1.11 Sulfidation and Mixed Gas Corrosion of Alloys

a ter, � 2010 Elsevier B.V. All rights reserved. n R tio dic OS ch 1.11.3 Oxidation/Nitridation by N2–O2 Gases 260 1.11.3.1 Types of Equipment and Processes Where Nitridation Occurs 260 concentrations for metals and alloys in equipment. 1. T ex products are sulfides. Sulfides will be found on the microstructure alloy and exp- ction and fluo- monmethods . An alternate 24 11.1.2 Corrosion Mechanism he key variables, which influence the kinetics are posure time, partial pressures PH2 and PH2S (for H2 exposed surface and within the alloy near to the surface, depending upon the osure conditions. X-ray analyses bydiffra rescence of a surface scale sample are com to determine the presences of sulfides 1.11.3.2 Thermochemistry and Corrosion Mechanism 262 1.11.3.3 Predicting Corrosion Product and Alloy Phase Formation in Nitriding/Oxidizing Conditions 262 1.11.4 Oxidation/Carburization by CH4–H2O Gases 265 1.11.4.1 Types of Equipment and Processes Where Carburization Occurs 265 1.11.4.2 Thermochemistry and Corrosion Mechanism 265 1.11.4.3 Corrosion Products Prediction 266 1.11.5 Summary 267 References 270 1.11.1 Sulfidation 1.11.1.1 Types of Equipment and Processes Corrosion of metals and alloys used in equipment processing high temperature, corrosive, gases contain- ing sulfur, H2S, and COS is a concern in processes used in gas processing, combustion gas process heaters, petroleum refineries (hydrocracking, coking, vacuum flashing, hydrotreating, and catalytic reforming) coal/ coke/oil gasification, petrochemical production, gasi- fication of black liquor in pulp/paper production, and fossil fuel-fired power generation. Corrosion is often the phenomenon, which defines the maximum allow- able temperature or maximum allowable gas species and H2S gases) or PCO and PCOS (for CO and COS gases), alloy composition and temperature. Sulfidation corrosion forms sulfide corrosion products, damages the metal and alloy by losses in wall thickness (penetra- tion), and occurs upon exposure of metals to gases containing sulfur, COS, or H2S. Sulfidation by H2S is predominately discussed in this chapter. Sulfidation reduces the useful thickness of the metal by the forma- tion of surface sulfide scale and internal sulfide phases. The sum of these two thicknesses is defined as the total metal penetration. The first step in determining the potential for equipment to sulfide is to assume that sul- fidation is the dominant corrosion mechanism. The key indicator of sulfidation is that most of the corrosion 1.11.1 Sulfidation 1.11.1.1 Types of Equipment and Processes 1.11.1.2 Corrosion Mechanism 1.11.1.3 Compilations of Sulfidation Corrosio 1.11.1.4 Time Dependence to Predict Sulfida 1.11.1.5 Laboratory Simulation 1.11.2 Corrosion by Mixed Gases 1.11.2.1 Thermochemical Calculations to Pre 1.11.2.2 Sulfidation/Oxidation by CO–CO2–C 1.11.2.3 Thermochemistry and Corrosion Me 1.11.2.4 Laboratory Simulation 1.11.2.5 Kinetics 1.11.2.6 Corrosion Influenced by Gas History 1.11 Sulfidation and Mixed G R. John Shell Global Solutions (US) Inc., Westhollow Technology Cen 0 s Corrosion of Alloys P.O. Box 4327, Houston, TX 77210, USA 240 240 240 ate Predictions 243 n 243 245 245 t Oxide–Sulfide–Carbide–Nitrides on Alloys 247 and H2–H2O–H2S Gases 249 anism 250 250 254 259 method is to evaluate the thermochemical interactions between the alloy and gas, and the compounds formed. Surface metal loss (scaling) by sulfidation can be detected by methods, which measure the metal thick- ness, such as ultrasonic and mechanical methods. Sound metal loss by internal sulfidation can only be detected by metallography, as shown schematically in Figure 1. The time dependence of sulfidation has been con- troversial1–5 with reports of a parabolic time depen- dence (metal loss proportional to time0.5), linear time dependence (metal loss proportional