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Volume 3 Paper 24


CO2 Corrosion of Pipelines under a Disbonded Coating in the Presence of a Precipitate

F.M. Song,a,b D.W. Kirk,a J.W. Graydon,a and D.E. Cormacka

a University of Toronto, Chemical Engineering and Applied Chemistry, 200 College Street, Toronto, Ontario, Canada M5S 3E5

b Present address: Metallurgical and Materials Eng./388, University of Nevada Reno, Reno, NV 89557 email: .

Abstract

CO2 corrosion is a significant problem for coated pipelines in the transport of petroleum, gas and water. The corrosion occurs often under a precipitate of either ferrous carbonate (FeCO3) or ferrous hydroxide (Fe(OH)2) or both. Although neither precipitate provides a good barrier to the transport of oxygen (O2), carbon dioxide (CO2) and ions since they are loose and porous, the precipitate(s) may affect corrosion by determination of the pH at the steel surface. The goal of this work is to explore conditions for the formation of the precipitates.

For steel corrosion under a disbonded coating, there is a critical pressure ratio of O2 to CO2 that separates the two precipitates. When O2 diffusion across the disbonded coating controls the corrosion rate, the ratio is 9/20, above which the precipitate is Fe(OH)2. The precipitate is FeCO3 otherwise. The corresponding ratio is 14 if O2 diffusion across the solution boundary layer under the disbonded coating controls the corrosion rate, when the coating may be deteriorated or itself permeable.

For steel corrosion not limited by O2 transport, charge transfer of hydrogen ion (H+) reduction due to dissolved CO2 in solution may control the corrosion rate. Thermodynamic calculations are permitted to determine the solution chemistry and conditions for precipitation. It was found that regardless of the initial amount of CO2 in the solution, the solution pH increases with the extent of corrosion due to H+ reduction. The formation of iron complexes in the system has an insignificant effect on the solution chemistry.

With zero CO2, the precipitate is only Fe(OH)2 and the saturated pH is about 9.05 with consideration of all complexes. With certain initial amount of CO2, the first precipitate is FeCO3 and the pH at the precipitation, which varies with the initial CO2 pressure, is 5.7 for a CO2 partial pressure of 0.23 atm. With continuation of the corrosion, FeCO3 and Fe(OH)2 co-precipitate and the pH is fixed at 8.8, regardless of further corrosion.

The thermodynamic calculations also show that the time for a precipitate to saturate a boundary layer of 0.5 mm in thickness is short. For a system with a partial pressure of CO2 below 0.23 atm, this time is not greater than 1.5 hours for a corrosion rate of 0.01 mm/y. This result suggests that pipeline corrosion under a disbonded coating indeed normally occurs under a precipitate.

Keywords: corrosion, steel, coating, CO2, O2, precipitation, FeCO3.

Introduction

CO2 and O2 are two crucial species that cause exterior corrosion of coated pipelines since they can diffuse through a disbonded coating. Two precipitates, FeCO3 and Fe(OH)2, are possible during the corrosion. Another precipitate, ferric hydroxide, is insignificant to pipeline corrosion as it is often present as a suspension in solution and does not affect the corrosion rate considerably.[1]

The precipitate(s) of either FeCO3 or Fe(OH)2 or both may have a significant impact on the pipe corrosion rate because it may determine the pH at the metal steel surface. The conditions for formation of a precipitate have been investigated in this work. The results provided useful information for a recent comprehensive CO2 corrosion model which was developed to include transport, chemical reactions, ionic interactions, precipitation and electrochemical reactions altogether.[2-4]

In this work, two cases are studied for the determination of the type of precipitate(s) under a disbonded coating: (1) corrosion control by O2 diffusion, and (2) corrosion control by charge transfer of H+ reduction. Both cases are described as below, starting with corrosion control by O2 diffusion.

