Effect of Iron Sulfide Deposits on Sour Corrosion of Carbon Steel

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1 Paper No Effect of Iron Sulfide Deposits on Sour Corrosion of Carbon Steel Jon Kvarekvål, Gaute Svenningsen Institute for Energy Technology (IFE) Instituttveien Kjeller Norway ABSTRACT The corrosive effects of iron monosulfide particles deposited on carbon steel surfaces were investigated in a series of autoclave experiments. The H 2 S and CO 2 partial pressures were varied in the range of 1-20 bar. Experiments were run at temperatures in the range of o C. The test solutions consisted of high-salinity brine (100 g/l NaCl, m bicarbonate) and low-salinity condensed water (0.1 g/l NaCl). The duration of the tests was typically 14 days. Both weight loss corrosion and localized corrosion data were obtained. The entire surfaces of the exposed coupons were scanned with a 3D profilometer, obtaining detailed data on localized corrosion morphology, pit frequency and pit depths. The presence of FeS deposits was found to significantly increase both weight loss and local penetration rates in experiments with high H 2 S/CO 2 partial pressures (20-25 bar total). The results are discussed on the background of available literature. Keywords: Sour corrosion, carbon steel, pitting, localized corrosion INTRODUCTION A significant part of the remaining natural gas reserves worldwide are sour, i.e. H 2 S-containing. Many oil and gas fields, especially in North America, Central Asia and the Middle East, are highly sour with an H 2 S fraction of several percent. Internal corrosion in sour environments is associated with a high risk of severe localized corrosion attacks. This has consequences for design and materials selection for sour oil and gas production facilities, where special requirements apply for use of carbon steels vs. CRAs as well as corrosion inhibition of carbon steels. However, the presence of H 2 S can also be an advantage for corrosion protection compared to purely sweet systems. Formation of more or less protective corrosion product layers will take place instantly on carbon steels under most conditions, which in some 1

2 cases has removed the necessity of further anti-corrosion treatment. Being able to identify these conditions will provide an economical benefit from the aspects of material selection and corrosion control. This paper contains results from an under-deposit corrosion (UDC) study carried out as part of a Joint Industry Project (JIP) on sour corrosion of carbon steel. Some results from the project have previously been published in , providing insight on the effect of time and temperature on corrosion with high H 2 S partial pressures in the range of 5-20 bar. Under-deposit corrosion (UDC) with iron sulfide deposits has been identified by the industry as a particularly problematic scenario, as the FeS deposit can increase corrosion rates by limiting inhibitor availability at the steel surface and act as a high area cathode. A number of papers have addressed this issue. In the review paper on iron sulfides by Smith & Miller 2 (1975) the harmful effect of suspended and deposited iron sulfides is described. Deposited FeS particles caused more corrosion than suspensions. The various types of iron sulfide were reported to have different corrosivity, with pyrite being the most aggressive, followed by smythite and greigite. In a paper by Achour et al. 3 pitting was reported to occur in a long-term sour corrosion inhibitor experiment as a result of heterogeneous iron sulfide coverage on the steel surface. Wang et al. 4 reported on failures of sour water injection pipelines without chemical treatment, and suggested iron sulfide deposits as a possible cause for localized corrosion attacks. Extreme local penetration rates, up to 350 mm/y, were recently reported by Moore et al. 5 in short-term under-deposit corrosion inhibitor tests with mixed deposits of sand and elemental sulfur. The corrosion rate increased with the fraction of sulfur in the mixture. The corrosion inhibitors tested were not effective against the under-deposit attack, but the use of sulfur dispersants was found to reduce the amount of sulfur in contact with the metal, thereby reducing the corrosion rate. A study of the effect under-deposit corrosion with iron sulfide particles was published in a recent paper by Menendez et al. 6 Three different types of iron sulfide were employed as deposits to conduct the corrosion experiments, including mackinawite synthesized under anaerobic conditions and commercially acquired FeS powders. While it was found that freshly precipitated iron sulfides lead to severely increased corrosion rates and pitting, the effect of the commercial FeS powders was much less. This was ascribed to the presence of nm thick oxidized layers (identified by XPS) on the commercial FeS particles, which can delay UDC during a short term sour corrosion test. EXPERIMENTAL UDC experiments with simulated formation and condensed water conditions have been carried out in Hastelloy autoclaves. The total volume of each autoclave was 2 litres, and a filling level of 75% was used (1.5 litres of test solution). The test conditions and results are given in Table 1. Two types of water chemistry were used: synthetic formation water (FW) with 100 g/l NaCl and 0.15 g/l HCO 3 -, and synthetic condensed water with 0.1 g/l NaCl (assuming mixing with small amounts of formation water due to mist/droplets) and no alkalinity. The initial ph for the various conditions was either measured or calculated using available software. The test coupons were made of UNS K03014 carbon steel with dimensions 24 mm x 3 mm. The exposed surface area was 11.3 cm 2. All test coupons were mounted horizontally in PTFE holders. 2-3 coupons, of which one was a weight loss corrosion coupon, were exposed in each experiment. The ratio between the test solution volume and exposed surface area was typically ml/cm 2. The autoclave and controller setup is shown in Figure 1. The target H 2 S and CO 2 partial pressures were achieved by loading and stabilizing with H 2 S, and then adding CO 2. Both regular (bare steel) and UDC coupons were exposed in the experiments. The UDC coupons were covered with FeS powder (a commercially available troilite/pyrrhotite mixture, 99.9%), sand, or a FeS/sand mixture deposited over the entire surface. Before the experiments were started, autoclave Trade Name 2

