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Materials Research

Print version ISSN 1516-1439On-line version ISSN 1980-5373

Mat. Res. vol.7 no.1 São Carlos Jan./Mar. 2004

http://dx.doi.org/10.1590/S1516-14392004000100022 

Assessing corrosion in oil refining and petrochemical processing

 

 

Randy C. JohnI, *; Arthur D. PeltonII; Arthur L. YoungIII; William T. ThompsonIV; Ian G. WrightV; Theodore M. BesmannV

IShell Global Solutions (US) Inc., P. O. Box 1380, Houston, Texas 77251-1380 USA
IICRCT, Ecole Polytechnique de Montréal, 447 Berwick, Montreal, suite 1410, 270 Scarlett Road, Toronto,Ontario M6N 4X7 Canada
IIIHumberside Solutions Ltd., Suite 1410, 270 Scarlett Road, Toronto, Ontario M6N 4X7 Canada
IV
Royal Military College of Canada, Kingston, Ontario, K7K 5LO Canada
VOak Ridge National Laboratory, P. O. Box 2008,Bldg. 4500S, MS-6156 Oak Ridge,Tennessee 37831-6156 USA

 

 


ABSTRACT

This paper summarizes the development of an information system used to manage corrosion of metals and alloys by high temperature gases found in many different oil refining, petrochemical, power generation, and chemical processes. The database currently represents about 7.9 million h of exposure time for about 5,500 tests with 89 commercial alloys for a temperature range of 200 - 1,200°C. The system manages corrosion data from well-defined exposures and determines corrosion product stabilities. New models used in the analysis of thermochemical data for the Fe-Ni-Cr-Co-C-O-S-N-H system are being compiled. All known phases based upon combinations of the elements have been analyzed to allow complete assessments of corrosion product stabilities. Use of these data allows prediction of stable corrosion products and hence identification of the possible dominant corrosion mechanisms. The system has the potential to be used in corrosion research, alloy development, failure analysis, lifetime prediction, and process operations evaluations. The corrosion mechanisms emphasized are oxidation, sulfidation, sulfidation/oxidation, and carburization.

Keywords: oxidation, sulfidation, sulfidation/oxidation, carburization, ASSET, database, high temperature corrosion, total metal penetration, and engineering lifetime prediction


 

 

1. Introduction

This paper describes the development of a high-temperature corrosion information system. The capabilities include the ability to identify corrosion mechanisms in complex gases by using thermochemical models, and to predict alloy corrosion for wide ranges of exposure conditions found in many different processes.

The applications of this technology to management of corrosion in high-temperature gases includes prediction of sound metal losses for a wide range of conditions. Corrosion by hot gases is possible in processes such as: petroleum refining, gas processing, fired equipment, process heaters, burners, flares, furnaces, boilers, hydrocracking, coking, oil refining, hydrotreating, coal/coke/oil gasifying, petrochemical production, waste incineration, hydrogen plants, heat treatment, and electric heaters.

The ability to predict corrosion of alloys in high-temperature corrosive gases in many processes aids management of corrosion in many types of equipment. An important obstacle to predicting corrosion is the variety of the combinations of alloys and corrosive environments. Another obstacle is a lack of agreement on how to generate and then use high-temperature corrosion data to assess engineering lifetimes. The technology discussed in this article offers suggestions to deal with many alloys and corrosive conditions and provides a systematic method to predict corrosion in diverse conditions. Many aspects of equipment/process design, process operation, alloy selection, alloy design, and plant maintenance are influenced by the expected lifetimes of equipment in high-temperature, corrosive gases. These lifetimes are greatly affected by the conditions present in process equipment, because process equipment usually has maximum allowable temperatures, or other process conditions, which are limited by the corrosion rates expected for the equipment.

