Corrosion Science 75 (2013) 193–200

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Corrosion Science

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Effect of pre-oxidization on the cyclic coking and carburizing resistanceof HP40 alloy: With and without yttrium modification

0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.06.002

⇑ Corresponding authors. Address: School of Materials Science and Engineering,Xi’an Jiaotong University, 28 Xianning West Road, Xi’an, Shaanxi Province 710049,PR China. Tel./fax: +86 29 82665479.

E-mail addresses: mlf@szet.com.cn (L. Ma), ymgao@mail.xjtu.edu.cn (Y. Gao).

Lifeng Ma a,⇑, Yimin Gao a,⇑, Jingbo Yan b, Xinxin Wang a, Liang Sun a, Xiuqing Li a

a State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049, PR Chinab Materials Research Deperartment, Xi’an Thermal Power Research Institute Co., Ltd., PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 January 2013Accepted 1 June 2013Available online 10 June 2013

Keywords:A. AlloyA. Rare earth elementsC. OxidationC. Carburization

The pre-oxidation results demonstrate that rare earth elements play an important role on the scaleadherence of alloy after the oxidation at elevated temperature. The better scale adherence of the yttriummodified alloy permits a superior cyclic coking and carburizing resistance. This is because the integritychromia layer, which prevents fast ion diffusion, can retard the outer catalytic coking and the inner car-burizing of pre-oxidized alloy. As for the alloy under the cast condition, severe coking and carburizationhas been detected, which was also observed on the pre-oxidized samples where the scale suffered fromsevere spalling.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The service conditions of pyrolysis tube in petrochemicalindustry are severe, including high temperature, moderate pres-sure and a carburizing environment. These conditions thereforedemand outstanding properties of tube alloy, such as creep andcarburization resistance. Because of superior creep-resistance,HP40 alloy is one of the most likely candidate to be used as tubealloy in the petrochemical pyrolysis industry [1,2]. However, thehigh concentration of Ni in HP40 alloy, which is required forsuperior creep-resistance [3,4], is reported to greatly facilitatecoking on the surface of material [5]. The coke deposited on theinner wall of the cracking tube gives rise to a loss of thermal effi-ciency and decreases the effective cross section of the tube andblocks cracking reactions. As a result, the coke greatly reducescracking output and increases production cost [6,7]. In addition,coking on the inner surface of the tube facilitates internal carbu-rizing which deteriorates the mechanical properties of the alloy[8–10]. Usually, this carbon deposit is removed by a methodcalled decoking, which improves burning the carbon deposit layerusing a steam and air mixture. However, the decoking process re-duces ethylene yield and deteriorates certain properties of thefurnace tube. Thus, retarding the carbon deposit and prolongingthe decoking cycle can have a significant effect on the service lifeof the materials. Surface coating on the superalloy is considered

as an effective method to inhibit direct contact between the alloyand the atmosphere, which decelerates coking on the tube surface[11–15]. However, because of increased cost and more complexprocessing, it is difficult to apply this method to industrial pro-duction. Oxide layer is reported as one of the effective coatingsthat can improve coking resistance [16,17], and costs and whilealso being easy to apply. The oxide scale can act as an obstacleto direct contact between the reacting gas and Ni, Fe elementsin the substrate which then retards catalytic coking. Unfortu-nately, researchers have reported that scale spallation inducedby the decoking process or creep of the material limits the effectof oxide scale [18,19]. Thus, improving the binding strength of theoxide scale is expected to have a positive effect on the coking andcarburizing resistance of alloy.

Yttrium is extensively reported to have positive effects on themechanical properties and corrosion resistance of alloy [20–22].As for HP40 alloy, the effect of yttrium is investigated in our pre-vious research [23–25] which found that yttrium refines themicrostructure of alloy and improves cyclic oxidation resistance.During these investigations, we found that yttrium can improvethe binding strength of oxide scale after relative short oxidationduration [24]. This is in agreement with other researches whichreported that yttrium can improve scale adherence during iso-thermal and cyclic oxidation [26,27]. However, to the best ofthe authors’ knowledge, the effect of yttrium is scarcely consid-ered on the coking behavior of pre-oxidized materials. Thus, thegoal of this work is to investigate the effect of pre-oxidation oncyclic coking and carburizing resistance of the yttrium adoptalloy.

Table 1Chemical composition.

Sample Amount (wt.%) of the following elements

C Ni Cr Mn Si Mo P S Y Fe

1 0.50 33.93 23.09 2.11 1.77 0.43 60.03 60.03 – Bal.2 0.51 34.93 23.14 1.94 1.81 0.51 60.03 60.03 0.06 Bal.

Fig. 1. Schematic diagram of experiment apparatus.