to time), power law dependence (metal loss proportional to timex), and various combinations of these dependencies. If the corrosion product sulfide scale remains undis- turbed on the alloy surface and sufficient time has passed (in excess of 1000 h), currently available infor- mation suggests that the time dependence is para- bolic. Removal or cracking of the surface sulfide scale the effect of several alloys widely used in process equipment. Dashed lines represent extrapolations of the available data, while solid lines represent interpo- lations of the available data. The line for carbon steel stops for low concentrations of H2S because FeS is not stable at low concentrations of H2S (as shown in Figure 4), and the carbon steel cannot corrode at combinations of sufficiently high temperature and suf- ficiently low partial pressure of H2S. It is incorrect to indicate that steels with significant concentrations of Cr cannot corrode at conditions representing low H2S concentrations and high temperatures indicated by this line. This is because alloys with Cr form CrS at lower H2S concentrations and higher temperatures 0.0 0.2 0.4 0.6 0 25 000 50 000 75 000 100 000 Time (h) P en et ra tio n (m m ) Figure 2 Effect of time upon the sulfidation corrosion of carbon steel (G10200) after 1 year in 0.05 atm H2S and H2 at 623K. 10.00 Sulfidation and Mixed Gas Corrosion of Alloys 241 tends to increase the rate of sulfidation, because the presence of the scale is a partial barrier, which tends to slow the sulfidation rate as time passes. The idea- lized time parabolic dependence of sulfidation of G10200 carbon steel at 350 �C in 0.05 atm H2S in H2 after several thousand hours is shown in Figure 2. Some studies suggest an initial linear time depen- dence for several thousand hours. Sulfidation data measured after several hundred hours (as is often the case for available data) are unlikely to be useful in estimating sulfidation corrosion rates for long-term service. Sulfidation data are properly used when the time dependence is considered. Increasing the concentration of H2S tends to increase the sulfidation rate of alloys. Figure 3 shows Uncorroded alloy Total penetration Internal penetration External scale Internal corrosion products Corroded grain boundaries Figure 1 Schematic view of total penetration measurement for a typical corrosion product morphology. 0.01 0.10 1.00 1.E − 04 1.E − 03 1.E − 02 1.E − 01 1.E + 00 P H2S (atm) P en et ra tio n (m m ) G10200 S50400 S41000 S30400 S31000 N08810 Figure 3 Effect of H2S partial pressure upon the sulfidation corrosion after 1 year in H2–H2S gases at a total pressure of 1 atm at 723K. 242 Types of High Temperature Corrosion 400 600 800 1000 1200 0.0001 0.001 0.01 0.1 1 10 Temperature (�F) H 2S c on ce nt ra tio n (% ) FeS Fe Cr Sulfidation of carbon/low alloy (<5% Cr) steels No sulfidation of carbon/low alloy (<5% Cr) steels Sulfidation of steels with >5% Cr Cr6S7 than that needed to form FeS on steels. Figure 4 shows the limits of H2S concentration and temperature corresponding to sulfide corrosion products of Fe and Cr, as shown beneath the lines. Alloys with greater than 5% Cr can corrode in conditions where low alloy steels cannot corrode. The corrosion rates may be low (such as 0.025–0.25mmyear�1) but still significant for conditions where steels are traditionally thought to be immune to sulfidation. The ranges of H2S concentration represented in Figures 3 and 4 span the low H2S range of catalytic reformers to the high H2S concentrations expected in modern hydro- treaters in crude oil distillation equipment in petro- leum refineries. These curves are in good agreement with the traditional data.1–5 Sulfidation caused by sulfur-containing compounds in liquid hydrocarbons in the absence of a gas phase (such as that found in petroleum distilling units) is not discussed in this chapter. The sulfur