ARTICLE pubs.acs.org/JPCB
Kinetics and Mechanism of the Electrochemical Formation of Iron Oxidation Products on Steel Immersed in Sour Acid Media Antonio Hernandez-Espejel,† Manuel Palomar-Pardave,*,‡ Roman Cabrera-Sierra,§ Mario Romero-Romo,‡ María Teresa Ramírez-Silva,|| and Elsa M. Arce-Estrada*,† †
Instituto Politecnico Nacional-ESIQIE, Departamento de Ingeniería en Metalurgia y Materiales, UPALM, Ed. 7. Mexico D. F., Mexico rea Ingeniería de Materiales, Mexico D. F., Mexico Universidad Autonoma Metropolitana-Azcapotzalco, Departamento de Materiales, A C.P. 02200 § Instituto Politecnico Nacional-ESIQIE, Departamento de Ingeniería Química Industrial, UPALM, Ed. 7. Mexico D. F., Mexico rea Química Analítica, Mexico D. F., Mexico C.P. 09340 Universidad Autonoma Metropolitana-Iztapalapa, Departamento de Química, A
)
‡
ABSTRACT: From electrochemical techniques (cyclic voltammetry, potential steps, and EIS), XRD, and SEM-EDX, the kinetics and mechanism of anodic film formation applying anodic potential steps on steel immersed in sour acid media was determined. It was found, from a thermodynamic analysis, based on equilibrium phase diagrams of the system considered in this work, that iron oxidation may produce different new solid phases, depending on the applied potential, the first being the iron oxidation associated with formation of FeS(c) species, which in turn can be reoxidized to FeS2(c) or even to Fe2O3(c) at higher potential values. From analysis of the corresponding experimental potentiostatic current density transients, it was concluded that the electrochemical anodic film formation involves an E1CE2 mechanism, whereby the first of the two simultaneous processes were the Fe electrochemical oxidation (E1) followed by FeS precipitation (C) that occurs by 3D nucleation and growth limited by mass transfer reaction and FeS oxidation (E2) forming a mix of different stoichiometry iron sulphides and oxides. From EIS measurements, it was revealed that the anodic film's charge transfer resistance diminishes as the potential applied for its formation becomes more anodic, thus behaving poorly against corrosion.
1. INTRODUCTION Normal operations in the petroleum and petrochemical industries give rise to quite a complex variety of corrosive environments. For instance, crude oil and gas normally contain, aside from water, considerable amounts of hydrogen sulphide or carbon dioxide, and more often than not, combinations of both.1-5 It has been rightly asserted that the presence of H2O, H2S, and CO2 added to impurities such as chlorides and cyanides, as well as the operating conditions, like the temperature, pressure, and flow velocity,6-9 promote the corrosion process.10-12 In the majority of cases, failure occurs after there has been excessive metal wastage, or when hydrogen-induced cracking (HIC), hydrogen embrittlement (HE), stress corrosion cracking (SCC), or sulphide stress corrosion cracking (SSCC) have occurred. Steel corrosion in the presence of H2S has been studied in the field and in the laboratory.13-16 Sosa et al.17 prepared corrosion films using potential pulse applications to an AISI-SAE 1018 steel in a brine intended to emulate alkaline-sour conditions and determined the electrochemical behavior, further establishing the morphology of the surface corrosion products by means of EIS, SEM, and SPECM methods. The EIS diagrams and the SPECM images showed relative homogeneity and associated it to the passivity displayed by the corrosion products’ films formed. Other studies reported formation of films prepared through immersion techniques and cyclic voltammetry at various inversion potentials using alkaline-sour media, thereby establishing that the r 2011 American Chemical Society
products formed depended on the immersion time or on the potential interval within which the steel was cycled during film formation.18 Veloz and Gonzalez,19 studied the behavior of AISI-SAE 1018 carbon steel, in a Cl--containing HAc buffer solution, with and without H2S obtaining polarization resistance (PR) plots through electrochemical methods. The authors established, on analyzing the features of polarization plots, that the anodic slope was not modified in the presence of H2S, however, the cathodic branch seemed to be more sensitive, displaying the influence of an accelerated reduction reaction taking place. The impedance spectra revealed the steel as being largely active in the solution studied, though at low frequencies there were some differences observed. Several authors suggested that the first corrosion product that forms over the steel’s surface is mackinawite (tetragonal, Fe(1þx)S).20-22 Shoesmith et al.23 reported the formation of three iron monosulphide phases: mackinawite, troilite (hexagonal, FeS) and the amorphous iron sulphide in H2S unstirred aqueous solutions. Tewari and Campbell24 observed the formation of a series of nonstoichiometric iron sulphides,25-28 the mackinawite being the initial product; while, Lucio-Garcia et al.29 suggested Received: July 22, 2010 Revised: January 5, 2011 Published: February 8, 2011 1833
