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INSIGHT March 22, 2002 Houston, Texas- Most corrosion data for alloys in hightemperature gases have been reported in terms of weight change / area for relatively short exposures and inadequately defined exposure conditions. Even though corrosion at high temperatures is often reported by weight change/ area information, this does not directly relate to the thickness of corroded metal, which is often needed in assessing the strength of equipment components. Corrosion is best reported in penetration units, which indicate the sound metal loss and can be directly related to engineering assessment of component strength. Corrosion in high-temperature gases is affected by parameters of the corrosive environments such as temperature, alloy composition, time, and gas composition. The ASSET (Alloy Selection System for Elevated Temperatures) system now contains information on about 80 commercial alloys, 4900 corrosion data measurements, and 6.4 million exposure hours. The system can predict metal thickness losses by several corrosion mechanisms at high temperatures as functions of gas composition, temperature, time, and alloy type. This article explains how the system can be used to manage corrosion in oxidizing, sulfidizing, sulfidizing/oxidizing, and carburizing conditions. Oxidation Metals are often oxidized upon exposure to tem peratures above 300 deg C (570 deg F) in gases containing more than 1 vol% oxygen. The first step in determining the potential for oxidation is to confirm that oxides are present. X-ray analysis by diffraction of the surface scale or analysis of the gas composition are common methods to confirm oxidation. Fig. 1 The dependence of corrosion upon exposure time for alloys after sufficient time has passed is commonly assumed to be proportional to the square root of time, a situation known as parabolic time dependence. Several thousand hours may be required to establish this time dependence, suggesting that oxidation measured after hundreds of hours is unlikely to be useful in estimating long term oxidation rates. It has been shown that many alloys establish parabolic time dependence after a time of 500 to 1000 hours in air at constant temperatures of 870 to 1090 deg C (1600 to 2000 deg F). Gas composition influences the rate of oxidation, in terns of variables such as oxygen. The influence of oxygen concentration is specific to each alloy. Most alloys do not show a strong influence of the oxygen concentration upon the total penetration. Alloys such as Special Metals Incoloy MA 956, and Haynes International HR-120 and 214 exhibit slower oxidation rates as the oxygen concentration increases. These alloys are rich in chromium or aluminum, whose oxides are stabilized by increasing levels of oxygen concentration. Some alloys that generally exhibit higher oxidation rates as oxygen concentration increases, are AISI 304, AISI 410, 9Cr-lMo, Incoloy DS, 617, AISI 446, and 253MA (of Avesta Sheffield). These alloys tend to form rapidly growing oxide scales, and increasing levels of oxygen effectively raise the growth rates of the corrosive species, thereby increasing the corrosion rate. Most alloys tend to have increasing penetration rates with increasing temperature for all oxygen concentrations. Some exceptions are alloys with 1 to 4% aluminum, such as Incoloy MA 956 and Haynes 214. These alloys need high temperatures to form Al^sub 2^O^sub 3^ as the dominant surface oxide, which grows more slowly than Cr^sub 2^O^sub 3^, which in turn dominates at the lower temperatures. Figure 1 summarizes oxidation after one year for some common alloys exposed to air. The metal temperature is the basis for assessing the oxidation rate of metals, and not the gas temperature. Most of the commercial heat-resistant alloys are based on combinations of iron, nickel, and chromium, and they show about 80 to 95% of the total penetration as subsurface oxidation. The amount of total penetration by subsurface oxidation changes as time passes, until long-term behavior is established, even though the corrosion product morphologies may remain constant. Sulfidation activity Sulfidation develops upon exposure of metals to temperatures above approximately 200 deg C (390 deg F) in gases containing H^sub 2^S at concentrations greater than 1 ppm. The presence of sulfides confirms sulfidation. X-ray analysis by diffraction of a scale sample or analysis of the gas are common methods to confirm the existence of sulfidation. Metals become sulfidized