Most data for corrosion of alloys in high-temperature gases have been reported in terms of weight change/area for relatively short exposures and inadequately defined exposure conditions. Unfortunately, the weight change/area information 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, as discussed earlier1-3, and is shown in Fig. 1. Metal loss needs to be directly related to the loss of metal thickness used in equipment design and operation decision-making. Corrosion in high-temperature gases is affected by parameters of the corrosive environments such as temperature, alloy composition, time, and gas composition.

 

 

Benefits in improving the ability to manage corrosion data for alloys in high-temperature, corrosive gases will be far reaching in many industries. Examples are improvement in process safety, reduction in costs of maintenance of process operation, more cost-effective use of expensive alloys in equipment designs, reduction in the use of energy, moderation in the release of CO2 into the atmosphere and, of course, more confidence in use of alloys in extreme operating conditions in terms of allowable temperatures and gas compositions. All of these benefits are expected as a direct result of accurate predictions of corrosion for a wide range of commercially available metals and alloys exposed to complex, high-temperature gases.

Reductions in energy usage are now possible by more efficient use of fossil fuels through higher allowable equipment temperatures for some processes, while other processes may benefit from lower allowable temperatures as lower cost alloys can be used with more confidence, once their corrosion rates can be more accurately predicted. Enhanced productivity can be a direct result of improved prediction of alloy corrosion rates in high-temperature chemical process equipment and also for other processes. Equipment maintenance is also better scheduled and unplanned outages due to unexpected corrosion are reduced, by using this project technology.

The applications of the technology are in the following:

• equipment failure analysis to reduce maintenance costs and improve process safety;

• alloy evaluations to select cost effective alloys to design new alloys;

• equipment design/operation guidelines to optimize process economics;

• corrosion data management to archive and exploit data; and lifetime prediction and equipment inspection plans.

Eighty-nine commercial alloys are currently represented in the software, which contains 5,500 corrosion measurement records with a total exposure time of 7.9 million h. The methodologies of corrosion mechanism determination and corrosion prediction are described in this paper. The general approach is the same for all mechanisms except that the form of the rate equation and the critical species differ. The system uses thermochemical calculations to predict the stable corrosion products and the equilibrium gas composition for many environments.

 

2. Corrosion Mechanisms

The stable corrosion products are used to determine the names of the corrosion mechanisms discussed in this paper. For example: oxides imply oxidation, sulfides imply sulfidation, sulfides plus oxides imply sulfidation/oxidation, and carbides imply carburization.

 

3. Oxidation

Oxidation often occurs upon exposure of metals to temperatures above 300 °C in oxidizing gases. Examples of the types of process equipment where oxidation is a concern are fired equipment, combustors, process heaters, burners, furnace structural equipment/instrumentation, and flares. Common methods to confirm oxidation are either X-ray analysis by diffraction of the surface scale or analysis of the gas composition.

The dependence of corrosion upon exposure time for alloys after sufficient time has passed is commonly assumed to be proportional to (time)0.5, which is known as parabolic time dependence. Several thousand h may be required to establish this time dependence. Oxidation measured after hundreds of h is unlikely to be useful in estimating long time oxidation rates. It is known that many alloys establish parabolic time dependence after times of 500 - 1,000 h in air at constant temperatures only of 870-1,090 °C 2-3.

Most alloys tend to have increasing penetration rates with increasing temperature for all O2 concentrations. Some exceptions will be alloys with 1-4% Al concentrations, such as S67956 and N07214. See Table 1 for compositions and alloy names. These alloys need high temperatures to form Al2O3 as the dominant surface oxide, which grows more slowly than the Cr2O3, which dominates at the lower temperatures. Figure 2 summarizes oxidation after one year for some widely used alloys exposed to air for one year. The metal temperature is used in assessing the oxidation rate of metals and not the gas temperature.

