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Collaborating Authors
CORROSION 2003
ABSTRACT Of critical importance in the proper application of inhibitors within oil producing wells is the delivery of the chemical throughout the system. A new application model is presented based on small scale laboratory measurements to determine corrosion inhibitor efficiency and film persistency. A forty foot well annulus simulator is used to determine the flow and fate of the applied chemical inhibitors for typical backside batch and flush application in a packerless completion. The delivery depends upon both chemical parameters such as density, viscosity, and surface tension as well as flush parameters including volume and rate. INTRODUCTION During the production of oil and gas, water (or brine) is typically produced as an undesirable byproduct. When brine is produced in the presence of carbon dioxide, carbonic acid species may occur in solution resulting corrosion or metal loss from metal goods utilized for production. As the well ages, the pressure in the reservoir slowly drops resulting in greater water production and increased corrosion failures. ~'2 In order to prevent failures due to metal loss, corrosion inhibiting chemical are frequently introduced into the producing wells. 3 There are several methods for applying corrosion inhibiting chemicals to protect metal goods. The corrosion inhibitor may be supplied as a free flowing liquid, a water external emulsion, an oil external emulsion, or solid state. The inhibiting chemical may either be applied intermittently (batch) or continuously into the producing well. In continuous application a liquid inhibitor is pumped either down the backside of a well or through capillary tubing to the bottom of a producing well. Solid and emulsion based products are not generally applied continuously though due to their physical properties their chemical return profile may simulate continuous injection. 4 Batch applications include applying the corrosion inhibitor down the annulus followed by a liquid flush, applying the inhibitor down the producing tubing and shutting the well until the fluid applied chemical reached the bottom of the well, displacing all the fluid from the production tubing, or squeezing the chemical into the formation. In batch and flush applications, the key is to deliver the chemical to the proper location at the proper time to provide protection from corrosion and metal loss. When using a standard liquid based corrosion inhibitor applied into the annulus, the flush is designed to displace the fluid column and result in the immediate delivery of the chemical inhibitor up the production tubing. As the batch of chemical inhibitor is produced through the tubing, a corrosion inhibiting film is formed on the metal surface of the well. As the well continues to produce, the inhibitor films are lost and another batch treatment will be required. Typically the time between batch treatments ranges from 3 days to 1 month depending upon both well production conditions and properties of corrosion inhibitors. However, emulsions and properly designed solid inhibitors form a reservoir of chemical in the well annulus, which slowly delivers a constant supply of corrosion inhibitor for a set period of time. The typical lifetime of solid and emulsion based corrosion inhibitors is greater than 1 month. The end result is the protection of tubing against corrosion for an extended time period compared to the standard batch type treatments. Correct placement of the required amount of inhibitor in the annulus or delivery into the tubing provides sufficient chemical to form the protective film coating on the tubing surface. There are several factors on which the success of treatment depends, which include inhibitor volume, flush rate
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
INTRODUCTION ABSTRACT Severe deterioration of a reinforced concrete cooling tower, which comprised pre-cast columns, beams, wall panels, and slab panels was noticed during the last few years. A diagnostic survey was conducted to identify the cause and extent of deterioration. The concrete deterioration was very advanced in some areas, particularly on the external side of the end walls and posing a safety hazard to plant personal. Internally, the level of defects was very low with only minor patches of delamination and cracks. Both chloride and sulfate ions were present in the concrete cover at rebar depth well in excess of their threshold levels. About 12% of the half-cell potential results indicated high (90%) corrosion risk and 53% of the results exhibited medium (50%) corrosion risk in all tested areas of the structure. The investigations concluded that deterioration of concrete has occurred mainly due to chloride- induced corrosion of the steel reinforcement. Patch repair and cathodic protection (CP) repair method was recommended to arrest the ongoing corrosion of the steel reinforcement. The CP system design, installation, and initial commissioning results are also described and discussed. The 25CTI cooling tower provides cooling system for three major plants in Saudi Petrochemical Company, located in Jubail industrial city, Saudi Arabia. The cooling tower (CT) provides potable water supply to ethylene (ETH), crude industrial ethanol (CIE), and styrene (STY) plants, hence a very important and critical structure to the plant operations. The CT structure comprises pre-cast concrete units mounted on a reinforced structural frame. The end walls consist of pre-cast beams and wall panels, and in-situ columns. The roof slab consists of 160 mm thick pre-cast elements fixed to a supporting frame. A schematic illustration of the CT with major components identified is given in Figure 1 and a view of north elevation and east side is shown in Figure 2. The CT was commissioned in 1983 and has been showing some signs of concrete distress in the form of cracking and spalling of concrete for the last few years, believed to be caused by corrosion of the reinforcement. In some areas, the extent of deterioration was very severe and posing a safety hazard to personnel and plant below. A condition survey was conducted to determine the cause and extent of deterioration and recommend appropriate repair methods for the rehabilitation of the structure. This paper describes and discusses the condition survey results, available repair options, and design, installation, and commissioning of the recommended patch repair and cathodic protection (CP) repair method. CONDITION SURVEY Standard condition survey techniques were employed throughout this investigation, which includes the following: ? Visual Inspection & Delamination survey of concrete surface ? Concrete powder samples for chloride and / or Sulfate content determination ? Cement content analysis ? Depth of carbonation & reinforcement ? Half-cell potential measurements Visual Inspection Visible defects particularly spalls and delaminations were evident in all areas of the slab and end elevations. Where steel was exposed, loss of steel section was noted, in some cases up to 100%. An 'aspect distribution' of defects was evident i.e. a greater incidence of defects were found on the south and west faces (on the lee side of the prevailing wind) than on the north and east faces. Minor defects in terms of extent and distribution included corrosion staining, failed old repairs, detached mortar fillets on beam upper surfaces and water leakage from 5~6 locations. Internally, the level of defect
- North America > United States (0.46)
- Asia > Middle East > Saudi Arabia > Eastern Province > Jubail Industrial City (0.25)
ABSTRACT Electrochemical chloride extraction (ECE) is a process used to extract or remove chloride ions from chloride contaminated reinforced concrete elements. The extraction is accomplished by application of an electrical field between a temporary external anode and steel embedded in concrete. The electric field induces an ionic current in the concrete electrolyte, a portion of which is carried by the negatively charged chloride ions moving away from the embedded steel and towards the external anode. Once sufficient amount of chloride ions are extracted the surface of the concrete element is sealed to prevent future ingress of chloride ions into the concrete. To ascertain the long-term effectiveness of ECE, a total of 10 laboratory specimens fabricated and treated in an earlier study were evaluated over a five year period. At the start of this study, these specimens had already undergone five years of post treatment evaluation. Since treatment, the specimens were continually exposed to Northern Virginia climate. The variables in the specimen matrix included current density, total charge passed, anode material, and electrolyte. Three different current densities ranging from 93 to 600 mA/m2, three different totalcharge- passed ranging from 645 to 3,226 A-hr/m2, two different types of anode material, and three different electrolytes had been used to obtain a total of 10 specimens. A bridge deck of one structure and columns of another, treated during the earlier study were also included in this evaluation. The results of the long-term evaluation suggest that ECE was effective in mitigating corrosion in the laboratory specimens for over 10 years. Corrosion continued unabated in the control specimen and exhibited corrosion induced damage. Whereas, only specimens receiving a total charge of 645 A-hr/m2 exhibited early signs of corrosion initiation after 10 years of exposure. Although, some remigration of chloride ions was observed in the concrete above the top mat of reinforcing steel, no such migration was noted at the depth of the top mat reinforcing steel. Although chloride ions had been successfully extracted from the bridge deck, no difference between the treated and the control area were observed due to insufficient passage of time after treatment. The ECE treatment on the columns of the bridge deck had encountered several problems and was not successful in mitigating corrosion. INTRODUCTION Project Background Electrochemical chloride extraction (ECE) is a process used to extract or remove chloride ions from chloride contaminated reinforced concrete elements. The extraction is accomplished by the application of an electrical field between a temporary external anode and the steel embedded in concrete. The electric field induces an ionic current in the concrete electrolyte, a portion of which is carried by the negatively charged chloride ions moving away from the embedded steel and towards the external anode. The process was originally referred to as electrochemical chloride removal (ECR). However, due to the popularity of the acronym (ECR) for epoxy coated rebar, the term ECE was adopted, particularly in North America. In Europe, the process is often referred to as Desalination or Electrochemical Chloride Migration (ECM). ECE is becoming increasingly popular as a rehabilitation option for chloride contaminated reinforced concrete structures. The technique of removing chloride ions from contaminated reinforced concrete by electrochemical means was first studied in the mid-1970?s by Kansas Department of Transportation and Battelle Columbus Laboratories1,2. Both studies were funded by the Federal Highway Administration (FHWA). Although, the technique was found to be promising, some adve