upon exposure of metals to gases containing CO-CO^sub 2^-COS-H^sub 2^-H20-H^sub 2^S. Variables that influence the sulfidation rate are the exposure time, partial pressures of H^sub 2^ and H^sub 2^S, and temperature. The time dependence of sulfidation is controversial, with reports of a parabolic time dependence (metal loss proportional to the square root of time), linear time dependence (metal loss directly proportional to time), power law dependence (metal loss proportional to time^sup x^), and combinations of these dependencies. An undisturbed sulfide scale and an exposure time in excess of 2000 hr, probably yield parabolic time dependence. However, some studies report linear time dependence after several thousand hours. Increasing the concentration of H^sub 2^S tends to increase the sulfidation rate of alloys, as illustrated for several alloys in Fig. 2. The line for carbon steel stops for lower concentrations of H^sub 2^S because FeS is not stable and the steel can not corrode. Sulfidation of high nickel alloys can be especially rapid and can yield corrosion rates greater than 2.5 mm/ yr, if the temperature exceeds 630 deg C (1165 deg F). This is the melting point of a potential corrosion product that forms as a mixture of nickel and nickel sulfide. Sulfidation/oxidation Sulfidation/oxidation takes place upon exposure of alloys containing elements such as chromium and aluminum to hot gases containing various combinations of CO-CO^sub 2^-COS-H^sub 2^- H2OH^sub 2^S gases. Sulfidation/oxidation is found in hydrocrackers, hydrotreaters, coal / coke / oil gasifiers, and Flexicokers, where alloys are exposed to complex gases. Sulfidation/oxidation develops when corrosion products are mixtures of sulfides and oxides. Elements such as chromium, aluminum, and silicon may be present in oxides, while iron, nickel, and cobalt may be present in sulfides, because they typically do not form both oxides and sulfides simultaneously. Pure metals such as iron, low alloy steels, or nickel form either sulfides or oxides, and rarely undergo both sulfidation / oxidation. X-ray analysis by diffraction is a common method to determine the presence of oxides and sulfides. Fig. 2 Fig. 3 Fig. 4 The important variables for sulfidation/oxidation of each alloy are the alloy composition, Po^sub 2^ and Ps^sub 2^, metal temperature, and time. The oxidation potential, Po2, is the partial pressure of oxygen and the sulfidation potential is Ps2. The Po2 can be calculated by using the partial pressure ratios of H2O/H^sub 2^ or CO^sub 2^ / CO and metal temperature. The Ps^sub 2^ can be calculated by using the partial pressure ratios of H^sub 2^S/H^sub 2^ or CO/ COS and metal temperature. The presence of oxidizing gases such as H2O or CO^sub 2^ slows the sulfidation rate below that expected if only H^sub 2^S-H^sub 2^ were present. This is important because gases that are thought to contain only H^sub 2^SH^sub 2^ often also contain some H2O because of exposure to liquid water. A gas exposed to water at room temperature (such as a water wash of a gas stream) may contain up to 2% water in the gas. Sulfidation rates predicted by the H^sub 2^S- H^sub 2^ concentrations might overestimate the rate, if H2O is present. This slowed corrosion rate is sulfidation/ oxidation, which is a transition between the rapid corrosion of sulfidation and the slow corrosion of oxidation. This is illustrated in Fig. 3 for AISI 304 at 700 deg C (1290 deg F) in a gas H^sub 2^S-H^sub 2^-H2O, based on the ASSET analysis methods discussed earlier. The right-hand Po^sub 2^ corresponds to air (normal oxidation), while the left-hand Po^sub 2^ corresponds to oxygen-depleted conditions (normal sulfidation). The minimum rate is the rate of oxidation in oxygen-containing gases, and the maximum rate is the rate of sulfidation in H^sub 2^S-H^sub 2^ gases. Carburization corrosion Carburization forms carbide corrosion products and develops upon exposure of metals to temperatures above approximately 760 deg C (1400 deg F) in gases containing CH^sub 4^, CO, hydrocarbons, and solid carbon. The first step in determining the potential for carburization is to confirm that carbides are present. X-ray analysis by diffraction of the surface scale, and analysis of the gas, are common methods to confirm carburization. Variables that influence the carburization rate are the temperature, exposure time, partial pressures of H2, CH, H2S, and alloy composition. Alloys tend to have more penetration with increasing temperature for all gas conditions. Figure 4 