3 loading trials indicated that the deposited powder would stay in the holder during liquid and gas loading. This was verified when the tests were finished, and is also reflected in the results. After exposure the coupons were immersed in isopropanol and dried. Coupons destined for corrosion film analysis with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were stored in a dry N 2 atmosphere. Coupons for weight loss and surface profilometry were stripped by ultrasonic cleaning in Multitreat 5537 sulfide solvent. Stripped coupons were scanned with a Nanovea ST400 optical pen profilometer, providing surface profiles for the entire exposed surface area of each coupon. An optical pen with a Z-range of 2 mm and resolution of 0.3 µm was used for most of the coupons. The raw profiles were post-processed with Mountain Professional 3D software to obtain pit counts, maximum pit depths and distribution of surface area on different depths. Figure 1: Autoclaves installed in vented cabinet. RESULTS Effect of Temperature on FeS under-deposit corrosion Average weight loss corrosion rates and maximum local penetration rates are plotted against temperature for regular coupons and coupons with iron sulfide deposits (1 g/cm 2 ) in Figures 2-3. Figure 2 contains the results for formation water, with a temperature range of o C, while Figure 3 covers the condensed water conditions with temperatures o C. With formation water (Figure 2) the trend is largely the same for average and maximum penetration rates, but with a marked difference between regular and FeS-deposit coupons. For both coupon types there is a marked increase in avg. and maximum penetration rates between 10 and 25 o C. However, on regular coupons the corrosion rates in the temperature range o C are significantly lower than at 25 o C, while on the FeS-deposit coupons both avg. and maximum penetration rates are almost constant in the range o C at 6-8 mm/y and ca Trade Name 3

4 20 mm/y respectively. For both coupon types a decrease in both avg. and maximum penetration rates is observed at o C, followed by a sharp increase when going to 120 o C. Less data on the effect of temperature on corrosion have been generated for condensed water (Figure 3), but it appears that corrosion on the FeS-deposit coupons is less sensitive to temperature in the range studied, while a minimum at 90 o C is observed for both avg. and maximum penetration rates on regular coupons. The most interesting part of this trend is the situation at 120 o C, where the regular and FeS-covered coupons have very similar corrosion rates. This was in fact the only condition where the presence of FeS did not lead to a marked increase in corrosion. Figure 2: Average corrosion rates (upper chart) and max corrosion rates (lower chart) plotted against temperature for regular and FeS-covered (1 g/cm 2 ) coupons. Formation water (100 g/l NaCl, m bicarbonate), 10 bar H 2 S, 10 bar CO 2. 4

5 10 Avg. corrosion rate (mm/y) 1 Reg CW FeS CW Temperature (C) 10 Max. corrosion rate (mm/y) 1 Reg CW FeS CW Temperature (C) Figure 3: Average corrosion rates (upper chart) and max. local penetration rates (lower chart) plotted against temperature for regular and FeS-covered (1 g/cm 2 ) coupons. Condensed water (0.1 g/l NaCl), 10 bar H 2 S, 10 bar CO 2. 5

