Electrochemical characterization of the different surface states formed in the corrosion of carbon steel in alkaline sour medium

Corrosion Science 43 (2001) 2305±2324

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Electrochemical characterization of the di€erent surface states formed in the corrosion of carbon steel in alkaline sour medium R. Cabrera-Sierra a, M. Miranda-Hern andez b,c, E. Sosa b, T. Oropeza b, I. Gonz alez b,* a

Escuela Superior de Ingenierõa Quõmica e Industrias Extractivas (ESIQIE-IPN), Departamento de Ingenierõa Metal urgica, A.P:75-874, C.P. 07338, M exico, DF, Mexico b  Departamento de Quõmica, Area de Electroquõmica, Universidad Aut onoma Metropolitana-Iztapalapa, Apartado Postal 55-534, C.P. 09340, M exico, DF, Mexico c Instituto Mexicano del Petr oleo, Coordinaci on de Simulaci on Molecular, Area de materiales y corrosi on, Eje central L azaro C ardenas No. 152, C.P. 07730, M exico, DF, Mexico Received 17 October 2000; accepted 2 February 2001

Abstract In this study the di€erent surface states that manifest in the corrosion process of 1018 carbon steel in alkaline sour environment, solution prepared speci®cally to mimic the sour waters occurring in the catalytic oil re®nery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH4 )2 S and 10 ppm NaCN at pH 9.2) were prepared and characterized. The surface states of the carbon steel were formed by treating the surface with cyclic voltammetry at di€erent switching potentials (Ek‡ ), commencing at the corrosion potential (Ecorr ˆ 0:890 V vs sulfate saturated electrode, SSE). The surface states thus obtained were characterized using electrochemical impedance spectroscopy and scanning electron microscopy techniques. It was found that for Ek‡ ˆ 0:7 and 0:6 V vs SSE a ®rst product of corrosion formed, characterized by a high passivity. Moreover, it was very compact (with a thickness of 0.047 lm). However, at more anodic potentials (Ek‡ > 0:5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 lm and very active) was observed. The di€usion of atomic hydrogen (H0 ) was identi®ed as the slowest step in the carbon steel corrosion process in the alkaline sour media. The H0 di€usion coecients in the ®rst and second products that formed at the carbon steel±sour medium interface were of the order of 10 15 and 10 12 cm2 /s respectively. Ó 2001 Elsevier Science Ltd. All rights reserved.

*

Corresponding author. Tel.: +52-5804-46-71; fax: +52-5804-46-66. E-mail address: [email protected] (I. GonzaÂlez).

0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 0 3 8 - 5

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Keywords: Sour environments; Iron sul®de ®lms; Carbon steel; Corrosion; Electrochemical impedance spectroscopy

1. Introduction Oil-re®ning catalytic plants have serious corrosion problems due to the aggressive environments they manage, which contain complex mixtures of organic and inorganic chemical species. Several previous studies have been made on the corrosion rate and growth of corrosion ®lms in a re®nery environment. Among these studies the most signi®cant were those in aqueous environments containing 1±3% of chlorides and with an H2 S concentration ranging from 1 to 10 mM (saturation) [1,2]; those containing NH‡ [3]; and those studying other aqueous en4 , H2 S and Cl vironments saturated with H2 S in which the e€ect of CN and Cl was evaluated [4]. It is generally established that the corrosion process is carried out through the following mechanism [5]: anodic reaction

Fe ! Fe2‡ ‡ 2e

cathodic reaction

2 HSm;ad ! H‡ m;ad ‡ S

0 H‡ m;ad ‡ e ! Hm;ad

…1† …2a† …2b†

H0m;ad ‡ H0m;ad ! H2…g†

surface chemical reactions H0m;ad ! H0m;abs

…3a† …3b†

chemical reaction xFe2‡ ‡ yHS ! Fex Sy ‡ yH‡ 4

FeS ‡ 6CN ! ‰Fe…CN6 †Š

‡ S2

…4† …5†

HSm;ad and H‡ m;ad correspond to the disul®de ion and the proton adsorbed on the metal surface. After adsorption of the disul®de ions on the iron electrode (Eq. (2a)), the proton reduction reaction occurs (Eq. (2b)). A portion of the atomic hydrogen produced from proton reduction (Eq. (2b)) is incorporated into the lattice (Eq. (3b)), leading to blisters. Blisters form as the volume fraction of near-surface absorbed hydrogen increases until the absorbed atomic hydrogen expands the lattice at high volume fractions. Some of the absorbed atomic hydrogen may di€use into the voids (blisters) in the lattice caused by the expansion, ultimately forming molecular hydrogen. The rest of the atomic hydrogen produced in the reaction of Eq. (2b) simply evolves on the surface as H2…g† (Eq. (3a)).

