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Flow-assisted corrosion of steel and the influence of Cr and Cu additions
259
Journal of Nuclear Materials 152 (1988) 259-264 North-Holland, Amsterdam
FLOW-ASSISTED CORROSION AND Cu ADDITIONS Daniel
OF STEEL AND THE INFLUENCE
OF Cr
CUBICCIO’MI
Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303, USA
Received 2 August 1987; accepted 1 December 1987
Flow-assisted corrosion (FAC) of steel feedwater lines occurs by dissolution of the surface oxide layer on the steel. The solubility of iron in water under FAC conditions is discussed through the use of potential-pH diagrams (Pourbaix diagrams). Alloying additions of chromium and copper both decrease FAC. An assessment is presented that Cr additions decrease FAC by forming a mixed oxide with iron instead of a pure iron oxide. The solubility of iron from the mixed oxide is smaller than for pure iron oxide and leads to a smaller FAC rate. The stable form of copper under FAC conditions is not the mixed iron-copper oxide but metallic copper, which may act in the underlying steel surface to impede FAC.
1. Introduction
Flow-assisted corrosion (FAC) of steel piping can be severe enough to cause failure of high-velocity hot water lines [l-3]. In a recent case, the walls of a carbon steel feedwater line had been thinned by FAC to the point of rupture by ductile overloading when the pipe was not able to sustain the operating pressure [4]. This kind of attack has also been called erosion-corrosion because of the worn appearance of the metal surface; however, FAC is more descriptive of the mechanism of the process, which does not involve mechanical erosion in carbon and low alloy steels. Substantial research has been performed to establish the key factors that influence FAC [S], which are consistent with a mechanism in which iron dissolves in the flowing water. The factors which affect FAC of steel in water can be grouped into the following: (1) flow rate (hydrodynamics), (2) material (alloying components), (3) environment (water chemistry). These can be represented schematically to indicate their conjoint action as in fig. 1. The mechanism for FAC, derived from laboratory studies [2], is schematically illustrated in fig. 2. Iron reacts with water to form a surface oxide layer. This oxide dissolves in the water, and the rate of iron removal (i.e. the rate of FAC) is controlled by the rate of diffusion of dissolved iron species through the boundary layer of water near the surface into the bulk water. This diffusion (or mass transport of iron away from the 0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
surface) depends directly on the concentration of soluble iron species at the oxide surface and inversely on the thickness of the boundary layer. Thus, a decrease of the boundary layer thickness because of increased water flow rate or because of local turbulence causes an increase of corrosion rate.
Environment
Fig. 1. Diagram of interaction of key controllable jointly lead to flow-assisted corrosion.
B.V.
factors
that
260
D. Cubicciotti
/ Flow assisted corrosion of steel
2. Solubility of iron oxide
1.0
r
The concentration of dissolved iron at the oxide surface is controlled by the solubility of the oxide, which can be evaluated as follows. The potential-pH diagram for the Fe-H,0 system, shown in fig. 3, presents the equilibrium information available for the solid oxides and water-soluble species for Fe. It is necessary to know the conditions that prevail in a typical system to specify the region of interest in this diagram. In a recent FAC rupture [4], the temperature was about 2OO’C and the pH of the water (measured at room temperature) was about 8.9 controlled by NH, addition. The inside surface of the pipe was coated with
Bulk water flow
Diffusion of corrosion products
Fig. 2. Schematic representation of the mechanism of flow-assisted corrosion of steel. Note
to fig.
2
4
6 pH at Temperature
8
10
Fig. 3. Potential-pH diagram (Pourbaix diagram) for the Fe-H,0 system at 473 K (200°C). Full lines separate the fields of stability for Fe species. (Activity of condensed phases is unity. Concentrations of dissolved species is 1O-6 mot/kg.) Dashed lines represent the 0, (1 atm) equilibrium - upper line - and the H, (1 atm) equilibrium - lower line.
