Alexander Kuzmak, Alexander Kozheurov, Larissa Frolova
Keywords: coulometric method, detection of corrosion products, protective effect of inhibitor
Abstract:
This paper proposes a method for assessment of the protective effect of an amine corrosion inhibitor in liquid and gas-vapor hydrogen sulfide containing media using coulometric determination of corrosion products (hereinafter, CDCP). The kinetic regularities of the protective effect depending on the phase transformations of corrosion products on steel surface in H2S-containing media have been determined. Qualitative and quantitative analyses of the corrosion product composition have been performed by high-precision determination of sulfur- and oxygen-containing compounds of bi- and trivalent iron (Fe+2, Fe+3 ions) within the range of 0.2 to 10 μg. The experimental technique using a glass-carbon indicator electrode-cell ensured high-precision registration of corrosion products (ions) at the sub-microcoulomb level (the discharge currents of the ions being determined are ~10-6 A). Voltammetric measurements were used to identify sulfides and oxygen-containing corrosion products. The results of coulometric tests compared with gravimetric estimation proved the high information value of the suggested approach.
Because you are not logged-in to the journal, it is now our policy to display a 'text-only' version of the preprint. This version is obtained by extracting the text from the PDF or HTML file, and it is not guaranteed that the text will be a true image of the text of the paper. The text-only version is intended to act as a reference for search engines when they index the site, and it is not designed to be read by humans!
If you wish to view the human-readable version of the preprint, then please Register (if you have not already done so) and Login. Registration is completely free.
ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 Use of Coulometry to Assess the Protective Effect of Inhibitors Alexander E. Kuzmak, Alexander V. Kozheurov, Larissa V. Frolova IPCE RAS, 119991, Moscow, Leninsky Prospect, 31. akuzmak@yandex.ru kuzmak@ipc.rssi.ru Abstract This paper proposes a method for assessment of the protective effect of an amine corrosion inhibitor in liquid and gas-vapor hydrogen sulfide containing media using coulometric determination of corrosion products (hereinafter, CDCP). The kinetic regularities of the protective effect depending on the phase transformations of corrosion products on steel surface in H2S-containing media have been determined. Qualitative and quantitative analyses of the corrosion product composition have been performed by high-precision determination of sulfur- and oxygen-containing compounds of bi- and trivalent iron (Fe+2, Fe+3 ions) within the range of 0.2 to 10 µg. The experimental technique using a glass-carbon indicator electrode-cell ensured high-precision registration of corrosion products (ions) at the sub-microcoulomb level (the discharge currents of the ions being determined are ~10-6 A). Voltammetric measurements were used to identify sulfides and oxygen-containing corrosion products. The results of coulometric tests compared with gravimetric estimation proved the high information value of the suggested approach. Keywords: coulometric method, detection of corrosion products, protective effect of inhibitor 1. Introduction Studies on the protective mechanism of inhibitors of the hydrogen sulfide corrosion of iron by various physical methods (reflection electron diffraction, Auger and X-ray photoelectron spectroscopy, secondary ion mass spectrometry) have shown [1] that corrosion inhibition can occur upon inhibitor interaction with the “dense” corrosion product, viz., fine-crystalline iron sulfide. As follows from the established surface layer structure, metal protection is effected by a complex iron compound containing inhibitor components and hydrosulfide groups that cover the sulfide crystals. Upon contact with a corrosive medium, this layer inhibits the recrystallization of fine-crystalline iron sulfide to a “loose” (coarse-crystalline) compound, thus