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The chemical reaction that occurs in these environments (Eq. (4)) forms a nonstoichiometric sul®de ®lm at the metal surface the stability of which depends on the environment. In alkaline media (pH > 7) these layers were determined to be adherent, protective and composed principally of Makinawite (Fe…1‡x† S) [4,6±8]. In environments where cyanides are also present sul®de layers are dissolved [3,4], in part due to a possible reaction between the iron sul®de ®lm and the cyanide ions CN [5] (reaction (5)) which does not occur in the acid media. In a previous study [9] the corrosion process of 1018 carbon steel in an alkaline sour environment was evaluated using a solution prepared to mimic the conditions of the sour waters occurring in the catalytic plants of the PEMEX re®nery (0.1 M (NH4 )2 S and 10 ppm NaCN at pH 9.2). In this study, it was found that after immersion, the corrosion rate was initially rapid and a ®lm of maximum thickness formed. After 50 h of immersion, growth and dissolution of the ®lm reached equilibrium. In the same study, electrochemical impedance spectroscopy (EIS) technique showed that over the immersion time of the carbon steel in the sour medium at least two di€erent corrosion products formed. Moreover, it was found that the corrosion process in these media involved a series of simultaneous steps: a charge transfer process at the interface between the metal and the ®lm of corrosion products and di€erent di€usion processes that occur through the corrosion products. Fe2‡ ions and atomic hydrogen (H0 ) di€use through the ®lm, the di€usion of atomic hydrogen being the limiting step of the corrosion process. This previous study of the carbon steel corrosion process in sour medium used the immersion time as a parameter. This method did not permit any control over the evolution of the corrosion process at short times. However, it was established, at these short times, that di€erent corrosion products formed and the important changes in the interface properties occurred. In the present study electrochemical techniques were used to induce surface damage. These techniques permit the study of the carbon steel corrosion process in a sour environment as they simulate the different steel surface states when it is exposed to a corrosive media for short immersion times. The characterization and study of the di€erent corrosion products formed in sour media is of great importance. An understanding of the electrical and di€usional properties displayed by the corrosion products formed on the steel surface would help to reduce the signi®cant losses caused by corrosion and provide a base for new methods of inhibition and control. The objective of the present study is the use of di€erent electrochemical techniques to induce, characterize and systematically study the di€erent surface states produced by corrosion. Of particular interest was the simulation of the surface states formed at short times in the corrosion of carbon steel 1018 immersed in the media present in the catalytic plants of PEMEX-REFINERIES. To achieve this a sour environment was used composed of 0.1 M of (NH4 )2 S and 10 ppm of NaCN at a temperature of 30°C with pH ˆ 9:2, these concentrations being similar to those encountered in the catalytic plants.

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2. Experimental procedure 2.1. Experimental device description A typical three-electrode electrochemical cell, consisting of a 50-ml glass ¯ask with controlled temperature, was used. The re®nery environment was simulated using a 0.1 M (NH4 )2 S, 10 ppm NaCN solution with pH 9.2, prepared from analytical grade reagents and deionized water. The temperature of the electrolytic solution was maintained at 30  0:1°C using a HAKKE circulating water bath. These experiments were performed using a mercurous sulfate/sulfate saturated electrode (SSE) as a reference electrode with a separate compartment (Luggin capillar) and a graphite bar counter electrode of 6 mm diameter, 5 cm length, with a separate compartment (glass tube with porous frit). 2.2. Preparation of testing specimens The working electrode consisted of a 1018 carbon steel disc with a surface area of 0.5026 cm2 (/ ˆ 0:8 cm). This material has a typical composition of 0.14±0.20% C, 0.60±0.9% Mn, 0.035% maximum S, 0.030% maximum P and the remainder Fe. The electrode was embedded in a Te¯on base. In order to avoid solution ®ltration within this device, ``curing'' with a polyester resin and styrene mixture was carried out. The polishing method used to prepare clean steel surfaces was the following. First the carbon steel electrode surface was polished with silicon carbide emery paper (grade 400) and water. The surface was then polished with emery paper (grade 600) until a homogeneous surface was obtained, rinsed with acetone and, ®nally, the specimen was subjected for 5 min to ultrasonic washing with acetone. 2.3. Inducing damage on the carbon steel surface In order to induce damage one carbon steel 1018 electrode was introduced into an alkaline solution (0.1 M (NH4 )2 S, 10 ppm of CN as sodium cyanide and pH ˆ 9:2) at 30°C and the electrochemical technique (cyclic voltammetry) was applied. The potential scan was initiated in the positive direction starting from the corrosion potential (Ecorr ), at a rate of 20 mV/s, until reaching a switching potential …Ek‡ †. The scan potential was then inverted, the scan being stopped at Ecorr . The surface damaged in this way was characterized using an impedance study. The studied surfaces were damaged using the following Ek‡ : 0:7, 0:6, 0:5, 0:3, 0:2, 0:1, 0.1, 0.2, 0.3 and 0.5 V vs SSE. The program for potential of damage was always carried out on freshly polished surfaces. The corrosion potential of carbon steel in the alkaline sour environment, before damaging and after polishing, was 0:880 to 0:890 V vs SSE. The small variation in corrosion potential before every damage test con®rms the reproducibility obtained in the surface polishing of the steel. After the damaging of each surface (before impedance test), the corrosion potential was more negative.