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The Journal of Physical Chemistry B the formation of FeS on steel exposed to the acid media containing H2S. In our previous work,30 the influence of different corrosion films formed by cyclic voltammetry and immersion tests on the steel through application of EIS and PR aided by surface characterization by means of SEM-EDX were studied. This work allows identification of the formation of both iron sulphides and oxides since the damage of the steel was favored by positive potentials and longer immersion times. However, hitherto, there is a lack of information regarding the mechanism of passivating corrosion products formation on steel, particularly in acid sour media. Therefore, the main goal of this work is to study the nucleation and growth kinetics of the corrosion products’ films formed on steel by means of the potential step technique.31-33 This electrochemical method has been successfully applied to characterize the nucleation and growth of PbSO4 on lead anodes in lead-acid batteries,31 as well as the formation of polypyrrole32 and hard gold electrodeposition.33
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Table 1. Thermodynamic Constants, 298 K, and the Chemical Species Considered during Construction of Figure 1 Using the Software MEDUSA and Database HYDRAa product
log K
reactants H
Fe3þ Fe(HS)2
were machined out directly from commercial steel with the following dimensions: 0.5 mm diameter 0.9 mm long, served as working electrodes; the nominal composition is as follows: 0.21 wt % C, 0.6 wt % Si, 1.2 wt % Mn, 0.03 wt % P, 0.036 wt % S, 0.025 wt % Cu, balance Fe. The samples were embedded in a Teflon base leaving only its cross-sectional area (0.196 cm2) exposed to the electrolyte. Prior to the potentiostatic study aimed at electrochemically forming the corrosion products in acid media, the electrode surface was prepared through conventional metallographic techniques, ground down with 600 grit emery paper; thereafter, the specimens were washed with water, degreased, and ultrasonically cleaned in acetone for 5 min. 2.2. Experimental Setup. The experiments were conducted in a conventional three-electrode cell (50 mL) connected to an AUTOLAB 30 potentiostat/galvanostat. The reference electrode was saturated calomel that was inserted separately in a compartment lodging the Lugging capillary and a large graphite bar, which served as counter electrode; all potentials reported in this work should refer to the said electrode. For the electrochemical tests, an NACE ID 182 solution containing 1.64 M NaCl, 0.055 M CaCl2, 0.018 M MgCl2 3 6H2O, 0.77 M CH3COOH, 0.29 M Na2S 9 H2O, and 0.93 mM Na2SO3 at pH 5.68 was prepared from analytical grade reagents dissolved in deionized water (18.2 MΩ cm). 2.3. Corrosion Film Formation. 2.3.1. Cyclic Voltammetry. In order to carry out kinetic studies on the API 5 L X52 carbon steel, it was first required that we establish through cyclic voltammetry (CV) the potentials as well as the current values generated during the oxidation and/or reduction process occurring at the metal-sour solution interface. The potentials were obtained from the voltammetry curves resulting after 6 complete successive cycles, which were needed to obtain a reproducible response by the system. First, the scans started at the open circuit potential in the positive direction until a switching potential was reached at -0.6 V; thereafter, the scan proceeded in the negative direction up to -1.0 V. Lastly, the potential scan was reversed toward the open circuit potential. In order to approach reproducibility criteria, different scan rates were considered during this work, we performed the experiments three times to verify that the results were reproducible.
