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Effects of thiosulphates and sulphite ions on steel corrosion
Corrosion Science xxx (xxxx) xxx–xxx
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Effects of thiosulphates and sulphite ions on steel corrosion☆ ⁎
M. Cabrini, S. Lorenzi , T. Pastore Department of Engineering and Applied Sciences, University of Bergamo, Viale Marconi 5, 24044, Dalmine (Bergamo), Italy
A B S T R A C T The paper deals with the general corrosion of carbon steel in geothermal plants for conveying and re-injecting condensates. The results of electrochemical and weight loss tests are discussed in function of sulphite and thiosulphate concentrations typical of Italian geothermal power plants. Thiosulphates can inhibit the reactions between oxygen and sulphites and increase the corrosion rate, but they control the acidification resulting from the reactions of sulphites with oxygen. In anaerobic solutions, both species modify mainly the cathodic process. In the absence of oxygen, the corrosion process leads to the formation of mackinawite scales, while iron sulphide forms due to the reduction of thiosulphate and sulphite.
1. Introduction In geothermal plants that generate electrical energy, the steam produced in the extraction wells is conveyed to steam turbines that transform mechanical energy into electrical energy. The condensed water is then sent to the cooling towers and, finally, to a well for reinjection into the ground, to avoid environmental pollution by substances present in the condensates. This also helps prevent subsidence that might be induced by the extraction of significant quantities of water, while supporting steam pressure in the geothermal reservoir. The process releases into the atmosphere approximately 75% of the water flow [1]. The extracted fluid also contains non-condensable gases (NCGs), in a variety of concentrations depending on the reservoir, with extracts from the Larderello geothermal wells having an NCG content of around 5% [2]. These fluids contain mainly carbon dioxide and significant trace amounts of hydrogen sulphide [3]. To limit their release into the atmosphere, and to maintain their levels to values that are significantly below those perceptible by the population, steam plants in Italy are equipped with abatement systems that oxidize hydrogen sulphide to SO2. This gas is subsequently washed with the condensation water produced by the plant, and this dissolves the SO2 in the form of sulphite. Subsequently, sulphites give rise to sulphates and thiosulphates by further reactions with oxygen and sulphur [1,4]. Literature reports have highlighted the influence of the thiosulphate ion on various forms of corrosion, from that of generalized carbon steel, to the localized corrosion of stainless steels a [4] and nickel alloys, intergranular corrosion of sensitized stainless steels [5] and nickel
alloys, and stress corrosion [6] caused by sulphides in high strength steels. Sulphur may be present in the form of numerous compounds that are characterized by different degrees of oxidation. Because of the instability of these compounds, they can become involved in oxidation and reduction processes that depend on the characteristics of the environment in which they are dissolved. As a consequence, sulphur can initiate a large variety of corrosion phenomena that are characterized by complex behaviour [5,6]. In addition to sulphides and sulphates, sulphur can also exist as sulphites. Of great importance to corrosion are the polythionic acids, of generic formula H2SxO6, which have at least one sulphur atom bridging two sulphonic acid units, and the thiosulphate group that consists of a sulphate group bound to a sulphur atom. Thiosulphate ions display strongly acidic behaviour and, in water, they are normally dissociated. These compounds are involved in most of the reactions involving polythionic acids, and can be formed by reduction on a metal surface [6–8]. The role of thiosulfates in corrosion has been the object of several studies, and their corrosive effects have been well known since the 1940s, coinciding with the first accidents in the oil and paper industries involving sensitized austenitic stainless steels [7] [4]. These failures were caused by stress corrosion promoted by polythionic acids and thiosulphates, and were caused by the entry of air and moisture during the plant maintenance [9]. Similar problems occurred several years later, in pressurized-water nuclear reactors, where solutions containing thiosulphates were used as emergency liquids for the absorption of radioactive iodine. In the paper industry, the addition of sodium thiosulphate, used as a bleaching agent, resulted in the corrosion of AISI
☆ ⁎
Consorzio INSTM, UdR “Materials and Corrosion”, Via G.Giusti 6, Firenze, Italy. Corresponding author. E-mail addresses: [email protected] (M. Cabrini), [email protected] (S. Lorenzi), [email protected] (T. Pastore).
https://doi.org/10.1016/j.corsci.2018.02.046 Received 13 January 2017; Received in revised form 19 February 2018; Accepted 20 February 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Cabrini, M., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.02.046
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Corrosion behaviour was studied by weight loss, and electrochemical measurements. Potentiodynamic polarization tests and cathodic potentiostatic polarization tests were also performed by using IVIUM Compactstat potentiostat. Weight loss measurements were performed by means of Sartorius BP211D scale on the disc specimens, described above. The surface of each disc was polished with emery paper of up to 2400 grit. The exposed surfaces were further polished to 1 μm using diamond paste, followed by rinsing and degreasing with acetone in an ultrasonic bath, and then drying. After exposure, the specimens were pickled in diluted hydrochloric acid inhibited with hexamethylenetetramine, before being weighed. During these experiments, two discs were immersed in a 400 mL closed glass container filled with the de-aerated solution, under continuous nitrogen bubbling. Electrochemical tests were performed under stagnant conditions using a rotating cylinder electrode EG&G mod. 616 RDE. In the first case, one litre ASTM cell was used, in which the exposed surface area of the specimen was 1 cm2. A double-junction Ag/AgCl/KClsat. from now on indicated by Ag/AgCl reference electrode and a graphite counterelectrode were placed in separate compartments with porous ceramic diaphragms. A Huber-Luggin capillary was used. The flux of the gases produced at the counter-electrode of the test cell was vented through a hole in the upper part of the compartment. In experiments using the rotating electrode, similar equipment to that described above was used, and the rotating shaft was sealed against external air exposure. Before any electrochemical testing, the solution was de-aerated by nitrogen bubbling for at least twelve hours. Subsequently, a constant flow of nitrogen was maintained throughout the test period. Potentiodynamic curves were plotted after about 5 min of immersion, at a scan rate of 10 mV/min, from the free corrosion potential to a potential of 100 mV higher, for anodic curves, or 700 mV lower, for cathodic curves. Ohmic drop compensation was applied to all potentiodynamic curves. The ohmic drop was evaluated by means of electrochemical impedance measurements at a frequency of about 103 Hz, with an amplitude of 10 mV around the free corrosion potential. This correction is significant only for current densities above 5 mA/cm2 and 1 mA/cm2 for the disc electrode and the rotating cylinder electrode, respectively. Potentiostatic experiments were carried out by detecting the value of the stationary current, at −800 mV vs. Ag/AgCl/saturated KCl, on a rotating electrode at increasing speeds between 0 and 6000 rpm. The composition and morphology of the corrosion scale was characterized by electron microscopy and X-ray analyses of specimens immediately after testing. The specimen surfaces were washed with distilled water and dried under a flow of nitrogen. Analyses of the powdered corrosion scale products were performed on specimens after long exposure.
304 stainless steel [8]. On corrosion-resistant alloys, thiosulphates tend to become reduced to sulphur in zones where the passive film breaks down, according to the following reaction:
S2 O32 − + 6H+ + 4e− = 2S + 3H2 O E 0 = 0.465 VSHE
(1)
On the one hand, the sulphur that is deposited during this reaction stimulates anodic processes on the active surface, and on the other, it prevents the adsorption of hydroxide ions. In this way, it prevents passive film restoration. However, the formation of sulphur takes place only on the active metal, since its formation is inhibited by the passive, chromium oxide film, and is favoured by acidity. The action of thiosulphate ions occurs mainly on active steel or whenever the passive film is damaged, i.e. at places of localized corrosion. Several reports agree that corrosion phenomena due to thiosulphates can occur only in the presence of other substances that are able to attack the passive film, or when the film is damaged by mechanical stress, i.e. at the apex of a crack formed by stress corrosion cracking or similar [7,9–18]. Some reports [19–21] indicate that thiosulphate can itself promote localized corrosion, even in the absence or presence of small amount of chloride. Many studies have been carried out on general corrosion in neutral or weakly acid solutions containing thiosulphate, but very few investigations have been carried out in solutions with high sulphite content. To the best of our knowledge, there are no data presented in the literature regarding the simultaneous presence of these two chemical species. The object of this research is the study of the general corrosion of the carbon steel used in geothermal plants for conveying and re-injecting condensates containing thiosulfates and other species produced by the oxidation of sulphites. The paper reports the results from our study into the corrosion mechanism of carbon steel in contact with oxidation species of sulphur. 2. Materials and methods All experiments were carried out on API 5L grade X65 steel, with ferritic-pearlitic structure. Table 1 shows the chemical composition determined by means of spectrographic analysis. Disk specimens of 15 mm diameter and 5 mm height, or cylinders of 12 mm diameter and 18 mm height were used. The composition of the test solutions used in this study is representative of that of the condensation water found in Italian geothermal plants [19], namely 3000 ppm of SO42−, 300 ppm of sulphites, 150 ppm of thiosulphate, with pH ranging between 5 and 7. The pH was adjusted by the addition of sulphuric acid. All experiments were carried out at room temperature. In some experiments, the sulphite and thiosulphate composition of the solution was modified in order to assess the effects of these ions on the dissoved oxygen concentration. All solutions were prepared using sodium salts. The stability of each solution was evaluated by measuring the oxygen concentration and pH before and after each experiment. Oxygen concentration was measured by means of WTW FDO® 925–optical oxygen sensor and pH was measured by means of AMEL 2335 pHmeter. Furthermore, monitoring was carried out for up to 24 h, with air or nitrogen bubbling, with and without addition of sulphite and thiosulphate. To evaluate the effect of the sulphite/thiosulphate ratio on oxygen content, the oxygen concentration of the solution exposed to air bubbling was measured after the progressive addition of 150 ppm of sulphite, every 5 min.