6 Effect of different FeS loads and mixed solid deposits Figure 4 shows the average and maximum local penetration rates plotted against FeS load (in grams/cm 2 ) for FeS-deposit coupons exposed in exps. A01-A08. FeS deposit loads of 0.03, 03 and 1 g/cm 2 were tested, corresponding to a deposit thickness of 0.15, 1.5 and 5 mm and total FeS amount per coupons of 0.2, 2 and 7 grams respectively. While a FeS load of 1 g/cm 2 gave increased average and local penetration rates in all tests, the results are not so clear for the lower FeS loads in the range g/cm 2. E.g., for the experiments at 90 o C the average corrosion rates increase with the FeS load in both condensed and formation waters, while the corresponding max. local penetration rates exhibit a very different behaviour. In the cases with more than 0.3g/cm 2 FeS formation water give higher corrosion rates than condensed water at 25 C, but not at 90 C. The iron sulfide used for deposits was a commercial product, while iron sulfide particles accumulating in pipelines will mostly be products of corrosion. An experiment was therefore carried out to compare the commercial FeS with FeS produced by H 2 S corrosion of pipeline steel, with conditions being 10 bar H 2 S, 10 bar CO 2, 100 g/l NaCl, m bicarbonate and 25 o C. The FeS load was 0.6 g/cm 2. The FeS corrosion product used was synthesized under the same conditions, by corrosion of pipeline steel shavings obtained from machining of test coupons. Figure 5 shows the effect of the different FeS qualities. Both average weight loss and maximum local penetration rates were higher with the FeS corrosion products than with the commercial FeS powder. In order to study how aggressive iron sulfide is when mixed with other solids, iron sulfide powder was mixed with sand at different ratios, including a sample with sand only (no iron sulfide). The mixtures were applied to a thickness of 5 mm (corresponding to 1 g/cm 2 pure FeS powder). Figure 6 shows the average and maximum penetration rates as function of the percentage/fraction of iron sulfide in the deposit. 50 % FeS in the deposit gave avg. and maximum penetration rates almost as high as pure FeS, while there is no clear trend at lower FeS percentages. At 16.7 % FeS (5:1 sand: FeS ratio) the attack was uniform with a corrosion rate of ca 0.6 mm/y. Maximum penetration rates of 3-4 mm/y was observed on the coupons with 4.8 % FeS and sand only. Despite the lack of trend, it is clear that deposits of iron sulfide mixed with other deposits may increase both weight loss and localized corrosion rates. Figure 7 shows SEM images of the spatial 2D distribution of sand and FeS. The distribution of FeS (sulfur) determined by EDS mapping is also shown. It appears that a continuous network of conductive FeS, which would be required for maximum cathodic activity on the deposit, is only achieved with a 1:1 FeS/sand mixture. 6

7 Figure 4: Average corrosion rates (upper chart) and max. local penetration rates (lower chart) plotted against FeS load on UDC coupons. FW = formation water (100 g/l NaCl, m added alkalinity), CW = condensed water (0.1 g/l NaCl). 25 o C, 10 bar H 2 S, 10 bar CO 2. 7

8 Figure 5: Comparison of average weight loss and maximum local penetration rates on FeSdeposit coupons with commercial FeS powder and FeS particles produced by corrosion. 10 bar H 2 S, 10 bar CO 2, 100 g/l NaCl, m added alkalinity, 25 o C, FeS load 0.6 g/cm 2. Figure 6: Average corrosion rates and maximum penetration rates plotted against percentage of FeS in FeS/sand mixture (balanced by sand). Formation water (100 g/l NaCl, m added alkalinity), 25 o C, 10 bar H 2 S, 10 bar CO 2. 8

9 Sand only Sand: FeS 20:1 Sand: FeS 5:1 Figure 7: SEM images and EDS mapping of sulfur (indicating FeS) for different mixtures of sand and FeS 9

