on the potential and the pH; they

could be represented by oblique lines in a Pourbaix diagram.

The number of reactions was 83 for the CrH2O system and

102 for the CrBrH2O system.

2. Electrochemical reactions not involving H+.These electrochemical

reactions were dependent on the potential and independent of

the pH; they could be represented by horizontal lines in a Pour-

baix diagram. The number of reactions was 5 for the CrH2O

system and 10 for the CrBrH2O system.

3. Chemical reactions involving H+. These chemical reactions were

independent of the potential and dependent on the pH; theycould be represented by vertical lines in a Pourbaix diagram.

The number of reactions was 27 for the CrH2O system and

34 for the CrBrH2O system.

4. Chemical reactions not involving H+. These chemical reactions

were independent both of the potential and the pH; they could

not be represented in a Pourbaix diagram, but they could be

considered when calculating the equilibrium conditions. The

number of reactions was 5 for the CrH2O system and 7 for

the CrBrH2O system.

The reactions were classified as: (1) homogeneous reactions(involving all the dissolved species), (2) heterogeneous reactions

with two solid species, and (3) heterogeneous reactions with one

solid species.

Equations of the reactions used for the construction of the Pour-

baix diagrams for the CrBrH2O system are shown inTables 25.

The reactions used for the simple CrH2O system are shaded.

Conventional procedures were followed to calculate the electro-

chemical and chemical equilibria from DGof data[24].

3. Results

Equilibria for the CrBrH2O system at 25C were deter-

mined for activities of bromide species representative of the test

Table 1

Standard Gibbs free energies of formation (DGof) at 25 C for the CrBrH2O system.

S pec ie s Ox ida tion numbe ra Stateb DGof (KJ/mol) Reference

H+ aq 0 [31,32]

H2 g 0 [31,32]

O2 g 0 [31,32]

H2O l 237.178 [31,32]

OH aq 157.293 [31,32]

Cr 0 s 0 [23]Cr(OH)2 II s 585.57 [23]

Cr2O3 III s 1053.09 [23]

Cr(OH)3(s) III s 873.17 [23]

CrO2 IV s 548 [23]

CrO3 VI s 510.04 [23]

Cr+2 II aq 174 [23]

Cr+3 III aq 215 [23]

CrOH+2 III aq 431.8 [23]

Cr(OH)2+ III aq 633.19 [23]

Cr(OH)3(aq) III aq 834.13 [23]

Cr(OH)4 III aq 1005.89 [23]

CrO42 VI aq 727.75 [23]

HCrO4 VI aq 765.14 [23]

H2CrO4 VI aq 764.00 [23]

Cr2O72 VI aq 1302.23 [23]

Br aq 103.97 [31,32]

CrBr2 II s

289 [31]CrBr+2 III aq 302 [31]

a Oxidation number for the chromium species.b aq, aqueous; s, solid; l, liquid; and g, gas.

Table 2

Electrochemical reactions not involving H+.

a

Solid species are typed in bold letters. Reactions used for the construction of thePourbaix diagram for the simple CrH2O system are shaded.

808 M.J. Muoz-Portero et al. / Corrosion Science 51 (2009) 807819


3/13

Table 3

Electrochemical reactions involving H+.

Table 4

Chemical reactions involving H+.

aSolid species are typed in bold letters. Reactions used for the construction of the

Pourbaix diagram for the simple CrH2O system are shaded.



M.J. Muoz-Portero et al. / Corrosion Science 51 (2009) 807819 809


4/13

solutions. Activities of the Br ion were calculated to be 15.61,

194.77, 650.06, and 2042.65, which corresponded to the 400,

700, 850, and 992 g/L (4.61, 8.06, 9.79, and 11.42 M) LiBr solu-

tions, respectively. These extremely high activities are a result

of the very large activity coefficients of the LiBr solutions. Calcu-

lation of activities of aqueous solutions of strong electrolytes

generally have been handicapped by a lack of activity coefficient

data. The DebyeHckel equation for predicting such activitycoefficients unfortunately applies only to dilute solutions having

concentrations far below those of common industrial interest.