 

 

The gas composition also influences the rate of oxidation, in terms of variables such as P O2 (oxygen partial pressure). The influence of P O2 on oxidation kinetics is specific to each alloy2-3. Figure 3 shows that some alloys show increasing corrosion with P O2, while others slow as the P O2 increases. Alloys such as S67956, N08120, and N07214 exhibit slower oxidation rates as the O2 concentration increases. These alloys form surface oxides rich in Cr or Al, whose oxides are stabilized by increasing levels of PO2. Some alloys, which generally exhibit increased oxidation rates as the O2 concentration increases, are S30400, S41000, S50400, Incoloy DS, N06617, S44600, and S30815. These alloys form rapidly growing oxide scales and increasing levels of O2 concentration increase the growth rates of the corroding alloy components, thereby increasing the corrosion rate.

 

 

Many commercial heat resistant alloys show about 80-95% of the total penetration as subsurface oxidation2-3. Some alloys differ in how much of the total penetration occurs by subsurface oxidation as time passes, until long-term behavior is established, even though the corrosion product morphologies may remain fairly constant.

 

4. Sulfidation

Sulfidation can occur upon exposure of metals to temperatures above approximately 200 °C in gases containing H2S at partial pressures greater than 1 × 10-6 atma. Examples of the types of process equipment where sulfidation is a concern are hydrotreater charge furnaces, crude distilling columns, vacuum flashers, petroleum coking units, and sulfur removal plants (gas sweetening plants). The presence of sulfides confirms sulfidation. Sulfidation occurs upon exposure of metals to gases containing CO-CO2-COS-H2-H2O-H2S. Common methods to confirm sulfidation are either X-ray analysis by diffraction of the surface scale or analysis of the gas composition. Variables that influence the sulfidation rate are the exposure time, and partial pressures of H2 and H2S, and temperature.

The time dependence of sulfidation is controversial4-12 with reports of a parabolic time dependence, linear time dependence (metal loss proportional to time), power law dependence (metal loss proportional to timex), and combinations of these dependencies. An undisturbed sulfide scale and exposure time in excess of 2,000 h, probably yield parabolic time dependence. Some studies, however, report linear time dependence after several thousand h4,5.

The first step in assessing the rate of sulfidation is to evaluate the potential for sulfide corrosion products to form. Confirmation of sulfides on existing equipment or a thermochemical evaluation of the corrosive gas to produce sulfides may be done by analyses of corrosion products, use of well-known compilations6-12, or thermochemical calculations13. Once sulfidation is expected, one can predict sulfidation rates by using either the traditional sulfidation curves for corrosion in H2S-H2 gases or the curves of the type shown in this presentation, including the effects of temperature, gas composition, and alloy composition. One important concept often assumed is that extensive sulfidation corrosion cannot occur below the line shown to progress from the lower left to the upper right, as illustrated in Fig. 4 for steels. This line represents the limiting conditions for formation of FeS, which is the primary corrosion product of carbon/low alloy steels (< 5% Cr) during sulfidation in contact with H2-H2S gases. H2S concentrations above the line favor formation of FeS, which is a requirement for sulfidation of iron and low alloy steels. It should be noted that it is incorrect to indicate that alloys with significant concentrations of Cr cannot corrode at conditions representing lower H2S concentrations and higher temperatures indicated by this line. Alloys with Cr can form CrS, which forms at lower H2S concentrations and higher temperatures than needed to form FeS on low alloy steels. Figure 4 shows the limits of H2S concentration and temperature corresponding to corrosion products of Cr. Alloys with greater than 5% Cr can corrode in conditions where low alloy steels cannot corrode. The corrosion rates may be low (such as 0.025-0.25 mm per year) but significant for conditions where steels have been traditionally thought to be immune to sulfidation. The effects of temperature and H2S concentration upon sulfidation of steels often used in oil refining services are shown in Figs. 5-6, which represent metal losses expected after one year of exposure. Figure 5 shows sulfidation predictions of several simple metals. Figure 6 shows sulfidation predications of a range of alloys for a range of H2S values. Increasing the temperature and H2S concentration increase the sulfidation rate. It is typical that a temperature increase of 50 °C will double the sulfidation rate, while increasing the H2S concentration by a factor of 10 may be needed to double the sulfidation rate. Therefore, changes of H2S concentration are generally less significant, in terms of influencing corrosion, than temperature variations. Increasing the Cr content of the alloy greatly slows the sulfidation, as seen in progression from 9% Cr (S50400), 12% Cr (S41000),18% Cr (S30400), 20% Cr (N08810), and 25% Cr (N08825 and N06625). The ranges of H2S concentration represented in these figures span the low H2S of catalytic reformers to the high H2S levels expected in modern hydrotreaters. These curves are in good agreement with the traditional data8-13.