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2.4. Electrochemical characterization tests 2.4.1. Electrochemical impedance spectroscopy tests After voltammetric damage was induced on the carbon steel surface, an EIS test was carried out on the same solution (containing 0.1 M (NH4 )2 S, 10 ppm of CN as NaCN). The impedance experiments were performed at the corrosion potential (Ecorr ) applying a sinusoidal signal of 10 mV amplitude within a frequency interval ranging from 10 kHz to 0.01 Hz. Experimental conditions were 30°C. The electrode was not rotated. An EG&G Parc 283 potentiostat/galvanostat, coupled to an SI-1260 Solartron impedance/gain-phase analyzer was used, results being recorded using a PC. Adjustment of experimental results was calculated using a non-linear regression algorithm created by Boukamp (equivalent circuit) [10]. 3. Results and discussion 3.1. Damage induction to carbon steel surface Fig. 1 shows the typical voltamperometric curves obtained during the carbon steel damage process in a sour environment, using the following switching potentials: 0:7, 0:6, 0:5, 0:3, 0:2, 0:1, 0.1, 0.2, 0.3 and 0.5 V vs SSE. Every voltammogram trace was executed separately, the surface being renovated by mechanical polishing before commencement for every experimental condition and potential scan. It is worth noting the reproducibility of the voltammograms obtained, above all in the initial scan. These arm the reproducibility of the surface polishing performed before every experiment. To achieve this reproducibility it was also necessary to continually renew the electrolyte solution. In the ®rst case (Ek‡ of 0:7 V), a current increase occurred in the direct potential scan. On inverting the potential scan the associated current was the same, indicating the same steel oxidation process. Di€erent voltammetric behavior was observed in the second case (Ek‡ of 0:6 V). On inverting the scan potential an apparent passivation was observed, this manifesting as a decrease in the magnitude of the current in the inverted potential scan. The subsequent voltammetric curves displayed the same electrochemical behavior (Fig. 1b). At the beginning of the forward potential scan a plateau in the oxidation was observed at a potential of 0:6 V vs SSE. This indicated a constant steel oxidation velocity that could be identi®ed with the appearance of a ®lm of oxidation products (partial passivation). After the plateau region an increase in current was observed, probably due to breakage of the ®lm of corrosion products that had previously formed, giving rise to another ®lm of corrosion products with completely di€erent properties. The magnitudes of the currents were notably greater for the inverse potential scan than for the direct scan, indicating that the ®lm that ultimately formed is by nature non-protective and very active.

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Fig. 1. Typical cyclic voltamperommetric curves obtained for 1018 carbon steel in alkaline sour environment (0.1 M (NH4 )2 S and 10 ppm NaCN, pH ˆ 9.2). The potential scan was initiated in positive direction from the corrosion potential (Ecorr ), to switching potential (Ek‡ ) then the scan was inverted and it was stopped at Ecorr . The Ek‡ was varied: (a) ( ) 0.7, ( ) 0:6, () 0:5; (b) (i) 0:3, (ii) 0:2, (iii) 0:1, (iv) 0.1, (v) 0.2, (vi) 0.3 and (vii) 0.5 V vs SSE.

It is worth mentioning that during the formation of the di€erent surface states, a blue precipitate was observed to appear on the surface. This precipitate could

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possibly be either NaFeS2 formed by the combination of Na‡ ions in the electrolyte and the sul®de ®lm formed in the oxidation process or Fe[Fe(CN)6 ]. The formation of these precipitates have been reported in the literature [11,12]. Owing to the low cyanide concentration in the solution, reaction (5) could modify the concentration of dissolved CN . Therefore, in order to obtain damage reproducibility a new solution must be used for each new experiment. The di€erences in these curves indicate the apparent formation of two diverse corrosion products on the substrate surface. The resulting product depended on the oxidation potential to which the carbon steel was subjected. In order to establish quantitatively the di€erence between the two corrosion products the charge associated with the damage process was determined by integrating the area under the curve from the corresponding voltammograms (Fig. 1). In Table 1, we present the charges associated with the oxidation process for carbon steel in the alkaline sour media. Comparing the anodic charges for every curve, it can be seen that the initially formed ®lm of corrosion products displayed certain characteristics of passivity. This arises because the anodic charge is very small at these damage potentials ( 0:7 and 0:6 V). Moreover, it increases only three times in passing from a potential of 0:7 to 0:6 V vs SSE. For more anodic switching potentials Ek‡ > 0:6 V vs SSE, the magnitude of the anodic charge started to increase considerably. An increase was observed for every damage condition, with an average of 130 mC for every 100 mV of damage. The properties of this second corrosion product indicated that it is completely active and apparently very porous. Finally, on passing from a potential of 0:6 to 0:5 V vs SSE, an increase in anodic charge was seen. This increase was four times greater than that observed in going from a potential of 0:7 to 0:6 V, indicating that growth of the second corrosion ®lm begins within the potential interval of 0:6 to 0:5 V. The damage obtained at 0.5 V therefore represents the intermediate point at which occurs the transition from one corrosion product to another. The formation of di€erent corrosion products with changing damage voltammetries was visually evident. For potentials of 0:7 and 0:6 V vs SSE formation of a homogeneous yellow ®lm was observed, the intensity of the ®lm increasing on Table 1 Anodic charge (Qa ) associated to the di€erent voltammetric curves (damage induction) performed on the carbon steel/sour environment as a function of positive switching potential (Ek‡ ) Ek‡ vs SSE (V) 0.7 0.6 0.5 0.3 0.2 0.1 0.1 0.2 0.3 0.5