- North America > United States > Kansas (0.34)
- North America > United States > Virginia (0.24)
ABSTRACT In this preliminary study, the complexity of the corrosion phenomenon of post-tensioning strands in grouted anchorage assemblies was examined and both physicochemical and electrochemical key technical issues were identified. Measurements of oxygen reduction efficiency in high pH electrolytes were conducted to obtain polarization parameters to be used in modeling. The time evolution of electrical resistivity of 5 low-bleed commercial grouts was measured also for model input. A mathematical model for a simple grout-strand system was proposed and dimensionless equations were formulated, to solve the combined polarization and oxygen transport problem. Within the range of validity of the model assumptions, initial computations indicated that oxygen availability was a key factor in determining corrosion severity while grout resistivity was secondary. Predicted corrosion rates were in general agreement with field and laboratory observations. Issues for subsequent model development were identified. INTRODUCTION In post-tensioning (PT) of concrete a compressive force is applied by stressing tendons (or bars) after the concrete is cast and cured so that deflection and cracking are minimal, even at full load. Severe corrosion distress and even complete failure of external PT was recently observed in tendons of pillars or superstructure of three Florida pre-cast segmental bridges over salt water. As PT tendons are critical to the integrity of these structures, the investigation described here was initiated to investigate the corrosion mechanism. Nature of the problem: The first reported incident was at the 18-year old Niles Channel Bridge in the Florida Keys (Powers, 1999, Sagüés, 2000) followed by the 7-year old Mid Bay Bridge (MBB) in the Western panhandle (Corven, 2001) and the 15-year old Sunshine Skyway Bridge over Tampa Bay (FDOT, 2002). In those structures, the tendons are a bundle of typically 19 or more seven-wire highstrength (ultimate tensile strength 1.86 GPa (270 ksi)) steel strands, contained for much of the length (including the part of the tendon that is external to the concrete) in a high-density polyethylene (HDPE) duct. The tendon terminates at anchorage assemblies (Figure 1) that transfer the longitudinal tendon force to the concrete. Portions of the tendon pass through pipes in intermediate deviation blocks or Ubends to provide lateral force transfer where needed. After tensioning the entire space between tendon and ducts or anchorage is pumped full with a cementitious grout. The grout is intended for corrosion protection of the steel by providing a highly alkaline environment, and also to allow for some force transfer between strand and anchorage in case of strand breakage or loosening. The observed damage consisted, in each of the three bridges, of one completely separated tendon plus one to several partially detensioned tendons. The completely separated tendons had failed in the anchorage region following severe corrosion of many of the strands, as illustrated in Figure 2. Mechanical failure appears to be the result of simple overload due to locally reduced cross-section, usually in the form of generalized corrosion along several cm with occasional pitting. In the partially detensioned tendons severe corrosion and mechanical failure had affected some but not all of the strands. In one instance the area of severe corrosion was about 2-3 m away from the anchorage. All cases of severe corrosion were associated with large void areas that should have been filled with grout. Significant chloride contamination of the remaining grout was observed at the corroded regions. In nearly all cases the affected anchorages were at bridge expansion joints, or underneath horizontal