 

 

 

 

 

 

5. Sulfidation/oxidation

Sulfidation/oxidation occurs upon exposure of alloys containing elements such as Cr and Al to hot gases containing various combinations of CO-CO2-COS-H2-H2O-H2S gases. Examples of the types of process equipment where sulfidation/oxidation is a concern are hydrocrackers, hydrotreaters, coal/coke/oil gasifiers, crude distilling columns, vacuum flashers, sulfur removal plants and Flexicokers. Sulfidation/oxidation occurs when corrosion products are mixtures of sulfides and oxides. Elements such as Cr, Al, and Si may be present in oxides, while Fe, Ni, and Co may be present in sulfides. Pure metals such as Fe, low alloy steels, or Ni form either sulfides or oxides and rarely undergo sulfidation/oxidation. Common methods to confirm sulfidation/oxidation are either X-ray analysis by diffraction of the surface scale or analysis of the gas composition.

The important variables for sulfidation/oxidation of each alloy are the alloy composition, P O2 and P S2, metal temperature, and time. The P O2 can be calculated by using the partial pressure ratios of H2O/H2 or CO2/CO and metal temperature13. The P S2 can be calculated by using the partial pressure ratios of H2S/H2 or CO/COS and metal temperature13.

The presence of oxidizing gases such as H2O or CO2 slows the sulfidation rate below that expected if only H2S-H2 were present. This is important because gases, which are thought to contain only H2S-H2 often also contain some H2O, because of exposure to liquid water. A gas exposed to water at room temperature (such as a water wash in a process) may contain up to 2% water in the gas, if saturated with water at room temperature. Sulfidation rates predicted using the H2S-H2 concentrations might overestimate the rate, if H2O is present. This slowed corrosion rate can be thought of as a transition between rapid corrosion of sulfidation and slow corrosion of oxidation. This is illustrated in Fig. 7 for S30400, S3100 and N08810 at 700 °C in a gas H2S-H2-H2O, based upon the analysis methods discussed earlier13 and reviewed in this article. The right-hand PO2 corresponds to air, while the left-hand PO2 corresponds to O2-depleted conditions (sulfidation). The minimum rate is rate of oxidation in O2-containing gases and the maximum rate is rate of sulfidation in H2S-H2 gases.

 

 

6. Carburization

Carburization forms carbide corrosion products and occurs upon exposure of metals to temperatures above approximately 700 °C in gases containing CH4, CO, hydrocarbons, and solid carbon. Examples of the types of process equipment where carburization is a concern are ethylene furnace tubes (steam cracking of hydrocarbons) and steam methane reforming tubes for hydrogen production. The first step in determining the potential for carburization is to confirm that carbides are present. Common methods to confirm carburization are either X-ray analysis by diffraction of the surface scale or analysis of the gas composition. Variables, which influence the carburization rate are the temperature, exposure time, partial pressures of H2, CH4, H2S, and alloy composition. Suitable data from previous work18-24 have been compiled and added into the databases. Alloys tend to have more penetration with increasing temperature for all gas conditions. Figure 8 summarizes carburization after one year for some alloys exposed to carbon and 2x10-4 atma H2S.

 

 

The time dependence of carburization is often parabolic. One thousand h may be required to establish the time dependence expected for long-term service, suggesting that carburization rates measured after periods of only hundreds of h (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 H2S tends to slow the carburization rate of alloys. Figure 9 shows the effect for several alloys widely used in petrochemical equipment. The H2S slows decomposition of the CH4, 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 upon the alloy and gas compositions. Approximately this means a concentration of 300 ppm (vol.) of H2S for ethylene furnace conditions containing hydrocarbons and steam at 980-1,090 °C.

