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Revised pourbaix diagrams for chromium at 25–300 °C
Ccwrosion Science, Vol. 39, No. 1,pp.43-57, 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 25300°C
B. BEVERSKOG
FOR CHROMIUM
AT
and I. PUIGDOMENECH
Stud&k Material AB, S-61 1 82 Nykoping, Sweden
Abstract-The Pourbaix diagrams (potential-pH diagrams) for chromium at 25-300°C 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> 150°C 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
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%17,2swh’ic h 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 25°C. 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 25°C 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 sand 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
45
Pourbaix diagrams for Cr Table 1. Considered species in the aqueous system of chromium Condition
Oxidation number
Included
crystalline solid crystalline solid 9,
0 I
Cr
gaseous crystalline “ 3, 7. 9. ,, ,, 3, 3. “ solid crystalline “ 3, dissolved “ >, 7, 7, 7, ,, ,, ,. “ 7, 1, 1. z
II ‘1
CrH cro
CrW-02 CrH2 CrH2
31
II/III III “
cr304
Cr203
Cr203. H20 CrzOs. 2 Ha0 Cr203. 3 Hz0
“
“
WOW3 Cr(OH)s
“
VI VIII X II III ‘1 “ “ “ “ “ V VI “ ‘1 “
n Hz0
CrOOH Cr2H7
“
III/IV IV ..
Excluded
CrOz Cr(OH)4 CrOs
Cr*+ Cr3 + CrOH’+ Cr(OH): Cr(OHMaq) Cr(OH); (?)
H2Cr04 HCrOi CrOiCr@ 6+10=16
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.22,23,39CrOi-(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
B. Beverskog and 1. Puigdomenech
46
Table 2. Thermodynamic data at 25°C 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 c(T)/(J
Species Cr(cr) Cr(OHMs) Cr203W
Cr(OHX(s) CrOa(cr) CQfcr) Cr2 + Cr’+ CrOH*’
Cr(OH)z + Cr(OH)s(aq) chromite aniod CrO:HCrO;
H$XUaq) Cr_@-
S” (J K-t mol-‘)
Arc0 (kJ mol-‘) 0.
23.6 El. 81.2 105 54 73.2
- 585.57 - 1053.09 -873.17 - 548 -510.04 -174 -215 -431.8 -633.19 -834.13 - 1005.89
21.76 116.064 119.37 127.612 94.6 71.76
-100
-11
-293 -151 -92 -38 -83.8
-30 160 340 480 410
50.21 195.2 311 299.5
- 727.75 -765.14 - 764.00 - 1302.23
a*
K-’ mol-‘) =a+bT+cT-= bx IO3
cxw6
8.98 8.648 9.20 41.639 17.2 87.86
-0.096: -2.874 - 1.565 -4.217 - 1.674
-251 -50 84 - 175
+For aqueous species “a” corresponds to the standard partial molar heat capacity at 25°C and the revised Helgeson-Kirkham-Flowers model has been used to obtain its temperature dependence. XC$Cr(cr), T)/(J K-’ mol-‘) = o + bT + CT-~ + dT*, 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 25°C 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 partial 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 and our methodology requires a value of Cj at 25°C. model of Helgeson et uZ.28--31
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 aL3* For have been used. The parameters for the T-equation of Cr02(s), the values listed in 33Y34 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,36Y37while for Cr(OH)3(s) the upper limit for the solubility constant proposed in the careful study of Rai et al.38 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(25”C) 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 25°C 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 Vasil’ev et a1.45 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 P J K-t mol-’ have been derived from the work of Spitzer et al.48-50 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 and chromatographic study” has confirmed the membranes.5’ A spectroscopic 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
48
B. Beverskog and I. Puigdomenech
