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INTRODUCTION ABSTRACT Duplex stainless steels (DSS) may experience environmental assisted cracking (EAC) under certain conditions in the kraft white liquor solutions used by the pulp and paper industry. Previous work showed that hydrogen absorption is possible in these solutions under applied cathodic potentials (CP) at room temperature, which is a concern because DSSs may become susceptible to hydrogen embrittlement (HE) under certain conditions; however, the effect of a CP on the EAC behavior of DSS in kraft white liquor solutions is currently unknown. Microhardness was used to evaluate the change in hardness upon exposure to white liquor (WL) composed of 150 g/L NaOH and 50 g/L Na2S at room temperature under CP. The hydrogen microprint technique (HMT) was used to observe the hydrogen distribution on the sample surface. Slow strain rate testing (SSRT) was conducted on grade 2205 DSS at a strain rate of 2 X 10 s in the range of -1500 mV vs. SCE to the corrosion potential (E corr) at room temperature. The affect of CP on the EAC susceptibility was evaluated as a function of the reduction in area and the time to failure. The fracture morphology was examined with scanning electron microscopy (SEM). The results indicate that an increase in the CP will enhance the severity of EAC below a threshold potential, which was found to be ~-1100 mVSCE. These findings suggest that care must be taken when DSS are used in pulping applications where there is a risk for galvanic coupling with a more active material, such as carbon or low alloy steel, or in plant equipment where cathodic protection is used. Duplex austenitic-ferritic stainless steels (DSSs) are widely used in chemical processing industries that utilize sulfide-containing, caustic solutions. The most common of these industrial processes include the modified Bayer processes used in alumina ore processing, the Girdler-sulfide (GS) process used in heavy water production, hydrocarbon processing for the treatment of acidic impurities such as hydrogen sulfide and mercaptans, and various process streams in pulp mills utilizing the kraft pulping process. DSSs have generally provided superior mechanical properties and corrosion resistance as compared to conventional austenitic stainless steels in these environments at room temperature and elevated temperatures; however, field and laboratory experience has shown that DSSs may experience EAC and general corrosion under certain conditions. Hydrogen embrittlement (HE) is a form of EAC that is particularly insidious, owing to the often sudden and unpredictable nature of failure. While there has been a modest amount of research on the susceptibility of DSSs to HE in acidic and neutral pH environments, there has been limited work in higher pH, caustic environments. Prior work showed that the threshold concentration of hydrogen required for HE in a 2304 DSS (23Cr-5Ni-3Mo) in a 0.1M NaOH solution under applied CP was only a few ppm of hydrogen, therefore even limited hydrogen activity could promote cracking. DSSs may become susceptible to HE where a high hydrogen fugacity is encountered on the metal surface.
- Geology > Mineral > Sulfide (0.76)
- Geology > Geological Subdiscipline > Geomechanics (0.49)
- Materials > Paper & Forest Products (1.00)
- Materials > Metals & Mining > Steel (1.00)
- Materials > Chemicals (1.00)
Influence Of Fatigue Pre-Loading Level On Hydrogen-Assisted Micro-Damage In Eutectoid Steel
Toribio, Jesús (Department of Materials Engineering University of Salamanca) | Lorenzo, Miguel (Department of Mechanical Engineering University of Salamanca) | Vergara, Diego (Department of Mechanical Engineering University of Salamanca)
ABSTRACT A fracture mechanics approach to hydrogen-assisted micro-damage in eutectoid steel is presented. Fractographic analysis revealed micromechanical effects of hydrogen in the form of tearing topography surface (TTS). The progress of this micro-damage is modeled as a macroscopic crack that extends the original fatigue pre-crack and involves linear elastic fracture mechanics principles. In this case, the change from hydrogen-assisted micro-damage (TTS) to cleavage-like topography takes place when a critical stress intensity factor (KH) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue pre-crack plus the TTS area). The present study analyzes the effect of the TTS zone aspect ratio on KH. 1. INTRODUCTION Previous research on hydrogen-induced fracture of pre-cracked and notched samples of high-strength pearlitic steel demonstrated the existence of a non conventional microscopic fracture mode: tearing topography surface (TTS), which can be associated with hydrogen-assisted micro-damage [1]. The TTS mode appears in fracture tests on pre-cracked and notched specimens when tested under hydrogen charging. Experimental results [2] showed phenomenological relations between the size of the TTS region and variables such as the electrochemical potential and the maximum stress intensity factor during fatigue pre-cracking (for cracked samples), or the time to failure and the geometry (for notched samples). A hydrogen diffusion model was proposed to explain these relations [3]. In this paper a fracture mechanics approach to hydrogen-assisted micro-damage in eutectoid steel is presented. The TTS area can be modelled as a macroscopic crack that extends the original fatigue pre-crack and involves linear elastic fracture mechanics principles. In this case, the change from TTS to cleavage takes place when a critical stress intensity factor (KH) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue pre-crack plus the TTS area). It is shown that the value KH depends on the fatigue pre-cracking regime and its value may be associated with a characteristic level of stress intensity factor in the crack growth kinetics curve. 