Black powder formation in thin water layers under stagnant conditions

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1 Paper No Black powder formation in thin water layers under stagnant conditions Gaute Svenningsen and Rolf Nyborg Institute for Energy Technology Instituttveien 18 NO-2027 Kjeller Norway Abdelmounam Sherik and Arnold L. Lewis Saudi Aramco Bldg Dhahran Saudi Arabia ABSTRACT Black powder formation was simulated using carbon steel samples with a thin water layer. Temperature, alkalinity, TEG content and the partial pressures of O 2, CO 2 and H 2 S were systematically varied to study the effect on corrosion rate and corrosion products formed. The corrosion rate was in most cases close to 0.08 mm/y and almost unaffected by variation in CO 2, H 2 S and O 2 partial pressure. The corrosion rate was significantly reduced in high TEG solution. Compared to distilled water, addition of alkalinity had just minor effect on the corrosion rate, except for CO 2 -free conditions. Two essentially different regimes of corrosion products were identified. Magnetite formation dominated in CO 2 -free conditions while siderite and goethite formed in the presence of CO 2. Key words: Black Powder, CO 2 Corrosion, H 2 S Corrosion; Carbon steel INTRODUCTION Black powder is a generic term for particulate matter that forms in gas pipelines. 1-3 Although the name indicates that it is always black, the color may also be brown or grey. The composition of black powder may vary from pipeline to pipeline, but it contains at least one of the following solid components: iron carbonate, iron sulfide or iron oxides, which are formed by internal corrosion of the pipeline. Mill scale (e.g. Fe 3 O 4 ) can also contribute to black powder in new pipelines. Black powder may contain other solid constituents such as elemental sulfur, salts, sand, clay, welding splatter, formation cuttings and metal debris. Black powder may additionally contain liquids, for example liquid hydrocarbons (condensate, 2011 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE 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 Association. 1

2 compressor oil), glycol, corrosion inhibitors and water. Black powder is causing problems in the gas industry by, for example, clogging and eroding valves and contaminating the customer s sales gas. Black powder is formed due to the simultaneous presence of corrosive gases and water in the pipeline. Water condensation is the result of high water content and low temperature (under dew point). Corrosion of carbon steel in water with pure gases like CO 2, H 2 S or O 2 has been extensively studied and their individual corrosion products are well known. However, corrosion in mixed gases has been studied to a much lesser extent, particularly in the very small water volumes (thin aqueous films) that typically exists in gas pipelines with water condensation and gas flow. The conditions in these thin films are principally different from large water volume ( bulk ) exposure due to many factors, such as less corrosion required to reach saturation/precipitation of corrosion products, limited convection in the water phase, corrosion products present all through the liquid film and possible evaporation or drying out in periods with no condensation. The present work studied formation of black powder in thin aqueous films by systematic investigating the effect of temperature, alkalinity, TEG content and the partial pressures of O 2, CO 2 and H 2 S in thin stagnant water layers. Alkalinity was included because ph adjustment could be a potential candidate to reduce black powder formation in a similar manner as it is used to reduce aqueous CO 2 corrosion. 4,5 Related research on black powder formation under flowing conditions has been published in a separate paper. 6 EXPERIMENTAL PROCEDURE Corrosion samples The samples were made of ferritic pearlitic X-65 pipeline steel, with the chemical composition given in Table 1. The samples were machined to the shape of cups/disks as shown in Figure 1 (22 mm internal diameter and 3 mm internal height). A total surface area of 5.9 cm² was wetted when the cups were filled with 1.2 ml liquid. This gave a very small liquid volume/wetted surface area ratio, which is important for simulation of corrosion in thin liquid films, e.g. because of significant accumulation of corrosion products. Table 1: Chemical composition of the sample alloy (wt%). C Si Mn S P Cr Ni V Mo Cu Al Sn Nb Sample preparation The corrosion samples were degreased in an ultrasonic bath (first acetone then isopropanol) and labeled on the back side (facing down). Samples used for mass loss measurements were additionally coated with epoxy on the external surfaces, to keep the exposed metal surface area constant if condensed water droplets occasionally should hit the samples. It was observed that the epoxy did take up water/moisture during exposure, but the water was completely released when heated. All samples were therefore kept at least one hour in an oven before weight measurements (initial, exposed and stripped weight). Corrosion experiments The corrosion tests were carried out in a glass cell, except for the experiment with TEG which was carried out in a closed autoclave. A 95 wt% TEG solution is not in equilibrium with water saturated gas at 26 C, and would therefore take up water in the glass cell. It was therefore decided to carry out the TEG experiment in a closed autoclave to avoid water uptake (i.e. dilution of the TEG). 2