2
path for iron removal by the flowing water is: Fe (metal) --) Fe*+ (oxide-metal interface); Fe2+ (oxide) diffuses through oxide to water interface; Fe*+ (oxide-water interface) + FeOH+ (dissolved in boundary layer); D: FeOH+ (dissolved) diffuses through boundary layer to bulk water; E: FeOH+ may carried away by bulk water, or E’: FeOH+ (bulk water) * Fe,O, (particles, suspended in bulk flowing water). Steps A and E’ involve oxidation of iron species. The presumed complementary reduction reactions are: A-l: e- (metal)-te(in oxide at metal interface); em (in oxide at metal interface) + e- (in oxide at water interface); e- (in oxide at water interface)+H,O 4 :H, (dissolved) + OH - (dissolved). Dissolved H, may simply be carried away by the water stream or may react with dissolved O,, as in F - 1. F-l: 0, (bulk water)+4e-+2H,O-+40H-. Based on the response of the FAC rate to parameters such as flow velocity, pH, and dissolved oxygen, the rate-determining step is D (diffusion of dissolved species through the water boundary layer). The A: B: C:
-1.0
black iron oxide (61 overlaid by a few crystallites of red oxide. These observations lead to the follo$ing estimates for the environmental conditions. The pH of the water at 473 K (200’ C) can be estimated to be about 6. (Calculated from the MULTEQ code [7] in which the ionization constant of water is 1 X lo-l4 and 5.14 X lo-‘* at 298 K and 473 K. The dissociation constants for NH, are 1.7 X lop5 and 3.14 x 10e6 at those temperatures.) The electrochemical potential of the surface (the corrosion potential) is taken as the value for which the black iron oxide (Fe,O,) is in equilibrium with the red oxide (Fe,O,). The potential-pH diagram (the Pourbaix diagram) for the Fe-H,0 system (fig. 3) was calculated from available thermodynamic data as described in the Appendix. From the diagram, one finds that for pH = 6, the two oxides of iron coexist at a potential of -0.4 V. The equilibrium dissolved species for iron is FeOH+. Thus, the equilibrium which sets the solubilify can be written: Fe,04+5H*+2e-e3FeOH++H20,
261
D. Cubicciotti / Flow msisted corrosion of steel
for which at 473 K (from the data in the Appendix): E(V,
SHE) = 0.051 - 0.235 pH - 0.141 log(FeOH+).
Since the concentration of FeOH+ in solution under these conditions is the same as the solubility of Fe,O,, this equation can be written: log(solubility
of Fe,O,)
= log(FeOH+) = 0.362 - 1.67 pH - 7.1E.
For the environmental conditions under consideration (473 K; pH = 6; E = -0.4 V), the equilibrium concentration of FeOH+ is 1.5 X lo-’ mol/kg (8 X 10e9 g Fe per g. H,O or 8 ppb of Fe) for unalloyed iron). The solubility equation indicates that the solubility of Fe,O,, and hence the FAC rate, depends on pH and E. Thus, as the pH is increased the FAC rate decreases. Also as the redox potential, E, is increased the FAC rate decreases (an increase in the 0, content of the water will give rise to an increase in E). These two factors for decreasing FAC have been discussed in ref.
-1.0 2
WI.
4
6
8
10
pH at Temperature
3. Effects of alloying additions The purpose of the present analysis is to consider the chemistry of FAC when the steel is alloyed with certain metals. The rate of FAC has been found to be decreased when Cu ahd especially Cr is present in the steel [8,9]. It will be shown that these elements decrease the solubility of iron which can account for the decreased FAC. The Pourbaix diagram for the Fe-Cr-H,O system is shown in fig. 4, calculated from data in the Appendix. The diagram shows that for the conditions considered (473 K, pH = 6, E = -0.4 v), the oxide FeCr,O, is stable, rather than Fe,O, + Fe,O,. Thus, with Cr present in the alloy the surface oxide should become FeCr,O, rather than Fe,O, + Fe,O, (as for unalloyed iron). The figure indicates that the solubility of iron, under the environmental conditions in question, is controlled by the equilibrium: FeCr,O,
+ H+ 8 FeOH+ + Cr,O,
for which the Gibbs energy equation AC = 22.2 kJ/mol+
RT log(FeOH+)
at 473 K is: - RT log(H+).
Under these conditions the solubility of Fe is the FeOH+ concentration, which depends on pH but not on corrosion potential. For pH = 6, the concentration of dissolved iron (i.e. FeOH+) is calculated, by equating the Gibbs energy equation to zero for equilibrium, to be 1.8 x lo-” mol/kg (0.01 ppb Fe).
Fig. 4. Potential-pH diagram for the Fe-Cr-H,O system at 473 K (200 ’ C). Full lines separate the fields of stability for Fe species. Dotted lines indicate the stability fields for Cr species in the absence of Fe. (Activities of condensed phases is unity. Concentrations of dissolved species are 10m6 mol/kg.) Dashed lines represent the equilibria for 0, (1 atm) - upper line - and H, (1 atm) - lower line.
This solubility is about l/1000 of the solubility of pure iron under the same conditions. Reduction of the solubility decreases the diffusion rate and the rate of FAC by approximately the same factor. It is thus proposed that the mechanism by which Cr in the steel decreases FAC is by formation of a surface oxide (FeCr*O,) that has a smaller solubility of iron species than that of the unalloyed metal oxide (Fe,O,). The solubility of chromium must also be considered to establish that chromium would remain in the surface oxide and not be leached out. The pertinent equation (based on the equilibrium phases in the Pourbaix diagram - fig. 4) is Cr,03+2e-+6H+@2Cr2++3HH20 for which the Nemst E(V,
equation
at 200 o C becomes
SHE) = 0.050 - 0.094 log(Cr’+)
- 0.282 pH.