2. HYDROGEN EMBRITTLEMENT TESTS The analysis is based on experimental results [2] of hydrogen embrittlement tests on pre-cracked cylindrical samples of eutectoid pearlitic steel (0.74% C, 0.70% Mn, 0.20% Si, 0.016% P, 0.023% S, 0.01% Cr, 0.01% Ni and 0.001% Mo) whose mechanical properties appear in Table I [4]. Slow strain rate testing—at very low strain rate—was conducted under simultaneous hydrogen charging by cathodic polarization in aqueous solution [2]. The main results are reproduced in another article [4] where it is shown phenomenological relations between the fracture load in hydrogen FH (divided by its reference value in air FO) and testing variables of an electrochemical nature (pH and potential E) and mechanical character (KmaxKO): (Equation in full paper) With regard to the TTS zone, Figure 1 shows the scheme of the three regions detected in a fracture surface of pre-cracked specimens tested under simultaneous hydrogen charging [2,4]: the fatigue pre-crack, a transition topography consisting of hydrogen-assisted micro-damage (TTS).
- Research Report > New Finding (0.68)
- Research Report > Experimental Study (0.68)
Role Of Drawing-Induced Residual Stresses And Strains On Hydrogen Embrittlement Of Cold Drawn Wire
Toribio, Jesús (Department of Materials Engineering University of Salamanca) | Vergara, Diego (Department of Mechanical Engineering University of Salamanca) | Lorenzo, Miguel (Department of Mechanical Engineering University of Salamanca) | Kharin, Viktor (Department of Materials Engineering University of Salamanca)
INTRODUCTION ABSTRACT Hydrogen Induced Fracture (HIF) plays an important role in the environmental cracking of cold drawn prestressings steel wires. The Standard Test in Ammonium Thiocyanate was proposed by the International Federation of Prestressing (FIP) as a suitable experimental method for checking the susceptibility of high-strength prestressing steels to hydrogen embrittlement. However, the FIP tests usually exhibit a high scattering of the results. It can be caused by the distribution of residual stresses and strains generated in the vicinity of the wire surface during the manufacturing (cold drawing) process. To this end, the knowledge of residual stresses and plastic strains in wires due to cold-drawing, as well as of wires hydrogenation from harsh environments, are the key to successful predictions of wire lives. Thus, the aim of this work is to improve previous analyses of HIF in cold-drawn prestressing wires via numerical modelling, first, comparing the distributions of residual stresses and plastic strains due to different real drawing processes, and next, analyzing the stress-strain assisted hydrogen diffusion in wires towards creation of the conditions for HIF nucleation. In order to achieve this goal, two industrial cold-drawing processes are analyzed. Basically the main differences between both are (i) the reduction of cross-sectional area performed at first drawing stage and (ii) the number of drawing steps used in the whole process. Generated results prove the importance of an adequate design of the cold drawing process with regard to the residual stress-and-strain field and its relevant role in hydrogen diffusion in the wires, as well as its possible consequences for HIF. Steel wires are used for concrete structures prestressing, which is a powerful civil engineering technique aiming to introduce desirable stresses and counterbalance undesirable ones so that the combination of prestressing and service loading produces stresses within specified limits [1]. Prestressing wires are usually made of eutectoid pearlitic steel heavily cold drawn to create elevated tensile properties. These wires, once subjected to high service stresses, remain such forever, usually in hostile environments, e.g., due to atmospheric humidity. All that makes them susceptible to surface cracking, in particular of the stress corrosion origin. Environmentally assisted fracture of prestressing steels has been the subject of extensive studies which substantiated the importance of hydrogen induced fracture (HIF), or hydrogen embrittlement (HE), in deterioration of structures [2]. The Ammonium Thiocyanate Test (ATT) adopted by the International Federation for Prestressing (FIP) is considered well suited for steel control and acceptance [3]. However, despite being in use as a standard test method, it reveals neither how HIF goes on in prestressing wires nor the roles of various manufacturing and service factors. To this end, contributions to a better interpretation of the ATT and understanding of HIF advancement in prestressing steel with account for substantial influencing factors are desired to gain an improvement of wires performance in hostile environments. With regard to manufacturing factors affecting the strength and life of prestressing wires, apart of the properties of material per se, the issue of residual stresses is essential [4,5].