summarizes carburization after one year for some alloys exposed to carbon and 200 ppm H^sub 2^S\. The time dependence of carburization has been reported to be parabolic (metal carburization proportional to the square root of time). One thousand hours may be required to establish the time dependence expected for long-term service, suggesting that carburization rates measured after penods of only hundreds of hours (as is often the case for available data) may not be useful in estimating carburization corrosion rates for long-term service. Increasing the concentration of H^sub 2^S tends to slow the carburization rate of alloys. Figure 5 shows the effect for several alloys widely found in petrochemical equipment. The H^sub 2^S slows decomposition of the CH^sub 4^, which adsorbs onto the metal surface, thus slowing the rate of carburization. Increasing concentrations will slow carburization, until the concentrations are high enough to sulfide the alloy. The conditions for the initiation of sulfidation depend on the alloy and gas compositions. This means a concentration of approximately 300 ppm of H2S for ethylene furnace conditions, hydrocarbons, and steam at 980 to 1090 deg C (1800 to 2000 deg F). The ASSET project plan The concepts we have just reviewed on corrosion data interpretation have been expanded and developed into a project to greatly expand an information system that provides alloy corrosion data for a wide range of conditions. An overview of the three-year project plan, which started in early 2000, is discussed here. The project is producing corrosion prediction software that includes a database and thermochemical calculation programs that are based on laboratory corrosion data from wellcontrolled conditions. The goal is to predict corrosion for alloys over a range of high- temperature, corrosive environments. The project is improving corrosion predictions for alloys in gases at temperatures of 250 to 1150 deg C (480 to 2100 deg F). The software operates under Microsoft Windows on a PC, and manages/ correlates corrosion data for alloys corroding by several mech anisms in high temperature gases. The applications are in the following: * equipment failure analysis to reduce maintenance costs and improve process safety * alloy evaluations to select cost effective alloys for equipment * equipment design/operation guidelines to optimize process economics * alloy design studies to optimize properties * corrosion research to archive and exploit data * process evaluations to look for several alloy corrosion concerns The project involves a diverse group of organizations: Caterpillar, USA; Usinor Industeel, France; Ecole Polytechnique de Montreal, Canada; Foster Wheeler Development Corporation, USA; Haynes International, USA; Humberside Solutions Ltd., Canada; KEMA, Netherlands; Kvaerner Pulping Oy - Finland; Materials Technology Institute, USA; Oak Ridge National Laboratory, Oak Ridge, USA; Royal Military College of Canada, Canada; Shell Global Solutions (US), USA; Special Metals Corporation, USA; Texaco, USA; and US Department of Energy - Office of Industrial Technologies, USA. The project goals for each of the three years involve four main tasks each year. The tasks are software development, thermochemical modeling, corrosion data generation, and information exchange. Details of these tasks are described below. Software development Humberside Solutions Ltd. is providing support to modernize the software, incorporate additional corrosion data, increase the number of corrosion mechanisms covered by the system, distribute software, and instruct participants in the operation of the system. Thermochemical modeling The Center for Research in Computational Thermochemistry of Ecole Polytechnic de Montreal, Quebec, Canada, is evaluating available thermochemical data to produce consistent data sets to be included in the software. This will improve the accuracy of predictions of corrosion mechanisms, by determining the most stable corrosion products formed by alloys in contact with the gas. The data will include all of the combinations in the system of Fe-Cr-Ni-Co-C-S-O, over the temperature range of 250 to 1200 deg C (480 to 2190 deg F), with the data covering conditions applicable to many different industrial processes. Corrosion testing Corrosion rates are being determined under welldefined conditions, according to guidelines that have been rigorously established, and the data stored in defined formats. Corrosion is evaluated via metallographic measurements of the maximum corrosion