For the present conditions (pH = 6, E = -0.4 V), the solubility of Cr (i.e. equilibrium concentration of Cr*+) is 1.8 lo-” mol/kg. The solubility of Cr is
262
D. Cubicciottr
/ Flow amsted corrosion ofsreel
about 0.1 that of Fe; thus iron would be removed by dissolution and transport by the flowing water stream about 10 times as fast as Cr. This analysis indicates that for unalloyed iron the surface oxide is Fe,O,, while for iron containing added Cr the steady-state oxide is FeCr,O,. In the latter oxide, the mole ratio of Fe to Cr is 1 to 2, while in the steel of the pipe the ratio can be 1000 or more (for a low-alloy steel) and about 5 (for a stainless steel). Presumably, the initial oxide formed on these Cr-containing steels is a mixture of Fe,O, and FeCr,O,, to maintain the Fe to Cr ratio of the base metal. The solubility of Fe,O, is much larger than that of FeCr,O,; therefore, in time the Fe,O, would be leached away by the flowing water, leaving FeCr,O, as the steady-state oxide. On this basis, one would expect the rate of FAC to decrease with time while the surface film changes from a mixture of Fe,O, and FeCr,O, to pure FeCr,O,. This discussion indicates that a better understanding of the corrosion process is obtained when the thermodynamics of the mixed oxide is included in the Pourbaix diagram for the system. The present treatment is based on the stoichiometric spine1 (FeCraO,) because thermodynamic data were available for it. Corrosion layer oxides on alloyed steels are generally more complicated and can involve non-stoichiometric spinels (Fe,Cr,,O,) which can contain other elements. When thermodynamic data are available for such phases better treatments will be possible.
1.0 r--
I
-1.0
2
4
6
8
10
pH at Temperature Fig. 5. Potential-pH diagram for the Fe-Cu-H,O system at 473 K (200 Q C). Full lines separate the fields of stability for Fe species. Dotted lines delineate fields of stability for Cu species in the absence of Fe (activities as in fig. 4). Dashed lines represent the equilibria for 0, (1 atm ) - upper line - and H, (1 atm) - lower line.
in the flowing
4. Effects of Cu in steel Small concentrations of copper in steel have also resulted in decreased FAC rates although the effectiveness of Cu is smaller than Cr [8,9]. The chemistry of the Fe-Cu-Hz0 system can be considered in the Pourbaix diagram of fig. 5. The stable compounds formed in this system under the given environmental conditions are Fe,O, + Fe,O, (the same as with unalloyed steel) and Cu metal. The diagram indicates that there is a mixed oxide, FeCuO,, but its field of stability does not extend to the environmental conditions under consideration. Thus, for the environment conditions discussed (473 K, pH = 6, E = -0.4 V), the solubility of iron in water is the same as for unalloyed steel. The Pourbaix indicates that Cu metal is the stable Cu compound in this environment. The decrease of FAC by copper in steel must occur by a different mechanism than for chromium, in view of these diagrams, possibly as follows. As copper-bearing steel corrodes, iron is oxidized to Fe,O, (and dissolves
water) but copper is not oxidized (see fig. 5). The copper remains in the metal and its concentration builds up in the metal underlying the corrosion film as the iron is removed by corrosion. The copper in this layer may mechanically impede the passage of iron to the corrosion film and thus slow down corrosion rates. Additional information about copper concentrations in the corrosion film may help resolve the mechanism.
Appendix. Method used for calculating diagrams Potential-pH diagrams (Pourbaix diagrams) represent the fields of thermodynamic stability of the various oxides or dissolved species that can form from an element in equilibrium with water [lo]. Such diagrams generally describe the behavior of a single metallic element in water. Occasionally, the influence of an electronegative element is included. For example, the effect of sulfur on the iron diagram has been considered because of the stability of compounds containing iron
D. Cubicciotti / Flow ahted
263
corrosion of steel
Table 1 Thermodynamic values used (Data from Refs. [12]-[16]) Substance
Enthalpy of formation at 298 K W/m4
Fe(s) Fe,%(s)
0.0 - 1118.0
Absolute entropy at 298 K (J/mol K)
Heat capacity constants CP = A + BT + C/T2 (J/mol K) cx10-5
B
A
27.28 146.4
12.72 86.3 98.28 - 1.08 79.1 19.4 - 29.8 27.28 30.0 75.44 0.0
0.032 0.209 0.0778 0.0012 - 0.22 - 0.021 0.27 0.0033 0.0042 _
H 20(l) H+(a@
- 285.9 0.0
87.45 - 137.7 - 316.0 - 142.3 - 29.3 130.6 205.0 70.0 0.0
OH- (aq) Cr(s)
- 230.1 0.0 1129.7 - 138.9 - 256.0 -881.2
- 10.9 23.64 81.2 18.0 - 307.5 50.2
506.4 17.7 119.4 - 102.3 32.7 667.0
-1.18 0.023 0.009 - 0.088 - 0.165 -1.25
