Yuan Song, Attila Palencsár, Gaute Svenningsen, and Jon Kvarekvål Institute for Energy Technology (IFE) P.O. Box 40 NO-2027, Kjeller Norway

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1 Paper No Effect of O 2 and Temperature on Sour Corrosion Yuan Song, ttila Palencsár, Gaute Svenningsen, and Jon Kvarekvål Institute for Energy Technology (IFE) P.O. Box 40 NO-2027, Kjeller Norway Tor Hemmingsen University of Stavanger 4036 Stavanger Norway BSTRCT The effects of O 2 and, implicitly, of oxidized sulfur species on sour corrosion were studied in a series of two-stage glass cell experiments at different temperatures (25, 40, 60, 80 C). In stage-i, aqueous solutions were purged with a mixed gas of H 2 S and air (O 2 ) for 1 day. The oxidized sulfur specie SO 4 formed at all temperature conditions. The amounts of SO 4 increased with increasing temperature. SO 3 was only found at 25 C. S 2 O 3 was detected at 25, 40 and 60 C. In addition, considerable amounts of elemental sulfur were formed at 40, 60 and 80 C. In stage-ii, corrosion coupons were exposed in the brines resulting from the stage-i. The highest corrosion rates were observed at 60 C. The presence of reaction products of H 2 S and O 2 had a more significant effect with regard to localized corrosion at high temperatures. Small pits were observed at lower temperatures (25, 40 C) while at 60 and 80 C large pits and deep localized attacks occurred. With the simultaneous presence of O 2 and H 2 S, high corrosion rates generated severe localized attacks. In the cells with oxidized sulfur species present from stage-i, a consistent increase in the severity of localized attacks was observed in the subsequent exposure periods, i.e. N 2, H 2 S, mixed gas (H 2 S and O 2 ) exposure period, when compared to the blank. Key words: sour corrosion, oxidized sulfur species, O 2, localized corrosion, temperature 2011 by NCE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NCE International, Publications Division, 1440 South Creek Drive, Houston, Texas The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the ssociation. 1

2 INTRODUCTION Ingress of O 2 in sour environments has a negative impact, being often associated with localized corrosion and further complicates prediction of sour corrosion which is a difficult task in itself. 1-3 The general mechanisms of corrosion in sour environment have been well studied, but the effect of O 2 in this context is not fully understood. 4 The presence of O 2 could affect sour corrosion in many ways, such as formation of various compounds containing sulfur in different oxidation states (oxidized sulfur species), oxygen reduction as an additional cathodic reaction, reaction with corrosion inhibitors etc. In sour environment, dissolution of H 2 S yields weakly dissociated acid. With presence of O 2, dissolved H 2 S can be further oxidized to elemental sulfur and different sulfur containing species. Various reaction pathways may yield strong acid such as H 2 SO 4, which enhances the corrosion rate by accelerating the cathodic reaction kinetics. 6-9 However the mechanisms by which oxidized sulfur species influence the corrosion is not fully understood. The presence of elemental sulfur in sour environment has been reported to be related to localized corrosion. 1-4 One of the proposed mechanisms is that the activation energy barrier for a metal atom from the surface to the solution is lowered by absorbing elemental sulfur on metal surface which wakens the metal-metal bonds. If elemental sulfur is not homogenously distributed over the surface but adsorbed in specific sites, localized corrosion occurs. 10 This may, however, not be the only mechanism involved in localized corrosion. This paper reports the results of the work aimed at studying how O 2 and oxidized sulfur species affect corrosion in sour environments in an effort to gain better understanding of localized corrosion mechanisms when O 2 and H 2 S are present. Experimental setup EXPERIMENTL PROCEDURE The experiments were performed in two 3 liter glass cells with airtight stainless steel lids. The ports for specimen holders and tubing were sealed with Swagelok feed-troughs on the lids. The setup permits withdrawal of specimens after various exposure times and continuous gas purging during the experiment. Materials and methods The specimens were made of X65 carbon steel. Two kinds of specimens were used in the experiments: rectangular coupons ( mm) for corrosion measurements and stationary cylinders (diameter 10 mm, height 10 mm) for electrochemical measurements. The initial test solution was 1 g/l NaCl (technical grade) in distilled water. The salt was added to provide adequate conductivity for the electrochemical measurements. Gases were of the following quality: H 2 S (purity: 99.8%), N 2 (purity: 99.9%). ir was used as a source of O 2. H 2 S and air was mixed at a volumetric ratio of 1:1 yielding a ratio of H 2 S to O 2 of ca. 5:1 (assuming 20.9 mol% O 2 in the air). The glass cell tests were performed at ambient pressure. The composition of the gas phase was the gas being bubbled and water vapor corresponding to the working temperature. 11 The experiments were performed in two parallel glass cells in two consecutive stages. During stage-i the test solution in one cell (cell ) was purged with H 2 S and air (O 2 ) while the other (cell B) was purged only with H 2 S to serve as a blank. The purpose of stage-i was to identify possible chemical reactions and products involving the oxidation of H 2 S on one hand and to produce test brines containing acidic oxidized sulfur species for subsequent corrosion experiments in stage-ii on the other hand. Stage-I was carried out as follows (Table 1): 2