3. Results 3.1. Oxygen–sulphite reaction The combined measurements of oxygen and pH permit the accurate description the rate of the reaction of sulphite with dissolved oxygen. Fig. 1 depicts the oxygen concentration of the of the solution at the beginning of the experiment. The addition of 300 ppm of sulphite, in the absence of thiosulphate ions, causes a rapid depletion of dissolved oxygen, and this reaches zero in a matter of minutes. This reaction leads to a variation in pH, with acidity increasing with the amount of reacted sulphite. In a solution containing 300 ppm of sulphite and continuous air bubbling, stable pH values between 3 and 4 were obtained (Fig. 2).
Table 1 Composition of steel used in this study (%w). Type
C
Mn
Si
P
S
Ni
Cr
Mo
Cu
Nb
Ceq
API 5L grade X65
0.09
1.64
0.24
0.003
0.002
0.02
0.03
0.01
0.01
0.049
0.366
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Fig. 4. Variations in oxygen concentration following the addition of increasing quantities of sulphites. Fig. 1. Variations in dissolved oxygen concentration following the addition of sulphite to solutions containing only sulphates.
Fig. 5. The dependence of oxygen reaction rate on the sulphite/thiosulphate ratio (decreases in dissolved oxygen content per time unit). Fig. 2. Time-dependence of the pH of aerated solutions.
amount of sulphite until a steady state was reached. Data derived by authors are in good agreement with those obtained by Mo et al. [27]. As this ratio increases by a factor of 5, the rate is observed to increase by a factor 6.5. 3.2. Weight loss experiments Fig. 6 summarizes the results from the weight loss measurements, as a function of pH and the number of days of exposure of the specimen, in the de-aerated solution. The corrosion rate is expressed as mass variation per surface unit and exposure period. It is the average value referred to the whole exposure period. It is evident that the corrosion rate decreases as the number of days of exposure increases. This decrease is more noticeable for low pH values, while at pH 7.2 the rate is Fig. 3. Variations in dissolved oxygen concentration following the addition of sulphite to a solution of sulphate containing thiosulphate.
The addition of 150 ppm of thiosulphate to the solution, prior to the addition of sulphite, significantly modifies the observed behaviour. The concentration of oxygen was observed to remain at values very close to its initial level (Fig. 3) and no acidification was observed. On the contrary, a slight increase in pH was detected (Fig. 2). Fig. 4 depicts the variation in oxygen concentration with further additions of fixed amounts of sulphite, equal to 150 ppm each. The oxygen concentration was monitored after 5 min, following the initial addition of sulphite. The variation in oxygen concentration between additions becomes greater with increasing sulphite content. Fig. 5 depicts the dependence of the estimated reaction rate on the weight ratio between sulphite and thiosulphate. The reaction rate was derived by the measurement of oxygen concentration during time, after the addition of constant
Fig. 6. Average corrosion rates from weight loss measurements as a function of exposure time and pH.
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cathodic curve is not modified at potentials approaching the free corrosion potential. At very low polarizations, Tafel behaviour is clearly seen and gradually becomes more evident with increasing rotational speed. The cathodic polarization curves for the solution containing sulphite and thiosulphate were significantly different from that of the de-aerated solution containing only sodium sulphate (Fig. 10). Without sulphite ions and thiosulphate, in the absence of oxygen, Tafel behaviour can be observed at very low potentials, due to the dissociation of water. On the other hand, in the presence of these ions, a complex modification of the cathodic curve is observed with an increase in the cathodic current density at less negative potentials, and greater overvoltage at very negative potentials. The Tafel curve for hydrogen reduction by water dissociation becomes visible at potentials more negative than 150–200 mV, compared to the curve for the solution containing only sulphates. The polarization resistance (Rp) was estimated through the Stern and Geary relationship, by calculating the slope of the potentiodynamic curves at polarizations of ± 20 mV. The constant of Stern and Geary relationship is assumed to be 26 mV for all experimental conditions. This value has been estimated by taking into account that the slope of the cathodic polarization curve is much higher than that of the anodic curve (ba), and that the slope of the latter is very close to 60 mV (Fig. 8). This value correlates well with that derived from the measurements of weight loss at day one. Fig. 11 plots the dependence of corrosion rate, estimated from the potentiodynamic experiments, on pH and rotational speed of the electrode. The results for the stationary electrode refer to tests performed on both cylindrical and disc electrodes. The results confirm that the corrosion rate increases as pH decreases. Moreover, the effect of the rotational speed of the electrode decreases with increasing pH. This effect is almost negligible at pH 7.2, but becomes predominant at pH 5, where a significant increase in current density, at a rotational speed of 1000 rpm, is observed.
Fig. 7. Dependence of pH on weight loss in a solution without oxygen.
approximately constant with time. The corrosion rates during the first day of exposure at pH 5 and 7.2 differ by a factor of about eight, after which this difference tends to become smaller. At the point of extraction, all specimens exhibited a thick black scale of poorly adhered corrosion products, except those exposed at pH 7.2. The data displayed in Fig. 7 show that pH increases slightly over time, except for samples immersed in solutions at pH 6.5 and 7.2. 3.3. Potentiodynamic experiments Fig. 8 shows the polarization curves for the steel sample in stagnant solutions. The anodic curves remain substantially unaffected by pH, so that trends are comparable. On the other hand, there is a clear shift of the cathodic curve towards higher values of current density as pH decreases. At pH 7.2, the cathodic branch of the polarization curve is characterized by Tafel behaviour and limiting currents at lower potentials. In the two most acid solutions, a significant increase in current density can be observed at potentials slightly below the free corrosion potential, with a progressive increase in slope on moving from pH 7.2 to pH 5. In addition, two different limiting processes can be easily identified. In these solutions, the curves tend toward a single Tafel slope, at around 180 mV/decade, at very negative potentials, due to reduction reactions involving hydrogen produced by the dissociation of water. The cathodic polarization curves are observed to change with changes in the fluid dynamic conditions. Fig. 9 shows the effect of the rotational speed of the electrode. The higher the rotational speed, the higher the cathodic limiting current density, in both the region close to the free corrosion potential, and at more negative potentials, confirming that transport phenomena are significant. In the solution at pH 7.2, the effect is significant only at the lowest potentials, whilst the
3.4. Potentiostatic experiments Fig. 12 shows the cathodic current density measured during potentiostatic experiments at −800 mV vs. Ag/AgCl. A sharp increase in cathodic current is observed, in a relatively short time, as the rotational speed increases. The stationary current density as a function of the rotational speed is shown in Fig. 13. The current density increases follow a power law, with an exponent that progressively increases from less than 0.1 in solution at pH 7.2 to about 0.3 at pH 5. While the kinetics of both anodic and cathodic processes, at pH 7.2, are affected by rotational speed, the corrosion rate remains substantially unchanged.
Fig. 8. Effect of pH on the polarization curves conducted with stationary electrodes. Anodic curve (left) and cathodic curve (right).
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Fig. 9. Effect of pH and rotation speed on the cathodic potentiodynamic curves for the rotating cylinder electrode.
Fig. 11. Dependence of current density on pH and rotation rate.
Fig. 10. Effect of the addition of sulphite and thiosulphate on the cathodic polarization curve.
chemical formula Fe(1+x)S, where x varies between 0.057 and 0.064 [20] and it is commonly observed in H2S-containing environments. The spectra obtained from the sample exposed for 10 days, and the powder obtained from the sample surface exposed for 30 days, reveal that the peak corresponding to amorphous iron sulphide is less pronounced after longer exposure, whilst the mackinawite peak becomes more evident.