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.48)
INTRODUCTION The Pipe System ABSTRACT High strength carbon steels are being used as the external reinforcement for a novel composite pipeline. This system combines a thin wall corrosion resistant alloy with the reinforcement to produce a high pressure pipeline which is manufactured on site giving a lower cost alternative to conventional thick-wall pipelines. Despite the widely known susceptibility of high strength ferritic-bainitic carbon steels to hydrogen embrittlement, there is little published data on the hydrogen embrittlement behaviour of martensitic carbon steels. Therefore, a series of tests was performed to establish the risk of cracking in a buried condition where cathodic protection is applied. Tests were performed to establish the plateau hydrogen content and the hydrogen flux under various charging conditions, and then a series of slow strain rate and constant load tests performed under polarised conditions to establish any reduction in ductility over samples tested in air. Microscopy analysis of the fracture surfaces was performed to identify any change in the fracture morphology. A stressed full ring test of a sample of the composite pipeline was also tested under simulated cathodic protection conditions. The results of this testing are discussed and the implications for the use of this novel composite pipeline system evaluated. The composite pipe system, known as X200 COMPOSITE PIPELINE CONSTRUCTION , is a steel strip laminate technology which uses high-performance adhesives to manufacture a metallic composite pipe. It offers a new method of low cost pipeline construction suitable for onshore gas and oil pipelines in a variety of configurations. The pipe is based on a thin wall liner that provides the fluid containment. The liner is formed from strip that is folded and welded to form a tube. This strip will vary according to service requirements; the current design is based on 316L stainless steel. The grade of steel or stainless steel or even reinforced thermoplastic can be user-defined to suit a given application. Fusion bonded epoxy (FBE) coated martensitic ultra-high strength steel strips are then pre-formed and helically wound around the liner to form a laminated high strength reinforcing layer providing the pipe's hoop strength. These are bonded using an adhesive. An image of the pipe construction is given in Figure 1. (Figure in full paper) The pipe may then be further coated with any suitable pipeline coating material, as required by a service condition. The combined system is considerably thinner and lighter than conventional linepipe products because of the beneficial strength-to-weight ratio. Unlike conventional linepipe that is manufactured in a pipe mill far from the construction site, this lightweight composite pipe can be produced at the construction facility using a portable manufacturing line. All components of the manufacturing process fit within standard ISO containers, each complete container weighing between 5 and 20 tonnes. This allows for easy transportation via truck, and handling or shipping (Figure 2). (Figure in full paper) (During Factory Acceptance Testing) CONTAINERISED COMPOSITE PIPELINE MANUFACTURING SYSTEM When buried, the martensitic steel strips are protected from corrosion by the FBE.