Meissner and Kusik have developed a model to calculate the

activity coefficients of strong electrolyte in aqueous solutions

[3,3335], which was described in detail in a previous work

[36]. Activity coefficients of 400, 700, 850, and 992 g/L aqueous

LiBr solutions were calculated to be 2.973, 19.095, 49.472, and

124.78, respectively.

Activities of the Br ion were obtained by converting the molar

concentrations of 4.61, 8.06, 9.79, and 11.42 M to their correspond-

ing molal concentrations of 5.25, 10.20, 13.14, and 16.37 m, and

using molality-dependent activity coefficients. The densities of

the 400, 700, 850, and 992 g/L LiBr solutions used for converting

the molarity to molality were 1.2766, 1.4904, 1.5948, and

1.6896 g/cm3, respectively, which were calculated using the corre-

lation proposed by Patterson and Prez-Blanco[37].

Activities of water in 400, 700, 850, and 992 g/L LiBr solutions

were calculated to be 0.715, 0.358, 0.216, and 0.118, respectively,

according to the method proposed by Meissner and Kusik, whichis described in detail in the Appendix [34,35,38,39]. Activities of

water in 400, 700, 850, and 992 g/L LiBr solutions were obtained

using the ionic strength of 5.25, 10.20, 13.14, and 16.37, and the re-

duced activity coefficient of the pure solution at 25 C of 2.973,

19.095, 49.472, and 124.78, respectively. The activities of all the

dissolved species containing chromium were plotted for 106,

104, 102, and 100.

The Pourbaix diagram for the CrH2O system at 25 C is shown

inFig. 1. The Pourbaix diagrams for CrBrH2O system at 25C for

Br activities of 15.61, 194.77, 650.06, and 2042.65, and water

activities of 0.715, 0.358, 0.216, and 0.118 are presented in

Figs. 25, respectively. Lines on the diagrams delimit the stability

regions of the solid phases in equilibrium with 106, 104, 102,

and 100 activities of the soluble chromium species.

Simplified Pourbaix diagrams for chromium in H2O at 25 C are

shown in the absence of Br ion (Fig. 6a), for a Br activity of 15.61

(Fig. 6b), for a Br activity of 194.77 (Fig. 6c), for a Br activity of

650.06 (Fig. 6d), and for a Br activity of 2042.65 (Fig. 6e). These

diagrams were plotted for a 106 activity of the soluble chromium

species, which was used for demarcation between corrosion,

immunity, and passivation. The corrosion areas were shaded to dif-

ferentiate them from immunity and passivation.

The predominance diagram for the dissolved chromium species

at 25C in the absence of Br ions is shown inFig. 7. The predom-

inance diagrams for the dissolved chromium species at 25 C for

Table 5Chemical reactions not involving H+.



2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Cr+3

Cr+2

CrOH+2

Cr(OH)4

_

HCrO4

_

CrO4

_2

111

24

20

113

127

4

134

10_ 6

10_ 4

10_ 2

100

135

Cr

137

10_ 6

3510

_ 4

80

79

44

64

Cr O2 3

78

Fig. 1. Pourbaix diagram for the CrH2O system at 25 C.

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5/13

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

aCr

+3

Cr+2

CrOH+2

Cr(OH)4

_

HCrO4

_

CrO4

_ 2

111

20

113

127

4

24

134

10_ 6

10_ 4 80

79

64

Cr O2 3

Cr

135

100

10_ 2

78

44

10_ 410

_ 6

137

35

Fig. 2. Pourbaix diagram for the CrBrH2O system at 25 C for a Br activity of 15.61, and a water activity of 0.715 (equivalent to 400 g/L LiBr solution).