Q anodic (mC) 6 19 57 169 298 428 702 875 1105 1642

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passing from a potential of 0:7 to 0:6 V. For the subsequent damage potentials (Ek‡ > 0:6 V vs SSE), the second corrosion product, black in color, began to form. As the damage potential was made more positive the degree of substrate coverage by this corrosion product increased. In order to better distinguish the properties of the diverse products formed in the corrosion process of carbon steel in an alkaline sour environment, a characterization using the scanning electron microscopy (SEM) technique was undertaken. 3.2. Physical characterization of the damage process using scanning electron microscopy Fig. 2 shows micrographs obtained from the carbon steel surface after voltammetric damage was induced in the alkaline sour environment. In Fig. 2a the micrograph obtained from a damage potential of 0:6 V is shown. In this micrograph can be seen the formation of a thin homogeneous ®lm over the surface of the sub-

Fig. 2. SEM images obtained, at di€erent magni®cations, from the surfaces of 1018 carbon steel previously damaged with cyclic voltammetric technique at di€erent Ek‡ in alkaline sour environment. (a) 2000, with Ek‡ ˆ 0:6 V vs SSE; (b) 50, with Ek‡ ˆ 0:3 V vs SSE, two di€erent zones are shown, a homogeneous layer of corrosion product (zone i) and one non-homogeneous zone (zone ii); (c) 500, zone i (Ek‡ ˆ 0:3 V), (d) 1000, zone ii (Ek‡ ˆ 0:3 V vs SSE).

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strate (the freshly polished surface), with scratches formed during mechanic polishing. Fig. 2b, on the other hand, shows in a general form the surface of the damaged substrate at a potential of 0.3 V vs SSE. Two di€erent zones are observed (Fig. 2b): one formed by a homogeneous layer of corrosion products (zone i), and another, non-homogeneous region (zone ii). The characteristics shown by both regions are more easily distinguished in magni®ed view of each zone given in Figs. 2c and d. In Fig. 2c the homogeneity of zone i of the ®lm can more clearly be seen. Its characteristics are very similar to those encountered for a damage potential of 0:6 V (Fig. 2a), con®rming the formation of the same corrosion product. In zone ii (Fig. 2d), however, the ®lm is rougher, indicating a change in porosity and the presence of another corrosion product. In this manner the formation of two di€erent corrosion products was con®rmed: one internally formed with highly protective properties and a small thickness; the other externally formed, at Ek‡ > 0:6 V vs SSE, with non-protective properties and a greater thickness. The degree of coverage of this ®nal corrosion product was a function of the damage potential. It is worth mentioning that at the beginning of the formation of the second corrosion product ®lm (Ek‡ > 0:6 V vs SSE), the interfacial properties also changed due to the continuous transformation in the damaged substrate surface as a function of switching potential. In this way di€erent zones formed on the surface, leading to the formation of a heterogeneous surface. This surface heterogeneity is responsible for the variations in the parameters evaluated using the EIS technique (see below). 3.3. Electrochemical impedance spectroscopy characterization Figs. 3 and 4 show typical Nyquist and Bode diagrams obtained over the surfaces of damaged steel. The damage was caused using voltammetry on carbon steel in a sour environment at various switching potentials. Every impedance diagram was obtained from a recently damaged surface and at the respective corrosion potential. For the surfaces damaged at di€erent switching potentials, three types of impedance diagrams can be distinguished. These represent the characteristic patterns of di€erent steel surface states in the corrosive medium. The ®rst type diagram presented (Fig. 3) is for surfaces damaged at potentials of 0:7 and 0:6 V. The second type diagram is for surfaces damaged at a potential of 0:5 V (Fig. 3), and the last type diagram is representative of the results for surfaces damaged at potentials greater than 0:5 V (Fig. 4). The ®rst diagram shows the same electrochemical behavior for surfaces damaged at potentials of 0:7 and 0:6 V over the entire frequency interval. This con®rms that at these damage potentials the same corrosion product formed. The impedance diagrams for damage potentials greater than 0:5 V also show a tendency toward the formation of the same corrosion product. They display the same basic electrochemical behavior with the di€erence that the magnitude of the impedance decreases with more positive damage potentials and higher degrees of coverage of the second corrosion product.

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Fig. 3. Typical impedance diagrams obtained for 1018 carbon steel in alkaline sour environment (0.1 M (NH4 )2 S and 10 ppm NaCN, pH ˆ 9:2), at 30°C. The surface was previously damaged by cyclic voltammetry technique with di€erent positive switching potentials: ( ) 0.7, ( ) 0.6, () 0.5 V vs SSE. (a) Nyquist diagrams, (b) enlargement of Nyquist diagrams presented in (a); some frecuencies (Hz) are indicated in the ®gures. (c) Bode diagram (phase angle (/) vs. frequency). Continuous lines represent adjustment of the experimental data through the equivalent circuit shown in Fig. 5.

Finally, the impedance diagram obtained over the damaged surface at a potential of 0:5 V demonstrates the transition in surface state that occurs on formation of the second corrosion product and the commencement of heterogeneous interface formation.