HS-
-13.02
0
-1
1
0
8.95
0
0
1
2
0 -2
0 0
1 1
3 0
Fe(OH)3-
-33.4
-3
0
1
0
Fe(OH)4-2
-46.35
-4
0
1
0
-10.2
0
-1
0
1
H2S
6.994
1
0
0
1
H2S(g)
7.991
1
0
0
1
S2-
-5.903
-2
-3
0
2
S22-
-12.734
-2
-2
0
2
S2O32-
-28.793
-8
-8
0
2
S32-
-7.265
-3
-4
0
3
S42-
-3.022
-4
-6
0
4
S52-
-1.007
-5
-8
0
5
0.844
-6
-10
0
6
SO32-
-37.089
-7
-6
0
1
SO42-
-33.692
-9
-8
0
1
Fe(OH)2þ
-18.69
-2
-1
1
0
Fe(OH)3
-25.58
-3
-1
1
0
Fe(OH)4-
-34.62
-4
-1
1
0
Fe(SO4)2-
-75.024
-18
-17
1
2
Fe2(OH)2þ4
-28.99
-2
-2
2
0
þ5
S62-
-45.36
-4
-3
3
0
FeHSO4þ2
-42.244
-8
-9
1
1
FeHSO4þ
-30.624
-8
-8
1
1
-125.87
-8
-4
1
0
Fe3(OH)4
FeO42-
-15.21
-1
-1
1
0
FeS2O3þ
-39.833
-8
-9
1
2
FeSO4
-31.442
-9
-8
1
1
FeSO4þ
-42.672
-9
-9
1
1
H2S2O3
-26.521
-6
-8
0
2
H2SO4
-33.692
-7
-8
0
1
HS2O3-
-27.108
-7
-8
0
2
HSO3-
-29.869
-6
-6
0
1
HSO4-
-31.712
-8
-8
0
1
Fe(c)
-16.097
0
2
1
0
Fe(OH)2(c)
-12.996
-2
0
1
0
Fe3O4(c)
-37.077
-8
-2
3
0
3.915
-1
0
1
1
FeS(c)
4.648
-1
0
1
1
FeS2(c)
18.479
-2
-2
1
2
FeOH
2þ
FeS(am)
2.145
-1
-2
0
1
Fe(OH)3(am)
-17.911
-3
-1
1
0
Fe2(SO4)3(c)
-130.696
-27
-26
2
3
-26.448
-6
-2
2
0
S(c)
Fe2O3(cr) 1834
Fe2þ
10.987 -20.8
Fe(OH)
2.1. Sample Preparation. API 5 L X52 carbon steel cylinders
e
-
Fe(HS)3Fe(OH)2
þ
2. EXPERIMENTAL SECTION
þ
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Table 1. Continued product
Fe3(OH)8(c) Fe3S4(c)
log K
reactants Hþ
e-
Fe2þ
HS-
-46.262
-8
-2
3
0
18.995
-4
-2
3
4
FeOOH(cr)
-14.02
-3
-1
1
0
FeSO4:7H2O(c)
-31.483
-9
-8
1
1
a
The equilibria associated with the constants (K) are stated as follows: wHþ þ xe- þ yFe2þ þ zHS- f product.
2.3.2. Potentiostatic Current Transients. The corrosion films' formation kinetics on steel immersed in the sour acid solutions were studied applying anodic potential pulses (Ea) in the -0.63 < Ea < -0.38 V potential range, at a starting potential of -0.82 V. 2.4. Corrosion Films Characterization. 2.4.1. Physical Characterization. In order to minimize the spontaneous oxidation of the corrosion products by air, see Boursiquot et al.,34 once the films were formed onto the steel samples, through application of a potential step, they were stored in a desiccator under a vacuum atmosphere no longer than 10 min, and then they were characterized by X-ray diffraction (XRD) technique within the 20° to 90° scanned range and a step-width of 0.02°, using a D8 Focus Bruker diffractometer with Cu KR radiation. Afterwards, the analysis of the XRD spectra was carried out using the CreaFit2.2 DRXWin program. Moreover, morphological and microanalysis studies were also carried out on the samples by scanning electron microscopy (SEM/EDS) in a JEOL JSM 6300 operated at 20 kV, 220 μA, and a work distance of 39 mm. 2.4.2. Electrochemical Characterization. Once the films were formed through application of a potential step, they were characterized by Electrochemical Impedance Spectroscopy (EIS). The EIS measurements were performed in the 10 0000.01 Hz frequency range with 10 mV amplitude of the sinusoidal voltage signal, from the open circuit potential. These electrochemical measurements were carried out at room temperature.