3 The experimental parameters are summarized in Table 2. Table 2: Summary of experimental parameters. ID Exposure time Partial pressure (mbar) Temperature TEG content Added alkalinity* (days) O 2 CO 2 H 2 S ( C) (wt%) (mmol/kg) BP and 5 BP and 5 BP and 5 BP and 5 BP and 5 BP and 5 BP and 5 BP and 5 * Alkalinity added as NaHCO 3. Glass cell experiments (BP01 BP07) The exposure chamber was a double-walled glass cell with connectors for gas feed and temperature monitoring, as shown in Figure 2. The temperature was maintained by circulation of water from a thermostated heating/cooling bath through the glass cell wall. The temperature was very stable and varied less than ± 1 C for all the experiments. The samples were placed on two plastic boards inside the glass cell. The gas flow was maintained by two gas controllers, one for compressed air (source of oxygen) and one for the other gases (pre-mixed gas in a pressure tank). The air flow was 1.0 ml/min in all experiments except BP02 which was O 2 free. A total flow (premixed gas and air) of about 17.7 ml/min was used for all experiments. The composition of the gas varied from experiment to experiment, as shown in Table 2. The pre-mixed gas and air was saturated with water moisture by letting it flow through two water-filled bubble flasks before the gas entered the glass cell. The bubble flasks were placed in the water bath and had therefore the same temperature as the glass cell. The air was mixed with the other gas right before it entered the glass cell, i.e. O 2 and H 2 S could not react before entering the cell. To further ensure complete water saturation, the bottom of the glass cell was filled with about 1 liter of distilled water. The exit-gas went through another bubble flask for visual observation of the gas flow and to avoid air ingress. The cup-shaped samples were filled with 1.2 ml of liquid each. Two types of liquid were used: Distilled water Distilled water with 5 mmol/kg NaHCO 3 (alkaline water) When the glass cell was closed, it was first flushed with nitrogen gas for a few minutes to remove air before the gas mixing system was connected. The glass cell was kept closed during the whole experiment. After exposure, the samples were taken out of the glass cell and submerged in isopropanol to remove/dilute water. The ph of the remaining liquid was measured with ph-indicator sticks (litmus paper) on four samples before they were put in isopropanol. The water wet atmospheric exposure was in all cases less than a few minutes. Finally the samples were dried in a hot air-circulated oven (~70 C). 3

4 Samples used for characterization of corrosion products were dried in nitrogen atmosphere after they had been immersed in isopropanol. A vacuum chamber (low pressure) that was constantly flushed with nitrogen was used to help evaporating remnants of water and isopropanol. The vacuum drying lasted at least one hour. A short stay in a hot air-circulated oven (~70 C) finalized the drying process. The uncoated BP01 experiment was carried out without vacuum drying; just immersion in isopropanol and then drying in a hot oven. Possible reaction between corrosion product and oxygen (air) during the transfer from glass cell to vacuum chamber and in the oven cannot be completely ruled out. However, all possible care was taken to keep the wet exposure time as short as possible, and no visual change could be observed during the air exposure. Figure 1: Unexposed sample with white epoxy coating on the external surfaces. Bottom view (left) and top view (right). Although some bubbles were trapped in the epoxy (left image) there was no direct exposure of bare metal. Figure 2: The exposure chamber (left) and the samples on the plastic board (right). 4

5 TEG autoclave experiment (BP08) There was no continuous gas supply to this experiment. The autoclave volume was 1.8 dm³. Images of the autoclave are shown in Figure 3. Experiment BP08 was carried out with two types of liquid: TEG with 5 wt% distilled water (TEG) TEG with 5 wt% distilled water and 5 mmol/kg NaOH (alkaline TEG). The samples were put into the autoclave and then filled with 1.2 ml TEG solution each. Additionally, about 150 ml TEG solution was put in the bottom of the autoclave as a reservoir to maintain equilibrium. Nitrogen flushing was applied for a few minutes when the autoclave was closed. The autoclave was then vacuumed (~0.1 bar a ) and filled with N 2 (~2 bar a ) to remove air. This was repeated eight times. The autoclave was then vacuumed and filled with the premixed gas (~2 bar a ) four times. Finally the autoclave was filled with premixed gas at a total pressure of 5 bar a, and partial pressures of 0.3 bar CO 2, 12 mbar O 2 and 0.1 mbar H 2 S. Figure 3: Images showing the autoclave that was used in the BP08 experiment. A small gas tank was used to fill the autoclave (blank cylinder on left image). The autoclave was submerged in a water bath (right) to keep the temperature constant. 5