3.5. Scale morphology and composition The corrosion scale from the weight-loss specimens was analysed immediately after the exposure period by means of X-ray diffraction (XRD), directly on the sample, or on the corrosion scale powder ground from the specimen surface after prolonged exposure. Fig. 14 shows the time-dependent variation of the spectrum of the corrosion products after 3 days, 10 days and 30 days of immersion in the solution at pH 5.8. The spectrum of the sample exposed for 3 days was compared those of the aluminium sample-holder and bare steel. The presence of at least four additional peaks can be noticed at 16.7°, 30°, 39° and 49.5°, and are attributable to the amorphous phase of FeS and mackinawite. The latter is a non-stoichiometric sulphide with a tetragonal structure and
4. Discussion In the test solutions considered in the present work, sulphur is present in the form of oxidized species. The instability of sulphites and thiosulphates can lead to other chemical species through reduction or disproportion, firstly to sulphur and hydrogen sulphide, and though reactions with other chemical species, mainly sulphites, thereby changing the composition of the environment. Moreover, iron sulphide 5
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complicated and can modify electrochemical processes. 4.1. Corrosion processes in neutral or weakly acid water The corrosion of steel in neutral or acidic aerated solutions occurs through anodic iron oxidation, supported by cathodic oxygen reduction. The cathodic process involving the direct reduction of the hydrogen ion becomes predominant with decreasing pH. In neutral aerated water, the corrosion rate is equal by the oxygen limiting current. Under these conditions, the corrosion rate increases proportionally to the concentration of dissolved oxygen and the water flux, because oxygen transport towards the metal surface increases with increasing flux. The latter decreases as deposits form, both calcareous and corrosion scale. Temperature has a complex effect. In the range from ambient to 40–50 °C, the corrosion rate generally increases, approximately doubling every 20–25 °C. Hydrogen can form according to different reactions. In acidic solutions, the hydrogen reduction reaction is prevalent due to the high concentration of hydrogen ions:
Fig. 12. Effect of rotational speed on the cathodic current density during the potentiostatic experiment at −800 mV vs. Ag/AgCl in solution at pH 5.8.
2H+ + 4e− → H2
(2)
In neutral water, the concentration of hydrogen ions is too low to support a relevant corrosion process. Hydrogen can be formed, however, through the dissociation of water:
2H2 O + 2e− → H2 + 2OH−
(3)
However, this process takes place only at very negative potentials, well below those that are usual for iron corrosion. In neutral or weakly acid solutions, in the absence of oxygen, the corrosion rate of steel is negligible, in the order of tens of microns per year. Higher corrosion rates are expected only in presence of substances that can promote the supply of hydrogen. These processes are different to those depicted in Eq. (2) and Eq. (3). Among these substances, carbon dioxide and hydrogen sulphide can be considered as prime candidates. The first is responsible for “sweet corrosion”, and is associated with high corrosion rates of carbon steel, well above those from oxygen attack observed in the oil industry, mining and geothermal environments. Carbon dioxide supports the cathodic development hydrogen, even at pH between 5 and 7, by direct reduction of carbonic acid, according to the reaction:
Fig. 13. Effect of rotation speed on the cathodic current density at −800 mV vs. Ag/AgCl, as a function of pH in de-aerated solutions (3000 ppm SO42−, 300 ppm SO32−, 150 ppm S2O32−).
corrosion scale is formed, which provides a much less protective scale compared to iron oxide. The corrosion of carbon steel in waters that contain high concentrations of sulphites and thiosulphates is therefore
Fig. 14. Dependence of the XRD spectrum of the corrosion products as a function of exposure time.
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2H2 CO3 + 2e− → H2 + 2HCO3−
(4)
The development of hydrogen by the direct reduction of hydrogen sulphide has been described by Bolmer [21] in the 1960s, and more recent studies can be found [22]. This reaction takes place according to Eq. (5): 2H2S + 2e− → H2 + 2HS−
(5)
4.2. Effect of thiosulphate on oxygen-induced corrosion Low amounts of sulphite, well below the concentrations considered in this work, are normally used in industrial plants for oxygenscavenging purposes. Sulphites react with oxygen dissolved in the water, causing its removal. The oxidation-reduction reaction, which is fast even at ambient temperature (Eq. (6) leads to the formation of sulphates by the reaction:
2HSO3− + O2 → H+ + 2SO42 −
Fig. 15. Effect of thiosulphate and sulphite on the corrosion rate, as a function of the rotational speed of the electrode, measured by the polarization resistance technique.
characteristic curves for both anodic and cathodic process, and increase the kinetics, with a major effect on the cathodic reaction, both for potentials close to the free corrosion potential, and at much lower potentials [23]. They also modify the cathodic polarization curve, causing the occurrence of a limiting process at low potential values. Similar behaviour is observed in solutions containing sulphites (Fig. 10). In the presence of sulphites and thiosulphates, the increase in the kinetics of the cathodic process is more evident with greater water flow and with lower the pH. As a consequence, the corrosion rate of steel increases by an order of magnitude on moving from pH 7.2–5 (Fig. 10 and 6). Kappes et al. [29] reported that the corrosion of carbon steel involves the formation of sulphur and H2S according to the reactions Eq. (7) and Eq. (8) or, alternatively, Eq. (9) and Eq. (10).
(6)
In industrial plants, sulphite is added, as a sequestering agent, in proportion to the oxygen content of the water. The presence of thiosulphates affects the oxygen-scavenging reaction promoted by sulphites (Fig. 3). Ulrich et al. and Mo et al. studied their inhibiting effect [26,27]. Thiosulphate acts as a free radical sequestering agent involved in the reactions sequence in which the overall reaction is articulated. In particular, they have been studied as inhibitors of the formation of sulphates in sulphur abatement implants for combustion gases. The reaction rate depends upon the nature of the cation and the acidity [26,27]. The inhibiting effect depends on the sulphite/thiosulphate ratio; the addition of increasing amounts of sulphite over thiosulphate hinders the effect of the thiosulphates, and gradually increases the oxygen-scavenging rate. When the sulphites/ thiosulphates ratio is around 1 or 2, the rate of this sequestering reaction becomes negligible or extremely low (Fig. 5). Therefore, the action of thiosulphate increases the levels of oxygen, even in sulphite-containing solutions, and may promote substantial increases in the corrosion rate of carbon steel, in proportion to the residual oxygen content. In addition, the oxidation of sulphite to sulphate causes a decrease in pH (Fig. 7), according to the reaction Eq. (6). In fact, the second dissociation constant of sulphuric acid is greater than that of sulphurous acid. The HSO3− ion is not substantially dissociated in the pH range of the water considered in this research. Upon reaction, it is replaced by the sulphate ion, which is completely dissociated under these conditions. In the experiments using solutions without thiosulphates, the pH was observed to rapidly reach of 3.4 after all the sulphite (300 ppm) had reacted. In this manner, the acid attack of steel can occur. Thiosulphate ions tend to block the oxygen-scavenging reaction and hence, they stabilize the pH, slowing down the production of acidity. However, corrosive attack, supported by cathodic oxygen reduction can still occur. The pH tends to increase if oxygen entry is prevented during the active corrosion process (Fig. 7).
S2 O32 − + 6H+ + 4e− = 2S 0 + 3H2 O E 0 = 0.465 VSHE
(7)
S° + 2H+ + 2e− = H2S E0 = 0.142 VSHE
(8)
S2 O32 −
(9)
+
H+
=
S0
+
HSO3−
HSO3− + 7H+ + 6e− = H2 S + 3H2 O E 0 = 0.366 VSHE
(10)
At the free corrosion potential, the cathodic current mainly promotes the reduction of thiosulphate, in sulphite-free waters. The formation of H2S is catalysed by the metal surface and otherwise it does not occur in bulk solution. It leads to the formation of a black film of iron sulphide, mainly mackinawite [22,23,29–31]. The formation of scale, associated with high corrosion rates, is reported even in saline waters without thiosulphates, but containing an excess of sulphites, compared to the values usually added to control oxygen-induced corrosion [32–35]. This reaction (Eq. (10) therefore represents an important process in waters where sulphites reach relevant concentrations, far greater than those of thiosulphates. The aggressiveness of these waters may not be related only to the content of thiosulphate, but also to that of sulphite. The formation of hydrogen sulphide on the metal surface can stimulate the cathodic process that produces hydrogen through two different mechanisms. On the one hand, it can increase the availability of the hydrogen ion due to its dissociation on the metal surface, according to the reaction:
4.3. Effect of thiosulphate on general corrosion, in the absence of oxygen In de-aerated neutral solutions devoid of particularly aggressive species, the corrosion rate of carbon steel is practically negligible. The addition of sulphites and thiosulphates, at the levels considered in this research work, results in an increase in the corrosion rate of about one order of magnitude (Fig. 15). This attack is enhanced by the rotational speed of the electrode. At 1000 rpm, the corrosion rate is almost double compared to that observed under stagnant conditions (Fig. 11). Ezuber [23] and Kappes et al. [24] studied the corrosion of carbon steels in solutions containing only thiosulphates, at pH levels comparable or lower than the solutions considered in this research work. The results of these studies highlighted that the corrosion rate of carbon steel increases with thiosulphate concentration. These ions modify the
2H2S → H+ + HS−
(11)
On the other hand, hydrogen evolution can occur owing to the direct reduction of hydrogen sulphide [24,25]: 2H2S + 2e− → H2 + 2HS−
(12)
The mackinawite scale is conductive, poorly protective and it is a great cathode for hydrogen production, further stimulating the corrosion of the metal due to galvanic coupling [23,28,29]. The scale formed during 7
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Fig. 16. Morphology of the corrosion products (solutions containing 3000 ppm SO42−, 300 ppm SO32−, 150 ppm S2O32−) as a function of pH and exposure time.