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Cr+3

Cr+2

CrOH+2

Cr(OH)4

_

HCrO4

_

CrO4

_ 2

111

20 127

24

80

79

64

Cr O2 3

Cr

44

10_ 6

137

35

4

78

113

134

135

100

10_ 6

10_ 4

10_ 2




6/13

Br

activities of 15.61, 194.77, 650.06, and 2042.65 and wateractivities of 0.715, 0.358, 0.216, and 0.118 are shown in Figs. 811, respectively. Lines on the diagrams only show equilibriabetween the dissolved species. The Pourbaix diagrams and

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

aCr

+3

CrOH+2

Cr(OH)4

_

HCrO4

_

CrO4

_ 2

111

20

113

127

24

79

64

Cr O2 3

Cr

44

10_ 6

137

35

4

134

135

100

10_ 6

10_ 4

10_ 2

Cr+2

78

80


2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

CrBr+2

Cr+2

CrOH+2

Cr(OH)4

_

HCrO4_

CrO4

_ 2

2

11

40

120

127

35

24

79

64

Cr O2 3

Cr

44

10_ 6

137

4

138

135

100

10 6

10_ 4

10_ 2

78

80_




7/13

predominance diagrams for the dissolved species are often super-

imposed to show the predominating dissolved species in each part

of the solid stability areas in a Pourbaix diagram. However, this canmake the diagram unclear and difficult to read, and it is avoided in

this work by separating the Pourbaix diagrams and the predomi-

nance diagrams for the dissolved species.

Coarse broken lines inFigs. 111,labelled a and b, limit thestability area of H2O at a partial pressure of gaseous species equal

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Alkalinecorrosion

Passivation

Immunity

Corrosion

Corrosion

Corrosion

Corrosion

Corrosion

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.0

b

E(V

)

SHE

a

Alkalinecorrosion

Passivation

Immunity

Corrosion

Corrosion

Corrosion

Corrosion

Corrosion

2.0

1.8

1.41.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8-2.0

b

E(V

)

SHE

a

Corrosion

Corrosion

Corrosion

Corrosion

Corrosion

Passivation

Alkalinecorrosion

Immunity

2.0

1.8

1.41.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8-2.0

b

E(V

)

SHE

a

Alkalinecorrosion

Passivation

Immunity

Corrosion

Corrosion

Corrosion

Corrosion

Corrosion

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.0

b

E(V

)

SHE

a

Alkalinecorrosion

Passivation

Immunity

Corrosion

Corrosion

Corrosion

Corrosion

Corrosion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

pH

a

c

e

d

b

Fig. 6. Simplified Pourbaix diagrams for chromium in H2O at 25 C (a) in the absence of Br ions, (b) for a Br activity of 15.61, (c) for a Br activity of 194.77, and (d) for a Br

activity of 650.06, and (e) for a Br activity of 2042.65, considering a 106 activity of the soluble chromium species.



8/13

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Cr+3

Cr+2

CrOH+2

Cr(OH)2

+

Cr(OH)3

(aq)

Cr(OH) 4

_

HCrO4

_

CrO4

_2

111

13

14

35

31

24

20

113

32

28

12

117

124

127

121

Fig. 7. Predominance diagram for the dissolved chromium species at 25 C.

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Cr+3

Cr+2

CrOH+2 Cr(OH)

2

+

Cr(OH)3

(aq)

Cr(OH)4

_

HCrO4

_

CrO4

_2

1 11

13

14

35

31

24

20

113

32

28

12

117

124

127

121

Fig. 8. Predominance diagram for the dissolved chromium species at 25 C for a Br activity of 15.61, and a water activity of 0.715 (equivalent to 400 g/L LiBr solution).



9/13

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Cr+3

Cr+2

CrOH+2

Cr(OH)2

+

Cr(OH)3

(aq)

Cr(OH) 4

_

HCrO4

_

CrO4

_2

111

13

14

35

31

24

20

113 23

27

12

117

124

127

121


2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

Cr+3

Cr+2

CrOH+2

Cr(OH)2

+

Cr(OH)3

(aq)

Cr(OH)4

_

HCrO4

_

CrO4

_2

111

13

14

35

31

24

20

113

23

27

12

117

124

127

121




10/13

to 1 atm. The upper line (a) represents the oxygen equilibrium line

(O2(g)/H2O(l)), and potentials above this line will oxidise H2O with

oxygen evolution. The lower line (b) represents the hydrogen equi-

librium line (H+(aq)/H2

(g)), and potentials below this line will re-

sult in hydrogen evolution. The potential values reported in this

work are always related to the standard hydrogen electrode

(SHE), which is considered to be zero at 25 C. All the diagrams

inFigs. 111were drawn using Autocad program.