6 Mass loss The samples were weighed three times: before exposure (initial weight), after exposure and drying (asexposed) and after the corrosion products had been stripped off (stripped weight). An analytical weight was used for all weight measurements. The stripped weight change (initial weight minus stripped weight) was used to calculate the corrosion rate. Reported mass loss is the average of five samples for each experiment, except for BP08 where four samples were used. It was clear that some corrosion products were lost during the immersion in isopropanol. This means that the as-exposed weight was too low and that the mass of corrosion products therefore is underestimated. Identification of constituents in the corrosion products The corrosion products were removed (scraping by a plastic tool) from the as-exposed samples and analyzed by X-ray powder diffraction (XRD). Some samples were also investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). RESULTS Mass loss, corrosion rate and ph A summary of the corrosion rates is shown in Table 3 and Figure 4. The 95% TEG solution (BP08) had clearly the lowest corrosion rate. The corrosion rates of the other experiments did not vary much, and most of them were within ±25% of the BP01 corrosion rate (0.08 mm/y). The effect of added alkalinity was relatively insignificant, except for the CO 2 -free experiment (BP03) where it reduced the corrosion rate to less than 0.02 mm/y. Increased H 2 S level from 0.1 to 0.3 mbar decreased the corrosion rate slightly to 0.05 mm/y. The corrosion rate was higher for the low temperature experiment (BP07), 0.11 mm/y. The ph measured in the sample liquid was in most cases similar for all experiments, around 6.5, as shown in Table 3. The CO 2 -free experiment (BP03) and high-co 2 experiment (BP04) were slightly more acidic with ph of 5.0 and 5.5 respectively. Samples with alkaline water had essentially the same ph as the samples with distilled water ( 0.5 ph unit more alkaline). 6

7 Table 3: The average corrosion rate, mass loss, mass of corrosion products and ph of exposed samples (epoxy coated). ID Corrosion rate Mass loss Mass of corr. Number of ph (mm/y) (mg/cm² d) prod. (mg/cm² d) test samples Distilled water BP BP BP BP BP BP BP BP N/A Alkaline water BP BP N/A BP BP BP BP BP BP N/A Distilled water Alkaline water Corrosion rate (mm/y) BP01 BP02 BP03 BP04 BP05 BP06 BP07 BP08 Figure 4. Mass loss corrosion rates for all experiments. 7

8 BP01 BP02 BP03 BP04 Alkaline water Distilled water Figure 5: Exposed and dried samples from experiment BP01 BP04. Inner diameter is 22 mm. BP05 BP06 BP07 BP08 Alkaline water Distilled water Figure 6: Exposed and dried samples from experiment BP07 and BP08. Inner diameter is 22 mm. 8

9 Table 4: Constituents in corrosion product identified by XRD and SEM/EDS. Alkaline and distilled water/teg are denoted A and D, respectively. An asterisk (*) indicates that the analysis is based on EDS only. Brackets indicates that the phase is a minor component (< 10 wt%). Question mark indicates that the phase identification is uncertain. BP01 BP02 BP03 BP04 BP05 BP06 BP07 BP08 Compound D D A D A D A D A D A D A D A Siderite FeCO 3 x x* x* x x (x) (x) x x x x x?* x?* Goethite -FeO(OH) x (x) x x x x x x Magnetite Fe 3 O 4 x x Lepidocrocite -FeO(OH) (x) (x) Pyrite anisotropic FeS 2 (x?) Sulphur S 8 (x?) (x) Iron sulphate FeSO 4 (x?) (x?) Iron sulphide Fe X S Y x* x* x?* x?* Figure 7: SEM image (left) and the corresponding EDS spectra (right) of corrosion products on the BP02 sample (distilled water). The locations of EDS spot analysis are indicated with black arrows. Figure 8: SEM image (left) and the corresponding EDS spectra (right) of corrosion products on the BP08 sample (95 wt% TEG). The location of EDS spot analysis is indicted with the black arrow. 9