INTRODUCTION ABSTRACT Crack propagation in the presence of sour gas is the conjunction of electrochemical reactions and metallurgical processes that increase hydrogen absorption at the crack tip and therefore produces embrittlement. To evaluate Oil Country Tubular Goods (OCTG) susceptibility to Sulfide Stress Cracking (SSC) in wet H2S environments NACE International adopted different testing methods. Particularly, Method D is used for design/fitness-for-service, material qualification and specification purposes. The material resistance to crack propagation in an aggressive environment is expressed in terms of a critical stress intensity factor, KISSC. Although this test is standardized, it is widely known that several variables can affect the obtained KISSC values: geometry of the specimen, initial applied intensity factor as well as the electrochemical (pH, temperature, H2S content) and metallurgical variables that modify the amount of absorbed hydrogen. In this work basic research regarding hydrogen insertion and transport in NACE solutions is used to explain KISSC of 1Cr-0.7Mo steel tested in different H/H2S environments. Experimental evidence relating KISSC to the concentration of absorbed hydrogen is provided. Concentration of absorbed hydrogen is related to proton and hydrogen sulfide concentrations as well as dislocation density and stress. A mechanism of hydrogen insertion in the presence of hydrogen sulfide is also presented. Hydrogen embrittlement produced by sour gas is a critical issue for Oil Country Tubular Goods (OCTG) products because it causes failure of these components at load levels much lower than the one determined by classical stress design. The detrimental effect of hydrogen in material toughness causes the growth of small cracks under service, producing premature failure of OCTG products. Understanding the phenomena is of critical importance to improve the design of the steels used in such aggressive environments. NACE International adopted different test methods to evaluate material susceptibility to SSC in wet H2S environments. In particular Method D expressed the material resistance to crack propagation in sour environments in terms of a critical stress intensity factor, KISSC. This factor is the resultant of several processes including transport of the deleterious specie to the crack tip, reactions of this specie with the newly created surfaces to produce hydrogen, diffusion of the formed hydrogen to the susceptible region ahead of the crack tip and finally achieving the critical hydrogen concentration that initiates crack propagation. Thus, crack growth response reflects a complex interplay among: a) mechanical variables which modify the driving force for crack growth such as arm displacement , specimen geometry ; b) metallurgical feactures ; c) the micro-mechanisms that controls crack growth process; and d) the mechanism of the relevant chemical reactions that modify hydrogen generation at the crack tip. All the mentioned variables affect the total amount of absorbed hydrogen at the crack tip and therefore modify cracking resistance. Though the dependence of KISSC with absorbed hydrogen is qualitatively known only few studies attempted to provide a mechanistic approach of such effect. Due to complexity of KISSC test, a multi disciplinary approach integrating chemistry.
- North America > United States > Texas (0.28)
- North America > United States > Virginia (0.28)
- Geology > Mineral > Sulfide (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.68)
INTRODUCTION ABSTRACT Zirconium is one of a very few materials of construction for processes involving high concentrations of sulfuric acid at temperatures above the atmospheric boiling point. In this severe environment only zirconium or tantalum can be used with any reliability. Most corrosion data for zirconium has been developed using welded wrought material as test coupons. The majority of this information is related to the corrosion resistance of Zr 702 with very little information available for Zr 705. In addition, almost no information is available as to the corrosion resistance of the cast forms of either alloy. This paper presents laboratory data comparing the corrosion resistance of Zr 702 and Zr 705 in the welded and wrought forms and in the cast form in sulfuric acid. A newly expanded iso-corrosion diagram is presented for both wrought and cast Zr 702 and Zr 705. Zr 705 is a zirconium alloy containing 2.5% Niobium. Because of the possibility of hydrogen absorption by the Niobium phase during the corrosion process, hydrogen absorption data is also presented for Zr 705. This hydrogen absorption data is compared with Zr 702 tested under the same conditions. Frequently users consider Zr 702 and Zr 705 as interchangeable in regards to corrosion. In some cases this is