concentrations in liquid hydrocarbons and the sulfur evolvable as gaseous H2S have not yet been clearly related. However, the effective H2 and H2S concentrations are likely domi- nant variables in the liquid hydrocarbons. If the Figure 4 Conditions for possible sulfidation, based upon H2S concentrations in H2–H2S gases and temperatures above the FeS/Fe line for carbon/low alloy (<5% Cr) steels and above the Cr6S7/Cr line for alloys with >5% Cr, 400–1200 �F (204–648 �C, 473–921K). effective gas concentration of H2S could be estimated, the approach discussed should be suitable for estimat- ing the metal losses, as related to the maximum allow- able temperatures/H2S concentrations and appropriate materials of construction. High-Ni alloys used either as base metals or as welding filler metals are a special concern in sulfida- tion conditions. Sulfidation of high-Ni alloys can be especially rapid and yield corrosion rates greater than 2.5mm year�1 if the temperature exceeds 530 �C, which is the melting point of a potential corrosion product which forms as a mixture of Ni and nickel sulfide. A reasonable approach for high-Ni alloys is that they should not be used in sulfidation conditions when metal temperatures will exceed 530 �C. High-Ni alloys with high-Cr levels (such as N06625 or N08825) can be very suitable, with low corrosion rates at lower temperatures.6–8 The phase behaviors of FeS and NiS are described in Figures 5–8. The first step in assessing the rate of sulfidation is to evaluate the potential for the formation of sulfide corrosion products. Confirmation of formation of sulfides on existing equipment or a thermochemical evaluation of the corrosive gas to produce sulfides may be done by the analyses of corrosion products, use of well-known compilations,7–13 or ASSET pro- gram.10 Once sulfidation is expected, one can predict sulfidation rates by using either the traditional sulfi- dation curves for corrosion in H2S–H2 gases or the curves of the type shown in previous work,7–13 including the effects of temperature, gas composition, and alloy composition. Most corrosion data for alloys exposed to high temperature gases have been reported in terms of weight change/area for relatively short exposures and inadequately defined exposure conditions. The weight change/area information is not directly relatable to the thickness of corroded metal (pen- etration), which is often needed to assess the strength of the equipment components. Corrosion is best reported in penetration units, which indicate the sound metal loss, as discussed earlier.12,13 Cor- rosion in high temperature gases is affected by key parameters of the corrosive environments such as temperature, alloy composition, time, and gas com- position. Summaries of metal penetrations are used in this chapter, which goes beyond the traditional corrosion weight change data by reporting total metal penetration for an extensive number of alloys over a wide range of conditions. Compositions of some alloys discussed in this chapter are shown in Table 1. 43 60 N i 9 S 8 Sulfidation and Mixed Gas Corrosion of Alloys 243 1455 t/ C 637 536 400 1455 801 FCC + liquid β 560 N i 2 S 3 N i 7 S 6 Liquid 0 200 400 600 800 1000 1200 1400 1600 1800 1.11.1.3 Compilations of Sulfidation Corrosion Rate Predictions Sulfidation rate data for some commercial alloys are summarized in Figures 9–12. Figure 9 shows sulfi- dation predictions of several simple metals (copper, carbon steel, and nickel). Figure 10 shows sulfidation predications of a range of alloys often used in these types of conditions. Figure 11 shows sulfidation pre- dictions of some of the most resistant alloys available. Figure 12 shows sulfidation predictions of several alloys similar to the 18%Cr–8%Ni steels. These steels experience different sulfidation rates, even though the Cr and Ni concentrations are similar. The differ- ent