the chromite ion, and therefore in erroneous Pourbaix diagrams at elevated temperatures, because this extrapolation relies (among other parameters) on the electric charge of the aqueous species, which is unknown for the chromite ion. The equilibrium constants (or lower limits) for the hydrolysis reactions of Cr(III) reported by Rai et a1.38 have been used to obtain the values of AfG”forCrOH2+, Cr(OH)l, Cr(OH),(aq), and Cr(OH), . The standard entropy changes for Cr3+ + HzO(1) + CrOH2+ + H+ CrOH2+ + H20(1) $ Cr(OH)l
+ H+
proposed by Dellien et ~1.~~(A,@ = 41.8 and 32.6 kJ . mol-‘, respectively) are used to obtain the entropies of the corresponding hydrolysis complexes. The data of Schug and King” are used to obtain the standard entropy of the chromite ion (assuming the stoichiometry Cr(OH);). The value of S”(Cr(OH),(aq)) has been estimated by assuming a smooth variation of A,@ for the stepwise hydrolysis reactions. The values of q for the hydrolysis products of Cr(II1) have been obtained by assuming the same standard heat capacity changes as for the hydrolysis of Fe(III).27 Values for the chromate ion, CrOi-, are those in Refs 34 and 29. The data for HCrO; and Cr@ have been calculated from the values for CrOz- and the values of A$ and the equilibrium constants in the temperature range 255175°C obtained by Palmer et d5’ It must be noted that the resulting values of S” differ from those in Ref. 34 and the C; values differ by as much as + 50 J K.-l mol-’ with those determined in Ref. 54. For H$ZrO,+(aq), the equilibrium constant at I= 0 recommended by Baes and Mesmer23 has been used to obtain ArG”(H2Cr04(aq)), and the reaction enthalpy quoted in Table 10.9 of the same reference has been used to obtain ~(H~~rO~(aq)), and a value for ($ for this aqueous complex has been estimated from the heat capacities of dissociation for oxy-acids proposed by Smith et aI.s5
CALCULATIONS The methods and assumptions used to caiculate the chemical equilibrium diagrams (including Pourbaix diagrams) have been described elsewhere.“’ Calculations to draw the Pourbaix diagrams presented in this work have been performed for eight temperatures in the interval 25-300°C (i.e. at 25,50,100, 150,200,250,285 and 3OO”Cj.Pourbaix diagrams have been calculated in this work at two concentration levels, 10m6 and 10-8molal, at every temperature. The former is the conventional corrosion limit stipulated by Pourbaix, and the latter is intended to be used in high purity waters, such as in nuclear reactors of the boiling water type. These concentrations are total concentration, i.e. the sum of all aqueous species containing chromium at each coordinate point (Es&pH-value). All values of pH given in this work are values at the specified temperature. The neutral pH value of pure water changes with the tem~rature with the ion product of water (HzO(Q e H’ + OH-, ~Hneutra~ = 1/2pK,,r). To facilitate reading the Pourbaix diagrams, the neutral pH value at the temperature of each diagram is given as a vertical dotted line.
Pourbaix diagrams for Cr
49
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 all 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 today’s 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 10m6and 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 25°C and the Pourbaix diagrams reported here at T > 25°C 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 60-500°C.
50
B. Beverskog
and I. Puigdomenech
Ziemniak5’ also gives calculated solubilities for CrOOH(s) based on unpublished results by Ziemniak et al. According to Laubengayer and Macune 58the 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
~Hac
7
7 PH~wo~~
pHam% Fig. 1.
Pourbaix
diagram
for chromium
and [Cr(aq)bt
= 10m6 m at 25, 100,200
and 300°C.