ensuring a high protective effect. Thus, the kinetic region of the structure transition from a dense layer to a loose one, or recrystallization region, is of special interest in the experimental assessment of the inhibitor protective effect. In the current laboratory practice, the protective effects of inhibitors are generally evaluated by gravimetric and electrochemical methods. Matching between the results of both methods is either achieved by calibration or not achieved at all, particularly in the case of prolonged tests. The reason for this mismatch is that polarization affects the specimen in the course of data recording, thus distorting its real corrosion behavior and introducing uncertainty in the interpretation of the results. In essence, only the gravimetric method meets the direct measurement requirement. However, the gravimetric method is not free of known limitations, and besides, it is fundamentally unable to ensure the measurement of an important parameter of the corrosion process, viz., the ion composition of the corrosion products. The use of the advantages of gravimetry as a direct measurement method, along with elimination of its restrictions, allowed us to formulate the following requirements for the criterion, parameter, and measurement method that does not require any calibrations or assumptions [2]: 1. Quantitative determination of a direct corrosion parameter based on a fundamental physical law; 2. Elimination of effects of the measuring system on the specimen; 3. Clarity of in situ measurement results; 4. Accuracy, selectivity, high sensitivity, and efficiency of the measurements. The above set of requirements can be satisfied by coulometry at a controlled potential; based on this method, 1 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 we developed a technique for the coulometric determination of corrosion products for studying the protective effect of inhibitors as described below. 2. Basis of the measurement methodology The proposed approach is based on the use of Faraday’s law, according to which the mass m (g) of a compound that undergoes the electrochemical conversion (in our case, corrosion products such as Fe2+ and Fe3+) is related to the consumed charge q (Cb) by the following equation: m = Ke· q = Ke ∫I(t)dt (1), where: Ke = M/nF (g/Cb) is the electrochemical equivalent of the compound or reaction; I(t) is the discharge current of the ions being determined on the indicator electrode (IE) – the measured parameter; M is the molecular mass of the oxidized or reduced component (g); n is the number of electrons participating in the electrochemical conversion of one atom, ion, or molecule of the compound; F is the Faraday constant. A CDCP experiment involves three main stages: 1. Exposure of the test specimen to a corrosive medium for a specified period of time. 2. Preparation of an aliquote containing the ions (corrosion products) for the coulometric analysis. At stage 2, the aliquotes with the ions (corrosion products) from the corrosive solution and from the metal surface are placed in a preserving solution in order to fix the oxidation states of the Fe2+ and Fe3+ ions. 3. 3. Coulometric analysis of the aliquote: discharge of the ions (corrosion products) on an indicator electrode of the measuring system containing the reference (blank) electrolyte. Coulometric analysis involves the determination of the compound being analyzed in a measuring cell at discharge potentials of the ions Ecat , Eanod for the Fe2+↔ Fe3+ reactions and processing of the results. The measuring cell represents the three-electrode potentiostatic scheme. A high-temperature glass carbon crucible that played the role of a measuring cell was used as the indicator electrode (IE). Carbon fiber was used as the auxiliary electrode. A silver chloride reference electrode was used. The coulometric measurements were carried out using an IPC – PRO electrochemical interface (manufactured by A.N.Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences) equipped with specialized software. 