ABSTRACT A series of 91 concrete G109 type specimens was exposed to cyclic wet-dry ponding with a 15 w/o NaCI solution for approximately four years. Mix design variables included 1) cement alkalinity (equivalent alkalinities of 0.97, 0.52, and 0.36), 2) water cement ratio (0.50, 0.41, and 0.37), 3) presence versus absence of coarse aggregate, and 4) presence versus absence of fly ash. Corrosion potential and macrocell current between bottom and top bars were monitored in order to determine the time at which active corrosion commenced. Subsequent to corrosion initiation, specimens were autopsied and determinations made of 1) the effective chloride diffusion coefficient, 2) pore water pH, and 3) the critical chloride concentration for corrosion initiation. Time-to-corrosion for specimens of the individual mix designs conformed to a distribution that was represented using Weibull analysis. Specimens fabricated using the highest alkalinity cement exhibited times-to-corrosion that exceeded those with the two lower alkalinity cements by as much as a factor of five. Chloride threshold concentration was also variable and for 1 Visiting Professor, Florida Atlantic University from Korea Maritime University, Pusan. 2 Formally Visiting Scientist, Florida Atlantic University. each mix design increased with increasing time-to-corrosion with data for all mixes conforming to a common concentration-time trend. Macro-voids that impinged upon the upper side of the top reinforcing bar facilitated corrosion initiation and may have contributed to the distributed nature of the time-to-corrosion and chloride threshold concentration data. INTRODUCTION Electrolyte composition is invariably a dominant factor affecting any corrosion process, including that for steel in concrete. Thus, the most prevalent corrosion activator in concrete pore water is normally chlorides, whereas hydroxides serve as a passivator. Consequently, time-to-corrosion, T,., for concrete structures is determined by the competing influences of these two species. On the one hand, pore water pH is affected by cement content, cement alkalinity, and exposures that promote carbonation. On the other, chloride concentration, [CI-], at the steel depth is determined by 1) composition of this species in the environment, 2) its ingress rate into the concrete, and 3) concrete cover over the steel, with onset of active corrosion being defined by a CI- threshold concentration for passive film breakdown, cth. Corrosion experiments intended to simulate steel in concrete have historically employed a saturated Ca(OH)2 solution, the pH of which is approximately 12.4. However, with the advent of pore water expression (1-3) and theoretical considerations, it was recognized that K + and Na + are the predominant cations; and the solubility and concentration of these is such that a pH in excess of 13 typically occurs. The mechanism of CI-intrusion into concrete invariably involves both capillary suction and diffusion; however, for situations where the depth to which the former (capillary suction) occurs is relatively shallow compared to the reinforcement cover, diffusion alone is normally assumed. Analysis of the latter (diffusion) is accomplished in terms of the solution to Fick's second law which for a one-dimensional system is, Equation 1 Where c(x,T) is the chloride concentration at depth x after time T, Co is the initial or background [CI-] in the concrete, cs is the [CI-] at the exposed surface, and Derf is the effective diffusion coefficient. Assumptions involved in arriving at this solution are, first, cs and Deff are constant with time and, second, the diffusion is "Fickian;" that is, there are no CI- so
ABSTRACT The article presents an example of a life-cycle cost analysis for reinforced concrete bridge decks based on the selected deterioration model. The calculations consider different construction/repair/rehabilitation options and several maintenance scenarios and yield annual cash outflows over the period of the deck?s service life for both direct and indirect (user costs). The analysis shows that indirect costs can exceed the direct costs by a factor 10 or more. Another significant conclusion is that at prevailing interest rates more corrosion resistant rebar designs are favored over the conventional ones, as the former minimizes corrosion-related deterioration and reduces the maintenance and repair expense. The complete version of the analysis, including an example of calculating the life-cycle costs, can be found in the ?Highway Bridges? sector of the report on the Corrosion Costs and Preventive Strategies in the United States, available at www.corrosioncost.com. INTRODUCTION When it comes to designing a reinforced-concrete bridge, bridge engineers have a variety of options to achieve the service requirements. There is a general understanding that cycle comparing the options on the initial cost basis is not a good predictor of life -cycle costs, i.e., corrosion maintenance costs are also important. Past economic analyses have treated lifecosting in different ways, with only a few estimating indirect costs, such as user costs associated with disruption caused by deteriorating deck surfaces and maintenance, repair, and replacement. (see References 1, 2, 3, and 4.) For a bridge that carries a high volume of traffic, indirect (user) costs can be substantially larger than materials and labor costs for bridge repair/rehabilitation. This means that to capture the total economic impact of the project, the analysis must include these indirect costs. The best way to compare bridges with different rebar materials and different corrosion maintenance practices is on the basis of annualized value (AV), which represents discounted cash outflows related to both the construction/maintenance costs and user costs associated with these activities. The following sections demonstrate this approach using direct and indirect cost calculations for several bridge deck designs [different rebar materials: black steel rebar, epoxy-coated rebar and stainless steel