10 Composition of corrosion products The type of corrosion products changed considerably between the different exposure conditions. The color of the corrosion products varied from black and rust brown to shiny metallic, as shown in Figure 5 and Figure 6. It was often observed that a crust (skin) formed on the air/liquid interface during exposure. In many cases this skin would remain intact as a thin flake after drying. Many of the corrosion products were electrostatic, particularly if present as thin flakes. Despite the moist conditions inside the glass cell, many samples appeared dried out, as liquid was not visible because of the corrosion products. However, the samples still contained liquid (observation during ph measurement). The results from identification of corrosion products by XRD and SEM/EDS are shown in Table 4. Samples from experiments BP02 and BP08 had too little corrosion products for XRD identification but SEM/EDS-analysis of exposed surfaces revealed the presence of siderite and iron sulfide for BP02, as shown in Figure 7. The BP08 samples were shiny metallic bright and had very low amounts of corrosion products. However, EDS-analysis indicated also here the presence of iron sulfide and possibly also iron carbonate (Figure 8). Quantitative compositional analysis of the corrosion products from BP02 and BP08 was not possible because the corrosion products were so small/thin that the metal substrate also contributed to the EDS signal. DISCUSSION Corrosion rate From a practical point of view, the mass of corrosion products (see Table 3) would probably be considered as most relevant for black powder formation. However, in the present experiments the weight of the as-exposed samples was inaccurate as some corrosion products clearly were lost in the drying process, resulting in underestimation of the mass of corrosion products. Therefore, only the corrosion rate (i.e. metal mass loss) will be used in the subsequent discussion. All the corroded iron will form solid corrosion products, except for a very small fraction that is dissolved in the aqueous phase as Fe 2+ ions. It should also be mentioned that these experiments were carried out in small stagnant liquid volumes that accumulated all corrosion products, quite different from standard corrosion tests in large electrolyte volumes where the corrosion products may fall off or dissolve in the solution. Effect of TEG Of the parameters tested, addition of 95 wt% TEG to the liquid had the largest influence on the corrosion rate (Figure 4). This was expected because glycol solutions are known to have an inhibiting effect on CO 2 corrosion. 7 The inhibiting effect of glycols is related to solution property changes with increasing glycol content, for example decreased solubility of CO 2, decreased water activity, increased viscosity and decreased diffusivity. Effect of H 2 S Increasing the H 2 S content (BP06) reduced the corrosion rate, most likely because the higher partial pressure of H 2 S stabilized iron sulfide, which formed a protective layer on the metal surface. Effect of temperature The corrosion rate increased when the temperature was reduced (BP07). While it is generally accepted that the potential corrosion rate is reduced with decreasing temperature, the observed phenomenon was probably related to slower formation of a (partly) protective siderite (FeCO 3 ) layer. 8 The kinetics for siderite formation is very temperature dependent and slow at low temperatures. Additionally, the siderite solubility increases with decreasing temperature so more iron has to corrode to reach the required (super)saturation. 10