3. Experimental An amine inhibitor at 4 g/l concentration in a NACE medium (0.5 g/l NaCl + 250 mg/l CH3COOH (pH=4), saturated with H2S to 2000 mg/l) was used as the test object. The corrosive medium (250 ml) was placed in a 600-ml test vessel. Three carbon steel specimens were placed in each the liquid and vapor-gas parts of the exposure space. Fresh steel specimens as well as fresh non-deaerated inhibited and non-inhibited media were used for each subsequent exposure interval. The specimens had the form of cylinders; the working area of each specimen available for coulometric and gravimetric measurements was 8 cm2. We used the following sources of Fe2+,3+ ions (corrosion products): dense fine-crystalline corrosion products adsorbed on the metal surface; loose coarse-crystalline products adsorbed on the dense layer, as well as loose corrosion products transferred from the specimens from the vapor-gas medium to the liquid one during the exposure. After exposure in the test medium for a specified time, corrosion products were removed from the specimen surfaces and preserved. Loose products were removed using a 4 M HCl washing solution at room temperature. Dense corrosion products were removed with an ammonium sulfosalicylate solution (C7H6O6S) with pH 4. The Fe2+,3+ ions from both washing solutions and from the liquid medium were determined on the IE (Table 1). 2 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 Table 1. Conditions of steel corrosion product determination by the CDCP method. Ion Fe2+,3+ source E, V Reference solution in the IE NACE medium 0.43 1 M HCl in isopropanol + 0.3 mg/ml inhibitor + 0.1 mg/ml H2S Loose 0.77 Aqueous 0.1 M HCl + 4 mg/ml C7H6O6S Dense 0.77 Aqueous 0.1 M HCl NACE medium 0.13 Aqueous 0.1 M HCl + 12 mg/ml C7H6O6S + (0.1-0.4) µg/ml Fe2+ Loose 0.13 Aqueous 0.1 M HCl + 4 mg/ml C7H6O6S Dense 0.13 Aqueous 0.1 M HCl + 12 mg/ml C7H6O6S + (0.1-0.4) µg/ml Fe2+ Fe2+ Fe3+ To identify the phase composition of the corrosion products (sulfur- and oxygen-containing iron compounds), voltammetric measurements were carried out from the corrosion potential Ecorr to E = - 1.8 V and from E = - 1.8 V to E = 0 V at potential scan rate vscan= 5 mV/s. Figure 1 shows a cathodic voltammetric curve for carbon steel without an inhibitor under the conditions specified in Table 1. -25 6 -20 6 5 I, mcА 4 -15 3 2 -10 1 -5 0 0 -0,25 -0,5 -0,75 -1 -1,25 -1,5 -1,75 -2 Е, v 3 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 Fig. 1. Reference cathodic voltammetric curve for the reduction of corrosion products: 1. Fe2+ → Fe0, E = -0.66 V; 2. Fe(OH)3 → Fe(OH)2, E = - 0.87 V; 3. Fe(OH)2 → Fe0, E = -1.19 V; 4. FeS (dense) → Fe0, E = -1.34 V; 5, 6. FeS (loose) → Fe0, E= -1.51 V and E = -1.7 V. Tables 2 and 3 show the measurement results for the kinetics of the total mass loss of ions determined gravimetrically, Σgr, and by CDCP, ΣCDCP = mFe2+ + mFe3+ , in the liquid and vapor-gas media. In evaluating the absolute mass loss, the additional corrosion products that were transferred from the vapor-gas phase to the liquid phase, ∆vg = ΣCDCP – Σgr, were also taken into account. Table 2. Total mass loss (g/m2) of specimens in inhibited and non-inhibited liquid media obtained by gravimetric and CDCP methods, taking ∆vg into account. Without inhibitor With inhibitor t, hours Grav. Σgr CDCP ΣCDCP ∆vg Grav. Σgr CDCP ΣCDCP ∆vg 0.75 0.52 0,48 0 0.09 0.07 0 1.5 3.61 3.27 0.34 0.18 0.21 0 3 7.26 6.58 0.68 0.60 0.59 0 6 14.48 14.00 0.48 0.,84 0.82 0 12 31.97 31.42 0.55 0.,84 0.83 0 18 29.71 28.42 1.29 1.62 1,52 0 19.5 29.07 26.50 2.57 3.61 3.63 0 21 37,.46 33.55 3.91 12.48 12.31 0 24 45,98 44.26 1.72 15.62 15.08 0 120 88.87 88.11 0.76 19.07 19.00 0 240 103.54 102.88 0.66 20.87 20.99 0 360 108.42 107.21 1.21 62.34 63.66 0 4 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 2 Table 3. Mass loss (g/m ) of specimens in inhibited and non-inhibited vapor-gas media obtained by gravimetric and CDCP methods. Without inhibitor Grav. CDCP hours Σgr ΣCDCP 0.75 0.03 0.01 1.5 0.66 3 With inhibitor ∆vg ∆vg Grav. CDCP Σgr ΣCDCP 0 0 0 0 1.02 0.36 0 0 0 1.30 2.00 0.70 0 0 0 6 3.74 4.25 