depth. Corrosion data are being measured for the following exposure conditions: * Temperature range of 250 to 1150 deg C (480 to 2100 deg F) * Exposure times of 500 to 20,000 hours * Oxygen partial pressure (Po) range of 0.01 to 1.0 atmospheres for oxidizing conditions Fig. 5 Fig. 6 * Hydrogen sulfide partial pressures (PH) of 0.001 to 0.1 atmosphere, and hydrogen partial pressures (P^sub H2^) of 0.1 to 10 atmospheres for sulfidizing conditions * P^sup O2^ of 1x10^sup -30^ to 1x10^sup -20^ atmospheres and P^sub S2^ of 1x10^sup -15^ to 1x10^sup -10^ atmospheres for sulfidizing/ oxidizing conditions P^sub H2S^ of 0 to 3x10^sup -4^ atmospheres, carbon activity of 0.5 to 1.0, and P^sub H2O^ of 0 to 0.5 atmospheres for carburizing conditions Each year has themes for the corrosion mechanism evaluated and the type of compounds assessed by the thermochemical modeling. Corrosion prediction ASSET is publicly available. The program stores the corrosion measurements, exposure conditions, and corrosion mechanisms. It predicts alloy corrosion by evaluating the stored data for that alloy and determining the parameters of the rate equation A different equation is listed for each corrosion mechanism for each alloy. The software includes the alloy composition and the corrosive environment information to calculate the stable corrosion products and the equilibrium gas composition, for a given combination of alloy and exposure conditions. The computations originally used the ChemSage program from F*A*C*T of Montreal, Quebec, Canada, and now use ChemApp from GTT of Aachen, Germany. Some of the potential solution or mixed corrosion product phases considered are:where M represents a combination of Fe, Ni, and Cr in the compound. The thermodynamic solution behaviors of the solid austenitic and ferritic alloys are also considered in the assessment of corrosion product stability. Also provided by the calculation are thermochemical characteristics such as the P^sub O2^, P^sub S2^, and carbon activity of the environment, which help determine corrosion product stabilities. The software assists identification of the likely corrosion mechanism, by knowing the stable corrosion products at the corrosion product/corrosive gas interface, the alloy in question, and the partial pressures of P^sub O2^ and P^sub S2^. Different alloys in the same exposure conditions may exhibit different stable corrosion products and different corrosion mechanisms. In the absence of experimental data for the specific conditions of interest, predictions made by the approach discussed here may be the best available for the corrosion mechanisms that are incorporated into the system, in comparison with those made by the traditional methods of literature review and data analysis. Corrosion predicted without familiarity with the specific environment should be experimentally confirmed if high confidence is required. Lifetime predictions as limited by corrosion depend strongly upon the corrosion rate predictions. Examples of the accuracy of the ASSET system are shown in Fig. 6. They show how large amounts of corrosion data can be well correlated. The correlations are quite good for three decades of variation in corrosion penetration for several alloys and corrosion mechanisms, considering the uncertainty associated with this type of data. R. C. John Shell Global Solutions (US), Houston, Texas A. D. Pelton CRCT, l;cole Polytechnique de Montreal, Montreal, Quebec A. L. Young Humberside Solutions Ltd., Toronto, Ontario W T Thompson Royal Military College of Canada, Kingston, Ontario I. G. Wright and T M. Besmann Oak Ridge National Laboratory, Oak Ridge, Tennessee For more information: Randy C. John, Shell Global Solutions, Houston TX 77251-1380; tel: 281-544-7229; e-mail: rcohn@shell.com. Acknowledgements: Participation and support by the following organizations are recognized and appreciated: US Department of Energy - Office of Industrial Technologies via cooperative agreement DE-FC02-0OCH11020, Shell Global Solutions (US), Humberside Solutions Ltd., Centre for Research in Computational Thermochemistry in Universite de Montreal, Royal Military College of Canada, Oak Ridge National Laboratory, Materials Technology Institute, Foster Wheeler Development Corporation, KEMA, Caterpillar, Special Metals Corporation, Texaco, Haynes International, Usinor Industeel, and Kvaerner Pulping Oy.
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Source: Copyright ASM International Mar 2002
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