- 878.2 - 469.4 - 1414.6 0.0 - 155.8 - 170.3 64.9 - 512.5 - 968.0
184.0 - 68.6 146.0 33.1 42.6 92.9 - 99.6 88.7 146.8
- 793.0 15.8 163.0 24.85 43.8 56.6 20.7 134.7 139.6
2.31 -0.13 0.022 - 0.0014 0.017 0.029 - 0.084 - 0.057 0.118
Fe@,(s) Fe’+(aq) Fe3+ (as) FeOH2+ (an) FeOH+ (aq)
- 825.5 - 89.12 - 48.53 - 291.2 - 324.7 0.0
H,Cd 02
0.0
(9)
Cr,O,W
Cr2+(aq) Cr3+ (as) CrO,Z- (aq) HCrOi (aq) CrOH’+ (aq) FeCr,O,(s) Cu(s) CuW) Cu 20(s) Cu2+ (aq) FeCuO, (s) Fe,CuO,(s)
-
and sulfur - especially the iron sulfides. No cases are known to this author in which the effect of a second metallic element is considered except for the case of Fe-Cr, which was reported earlier [ll]. Various mixed metal oxides are known to be stable and can have an impact on the chemistry of corrosion processes, as illustrated in the present paper, so that it is worthwhile considering Pourbaix diagrams for more than one metal. The diagrams presented in figs. 3 to 5 were prepared from the FACT computer program [12]. Room-temperature thermodynamic quantities together with heat capacities for compounds and aqueous species are used by the program to calculate Gibbs energies at temperature. The thermodynamic quantities used for the present calculations are given in table 1. For a single-element diagram, the FACT program computes the fields in E, pH space in which a given compound or aqueous species of the element is the most
2.51 _ - 14.85 40.0 15.4 29.1 26.6 0.5 -1.7 _ - 246.0 - 0.4 - 15.6 93.0 46.0 - 414.0 402.0 25.0 -31.9 0.0 - 5.9 0.0 22.0 - 34.0 - 23.4
stable form. The results of such a computation are shown in fig. 3 for iron. When more than one element is considered, the procedure is somewhat more complex. For example, for a diagram for an element A with a second element, B, present, one proceeds as follows. First, the stability fields for compounds of only element B are determined, at the given temperature. Then for each compound of B (i.e. for each field of the diagram for B), the diagram for A plus B is computed. (The activities of dissolved species were assumed to be 10e6 M. The activities of the two metals in the alloy were each assumed to be unity, which is an oversimplification. For a more accurate treatment one should use the actual activities of the alloying eleme,nts and actual values for dissolved species.) The final diagram for A in the presence of B is obtained as the sum of the A-B diagrams for the fields
264
D. Cubicciotti
/ Flow assured corrosion o/steel
in which the particular
B compound is stable. Thus, the Fe-Cr diagram in fig. 4 was assembled from the Fe + Cr diagrams for the fields in which HCrO; , CrOj- , Cr,O,, Cr2+, and Cr(OH)‘+ were stable.
References [l] IS. Woolsey, Erosion-corrosion in PWR secondary circuits, CERL Rep. TPRD/L/3114/R87 (Mar. 1987). [2] Ph. Berge and F. Khan, Eds., Specialists Meeting on Corrosion Erosion of Steels in High-Temperature Water and Wet Steam, Les Renardieres, France, Electricite de France, Paris, May 1982. [3] G. Cragnolino, Erosion-corrosion in nuclear power systems - An overview, Paper 86, Corrosion 87, NACE, Houston, Texas (1987). [4] Surry Unit 2 Reactor Trip/Feedwater Pipe Failure, Virginia Power Co., Richmond, Virginia (Jan., 1987). [5] R. Jones, B. Chexal, M. Behrahvesh and K. Stahlkopf, Single-phase erosion-corrosion of carbon steel piping, Electric Power Research Institute, Nuclear Power Division Report, Palo Alto, CA (Febr. 1987). [6] J. McAvoy, Discussion of Survey Plant Findings, EPRI Workshop on Erosion-Corrosion of Carbon Steel Piping, Electric Power Research Institute, Palo Alto, CA, April 1987.
[7] J.H. Alexander, MULTEQ User’s Manual, EPRI-NP5561CCM, Electric Power Research Institute, Palo Alto. CA (Febr. 1988). [8] M. Bouchacourt, Identification of Key Variables, EDF Studies, EPRI workshop cited in ref. 161. 191 J. Ducreaux, Theoretical and experimental investigation of the effect of chemical composition of steels on their erosion-corrosion resistance, ref. [2], paper 19. [lo] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, (NACE, Houston, Texas, 1974). [ll] D. Cubicciotti and L. Ljungberg, J. Electrochem. Sot. 132 (1985) 987. (121 W.T. Thompson, A.D. Pelton and C.W. Bale, FACT Facility for the Analysis of Chemical Thermodynamics, Thermfact Ltd., Quebec, Canada. (131 I. Barin, 0. Knacke and 0. Kubaschewski, Thermochemical Properties of Inorganic Substances (Springer-Verlag, Berlin, 1977). 114) D.D. Wagman et al., NBS Tech Note 270, US Dept. of Commerce, Washington, DC (1968 to 1981). [15] 0. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5th Ed. (Pergamon Press, London, New York, 1979). [16] H.E. Bamer and R.V. Scheuerman, Handbook of Thermochemical Data for Compounds and Aqueous Species (John Wiley & Sons, New York, 1978).