Pourbaix diagrams for Cr
51
passive film at prolonged exposures. This view is indirectly confirmed by several studies which show that the passive film on chromium consists of Cr203.60’6’ CrOOH has been found in short time experiments analysing the passive layer ex situ by XPS (X-ray Photoelectron Spectroscopy).62 In situ 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 diagrams879Y’5incorrectly 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 25°C 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) 25°C
50°C
100°C
150°C
200°C
250°C
285°C
3WC
Cr(cr) Cr(OHMcr) Cr(OHMcr)
4
Pp
PP
PP
PP
PP
Pp
PP
Cr203@d
PP
PP
PP
4
PP
PP
PP
Pp
Species
Cfl2W C*3W
C;L+(aq) Cr3 + (aq) CrOH’+(aq) Cr(OH):(aq) Cr(OHMas) Cr(OH);(aq) HDWaq) HCrWaq) Cro2,-(aq) CrzO:-(aq)
Ppd W W Ppd d Ppd Ppd F$
Ppd
Ppd
W W
4d Ppd 4d
w
&
W Ppd Ppd
:d
Ppd Ppd 4d
B. Beverskog
and I. Puigdomenech
2
1
c5
t L
0
W -1
7
0
14
-2
0
Mnc
0
Fig. 2.
diagram
14
PHlaJS
7 PHNOOOC Pourbaix
7
for chromium
0
7 PHsEos
and [Cr(aq)ltot
= lo-’
m at 25, 100,200
and 300°C.
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
53
Pourbaix diagrams for Cr
-2
0
7
0
14
pHz5c
7 PHNIOF
2
1
2
0
w
-1
-2 0
7
14
0
7
Pkc Fig. 3. Predominance
diagram for dissolved chromium species and lCr(aq)]tOt < IO@ m at 25, 100, 200 and 3OQ”C.
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
54
B. Beverskogand 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 300°C 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, 1I, 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.
Pourbaix diagrams for Cr
55
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 > 150°C 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 25°C.
CONCLUSIONS The revised Pourbaix diagrams for the system chromium-water concentrations of 10v6 and lo-* molal show that:
at 25-300°C and
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 > 150°C 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.
REFERENCES 1. E. Deltombe, N. de Zubov and M. Poutbaix, in Atlas of Electrochemical Equilibria in Aqueous Solution (ed. M. Pourbaix), Pergamon Press, New York, (1966), CEBELCOR, Brussels, (1966), NACE, Houston, TX, (1974). 2. P. A. Brook, Corr. Sci. 12, 291 (1972). 3. M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum Press, London, (1973). 4. M. Pourbaix, J. Electrochem. Sot. 123, 25C (1976). 5. J. D. Hem, Geochim. Cosmochim. Acta 41, 527 (1979). 6. D. D. Macdonald, B. C. Syrett and S. S. Wings, Corr. 35, 1 (1979). I. B. C. Syrett, D. D. Macdonald and H. Shih, Corr. 36, 130 (1980). 8. J. B. Lee, Corr. 37, 467 (1981). 9. H. Hirano, M. Mayuzmni and T. Kurosawa, Boshoku Gijutsu 31, 517 (1982). IO. P. Radkakrishnamurty and P. Adaikkalam, Corr. Sci. 22, 753 (1982). 11. C. M. Chen, K. Aral and G. J. Theus, EPRI-NP-3137,l & 2, (1983). 12. B. Maxxa and P. Pedeferri, 84 Riunione Annuale-Papers-Associazione Elettrotecnica ed Elettronica Italiana, Cagliari, Italy, paper A30, (Oct. 1983). 13. D. C. Silverman, Corr. 39, 488 (1983).