0 0 0 12 5.41 5.92 0.51 0 0 0 18 6.94 8.27 1.33 0 0 0 19.5 9.20 11.70 2,50 0 0 0 21 12.36 16.18 3..82 0 0 0 24 12.99 14.61 1.69 0 0 0 120 18.14 18.83 0.69 11.98 11.99 0 240 34.01 34.72 0.71 18.00 17.60 0 360 63.89 65.03 1.14 30.88 31.30 0 t, 0.1 3.1. CDCP measurements in liquid medium As one can see from Tables 2 and 3, the results of integral kinetics measurements by both methods are in good agreement, taking the ∆vgm in non-inhibited medium into account. For illustration purposes, Figures 2 and 3 show the results of mass loss kinetics measurements by the CDCP method in the liquid medium for t = 0 - 24 h and t = 0 - 360 h exposures. 5 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. Volume 15, Preprint 29 m, g/м2 m, г/м 2 ISSN 1466-8858 14 12 submitted 28 May 2012 70 60 10 50 8 40 6 30 4 20 2 10 0 0 1 2 3 4 5 6 7 8 9 0 10 0 50 100 150 200 250 t, h 300 350 400 t, h Fig. 2. Fig. 3. Mass loss kinetics in the liquid medium in the tests for 24 and 360 hours: ● – with inhibitor; ○ – without inhibitor, respectively Table 4 presents the kinetics of the total amounts of the corrosion products (Fe2+ + Fe3+) on the specimen surface, Σs, and in the liquid medium, Σlm , as well as their ratios: σFe = Σs / Σlm .Table 4. Kinetics of the amount of combined corrosion products and their ratio σFe obtained by CDCP method. σFe t, h Σ s on the surface Σ lm in the liquid medium 0.75 0 0.01 0 1.5 0.47 0.19 2.5 3 0.77 0.53 1.5 6 1.95 1.99 1.0 12 2.72 2.69 1.0 18 3.40 3.54 1.0 19.5 4.23 4.97 0.8 21 5.64 6.72 0.8 24 6.51 6.48 1.0 120 10.99 7.15 1.5 240 15.15 19.57 0.8 360 8.58 56.45 0.2 6 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 I, mcA As one can see from Table 4, the corrosion process in the liquid non-inhibited medium is controlled by a “protective” layer of corrosion products on the metal surface. This layer is formed within the first three hours of the test. This period is followed by regions of slower (from 6 to 18 h and from 24 to 120 h, σFe ≥ 1.0) and faster corrosion (from 18 to 24 h and from 120 to 360 h, σFe ≤ 0.8). On transition from a region with slower corrosion to a region with faster corrosion, the layer of corrosion products reaches a “critical” thickness and starts to undergo mechanical degradation. A fraction of the corrosion products are transferred from the specimen surface to the liquid medium. During this period, an increase in the metal mass loss is observed, which characterizes the recrystallization of dense iron sulfide. According to CDCP data, fast accumulation of loose products in Fe2+ form (~ 85%) and in oxidized Fe3+ form (~ 10-15%) occurs. However, the amount of dense products is at the detection level. The sulfide nature of the loose Fe2+ products is confirmed by the cathodic voltammetric curve (Fig. 4) that contains a signal of the reduction of a mixture of various sulfides, which is by an order of magnitude higher than the signal of the reduction of oxygen-containing Fe3+ compounds. -18 -16 -14 2 -12 -10 -8 -6 -4 1 -2 0 0 -0,5 -1 -1,5 -2 Е, v Fig. 4. Cathodic voltammetric curve of reduction of corrosion products in the liquid non-inhibited medium after exposure for 21 h: 1. Fe(OH)3 → Fe(OH)2, E = -0.87 V; 2. FeS (loose) → Fe0, E = -1.58 V. As shown previously (Fig. 2), the protective effect in inhibited liquid medium for 24 h amounted to ~ 100%. However, it follows from further studies that the start of recrystallization is accompanied by the formation of micro amounts of only loose Fe2+ compounds in the form of sulfides that are partially transferred to the liquid medium (Table 5). 