Precipitation with Corrosion Control by O2 Diffusion

The rate of pipe corrosion under a disbonded coating is often controlled by O2 diffusion through the disbonded coating (Figure 1). Proportional to corrosion rate, the flux of O2 diffusion obeys Fick�s first law:[5]

                                                           (1)

where JO2, PcO2, pO2, ΔpO2�and pO2o�are, respectively, diffusion flux, permeability of the coating, partial pressure within the coating, pressure drop across the coating and partial pressure in soil, all for O2. y and dc are respectively distance from the pipe side of and thickness of the coating.

The O2 diffusion flux can be converted to an actual corrosion rate through the relation: r=1.173 nO2 FJO2, where r, nO2 and F are corrosion rate, number of electrons transferred for O2 reduction and Faraday constant, respectively. 1.173 is a conversion coefficient for iron corrosion from current density unit, A/m2, to corrosion rate unit, mm/y.

When O2 diffusion across a solution boundary layer under the disbonded coating controls the corrosion rate (Figure 1), the diffusion flux can be calculated from:

                                                                            (2)

where ΔpO2, DO2 and HO2 are respectively pressure drop across the boundary layer, diffusion coefficient within the boundary layer and Henry�s law constant in solution, all for O2. δs is thickness of the boundary layer.

Equations (1-2) can be applied to CO2 directly. Then, the subscript �O2� for O2 must be replaced by �CO2�.

When diffusion of either O2 or CO2 is across a disbonded coating and at the same time across the solution boundary layer under the coating, the overall diffusion flux of either gas Jg can be calculated from: . Jg-coating is the diffusion flux of either gas across the disbonded coating and Jg-H2O is the diffusion flux across the solution boundary layer. Jg-coating and Jg-H2O can be calculated respectively from Equations (1) and (2).  This overall diffusion will not be considered in this work.

When O2 and CO2 both diffuse across a disbonded coating or a boundary layer, the type of precipitate on the pipe surface can be determined from comparison of the diffusion fluxes of O2 and CO2 through either the disbonded coating or the boundary layer. The comparison of diffusion fluxes uses the following stoichiometric reactions occurring in solution:

                                                                                    (3)

Equation (3) represents a combination of the overall processes of CO2 dissolution including the two-step carbonic acid dissociation and the overall corrosion reactions including O2 reduction and iron oxidation. Reductions of H+ and carbonic acid are neglected due to their less significant role than O2 in the corrosion process. Equation (3) indicates that at steady-state, if O2 diffusion controls the corrosion rate and if the diffusion rate is greater than half the CO2 diffusion rate, or JO2/JCO2>0.5, the steel surface is in direct contact with the precipitate of Fe(OH)2. This is because carbonate ions formed from dissolved CO2 cannot combine all ferrous ions (Fe2+) to form FeCO3 and the excessive Fe2+ are present as Fe(OH)2 on the steel surface. The steel surface is in direct contact with FeCO3 if the O2 diffusion rate is less than half the CO2 diffusion rate or JO2/JCO2<0.5, when there are excessive carbonate ions.

The diffusion rate ratio of O2 to CO2, JO2/JCO2=0.5, can be converted, by use of Henry�s law, to a pressure drop ratio through a coating or through a solution boundary layer.

 (1) If the diffusion of O2 and CO2 is across a new high density polyethylene (HDPE) coating, the pressure drop ratio of O2 to CO2 is, based on =0.5 (Equation (1)), =9/20. The parameters useful for the above calculation are given in Table 1. This pressure drop ratio indicates that the precipitate is Fe(OH)2 if O2 pressure in soil is greater than 9/20 times of CO2 pressure or pO2/pCO2>9/20, when O2 and CO2 are assumed greatly consumed under the disbonded coating to have nearly a zero pressure. The precipitate is FeCO3 otherwise.