56
B. Beverskog and 1. Puigdomenech
14. D. Cubicciotti and L. Ljungberg, J. Electrochem. Sot. 132, 987 (1985). 15. H. Hirano and T. Kurosawa, Boshoku Gijursu 34, 92 (1985). 16. C. W. Bale, A. D. Pelton and W. T. Thompson, Cornpurer aided aquisi~ionand analysis of corrosion data (ed. M. W. Kendig), ISSN 0161-6374 85-3, 165, (1985). 17. P. L. Daniel and S. L. Harper, EPRI-NP-4831, (1986). 18. J. C. Angus, B. Lu and M. J. Zappia, .I. E~e~~ro~hem.Sot. 17, 1 (1987). 19. D. G. Brookins, Eh-pH diagrams for geochemistry, Springer-Verlag, (1988). 20. D. Cubicciotti, J. Nut. Mat. 167, 241 (1989). 21. D. Cubicciotti, J. Nucl. Mat. 201, 176 (1993). 22. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, John Wiley, New York, 832, (1976). 23. C. F. Baes Jr and R. E. Mesmer, The Hydrolysis of Cations, John Wiley, New York, (1976). 24. G. Wolf, Phys. Stat. Sol. (a] 5, 627 (1971). 25. G. Wolf, 2. Phys. Chem. (Leipzig] 246, 403 (1971). 26. B. Baranowski and K. Bojarski, Roczniki Chem. 46, 1403 (1972). 27. B. Beverskog and I. Puigdomenech, Corros. SC;., in press (1996). 28. J. C. Tanger IV and H. C. Helgeson, Am. 1. Sci., t88, 19, (1988). 29. E. L. Shock and H. C. Helgeson, Geochim. Cosmochim. Acta, 52, 2009, (1988) Errata: 53, 215, (1989). 30. E. L. Shock, H. C. Helgeson and D. A. Sverjensky, Geochim. Cosmochim. Acra 53, 2157 (1989). 31. E. L. Shock, E. H. Oelkers, J. W. Johnson, D. A. Sverjensky and H. C. Heigeson, J. Chem. Sot., Faraday Trans. 88, 803 (1992). 32. 0. Kubaschewski, C. B. Afcock and P. J. Spencer, Materials rhermochemjsfry, Pergamon Press, Oxford, 6th edition, (1993). 33. 1. Dellien, F. M. Hall and L. G. Hepler, Chem. Rev. 76, 283 (1976). 34. K. Niki, Chromium, in Standard Potentials in Aqueous Solution (eds: A. J. Bard, R. Parsons and J. Jordan), Int. Union Pure Appl. Chem., Marcel Dekker, New York, 453, (1985). 35. I. Barin, Thermochemical Data of Pure Substances, VCH Verlagsgesellschaft. Weinheim, Germany, 2nd ed., Part I, 433, (1993). 36. D. N. Hume and H. W. Stone, J. Am. Chem. Sot. 63, 1197 (1941). 37. W. M. Latimer, The oxidation states ofthe elements and their potentials in aqueous solutions, Prentice-Hall, Englewood Cliffs, N. J., 2nd edition, (1952). 38. D. Rai, B. M. Sass and D. A. Moore, Inorg. Chem. 26, 345 (1987). 39. G. B. Naumov, B. N. Ryzhenko and I. L. Khodakovskiy, Handbook of thermodynamic data, Atomizdat, Moscow, 1971 (in Russian). Engl. transl. : report USGS-WRD-74-001 (eds. G. J. Soleimani, I. Barnes, and V. Speltz), U.S. Geological Survey, Menlo Park, California, (1974). 40. 0. Knacke, 0. Kubaschewski and K. Hesselmann, (eds.), Thermochemicalproperties inorganicsubstances, Springer-Verlag, Berlin, 2nd edition, (1991). 41. J. D. Cox, D. D. Wa~an and V. A. Medvedev, CODATA key valuesfor fhermodynum~cs, Hemisphere Publ. Co., New York, (1989). 42. A. Saul and W. Wagner, J. Phys. Chem. Ref Data 18, 1537 (1989). 43. D. G. Archer and P. Wang, f. Phys. Chem. Ref. Data 19, 371 (1990). 44. I. Dellien and L. G. Hepler, Can. J. Chem. 54, 1383 (1976). 45. V. P. Vasil’ev, V. N. Vasil’eva, 0. G. Raskova and V. A. Medvedev, Russ. J. Inorg. Chem. 25, 861 (1980). 