3 1. Continuous N 2 purging for 1 day in both glass cells to remove dissolved O 2 in the solution. 2. Cell : continuous purging mixed gas H 2 S and air (O 2 ) for 1 day. Cell B: continuous purging H 2 S for 1 day. 3. Continuous purging N 2 in both glass cells to remove the dissolved H 2 S in the solution. 5 The experimental procedures for stage-ii were as follows (Table 2): 1. Steel specimens inserted into both glass cells immediately after stage-i. The same cells and test solutions that resulted from stage-i were used. 2. N 2 purged for 5 days. One specimen removed from each cell for corrosion assessment. 3. H 2 S purged for 5 days. One specimen removed from each cell for corrosion assessment. 4. Mixed gas H 2 S and air (O 2 ) purged for 5 days. Remaining specimens removed for corrosion measurements. The above periods will be referred to hereafter as N 2 -exposure, H 2 S-exposure and mixed gas of H 2 S and O 2 exposure period. It should be noted that all specimens were introduced at the beginning of stage-ii, which implies that specimens withdrawn at the end of a certain exposure period had been exposed in all previous periods. Removed specimens were not replaced with fresh ones. ll the experiments (both stage-i and stage-ii) were performed at 25, 40, 60, 80 C. The ph of the solutions was measured using probes calibrated at the respective temperatures before starting the experiments. The temperature was measured by temperature sensors inserted into the glass cells. ll the experiments were performed with magnetic stirring. t the end of stage-i, liquid samples were taken out for analysis by ion chromatography (IC). During stage-ii, steel specimens were taken out at the end of each exposure period. fter removal, the specimens were immediately submerged in isopropanol and dried in a hot oven (~70 C). Films were studied by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). To determine the mass loss, specimens were weighed before and after exposure and again after the surface film had been chemically removed (stripped). Surface morphology of the samples after stripping was characterized using optical microscope and 3D surface profilometer. The corrosion rate was monitored by linear polarization resistance (LPR) in a three electrode configuration, using a Ti coil as auxiliary electrode and a g/gcl electrode as a reference. n empirical B-value of 20 mv was used for evaluating LPR corrosion rates. ll corrosion rates are reported as mm/year. Physico-chemical observations RESULTS ND DISCUSSION nalysis of liquid samples taken at the end of stage-i identified small amounts of oxidized sulfur species formed in cell at all temperatures. s shown in Figure 1, at 25 C sulfate (SO 4 ), sulfite (SO 3 ) and thiosulfate (S 2 O 3 ) were detected. Sulfite was not found at any of the higher temperatures, indicating its increasing reactivity to oxygen. The concentrations of both sulfate and thiosulfate showed an increasing trend with temperature, albeit at 80 C thiosulfate also appears to be consumed quickly, as sulfate was the only oxidized sulfur species present. While only anions were detected by the IC analysis, the findings indicate that the oxidation of H 2 S yielded strong acids (H 2 SO 4, H 2 SO 3 and H 2 S 2 O 3 ). 3