rebar]. The analysis focuses on decks rather than other bridge elements because the corrosion-related problems are most obvious on the deck, the most visible part of the bridge. In addition, only new construction alternatives are examined. Even for new construction, several alternatives such as inhibitors or high-performance silica fume concrete were not examined. Also not examined were several rehabilitation options, such as CP and electrochemical chloride removal. This does not mean that these options are not viable; certainly CP has proven to be success in mitigating ongoing corrosion on bridge decks and is an effective long-term rehabilitation alternative to replacement. The life-cycle costs given here are an example of how life-cycle costing can be accomplished. The values used in this example, while being realistic estimates and originating from referenced sources, are meant to illustrate the relative magnitude of the components of the total economic impact of bridge construction and maintenance. The readers are encouraged to view this data as an example and are encouraged to input their own data based on their experience to evaluate life-cycle costs. The complete version of the analysis, including an example of calculating the life-cycle costs, can be found in the ?Highway Bridges? sector of the report on the Corrosion Costs and Preve
- Construction & Engineering (1.00)
- Materials > Construction Materials (0.90)
- Materials > Metals & Mining > Steel (0.38)
- Management > Asset and Portfolio Management (0.89)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (0.86)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (0.86)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (0.68)
ABSTRACT Plain carbon steel reinforcing bars (rebar) were embedded in concrete blocks of varying waterto- cement ratios (w/c) and exposed to chlorides. A fraction of these blocks had sodium chloride added as an admixture, with all of the blocks subjected to cyclical ponding with a saturated solution of sodium chloride. Electrochemical chloride extraction (ECE) was then used to remove the chlorides from these blocks while making electrical measurements in the different layers between the anode and cathode. It was revealed that the resistance of the concrete surface layer increases during ECE, inevitably restricting current flow, while the resistance of the underlying concrete decreases or remains constant. A surface residue was observed on the concrete following ECE. Analyses of the residue revealed that it contains calcium carbonate, calcium chloride, and other yet unidentified minor components. INTRODUCTION Emerging from Kansas Department of Transportation (KDOT) experiments on the electrostabilization of clayey soils, ECE provides a means of removing chloride ions without requiring the excavation of the otherwise structurally sound concrete.[1] In addition, the alkalinity adjacent to the rebar is increased during the ECE process. Despite ECE?s documented successes in chloride removal and realkalization, it has been established that the amount of current passing through the concrete, and therefore the quantity of chlorides removed, diminishes as the treatment progresses.[2-5] Previous research has focused on the movement of ions within concrete?s porous network. Although the work by Christensen, et al., was performed on samples that did not undergo ECE, it suggested that the binding of ionic species and the increase in alkalinity during the hydration process has a strong influence on the conductivity.[6] During the early stages of hydration of a chloride free cement (¡Ü100hr.), conductivity is dominated by Na+, K+, Ca2+, OH-, and SO4 -2. As the hydration process continues, Christensen, et al., found that only Na+, K+, and OH- contribute significantly to the conductivity in these samples.[6] In chloride-contaminated concrete, it is known that the chloride ions contribute to the conductivity and thus ECE is possible. With Equation 1 and using the ionic conductivity values given in Table 1, Banfill calculated the transference values for a mixture containing 0.5 mol/liter sodium hydroxide and 0.5 mol/liter sodium chloride, which are given in Table 2.[7] Based on these values, it was concluded that the current flow from the reinforcing steel toward the anode was composed of 72% hydroxyl ions and 28% chloride ions during electromigration.[7] Equation (1) The calculation by Banfill assumed that for every 96,500 coulombs of charge passed, one mole or 35.5 g of chloride ions successfully migrated to the anode.[7] However, after calculating the efficiency of an early Strategic Highway Research Program study (6.9-7.8%), Banfill concluded that other negative ions (i.e. OH-, SO4 -2) and resistive heat generation must account for the efficiency loss.[7] Tritthart demonstrated that the concentration of hydroxyl ions in concrete increased during ECE, and suggested that during ECE the hydroxyl ion influences the rate of removal of the chloride ion.[8] This is because as hydroxyl ions are being produced at the cathode and migrating towards the anode during ECE, these ions will compete with chloride ions as charge carriers. Therefore, based on the calculations by Banfill and the results from Tritthart, it is not surprising that the efficiency of chloride removal would decrease as the treatment progressed.[7, 8] Chatterji suggested that although free chloride ions can participate in el