Journal of Nuclear Materials 152 (1988) 259-264 North-Holland, Amsterdam
FLOW-ASSISTED CORROSION AND Cu ADDITIONS Daniel
OF STEEL AND THE INFLUENCE
OF Cr
CUBICCIO’MI
Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303, USA
Received 2 August 1987; accepted 1 December 1987
Flow-assisted corrosion (FAC) of steel feedwater lines occurs by dissolution of the surface oxide layer on the steel. The solubility of iron in water under FAC conditions is discussed through the use of potential-pH diagrams (Pourbaix diagrams). Alloying additions of chromium and copper both decrease FAC. An assessment is presented that Cr additions decrease FAC by forming a mixed oxide with iron instead of a pure iron oxide. The solubility of iron from the mixed oxide is smaller than for pure iron oxide and leads to a smaller FAC rate. The stable form of copper under FAC conditions is not the mixed iron-copper oxide but metallic copper, which may act in the underlying steel surface to impede FAC.
1. Introduction
Flow-assisted corrosion (FAC) of steel piping can be severe enough to cause failure of high-velocity hot water lines [l-3]. In a recent case, the walls of a carbon steel feedwater line had been thinned by FAC to the point of rupture by ductile overloading when the pipe was not able to sustain the operating pressure [4]. This kind of attack has also been called erosion-corrosion because of the worn appearance of the metal surface; however, FAC is more descriptive of the mechanism of the process, which does not involve mechanical erosion in carbon and low alloy steels. Substantial research has been performed to establish the key factors that influence FAC [S], which are consistent with a mechanism in which iron dissolves in the flowing water. The factors which affect FAC of steel in water can be grouped into the following: (1) flow rate (hydrodynamics), (2) material (alloying components), (3) environment (water chemistry). These can be represented schematically to indicate their conjoint action as in fig. 1. The mechanism for FAC, derived from laboratory studies [2], is schematically illustrated in fig. 2. Iron reacts with water to form a surface oxide layer. This oxide dissolves in the water, and the rate of iron removal (i.e. the rate of FAC) is controlled by the rate of diffusion of dissolved iron species through the boundary layer of water near the surface into the bulk water. This diffusion (or mass transport of iron away from the 0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
surface) depends directly on the concentration of soluble iron species at the oxide surface and inversely on the thickness of the boundary layer. Thus, a decrease of the boundary layer thickness because of increased water flow rate or because of local turbulence causes an increase of corrosion rate.
Environment
Fig. 1. Diagram of interaction of key controllable jointly lead to flow-assisted corrosion.
B.V.
factors
that
260
D. Cubicciotti
/ Flow assisted corrosion of steel
2. Solubility of iron oxide
1.0
r
The concentration of dissolved iron at the oxide surface is controlled by the solubility of the oxide, which can be evaluated as follows. The potential-pH diagram for the Fe-H,0 system, shown in fig. 3, presents the equilibrium information available for the solid oxides and water-soluble species for Fe. It is necessary to know the conditions that prevail in a typical system to specify the region of interest in this diagram. In a recent FAC rupture [4], the temperature was about 2OO’C and the pH of the water (measured at room temperature) was about 8.9 controlled by NH, addition. The inside surface of the pipe was coated with
Bulk water flow
Diffusion of corrosion products
Fig. 2. Schematic representation of the mechanism of flow-assisted corrosion of steel. Note
to fig.
2
4
6 pH at Temperature
8
10
Fig. 3. Potential-pH diagram (Pourbaix diagram) for the Fe-H,0 system at 473 K (200°C). Full lines separate the fields of stability for Fe species. (Activity of condensed phases is unity. Concentrations of dissolved species is 1O-6 mot/kg.) Dashed lines represent the 0, (1 atm) equilibrium - upper line - and the H, (1 atm) equilibrium - lower line.