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Fig. 4. Typical impedance diagrams for 1018 carbon steel in alkaline sour environment (0.1 M (NH4 )2 S and 10 ppm NaCN, pH ˆ 9:2), at 30°C. The surface was previously damaged by cyclic voltammetry technique with di€erent positive switching potentials (Ek‡ ): (M) 0.3, ( ) 0.2, (Ð) 0.1, ( ) 0.3 and (- - -) 0.5 V vs SSE. (a) Nyquist diagrams, (b) enlargement of Nyquist diagrams presented in (a); some frequencies (Hz) are indicated in the ®gures. (c) Bode diagram (phase angle (/) vs. frequency). Continuous lines represent adjustment of the experimental data through the equivalent circuit shown in Fig. 5.

3.4. Discussion of electrochemical impedance spectroscopy diagrams In the curves at potentials of 0:7 and 0:6 V in the Nyquist diagram (Fig. 3a) at least two time constants in an intermediate to low frequency interval can be

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observed. Fig. 3b presents an enlargement of the Nyquist diagram, in which can be detected an additional time constant for all cases due to the presence of a capacitive loop at high frequencies. For more positive potentials, Ek‡ > 0:5 V, a considerable change is observed in comparison with the curves at 0:7 and 0:6 V. This change occurs both in the Nyquist diagram and the Bode diagram, / vs frequency (Fig. 4). In both diagrams it is possible to distinguish two di€erent regions in an intermediate to low frequency interval. In the Nyquist diagrams (Fig. 4a), the impedance spectra seem to show the same behavior, the only di€erence being in the magnitudes of the impedance. These magnitudes decreased as the switching potential at which the ®lms were prepared was made more positive. This feature is better observed in the Bode diagram (Fig. 4c) through the presence of a displacement of the time constants (formation of two maxima) to lower frequencies. Fig. 4b shows a magni®cation of the Nyquist diagram, in which another stage in the corrosion process can be identi®ed from the presence of a capacitive loop at high frequencies. Finally, the damaged surface at a potential of 0:5 V displays a characteristic spectrum that is primarily due to the transition from one corrosion product to another. For this damage potential, in all of the frequency intervals, the time constants obtained were the same as those of the initial potentials ( 0:7 and 0:6 V). The exception to this is a deviation in the time constant at low frequencies in the Nyquist diagram (Fig. 3a) and in the Bode diagram due to a change in the phase angle. After carrying out the impedance study it was found that there are three time constants for all of the damage conditions. In order to adjust the experimental diagrams for all damage conditions of the steel in the corrosive medium, we used the equivalent electrical circuit as that in Fig. 5. This circuit has previously been proposed for the study of the carbon steel corrosion process in these environments [9]. The electric elements of the circuit of Fig. 5 can be associated to: Rs , is the solution resistance, R1 , is charge transfer resistance, Q1 , is the non-ideal double layer capacitance, used in order to consider the roughness of the interface. R2 and Q2 are an

Fig. 5. Equivalent circuit used to ®t the experimental data of EIS diagrams for carbon steel±alkaline sour environment interface. The carbon steel surface was previously damaged at di€erent switching potentials (Ek‡ ).

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electrical arrangement that describes a di€usional process of Fe2‡ ions through the corrosion products. The terms R3 and Q3 represent the di€usion process of atomic hydrogen (H0 ) through the corrosion products. Continuous lines in Figs. 3 and 4 represent the ®ts to the Nyquist and Bode impedance diagrams using the circuit in Fig. 5. These ®gures demonstrate a good correspondence between the proposed circuit and the experimental data. Table 2 summarizes the values of the circuit parameters obtained from the best ®t with the experimental impedance diagrams. We note that for the case of 0:5 V, the simulation of experiment was not very successful. Hence, the values at this potential are not given in Table 2. The solution resistance value (Rs ) for all experimental conditions varies within a 26±32 X interval, indicating minimal variation in solution resistance. On the other hand, the variation found in the values of the electrical elements for Ek‡ > 0:6 V can be attributed to the formation of a heterogeneous surface at these potentials. As can be seen in Table 2, the values for the elements of the equivalent circuit are very similar to those of the initial damage potentials (Ek‡ of 0.7 and 0.6 V). This con®rms that the corrosion products formed under these conditions are the same. Under other conditions the variation in the electrical elements is due to the presence of a di€erent corrosion product. The increase in term R1 goes with the increase in damage potential, up to a potential of 0:6 V. Above this value it begins to diminish, and in some cases to increase, until reaching an equilibrium in the value of this term, varying from 1 to 10 X. The value of term R2 ¯uctuates greatly, reaching a constant value for Ek‡ P 0:2 V. Finally, the term R3 displays the highest values for the ®rst two inversion potentials ( 0:7 and 0:6 V). For more positive potentials it varies indiscriminately without reaching a constant value. In the term Q1 an increase in the value of constant phase element parameter (Y0 ) is observed with increasing damage potential. In contrast, the value of n1, which is a parameter describing the width of the material property distribution (dielectric Table 2 Values for the electric circuit elements obtained from the best ®t to the impedance diagrams of the carbon steel±alkaline sour environment interface, as a function of switching potential (Ek‡ ) for the damage process Ek‡ vs SSE (V)

R1 (X)

R2 (X)

R3 (X) Q1

Q2 3

0.7 0.6 0.4 0.3 0.2 0.1 0.2 0.3 0.5

7 16 12 28 6 3 4 3 8

271 262 339 652 59 77 127 131 129

8172 28 060 2453 2318 1467 807 815 445 1523

Q3 3

Y0  10 (siemens)

n1

Y0  10 (siemens)

n2

Y0  103 (siemens)

n3

0.1 0.17 3 3 4 10 8 11 5

1 0.93 0.70 0.58 0.68 0.65 0.63 0.62 0.61

0.3 0.2 4.3 3.6 8.8 12.8 19.6 22.1 10.9

0.81 0.82 1 1 0.95 1 1 1 1

1 1 0.52 0.4 1 3.9 5.2 6.58 2.9

0.43 0.36 1 1 0.96 1 1 1 1

The ®tting process was carried out using the equivalent circuit of Fig. 5.