- North America > United States > Virginia (0.29)
- North America > United States > Texas (0.28)
- North America > United States > Kansas (0.24)
- Materials > Chemicals (1.00)
- Materials > Metals & Mining > Steel (0.34)
ABSTRACT A brief history of the development of the rotating cage as a promising and reliable laboratory methodology for inhibitor evaluation is presented. The influence of the geometry of the rotating cage on the flow pattern as well as on corrosion rates has been investigated. The importance of vessel length and diameter, rotating cage length (and, as a consequence, the sample length), rotating cage diameter, rotation speed, volume of liquid, and flow pattern in determining the corrosion rates, and hence, inhibitor efficiency has been established. INTRODUCTION In 1990, the rotating cage was introduced as a promising laboratory methodology, to simulate pipe flow in the laboratory by rotating the specimens at speeds up to 1500 rpm ~4. In the literature, rotating cage experiments are also reported as high-speed autoclave tests (HSAT) 6'7 or rotating probe 4'5 experiments. In 1999, the atmospheric pressure rotating cage was described, together with a systematic analysis of flow patterns 8'9. Depending on the rotation speeds and liquid level, flow is classified in one of four categories, homogeneous, side-wall affected, top-cover affected, and turbulent. In the homogeneous zone, the vortex dimensions (length and width) increase with rotation speed. Equation (1) is used to determine the wall shear stress in the homogeneous zone. EQUATION (1) where z is the wall shear stress; Re is the Reynolds number; p is the density of the fluid; r is the radius of the rotating cage; and co angular velocity. In the side-wall affected zone and top-cover affected zone, the wall shear stress is less than that calculated by Eqn. 1, due to the movement of fluids in the opposite direction. In the turbulent zone, on the other hand, the wall shear stress may be higher than that calculated by Eqn. 1, due to the penetration of the vortex through the cage. In the results of a study published in 2001, the rotating cage was identified as the preferred methodology for evaluating corrosion inhibitors 1°. This assessment was based on a quantitative comparison of field and laboratory data on general corrosion rates, pitting corrosion rates, and percentage inhibition (calculated from general and pitting corrosion rates) under three different field conditions using three continuous and three batch inhibitors. The rotating cage methodology was also identified as an inexpensive and relatively simple methodology to carry out. In 2001, ASTM published a new standard, ASTM G 170-0 la, "Evaluating and Qualifying Oilfield and Refinery Corrosion Inhibitors in the Laboratory". This ASTM Standard describes three methodologies to evaluate the efficiency of inhibitors in the laboratory: Rotating Cylinder Electrode (RCE), Rotating Cage (RC), and Jet Impingement (JI). They are compact, inexpensive, hydrodynamically characterized, and scalable, i.e., they can be carried out under various flow conditions. Using these methodologies, several variables that influence inhibitor performance in the field can be simulated, including composition (of the steel, brine, oil, and gas); temperature; pressure; and flow. In 2002, the application of the atmospheric pressure rotating cage for simultaneous determination of inhibitor efficiency and drag reduction properties of chemicals was demonstrated ~5. In this study, the experiments to determine the inhibitor efficiency were carried out at 140 F (60°C) and 1,725 kPa (250 psi) (20% HzS , 2.5% CO2, balance argon), with a rotation speed of 500 rpm. On the other hand, the experiments for determining drag reduction properties were carried out at atmospheric pressure. The experimental setup was the same as that for experiments at elevated pressure except that, for ex
- North America > Canada (0.68)
- North America > United States > Texas (0.16)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
ABSTRACT The kraft papermaking process, which is by far the predominant method used in North America, depends on recovery of the chemicals used in the digesting or pulping operation. Currently, the black liquor recovery boiler is the heart of this recovery process, but replacement of this boiler with a gasification system is being intensively studied. A switch to gasification can be justified based on capital investment, energy efficiency, and safety, but there are a number of significant obstacles to its successful development and implementation. Identification of suitable materials for the containment of the reaction and the reaction products is considered one of the most critical issues. There are currently two distinct gasification processes being actively developed; a low temperature process in which the alkali salts are kept below their melting point, and a high temperature process in which these compounds are handled in the molten state. Each process has unique materials requirements and issues that are described in this paper. In addition, results from recent studies of the compatibility of gasifier components with the respective environments