concentrations of Mo, Ti, and Nb among these alloys do significantly influence the sulfidation kinet- ics which is not well known; it is widely assumed that these steels corrode with kinetics similar to the com- mon 18% Cr–8% Ni steel UNS S30400. The effects of temperature and H2S concentration upon sulfidation of steels often used in oil refining services are shown in Figures 3 and 9–12, which represent metal losses expected after 1 year of exposure. Increasing the temperature and H2S concentration 0 0.1 0.2 0.3 0.4 Figure 5 Calculated condensed phase diagram with experime 1023 115 353 1001 302 1596 7 0 Liquid(1) + Liquid(2) N iS N i 3 S 4 N iS 2Ni1-xS 993 increase the sulfidation rate. Temperature increases of 50 �C will typically double the sulfidation rate, while increasing the H2S concentration by a factor of 10 may be needed to double the sulfidation rate. Therefore, likely changes of H2S concentrations in processes are generally less significant, in terms of influencing corro- sion, than the likely 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 range of catalytic reformers to the high H2S concentrations expected in modern hydrotreaters. These curves are in good agreement with the tradi- tional data.8–13 1.11.1.4 Time Dependence to Predict Sulfidation A large compilation of test data was evaluated with respect to the time dependence of the corrosion. The following rate equation can be used to calculate cor- rosion by sulfidation.8 The equation incorporates the xs 0.5 0.6 0.7 0.8 0.9 1 ntal data for Ni–S system. S 244 Types of High Temperature Corrosion 0. 5 0. 6 0. 7 0. 8 0. 9 x S M1-xS well-known time parabolic and linear behaviors, which have been observed. Parabolic behavior occurs initially and then tends towards linear time depen- dence as the surface scales crack or partially spall as they thicken and become less adherent. The transi- tion towards linear dependence often results from development of an effective steady-state thickness of surface corrosion product, as caused by cracking, spalling, or other failure mechanisms. logððPenetration=Hours0:5Þ þ ðM �HoursÞÞ ¼ Aþ B � logðPH2SÞ þ C � logðPH2Þ þ D=T ½1� In this equation, penetration represents sound metal loss by corrosion, T is the absolute temperature, and PH2S and PH2 represent the partial pressures of the respective species. A, B, C, D, andM are constants that 0. 1 0. 2 0. 3 0. 4 0.50.60.70.80.9Fe FCC+ M1-xS+ β xFe Figure 6 Calculated isotherm of the Fe–Ni–S system at 998K 0.1 0.2 0.3 0.4 0.5 x Ni are calculated separately for each alloy. The form of the equation includes both linear and parabolic time dependencies. The M factor empirically describes how much time passes before the time dependence becomes linear and varies for each alloy. Graphical illustrations of correlations for a couple of example alloys (S30400 and N08810) are shown in Figure 13. These alloys are fairly typical and in that Fe–Ni–Cr alloys are widely used in many different types of process equipment, and they are also well correlated with the methodology described in this article. In fact, the data are well correlated over about three orders of magnitude of variation in the measured corrosion for wide ranges in the important environmental variables of temperature, partial pres- sure of H2, and partial pressure of H2S. 0.10.20.30.4 0.6 0.7 0.8 0.9 Ni β . M1-xS + β Liquid + M1-xS Liquid M9S8+ β X t( �C ) 0 0.2 0.4 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 Figure 7 Calculated section of the FeS–Ni3S2 with experimen 400 600 800 1 000 1 200 0.0001 0.001 0.01 0.1 1 10 Temperature (�F) H 2S c on ce nt ra tio n (% ) Ni3S2 Ni Sulfidation of nickel No sulfidation of nickel Figure 8 Conditions for possible sulfidation of nickel based upon combinations of H2S concentrations in H2–H2S gases and temperatures above the Ni/Ni3S2 line for Ni-base alloys. 