10 Corrosion attack morphology with iron sulfide deposits The presence of iron sulfide deposits substantially increased both average and local penetration rates under almost all conditions tested. Localized attacks took place in shape of pits, large/wide local attacks and level differences with magnitude of several hundred µm. The effect of the FeS deposit on the corrosion morphology is illustrated in Figure 8, showing 3D profilograms of coupons exposed at identical conditions (100 g/l NaCl, m added alkalinity, 60 o C, 10 bar H 2 S, 10 bar CO 2 ). While the regular non-deposit coupon has a very uniform corrosion attack with a surface roughness of a few µm, the FeS-deposit coupon had deep localized attacks with depths up to 750 µm (corresponding to more than 20 mm/y local penetration rate. Figure 8: Regular coupon (left) and FeS-deposit coupon (right, 1 g FeS/cm 2 ). Exposure time 2 weeks, 100 g/l NaCl, m added alkalinity, 60 o C, 10 bar H 2 S, 10 bar CO 2. Structure and Composition of the Iron Sulfide Layers Cross-sectioned coupons with corrosion product layers were examined with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The corrosion layer structures on the FeS-deposit coupons were as expected more complex than on regular coupons, showing the remnants of the pre-applied FeS powder combined with precipitated FeS layers formed during the corrosion process. Figure 9 shows a FeS layer structure consisting of corrosion film and remaining iron sulfide powder on the FeS-deposit coupon exposed in synthetic formation water (100 g/l NaCl, m added alkalinity) at 25 o C, 5 bar H 2 S and 20 bar CO 2. The different components of the layer are indicated in the figure. Starting from the top of the layer, remainders of the applied FeS powder can be seen. Further towards the steel surface there are some lumps of iron sulfide that probably consist of FeS powder fused with precipitated FeS corrosion products. About 400 µm from the steel surface a border indicating the original surface level before exposure can be identified. Below this level the native corrosion film can be seen. This part of the layer has about the same thickness as the film on the regular specimen, but appears to be much less protective due to a high concentration of pores, cracks etc. On the innermost part of the film it is possible to distinguish between two phases of film material, as shown in Figure 10. EDS spot analysis indicates that FeCO 3 is present in addition to FeS. This observation is interesting as it indicates local depletion of H 2 S that, probably in combination with a locally high ph, shifted the thermodynamic conditions in favour of FeCO 3 formation. Locally high chloride content was also identified in this part of the film. 10

11 SEM images for regular and FeS-deposit coupons exposed condensed water also had a dense inner layer with a porous layer on top, but the thickness of each layer was much higher (around 200 µm). On the FeS-deposit coupons exposed in 0.1 g/l NaCl solution at 90 o C, 20 bar H 2 S and 5 bar CO 2 (see Figure 11) native corrosion film could be identified between the remaining FeS powder and the metal surface. The native corrosion films appeared to adhere very strongly to the metal surface, and the corrosion rates were lower than with corresponding conditions in formation water. Applied FeS powder Applied FeS powder fused with corrosion products Original surface level Native corrosion film Figure 9: SEM image of an FeS-deposit coupon with corrosion film (cross-section). Exp. A22, formation water, 25 o C, 5 bar H 2 S, 20 bar CO 2.. C-K O-K Al-K Si-K S-K Cl-K Fe-K Ni-K A22_4(2)_pt A22_4(2)_pt Figure 10 SEM image and EDS analysis of corrosion film close to the steel surface on an FeS-deposit coupon (cross-section), indicating the presence of both FeS and FeCO g/l NaCl, m added alkalinity, 25 o C, 5 bar H 2 S, 20 bar CO 2. NACE International, Publications Division, FeS Park Ten Place, Houston, Texas FeCO 3?

12 1.47 mm/y 0.85 mm/y Figure 11 SEM images of corrosion layers on FeS-deposit coupons exposed in condensed water (0.1 g/l NaCl). 90 o C, 20 bar H 2 S DISCUSSION A number of sources in literature have described the corrosive effect of iron sulfide particles and deposits in contact with the steel surface. Nevertheless, the tests with FeS deposits in this project showed a remarkably aggressive effect, considering that the iron monosulfides used have been reported to be less aggressive than e.g. pyrites. The presence of FeS deposits greatly increased both weight loss and local penetration rates in autoclave experiments with high H 2 S/CO 2 partial pressures (20-25 bar total). At lower acid gas pressures the increase was less dramatic. The addition of an iron sulfide deposit will affect several factors in the corrosion process. Iron sulfides with electron-donating semiconductor properties will increase the cathodic area and possibly the cathodic reaction rate, leading to increased corrosion rates. This effect will increase with the FeS load. On the other hand, the deposit will act a porous barrier limiting transport of corroding agents and corrosion products, which may lead to a decrease in corrosion rates. Also this effect increases with the FeS load. It is therefore not given that the corrosivity will increase with the FeS load, especially if the oxidizing agents are reduced and depleted in the outer part of the deposit. Furthermore, thick iron sulfide layers may facilitate formation of local cells and on the metal surface, which may lead to formation of pits and other localized attacks. This is consistent with the observations made in this study. It was also demonstrated that fresh FeS powder produced by corrosion in H 2 S environments was at least as aggressive as the commercial FeS used in most of the tests. Results from other studies 6 have shown that the aggressive cathodic effect of iron sulfide deposits may be mitigated by the presence of thin oxidized layers on the FeS particles; however, the results in this project indicated no such layers on any of the iron sulfide batches used. The effect of sand deposits was considerably less harmful than irons sulfide deposits, although it did cause some localized corrosion. Mixtures of iron sulfide and sand mixtures were found to a cause a significant increase in corrosion when the iron sulfide content was 50 % or more. In order for the FeS to act as a high-area cathode it must be able to form a continuous network within the deposit. With a 1:5 FeS/sand mixture the FeS particles appeared to fill in the voids between the larger sand grains without forming a continuous network, thus improving the diffusion barrier properties of the deposit without contributing to the cathodic reactions. With 1:20 FeS/sand and sand alone the voids between the sand 12