Table 2Alloy oxidation behavior at different temperature.

Oxidation temperature Mass gain (g/m2) Mass of ruptured scale (g/m2)

HP40 HP40Y HP40 HP40Y

1173 K 3.40 3.39 0.09 0.081373 K 16.90 15.61 4.69 1.51

194 L. Ma et al. / Corrosion Science 75 (2013) 193–200

2. Materials and methods

The chemical composition of the sample is shown in Table.1.Samples 1 and 2 are the HP40 alloy without and with Y additionrespectively. The impurities of P, S were controlled at the extentbelow 0.03 wt.%. The oxidation specimens were sectioned into32 mm � 16 mm � 3 mm shapes and each surface was ground

Fig. 2. XRD analysis of samples oxidize

using SiC sand paper up to 1000 #, then polished by flannelettefor less than 1 min and cleaned in acetone for 10 min.

The pre-oxidation experiment was conducted in a box-typeresistance furnace at 1173 K (within ±3 K) for 10 h. To betterunderstand the effect of oxide scale adherence on the coking resis-tance, a comparative oxidation test was conducted at 1373 K for10 h. The mass gain of the sample and the weight of the spalledoxide were measured. The detail of the oxidation experimentalprocedure refers to Chinese national standards (GB/T 13303-91).The coking experiment was conducted in a tubular resistance fur-nace at 1173 K. Three batches of the carburizing experiment wereconducted. Batch 1 consists of Samples 1 and 2 under cast condi-tion, while batches 2 and 3 consist of samples pre-oxidized at1173 K and 1373 K respectively. A schematic of the equipment isshown in Fig. 1. Before the carburizing experiment, all of the castand pre-oxidized samples were weighted and their dimensionsmeasured. Then, each sample was placed on a small ceramic cruci-ble with wholes on each side parallel to the flow of the reactive gas.They samples were placed in a zone that ensured the temperaturevariance was within ±3 K. The reactive gas for carburization was amixture of 10%C3H8 + H2 at a flow rate of 110 mL/min during test-ing. The partial pressure of oxygen impurities was less than100 ppm. During the experiment, each sample was cooled off every10 h and the mass gain was measured on an electronic balancewith a precision of 0.01 mg at room temperature. Then, the cruci-ble holding the sample was put back into the furnace. Argon wasused as protective gas during heating and cooling. Three replicateexperiments were conducted on each composition. Averages ofthe weight changes were calculated from the three specimens.

The surface morphology of the scale was observed by scanningelectron microscopy (SEM) and the composition of the scale wasdetermined by energy dispersive X-ray spectrometer (EDS) andX-ray diffraction (XRD). Each sample was sectioned, nickel plated,mounted in resin, grounded by SiC sandpaper up to 1000 # andpolished using flannelette for no more than 1 min. Back scattering

d under (a) 1173 K and (b) 1373 K.

Fig. 3. Surface morphology of (a) Sample 1, (b) Sample 2 oxidized at 1173 K for 10 h and (c) Sample 1, (d) Sample 2 oxidized at 1373 K for 10 h.

Fig. 4. Cross section profile of (a) Sample 1, (b) Sample 2 oxidized at 1173 K for 10 h and (c) Sample 1, (d) Sample 2 oxidized at 1373 K for 10 h.

L. Ma et al. / Corrosion Science 75 (2013) 193–200 195

Fig. 5. Mass gain versus time curves of different samples (batch 1 and 2) carburizedat 1173 K for 60 h.

Table 3EDS analyzing results on the points shown in Fig. 6.

Sample Amount (at.%) of the following elements

C O Fe Cr Mn Si

1 A 30.74 5.53 37.60 24.33 1.80B 40.39 6.67 1.24 51.70C 100

2 D 15.02 50.05 1.75 29.48 2.29 1.41E 7.69 39.73 27.85 24.73

196 L. Ma et al. / Corrosion Science 75 (2013) 193–200

electron microscopy (BSE) and EDS technology were used to exam-ine the cross-section of the scale.

3. Results and discussion

3.1. Effect of yttrium on the pre-oxidation behavior of the alloy

The results of the oxidation experiment are shown in Table. 2. Itcan be inferred that higher oxidation temperature results in in-creased oxidation and scale spallation. However, the difference ofthe mass gain between the two samples under the same oxidationtemperature is not obvious. XRD analysis on the surface of oxidescale demonstrates that the external oxide is primarily composedof chromia and Mn1.5Cr1.5O4 (Fig. 2). Moreover, compared with

Fig. 6. Surface morphology of (a) cast Sample 1, (b) Sample 2 carburized at 1173 K for 60

Sample 1, the spallation of oxide scale on Sample 2 decreasesremarkably after oxidation at 1373 K. Surface morphology of oxidescale under different oxidation conditions confirms this presump-tion (Fig. 3). Severe spalling can be detected on the Sample 1 oxi-dized at 1373 K, while a smooth oxide layer on Sample 2 can beobserved. The cross-section of oxide scale shows that internal oxi-dation (silica) took place on Sample 2 (Fig. 4). As we discussed pre-viously, the formation of silica is caused by the yttrium additionchanging the ion diffusion mechanism [23]. In addition, silica atthe scale-matrix interface greatly improves the binding strengthof the oxide scale [24].