2
path for iron removal by the flowing water is: Fe (metal) --) Fe*+ (oxide-metal interface); Fe2+ (oxide) diffuses through oxide to water interface; Fe*+ (oxide-water interface) + FeOH+ (dissolved in boundary layer); D: FeOH+ (dissolved) diffuses through boundary layer to bulk water; E: FeOH+ may carried away by bulk water, or E’: FeOH+ (bulk water) * Fe,O, (particles, suspended in bulk flowing water). Steps A and E’ involve oxidation of iron species. The presumed complementary reduction reactions are: A-l: e- (metal)-te(in oxide at metal interface); em (in oxide at metal interface) + e- (in oxide at water interface); e- (in oxide at water interface)+H,O 4 :H, (dissolved) + OH - (dissolved). Dissolved H, may simply be carried away by the water stream or may react with dissolved O,, as in F - 1. F-l: 0, (bulk water)+4e-+2H,O-+40H-. Based on the response of the FAC rate to parameters such as flow velocity, pH, and dissolved oxygen, the rate-determining step is D (diffusion of dissolved species through the water boundary layer). The A: B: C:
-1.0
black iron oxide (61 overlaid by a few crystallites of red oxide. These observations lead to the follo$ing estimates for the environmental conditions. The pH of the water at 473 K (200’ C) can be estimated to be about 6. (Calculated from the MULTEQ code [7] in which the ionization constant of water is 1 X lo-l4 and 5.14 X lo-‘* at 298 K and 473 K. The dissociation constants for NH, are 1.7 X lop5 and 3.14 x 10e6 at those temperatures.) The electrochemical potential of the surface (the corrosion potential) is taken as the value for which the black iron oxide (Fe,O,) is in equilibrium with the red oxide (Fe,O,). The potential-pH diagram (the Pourbaix diagram) for the Fe-H,0 system (fig. 3) was calculated from available thermodynamic data as described in the Appendix. From the diagram, one finds that for pH = 6, the two oxides of iron coexist at a potential of -0.4 V. The equilibrium dissolved species for iron is FeOH+. Thus, the equilibrium which sets the solubilify can be written: Fe,04+5H*+2e-e3FeOH++H20,
261
D. Cubicciotti / Flow msisted corrosion of steel
for which at 473 K (from the data in the Appendix): E(V,
SHE) = 0.051 - 0.235 pH - 0.141 log(FeOH+).
Since the concentration of FeOH+ in solution under these conditions is the same as the solubility of Fe,O,, this equation can be written: log(solubility
of Fe,O,)
= log(FeOH+) = 0.362 - 1.67 pH - 7.1E.
For the environmental conditions under consideration (473 K; pH = 6; E = -0.4 V), the equilibrium concentration of FeOH+ is 1.5 X lo-’ mol/kg (8 X 10e9 g Fe per g. H,O or 8 ppb of Fe) for unalloyed iron). The solubility equation indicates that the solubility of Fe,O,, and hence the FAC rate, depends on pH and E. Thus, as the pH is increased the FAC rate decreases. Also as the redox potential, E, is increased the FAC rate decreases (an increase in the 0, content of the water will give rise to an increase in E). These two factors for decreasing FAC have been discussed in ref.
-1.0 2
WI.
4
6
8
10
pH at Temperature
3. Effects of alloying additions The purpose of the present analysis is to consider the chemistry of FAC when the steel is alloyed with certain metals. The rate of FAC has been found to be decreased when Cu ahd especially Cr is present in the steel [8,9]. It will be shown that these elements decrease the solubility of iron which can account for the decreased FAC. The Pourbaix diagram for the Fe-Cr-H,O system is shown in fig. 4, calculated from data in the Appendix. The diagram shows that for the conditions considered (473 K, pH = 6, E = -0.4 v), the oxide FeCr,O, is stable, rather than Fe,O, + Fe,O,. Thus, with Cr present in the alloy the surface oxide should become FeCr,O, rather than Fe,O, + Fe,O, (as for unalloyed iron). The figure indicates that the solubility of iron, under the environmental conditions in question, is controlled by the equilibrium: FeCr,O,
+ H+ 8 FeOH+ + Cr,O,
for which the Gibbs energy equation AC = 22.2 kJ/mol+
RT log(FeOH+)
at 473 K is: - RT log(H+).
Under these conditions the solubility of Fe is the FeOH+ concentration, which depends on pH but not on corrosion potential. For pH = 6, the concentration of dissolved iron (i.e. FeOH+) is calculated, by equating the Gibbs energy equation to zero for equilibrium, to be 1.8 x lo-” mol/kg (0.01 ppb Fe).
Fig. 4. Potential-pH diagram for the Fe-Cr-H,O system at 473 K (200 ’ C). Full lines separate the fields of stability for Fe species. Dotted lines indicate the stability fields for Cr species in the absence of Fe. (Activities of condensed phases is unity. Concentrations of dissolved species are 10m6 mol/kg.) Dashed lines represent the equilibria for 0, (1 atm) - upper line - and H, (1 atm) - lower line.
This solubility is about l/1000 of the solubility of pure iron under the same conditions. Reduction of the solubility decreases the diffusion rate and the rate of FAC by approximately the same factor. It is thus proposed that the mechanism by which Cr in the steel decreases FAC is by formation of a surface oxide (FeCr*O,) that has a smaller solubility of iron species than that of the unalloyed metal oxide (Fe,O,). The solubility of chromium must also be considered to establish that chromium would remain in the surface oxide and not be leached out. The pertinent equation (based on the equilibrium phases in the Pourbaix diagram - fig. 4) is Cr,03+2e-+6H+@2Cr2++3HH20 for which the Nemst E(V,
equation
at 200 o C becomes
SHE) = 0.050 - 0.094 log(Cr’+)
- 0.282 pH.