13 grains were larger. This probably explains why the corrosion rates were lower with a 1:5 FeS/sand ratio than with 1:20 and sand only. The FeS deposits were applied at a thickness up to 5 mm in this study, which corresponds to about 1 gram FeS per cm 2 surface area. As an example of the amounts of FeS particles that will be available in the field, we will consider the example of a 1 km stretch of a 36 in (915 mm) diameter sour gas pipeline producing Fe 2+ by internal corrosion at an average rate of 0.1 mm/y (with respect to both time and surface area). With an internal surface area of 2873 m 2 this corrosion rate would produce ca. 0.3 m 3 iron per year, corresponding to 0.7 m 3 FeS. Assuming that 70 % of the FeS is captured as surface layers (a value typical for corrosion tests with low to intermediate ph or temperatures) the production rate of free FeS (e.g. particles) will be about 0.2 m 3 /year. If accumulated as porous deposits, this amount will be enough to cover a surface area of ca. 100 m 2 with a 5 mm thick deposit. In reality, the amounts of location of FeS deposits will of course depend on other factors such as flow velocity and regime, liquid production rates and pipeline geometry. CONCLUDING REMARKS A series of sour UDC autoclave experiments has been run with combined H 2 S and CO 2 levels of bar at temperatures in the range o C. The effects of FeS deposits and mixed FeS/sand deposits were investigated. The reference carbon steel coupons without deposits generally exhibited a low susceptibility to localized corrosion. With iron sulfide deposits significantly increased weight loss corrosion and severe localized corrosion attacks were observed for conditions with high H 2 S and CO 2 levels. The presence of 1 g/cm 2 FeS powder increased local attacks and maximum penetration rates by a factor up to 20 compared to regular coupons. Also, mixed FeS/sand deposits may accelerate corrosion and cause local attacks, while accumulation of sand alone is less detrimental. The highest local penetration rates with FeS deposits took place in simulated formation water with high chloride levels. SEM examination indicates that the presence of chlorides may prevent adhesion of corrosion film to the metal surface, and also lead to formation of pores in the film (with FeS deposits). The galvanic effect of conductive deposits such as FeS is also enhanced by the solution conductivity provided by high ionic strength. ACKNOWLEDGMENTS The authors wish to thank the following participants in the Kjeller Localized Sour Corrosion project for permission to publish the results: Total, Shell, ConocoPhillips, ExxonMobil, Chevron, Saudi Aramco, Champion, Clariant, Baker Hughes, Statoil, BP, BG Group, JOGMEC and Petrobras. 13

14 REFERENCES 1. Jon Kvarekvål, Gaute Svenningsen: Effect of High H 2 S Partial Pressures on Localized Corrosion of Carbon Steel. CORROSION/2015, Paper No. C , Dallas, March Smith, J. S. and Miller, J. D. A. (University of Manchester Institute of Science and Technology, Manchester). Nature of Sulphides and their Corrosive Effect on Ferrous Metals - A Review. Br. Corros. J. 1975; 10(3):pp Achour, Mohsen; Kolts, Juri; Humble, Phillip, and Hudgins, Roger. Experimental evaluation of corrosion inhibitor performance in presence of iron sulfide in CO 2 /H 2 S environment. NACE; Wang, Hongbin; Fell, David, and Bailey, Steven. Corrosion and Inhibition Study for a H 2 S Containing Water Injection System: A Field Investigation Using Electrochemical Techniques. NACE; J. Moore, J-F Liu, X. Tang: Corrosion Inhibitors in the Presence of Elemental Sulfur, paper no , CORROSION/2009, NACE, C. M. Menendez, V. Jovancicevic, S. Ramachandran, M. Morton, D. Stegmann, New Method for Assessing Corrosion under Iron Sulfide Deposits and CO 2 /H 2 S Conditions, Paper no , CORROSION/2011, NACE International, Houston, TX,