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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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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
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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.
aSolid species are typed in bold letters. Reactions used for the construction of the
Pourbaix diagram for the simple CrH2O system are shaded.
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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+.
aSolid species are typed in bold letters. Reactions used for the construction of the
Pourbaix diagram for the simple CrH2O system are shaded.
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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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
Fig. 3. Pourbaix diagram for the CrBrH2O system at 25 C for a Br activity of 194.77, and a water activity of 0.358 (equivalent to 700 g/L LiBr solution).
M.J. Muoz-Portero et al. / Corrosion Science 51 (2009) 807819 811
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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
Fig. 4. Pourbaix diagram for the CrBrH2O system at 25 C for a Br activity of 650.06, and a water activity of 0.216 (equivalent to 850 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
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_
Fig. 5. Pourbaix diagram for the CrBrH2O system at 25 C for a Br activity of 2042.65, and a water activity of 0.118 (equivalent to 992 g/L LiBr solution).
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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.
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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).
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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
Fig. 9. Predominance diagram for the dissolved chromium species at 25 C for a Br activity of 194.77, and a water activity of 0.358 (equivalent to 700 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)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
Fig. 10. Predominance diagram for the dissolved chromium species at 25 C for a Br activity of 650.06, and a water activity of 0.216 (equivalent to 850 g/L LiBr solution).
M.J. Muoz-Portero et al. / Corrosion Science 51 (2009) 807819 815
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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
Fig. 11. Predominance diagram for the dissolved chromium species at 25 C for a Br activity of 2042.65, and a water activity of 0.118 (equivalent to 992 g/L LiBr solution).
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
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Table 7
Calculated thermodynamic stability of chromium species in the CrBrH2O system, considering a 104 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)2
Cr2O3 P P P P PCr(OH)3(s)
CrO2CrO3Cr+2 Pd Pd Pd Pd Pd
Cr+3 Pd Pd Pd Pd
CrOH+2 Pd Pd Pd Pd d
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
Table 8
Calculated thermodynamic stability of chromium species in the CrBrH2O system, considering a 102 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 d d d d d
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
Table 9
Calculated thermodynamic stability of chromium species in the CrBrH2O system, considering a 100 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 d d d d d
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
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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
818 M.J. Muoz-Portero et al. / Corrosion Science 51 (2009) 807819
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7/25/2019 Distribucon Especies Cr 3
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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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