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relaxation times in frequency space) tends to decrease. The term Q2 displayed values of the same order for the ®rst two cases ( 0:7 and 0:6 V). For subsequent potentials the magnitude of Q2 , in some cases the values were one to two orders of magnitude greater than those obtained for the ®rst two damage potentials. The term n2 was constant under the ®rst two conditions. For all other cases it increased to a constant value of 1. Finally, term Q3 increased for the ®rst two points, then diminished and ®nally increased again. At the same time its corresponding values of n3 commenced with small values, ®nally reaching values close to 1. As mentioned above, the term R1 is associated with the charge transfer process at the interface between the metal and the corrosion products. It can be said that this stage occurs in a rapid manner in all of the voltammetrically formed ®lms. The increase in term Q1 , associated with the capacitance of the double layer, is due to the presence of the corrosion products formed in the interface. Moreover, these same corrosion products cause the value of n1 to change up to values of 0.6, indicating the high degree of roughness on the substrate. Pseudocapacitance values were determined from the constant phase element value Q1 (Table 2) using BoukampÕs ex1=n pression [10]: CHF ˆ …Y0 RHF † =RHF , where CHF is the capacitance at high frequency, Y0 is a constant phase element parameter (``mho'' sn ), RHF is the resistance at high frequency …R1 ; X† and n (0 6 n 6 1) is a parameter describing the width of the material property distribution (dielectric relaxation times in frequency space). The estimated values were 100±109 lF for surfaces damaged with Ek‡ of 0.7 and 0.6 V, and 540±1500 lF for surfaces damaged with Ek‡ > 0:5 V. The di€erence in the capacitance magnitudes is another variable that reveals the existence of di€erent voltammetrically formed corrosion products on the substrate, in agreement with the EIS and SEM results. The magnitude of these values for Ek‡ > 0:6 V were of the same order of magnitude as those reported for interfaces with corrosion product ®lms [13]. In these studies the high capacitance values (500±1750 lF) originated from corrosion products formed on the substrate which displayed porous and conducting characteristics [13]. These properties have already been clearly substantiated by SEM (Fig. 2d), and previous studies that show that iron sul®de ®lms behave as metallic conductors [14]. The variations in the values of the equivalent circuit electrical elements obtained after the simulation (terms R1 , R2 and R3 , Table 2), for Ek‡ > 0:6 V, are due in part to the corrosion products formed in these media having a certain degree of solubility. However, as will be seen below, this variation originates principally from the heterogeneity at the surface and the increase in this heterogeneity with more positive damage potentials. 3.5. Analysis of the di€usion processes through the corrosion products As already stated in the Ref. [9], there are at least two di€usional processes through the corrosion products in these alkaline sour media. One is the di€usion through the corrosion products of iron ions formed at the interface between metal and corrosion products, this being a ``chemical di€usion''. The second di€usion process is that which causes the blistering of the steel found in re®neries (catalytic

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plants). It involves the di€usion of atomic hydrogen, formed at the interface between corrosion products and sour media, toward the interior of the ®lm. 3.6. Evaluation of the corrosion products thicknesses As the di€usion process of the ferrous ions depends on the surface state, the values of R2 for Ek‡ of 0:7 and 0:6 V arise entirely from one corrosion product. The ®lm of this product is less porous than that of the second product. For Ek‡ > 0:5 V vs SSE, the term R2 decreases due to the presence of this second corrosion product which forms a porous, heterogeneous surface. In this way an analysis can be made of the experimental impedance data for monitoring the evolution of the ®lm thickness as a function of the di€erent damage potentials used to prepare the di€erent iron sul®de ®lms on the carbon steel surface. Determination of the ®lm thickness was made according to the derivations of Dawson et al. [15,16] and following the methodology proposed in Appendix A. For this determination the R2 values used were those obtained from the simulation of the experimental data, as shown in Table 2. The value used for the di€usion coecient was 4:3  10 11 cm2 /s, as reported by Vedage et al. [1] for the di€usion of ferrous ions (Fe2‡ ) through a ®lm of products containing iron sul®des. The results obtained for ®lm thickness are presented in Table 3. As seen in Table 3, the thicknesses evaluated for the ®rst two conditions ( 0:7 and 0:6 V) are very similar in value and are an order of magnitude smaller than those obtained for the subsequent damage potentials. For Ek‡ > 0:5 V the thicknesses vary from 0.31 to 0.47, indicating that they have the properties of the second ®lm. The average thickness of these ®lms was 0.4 lm. Another method for approximately estimating the ®lm thickness (X ) has been reported in Ref. [17]. In this method the charge (Q) involved in the damage process in potentiostatic experiments was used for this estimation, towards the following expression: X ˆ

QM nF cq

…6†

Table 3 Corrosion product ®lm thickness formed at voltammetric damage induction of the carbon steel surface in alkaline sour environment as a function of switching potential (Ek‡ ) of damage induction Switching potential, Ek‡ (V vs SSE) 0.7 0.6 0.5 0.4 0.3 0.2 0.3 0.5

Film thickness, X (lm) 0.054 0.047 0.46 0.31 0.43 0.40 0.47 0.31

The evaluation of thickness was performance from EIS measurements.