true, but not for sulfuric acid applications. For many applications in the chemical processing industry (CPI) involving sulfuric acid, zirconium is the material of choice. This is especially true in concentrated sulfuric acid and when the operating temperature is above the atmospheric boiling point. Although Zr 702 is the most corrosion resistant of the zirconium alloys for this application, Zr 705 can be used in the form of cast pump parts and for fasteners of various types. There are, however, many differences between the two alloys. Zr 702 is generally more corrosion resistant than Zr 705 but Zr 705 has considerably higher strength and is more formable than Zr 702. Both alloys can be produced into large castings by the rammed graphite casting method . Cast zirconium has similar metallographic structure to that found in the welded wrought material. In both cases, the corrosion resistance of the cast material is somewhat lower than the wrought material especially at higher concentrations and temperatures in sulfuric acid. In this paper, all corrosion and mechanical data, including Table 4, used Zircadyne® zirconium. PHYSICAL AND MECHANICAL DIFFERENCES CHEMICAL COMPOSITION DIFFERENCES (Table in full paper) Zr 702C and Zr 705C cast zirconium is normally produced according to ASTM B752 “Standard Specification for Castings, Zirconium-Base, Corrosion Resistant, for General Application”. Zirconium sheet and plate is normally produced according to ASTM B751 “Standard Specification for Zirconium and Zirconium Alloy Strip, Sheet, and Plate”. Table 1 gives the chemical requirements as specified in these ASTM standards. Table 2 gives the physical and mechanical differences between these two zirconium alloys . Notice that the Zr 705 is actually a two phase alloy as opposed to Zr 702 which, for practical purposes, has a single phase.
INTRODUCTION ABSTRACT Welded API 5L Gr X65 is being used in flow line applications, the flow lines are laid by J laying, S laying, or by a reeling operation. The influence of the reeling on the corrosion fatigue performance of welded X65 pipe has not been well documented. This work is aimed at quantifying the effect of reeling on the corrosion fatigue performance of welded X65 pipe in air as well as sour environments. Fatigue crack growth measurements were performed on SENB specimens to determine the DK vs da/dN curves in air with the fatigue crack propagating through the thickness of the pipe. Parent pipe material, heat affected zone (HAZ) and weld center line were tested in air. Frequency scanning experiments from 1Hz to 1mHz were performed on Single Edge Notch Beam (SENB). Specimens of the HAZ and weld centerline were tested in sour environment to determine the knockdown factor associated with the sour environment. The results of the test program will be presented along with a discussion on the effects of reeling on the fatigue performance of welded pipe in air and sour environments. Reel lay techniques are used for installation of rigid pipelines from 101mm to 457mm (4 to 18 inch) diameter in a wide range of water depths. The significant benefit of the reel-lay method is that it allows a significant if not all of the pipeline fabrication activity to be performed onshore in a controlled environment, well ahead of the critical offshore phase of any pipeline installation project. This ultimately leads to a higher quality and a more cost effective installation solution. C-Mn steels, when subjected to reeling operations can experience a decrease in the mechanical properties. The effect of sour environments on the fatigue and fracture performance of welded pipe which is reeled is of interest because of the potential impact it could have of the knock down factors that will be used for an Engineering Criticality Analysis (ECA), which would in turn dictate the dimensions of tolerable flaw sizes. There is extensive data that has explored the effects of various environmental and certain material parameters on the corrosion fatigue performance of steels in sour environments. The effect of H2S on steels is an increase in the fatigue crack growth rate (FCGR). Typically at high ?K values the FCGR can be as high as 50x the values in air. FCGR studies in a range of environments indicate that FCGR increases with increasing H2S concentration. The effect of decreasing ?K leads to a lower increase in the FCGR compared to the values in air. In fact the threshold limit is not strongly influenced by the specific environment. Another important effect in sour environments on FCGR is the effect of cycle frequency; usually it is found that with decreasing frequency the FCGR increases and reaches a plateau. This is most likely due to the enhanced time for diffusion of hydrogen between cycles, which leads to an increase in hydrogen in the crack tip plastic zone.