4 Elemental sulfur formed in observable quantities at all temperatures except 25 C. The amount of elemental sulfur visibly increased as temperature increased. None of the species found in cell was present in the blank (cell B). The typical evolution of the solution ph is shown in the example in Figure 2 for 40 C. t the outset of stage-i, the ph values were close to neutral in both cells during N 2 purging. Dissolution of H 2 S followed by dissociation (weak acid) rapidly yielded ph values around 4, consistent with the theoretical ph values (4.0 and 4.1 respectively for 1.0 and 0.5 bar H 2 S at 40 C). The ph remained constant in cell B throughout the period of H 2 S purging, indicating no further reactions. The generation of strong acids by oxidation of H 2 S was, however, evidenced by the steady decrease in ph observed in the presence of O 2 in cell to values of ca Subsequent purging with N 2 gas again resulted in the gradual increase of the ph in cell B back to neutral as H 2 S was removed from the solution. The ph remained low in cell, confirming the presence of strong acids that were not removed by N 2 purging. The ph curves at other temperatures showed a similar trend. t the end of stage-i the ph values were 3.6 (25 C), 3.1 (60 C) and 2.8 (80 C) respectively in cell indicating lower values at higher temperatures. This is in agreement with the results obtained by IC (Figure 1) that indicate a higher concentration of acidic oxidized sulfur species with increasing temperature. The ph in cell B was approximately neutral at the end of stage-i at all temperatures. The acidic species present at the end of stage-i in cell were consumed rapidly upon insertion of steel coupons at the start of stage-ii, as shown by the ph increasing to near neutral levels (Figure 2). The ph in cell B was essentially unchanged upon insertion of the corrosion coupons. slight increase of ph was observed in both cells during the period of N 2 -purging. The ph dropped to stable values of approximately 4 in both cells during the subsequent period of H 2 S purging. During the final exposure period (H 2 S and O 2 purging), the ph in both cells decreased (to 3.5 in cell B and 3 in cell ) in a manner similar to that observed in cell in stage-i as a result of generating strong acidic species. The corrosion rates measured by LPR under various exposure conditions during stage-ii are shown in Figure 3. The average mass loss corrosion rates evaluated for coupons withdrawn at the end of each exposure period, presented in Figure 4, are in agreement with the LPR data. The corrosion rates generally show an increasing trend with temperature under similar conditions. The composition of the corrosion films as determined by XRD are presented in Table 3. Table 4, figures 6, 7 and 8 show the surface morphology of specimens after chemical removal (stripping) of the surface films. Corrosion while purging N 2 It is notable that the sulfur species formed in stage-i (elemental sulfur, acidic oxidized species) had a significant effect on the corrosion rate in N 2 exposure period (Figure 3 and Figure 4) from the beginning of stage-ii, shown as corrosion rates higher by ca. an order of magnitude or more in cell compared to the blank cell B at all temperatures. The corrosion rates increased with temperature, which is consistent with faster kinetics as well as increased formation of acidic oxidized sulfur species (Figure 1). The difference between cells and B becomes even more evident in view of the appearance of the specimen surfaces: the presence of oxidized species caused film formation (cell ) during N 2 exposure, while no films formed in the absence of those species (cell B). The thickness of the films increased with temperature. XRD analysis (see Table 3) shows that films formed in the N 2 exposure period on specimens in cell contained pyrite (FeS 2 ) and traces of hematite (Fe 2 O 3 ). 4