TABLE 1 Transport Parameters of O2 and CO2 at 25 �C[6-7]

 

Pc  �1011 (in HDPE)

H (in H2O)

D �109 (in H2O)

O2

    1.36

     0.8074

    1.96

reference

    5

    6

    7

CO2

    1.22

     0.02952

    1.96

reference:

    5 

    6

    7

The unit of Pc: mol/m/s/atm, H: atm�m3/mol and D: m2/s.

Since O2 pressure in soil is up to 0.21 atm, the precipitate must be FeCO3 if CO2 pressure is higher than 0.467 atm. These corresponding pressures of O2 and CO2 are given in Table 2, together with another O2 pressure of 0.45 atm and its corresponding CO2 pressure of 1 atm.

TABLE 2 Corresponding O2 and CO2 Pressures at Transition between Precipitates

 

Across HDPE Coating

Across HDPE Coating

Across H2O Layer

O2 (atm)

0.21

0.45

0.21

CO2 (atm)

0.467

1

0.015

 (2) If the diffusion is across a solution boundary layer, based on =0.5 (Equation (2)), the pressure drop ratio of O2 to CO2 becomes =14, where DCO2 and HCO2 are CO2 diffusion coefficient and Henry�s law constant in the solution (Table 1). The pressure drop ratio indicates that at steady-state, if O2 pressure in soil is greater than 14 times of CO2 pressure, the precipitate is Fe(OH)2. The precipitate is FeCO3 otherwise.

If O2 pressure is 0.21 atm, the CO2 pressure is 0.015 atm (Table 2), based on pO2/pCO2=14. Since this CO2 pressure is smaller than its partial pressure in atmosphere (0.03 atm), the precipitate for atmospheric corrosion of steel should be FeCO3 if Fe2+ oxidation is not accounted for. The product is however normally ferric hydroxide or ferric carbonate because Fe2+ oxidation does occur and the ferric precipitates have very low solubilities.[1]

(3) The above results indicate that the pressure drop ratio of O2 to CO2 across a boundary layer is much greater than that across an HDPE coating. The reason is that CO2 has an equivalent diffusion coefficient but a much greater solubility than O2 in solution, but the permeabilities of both species in HDPE coating are equivalent. These substantially different ratios of pressure drop indicate that a much smaller CO2 pressure than O2 is sufficient for precipitation of FeCO3 in a solution layer. However, this rule can be inaccurate if considering CO2 hydration which is a slower step in CO2 corrosion. In any event, this rule may become a useful guide when only transport parameters are available. The combination of diffusion of O2 and CO2 together with CO2 hydration and corrosion was modeled elsewhere.[3]

Precipitation with Corrosion Control by H+ Reduction

When charge transfer of H+ reduction controls the corrosion rate, the transport of each species in the solution boundary layer under a disbonded coating is fast and each concentration is uniform. Then, thermodynamic calculations are permitted for investigation of steel corrosion in the solution layer.

Analytical thermodynamic calculations have been extensively used to describe the equilibrium conditions for metal corrosion,[8] where the relative abundance of the species is normally described for a single reaction. Numerical methods have greatly expanded the use of thermodynamics since they make it possible to minimize the total free energy of all species in a system, important for determination of the equilibrium properties of a multiphase, multicomponent system. The �VCS� software (version 2.0),[9] an algorithm developed using the free-energy minimization method, has been used in this work to calculate the thermodynamic chemistry in an iron-corroding solution with and without dissolved CO2. The effects of iron complex ions on the solution chemistry have also been investigated.

The calculation for iron corrosion is performed for a total pressure of 1 atm and a temperature of 25�C and for the two cases: (1) corrosion in deionized water and (2) corrosion in a solution with only dissolved CO2.