7 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 Table 5. Kinetics of the phase composition of corrosion products in the liquid inhibited medium, g/m2. t, h Fe2+ compounds dense Fe3+ compounds loose dense Fe2+ compounds loose dense loose Fe3+ compounds dense loose 0.75 0 0.28 0 0.20 0.07 0 0 0 1.5 0 3.16 0 0.45 0.16 0 0 0.05 3 0.10 6.07 0 1.09 0.50 0 0 0.09 6 0.06 12.89 0 1.53 0.63 0 0 0.19 12 0.05 29.01 0 2.91 0.55 0.06 0 0.22 18 0.29 27.13 0 2.29 0.47 0.32 0.46 0.27 19.5 0.27 26.31 0 2.49 0.43 2.56 0.27 0.37 21 0.12 33.34 0 4.00 0.04 11.05 0.11 1.11 24 0.17 42.79 0 3.02 0.17 13.42 0 1.49 Figure 5 demonstrates the cathodic voltammetric curve of corrosion products in the liquid inhibited medium. I, mcA 18 16 14 12 1 10 8 6 4 2 0 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 E,v Fig. 5. Cathodic voltammetric curve of reduction of corrosion products in the liquid inhibited medium after exposure for 240 h: 1. FeS (loose) → Fe0, E = -1.55 V. 8 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 Thus, the loss of protective effect can be detected by determining micro amounts of loose Fe2+ compounds both on the metal surface and in the liquid medium. 2 50 45 40 35 30 25 20 15 10 5 0 m, g/m m, g/m 2 3.2. CDCP measurements in vapor-gas medium As shown in Figures 6 and 7, the corrosion process in the vapor-gas phase without an inhibitor contains regions where the mass loss kinetics is stabilized, i.e., 12 – 19.5 h and 120 – 360 h, and regions where it increases, i.e., 0.75 – 12 h and 19.5 – 120 h. 120 100 80 60 40 20 0 5 10 15 20 25 30 0 0 50 100 150 200 250 300 350 400 t, h t, h Fig. 6. Fig. 7. Kinetics of m(Fe2+ + Fe3+) mass loss in the vapor-gas medium during exposure for 24 h and 360 h: ● – with inhibitor; ○ – without inhibitor, respectively Table 6 shows the kinetics of the phase composition of corrosion products for the dense and loose phase layers in the vapor-gas medium. 9 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 Table 6. Kinetics of the phase composition of corrosion products in the vapor-gas medium, /m2. t, h Non-inhibited medium Fe2+ compounds Inhibited medium Fe3+ compounds dense loose dense loose 0.75 0 0.28 0 0.20 1.5 0 3.16 0 3 0.10 6.07 6 0.06 12 Fe2+ compounds dense Fe3+ compounds loose dense loose 0.07 0 0 0 0.45 0.16 0 0 0.05 0 1.09 0.50 0 0 0.09 12.89 0 1.53 0.63 0 0 0.19 0.05 29.01 0 2.91 0.55 0.06 0 0.22 18 0.29 27.13 0 2.29 0.47 0.32 0.46 0.27 19.5 0.27 26.31 0 2.49 0.43 2.56 0.27 0.37 21 0.12 33.34 0 4.00 0.04 11.05 0.11 1.11 24 0.17 42.79 0 3.02 0.17 13.42 0 1.49 120 1.77 4.37 8.55 4.28 0.46 6.65 0 1.89 240 12.17 75.58 10.49 4.64 1.82 17.44 0 1.73 360 10.35 8.90 7.15 12.02 1.84 8.08 0.36 2.06 One can see from Figures 6 and 7 and from Table 6 that the regions of mass loss in the medium are presumably due to recrystallization of dense sulfide, since the amount of loose Fe2+ products exceeds the amount of dense Fe2+ products by two orders. Corrosion is hindered due to a 5-10-fold increase in the amount of dense corrosion products in the form of Fe2+, while the amount of loose products on these areas remains almost unchanged. It follows from the cathodic voltammetric curves obtained in the slow-down regions (Fig. 8), e.g., after 18 hours, that the dense Fe2+ products formed in the vapor-gas phase without an inhibitor are oxygen-containing compounds. A signal with a maximum at E = - 0.95 V is observed which combines the reduction regions of oxygen-containing Fe2+ and Fe3+ compounds; a signal from sulfides is almost not observed here 10 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. Volume 15, Preprint 29 I, mcA ISSN 1466-8858 submitted 28 May 2012 -25 -20 -15 1 . -10 -5 0 0 -0,5 -1 -1,5 -2 Е, v Fig. 8. Cathodic voltammetric curve of reduction of corrosion products in the vapor-gas non-inhibited medium after exposure for 18 h: 1. Fe(OH)3 → Fe(OH)2, Fe(OH)2 → Fe0, E = - 0.95 V. Figures 9a and 9b show the kinetics of the inhibitor protective effect (Z, %) in the vapor-gas medium: a) from 0 to 25 h; b) from 25 to 360 h. 110 100 90 90 80 80 70 60 60 50 Z, % Z, % 70 40 30 50 40 20 30 10 20 0 10 0 2 4 6 8 10 12 14 16 18 20 22 24 26 0 25 t, h Fig. 9a. 125 225 325 425 t, h. Fig. 9b. Kinetics of (Fe2+ + Fe3+) integral mass loss in the vapor-gas medium during exposure for 25 h (9a) and 360 h (9b), respectively. 