It must be remarked that the Pourbaix diagrams for chromium

in concentrated aqueous LiBr solutions of 400, 700, 850, and 992 g/

L at 25 C are not hitherto available in the literature, and are origi-

nal in this work.

Thermodynamic stability of the chromium species in the Cr

BrH2

O system is summarised in Tables 69, for 106, 104,

102, and 100 activities of the soluble chromium species, respec-

tively, where P stands for stability in the Pourbaix diagram, and

d stands for stability in the predominance diagram for the dis-

solved chromium species. Unmarked species do not appear in the

diagrams at any Br activity at the activity values of the soluble

chromium species used.

2.0

1.8

1.4

1.6

0.8

1.0

0.4

-0.2

1.2

0.2

0.0

0.6

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 170

b

pH

E(V

)

SHE

a

CrBr+2

Cr+2

CrOH+2

Cr(OH)2

+

Cr(OH)3

(aq)

Cr(OH)4

_

HCrO4

_

CrO4

_ 2

211

13

14

35

31

24

40

12023

27

12

117

124

127

121


Table 6

Calculated thermodynamic stability of chromium species in the CrBrH2O system, considering a 106 activity of the soluble chromium species (P, appears in the Pourbaix

diagram; d, appears in the predominance diagram for the dissolved chromium species).

Species (Br) = 0 (Br) = 15.61 (Br) = 194.77 (Br) = 650.06 (Br) = 2042.65

(H2O) = 1 (H2O) = 0.715 (H2O) = 0.358 (H2O) = 0.216 (H2O) = 0.118

[LiBr] = 0 g/L [LiBr] = 400 g/L [LiBr] = 700 g/L [LiBr] = 850 g/L [LiBr] = 992 g/L

Cr P P P P P

Cr(OH)2Cr2O3 P P P P P

Cr(OH)3(s)

CrO2CrO3Cr+2 Pd Pd Pd Pd Pd

Cr+3 Pd Pd Pd Pd

CrOH+2 Pd Pd Pd Pd Pd

Cr(OH)2+ d d d d d

Cr(OH)3(aq) d d d d d

Cr(OH)4 Pd Pd Pd Pd Pd

CrO42 Pd Pd Pd Pd Pd

HCrO4 Pd Pd Pd Pd Pd

H2CrO4Cr2O7

2

CrBr2CrBr+2 Pd



11/13

Table 7




(H2O) = 1 (H2O) = 0.715 (H2O) = 0.358 (H2O) = 0.216 (H2O) = 0.118


Cr P P P P P

Cr(OH)2

Cr2O3 P P P P PCr(OH)3(s)


Cr+3 Pd Pd Pd Pd

CrOH+2 Pd Pd Pd Pd d

Cr(OH)2+ d d d d d





H2CrO4Cr2O7

2

CrBr2CrBr+2 Pd

Table 8




(H2O) = 1 (H2O) = 0.715 (H2O) = 0.358 (H2O) = 0.216 (H2O) = 0.118


Cr P P P P P


Cr(OH)3(s)


Cr+3 Pd Pd Pd Pd

CrOH+2 d d d d d

Cr(OH)2

+ d d d d d





H2CrO4Cr2O7

2

CrBr2CrBr+2 Pd

Table 9




(H2O) = 1 (H2O) = 0.715 (H2O) = 0.358 (H2O) = 0.216 (H2O) = 0.118


Cr P P P P P


Cr(OH)3(s)


Cr+3 Pd Pd Pd Pd

CrOH+2 d d d d d

Cr(OH)2+ d d d d d





H2CrO4Cr2O7

2

CrBr2CrBr+2 Pd



12/13

4. Discussion

The Pourbaix diagram for the simple CrH2O system at 25 C

(Fig. 1) shows that chromium is a very reactive metal, as the immu-

nity region is situated below the hydrogen equilibrium line

(H+(aq)/H2(g)). Chromium corrodes in acid-to-neutral solutions to

form Cr+2, which is unstable and can oxidise further to three or

six valent forms. Depending on pH and activity of the dissolvedchromium species, Cr(III) species can be either aqueous species