are presented. INTRODUCTION AND BACKGROUND Papermaking by the kraft process involves treatment of wood chips in the dissolver vessel with a steam-sodium sulfide-sodium hydroxide mixture to separate the cellulose fibers from the lignin that binds them together. The streams exiting the digester vessel include a fiber-rich stream that is further treated to provide the fibers that are used to form paper or other cellulose-based product. The other stream is identified as black liquor, which is an aqueous solution containing the waste organic material including the lignin as well as the spent pulping chemicals that are primarily sodium carbonate and sodium sulfate. Other inorganic components include salts containing potassium and chloride. In order to regenerate the pulping chemicals as well as recover some of the heating value contained in the organic components, the black liquor stream is burned in the black liquor recovery boiler. Concentrated black liquor is injected into this boiler where the water is evaporated, the organic materials are burned to produce heat and steam, and the inorganic components are recovered in the bottom of the boiler in a partially reduced state, primarily as sodium carbonate and sodium sulfide. After an additional processing step, the sodium sulfide-sodium hydroxide white liquor is completely regenerated and is ready for reuse in the pulping process. A diagram highlighting the steps in papermaking and chemical recovery is shown in Fig. 1. Recovery boilers have been used very successfully for many years, but there are a number of shortcomings. First, as the most expensive component in a paper mill, the boiler is very capital intensive. In addition, the boiler is relatively inefficient in producing electric power from black liquor. Besides these economic issues, there is a safety issue because of the potential for recovery boiler explosions if the pressurized water contained in the tubes that form a boiler's walls, floor and ceiling escapes through a leak and contacts the bed of molten salt (referred to as smelt in the paper industry) that is present on the floor of the boiler. Contact of this hot water with the molten smelt could result in a violent explosion that can severely damage a boiler and cause serious injuries to personnel. Recovery boiler explosions in the U.S. have been fairly infrequent, but during the first eight months of 2002, there were two boiler explosions in the U.S. with one fatality and several injuries. The U.S. Department of Energy (DOE) is providing support for a number of projects that have
- Materials > Paper & Forest Products (1.00)
- Materials > Chemicals (1.00)
- Energy > Oil & Gas > Upstream (0.94)
- Government > Regional Government > North America Government > United States Government (0.69)
INTRODUCTION
ABSTRACT Experiments in a large-scale flow loop provide an example of a somewhat increased corrosion rate under multiphase flow conditions caused by a low concentration of H2S in a CO2 saturated environment. Under operating conditions that prevent scale formation, the addition of 3ppm H2S in the gas phase increased the corrosion rate of AISI C 1018 and API 5L X-65 carbon steels, while the addition of H2S concentrations of 15ppm and greater tended to retard corrosion. The same effect was not found in single-phase flow. A vapor-liquid equilibrium model for dilute aqueous solutions of H2S/CO2 was developed to provide a tool for estimating water chemistry of a closed system such as a flow loop or an autoclave.
The simultaneous presence of CO2 and H2S in produced fluids makes for a very aggressive environment, which can lead to severe corrosion of mild steel. H2S and CO2 have been shown to produce competing films at temperatures between 20 and 60ºC1. When hydrogen sulfide is present in low concentrations in a CO2 dominated system, some have noted that the iron sulfide (FeS) film interferes with the formation of the iron carbonate scale (FeCO3). However, the iron sulfide film is considered to have a protective effect at about 60°C1. Although a protective film forms in the presence of H2S, observations by Videm and Kvarekval2 demonstrated that small amounts of H2S increased the corrosion rate because the film could easily be disturbed by surface defects and the attack did not produce uniform corrosion. This is of interest because the iron sulfide film is more easily removed from the pipe wall than the iron carbonate scale. Under turbulent conditions (i.e. slug flow), removal of the protective scale can occur and lead to an increased corrosion rate and possibly pitting corrosion.
There are a limited number of studies that cover H2S corrosion, particularly when compared to the extensive literature available on sweet CO2 corrosion. The few experimental studies that have been published in open literature2,3,4,5 are limited to autoclaves and glass cells. Previous research has shown that low concentrations of H2S (<30 ppm) in a CO2 saturated water solution can accelerate the corrosion rate. The effect seems to vanish at higher H2S concentrations and high temperatures3,5 (>80oC) when a protective film forms. In one study it was also suggested that the effect of H2S could be significant only in the low pH range4 (
- North America > United States > Texas (0.28)
- Europe > Slovenia > Dobrepolje > Videm (0.24)