(1) Steel Corrosion in Deionized Water

For steel corrosion in deionized water, the corrosion system is schematically shown in Figure 2. For the corrosion starting with a fresh specimen, the variation of species concentration vs. extent of corrosion or pH is shown in Figures 3-4. Reading from left to right, the initial solution pH is 7. As iron corrodes, the corrosion product, Fe2+, dissolves into solution and at the same time, H+ is reduced to hydrogen at the steel surface. The concentrations of iron ions and pH increase with the extent of corrosion. The solution finally becomes saturated at a pH of 9.05 and a total of all iron species concentrations is 7.66 � 10-6 mol/l. The solution pH and the concentration of each species thereafter remain fixed even the corrosion continues. This can be explained from the phase rule of Gibbs, f=c-p+n where f, c, p and n are respectively the system degree of freedom, number of independent species, number of phases and number of environmental factors (temperature and pressure). In the thermodynamic system, the degree of freedom is zero because in this binary system (H2O and iron) or c=2, there are two phases (liquid phase and solid-Fe(OH)2) or p=2 as Fe(OH)2 precipitates. n=0 at given system temperature and pressure. Figures 3-4 also show that in this corrosion process, the concentration of each iron complex is much smaller than that of the free Fe2+.

The time required for Fe(OH)2 to saturate a 0.5 mm thick water layer was calculated, which provides information on whether iron corrosion normally occurs under a precipitate. To saturate the boundary layer with Fe(OH)2, it requires the dissolution of 2.751�10-8 mm thickness of iron. This takes 1.4 minutes at a corrosion rate of 0.01 mm/y.[10] This suggests that corrosion of steel may normally occur under Fe(OH)2 precipitate because the water layer can be quickly saturated after an iron surface is exposed to the layer.

(2) Steel Corrosion in a Solution with Dissolved CO2

For steel corrosion under a solution boundary layer containing dissolved CO2 (Figure 5), the variation of the concentration of each species with pH is calculated and shown in Figure 6. The solution is initially saturated with CO2 of 0.23 atm and the solution pH is 4.22. With increasing extent of corrosion or as the abscissa shifts from left to right, the Fe2+ concentration and pH increase. Fe2+ concentration increases because Fe2+ is soluble; pH increases because H+ is reduced to hydrogen. At a certain corrosion extent, the Fe2+ concentration reaches a maximum of 6.8 �10-4 mol/l and the pH then is 5.7. At the maximum concentration of Fe2+, FeCO3 begins to precipitate. As corrosion continues, Fe2+ concentration decreases while pH continues to increase. This increase of pH results from continuous H+ reduction. Fe2+ concentration decreases for the following reasons. Fe2+ forms from iron oxidation, and by charge, the formation of one Fe2+ requires reduction of two H+. Since H+ is formed from dissociation of both carbonic acid and bicarbonate ion, the reduction of two H+ results in formation of more than one carbonate ion. Therefore, there is a deficit for Fe2+ as they combine with carbonate ions to form FeCO3 to precipitate. To compensate, Fe2+ in the solution must be partially consumed and the concentration of Fe2+ must decrease. When the corrosion further continues, FeCO3 and Fe(OH)2 begin to co-precipitate and the pH is fixed at 8.8 and Fe2+ concentration at 2.04 �10-4 mol/l. Regardless of continuation of the corrosion, the concentration of each species is fixed in the system because in the phase rule of Gibbs, f=c-p+n=0. In this system, there are three independent components (iron, CO2 and water) or c=3, three phases (liquid phase and two solid phases, FeCO3 and Fe(OH)2) or p=3, and the system temperature and pressure are given or n=0.

In the corrosion process, iron complexes are insignificant because each of their concentrations is much smaller than that of free Fe2+ (Figure 7). Compared to corrosion in de-ionized water, the free Fe2+ concentration is greater in the presence of dissolved CO2 and the principal anion is bicarbonate ion.

The time required for FeCO3 to saturate a 0.5 mm thick solution layer is about 88.8 minutes at a corrosion rate of 0.01 mm/y.[10] This limited time indicates that pipe corrosion should usually occur under FeCO3 precipitate or under the co-precipitate of FeCO3 and Fe(OH)2.