11 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 15, Preprint 29 submitted 28 May 2012 I, mcA The composition of the corrosion products for 6-12 h exposure is shown on the voltammetric curve (Fig. 10) where one can see signals from dense iron sulfide and oxygen-containing Fe3+ compounds. -14 -12 -10 -8 -6 2 -4 -2 1 0 0 -0,5 -1 -1,5 -2 Е,v Fig. 10. Cathodic voltammetric curve for the reduction of corrosion products in the vapor-gas inhibited medium after exposure for 6 h: 1. Fe(OH)3 → Fe(OH)2, E = -0.84 V; 2. FeS (dense) → Fe0, E = -1.35 V. Thus, the effect of the aforementioned dense oxygen-containing corrosion products is obvious, since, in addition to a shift of the iron dissolution signal, a signal of oxidation Fe(OH)2→ Fe(OH)3 at E = - 0.84 V is also observed. It also follows from Table 6 that, after 12 h of exposure, the phase composition of the corrosion products changes as follows: ~ 10% of dense Fe2+ products is converted to loose products, while the total amount of Fe2+ products is the same as that after exposure for 6 h. This change in the phase composition suggests that sulfide recrystallization starts, which stimulates corrosion acceleration. In the period from 12 to 24 h, the content of dense Fe2+ products on the specimen surface decreases by an order. The amount of loose Fe2+ products increases by approximately the same value. Exposure to oxygen results in the formation of dense oxygen-containing Fe3+ compounds. However, the latter provide no protective effect; hence, it may be assumed (Table 6) that only dense oxygen-containing Fe2+ compounds have protective ability. The recrystallization is mostly completed by 21 hours of exposure. During this process, the metal mass loss increases 15-fold, whereas the protective effect decreases from 95% to 67% (Fig. 9a). However, the kinetics of the subsequent corrosion process does not repeat that of the non-protected metal. The process slows down in the period from 24 to 240 h (Fig. 6, Table 3), so the mass loss increases no more than 1.4-fold. During this period, the amount of solid Fe2+ products again increases, whereas the amount of loose products increases only a little (Table 6). The efficiency of metal protection during this period is rather high, i.e., 70-80% (Fig. 9b). However, the protection that is achieved is not related to inhibitors, as data of the cathodic voltammetric curve recorded after 120 h suggest (Fig. 11). 12 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. Volume 15, Preprint 29 I, mcA ISSN 1466-8858 submitted 28 May 2012 25 2 20 15 10 1 5 0 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 E, v Fig. 11. Cathodic voltammetric curve for the reduction of corrosion products in the vapor-gas inhibited medium after exposure for 120 h: 1. Fe(OH)3 → Fe(OH)2, Fe(OH)2 → Fe0, E = -0.98 V; 2. FeS (loose) → Fe0, E = -1.65 V. These results show that metal protection is provided by a layer of dense oxygen-containing Fe2+ compounds. 4. Conclusions 1. In this paper we propose a method for the coulometric determination of corrosion products (CDCP) for assessment of the protective effect of corrosion inhibitors in liquid and vapor-gas H2S-containing media. 2. The kinetic regularities of changes in the inhibitor protective effect depending on the phase transformations of sulfur- and oxygen-containing compounds on the metal surface have been determined. 3. Qualitative and quantitative analyses of the corrosion product composition have been performed by highprecision determination of sulfur- and oxygen-containing compounds of bi- and trivalent iron. 4. The main feature of the inhibitor protective effect has been identified: kinetic recrystallization region of fine-crystalline to coarse-crystalline sulfide. AKNOWLEDGMENTS The authors are grateful to Prof. Y. I. Kuznetsov and Dr. N. N. Andreev for valuable comments. REFERENCES [1]. “The Formation of Protective Films on Iron under the Action of the Inhibitor IFHANGAZ-1 in an Aqueous Solution Saturated with Hydrogen Sulfide”, Rozenfel’d I.L., Bogomolov D.B., Gorodetsky A.E., Kazansky L.P., Frolova L.V., Shamova L.I. ” Zashchita metallov”, 18, 2, pp. 163-168, 1982.. [2]. “Coulometric Estimation of Corrosion Rate of Carbon Steel”, Kuzmak A.E., Kozheurov A.V. “Protection of Metal and Physical Chemistry of Surfaces”, 40, 3, pp. 315 – 320, 2004. 13 © 2012 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.