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where Q is the charge (C/cm2 ), M is the molecular weight (g/mol), and assuming a surface roughness of c ˆ 1:1, a ®lm density of q ˆ 4:61 g/cm3 for a ®lm of FeS or 4.98 g/cm3 for a ®lm of FeS2 , and gF ˆ 19 300 C/mol. As this expression is valid only for very compact, non-porous ®lms it could be applied only to the ®rst two damage potentials ( 0:7 and 0:6 V). As the corrosion products formed in this medium shows an intermediate stoichiometric relationship, and with the aim of comparing and validating the thicknesses obtained by the impedance technique, the determinations estimated below relate to ®lms with two stoichiometric relationships such as FeS and FeS2 . The values found for Ek‡ ˆ 0:7 V are 0.0107 and 0.014 lm, while for Ek‡ ˆ 0:6 V they vary between 0.034 and 0.043 lm. The values determined using expression (6) are very similar to those evaluated using the impedance technique, as shown for Ek‡ ˆ 0:6 V in Table 3. This supports the validity of the impedance technique results. For a potential of Ek‡ ˆ 0:7 V, the calculated thickness varied due to incomplete coverage of the electrode by the corrosive ®lm. This is con®rmed by the voltammetric curves for the two cases (Fig. 1a). From the calculated thicknesses it can be seen that the ®rst corrosion products are both protective and very compact. For the second corrosion product formed at the interface the variation in the calculated thickness con®rms the presence of a dissolution process in the ®lm, as has been reported previously [9,18]. Moreover, this ®lm is completely non-protective. 3.7. Evaluation of the di€usion coecient for the atomic hydrogen (H 0 ) As in a previous study [9], it was found that H0 di€uses through the ®lm of corrosion products formed in this alkaline sour medium. The rate of this di€usion depends on the product properties. In the present study it was found that the magnitude of the di€usion of H0 through the ®rst corrosion product (formed at 0.7 and 0.6 V vs SSE) was less than that through the second ®lm of corrosion products, formed at Ek‡ > 0:5 V vs SSE. This appears to be supported by the values of R3 in Table 2. The R3 values obtained for 0:7 to 0:6 V are one or two orders of magnitude greater than those obtained at Ek‡ > 0:5 V. This can be better observed by evaluating the di€usion coecient at various switching potentials using an expression employed in many previous studies [19,20] to estimate the di€usion coecient across an amorphous and porous solid: " # tanh …sX 2 =D†1=2 ZD ˆ R …7† 1=2 …sX 2 =D† where ZD is the total impedance of the di€usion process (X), R is an e€ective diffusion resistance (X), D is the di€usion coecient (cm2 /s), X is the ®lm thickness of the corrosion products (lm) and s is the product of jx. The di€usion impedance can be estimated from the experimental impedance diagrams at low frequencies. The value of R corresponds to that of R3 , obtained from the simulation of the experimental data using an equivalent circuit (Fig. 5) and using

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Table 4 Evaluation of atomic hydrogen (H0 ) di€usion coecient through the corrosion products ®lm, using Eq. (7), as a function of the switching potential of the damage induction Switching potential, Ek‡ (V vs SSE) 0.7 0.6 0.4 0.3 0.2

Di€usion coecient, D (cm2 /s) 2:6  10 1:9  10 2:3  10 3:6  10 3:1  10

14 15 12 12 12

the previously calculated ®lm thickness (Table 3). Table 4 gives the di€usion coef®cient as a function of the Ek‡ used to damage the surface. While similar electrochemical behavior was observed using the impedance technique for surfaces damaged at values of Ek‡ of 0.7 and 0.6 V, the di€usion coecients of atomic hydrogen (H0 ) under these conditions were di€erent. This di€erence can be attributed to an incomplete covering of the electrode surface by the ®lm at 0:7 V meanwhile the ®lm formed at 0:6 V was more homogeneous. This statement is based for the voltammetric behavior of damage induction of 0:7 V (Fig. 1a). The associated current in both direct and inverse potential scan was similar, meanwhile for 0:6 V the current in the inverse potential scan is lower than that for the direct potential scan (Fig. 1a). The ®lms of corrosion products formed at Ek‡ > 0:5 V (i.e. 0.4, 0.3 and 0.2 V), an increase was observed in the di€usion coecient of H0 through the surface states, showing that the ®lms were very porous in comparison with those obtained at 0:7 and 0:6 V. It is noteworthy that the magnitudes obtained were within the same order of magnitude, demonstrating good agreement between the results. It is therefore possible to form a range of surface states on carbon steel in these corrosive media by using the potential as a variable (voltammetry technique for damage induction). These states should be similar to those obtained by varying the immersion time. This is since, by using the potential as a variable, the evolution of the damage on the substrate in the corrosive medium can be controlled. In a previous study of the same interface (carbon steel±sour media) [9] the steel surface state was monitored over changing immersion time using the EIS technique. It was found that between an initial time (0 h) and the ®rst impedance measurement, a protective ®rst corrosion product formed. For immersion times greater than 7 h a considerable decrease was observed in the Nyquist impedance spectra, ®nally reaching a constant value for times greater than 20 h [9]. The transition encountered at short immersion times was also seen in the present study, since the spectra for systems with damage potentials of 0.7 and 0.6 V are similar to those for an immersion time of 0 h. In addition, the spectra for potentials greater than 0.3 V were similar to those found for immersion times greater than 7 h. In addition to similarities in the impedance spectra, the ®lm thickness properties from the previous study also displayed similarities with the results presented above. The ®lm thickness varied with immersion time, reaching a constant value of 0.4 lm at 50 h [9], this value being similar to that of the second ®lm reported in the present