under conditions that promote high corrosion rates [25]. The weight loss experiments (Fig. 7) substantially confirm this mechanism. The reactions depicted in Eq. (7), Eq. (8), Eq. (9) and Eq. (10) are favoured in acidic solutions and become more aggressiveness as the pH diminishes. In particular, the corrosion rate significantly increases on moving from pH 7.2 to 5, due to increases in the kinetics of the cathodic processes, and are associated with the more rapid growth of corrosion scale (Figs. 6, 16 and 18).
our laboratory experiments is fragile and multi-layered. Underneath the scale, the metallographic structure of the steel can be evidenced, with the formation of localized attack sites (Fig. 16) after long exposure times. The sulphur layer favours the formation of H2S on the metal surface according to the reaction [25]:
(x − 1) Fe + Sy − 1 S 2 − + 2H+ = (x − 1) FeS + H2 S + Sy − x
(13)
The stability diagram for various sulphur compounds confirms that the free corrosion potential of steel falls inside the stability region of H2S [22,36] (Fig. 17). The concentration of hydrogen sulphide on the surface of steel in acidic solutions containing thiosulphates has been evaluated by Kappes [29,30]. The author points out that the corrosion rate of carbon steel in solutions containing thiosulphates is higher than that found in the same solution devoid of these ions, but saturated with an H2S partial pressure equal to that estimated on the surface in the presence of thiosulphates. The reduction of thiosulphate, firstly to sulphur and then to hydrogen sulphide, and that of sulphite, both reduce the concentration hydrogen ions and tend to increase pH on the metal surface, especially
5. Conclusions The corrosion of carbon steel in neutral or weakly acid solutions containing sulphites and thiosulphates in significant concentrations occur with different mechanisms. Thiosulphates can inhibit the reactions between oxygen and sulphites and promote a substantial increase in the corrosion rate, in proportion to the residual oxygen content. On the other hand, they control the acidification resulting from the reactions of sulphites with oxygen, which may, in turn, induce acid corrosion. In anaerobic solutions, both sulphites and thiosulphates modify the characteristic anodic and cathodic potentiodynamic curves, with a
Fig. 17. Stability regions of the oxidation species of sulphur [37], and the range (in red) of the corrosion potential of steel, measured in solutions containing 3000 ppm SO42−, 300 ppm SO32−, 150 ppm S2O62−.
Fig. 18. Dependence of current density on pH and electrode rotational speed.
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marked effect on the cathodic process at potentials approaching the free corrosion potential, and under conditions of water flow. In the absence of oxygen, the corrosion process leads to the formation of thick scales of mackinawite. The iron sulphide forms as a result of the reduction of thiosulphate and sulphite, leading to the formation of hydrogen sulphide and the precipitation of the corrosion scale.
study, Corros. Sci. 39 (1996) 913–933. [15] P. Marcus, Surface science approach of corrosion phenomena, Electrochim. Acta 43 (1998) 109–118. [16] D. Tromans, L. Frederick, Effect of thiosulfate on crevice corrosion of stainless steels, Nace 40 (1984) 633–639. [17] Y.-S. Choi, J.-G. Kim, Investigation of passivity and its breakdown on Fe3Al-Cr-Mo intermetallics in thiosulfate-chloride solution, Mat. Sci. Eng. A 333 (2002) 336–342. [18] Y.M. Liou, S.Y. Chiu, C.L. Lee, H.C. Shih, Electrochemical pitting behaviour of type 321 stainless steel in sulfide-containing chloride solutions, J. Appl. Electrochem. 29 (1999) 1377–1381. [19] R. Newman, H. Isaacs, B. Alman, Effects of sulfur compounds on the pitting behavior of type 304 stainless steel in near-neutral chloride solutions, Corrosion 38 (1982) 261–264. [20] A. Garner, Thiosulfate corrosion in paper-machine white water, Corrosion 41 (1985) 587–591. [21] R.C. Newman, Pitting of stainless alloys in sulfate solutions containing thiosulfate ions, Corrosion 41 (1985) 450–453. [22] M. Cabrini, M. Favilla, S. Lorenzi, B. Tarquini, T. Pastore, R. Perini, Corrosione Da Tiosolfati Negli Impianti Geotermici, Pietro Pedeferri e la scuola di corrosione e protezione dei materiali al Politecnico di Milano, Milano, 2013. [23] N. Sridhar, D. Dunn, A. Anderko, M. Lencka, H.U. Schutt, Effects of water and gas compositions on the internal corrosion of gas pipelines – modeling and experimental studies, Corrosion 57 (2001) 221–235. [24] P.W. Bolmer, Polarization of iron in H2S.NaHS buffers, Corrosion 21 (1965) 69–75. [25] Y. Zheng, B. Brown, S. Nesic, Electrochemical study and modeling of H2S corrosion of mild steel, Corrosion 70 (2013) 351–365. [26] R.K. Ulrich, G.T. Rochelle, R.E. Prada, Enhanced oxygen absorption into bisulphite solutions containing transition metal ion catalysts, Chem. Eng. Sci. 41 (1986) 2183–2191. [27] J. Mo, Z. Wu, C. Cheng, B. Guan, W. Zhao, Oxidation inhibition of sulfite in dual alkali flue gas desulfurization system, J. Environ. Sci. 19 (2007) 226–231. [28] H.M. Ezuber, Influence of temperature and thiosulfate on the corrosion behaviour of steel in chloride solutions saturated in CO2, Mater. Des. 30 (2009) 3420–3427. [29] M. Kappes, G.S. Frankel, N. Sridhar, R.M. Carranza, Corrosion behavior of carbon steel in acidified, thiosulfate-containing brines, Corrosion 10 (2012) 872–884. [30] M. Kappes, G. S.Frankel, N. Sridhar, R.M. Carranza, Reaction paths of thiosulfate during corrosion of carbon steel in acidified brines, J. Electrochem. Soc. (2012) C195–C204. [31] J. Banas, U. Lelek-Borkowska, B. Mazurkiewicz, W. Solarski, Effect of CO2 and H2S on the composition and stability of passive film on iron alloys in geothermal water, Electrochim. Acta 152 (2007) 5704–5714. [32] T. Hemmingsen, H. Vangdal, T. Våland, Formation of ferrous sulfide film from sulfite on steel under anaerobic conditions, Corrosion 48 (1992) 475–481. [33] T. Hemmingsen, The electrochemical reaction of sulphur-oxigen compounds – part II. Voltammetric investigation performed on platinum, Electrochim. Acta 37 (1992) 2785–2790. [34] T. Hemmingsen, H. Lima, Electrochemical and optical studies of sulphide, Electrochim. Acta 43 (1998) 35–40. [35] T. Hemmingsen, T. Våland, An investigation of the effect of bisulphite/sulphite on steel under anaerobic conditions, 11th Scandinavian Corrosion Congress, Stavanger, 1989. [36] P. Marcus, E. Protopopoff, Thermodynamics of thiosulfate reduction on surfaces of iron, nickel and chromium in water at 25 and 300 °C, Corros. Sci. 39 (1997) 1741–1752.