Figure 1. E-pH diagram constructed using MEDUSA software35 and the species and thermodynamic constants reported in Table 1, at a constant temperature of 298 K, [Fe(II)] = 100 mM and [S(-II)] = 300 mM.
Figure 2. Theoretical predominant chemical species zone formed in the steel surfaces immersed in the NACE ID 182 solution as a function of the applied potential.
3. RESULTS AND DISCUSSION 3.1. Thermodynamic Analysis of the System. Considering that oxidation of steel immersed in the NACE ID 182 aqueous solution involves a multicomponent and multireactive system in which acid-base (proton, Hþ, exchange) and complexation reactions may occur along with charge, e-, transfer, and that this may conduce to the formation of a large number of chemical species (see Table 1), therefore, it is very convenient to analyze, at the experimental conditions, which species are the thermodynamically most favored to be formed. In order to do this, we construct, using MEDUSA35 software with its thermodynamic constants HYDRA database, a Pourbaix-type diagram (E-pH), see Figure 1. For the construction of this diagram, 55 soluble and 14 solid species were considered along with the experimental conditions. Table 1 summarizes the data corresponding only to the most significant products. Using the data shown in Figure 1 and the experimental pH (5.68), it is possible to construct the predominance zone diagram shown in Figure 2. From it, one could note that in the system considered in this work, iron oxidation may produce different new solid phases, depending on the applied potential, the first being iron oxidation associated with the formation of FeS(c)
Figure 3. Experimental cyclic voltammetry plots recorded in the system (a) API 5 L X52/NACE ID 182 solution and (b) API 5 L X52/NACE ID 182 solution without Na2S. The scan rate was 75 mVs-1. The inset depicts the variation of the current density peak as a function of the square root of the potential scan rate.
species, which in turn can be reoxidized to FeS2(c) or even to Fe2O3(cr) at high potential values. It is important to stress, from Figure 1, that sulfur may be oxidized, producing sulfate ions. 3.2. Voltammetric Study. Figure 3a shows a typical CV recorded in the system API 5 L X52/NACE ID 182 solution. It is possible to note the formation of a well-defined current density peak at around -0.67 V during the anodic segment, which could be related to the formation of a corrosion layer on 1835
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The Journal of Physical Chemistry B the steel surface mainly composed by iron sulphides.30 For potential values above -0.65 V, a transpassive behavior is evident, because the current increase indicates an active condition of the passive layer previously formed. Similar CVs were recorded when different potential scan rates were applied, not shown. As is possible to note from the inset in Figure 3, the corresponding current density peaks vary linearly with the square root of the potential scan rate, thus indicating that the oxidation reaction is mass transport-controlled. It is important to mention that when sulphide ions are not present in the aqueous solution, see Figure 3b, the passive zone is not observed and iron oxidation occurs at more positive potential values. This experimental evidence may support that, in the system under study, the electrochemical formation of FexSy occurs before an FexOy-phase can be formed as was predicted in Section 3.1. This was also observed by Hansson et al.36 in the case of high purity iron oxidation. 3.3. Potentiostatic Study. Potentiostatic current density transients (j vs t) were obtained for the oxidation of the API 5 L X52 steel by applying potential steps starting in all cases at E1 = -0.82 V to a second more positive potential Ea, such that this Ea is located within the -0.63 e Ea e -0.38 V range (transpassive zone). As can be seen in Figure 4, in all cases, the j-t transients reach a limiting current and in some cases, for the more positive potential values, this is so after going through an early maximum. In the literature37-46 different theoretical models to
Figure 4. Experimental current density transients recorded in the system API 5 L X52/NACE ID 182 solution after applying different potentials steps, as indicated.