INTRODUCTION ABSTRACT This paper provides a concise review of 'hydrogen promoters', aqueous corrodants known to generate hydrogen flux through mild steel at low temperatures. A new theory of hydrogen promotion will be presented. Hydrogen promoters will be ranked according to their severity. The role of temperature, scale, surface coatings and surface condition will also be considered. The paper should provide useful reference material for those interested in field and laboratory measurements of hydrogen flux, and the relation between the two. Certain non-metal hydrides are known to be strong hydrogen promoters . These cause enhanced entry of hydrogen into corroding metal, in particular carbon steel, and subsequent hydrogen flux through the metal which is orders of magnitude higher than the flux generated by a comparably corrosive, non- hydrogen promoting corrodants under the same conditions. Hydrogen is formed on metals, as a result of impressed or corrosive cathodic reduction of aqueous protons: (Equation in full paper) In 1969 Newman and Shreir presented authoritative data on the relative strength of certain strong hydrogen promoters identified by Smialowski et al. and found them to be active as the hydrides H2S, H2Se, H2Te, PH= and AsH, a conclusion supported by Bockris. The chief industrial interest in promoters then, as now, was the risk of hydrogen embrittlement and damage caused to steel by hydrogen entering steel at very high activities due to the very powerful promoter hydrogen sulfide. Of course, much has since been done to develop corrosion inhibitors which prevent sour corrosion, and to manufacture steels and welds which are not susceptible to sour corrosion induced hydrogen damage. However, as oil is produced from older and deeper reservoirs, so, it appears, the sulfur content of produced crude has increased, leading to more severe sour corrosive service environments that have to be faced in oil production facilities and refineries. In the case of refineries, the effect is more severe as equipment was not necessarily designed to cope with severe sour corrosion. In recent years there has come to light much more data on promoter action, including data for hydrogen fluoride, which provides a vital clue as to the mechanism of hydrogen promotion. By considering old and contemporary data, a mechanism for promoter action can be developed, which fits the facts much better than the conventional theory of promoters acting to 'poison the hydrogen association reaction' (ii). Moreover, with contemporary measurements of hydrogen flux, it is possible to now offer a universal index of promoter strength. THE MECHANISM OF HYDROGEN PROMOTION Recent studies agree as to the very weak dependency of hydrogen entry on the concentration of a promoter , several authors concluding that steady state hydrogen flux, J8 varies as [H2S]. It therefore seems likely that the promoter acts catalytically at the metal surface to facilitate hydrogen entry. The chemical elements associated with hydrogen promotion are identified in Figure 1. The broader chemistry of hydrogen with the chemical elements is also shown.
- 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)
Quantitative Measurement Of Hydrogen Diffusion And Trapping Parameters For Linepipe Steel Under Constant Load In Sour Environment
Park, Gyu Tae (Graduate Institute of Ferrous Technology, Pohang University of Science and Technology) | Kim, Sung Jin (Graduate Institute of Ferrous Technology, Pohang University of Science and Technology) | Kim, Kyoo Young (Graduate Institute of Ferrous Technology, Pohang University of Science and Technology)
INTRODUCTION ABSTRACT There have been many attempts to measure the diffusivity and permeability of hydrogen in the steel under tensile straining condition using the electrochemical hydrogen permeation technique (HPT). However, the different experimental conditions relating to strain rate and applied electrochemical parameters often lead to unrealistic data. In this work API X65 grade linepipe steel having a high sulfide stress cracking (SSC) resistance was used and most experimental conditions were determined according to the ISO17081 standard. NACE-A solution was selected as the hydrogen charging solution according to the NACE procedure TM0177A. Applied stresses were smaller than threshold stress to avoid failure of the specimen. In this study, the permeability, apparent diffusivity, apparent solubility of diffusible hydrogen and the amount of irreversibly trapped hydrogen was measured by a modified HPT, and were are correlated with the SSC resistance to investigate the role of hydrogen in causing SSC. When high strength low alloy (HSLA) steel is under the combined condition of both tensile stress and corrosion in an aqueous environment containing hydrogen sulfide (H2S) gas, cracking failure identifiable as SSC occurs even though the applied stress is not high enough to produce macroscopic failure of steel. A considerable body of literature indicates that atomic American Petroleum