Acknowledgment This research project was made possible by funding from the ITALY® PROJECT of the University of Bergamo. References [1] A. Baldacci, M. Mannari, F. Sansone, Greening of geothermal power: an innovative technology for abatement of hydrogen sulphide and mercury emission, Proceedings World Geothermal Congress, Antalya, Turchia, 2005. [2] M. Cabrini, S. Lorenzi, T. Pastore, Corrosion behavior of carbon steel in acidic solutions containing thiosulfate and sulfiteions, Metallurgia Italiana 106 (11–12) (2014) 3–10. [3] V. Duchi, A. Minissale, M. Manganelli, Chemical composition of natural deep and shallow hydrothermal fluids in the Larderello geothermal fields, J. Volcanol. Geotherm. Res. 49 (1992) 313–328. [4] F. Sabatelli, M. Mannari, R. Parri, Hydrogen sulfide and mercury abatement: development and successful operation of AMIS technology, GRC Trans. 33 (2009) 343–347. [5] D. Macdonald, Critical Issues in the Use of Metals and Alloys in Sulphur-Containing Aqueous Systems, (1993). [6] H. Horowitz, Chemical studies of polythionic acid stress corrosion cracking, Corros. Sci. 23 (1983) 353–362. [7] R.C. Newman, K. Sieradzki, H.S. Isaacs, Stress corrosion cracking of sensitized type 304 stainless steel in thiosulfate solutions, Metall. Trans. A 13 (1982) 2015–2026. [8] E.A. Abd El Meguid, N.A. Mahmoud, S.S. Abd El Rehim, The effect of some sulphur compounds on the pitting corrosion of type 304 SS Mater, Chem. Phys. 63 (2000) 63–74. [9] J.O. Park, M. Verhoff, R. Alkire, Microscopic behavior of single corrosion pits: the effect of thiosulfate on corrosion of stainless steel in NaCl, Electrochim. Acta 42 (1997) 3281–3291. [10] H.-S. Kuo, H. Chang, W.-T. Tsai, The corrosion behavior of AISI 310 stainless steel in thiosulfate ion containing saturated ammonium chrolide solution, Corros. Sci 41 (1999) 669–684. [11] T. Laitinen, Localized corrosion of stainless steel in chloride, sulfate and thiosulfate containing environments, Corros. Sci. 42 (2000) 421–441. [12] A.I. Almarshad, D. Jamal, Electrochemical investigations of pitting corrosion behaviour of type UNS S31603 stainless steel in thiosulfate-chloride environment, J. Appl. Electrochem. 34 (2004) 67–70. [13] W.T. Tsai, M.J. Sheu, J.T. Lee, The stress corrosion crack growth rate in sensitized alloy 600 in thiosulfate solution, Corros. Sci. 38 (1996) 33–45. [14] C. Duret-Thual, D. Costa, W. Yang, P. Marcus, The role of thiosulfates in the pitting corrosion of Fe-17Cr alloys in neutral chloride solution: electrochemical and XPS
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effects of thiosulphates and sulphite ions on steel corrosion☆ ⁎
M. Cabrini, S. Lorenzi , T. Pastore Department of Engineering and Applied Sciences, University of Bergamo, Viale Marconi 5, 24044, Dalmine (Bergamo), Italy
A B S T R A C T The paper deals with the general corrosion of carbon steel in geothermal plants for conveying and re-injecting condensates. The results of electrochemical and weight loss tests are discussed in function of sulphite and thiosulphate concentrations typical of Italian geothermal power plants. Thiosulphates can inhibit the reactions between oxygen and sulphites and increase the corrosion rate, but they control the acidification resulting from the reactions of sulphites with oxygen. In anaerobic solutions, both species modify mainly the cathodic process. In the absence of oxygen, the corrosion process leads to the formation of mackinawite scales, while iron sulphide forms due to the reduction of thiosulphate and sulphite.
1. Introduction In geothermal plants that generate electrical energy, the steam produced in the extraction wells is conveyed to steam turbines that transform mechanical energy into electrical energy. The condensed water is then sent to the cooling towers and, finally, to a well for reinjection into the ground, to avoid environmental pollution by substances present in the condensates. This also helps prevent subsidence that might be induced by the extraction of significant quantities of water, while supporting steam pressure in the geothermal reservoir. The process releases into the atmosphere approximately 75% of the water flow [1]. The extracted fluid also contains non-condensable gases (NCGs), in a variety of concentrations depending on the reservoir, with extracts from the Larderello geothermal wells having an NCG content of around 5% [2]. These fluids contain mainly carbon dioxide and significant trace amounts of hydrogen sulphide [3]. To limit their release into the atmosphere, and to maintain their levels to values that are significantly below those perceptible by the population, steam plants in Italy are equipped with abatement systems that oxidize hydrogen sulphide to SO2. This gas is subsequently washed with the condensation water produced by the plant, and this dissolves the SO2 in the form of sulphite. Subsequently, sulphites give rise to sulphates and thiosulphates by further reactions with oxygen and sulphur [1,4]. Literature reports have highlighted the influence of the thiosulphate ion on various forms of corrosion, from that of generalized carbon steel, to the localized corrosion of stainless steels a [4] and nickel alloys, intergranular corrosion of sensitized stainless steels [5] and nickel
alloys, and stress corrosion [6] caused by sulphides in high strength steels. Sulphur may be present in the form of numerous compounds that are characterized by different degrees of oxidation. Because of the instability of these compounds, they can become involved in oxidation and reduction processes that depend on the characteristics of the environment in which they are dissolved. As a consequence, sulphur can initiate a large variety of corrosion phenomena that are characterized by complex behaviour [5,6]. In addition to sulphides and sulphates, sulphur can also exist as sulphites. Of great importance to corrosion are the polythionic acids, of generic formula H2SxO6, which have at least one sulphur atom bridging two sulphonic acid units, and the thiosulphate group that consists of a sulphate group bound to a sulphur atom. Thiosulphate ions display strongly acidic behaviour and, in water, they are normally dissociated. These compounds are involved in most of the reactions involving polythionic acids, and can be formed by reduction on a metal surface [6–8]. The role of thiosulfates in corrosion has been the object of several studies, and their corrosive effects have been well known since the 1940s, coinciding with the first accidents in the oil and paper industries involving sensitized austenitic stainless steels [7] [4]. These failures were caused by stress corrosion promoted by polythionic acids and thiosulphates, and were caused by the entry of air and moisture during the plant maintenance [9]. Similar problems occurred several years later, in pressurized-water nuclear reactors, where solutions containing thiosulphates were used as emergency liquids for the absorption of radioactive iodine. In the paper industry, the addition of sodium thiosulphate, used as a bleaching agent, resulted in the corrosion of AISI
☆ ⁎
Consorzio INSTM, UdR “Materials and Corrosion”, Via G.Giusti 6, Firenze, Italy. Corresponding author. E-mail addresses: [email protected] (M. Cabrini), [email protected] (S. Lorenzi), [email protected] (T. Pastore).
https://doi.org/10.1016/j.corsci.2018.02.046 Received 13 January 2017; Received in revised form 19 February 2018; Accepted 20 February 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Cabrini, M., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.02.046
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Corrosion behaviour was studied by weight loss, and electrochemical measurements. Potentiodynamic polarization tests and cathodic potentiostatic polarization tests were also performed by using IVIUM Compactstat potentiostat. Weight loss measurements were performed by means of Sartorius BP211D scale on the disc specimens, described above. The surface of each disc was polished with emery paper of up to 2400 grit. The exposed surfaces were further polished to 1 μm using diamond paste, followed by rinsing and degreasing with acetone in an ultrasonic bath, and then drying. After exposure, the specimens were pickled in diluted hydrochloric acid inhibited with hexamethylenetetramine, before being weighed. During these experiments, two discs were immersed in a 400 mL closed glass container filled with the de-aerated solution, under continuous nitrogen bubbling. Electrochemical tests were performed under stagnant conditions using a rotating cylinder electrode EG&G mod. 616 RDE. In the first case, one litre ASTM cell was used, in which the exposed surface area of the specimen was 1 cm2. A double-junction Ag/AgCl/KClsat. from now on indicated by Ag/AgCl reference electrode and a graphite counterelectrode were placed in separate compartments with porous ceramic diaphragms. A Huber-Luggin capillary was used. The flux of the gases produced at the counter-electrode of the test cell was vented through a hole in the upper part of the compartment. In experiments using the rotating electrode, similar equipment to that described above was used, and the rotating shaft was sealed against external air exposure. Before any electrochemical testing, the solution was de-aerated by nitrogen bubbling for at least twelve hours. Subsequently, a constant flow of nitrogen was maintained throughout the test period. Potentiodynamic curves were plotted after about 5 min of immersion, at a scan rate of 10 mV/min, from the free corrosion potential to a potential of 100 mV higher, for anodic curves, or 700 mV lower, for cathodic curves. Ohmic drop compensation was applied to all potentiodynamic curves. The ohmic drop was evaluated by means of electrochemical impedance measurements at a frequency of about 103 Hz, with an amplitude of 10 mV around the free corrosion potential. This correction is significant only for current densities above 5 mA/cm2 and 1 mA/cm2 for the disc electrode and the rotating cylinder electrode, respectively. Potentiostatic experiments were carried out by detecting the value of the stationary current, at −800 mV vs. Ag/AgCl/saturated KCl, on a rotating electrode at increasing speeds between 0 and 6000 rpm. The composition and morphology of the corrosion scale was characterized by electron microscopy and X-ray analyses of specimens immediately after testing. The specimen surfaces were washed with distilled water and dried under a flow of nitrogen. Analyses of the powdered corrosion scale products were performed on specimens after long exposure.