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describe the nucleation and growth for the new phases taking place on the electrode after applying potential step perturbations to the electrode surfaces have been established for both: anodic films formation (namely: PbSO4 onto Pb31 and Polypyrrole on graphite32) and metal deposition (namely: Au,33 Ag39,42 and Co38,41,45). In particular, Palomar-Pardave et al.,45 have proposed a theoretical framework that describes j-t plots with very similar shapes as those shown in Figure 4. According to the authors,45 this sort of j-t plots is obtained when new phases are potentiostatically formed on the electrode surface by a multiple hemispherical 3D nuclei that grew limited by the mass transfer reaction, simultaneously with another faradaic process, like an oxidation reaction, that is occurring on the growing surface of the new phases. On the basis of the Palomar-Pardave et al. model45 and taking into account the possibility of iron sulphides and oxides mixed into the formation in this system, see Section 3.1, in our case, the equation derived to describe this process is as follows: ð1Þ jtotal ðtÞ ¼ j3D ðtÞ þ jSO ðtÞ The overall current density-time transient, jtotal(t), is given by the sum of two contributions, one due to the formation and
Figure 5. Comparison between an experimental potentiostatic current transient (OOO) obtained during the oxidation process of the steel API 5 L X52 immersed in the NACE ID 182 solution at an applied potential of -0.48 V, and a theoretical transient (solid line) obtained through nonlinear fitting of eq 6 to experimental data.
Scheme 1. Schematic Representation of the Cross Section of the Steel Electrode during the Anodic Film Formation in Sour Acid Mediaa
a
The different anodic current contributions, j3D and jSO are indicated. 1836
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Table 2. Best Fit Parameters Obtained during the Simulation of the Experimental Current Density Transients Shown in Figure 2 with Nonlinear Fitting of eq 6 to the Experimental Data -E (mV) P1 (μA cm-2) 103P2 (s-1)
P3 (s-1)
P4 (μA cm-2s1/2)
0.38
300.82 ( 0.01
23.3 ( 0.1
0.18 ( 0.02
14.5 ( 0.1
0.43
230.90 ( 0.28
59.4 ( 0.1
0.11 ( 0.03
14.3 ( 0.3
0.48
190.2 ( 0.2
21.4 ( 0.1
0.12 ( 0.01
15.2 ( 0.8
0.53
160.7 ( 0.6
7.5 ( 0.3
0.02 ( 0.04
13 0.4 ( 0.8
0.58
110.84 ( 0.03
7.90 ( 0.02
0.30 ( 0.03
14.4 ( 0.4
0.63
90.69 ( 0.21
4.1 ( 0.8
0.30 ( 0.02
14.6 ( 0.6
growth of the FeS deposit, j3D(t), which involves an E1C mechanism31 where the electrochemical reaction (E1) is due to iron oxidation and the chemical reaction (C) is related with FeS formation and precipitation (see reaction 2) and a second electrochemical reaction (E2) due to the FeS oxidation, jSO(t) forming other iron compounds on the steel. This subsequent oxidation (E2) could be related with the formation of orthorhombic Marcasite (see reaction 3), cubic Maghemite, iron oxide, or rhombohedral Hematite (see reaction 4) and cubic Magnetite (see reaction 5). FeðcÞ þ H2 SðaqÞ a FeSðcÞ þ 2Hþ ðaqÞ þ 2e
ð2Þ
FeSðcÞ þ H2 SðaqÞ a FeS2ðcÞ þ 2Hþ ðaqÞ þ 2e
ð3Þ
2FeSðcÞ þ 3H2 OðaqÞ a Fe2 O3ðcrÞ þ 2H2 SðaqÞ þ 2Hþ ðaqÞ þ 2e
ð4Þ 3FeSðcÞ þ 4H2 OðaqÞ a Fe3 O4ðcrÞ þ 3H2 SðaqÞ þ 2Hþ ðaqÞ þ 2e
ð5Þ
Figure 6. Variation of ln (P1) as a function of the applied potential. The linear fit (line) of the experimental data is also shown.