400–1200 �F (204–628 �C). Sulfidation and Mixed Gas Corrosion of Alloys 245 β β + Ni3S2 M S + β + Ni S 1.11.1.5 Laboratory Simulation Laboratory simulation of sulfidation corrosion is generally carried out with flowing H2–H2S gases or CO–COS gases at constant temperature, starting with a polished metal surface. The combination of gas flow rate and H2S (or COS) concentration must be high enough to prevent significant consumption of H2S during the test exposures. Post-exposure examination is carried out with weight change and microscopic methods to assess the amount of metal consumed by scale formation and also the amount of the metal containing internal corrosion products. 1.11.2 Corrosion by Mixed Gases The corrosion mechanism names for mixed gas cor- rosion indicate the stable alloy corrosion products formed and result from a competition between the different possibilities and are named after the corrosion 9 8 3 2 FCC + M9S8+ Ni3S2 N3S2 0.6 0.8 1 tal data. 0 0 9 246 Types of High Temperature Corrosion Table 1 Compositions of alloys discussed UNS Fe Cr Ni Co Mo Al S30415 69.66 18.30 9.50 0.00 0.42 0.0 S30815 65.60 20.90 11.00 0.00 0.00 0.0 N06025 9.45 25.35 62.63 0.00 0.00 2.0 products, which dominate. Examples are: sulfidation/ oxidation means that sulfides and oxides form, oxida- tion/carburization means that carbides and oxides form, oxidation/nitridation means that nitrides and oxides form, and so on. Corrosion leads to the S41000 86.50 12.30 0.50 0.00 0.10 0.00 G10200 99.42 0.00 0.00 0.00 0.00 0.00 S30400 71.07 18.28 8.13 0.14 0.17 0.00 S31000 52.41 24.87 19.72 0.05 0.16 0.00 S31600 68.75 17.00 12.00 0.00 2.25 0.00 S34700 68.14 17.75 10.75 0.00 0.00 0.00 S44600 74.12 24.36 0.36 0.02 0.20 0.00 R30188 1.32 21.98 22.82 38.00 0.00 0.00 N07214 2.49 16.04 76.09 0.14 0.10 4.71 N06230 1.30 21.90 59.70 0.28 1.20 0.38 R30556 32.50 21.27 21.31 18.09 2.88 0.17 N06600 7.66 15.40 75.81 0.00 0.00 0.32 N06601 13.53 23.48 60.00 0.06 0.16 1.26 N06617 0.76 22.63 53.20 12.33 9.38 1.15 N06625 2.66 21.74 62.79 0.00 8.46 0.10 N08810 44.22 21.22 31.71 0.00 0.00 0.33 C11000 0.00 0.00 0.00 0.00 0.00 0.00 N08120 34.53 25.12 37.44 0.11 0.37 0.11 N12160 8.00 28.00 34.30 27.00 0.00 0.00 S67956 75.22 19.40 0.28 0.05 0.00 4.50 N02270 0.00 0.00 99.99 0.00 0.00 0.00 RE means rare earth elements. 1.E – 2 1.E – 01 1.E + 00 1.E + 01 1.E + 02 200 300 400 500 600 700 Temperature (�C) P en et ra tio n (m m ) C11000 Nickel 201 G10200 Figure 9 Effect of temperature upon sulfidation of some simple metals after 1 year in 0.5 atm H2 and 0.05 atm H2S. Si Ti W Mn Cu RE Nb 1.23 0.00 0.00 0.56 0.23 0.05 0.00 1.77 0.00 0.00 0.64 0.00 0.00 0.00 0.06 0.14 0.00 0.09 0.01 0.00 0.00 0.60 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.38 0.00 0.00 0.00 0.49 0.00 0.00 1.48 0.19 0.00 0.00 0.68 0.00 0.00 1.94 0.11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.55 0.00 0.00 1.80 0.00 0.00 0.96 0.33 0.00 0.00 0.45 0.10 0.00 0.00 0.37 0.00 14.55 0.82 0.00 0.04 0.00 0.10 0.00 0.10 0.20 0.00 0.00 0.00 0.42 0.02 14.20 0.49 0.01 0.00 0.00 0.33 0.00 2.38 0.96 0.00 0.00 0.00 0.16 0.00 0.00 0.29 0.32 0.00 0.00 0.50 0.27 0.00 0.31 0.38 0.00 0.00 0.15 0.27 0.00 0.02 0.05 0.00 0.00 0.41 0.19 0.00 0.10 0.00 0.00 3.52 0.60 0.41 0.00 0.92 0.51 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.57 0.02 0.10 0.73 0.18 0.00 0.66 2.70 0.00 0.00 0.00 0.00 0.00 0.00 0.11 0.33 0.00 0.09 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 formation of corrosion products, which leads to metal loss (penetration), and occurs upon exposure of metals to gases, which contain the reactive species needed to form the corrosion products. An X-ray analysis by diffraction of a surface scale sample is a common method to determine the identity of corrosion pro- ducts. An alternate method is to evaluate the gas composition. Surface metal loss (scaling) can be detected by methods, which measure the metal thick- ness, such as ultrasonic and