5 During the period with N 2 purging the corrosion was uniform in both cells at the lower temperatures (25, 40 C). t high temperatures (60, 80 C) were found on the samples from cell with preexisting acidic oxidized sulfur species. Corrosion while purging H 2 S The corrosion rates increased rapidly after H 2 S was introduced to the system (Figure 3). lthough slightly varying with temperature, the general trend was that the solution with pre-existing acidic sulfur species displayed higher corrosion rates. Corrosion films formed in both cells and B as expected for the H 2 S exposure conditions. It is interesting to note that the film thickness in cell was about twice that in cell B. In general the films developed during this period appear to confer little protectiveness, evidenced by the relatively limited decrease in corrosion rates. s shown in Table 3, films that developed during the H 2 S exposure period contained mackinawite (FeS), pyrite (FeS 2 ) in both cells. Traces of hematite (Fe 2 O 3 ) were only found on the samples from cell. During the period of H 2 S purging, the severity of localized attacks appears to increase in the cells containing acidic oxidized sulfur species (Table 4 and Figure 7), particularly at high temperatures (60 and 80 C). In this period, deep and wide localized attacks were observed on the samples from cell at high temperature (60, 80 C), while for the samples from cell B there were island shaped areas on the sample at 60 C and on the samples at 80 C (Figure 7). Corrosion while purging mixed gas of H 2 S + air (O 2 ) The corrosion rates increased again as mixed gas of H 2 S and air (O 2 ) was purged to both cells. (Figure 3 and Figure 4). Several factors could in principle contribute: corrosion by elemental sulfur, oxygen reduction as an additional cathodic reaction or increased corrosion due to formation of strong acids. The films observed after the mixed gas of H 2 S and air (O 2 ) exposure were thicker than in the previous H 2 S exposure period. Figure 5 shows the cross section FE-SEM images of samples from cell, after the period of purging H 2 S and air (O 2 ) at different temperatures. Films at higher temperatures (60 and 80 C) were thicker and appear denser than at lower temperatures. The protectiveness is rather poor in all cases, allowing corrosion rates of several mm/yr even though it is usually expected that thicker and tenacious iron sulfide films yield better corrosion protection. Despite the differences found after the previous two exposure periods (N 2 exposure and H 2 S exposure), the film composition at the end of the mixed gas of H 2 S and air (O 2 ) exposure period was similar for specimens from cell and cell B, which contained both iron sulfides and iron oxides (Table 3). Even though the corrosion rates in cell and B were essentially similar in mixed gas of H 2 S and air (O 2 ) exposure period (Figure 3), the corrosion morphology in the two cells was very different (Table 4). The corrosion attacks in cell were more severe and appeared more localized (Table 4), especially at the high temperatures. In mixed gas of H 2 S and air (O 2 ) exposure period, since both cells were continuously supplied with equal amounts of O 2 and H 2 S, the formation of elemental sulfur and oxidized sulfur species (acids) should be equivalent. The different corrosion morphology in two cells may therefore be caused by pre-existence of acidic oxidized sulfur species in cell, possibly related to surface films formed in cell during the N 2 exposure period. fter mixed gas of H 2 S and air (O 2 ) exposure, large pits were observed on the samples from cell at low temperatures (25, 40 C), while at high temperatures (60, 80 C) the surfaces of samples from both cells (Figure 8(c) and (d)) were extremely uneven and rough, practically obscuring individual pits. The localized attacks were deeper and wider as temperature increased (Figure 8). 5