For the present conditions (pH = 6, E = -0.4 V), the solubility of Cr (i.e. equilibrium concentration of Cr*+) is 1.8 lo-” mol/kg. The solubility of Cr is
262
D. Cubicciottr
/ Flow amsted corrosion ofsreel
about 0.1 that of Fe; thus iron would be removed by dissolution and transport by the flowing water stream about 10 times as fast as Cr. This analysis indicates that for unalloyed iron the surface oxide is Fe,O,, while for iron containing added Cr the steady-state oxide is FeCr,O,. In the latter oxide, the mole ratio of Fe to Cr is 1 to 2, while in the steel of the pipe the ratio can be 1000 or more (for a low-alloy steel) and about 5 (for a stainless steel). Presumably, the initial oxide formed on these Cr-containing steels is a mixture of Fe,O, and FeCr,O,, to maintain the Fe to Cr ratio of the base metal. The solubility of Fe,O, is much larger than that of FeCr,O,; therefore, in time the Fe,O, would be leached away by the flowing water, leaving FeCr,O, as the steady-state oxide. On this basis, one would expect the rate of FAC to decrease with time while the surface film changes from a mixture of Fe,O, and FeCr,O, to pure FeCr,O,. This discussion indicates that a better understanding of the corrosion process is obtained when the thermodynamics of the mixed oxide is included in the Pourbaix diagram for the system. The present treatment is based on the stoichiometric spine1 (FeCraO,) because thermodynamic data were available for it. Corrosion layer oxides on alloyed steels are generally more complicated and can involve non-stoichiometric spinels (Fe,Cr,,O,) which can contain other elements. When thermodynamic data are available for such phases better treatments will be possible.
1.0 r--
I
-1.0
2
4
6
8
10
pH at Temperature Fig. 5. Potential-pH diagram for the Fe-Cu-H,O system at 473 K (200 Q C). Full lines separate the fields of stability for Fe species. Dotted lines delineate fields of stability for Cu species in the absence of Fe (activities as in fig. 4). Dashed lines represent the equilibria for 0, (1 atm ) - upper line - and H, (1 atm) - lower line.
in the flowing
4. Effects of Cu in steel Small concentrations of copper in steel have also resulted in decreased FAC rates although the effectiveness of Cu is smaller than Cr [8,9]. The chemistry of the Fe-Cu-Hz0 system can be considered in the Pourbaix diagram of fig. 5. The stable compounds formed in this system under the given environmental conditions are Fe,O, + Fe,O, (the same as with unalloyed steel) and Cu metal. The diagram indicates that there is a mixed oxide, FeCuO,, but its field of stability does not extend to the environmental conditions under consideration. Thus, for the environment conditions discussed (473 K, pH = 6, E = -0.4 V), the solubility of iron in water is the same as for unalloyed steel. The Pourbaix indicates that Cu metal is the stable Cu compound in this environment. The decrease of FAC by copper in steel must occur by a different mechanism than for chromium, in view of these diagrams, possibly as follows. As copper-bearing steel corrodes, iron is oxidized to Fe,O, (and dissolves
water) but copper is not oxidized (see fig. 5). The copper remains in the metal and its concentration builds up in the metal underlying the corrosion film as the iron is removed by corrosion. The copper in this layer may mechanically impede the passage of iron to the corrosion film and thus slow down corrosion rates. Additional information about copper concentrations in the corrosion film may help resolve the mechanism.
Appendix. Method used for calculating diagrams Potential-pH diagrams (Pourbaix diagrams) represent the fields of thermodynamic stability of the various oxides or dissolved species that can form from an element in equilibrium with water [lo]. Such diagrams generally describe the behavior of a single metallic element in water. Occasionally, the influence of an electronegative element is included. For example, the effect of sulfur on the iron diagram has been considered because of the stability of compounds containing iron
D. Cubicciotti / Flow ahted
263
corrosion of steel
Table 1 Thermodynamic values used (Data from Refs. [12]-[16]) Substance
Enthalpy of formation at 298 K W/m4
Fe(s) Fe,%(s)
0.0 - 1118.0
Absolute entropy at 298 K (J/mol K)
Heat capacity constants CP = A + BT + C/T2 (J/mol K) cx10-5
B
A
27.28 146.4
12.72 86.3 98.28 - 1.08 79.1 19.4 - 29.8 27.28 30.0 75.44 0.0
0.032 0.209 0.0778 0.0012 - 0.22 - 0.021 0.27 0.0033 0.0042 _
H 20(l) H+(a@
- 285.9 0.0
87.45 - 137.7 - 316.0 - 142.3 - 29.3 130.6 205.0 70.0 0.0
OH- (aq) Cr(s)
- 230.1 0.0 1129.7 - 138.9 - 256.0 -881.2
- 10.9 23.64 81.2 18.0 - 307.5 50.2
506.4 17.7 119.4 - 102.3 32.7 667.0
-1.18 0.023 0.009 - 0.088 - 0.165 -1.25