(Cr+3 and CrOH+2), which represents corrosion, or a solid com-

pound (Cr2O3), which represents passivation. An increase in the

activity of the dissolved chromium species from 106 to 100 results

in a decrease of pH value for the formation of Cr 2O3. For 102 and

100 activities of the dissolved chromium species, CrOH+2 is not sta-

ble. For very strong alkaline solutions, chromium corrodes through

the formation of Cr(OH)4. At high potentials, Cr(III) species oxidis-

es to form aqueous Cr(VI) species (HCrO4 and CrO4

2), which

establishes a corrosion area at all pH-values. An increase in the

activity of the dissolved chromium species from 106 to 100 results

in an increase of electrochemical potential for a given pH value at

which there is equilibrium between Cr2O3and the aqueous Cr(VI)

species. It can be concluded that the activity of the dissolved chro-

mium species changes the size of the different stability areas of

immunity, passivation, and corrosion. The immunity area (stability

of the metal itself) and the passivation area (stability of solid com-

pounds) increase with increasing the activity of the dissolved chro-

mium species. The corrosion area (stability of the dissolved

species) at acid-neutral pH, the corrosion area at alkaline pH, and

the pH-independent corrosion area at high potentials decrease by

increasing the activity of the dissolved chromium species.

Comparison of the simple CrH2O system at 25 C inFig. 1with

the diagrams for the CrBrH2O system at 25 C inFigs. 25show

that the dominant aqueous Cr(III) species in acid solutions is Cr+3

for Br activities of 15.61, 194.77, and 650.06, whereas it is CrBr +2

for a Br activity of 2042.65.

Chemical equilibrium between Cr+3 and CrBr+2 is shown in Eq.

(2). The equilibrium constant (K) at 25 C is given by Eq. (3):

Cr3 BrCrBr2 2

K CrBr2

Cr3Br

" #1:0604 103 3

Consequently, aqueous CrBr+2 forms at a Br activity higher

than 943.05, which corresponds to the 896.3 g/L (10.32 M) LiBr

solution.

Figs. 25 show that the chromium solubility range in the acid

area of the diagrams extends slightly to higher pH values with

increasing Br activity and decreasing water activity, as a result

of destabilisation of Cr2O3. For a 106 activity of the dissolved

chromium species, the precipitation of Cr2O3 is governed by the

chemical equilibrium with CrOH+2. The pH value of the precipita-tion of Cr2O3 decreases with increasing activity of the dissolved

chromium species from 106 to 100. For Br activities of 15.61,

194.77, and 650.06, CrOH+2 is not stable for 102 and 100 activi-

ties of the dissolved chromium species, as shown inFigs. 24; in

these cases, the precipitation of Cr2O3 is governed by the chemi-

cal equilibrium with Cr+3. For a Br activity of 2042.65, CrOH+2 is

not stable for 104, 102, and 100 activities of the dissolved chro-

mium species, as shown inFig. 5; in these cases, the precipitation

of Cr2O3 is governed by the chemical equilibrium between CrBr+2

and Br species. At high potentials, the Cr(III) species in acid solu-

tions (Cr+3 or CrBr+2) oxidise to form aqueous HCrO4. The oxida-

tion of Cr+3 or CrBr+2 to form HCrO4 occurs at both higher

potentials and higher pH values with the increase of Br ions

activity and decrease of water activity, as shown in Figs. 25.

Comparison of the simplified Pourbaix diagrams for chromium

in H2O in the absence and presence of Br ions (Fig. 6ae) shows

that the increase in Br activity and the decrease in water activity

shift the corrosion area at acid pH to higher pH values. The size of

the passivation area increases with increasing Br activity and

decreasing water activity, because the corrosion area at alkaline

pH shifts to higher pH values with increasing Br activity. The

immunity area increases slightly and the pH-independent corro-sion area at high potentials decreases slightly with increasing Br

activity and decreasing water activity.

Predominance diagram for the dissolved chromium species at

25C(Fig. 7) contains oxidation numbers II, III, and VI. The chro-

mium species with valency two is Cr+2. Oxidation number III is rep-

resented by Cr+3, CrOH+2, Cr(OH)2+, Cr(OH)3(aq), and Cr(OH)4

. Six

valent chromium species are HCrO4 and CrO4

2.