11 Effect of O 2 Previous literature reports suggest that the corrosion rate should go down with reduced oxygen content 2,9,10, and it is surprising that the BP01 and oxygen free BP02 experiments were similar in corrosion rates. This is probably related to the small amounts of oxygen (12 mbar) compared to CO 2 (300 mbar). Factors affecting the corrosion rate The corrosion rate remained essentially unchanged when each of the three corrosive gases was individually absent (no O 2 in BP02, no CO 2 in BP03 and no H 2 S in BP05) or present at different partial pressures (BP04 and BP06). Therefore, none of these species individually (at the tested partial pressures) had significant effect on the corrosion rate. This could indicate that transport mechanisms are rate limiting. Further support for this hypothesis is the relative small increase of corrosion rate when the CO 2 partial pressure was increased from 0.3 bar to 1 bar (BP04). Transport limitations could occur on several locations: On the gas/liquid interface (dissolution of reactive species), particularly if a skin is present. Transport through the liquid phase Transport through pores/cracks in the corrosion products that build up Transport through thin layers/films, such as FeS or FeCO 3, on the metal surface. In a purely diffusion controlled system the corrosion rate would go down with decreasing temperature, opposite to the observation in BP07 where the corrosion rate increased when the temperature was reduced from 26 to 13 C. This means that the corrosion rate was not only controlled by diffusion through the liquid phase. It is likely that the formation rate of protective corrosion films also is affecting the corrosion rate. Usually, the corrosion rate is initially high and is reduced when a (partly) protective film builds up. For short exposure times the initial period with high corrosion rate would give significant contributions to the average corrosion rate. It is also possible that the properties of protective corrosion films could change during the exposure, for example if one specie transforms to another specie. Corrosion products Siderite (FeCO 3 ) was formed in all experiments with CO 2, indicating that CO 2 contributed to the total corrosion process. Magnetite (Fe 3 O 4 ) and lepidocrocite ( -FeOOH) were only observed in the CO 2 -free experiment (BP03), indicating that CO 2 and siderite precipitation compete with the formation of these constituents. It was reported in literature that lepidocrocite may transform to magnetite, 11,12 so their coexistence was not unexpected. Iron carbonate can undergo transformation to magnetite in the presence of oxygen through the slow reaction: 6FeCO 3 + O 2 = 2Fe 3 O 4 + 6CO 2. 2,11-15 There were no indications on this happening in the laboratory experiments as magnetite only formed in the CO 2 -free experiment (BP03) where siderite obviously was not present. It is not possible to conclude on whether the siderite to magnetite transformation was not observed because of short exposure time or if the reaction was prevented due to unfavorable conditions. Most corrosion products contained only traces of sulfur compounds. Elemental sulfur (S 8 ) was detected in the high H 2 S experiment (BP06). Experiments BP02, BP06, BP07 and BP08 contained some iron sulfur species, but the concentration was in all cases very low (less than 10 wt%). This indicates that the BP01 conditions don t favor oxidation of H 2 S or H 2 S-corrosion, but these reactions may occur if the H 2 S content is increased to 0.3 mbar. The very limited amount of sulfur containing compounds in the corrosion products suggest that the sulfur reactions are minor compared to O 2 and CO 2 corrosion. Effect of ph It was expected that increasing the ph (added alkalinity) would reduce the corrosion rate, as commonly observed in sweet systems, 4,5 because the high ph helps form a protective siderite film. However, addition of 5 mmol/kg alkalinity did not significantly affect the corrosion rate in the present work, except 11

12 for BP03 where it reduced corrosion with about 75%. The ph after exposure was essentially similar regardless of alkalinity being added or not (Table 3). This is related to ph increase during corrosion of iron, as shown in the simplified reactions: Fe = Fe e - O 2 + 4e - + H 2 O = 4OH - (four hydroxide ions formed) 2H 2 CO 3 + 2e - = 2HCO H 2 (two bicarbonate ion formed) H 2 S + 2e - = S 2- + H 2 (one sulphide ion formed) As these reactions show, the ph will increase until corrosion products start to precipitate out, after which the ph becomes stable (formation and precipitation of alkaline species is equal). In the current experiments the liquid volume was limited so the solution ph was largely governed by the corrosion reactions. Hence the added alkalinity (initial ph) did not have significant effect on the final ph and corrosion rate. However, the corrosion rate of the CO 2 -free experiment BP03 was affected by addition of alkalinity, most likely because the alkalinity aided the formation of a protective iron oxide layer (since PB03 was CO 2 -free, formation of a protective siderite film was not possible for that experiment). CONCLUSIONS Metal cups with small volumes of stagnant liquid were used to measure the corrosion rate (mass loss) under simulated black power generation conditions at 0.3 bar CO 2, 0.1 mbar H 2 S, 12 mbar O 2 saturated with water at 26 C. The corrosion rate was 0.08 mm/y and it was essentially unaffected by the variations in the CO 2, H 2 S or O 2 partial pressures tested here. Increasing the H 2 S partial pressure from mbar (with 0.3 bar CO 2 ) decreased the corrosion rate from 0.08 mm/y to 0.05 mm/y. The corrosion rate increased to 0.11 mm/y when the temperature was reduced from 26 to 13 C (with 0.3 bar CO 2 ), most likely because of slower formation of a protective iron carbonate film. The corrosion rate was significantly reduced in 95 wt% tri ethylene glycol (TEG) solution to mm/y. Addition of 5 mmol/kg alkalinity to the test solution did not significantly affect the corrosion rate, except for the CO 2 -free experiment where the corrosion rate was reduced with about 75% most likely due to increased formation of protective iron oxide(s). A mixture of O 2 and CO 2 corrosion occurred at baseline conditions (12 mbar O 2 ), while CO 2 corrosion was the dominating corrosion reaction in O 2 -free conditions. Goethite ( -FeO(OH)) and siderite (FeCO 3 ) were the main corrosion products formed in the presence of CO 2, while magnetite (Fe 3 O 4 ) was the main corrosion product in CO 2 -free conditions. 12

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