The calculated results have shown that free Fe2+ concentration in the FeCO3 saturated solution is much greater than any iron complex ion. Hence, in the calculation of equilibrium concentrations, the multiphase, multicomponent system may be simplified by neglecting the complex ions.

Conclusions

For pipeline corrosion, with its rate controlled by O2 diffusion across a disbonded HDPE coating, the precipitate is Fe(OH)2 if O2 pressure is greater than 9/20 times of CO2. The precipitate is FeCO3 otherwise. If the corrosion rate is controlled by O2 diffusion across a solution layer, the precipitate is Fe(OH)2 if O2 pressure is greater than 14 times of CO2. The precipitate is FeCO3 otherwise. This estimation for the type of precipitate may not be accurate with consideration of CO2 hydration (slow) but would be useful when the hydration information is not available.

The solution pH increases with increasing extent of corrosion. During corrosion, the concentration of each iron complex is significantly smaller than free Fe2+. The role of complex species in iron corrosion is insignificant and may be neglected.

The time is short, less than 1.5 hours, for a 0.5 mm thick water layer on the iron surface to be saturated with a precipitate, Fe(OH)2 or FeCO3. Pipeline corrosion may often occur under a precipitate.

References

1.      F.M. Song, D.W. Kirk, J.W. Graydon, D.E. Cormack, Corrosion 58 (2) (2002) 145-155.

2.      F.M. Song, D.W. Kirk, J.W. Graydon, D.E. Cormack, �Prediction for CO2 Corrosion of Active Steel under FeCO3 Precipitate�, Corrosion/04, Paper #04382 (New Orleans, LA: NACE, 2004).

3.      F.M. Song, D.W. Kirk, J.W. Graydon, D.E. Cormack, Journal of the Electrochemical Society, 149(11) (2002) B479-B486.

4.      F.M. Song, �Corrosion of Coated Pipelines with Cathodic Protection�, Ph.D. Dissertation (2002), University of Toronto, pp. 114-129.

5.      J. Brandrup, E.H. Immergut, Polymer Handbook, 3rd (ed) (John Wiley & Sons, Inc, 1989) VI/435-444.

6.      M.D. Koretsky, F. Abooameri, J.C. Westall, Corrosion 55(1) (1999): 52-64.

7.      P. Lorbeer, W.J. Lorenz, Electrochimica Acta 25 (1980): 375-381.

8.  G.M. Murray, G.K. Schweitzer, F.K. Heacker, Corrosion 46 (2) (1990) 95-99.

9.  J. Smith, T. Riley, VCS Software (1991), Department of Chemical Engineering, University of Toronto, Toronto, Ontario, Canada.

10.  D. Behrens, DECHEMA Corrosion Handbook: Corrosive Agents and Their Interaction with Materials, vol. 2 (Weinheim, Federal Republic of Germany: Frankfurt am Main: DECHEMA, 1988) 269.

Figures

 

Figure 1: Oxygen and carbon dioxide diffusion through a disbonded coating and a solution layer underneath to cause corrosion of pipe steel and to form a ferrous carbonate or ferrous hydroxide precipitate (oxygen diffusion controls the corrosion rate).

Figure 2: Corrosion and precipitation of a pipe steel in deionized water under a disbonded coating.

Figure 3: Variation of concentrations of iron species with pH as the pipe steel corrosion continues in deionized water.

Figure 4: Variation of concentrations of iron species in logarithm with pH as the pipe steel corrosion continues in deionized water.

Figure 5: Corrosion and precipitation(s) of a pipe steel in a solution with an initial carbon dioxide pressure under a disbonded coating.

Figure 6: Variation of concentrations of iron species with pH as the pipe steel corrosion continues in a solution with dissolved carbon dioxide. Initial carbon dioxide pressure is 0.23 atm.

Figure 7: Variation of concentrations of iron species in logarithm with pH as the pipe steel corrosion continues in a solution with dissolved carbon dioxide. Initial carbon dioxide pressure is 0.23 atm.