Eventually, these processes will drive to formation of different stoichiometry iron compounds, assuming the oxidation of the FeS initially formed on steel through an E1CE2 mechanism. According to reactions (3)-(5), the different iron compounds could be formed simultaneously on steel, resulting in the presence of heterogeneous corrosion products, see Scheme 1. Equation 1 can be parametrized as follows:45 1-expð-P3 tÞ jtotal ðtÞ ¼ ðP1 þ P4 t-1=2 Þ 1-exp -P2 tP3 ð6Þ
Figure 7. X-ray diffractograms, with intensity in arbitrary units, obtained on the steel samples that were immersed for ∼2000 s in the NACEID 182 solution after applying different potential steps, as indicated. The XRD pattern of the steel substrate, blank, before immersion is also shown. 1837
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Figure 8. Secondary electron images and their corresponding EDX analyses, of the API X52 surface immersed in the NACE- ID 182 solution, after imposing two different anodic potentials (a,b) -0.53 and (c,d) -0.43 V.
With
2c0 M 1=2 zSO FkSO P1 ¼ πF
ð7Þ
P2 ¼ N0 πk0 D
ð8Þ
P3 ¼ A
ð9Þ
P4 ¼
zFD1=2 c0 π1=2
k0 ¼ ð8πc0 =FÞ1=2
ð10Þ ð11Þ
Where, in the present case, c0 is the H2S concentration of the bulk solution, F is the Faraday constant, F is the density of the deposit (FFeS = 4.83 gcm-3), M is its molar mass (MFeS = 87.91 g mol-1), zSOF is the molar charge transferred during the iron oxidation process, kSO is the rate constant of the FeS oxidation reaction, D is the H2S diffusion coefficient, A is the FeS nucleation rate, and N0 number density of active sites for FeS nucleation on the electrode surface. Figure 5 shows a comparison of an experimental j-t plot recorded during nucleation and growth of the film formed on the steel immersed in the NACE ID solution, see Figure 4; when
a potential of -0.48 V was imposed to the electrode surface and a theoretical current density transient generated by a nonlinear fit of eq 6 to the experimental data. It is possible to note that the theoretical model represented by eq 1 adequately describes the whole current density transient recorded. Similar results were obtained for all j-t plots shown in Figure 4. The best fit parameters obtained during the simulation process are reported in Table 2. The variation of P1 parameter as a function of the applied potential is depicted in Figure 6, where a linear relationship is clearly revealed. As is possible to note from eq 7, P1 is proportional to the rate constant of the FeS oxidation reaction, kSO. Therefore, this indicated that kSO satisfied a Butler-Volmer relationship that can be modeled by eq 12.45 -Rso zFE kSO ¼ k0SO exp ð12Þ RT From the slope of the linear fitting presented in Figure 6, a value of 0.25 was obtained for the FeS oxidation charge transfer coefficient, RSO, considering a temperature of 298 K. Moreover, from the average of P4 parameter it is possible to calculate the diffusion coefficient, D, of H2S (2.5 10-5 cm2s-1) which is in close agreement with that reported by Tamimi et al., using a wetted-sphere absorption apparatus.46 1838
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The Journal of Physical Chemistry B According to this potentiostatic study, the different iron compounds externally formed on the steel see reactions 2-5 occur simultaneously through an oxidation-precipitation mechanism. 3.4. XRD Analysis. Figure 7 shows the XRD patterns obtained after the formation of corrosion films applying different potential steps to the steel surface immersed in the sour solution. The signals correspond to orthorhombic marcasite (FeS2, 2θ = 44.77° and 65.03°, PDF Card 370475), cubic maghemite (Fe2O3, 2θ = 44.81° and 65.08°, PDF Card 391346), hexagonal troilite (FeS, 2θ = 65.03° and 82.43°, PDF Card 370477), iron oxide (β-Fe2O3, 2θ = 65.08°, PDF Card 390238), cubic magnetite (Fe3O4, 2θ = 65.08°, PDF Card 190629), rhombohedral hematite (Fe2O3, 2θ = 82.391°, PDF Card 330664), tetragonal mackinawite (FeS, 2θ = 82.43°, PDF Card 150037), and cubic pyrite (FeS2, at 2θ = 98.92°, PDF Card 060710) peaks. It is important to emphasize that even when XRD results may give some experimental support to the existence of the compounds formed by reactions 2-5 which were initially proposed to describe the experimental current density transients, see Section 3.3, a deep study is still required in order to confirm the presence of the assumed corrosion products, see for instance Hansson et al.,36 where the XPS technique is used to fully elucidate the presence of the type of sulphides formed: mono or nonstoichiometric sulphide species. Moreover, it is important to clarify that such study is out of the scope of the present work because it must be carried out within the potential that corresponds to the passive zone in order to avoid oxidation of the FexSy products. Under the conditions considered in this work, the transpassive zone, such work will be useless since most of the FexSy passive layer formed is oxidized to FexOy, making it difficult to fully demonstrate its presence. 