 

 

7. Technology Development

Now that we have reviewed some concepts on corrosion data interpretation, we'll now discuss the development of a technology to provide alloy corrosion predictions. The project started in early 2000 and is producing software that includes a database and thermochemical calculation programs that use corrosion data from well-controlled conditions 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 250-1,150 °C. The project goals involve four main tasks, as described.

 

8. Software Development

Humberside Solutions provides the software development by incorporating additional corrosion data, distributing software, and instructing participants in the operation of the software.

 

9. Thermochemical Modeling

The Center for Research in Computational Thermochemistry of Ecole Polytechnic de Montreal, evaluates available thermochemical data to produce consistent data sets. This will improve the accuracy of predictions of corrosion mechanism, by determining the most stable corrosion products formed by alloys in contact with the gas. The data include the system of Fe-Cr-Ni-Co-C-S-O-N-H, over the temperature range of 250 - 1,200 °C, with the data covering conditions applicable to many industrial processes.

The new thermochemical database for the Fe-Co-Ni-Cr-C-N-O system is now complete. All thermodynamic and phase diagram data have been critically evaluated and modeled for all metallic carbide and nitride compounds and solutions. Maximum carbon, cobalt, chromium and nickel concentrations apply to commercial alloys. The new oxide models take into account the latest experimental data, magnetic contributions to thermodynamic properties, cation distributions and vacancies in spinel phases. The new database includes a complete re-evaluation and optimization of non-stoichiometric wustite and magnetite. Work on the updating of the sulfide phases in the Fe-Cr-Ni-Co-S system is in progress. The routine that calculates equilibria of alloys, corrosion products, and gases is "ChemApp", which consists of the latest Gibbs' energy minimization routine of FACT25 as well as all FACT solution model subroutines.

 

10. Corrosion Testing Plan for the Project

Corrosion is being determined under well-defined conditions, according to guidelines, which have been rigorously established, and the data stored in defined formats.The testing plan is shown in Table 2.

Oxidation

The oxidation testing generates data in three environments: air, N2 + 1% O2, and O2, at the temperatures of 300-1,180 °C, Oxygen partial pressure (PO2) range of 0.01-1.0 atma for oxidizing conditions, and exposure times of up to 24,000 h. Examples of the alloys tested are:

Copper, S30415, S30815, S34700, Incoloy DS, Incoloy 803, N06625, N06600, N12160, Nickel, S30400, S31000, S31600, N08810, and S67956.

Sulfidation

The sulfidation testing generates data in environments containing H2 and H2S. The H2S partial pressures range from 0.001-0.20 atma and the PH2 ranges from 0.05 to 1 atma, at temperatures of 300-800 °C.

Examples of the alloys tested are:

S30415, S30815, S30400, S31000, S34700, S44600, N06600, N08810, N12160, N08120, Incoloy DS, S41000, S31600, and S67956

Sulfidation/Oxidation

The sulfidation/oxidation tests were done with gas mixtures of CO-CO2-COS. The PS2 ranges from 1 × 10-5 to 1 × 10-3 atma and the PO2 ranges from 1 × 0-24 to 1 × 10-14 atma at temperatures of 300-900 °C. Exposure times were up to 6,000 h. Examples of the alloys tested are:

S30400, S31400, S31600, S31700, S34700, S41000, N06600, N06601, N06025, N06617, 803, DS, N12160, N08120, S30415, S30815, S67956, and N0214

Carburization

The carburization tests are in environments containing mixtures of CH4 - H2 - H2S. The carbon activities will range from 0.5-1.0 and the PH2S ranges up to 3 × 104 atma. Exposure times are up to 6,000 h and test temperatures are 800-1, 150 °C. Examples of the alloys being tested are:

S30400, S31000, S41000, N06600, 602CA, Incoloy 803, Incoloy DS, N12160, N08120, S30415, S30815, S67956, N07718, N008810, S50400, N06025, ACI HK40, and ACI HP NB.