304 stainless steel [8]. On corrosion-resistant alloys, thiosulphates tend to become reduced to sulphur in zones where the passive film breaks down, according to the following reaction:
S2 O32 − + 6H+ + 4e− = 2S + 3H2 O E 0 = 0.465 VSHE
(1)
On the one hand, the sulphur that is deposited during this reaction stimulates anodic processes on the active surface, and on the other, it prevents the adsorption of hydroxide ions. In this way, it prevents passive film restoration. However, the formation of sulphur takes place only on the active metal, since its formation is inhibited by the passive, chromium oxide film, and is favoured by acidity. The action of thiosulphate ions occurs mainly on active steel or whenever the passive film is damaged, i.e. at places of localized corrosion. Several reports agree that corrosion phenomena due to thiosulphates can occur only in the presence of other substances that are able to attack the passive film, or when the film is damaged by mechanical stress, i.e. at the apex of a crack formed by stress corrosion cracking or similar [7,9–18]. Some reports [19–21] indicate that thiosulphate can itself promote localized corrosion, even in the absence or presence of small amount of chloride. Many studies have been carried out on general corrosion in neutral or weakly acid solutions containing thiosulphate, but very few investigations have been carried out in solutions with high sulphite content. To the best of our knowledge, there are no data presented in the literature regarding the simultaneous presence of these two chemical species. The object of this research is the study of the general corrosion of the carbon steel used in geothermal plants for conveying and re-injecting condensates containing thiosulfates and other species produced by the oxidation of sulphites. The paper reports the results from our study into the corrosion mechanism of carbon steel in contact with oxidation species of sulphur. 2. Materials and methods All experiments were carried out on API 5L grade X65 steel, with ferritic-pearlitic structure. Table 1 shows the chemical composition determined by means of spectrographic analysis. Disk specimens of 15 mm diameter and 5 mm height, or cylinders of 12 mm diameter and 18 mm height were used. The composition of the test solutions used in this study is representative of that of the condensation water found in Italian geothermal plants [19], namely 3000 ppm of SO42−, 300 ppm of sulphites, 150 ppm of thiosulphate, with pH ranging between 5 and 7. The pH was adjusted by the addition of sulphuric acid. All experiments were carried out at room temperature. In some experiments, the sulphite and thiosulphate composition of the solution was modified in order to assess the effects of these ions on the dissoved oxygen concentration. All solutions were prepared using sodium salts. The stability of each solution was evaluated by measuring the oxygen concentration and pH before and after each experiment. Oxygen concentration was measured by means of WTW FDO® 925–optical oxygen sensor and pH was measured by means of AMEL 2335 pHmeter. Furthermore, monitoring was carried out for up to 24 h, with air or nitrogen bubbling, with and without addition of sulphite and thiosulphate. To evaluate the effect of the sulphite/thiosulphate ratio on oxygen content, the oxygen concentration of the solution exposed to air bubbling was measured after the progressive addition of 150 ppm of sulphite, every 5 min.
3. Results 3.1. Oxygen–sulphite reaction The combined measurements of oxygen and pH permit the accurate description the rate of the reaction of sulphite with dissolved oxygen. Fig. 1 depicts the oxygen concentration of the of the solution at the beginning of the experiment. The addition of 300 ppm of sulphite, in the absence of thiosulphate ions, causes a rapid depletion of dissolved oxygen, and this reaches zero in a matter of minutes. This reaction leads to a variation in pH, with acidity increasing with the amount of reacted sulphite. In a solution containing 300 ppm of sulphite and continuous air bubbling, stable pH values between 3 and 4 were obtained (Fig. 2).
Table 1 Composition of steel used in this study (%w). Type
C
Mn
Si
P
S
Ni
Cr
Mo
Cu
Nb
Ceq
API 5L grade X65
0.09
1.64
0.24
0.003
0.002
0.02
0.03
0.01
0.01
0.049
0.366
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Fig. 4. Variations in oxygen concentration following the addition of increasing quantities of sulphites. Fig. 1. Variations in dissolved oxygen concentration following the addition of sulphite to solutions containing only sulphates.
Fig. 5. The dependence of oxygen reaction rate on the sulphite/thiosulphate ratio (decreases in dissolved oxygen content per time unit). Fig. 2. Time-dependence of the pH of aerated solutions.
amount of sulphite until a steady state was reached. Data derived by authors are in good agreement with those obtained by Mo et al. [27]. As this ratio increases by a factor of 5, the rate is observed to increase by a factor 6.5. 3.2. Weight loss experiments Fig. 6 summarizes the results from the weight loss measurements, as a function of pH and the number of days of exposure of the specimen, in the de-aerated solution. The corrosion rate is expressed as mass variation per surface unit and exposure period. It is the average value referred to the whole exposure period. It is evident that the corrosion rate decreases as the number of days of exposure increases. This decrease is more noticeable for low pH values, while at pH 7.2 the rate is Fig. 3. Variations in dissolved oxygen concentration following the addition of sulphite to a solution of sulphate containing thiosulphate.
The addition of 150 ppm of thiosulphate to the solution, prior to the addition of sulphite, significantly modifies the observed behaviour. The concentration of oxygen was observed to remain at values very close to its initial level (Fig. 3) and no acidification was observed. On the contrary, a slight increase in pH was detected (Fig. 2). Fig. 4 depicts the variation in oxygen concentration with further additions of fixed amounts of sulphite, equal to 150 ppm each. The oxygen concentration was monitored after 5 min, following the initial addition of sulphite. The variation in oxygen concentration between additions becomes greater with increasing sulphite content. Fig. 5 depicts the dependence of the estimated reaction rate on the weight ratio between sulphite and thiosulphate. The reaction rate was derived by the measurement of oxygen concentration during time, after the addition of constant
Fig. 6. Average corrosion rates from weight loss measurements as a function of exposure time and pH.
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cathodic curve is not modified at potentials approaching the free corrosion potential. At very low polarizations, Tafel behaviour is clearly seen and gradually becomes more evident with increasing rotational speed. The cathodic polarization curves for the solution containing sulphite and thiosulphate were significantly different from that of the de-aerated solution containing only sodium sulphate (Fig. 10). Without sulphite ions and thiosulphate, in the absence of oxygen, Tafel behaviour can be observed at very low potentials, due to the dissociation of water. On the other hand, in the presence of these ions, a complex modification of the cathodic curve is observed with an increase in the cathodic current density at less negative potentials, and greater overvoltage at very negative potentials. The Tafel curve for hydrogen reduction by water dissociation becomes visible at potentials more negative than 150–200 mV, compared to the curve for the solution containing only sulphates. The polarization resistance (Rp) was estimated through the Stern and Geary relationship, by calculating the slope of the potentiodynamic curves at polarizations of ± 20 mV. The constant of Stern and Geary relationship is assumed to be 26 mV for all experimental conditions. This value has been estimated by taking into account that the slope of the cathodic polarization curve is much higher than that of the anodic curve (ba), and that the slope of the latter is very close to 60 mV (Fig. 8). This value correlates well with that derived from the measurements of weight loss at day one. Fig. 11 plots the dependence of corrosion rate, estimated from the potentiodynamic experiments, on pH and rotational speed of the electrode. The results for the stationary electrode refer to tests performed on both cylindrical and disc electrodes. The results confirm that the corrosion rate increases as pH decreases. Moreover, the effect of the rotational speed of the electrode decreases with increasing pH. This effect is almost negligible at pH 7.2, but becomes predominant at pH 5, where a significant increase in current density, at a rotational speed of 1000 rpm, is observed.
