Assessing Corrosion in Oil Refining and Petrochemical Processing

a Shell Global Solutions (US) Inc., P. O. Box 1380, Houston, Texas 77251-1380 USA b CRCT, Ecole Polytechnique de Montréal, 447 Berwick, Montreal H3R 1Z8 Canada c Humberside Solutions Ltd., Suite 1410, 270 Scarlett Road, Toronto, Ontario M6N 4X7 Canada d Royal Military College of Canada, Kingston, Ontario, K7K 5LO Canada e Oak Ridge National Laboratory, P. O. Box 2008,Bldg. 4500S, MS-6156 Oak Ridge,Tennessee 37831-6156 USA


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 earlier [1][2][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 CO 2 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.

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.

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 O 2 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 Al 2 O 3 as the dominant surface oxide, which grows more slowly than the Cr 2 O 3 , 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 O 2 (oxygen partial pressure).The influence of P O 2 on oxidation kinetics is specific to each alloy [2][3] .Figure 3 shows that some alloys show increasing corrosion with P O 2 , while others slow as the P O 2 increases.Alloys such as S67956, N08120, and N07214 exhibit slower oxidation rates as the O 2 concentration increases.These alloys form surface oxides rich in Cr or Al, whose oxides are stabilized by increasing levels of PO 2 .Some alloys, which generally exhibit increased oxidation rates as the O 2 concentration increases, are S30400, S41000, S50400, Incoloy DS, N06617, S44600, and S30815.These alloys form rapidly growing oxide scales and increasing levels of O 2 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 oxidation [2][3] .Some alloys differ in how much of the total penetration occurs by subsurface oxidation as time passes, until longterm behavior is established, even though the corrosion product morphologies may remain fairly constant.

Sulfidation
Sulfidation can occur upon exposure of metals to temperatures above approximately 200 °C in gases containing H 2 S 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-CO 2 -COS-H 2 -H 2 O-H 2 S. 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 H 2 and H 2 S, and temperature.
The time dependence of sulfidation is controversial 4-12 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 h 4,5 .
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 compilations [6][7][8][9][10][11][12] , or thermochemical calculations 13 .Once sulfidation is expected, one can predict sulfidation rates by using either the traditional sulfidation curves for corrosion in H 2 S-H 2 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 Figure 4 shows the limits of H 2 S 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 H 2 S 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 H 2 S values.Increasing the temperature and H 2 S concentration increase the sulfidation rate.It is typical that a temperature increase of 50 °C will double the sulfidation rate, while increasing the H 2 S concentration by a factor of 10 may be needed to double the sulfidation rate.Therefore, changes of H 2 S concentration are generally less significant, in terms of influencing corrosion, than temperature varia-  tions.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 H 2 S concen- tration represented in these figures span the low H 2 S of catalytic reformers to the high H 2 S levels expected in modern hydrotreaters.These curves are in good agreement with the traditional data [8][9][10][11][12][13] .

Sulfidation/oxidation
Sulfidation/oxidation occurs upon exposure of alloys containing elements such as Cr and Al to hot gases containing various combinations of CO-CO 2 -COS-H 2 -H 2 O-H 2 S 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 O 2 and P S 2 , metal temperature, and time.The P O 2 can be calculated by using the partial pressure ratios of H 2 O/H 2 or CO 2 /CO and metal temperature 13 .The P S 2 can be calculated by using the partial pressure ratios of H 2 S/H 2 or CO/COS and metal temperature 13 .The presence of oxidizing gases such as H 2 O or CO 2 slows the sulfidation rate below that expected if only H 2 S-H 2 were present.This is important because gases, which are thought to contain only H 2 S-H 2 often also contain some H 2 O, 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 H 2 S-H 2 concentrations might overestimate the rate, if H 2 O 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 H 2 S-H 2 -H 2 O, based upon the analysis methods discussed earlier 13 and reviewed in this article.The right-hand PO 2 corresponds to air, while the left-hand PO 2 corresponds to O 2 -depleted conditions (sulfidation).The minimum rate is rate of oxidation in O 2 -containing gases and the maximum rate is rate of sulfidation in H 2 S-H 2 gases.

Carburization
Carburization forms carbide corrosion products and occurs upon exposure of metals to temperatures above approximately 700 °C in gases containing CH 4 , 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 H 2 , CH 4 , H 2 S, and alloy composition.Suitable data from previous work 18- 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 H 2 S.
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 longterm service.
Increasing the concentration of H 2 S tends to slow the carburization rate of alloys.Figure 9 shows the effect for several alloys widely used in petrochemical equipment.The H 2 S slows decomposition of the CH 4 , which adsorbs onto the metal surface, thus slowing the rate of carburiza-tion.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 H 2 S for ethylene furnace conditions containing hydrocarbons and steam at 980-1,090 °C.

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.

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

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 FACT 25 as well as all FACT solution model subroutines.

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.

Generalized Procedure for Corrosion Predictions
The methodologies discussed in this paper have been built into software [26][27][28][29][30][31][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*T 25 .The thermodynamic solution behaviors of the solid and liquid alloys and the solid and liquid corrosion products are considered.Thermochemical characteristics such as the P O 2 , P S 2 , 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 cor- rosion 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.

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 ar-chive 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.

Figure 1 .
Figure 1.Schematic View of Total Penetration Measurement for a Typical Corrosion Product Morphology

Figure 2 .
Figure 2. Effect of Temperature Upon Metal Penetration of Some Common Alloys by Oxidation After Exposure for One Year to Air (P O 2 of 0.21 atma).

Figure 3 .
Figure 3.Effect of P O 2 Upon Oxidation Corrosion of a Range of Alloys at 900 °C After One Year.

Figure 4 .
Figure 4. Conditions for Possible Sulfidation, Based Upon H 2 S Concentrations in H 2 -H 2 S Gases and Temperatures Above the FeS/Fe Line for Carbon/Low Alloy (5% Cr) Steels and Above the Cr 6 S 7 /Cr Line for Alloys with>5%Cr.400-1,200 °F = 204-648 °C.

Figure 6 .
Figure 6.Effect of H 2 S Partial Pressure Upon the Sulfidation Corrosion After One Year in H 2 S Gases at a total Pressure of One Atma at 500.

Figure 5 .
Figure 5.Effect of Temperature Upon Sulfidation Corrosion of Some Alloy Commonly Used in Sulfidizing Conditions After One Year 0.05 atma H2S and 0.5 atma H 2 S and 0.5 atma H 2 .

Figure 8 .
Figure 8.Effect of Temperature Upon Carburization of Several Alloys Exposed to Solid Carbon and 2 × 10 -4 atma H 2 S After One Year.

Figure 7 .
Figure 7. Effect of H 2 O or CO 2 , as Indicated in the PO 2 Upon the Extent of Sulfidation/Oxidation of Several Alloys at 700 °C in H 2 -H 2 S-H 2 O Gases, after one year.

Figure 9 .
Figure 9.Effect of PH 2 S Upon Carburization of Several Alloys Exposed to Carbon Activity of 1.0 in H 2 S -Containing Gases at 1000 °C, after one year.

Figure 10 .
Figure 10.Example Screens for Alloy S30400 Exposed to a Complex Gas Analyzed to Predict the Stable.

Figure 11 .
Figure 11.Examples of Correlations of Corrosion Data for Several Alloy/Corrosion Mechanisms Combinations.

Table 1 .
Compositions in wt% of Some Alloys in The Databases.
sulfidation testing generates data in environments containing H 2 and H 2 S. The H 2 S partial pressures range from

Table 2 .
Test Plan for conditions being used to generate corrosion data.