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study. At the same time, the di€usion coecient for atomic hydrogen from the immersion time study was 2:3  10 13 cm2 /s [9], while in the present study its value depended on the formed ®lm, displaying values ranging from 10 15 to 10 12 . This indicates that the parameters measured previously for varying immersion time were averages. The present study has therefore succeeded in one of its objectives: the separation of the contributions from di€erent corrosion products. In this manner it was con®rmed that voltammetry can reproducibly induce interfacial damage representative of the steel corrosion process in sour media using immersion time.

4. Conclusions The formation of the surface states that occur in the corrosion process of 1018 carbon steel in alkaline sour media was achieved using cyclic voltammetry, which generated di€erent states by changing the switching potential (Ek‡ ). These systems were chosen to simulate the sour waters found in catalytic plants (PEMEX-REFINERIES). The surface states generated using voltammetry were equivalent to those obtained using di€erent immersion times of the steel in the sour media. The characteristics of the damaged surfaces were evaluated using EIS and SEM. It was found that for values of Ek‡ of 0:7 and 0:6 V vs SSE a ®rst corrosion product formed. It was of high passivity and very compact (0.047 lm). For more anodic potentials (Ek‡ > 0:5 V vs SSE) a second corrosion product formed that had nonprotective properties (porous and very active). The calculated di€usion coecients for atomic hydrogen (H0 ) for the di€erent corrosion products varied in order from 10 15 to 10 12 cm2 /s. The methodology proposed in this study was principally developed in order to separately generate and characterize the di€erent corrosion products.

Acknowledgements E. Sosa is grateful to Conacyt (project 32689 E) and IMP (FIES 98-13-II) for his postgraduate grant. R. Cabrera-Sierra is grateful for IMP (FIES 98-13-II) for his grant. We acknowledge the ®nancial aid of Conacyt (project no. 32689 E) and FIES 98-13-II.

Appendix A To monitor ®lm thickness as a function of Ek‡ , we employed the Warburg impedance, which is expressed in terms of its real …ZD0 † and imaginary components …ZD00 †, considering a di€usion factor K where ®lm thickness is involved [15,16]. Additionally, this factor is responsible for the response of the system.

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 KˆX

2 D

2323

1=2 …A:1†

where X is the ®lm thickness of corrosion products (lm) and D is the di€usion coecient of the ferrous ions (Fe2‡ ) through the sul®de ®lm   r sinh …Kx1=2 † ‡ sin …Kx1=2 † ZD0 ˆ 1=2 …A:2† x cosh …Kx1=2 † ‡ cos…Kx1=2 † ZD00

r ˆ 1=2 x



sinh …Kx1=2 † sin …Kx1=2 † cosh …Kx1=2 † ‡ cos…Kx1=2 †

 …A:3†

where r is the Warburg coecient for the ®lm di€usion process, x is the frequency and K is a di€usion factor de®ned by expression (A.1). The limits of ZD0 and ZD00 at low frequencies agree with the following expression: lim ZD0 ˆ rK

…A:4†

lim ZD00 ˆ 0

…A:5†

x!0

x!0

In addition to the following expressions: ZD00 max ˆ 0:417225 rK

…A:6†

K 2 xmax ˆ 5:069

…A:7†

The above considerations apply when di€usion of one species through the sul®de ®lm exists and when the characteristic spectrum in the complex diagram shows a well de®ned capacitive loop. Where no well-de®ned time constant appears, the impedance measurements are in¯uenced by others processes. In this case the following procedure is used: 1. Assuming a frequency with its respective imaginary impedance, calculate K based on expression (A.7). 2. Evaluate r using expression (A.3) based on the imaginary impedance, x and K. 3. The product of r and K, should be a value similar to the term R2 obtained from the adjustment due to di€usion in the ®lm. The terms R2 and Q2 are associated with the formation of a single ®lm and the di€usion of the iron ions (Fe2‡ ) across that ®lm. To evaluate the data presented in Table 3, a di€usion coecient value of 4:3  10 11 cm2 /s, (reported by Vedage et al. [1]) is used for the di€usion of iron ions, Fe2‡ , across the corrosion product ®lm (pyrrhotite) at 30°C.

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