Fig. 7. Dependence of pH on weight loss in a solution without oxygen.
approximately constant with time. The corrosion rates during the first day of exposure at pH 5 and 7.2 differ by a factor of about eight, after which this difference tends to become smaller. At the point of extraction, all specimens exhibited a thick black scale of poorly adhered corrosion products, except those exposed at pH 7.2. The data displayed in Fig. 7 show that pH increases slightly over time, except for samples immersed in solutions at pH 6.5 and 7.2. 3.3. Potentiodynamic experiments Fig. 8 shows the polarization curves for the steel sample in stagnant solutions. The anodic curves remain substantially unaffected by pH, so that trends are comparable. On the other hand, there is a clear shift of the cathodic curve towards higher values of current density as pH decreases. At pH 7.2, the cathodic branch of the polarization curve is characterized by Tafel behaviour and limiting currents at lower potentials. In the two most acid solutions, a significant increase in current density can be observed at potentials slightly below the free corrosion potential, with a progressive increase in slope on moving from pH 7.2 to pH 5. In addition, two different limiting processes can be easily identified. In these solutions, the curves tend toward a single Tafel slope, at around 180 mV/decade, at very negative potentials, due to reduction reactions involving hydrogen produced by the dissociation of water. The cathodic polarization curves are observed to change with changes in the fluid dynamic conditions. Fig. 9 shows the effect of the rotational speed of the electrode. The higher the rotational speed, the higher the cathodic limiting current density, in both the region close to the free corrosion potential, and at more negative potentials, confirming that transport phenomena are significant. In the solution at pH 7.2, the effect is significant only at the lowest potentials, whilst the
3.4. Potentiostatic experiments Fig. 12 shows the cathodic current density measured during potentiostatic experiments at −800 mV vs. Ag/AgCl. A sharp increase in cathodic current is observed, in a relatively short time, as the rotational speed increases. The stationary current density as a function of the rotational speed is shown in Fig. 13. The current density increases follow a power law, with an exponent that progressively increases from less than 0.1 in solution at pH 7.2 to about 0.3 at pH 5. While the kinetics of both anodic and cathodic processes, at pH 7.2, are affected by rotational speed, the corrosion rate remains substantially unchanged.
Fig. 8. Effect of pH on the polarization curves conducted with stationary electrodes. Anodic curve (left) and cathodic curve (right).
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Fig. 9. Effect of pH and rotation speed on the cathodic potentiodynamic curves for the rotating cylinder electrode.
Fig. 11. Dependence of current density on pH and rotation rate.
Fig. 10. Effect of the addition of sulphite and thiosulphate on the cathodic polarization curve.
chemical formula Fe(1+x)S, where x varies between 0.057 and 0.064 [20] and it is commonly observed in H2S-containing environments. The spectra obtained from the sample exposed for 10 days, and the powder obtained from the sample surface exposed for 30 days, reveal that the peak corresponding to amorphous iron sulphide is less pronounced after longer exposure, whilst the mackinawite peak becomes more evident.
3.5. Scale morphology and composition The corrosion scale from the weight-loss specimens was analysed immediately after the exposure period by means of X-ray diffraction (XRD), directly on the sample, or on the corrosion scale powder ground from the specimen surface after prolonged exposure. Fig. 14 shows the time-dependent variation of the spectrum of the corrosion products after 3 days, 10 days and 30 days of immersion in the solution at pH 5.8. The spectrum of the sample exposed for 3 days was compared those of the aluminium sample-holder and bare steel. The presence of at least four additional peaks can be noticed at 16.7°, 30°, 39° and 49.5°, and are attributable to the amorphous phase of FeS and mackinawite. The latter is a non-stoichiometric sulphide with a tetragonal structure and
4. Discussion In the test solutions considered in the present work, sulphur is present in the form of oxidized species. The instability of sulphites and thiosulphates can lead to other chemical species through reduction or disproportion, firstly to sulphur and hydrogen sulphide, and though reactions with other chemical species, mainly sulphites, thereby changing the composition of the environment. Moreover, iron sulphide 5
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complicated and can modify electrochemical processes. 4.1. Corrosion processes in neutral or weakly acid water The corrosion of steel in neutral or acidic aerated solutions occurs through anodic iron oxidation, supported by cathodic oxygen reduction. The cathodic process involving the direct reduction of the hydrogen ion becomes predominant with decreasing pH. In neutral aerated water, the corrosion rate is equal by the oxygen limiting current. Under these conditions, the corrosion rate increases proportionally to the concentration of dissolved oxygen and the water flux, because oxygen transport towards the metal surface increases with increasing flux. The latter decreases as deposits form, both calcareous and corrosion scale. Temperature has a complex effect. In the range from ambient to 40–50 °C, the corrosion rate generally increases, approximately doubling every 20–25 °C. Hydrogen can form according to different reactions. In acidic solutions, the hydrogen reduction reaction is prevalent due to the high concentration of hydrogen ions:
Fig. 12. Effect of rotational speed on the cathodic current density during the potentiostatic experiment at −800 mV vs. Ag/AgCl in solution at pH 5.8.
2H+ + 4e− → H2
(2)
In neutral water, the concentration of hydrogen ions is too low to support a relevant corrosion process. Hydrogen can be formed, however, through the dissociation of water:
2H2 O + 2e− → H2 + 2OH−
(3)
However, this process takes place only at very negative potentials, well below those that are usual for iron corrosion. In neutral or weakly acid solutions, in the absence of oxygen, the corrosion rate of steel is negligible, in the order of tens of microns per year. Higher corrosion rates are expected only in presence of substances that can promote the supply of hydrogen. These processes are different to those depicted in Eq. (2) and Eq. (3). Among these substances, carbon dioxide and hydrogen sulphide can be considered as prime candidates. The first is responsible for “sweet corrosion”, and is associated with high corrosion rates of carbon steel, well above those from oxygen attack observed in the oil industry, mining and geothermal environments. Carbon dioxide supports the cathodic development hydrogen, even at pH between 5 and 7, by direct reduction of carbonic acid, according to the reaction:
Fig. 13. Effect of rotation speed on the cathodic current density at −800 mV vs. Ag/AgCl, as a function of pH in de-aerated solutions (3000 ppm SO42−, 300 ppm SO32−, 150 ppm S2O32−).
corrosion scale is formed, which provides a much less protective scale compared to iron oxide. The corrosion of carbon steel in waters that contain high concentrations of sulphites and thiosulphates is therefore
Fig. 14. Dependence of the XRD spectrum of the corrosion products as a function of exposure time.
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2H2 CO3 + 2e− → H2 + 2HCO3−
(4)
The development of hydrogen by the direct reduction of hydrogen sulphide has been described by Bolmer [21] in the 1960s, and more recent studies can be found [22]. This reaction takes place according to Eq. (5): 2H2S + 2e− → H2 + 2HS−
(5)
4.2. Effect of thiosulphate on oxygen-induced corrosion Low amounts of sulphite, well below the concentrations considered in this work, are normally used in industrial plants for oxygenscavenging purposes. Sulphites react with oxygen dissolved in the water, causing its removal. The oxidation-reduction reaction, which is fast even at ambient temperature (Eq. (6) leads to the formation of sulphates by the reaction:
2HSO3− + O2 → H+ + 2SO42 −
Fig. 15. Effect of thiosulphate and sulphite on the corrosion rate, as a function of the rotational speed of the electrode, measured by the polarization resistance technique.
characteristic curves for both anodic and cathodic process, and increase the kinetics, with a major effect on the cathodic reaction, both for potentials close to the free corrosion potential, and at much lower potentials [23]. They also modify the cathodic polarization curve, causing the occurrence of a limiting process at low potential values. Similar behaviour is observed in solutions containing sulphites (Fig. 10). In the presence of sulphites and thiosulphates, the increase in the kinetics of the cathodic process is more evident with greater water flow and with lower the pH. As a consequence, the corrosion rate of steel increases by an order of magnitude on moving from pH 7.2–5 (Fig. 10 and 6). Kappes et al. [29] reported that the corrosion of carbon steel involves the formation of sulphur and H2S according to the reactions Eq. (7) and Eq. (8) or, alternatively, Eq. (9) and Eq. (10).
(6)
In industrial plants, sulphite is added, as a sequestering agent, in proportion to the oxygen content of the water. The presence of thiosulphates affects the oxygen-scavenging reaction promoted by sulphites (Fig. 3). Ulrich et al. and Mo et al. studied their inhibiting effect [26,27]. Thiosulphate acts as a free radical sequestering agent involved in the reactions sequence in which the overall reaction is articulated. In particular, they have been studied as inhibitors of the formation of sulphates in sulphur abatement implants for combustion gases. The reaction rate depends upon the nature of the cation and the acidity [26,27]. The inhibiting effect depends on the sulphite/thiosulphate ratio; the addition of increasing amounts of sulphite over thiosulphate hinders the effect of the thiosulphates, and gradually increases the oxygen-scavenging rate. When the sulphites/ thiosulphates ratio is around 1 or 2, the rate of this sequestering reaction becomes negligible or extremely low (Fig. 5). Therefore, the action of thiosulphate increases the levels of oxygen, even in sulphite-containing solutions, and may promote substantial increases in the corrosion rate of carbon steel, in proportion to the residual oxygen content. In addition, the oxidation of sulphite to sulphate causes a decrease in pH (Fig. 7), according to the reaction Eq. (6). In fact, the second dissociation constant of sulphuric acid is greater than that of sulphurous acid. The HSO3− ion is not substantially dissociated in the pH range of the water considered in this research. Upon reaction, it is replaced by the sulphate ion, which is completely dissociated under these conditions. In the experiments using solutions without thiosulphates, the pH was observed to rapidly reach of 3.4 after all the sulphite (300 ppm) had reacted. In this manner, the acid attack of steel can occur. Thiosulphate ions tend to block the oxygen-scavenging reaction and hence, they stabilize the pH, slowing down the production of acidity. However, corrosive attack, supported by cathodic oxygen reduction can still occur. The pH tends to increase if oxygen entry is prevented during the active corrosion process (Fig. 7).