6 CONCLUSIONS cidic oxidized sulfur species formed in aqueous solution purged with H 2 S and air (O 2 ) at temperatures ranging from 25 to 80 C. SO 4 was present at all temperatures. The amount of SO 4 increased with increasing temperature. SO 3 was only found at 25 C. S 2 O 3 was only present at 25, 40 and 60 C. In addition, elemental sulfur formed quickly in observable quantities at the higher temperatures (40, 60, and 80 C), indicating sluggish kinetics of this reaction around room temperature. Pre-existing reaction products of H 2 S and air (O 2 ), consistently increased the severity of attacks compared to the blanks, under otherwise similar exposure conditions. The present work indicates that elemental sulfur is likely not the only species responsible for the development of localized attacks under conditions of O 2 ingress in sour corrosion. One or more of the reaction products between H 2 S and O 2, including SO 4, SO 3, S 2 O 3, H + had effects on localized corrosion as well. These effects appear to be independent of pitting caused by O 2. The results show that elemental sulfur may not necessarily be an intermediate in the formation of sulfur species of higher oxidation state. Since changes in the environment marked by the presence of acidic oxidized sulfur species have significant effects, these findings warrant further investigation into the mechanistic pathways of their formations, consumptions as well as their roles in localized corrosion. CKNOWLEDGEMENTS The authors would like to express their gratitude to all the technicians involved in this project and also the Norwegian Research Council for funding this project. 6

7 REFERENCES 1. R.L. Martin, Corrosion Consequences of Oxygen Entry into Oilfield Brines, CORROSION/2002, paper no NCE, S. Kapusta, Managing Corrosion in Sour Gas System: Testing, Design, Implementation and Field Experience, CORROSION/2008, paper no NCE, N.G. Park, L. Morello, J.E. Wong and S.. Maksoud, The Effect of Oxygenated Methanol on Corrosion of Carbon Steel in Sour Wet Gas Environments, CORROSION/2007, paper no NCE, G. Schmitt, Present Day Knowledge of The Effect of Elemental Sulfur on Corrosion in Sour Gas System, CORROSION/1990, paper no NCE, Palencsár, Y. Song, Effect of Oxygen on queous Sour Corrosion System, Eurrocorr, Nice, France, 6-10 sep J.H. Karchmer, The nalytical Chemistry of Sulfur and its Compounds, Wiley-Interscience, New York G. Nicklessa, Inorganic Sulfur Chemistry, Elsevier, msterdam (1968): p T. Hemmingsen, The Electrochemical Reaction of Sulfur-Oxygen Compounds Part I. Review of Literature on The Electrochemical Properties of Sulfur/Sulfur-Oxygen Compounds, Electrochemica cta (1992). Vol. 37, No. 15, p G. Cragnolino, O.H. Tuovinen, The Role of Sulfate Reducing and Sulfur Oxidizing Bacteria in The Localized Corrosion of Iron-Base lloys Review, Int.Biodeterioration 20 (1984): p P. Marcus, dvances in Localized Corrosion, NCE, 1990: p R.C. Weast, CRC Handbook of Chemistry and Physics, 66 th ed. (Boca Raton, Florida: CRC Press, 1985), p. D

8 Table 1 Experimental matrix of stage-i Purge gas Glass cell 1 day 1 day 1-2 days N 2 H 2 S + O 2 volumetric ratio 5:1 N 2 B N 2 H 2 S N 2 Table 2 Experimental matrix of stage-ii Purge gas Glass cell Test solution 1 st 5 th day 6 th 10 th day 11 th 15 th day NaCl (1 g/l) Oxidized sulfur species (formed during stage-i ) N 2 H 2 S H 2 S + O 2 volumetric ratio 5:1 B NaCl (1 g/l) N 2 H 2 S H 2 S + O 2 volumetric ratio 5:1 Table 3 Film compositions of samples exposure at 40 C Exposure conditions Composition N 2 H 2 S H 2 S+O 2 B B B Troilite (FeS) X X X Mackinawite (FeS) X X X X Pyrite anisotropic (FeS 2 ) X X X X X Greigite (Fe 3 S 4 ) X X Hematite (Fe 2 O 3 ) trace trace X X Magnetite (Fe 3 O 4 ) X X 8