 

11. Generalized Procedure for Corrosion Predictions

The methodologies discussed in this paper have been built into software26-32. The software uses 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. This is illustrated in Fig. 10. The gas composition and stable corrosion products are calculated after providing information on the alloy gas composition and the temperature. The stable corrosion products are calculated assuming a small amount of metal and a large amount of gas, which is intended to approximate the corrosion product/gas interface. The computations use the ChemApp program from F*A*C*T25. The thermodynamic solution behaviors of the solid and liquid alloys and the solid and liquid corrosion products are considered. Thermochemical characteristics such as the PO2, PS2, and carbon activity of the environment, which help determine corrosion product stabilities are also determined by the software. 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 gas thermochemistry. 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 using the approach discussed here may be the best available for the corrosion mechanisms an alloys that are incorporated into the system, in comparison with those made using 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.

Equipment lifetime predictions as limited by corrosion depend strongly upon the corrosion rate predictions. Examples of the accuracy possible using the system are shown in Fig. 11, for several different examples of alloys and corrosion mechanisms. The correlations are quite good for three decades of variation in corrosion penetration for many alloys. The uncertainty in the predicted corrosion is typically only + 50% of the predicted value.

 

12. Conclusions

This paper has reviewed a new technology delivery vehicle for high temperature corrosion science to assess metal loss information for alloys corroding in high temperature, corrosive environments. The technology has the following goals:

• Develop an objective and consistent approach to archive and use data for alloys corroding in diverse conditions and by different mechanisms;

• Create a database to manage data on corrosion of metals and alloys corroding in complex, corrosive, and high temperature gases;

• Form a state-of-the-art capability in thermochemical calculations to determine corrosion mechanisms of alloys in industrial processes;

• Create an information system which can be applied to the following uses: equipment failure analysis, alloy evaluations to select cost effective alloys for equipment, guidelines to optimize equipment design/operation process economics, alloy design studies, and corrosion research.

 

Acknowledgements

Participation and support by the following organizations are recognized and appreciated: US Department of Energy - Office of Industrial Technologies via cooperative agreement DE-FC02-00CH11020, Shell Global Solutions (US) Inc., Humberside Solutions Ltd., Centre for Research in Computational Thermochemistry in Université de Montréal, 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. Unlimited thanks are given to Owen Kriegel, who performed tests and compiled corrosion data results at Shell in Houston, Texas.

 

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29. John, R. C; Pelton, A. D; Young, A. L; Thompson, W. T; Wright, I. G "The ASSET Project - A Corrosion Engineering Information System for Metals in Hot Corrosive Gases", presented at the EFC – Workshop on Life Time Modeling of High Temperature Corrosion Processes, 22 and 23 February 2001, Frankfurt/Main, Germany.        [ Links ]

30. John, R. C; Pelton, A. D.; Young, A. L.; Thompson, W. T.; Wright, I. G "ASSET, An Information System for Alloy Corrosion in High-Temperature Gases", presented at the Electrochemical Society Meeting, Electrochemical Society - High Temperature Corrosion Materials Chemistry III, March 25-30, 2001, Washington, D.C.        [ Links ]

31. John, R. C.; Pelton, A. D.; Young, A. L.; Thompson, W. T.; Wright, I. G "Managing Corrosion Engineering Information for Metals in Hot Corrosive Gases", to be presented at the TMS and the ASM meetings in November 4-8, 2001, Indianapolis, Indiana.        [ Links ]

32. "Database helps combat high temperature gas corrosion", Hydrocarbon Processing, May 2001, page 93.        [ Links ]

 

 

Received: September 2, 2002
Revised: September 4, 2002
Presented at the International Symposium on High Temperature Corrosion in Energy Related Systems, Angra dos Reis - RJ, September 2002

 

 

* e-mail: randy.john@shell.com

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