Institute (API), 1220 L S t. NW, Washington, DC 20005. hydrogen, trapped in microstructural defects such as inclusion/matrix interfaces and internal voids, is primary reason for SSC and several experimental techniques have been utilized to study the role of hydrogen on the SSC of steel. Combining the slow strain rate test (SSRT) with HPT has been often utilized and it has been known that dislocation produced by plastic deformation during tensile straining increases the hydrogen solubility in steel. However, it is not enough simply to explain SSC phenomena because the straining regime of SSRT is different from NACE TM0177A, which is usually used to evaluate resistance to SSC, and the straining regime affects the mechanism of cracking failure.In this research, a modified HPT is undertaken in combination with a constant load test (CLT), also known as a dead weight test. The HPT engaged no palladium (Pd) coating because a Pd coating can be easily deformed during CLT, leading to unreliable HPT results. Stable iron oxide formed in 0.1M NaOH by potentiostatic polarization was used instead of the Pd coating. The permeability, apparent diffusivity, apparent solubility of diffusible hydrogen and the amount of irreversibly trapped hydrogen were measured and correlated with the SSC resistance. Specimen Preparation EXPERIMENTAL PROCEDURE To avoid the failure of specimen during HPT combined with CLT, sour resistant HSLA steel equivalent to API X65 grade were used as specimens. TABLE 1 shows the chemical composition of tested steel. The steel showed very high hydrogen induced cracking (HIC) resistance, exhibiting no internal cracks after a standard NACE TM0284 test.8 The tested steel also indicated high SSC resistance with a threshold stress level evaluated by TM0177A method of 87% vs. yield strength (YS).
INTRODUCTION ABSTRACT Over a six month period, UT measurements at a matrix of measurement sites on a Fractionation column indicated that it was suffering from severe internal corrosion. It was decided to monitor hydrogen flux at all measurement points on a frequent basis. The resulting flux profiles broadly co-trended. Time averaged flux and longer term corrosion rates correlated well. In a more sophisticated treatment, flux data was converted into hydrogen activity at the corroding face, to compensate for hydrogen permeation through the steel, thus normalising any variations in steel temperature and thickness. Again the correlation of activity and shorter term corrosion rate was good, with a correlation factor that was closely comparable to that obtained from laboratory data. In short, the data illustrates that, at temperatures where naphthenic acid corrosion can take place, hydrogen flux may provide a keen indication of the corrosion rate in near real time, with the diffusive delay in flux stabilization upon a corrosive change typically being one hour. Hydrogen flux monitoring at high temperatures has only been carried out in the field since 2002 . In particular, the technique has proven with the authors to be highly effective in the monitoring of the need for, effectiveness and completion of hydrogen bakeouts. In this paper and a contemporary paper, we report upon a case of naphthenic acid corrosion (NAC) in an oil refinery visbreaker fractionator, which was monitored over time using hydrogen flux measurements. In this paper, we will concentrate on correlation of the corrosion with the flux. Specifically, we will consider the errors and assumptions associated with every step in the sequence from the generation of naphthenic acid corrosion through to the recording of a measurement of a high temperature hydrogen flux. By this means, we will know how confident we can be in assessing NAC from flux measurements and how the correlation between the two should be qualified. The sequence of chemical reactions and movements of the hydrogen atom leading to flux measurement of NAC is depicted in Figure 1. These can be summarized as follows: Corrosive formation of hydrogen on a corroding steel surface Hydrogen entry into the steel wall Hydrogen permeating the steel wall Hydrogen exit onto an external steel surface Hydrogen association and desorption on the external surface Hydrogen flux measurement. FORMATION OF ATOMIC HYDROGEN IN STEEL During NAC, naphthenic acid, represented as NpCOOH, is considered to physically adsorb on steel (i), then chemically react, (ii): NpCOOH(solution) => NpCOOH (ads) (i) 2NpCOOH(ads) + Fe => Fe(NpCOO)2 + 2Hads (ii) The subscript 'ads' is used to depict chemical adsorption. It has been suggested that corrosion reaction (ii) could proceed electrochemically. However, given the very low conductivity of production fluids subject to NAC, any anodic and cathodic reactions would have to take place over very small distances, and therefore it is almost certain that, during NAC, hydrogen only forms on steel according to (i), (ii), and not through any semiconducting product layer.
- North America > United States > Texas (0.29)
- Europe > United Kingdom (0.28)