S2 O32 − + 6H+ + 4e− = 2S 0 + 3H2 O E 0 = 0.465 VSHE
(7)
S° + 2H+ + 2e− = H2S E0 = 0.142 VSHE
(8)
S2 O32 −
(9)
+
H+
=
S0
+
HSO3−
HSO3− + 7H+ + 6e− = H2 S + 3H2 O E 0 = 0.366 VSHE
(10)
At the free corrosion potential, the cathodic current mainly promotes the reduction of thiosulphate, in sulphite-free waters. The formation of H2S is catalysed by the metal surface and otherwise it does not occur in bulk solution. It leads to the formation of a black film of iron sulphide, mainly mackinawite [22,23,29–31]. The formation of scale, associated with high corrosion rates, is reported even in saline waters without thiosulphates, but containing an excess of sulphites, compared to the values usually added to control oxygen-induced corrosion [32–35]. This reaction (Eq. (10) therefore represents an important process in waters where sulphites reach relevant concentrations, far greater than those of thiosulphates. The aggressiveness of these waters may not be related only to the content of thiosulphate, but also to that of sulphite. The formation of hydrogen sulphide on the metal surface can stimulate the cathodic process that produces hydrogen through two different mechanisms. On the one hand, it can increase the availability of the hydrogen ion due to its dissociation on the metal surface, according to the reaction:
4.3. Effect of thiosulphate on general corrosion, in the absence of oxygen In de-aerated neutral solutions devoid of particularly aggressive species, the corrosion rate of carbon steel is practically negligible. The addition of sulphites and thiosulphates, at the levels considered in this research work, results in an increase in the corrosion rate of about one order of magnitude (Fig. 15). This attack is enhanced by the rotational speed of the electrode. At 1000 rpm, the corrosion rate is almost double compared to that observed under stagnant conditions (Fig. 11). Ezuber [23] and Kappes et al. [24] studied the corrosion of carbon steels in solutions containing only thiosulphates, at pH levels comparable or lower than the solutions considered in this research work. The results of these studies highlighted that the corrosion rate of carbon steel increases with thiosulphate concentration. These ions modify the
2H2S → H+ + HS−
(11)
On the other hand, hydrogen evolution can occur owing to the direct reduction of hydrogen sulphide [24,25]: 2H2S + 2e− → H2 + 2HS−
(12)
The mackinawite scale is conductive, poorly protective and it is a great cathode for hydrogen production, further stimulating the corrosion of the metal due to galvanic coupling [23,28,29]. The scale formed during 7
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Fig. 16. Morphology of the corrosion products (solutions containing 3000 ppm SO42−, 300 ppm SO32−, 150 ppm S2O32−) as a function of pH and exposure time.
under conditions that promote high corrosion rates [25]. The weight loss experiments (Fig. 7) substantially confirm this mechanism. The reactions depicted in Eq. (7), Eq. (8), Eq. (9) and Eq. (10) are favoured in acidic solutions and become more aggressiveness as the pH diminishes. In particular, the corrosion rate significantly increases on moving from pH 7.2 to 5, due to increases in the kinetics of the cathodic processes, and are associated with the more rapid growth of corrosion scale (Figs. 6, 16 and 18).
our laboratory experiments is fragile and multi-layered. Underneath the scale, the metallographic structure of the steel can be evidenced, with the formation of localized attack sites (Fig. 16) after long exposure times. The sulphur layer favours the formation of H2S on the metal surface according to the reaction [25]:
(x − 1) Fe + Sy − 1 S 2 − + 2H+ = (x − 1) FeS + H2 S + Sy − x
(13)
The stability diagram for various sulphur compounds confirms that the free corrosion potential of steel falls inside the stability region of H2S [22,36] (Fig. 17). The concentration of hydrogen sulphide on the surface of steel in acidic solutions containing thiosulphates has been evaluated by Kappes [29,30]. The author points out that the corrosion rate of carbon steel in solutions containing thiosulphates is higher than that found in the same solution devoid of these ions, but saturated with an H2S partial pressure equal to that estimated on the surface in the presence of thiosulphates. The reduction of thiosulphate, firstly to sulphur and then to hydrogen sulphide, and that of sulphite, both reduce the concentration hydrogen ions and tend to increase pH on the metal surface, especially
5. Conclusions The corrosion of carbon steel in neutral or weakly acid solutions containing sulphites and thiosulphates in significant concentrations occur with different mechanisms. Thiosulphates can inhibit the reactions between oxygen and sulphites and promote a substantial increase in the corrosion rate, in proportion to the residual oxygen content. On the other hand, they control the acidification resulting from the reactions of sulphites with oxygen, which may, in turn, induce acid corrosion. In anaerobic solutions, both sulphites and thiosulphates modify the characteristic anodic and cathodic potentiodynamic curves, with a
Fig. 17. Stability regions of the oxidation species of sulphur [37], and the range (in red) of the corrosion potential of steel, measured in solutions containing 3000 ppm SO42−, 300 ppm SO32−, 150 ppm S2O62−.
Fig. 18. Dependence of current density on pH and electrode rotational speed.
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marked effect on the cathodic process at potentials approaching the free corrosion potential, and under conditions of water flow. In the absence of oxygen, the corrosion process leads to the formation of thick scales of mackinawite. The iron sulphide forms as a result of the reduction of thiosulphate and sulphite, leading to the formation of hydrogen sulphide and the precipitation of the corrosion scale.
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Acknowledgment This research project was made possible by funding from the ITALY® PROJECT of the University of Bergamo. References [1] A. Baldacci, M. Mannari, F. Sansone, Greening of geothermal power: an innovative technology for abatement of hydrogen sulphide and mercury emission, Proceedings World Geothermal Congress, Antalya, Turchia, 2005. [2] M. Cabrini, S. Lorenzi, T. Pastore, Corrosion behavior of carbon steel in acidic solutions containing thiosulfate and sulfiteions, Metallurgia Italiana 106 (11–12) (2014) 3–10. [3] V. Duchi, A. Minissale, M. Manganelli, Chemical composition of natural deep and shallow hydrothermal fluids in the Larderello geothermal fields, J. Volcanol. Geotherm. Res. 49 (1992) 313–328. [4] F. Sabatelli, M. Mannari, R. Parri, Hydrogen sulfide and mercury abatement: development and successful operation of AMIS technology, GRC Trans. 33 (2009) 343–347. [5] D. Macdonald, Critical Issues in the Use of Metals and Alloys in Sulphur-Containing Aqueous Systems, (1993). [6] H. Horowitz, Chemical studies of polythionic acid stress corrosion cracking, Corros. Sci. 23 (1983) 353–362. [7] R.C. Newman, K. Sieradzki, H.S. Isaacs, Stress corrosion cracking of sensitized type 304 stainless steel in thiosulfate solutions, Metall. Trans. A 13 (1982) 2015–2026. [8] E.A. Abd El Meguid, N.A. Mahmoud, S.S. Abd El Rehim, The effect of some sulphur compounds on the pitting corrosion of type 304 SS Mater, Chem. Phys. 63 (2000) 63–74. [9] J.O. Park, M. Verhoff, R. Alkire, Microscopic behavior of single corrosion pits: the effect of thiosulfate on corrosion of stainless steel in NaCl, Electrochim. Acta 42 (1997) 3281–3291. [10] H.-S. Kuo, H. Chang, W.-T. Tsai, The corrosion behavior of AISI 310 stainless steel in thiosulfate ion containing saturated ammonium chrolide solution, Corros. Sci 41 (1999) 669–684. [11] T. Laitinen, Localized corrosion of stainless steel in chloride, sulfate and thiosulfate containing environments, Corros. Sci. 42 (2000) 421–441. [12] A.I. Almarshad, D. Jamal, Electrochemical investigations of pitting corrosion behaviour of type UNS S31603 stainless steel in thiosulfate-chloride environment, J. Appl. Electrochem. 34 (2004) 67–70. [13] W.T. Tsai, M.J. Sheu, J.T. Lee, The stress corrosion crack growth rate in sensitized alloy 600 in thiosulfate solution, Corros. Sci. 38 (1996) 33–45. [14] C. Duret-Thual, D. Costa, W. Yang, P. Marcus, The role of thiosulfates in the pitting corrosion of Fe-17Cr alloys in neutral chloride solution: electrochemical and XPS
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