Spallation can hardly be detected on samples oxidized at1173 K. In increase in thermal stress in the oxide scale was relatedto decreasing temperature [28]. The thermal stress can be calcu-lated as:

rox ¼Eox

ð1� mÞ

Z T

TO

ðaM � aoxÞdT ð1Þ

here rox is the stress caused by cooling, Eox is Young’s modulus ofthe oxide, m is Poisson’s ratio, aox and aM are thermal expansioncoefficients of scale and matrix respectively. Spalling cannot be de-tected on the 1173 k pre-oxidized sample, probably due to the

h and (c) 1173 K pre-oxidized Sample 1, (d) Sample 2 carburized at 1173 K for 60 h.

L. Ma et al. / Corrosion Science 75 (2013) 193–200 197

lower oxidation temperature inducing lower thermal stress duringcooling

3.2. Effect of the integrity oxide scale on the coking and carburizingresistance of alloy (coking and carburizing of batches 1 and 2)

Based on the superior adherence of oxide scale generated dur-ing 1173 K pre-oxidation, the effect of oxide scale on coking andcarburizing behaviors can be investigated. The mass gain versustime curve of samples carburized for 60 h at 1173 K is shown inFig. 5. Compared with the alloy pre-oxidized at 1173 K (batch 2),a relatively large mass gain was observed on the cast sample (batch1) during carburization. In addition, the mass gain of the sample ofbatch 1 started to decrease when it reached a critical value. As

Fig. 7. XRD analysis of (a) cast samples and (b) 1173 K

Fig. 8. Cross section profile of (a) cast Sample 1, (b) Sample 2 carburized at 1173 K for 60

comparison, the decrease of the mass gain was barely detectedon the sample of batch 2.

Surface morphology of the carburizing samples is shown inFig. 6. Severe spalling occurred on the cast samples (batch 1), asshown in Fig. 6a and b. EDS analysis on the carburized samplesof batch 1 demonstrated that the spalling area was Mn, Cr-richoxide, Cr-rich carbide and carbon (Table. 3). XRD analysis con-firmed they were Mn1.5Cr1.5O4 oxide, Cr3C2 and graphite, respec-tively (Fig. 7a). However, the chromia layer, which is commonlydeveloped on the alloy surface at elevated temperatures, was notdetected after carburizing for 60 h. We agreed with Hao et al.[29,30] who suggested that the thermal stability of Mn1.5Cr1.5O4

is higher than chromia under carburization conditions. Althoughthe spinel oxide (Mn1.5Cr1.5O4) layer is considered to improve thecarburization resistance of alloy [31], severe internal carburization

pre-oxidized samples after 60 h of carburization.

h and (c) 1173 K pre-oxidized Sample 1, (d) Sample 2 carburized at 1173 K for 60 h.

Fig. 9. Mass gain versus time curves of different samples of batch 3 carburized at1173 K for 60 h.

Table 4Calculation results of the critical thickness.

Sample Mass gain(mg)

Surface area(mm2)

Density(g/cm3)

Criticalthickness (lm)

1 Batch 1 85.42 1269.49 2.21 30.45Batch 2 46.80 1214.32 17.44Batch 3 38.39 1267.9 13.70

2 Batch 1 87.73 1181.93 33.59Batch 2 66.49 1218.93 24.68Batch 3 86.57 1185.53 33.04

Table 5EDS analyzing results on the points shown in Fig. 10.

Point Amount (at.%) of the following elements

C O Ni Fe Cr Mn Si

A 11.96 36.26 32.29 16.46 3.03B 8.74 40.05 38.86 12.35C 7.02 38.50 54.48

198 L. Ma et al. / Corrosion Science 75 (2013) 193–200

on samples of batch 1 was founded (Fig. 8a and b). Cross-sectionprofile showed that the spinel layer discontinued, which reducedprotection of the alloy during cyclic carburization. In addition, itwas interesting to note that the filamentary morphology of graph-ite was observed on the area where the graphite layer was thinner,or near the crack within the graphite layer (Fig. 6a). The formationof such filamentary graphite has been investigated by manyresearchers who suggest that it is because the catalytic cokingbehavior is induced by Ni and Fe rich particle [32,33]. Thus, itcan be inferred that the outer layer of the samples of batch 1 couldnot effectively inhibit the outward diffusion of Ni and Fe elementsand facilitate the formation of filamentary coke. The formation ofsuch the filamentary graphite confirms that the discontinuousMn1.5Cr1.5O4 layer cannot inhibit ion diffusion effectively.