46. R. L. Schmidt, Technical Report PNL-4881, Pacilic Northwest Lab., Richland, Washington, (1984). 47. D. C. Sassani and E. L. Shock, Geochim. Cosmochim. Acta, 56, 3895, (1992): Errata: 58, 2756, (1994). 48. J. J. Spitzer, P. P. Singh, I. V. Olofsson and L. G. Hepler, J. Solution Chem. 7, 623 (1978). 49. J. J. Spitzer, I. V. Olofsson, P. P. Singh and L. G. Hepler, J. Chem. Thermody~ics 11, 233 (1979). 50. J. J. Spitzer, I. V. Olofsson, P. P. Singh and L. G. Hepler, Can. J. Chem. 57, 2798 (1979). 51. K. Schug and E. L. King, J. Am. Chem. Sot. 80, 1089 (1958). 52. S. M. Bradley, C. R. Lehr and R. A. Kydd, J. Chem. Sot., Dalton Trans., 2415, (1993). 53. D. A. Palmer, D. Wesolowski and R. E. Mesmer, J. Soiurion Chem. 16,443 (1987). 54. J. K. Hovey and L. G. Hepler, J. Phys. Chem. 94, 7821 (1990). 55. R. W. Smith, C. J. Popp and D. I. Norman, Geochim. Cosmochim. Acta 50, 137 (1986). 56. M. W. Shafer and R. Roy, Zh. Anorg. Allgem. Chem. 276,275 (1954). 57. S. E. Ziemniak, J. Solution Chem. 21, 745 (1992). 58. A. W. Laubengayer and H. W. Macune, J. Am. Chem. 74,2362 (1974). 59. D. Costa, W. P. Wang and P. Marcus, Mat. Sci. Forum, 185-188, 325, (1995).
of
Pourbaix diagrams for Cr 60. 61. 62. 63. 64. 65.
57
M. A. Genshaw and R. S. Sirohi, J. Electrochem. Sot 118, 1558 (1971). M. Seo, R. Saito and N. Sato, J. Electrochem. Sot 127, 1909 (1980). L. Bjiimkvist and I. Olefjord, Corr. Sci. 32, 231 (1991). A. M. Sukhotin, E. I. Antonovskaya and A. A. Pozdeeva, Russ. J. Phys. Chem. 36, 1284 (1962). A. M. Sukhotin and A. A. Borodkina, Electrokim. 13, 296 (1976). A. M. Sukhotin, N. K. Khoreva, Yu. P. Kostikov, I. G. Vasilkova and M. N. Shlepakov, Elektrokim. 16, 1403 (1979). 66. B. Beverskog, Studsvik/M-91/52, Restricted distribution. In Swedish, (1992). 67. R. E. Connick and R. E. Powell, J. Chem. Phys. 21, 2206 (1953).
PII: s~l~9~x(%~lol-l
REVISED
POURBAIX
DIALOGS 25300°C
B. BEVERSKOG
FOR CHROMIUM
AT
and I. PUIGDOMENECH
Stud&k Material AB, S-61 1 82 Nykoping, Sweden
Abstract-The Pourbaix diagrams (potential-pH diagrams) for chromium at 25-300°C 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> 150°C 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
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%17,2swh’ic h 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 25°C. 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 25°C 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 sand 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
45
Pourbaix diagrams for Cr Table 1. Considered species in the aqueous system of chromium Condition
Oxidation number
Included
crystalline solid crystalline solid 9,
0 I
Cr
gaseous crystalline “ 3, 7. 9. ,, ,, 3, 3. “ solid crystalline “ 3, dissolved “ >, 7, 7, 7, ,, ,, ,. “ 7, 1, 1. z
II ‘1
CrH cro
CrW-02 CrH2 CrH2
31
II/III III “
cr304
Cr203
Cr203. H20 CrzOs. 2 Ha0 Cr203. 3 Hz0
“
“
WOW3 Cr(OH)s
“
VI VIII X II III ‘1 “ “ “ “ “ V VI “ ‘1 “
n Hz0
CrOOH Cr2H7
“
III/IV IV ..
Excluded
CrOz Cr(OH)4 CrOs
Cr*+ Cr3 + CrOH’+ Cr(OH): Cr(OHMaq) Cr(OH); (?)