3.5. SEM and EDS Analysis. In Figure 8, one could notice the evolution of the morphology of the corrosion films formed on the steel at two different anodic potential pulses (Ea), namely 0.53 V (Figure 8a) and -0.43 V (Figure 8c). At -0.53 V, the scratch lines of the mechanical grinding on the steel are still evident; whereas for the more positive potential, an increase in the roughness of the film and the presence of sulfur on the electrode surface (see Figure 8d) is observed . According to the EDS analysis, this sulfur peak is related to the presence of iron sulphides, as predicted in Section 3.1 and used in the mathematical model described in Section 3.3. 3.6. EIS Characterization. Figure 9 shows the EIS diagrams (complex, Figure 9a and phase angle plots, Figure 9b) recorded after film formation by imposing different oxidation potentials, namely: -0.63, -0.43, and -0.38 V. This figure shows a similar electrochemical behavior; the main difference is observed at low frequencies because the complex plots are compressed as the potential becomes more positive, see Figure 9a. As suggested above, this fact is related to the steel dissolution through the iron sulphide films because their poor behavior against corrosion when the potential becomes more positive; this assumption supports the heterogeneous composition of the corrosion films formed by potentiostatic methods in agreement with XRD results (see Figure 7). In contrast, the Bode diagrams exhibit negligible variations as a function of the frequency interval, making it difficult to assign the time constants occurring simultaneously. Different equivalent circuits have been reported,47-50 taking into account the features of the corrosion films; when porous films are in contact with an aqueous solution the characterization
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Figure 9. (a) Typical Nyquist diagrams obtained for the API X52 steel samples in acid sour media after imposing different potentials: -0.63, -0.43, and -0.38 V, the solid lines represent the best fitting for the experimental data through the equivalent circuit proposed in Figure 10. The various symbols on the traces correspond to the experimental data. (b) Corresponding Bode plots.
by EIS reached higher values in the complex diagrams. However, in these works, the porous films are thicker by more than 100 μm and formed by different accelerated conditions. In these works, the characterization of iron oxides is controlled by the diffusion of molecular oxygen following a semi-infinite porous electrode. Thus, considering the chemical composition of the iron sulphides and oxides formed on the steel (different semiconductor properties), the morphology, as well as the voltammetry characterization, it can be suggested that the diffusion occurring through the potentiostatically grown films is different. To address the mix formation of iron sulphides and oxides, the most likely explanation is related to the diffusion of iron ions through the films. Figure 10 shows the equivalent circuit proposed to describe the corrosion process occurring over the steel’s electrode surface. This circuit considers the following elements: the solution resistance, RS, the charge transfer resistance, RCT, and the capacitance of the film, Cfilm. It is important to note that to simulate the diffusion through the corrosion films, an arrangement of the Qdif and Rdif and Boukamp program49 is used to 1839
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Table 3. Parameter Values Obtained after the Fitting Procedure of the EIS Diagrams Using Boukamp Program Cfilm 105 (μFcm-2)
Cdif 105 (μFcm-2)
-E (V)
RS (Ω cm2)
RCT (Ω cm2)
Rdif (Ω cm2)
0.63
2.59
8.82
2.74
34.79
82.90
0.58
2.82
10.30
32.73
41.63
75.65
0.53
1.94
10.91
31.75
40.30
83.88
0.48
1.44
11.78
25.28
38.21
78.98
0.43
1.42
12.85
24.89
41.12
70.16
0.38
1.44
15.76
19.60
44.89
56.44
the diffusion element (Rdif) extrapolated by the Boukamp program supports this assumption, because a decrease is obtained as the potential becomes positive, favoring the oxidation of the steel. Finally, the E1C1E2 mechanism used to discuss the oxidation of the API X52 steel seems reasonabe, considering the formation of iron sulphides and oxides and their poor behavior against corrosion in the sour solution.