- 878.2 - 469.4 - 1414.6 0.0 - 155.8 - 170.3 64.9 - 512.5 - 968.0
184.0 - 68.6 146.0 33.1 42.6 92.9 - 99.6 88.7 146.8
- 793.0 15.8 163.0 24.85 43.8 56.6 20.7 134.7 139.6
2.31 -0.13 0.022 - 0.0014 0.017 0.029 - 0.084 - 0.057 0.118
Fe@,(s) Fe’+(aq) Fe3+ (as) FeOH2+ (an) FeOH+ (aq)
- 825.5 - 89.12 - 48.53 - 291.2 - 324.7 0.0
H,Cd 02
0.0
(9)
Cr,O,W
Cr2+(aq) Cr3+ (as) CrO,Z- (aq) HCrOi (aq) CrOH’+ (aq) FeCr,O,(s) Cu(s) CuW) Cu 20(s) Cu2+ (aq) FeCuO, (s) Fe,CuO,(s)
-
and sulfur - especially the iron sulfides. No cases are known to this author in which the effect of a second metallic element is considered except for the case of Fe-Cr, which was reported earlier [ll]. Various mixed metal oxides are known to be stable and can have an impact on the chemistry of corrosion processes, as illustrated in the present paper, so that it is worthwhile considering Pourbaix diagrams for more than one metal. The diagrams presented in figs. 3 to 5 were prepared from the FACT computer program [12]. Room-temperature thermodynamic quantities together with heat capacities for compounds and aqueous species are used by the program to calculate Gibbs energies at temperature. The thermodynamic quantities used for the present calculations are given in table 1. For a single-element diagram, the FACT program computes the fields in E, pH space in which a given compound or aqueous species of the element is the most
2.51 _ - 14.85 40.0 15.4 29.1 26.6 0.5 -1.7 _ - 246.0 - 0.4 - 15.6 93.0 46.0 - 414.0 402.0 25.0 -31.9 0.0 - 5.9 0.0 22.0 - 34.0 - 23.4
stable form. The results of such a computation are shown in fig. 3 for iron. When more than one element is considered, the procedure is somewhat more complex. For example, for a diagram for an element A with a second element, B, present, one proceeds as follows. First, the stability fields for compounds of only element B are determined, at the given temperature. Then for each compound of B (i.e. for each field of the diagram for B), the diagram for A plus B is computed. (The activities of dissolved species were assumed to be 10e6 M. The activities of the two metals in the alloy were each assumed to be unity, which is an oversimplification. For a more accurate treatment one should use the actual activities of the alloying eleme,nts and actual values for dissolved species.) The final diagram for A in the presence of B is obtained as the sum of the A-B diagrams for the fields
264
D. Cubicciotti
/ Flow assured corrosion o/steel
in which the particular
B compound is stable. Thus, the Fe-Cr diagram in fig. 4 was assembled from the Fe + Cr diagrams for the fields in which HCrO; , CrOj- , Cr,O,, Cr2+, and Cr(OH)‘+ were stable.
References [l] IS. Woolsey, Erosion-corrosion in PWR secondary circuits, CERL Rep. TPRD/L/3114/R87 (Mar. 1987). [2] Ph. Berge and F. Khan, Eds., Specialists Meeting on Corrosion Erosion of Steels in High-Temperature Water and Wet Steam, Les Renardieres, France, Electricite de France, Paris, May 1982. [3] G. Cragnolino, Erosion-corrosion in nuclear power systems - An overview, Paper 86, Corrosion 87, NACE, Houston, Texas (1987). [4] Surry Unit 2 Reactor Trip/Feedwater Pipe Failure, Virginia Power Co., Richmond, Virginia (Jan., 1987). [5] R. Jones, B. Chexal, M. Behrahvesh and K. Stahlkopf, Single-phase erosion-corrosion of carbon steel piping, Electric Power Research Institute, Nuclear Power Division Report, Palo Alto, CA (Febr. 1987). [6] J. McAvoy, Discussion of Survey Plant Findings, EPRI Workshop on Erosion-Corrosion of Carbon Steel Piping, Electric Power Research Institute, Palo Alto, CA, April 1987.
[7] J.H. Alexander, MULTEQ User’s Manual, EPRI-NP5561CCM, Electric Power Research Institute, Palo Alto. CA (Febr. 1988). [8] M. Bouchacourt, Identification of Key Variables, EDF Studies, EPRI workshop cited in ref. 161. 191 J. Ducreaux, Theoretical and experimental investigation of the effect of chemical composition of steels on their erosion-corrosion resistance, ref. [2], paper 19. [lo] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, (NACE, Houston, Texas, 1974). [ll] D. Cubicciotti and L. Ljungberg, J. Electrochem. Sot. 132 (1985) 987. (121 W.T. Thompson, A.D. Pelton and C.W. Bale, FACT Facility for the Analysis of Chemical Thermodynamics, Thermfact Ltd., Quebec, Canada. (131 I. Barin, 0. Knacke and 0. Kubaschewski, Thermochemical Properties of Inorganic Substances (Springer-Verlag, Berlin, 1977). 114) D.D. Wagman et al., NBS Tech Note 270, US Dept. of Commerce, Washington, DC (1968 to 1981). [15] 0. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5th Ed. (Pergamon Press, London, New York, 1979). [16] H.E. Bamer and R.V. Scheuerman, Handbook of Thermochemical Data for Compounds and Aqueous Species (John Wiley & Sons, New York, 1978).