Figs. 811show that the dissolved chromium species at 25 C in

the presence and absence of Br ions are the same, except for Br

activity of 2042.65, where Cr+3 is not stable and CrBr+2 is the stable

dissolved chromium species with valency three in acid solutions.

Pourbaix diagrams for chromium in concentrated aqueous LiBr

solutions of 400, 700, 850, and 992 g/L at 25 C with precise delin-

eation of immunity, passivation, and corrosion regions provide ba-

sic data for studying the corrosion behaviour of high-alloy stainless

steels in refrigeration systems, as chromium plays an important

part in their corrosion resistance, and to interpret the results ob-

tained [13,15,20,21].

5. Conclusions

1. This article has described the construction of Pourbaix diagrams

for chromium in concentrated aqueous LiBr solutions of 400,

700, 850, and 992 g/L at 25C, based on Br activities of

15.61, 194.77, 650.06, and 2042.65, and water activities of

0.715, 0.358, 0.216, and 0.118, respectively, and on dissolved

chromium species activities of 106, 104, 102, and 100, which

were not hitherto available in the literature. Pourbaix diagrams

for chromium in concentrated aqueous LiBr solutions could beparticularly useful to study the corrosion behaviour of alloys

with high chromium content in refrigeration systems.

2. Comparison of the Pourbaix diagrams for the CrBrH2O sys-

tem at 25 C in 400, 700, 850, and 992 g/L LiBr with the simple

CrH2O system at 25 C shows that:

A. The dominant aqueous Cr(III) species in acid solutions is Cr+3

for Br activities of 15.61, 194.77, and 650.06 (equivalent to

the 400, 700, and 850 g/L LiBr solutions, respectively),

whereas it is CrBr+2 for a Br activity of 2042.65 (equivalent

to 992 g/L LiBr solution). Aqueous CrBr+2 forms at a Br activ-

ity higher than 943.05.

B. The chromium solubility range in the acid area of the dia-

grams extends slightly to higher pH values with increasing

Br activity and decreasing water activity, as a result ofdestabilization of Cr2O3.

C. The size of the passivation area increases with increasing Br

activity and decreasing water activity, because the corrosion

area at alkaline pH shifts to higher pH values with increasing

Br activity. The immunity area increases slightly and the

pH-independent corrosion area at high potentials decreases

slightly with increasing Br activity and decreasing water

activity.

Acknowledgements

The authors acknowledge the DGI (Direccin General de Inves-

tigacin, Convention no. CTQ2006-07820) and FEDER (Fondo

http://-/?-http://-/?-


13/13

Europeo de Desarrollo Regional) for the support of this work, Dra.

M. Asuncin Jaime for her translation assistance, and Antonio Jun-

cos for his assistance in plotting the diagrams.

Appendix A

A.1. Activity of water in strong electrolyte solutions

The activity of water (aow) is expressed as pw/pow, namely the ra-

tio of the vapor pressures over the single electrolyte solution and

over pure water, all at the temperature of interest. For a pure solu-

tion of a single strong electrolyte in aqueous solution, aow can be

calculated at any temperature by rearrangement and integration

of the GibbsDuhem equation, according with the method pro-

posed by Meissner and Kusik[34,35,38,39]:

logaow 0:0156I

z1z2 0:036

Z C

o

1

Id log Co A:1

whereIis the ionic strength, Co is the reduced activity coefficient of

the pure solution at 25 C,z1is the number of charges on the cation,

andz2is the number of charges on the anion (z1= z2= 1 for LiBr/H2O

solution).The ionic strength of a single electrolyte 12 composed of ions 1

and 2 in aqueous solution is related to its molality as follows:

I0:5m12m12z1z2 A:2wherem12 is the molality (moles per kilogram of water), and m12is

the moles the ions formed upon dissociation of one mole of electro-

lyte. For LiBr/H2O solution,I= m12.

For a single electrolyte solution at 25 C:

Co 1 B1 0:1Iq BC A:3

with:

B0:75 0:065q A:4

log C0:5107 ffiffiIp1 C

ffiffiIp A:5

C1 0:055q exp0:023I 3 A:6whereq is the Meissners parameter (q= 7.27 for LiBr).

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distribucon especies cr 3

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