Figure 10. Equivalent circuit model for the API X52 steel to enable interpretation of the system’s behavior API 5 L X52/NACE- ID 182. Rs = solution’s resistance, Cfilm = capacitance of the film, RCT = charge transfer resistance, Qdif and Rdif is used to describe the iron ions diffusion through the corrosion films.
extrapolate the low frequency diffusion impedance. Afterward, the diffusion only is discussed and plotted against the resistive element, for the different experimental conditions. Table 3 summarizes the parameter values obtained after the fitting procedure using the equivalent circuit shown in Figure 10. The solution resistance showed slight modifications for the different potentials varying from 7 to 11 Ω.cm2. According to the charge transfer resistance values, minor modifications could be suggested in the coverage extent of the corrosion films formed on the steel. The literature has reported typical values, related to the formation of iron oxides,50 sulphides,22 and both, using different methods and electrolytic media. In most of these works, it has been established that those films involving oxides, sulphides or both, the capacitance values increase in the order of μF to mF.22,50 According to these results, the capacitance values obtained for -0.63, -0.58, -0.53, -0.48, -0.43, and -0.38 V varied from 90-171 μF 3 cm-2, typical of iron sulphides and oxides. Besides, this variation in the capacitance toward more positive potentials can be related to the predominant formation of iron sulphides (FeS2) from the beginning of steel oxidation; also, the presence of iron oxides, to a minor extent, are in agreement with XRD and EDS results mentioned above. Alternatively, the formation of iron compounds on the steel surface following an E1C1E2, favors the formation of corrosion films with a heterogeneous composition; however, the formation of new phases (presumably oxides) externally on the steel surface modify the precipitation (C1) and reoxidation (E2) steps during steel oxidation. In this way, the diffusion process occurring from the steel-corrosion films to corrosion films-aqueous solution interfaces is only associated with iron ions. Thus, the variation of
4. CONCLUSIONS The kinetics and mechanism of anodic film formation on steel immersed in sour acid media were determined. It was found that this anodic film is formed by a mix of different stoichiometric iron sulphides and oxides that were formed after applying anodic potential steps to the steel surface. From analysis of the corresponding experimental potentiostatic current density transients, it was concluded that the electrochemical anodic film formation involves two simultaneous processes associated with iron oxidation, precipitation of FeS, and its oxidation. The protective properties of the anodic films diminish as the potential applied for its formation becomes more anodic. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (E.M.A.-C.);
[email protected] (M.P.-P).
’ ACKNOWLEDGMENT A.H.E. is grateful for his postgraduate studentship to CONACYT and SIP-IPN. The authors are also grateful for financial support from the SIP 2009-0561 project and SNI-CONACYT. M.P.P. and M.R.R. thanks CONACYT for Project Nos. 24658 (“Nucleacion y crecimiento electroquímico de nuevas fases”) and 131432. Also E.M.A.-E. M.P.P., M.T.R.S., R.C.S., and M.R.R. gratefully thank the SNI for the distinction of their membership and the stipend received. M.R.R. and M.P.P. wish to thank the Departamento de Materiales, UAM-A, for financial support given through Project Nos. 2261203, 2261204, 2261205. ’ REFERENCES (1) Rena, C.; Liu, D.; Bai, Z.; Li, T. Mater. Chem. Phys. 2005, 93, 305. (2) Yin, Z. F.; Zhao, W. Z.; Bai, Z. Q.; Feng, Y. R.; Zhou, W. J. Electrochim. Acta 2008, 53, 3690. (3) Amri, J.; Gulbrandsen, E.; Nogueira, R. P. Electrochem. Commun. 2008, 10, 200. (4) Lopez, D. A.; Schreiner, W. H.; de Sanchez, S. R.; Simison, S. N. Appl. Surf. Sci. 2003, 207, 69. (5) Natividad, C.; Salazar, M.; Contreras, A.; Albite, A.; Perez, R.; Gonzalez-Rodríguez, J. G. Corros. 2006, 62, 375. (6) Ateya, B. G.; Al Kharafi, F. M.; Abdalla, R. M. Mater. Chem. Phys. 2002, 78, 534. 1840
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