9 Table 4 Surface morphology of samples after stripping Temperature 25 C 40 C Cell test solution formed at Stage-I NaCl, SO 4, SO 3, S 2 O 3 H + N 2 1 st - 5 th day general corrosion Exposure condition at Stage-II H 2 S 6 th -10 th day ( d max = 27 μm) B NaCl general corrosion general corrosion NaCl, SO 4, S 2 O 3 elemental sulfur H + general corrosion B NaCl general corrosion ( d max = 53 μm) ( d max = 17 μm) H 2 S + O 2 11 th - 15 th day large pits ( d max = 63 μm) (see Fig. 8(a)) ( d max = 30 μm) large pits ( d max = 157 μm) (see Fig. 8(b)) ( d max = 34 μm) 60 C NaCl, SO 4, S 2 O 3 elemental sulfur H + ( d max = 18 μm) B NaCl general corrosion NaCl SO 4 elemental sulfur H + ( d max = 20 μm) deep, wide localized attacks ( d max = 35 μm) Fig. 7(a) island shaped area Fig. 7(b) deep localized attacks ( d max = 139 μm) Fig. 7(c) extremely uneven rough surface ( d max = 400 μm) Fig. 8(c) uneven rough surface extremely uneven rough surface Fig. 8(d) 80 C B NaCl general corrosion ( d max = 35 μm) Fig. 7(d) island shaped area large pits ( d max = 140 μm) 9

10 Figure 1: Oxidized sulfur species in cell at the end of stage-i. Figure 2: Trend of ph values during the experiment at 40 C. 10

11 100 Corrosion rate / (mm/y) N 2 H 2 S H 2 S + ir (O 2 ) Cell Cell B Time / day (a) at 25 C Corrosion rate / (mm/y) Corrosion rate / (mm/y) Corrosion rate / (mm/y) N 2 H 2 S H 2 S + ir (O 2 ) Cell Cell B Time / day (b) at 40 C N 2 H 2 S H 2 S + ir ( O 2 ) Cell Cell B Time / day (c) at 60 C N 2 H 2 S H 2 S + ir (O 2 ) Cell Cell B Time / day (d) at 80 C Figure 3: Corrosion rates measured by LPR. 11

Cell Cell B Figure 4: verage corrosion rates

cell exposed throughout the experiments,

12 Cell Cell B Figure 4: verage corrosion rates evaluated by mass loss. (a) 25 C (b) 40 C (c) 60 C (d) 80 C Figure 5: Cross section FE-SEM images of samples from cell exposed throughout the experiments, including consecutively in N 2, H 2 S and the mixed gas (H 2 S + O 2 ) environment. 12

13 25 C 40 C 60 C 80 C B (a) Exposed in N 2 purging cell from 1 st -5 th day. B (b) Exposed in H 2 S purging cell from 6 th -10 th day. B (c) Exposed in H 2 S + O 2 purging cell from 11 th -15 th day. Figure 6: Images of stripped samples. 13

14 mm (a) 60 C Cell mm (b) 60 C Cell B mm (c) 80 C Cell mm (d) 80 C Cell B Figure 7: Surface morphology images of samples after H 2 S exposure period. Height differences are represented by colors where red is highest (top) and blue/black is the lowest (bottom). 14

15 mm (a) 25 C mm (b) 40 C mm (c) 60 C mm (d) 80 C Figure 8: Surface morphology images of samples from cell after mixed gas (H 2 S + O 2 ) exposure period. Height differences are represented by colors where red is highest (top) and blue/black is the lowest (bottom). 15

Effect of Iron Sulfide Deposits on Sour Corrosion of Carbon Steel

Paper No. 7313 Effect of Iron Sulfide Deposits on Sour Corrosion of Carbon Steel Jon Kvarekvål, Gaute Svenningsen Institute for Energy Technology (IFE) Instituttveien 18 2027 Kjeller Norway ABSTRACT The