H2Cr04 HCrOi CrOiCr@ 6+10=16
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.22,23,39CrOi-(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
B. Beverskog and 1. Puigdomenech
46
Table 2. Thermodynamic data at 25°C 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 c(T)/(J
Species Cr(cr) Cr(OHMs) Cr203W
Cr(OHX(s) CrOa(cr) CQfcr) Cr2 + Cr’+ CrOH*’
Cr(OH)z + Cr(OH)s(aq) chromite aniod CrO:HCrO;
H$XUaq) Cr_@-
S” (J K-t mol-‘)
Arc0 (kJ mol-‘) 0.
23.6 El. 81.2 105 54 73.2
- 585.57 - 1053.09 -873.17 - 548 -510.04 -174 -215 -431.8 -633.19 -834.13 - 1005.89
21.76 116.064 119.37 127.612 94.6 71.76
-100
-11
-293 -151 -92 -38 -83.8
-30 160 340 480 410
50.21 195.2 311 299.5
- 727.75 -765.14 - 764.00 - 1302.23
a*
K-’ mol-‘) =a+bT+cT-= bx IO3
cxw6
8.98 8.648 9.20 41.639 17.2 87.86
-0.096: -2.874 - 1.565 -4.217 - 1.674
-251 -50 84 - 175
+For aqueous species “a” corresponds to the standard partial molar heat capacity at 25°C and the revised Helgeson-Kirkham-Flowers model has been used to obtain its temperature dependence. XC$Cr(cr), T)/(J K-’ mol-‘) = o + bT + CT-~ + dT*, 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 25°C 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 partial 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 and our methodology requires a value of Cj at 25°C. model of Helgeson et uZ.28--31
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 aL3* For have been used. The parameters for the T-equation of Cr02(s), the values listed in 33Y34 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,36Y37while for Cr(OH)3(s) the upper limit for the solubility constant proposed in the careful study of Rai et al.38 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(25”C) 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 25°C 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 Vasil’ev et a1.45 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 P J K-t mol-’ have been derived from the work of Spitzer et al.48-50 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 and chromatographic study” has confirmed the membranes.5’ A spectroscopic 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
48
B. Beverskog and I. Puigdomenech
the chromite ion, and therefore in erroneous Pourbaix diagrams at elevated temperatures, because this extrapolation relies (among other parameters) on the electric charge of the aqueous species, which is unknown for the chromite ion. The equilibrium constants (or lower limits) for the hydrolysis reactions of Cr(III) reported by Rai et a1.38 have been used to obtain the values of AfG”forCrOH2+, Cr(OH)l, Cr(OH),(aq), and Cr(OH), . The standard entropy changes for Cr3+ + HzO(1) + CrOH2+ + H+ CrOH2+ + H20(1) $ Cr(OH)l
+ H+
proposed by Dellien et ~1.~~(A,@ = 41.8 and 32.6 kJ . mol-‘, respectively) are used to obtain the entropies of the corresponding hydrolysis complexes. The data of Schug and King” are used to obtain the standard entropy of the chromite ion (assuming the stoichiometry Cr(OH);). The value of S”(Cr(OH),(aq)) has been estimated by assuming a smooth variation of A,@ for the stepwise hydrolysis reactions. The values of q for the hydrolysis products of Cr(II1) have been obtained by assuming the same standard heat capacity changes as for the hydrolysis of Fe(III).27 Values for the chromate ion, CrOi-, are those in Refs 34 and 29. The data for HCrO; and Cr@ have been calculated from the values for CrOz- and the values of A$ and the equilibrium constants in the temperature range 255175°C obtained by Palmer et d5’ It must be noted that the resulting values of S” differ from those in Ref. 34 and the C; values differ by as much as + 50 J K.-l mol-’ with those determined in Ref. 54. For H$ZrO,+(aq), the equilibrium constant at I= 0 recommended by Baes and Mesmer23 has been used to obtain ArG”(H2Cr04(aq)), and the reaction enthalpy quoted in Table 10.9 of the same reference has been used to obtain ~(H~~rO~(aq)), and a value for ($ for this aqueous complex has been estimated from the heat capacities of dissociation for oxy-acids proposed by Smith et aI.s5
CALCULATIONS The methods and assumptions used to caiculate the chemical equilibrium diagrams (including Pourbaix diagrams) have been described elsewhere.“’ Calculations to draw the Pourbaix diagrams presented in this work have been performed for eight temperatures in the interval 25-300°C (i.e. at 25,50,100, 150,200,250,285 and 3OO”Cj.Pourbaix diagrams have been calculated in this work at two concentration levels, 10m6 and 10-8molal, at every temperature. The former is the conventional corrosion limit stipulated by Pourbaix, and the latter is intended to be used in high purity waters, such as in nuclear reactors of the boiling water type. These concentrations are total concentration, i.e. the sum of all aqueous species containing chromium at each coordinate point (Es&pH-value). All values of pH given in this work are values at the specified temperature. The neutral pH value of pure water changes with the tem~rature with the ion product of water (HzO(Q e H’ + OH-, ~Hneutra~ = 1/2pK,,r). To facilitate reading the Pourbaix diagrams, the neutral pH value at the temperature of each diagram is given as a vertical dotted line.
Pourbaix diagrams for Cr
49
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 all 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 today’s 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 10m6and 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 25°C and the Pourbaix diagrams reported here at T > 25°C 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 60-500°C.
50
B. Beverskog
and I. Puigdomenech
Ziemniak5’ also gives calculated solubilities for CrOOH(s) based on unpublished results by Ziemniak et al. According to Laubengayer and Macune 58the 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
~Hac
7
7 PH~wo~~
pHam% Fig. 1.
Pourbaix
diagram
for chromium
and [Cr(aq)bt
= 10m6 m at 25, 100,200
and 300°C.
Pourbaix diagrams for Cr
51
passive film at prolonged exposures. This view is indirectly confirmed by several studies which show that the passive film on chromium consists of Cr203.60’6’ CrOOH has been found in short time experiments analysing the passive layer ex situ by XPS (X-ray Photoelectron Spectroscopy).62 In situ 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 diagrams879Y’5incorrectly 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 25°C 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) 25°C
50°C
100°C
150°C
200°C
250°C
285°C
3WC
Cr(cr) Cr(OHMcr) Cr(OHMcr)
4
Pp
PP
PP
PP
PP
Pp
PP
Cr203@d
PP
PP
PP
4
PP
PP
PP
Pp
Species
Cfl2W C*3W
C;L+(aq) Cr3 + (aq) CrOH’+(aq) Cr(OH):(aq) Cr(OHMas) Cr(OH);(aq) HDWaq) HCrWaq) Cro2,-(aq) CrzO:-(aq)
Ppd W W Ppd d Ppd Ppd F$
Ppd
Ppd
W W
4d Ppd 4d
w
&
W Ppd Ppd
:d
Ppd Ppd 4d
B. Beverskog
and I. Puigdomenech
2
1
c5
t L
0
W -1
7
0
14
-2
0
Mnc
0
Fig. 2.
diagram
14
PHlaJS
7 PHNOOOC Pourbaix
7
for chromium
0
7 PHsEos
and [Cr(aq)ltot
= lo-’
m at 25, 100,200
and 300°C.
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
53
Pourbaix diagrams for Cr
-2
0
7
0
14
pHz5c
7 PHNIOF
2
1
2
0
w
-1
-2 0
7
14
0
7
Pkc Fig. 3. Predominance
diagram for dissolved chromium species and lCr(aq)]tOt < IO@ m at 25, 100, 200 and 3OQ”C.
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
54
B. Beverskogand 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 300°C 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, 1I, 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.
Pourbaix diagrams for Cr
55
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 > 150°C 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 25°C.
CONCLUSIONS The revised Pourbaix diagrams for the system chromium-water concentrations of 10v6 and lo-* molal show that:
at 25-300°C and
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 > 150°C 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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of
Pourbaix diagrams for Cr 60. 61. 62. 63. 64. 65.
57
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