As comparison, the surface of 1173 K pre-oxidized samples(batch 2) was covered by uniform spherical graphite rather thanfilamentary graphite (Fig. 6c and d), indicating the changes of thecoking mechanism. Wu et al. [34] suggested that the formationof spherical coke results from the free radical coking process,which can prevents the catalytic metal or alloy particles from con-tacting with hydrocarbon. It can be inferred that the outward dif-fusion of Ni and Fe ion was retarded, and inhibited the formationof filamentary graphite. In addition, a few spalling areas were ob-served on the surface of Sample 1 of batch 2 after carburizing.XRD analysis demonstrated that the spalling area was primarychromia and Mn1.5Cr1.5O4 (Fig. 7b). Moreover, Cr3C2 was not de-tected on the surface of the samples of batch 2, indicating the out-standing carburization resistance of the alloy. The cross-section

Fig. 10. Surface morphology of (a) 1373 K pre-oxidized Sam

profile showed that the integrity chromia layer developed on thesurface of both pre-oxidized alloys, as shown in Fig. 8c and d. Inaddition, during nickel plating, the detachment of the outer graph-ite layer from the integrity oxide layer demonstrated the superiorbinding strength of oxide scale (Fig. 8c). Thus, it can be inferredthat the uniform chromia layer developed on the alloy improvedthe coking resistance by inhibiting the outward diffusion of Feand Ni elements. Moreover, internal carburization decreasedwhich indicated that the chromia layer on the tube can slow downthe rate of carburization of Fe–Cr–Ni-based alloy because of thenegligible solubility and diffusivity of carbon in dense Cr2O3. Thisagreed with Wolf’s suggestion [35].

3.3. Effect of yttrium on the coking and carburizing resistance of pre-oxidized alloy (coking and carburizing of batch 3)

Although the oxide scale had a positive effect on the anti-cokingproperty of the alloy, failure of the scale can be caused by decokingor creep process. Moreover, we noticed that the scale bindingstrength of Sample 2 pre-oxidized at 1373 K is higher than Sample1 (Fig. 3). This is because yttrium facilitates the formation of inter-nal oxide, and then increases the critical strain energy for scale fail-ure by keying matrix [23], as shown in Fig. 4. Although internaloxidation was observed on Sample 2 of batch 2 after the carburiza-tion experiment (Fig. 8d), the effect of internal oxide on carburizingresistance was still not clear. Thus, the cyclic carburization

ple 1 and (b) Sample 2 carburized at 1173 K for 60 h.

Fig. 11. Cross section profile of (a) 1373 K pre-oxidized Sample 1 and (b) Sample 2 carburized at 1173 K for 60 h.

L. Ma et al. / Corrosion Science 75 (2013) 193–200 199

experiment should be conducted on the alloy with a higher pre-oxidation temperature. This is because the higher pre-oxidationtemperature can result in higher thermal stress within the oxidewhich facilitates spalling of the oxide scale [28].

Similar to the carburizing behavior of samples of batch 1, themass gain of the 1373 K pre-oxidized Sample 1 (batch 3) reacheda critical value during carburization and then started to decreaseas the experiment continued (Fig. 9). Surface morphology observa-tion confirmed that spalling took place on the Sample 1 of batch 3(Fig. 10a). Based on the mass gain in unit area and the density ofgraphite, the critical thickness of outer carbon layer can be approx-imately calculated according to:

n ¼ DmAl

ð2Þ

here n is the thickness of the coking layer, Dm is the mass gain ofthe alloy during the carburizing experiment, A is the surface areaof the alloy and ‘ is the density of the carbon. The relative parame-ters and the calculation results are shown in Table. 4. The resultshows that, compared with the Sample 1 of batch 3, the criticalthickness of the graphite layer on the samples of batch 1 was muchlarger. Because of the lower thermal expansion coefficient of graph-ite (4.9 � 10�6 K�1 [36]) compared with matrix (17.78 � 10�6 K�1

[37]), the outer layer on the alloy can suffered from thermal com-pressive stress during cooling. When stress exceeds a critical value,spalling takes place at the place where the strength is lowest. Refer-ring to Walter’s suggestion [38], the critical strain energy scale fail-ure under compressive deformation can be calculated according to:

�ecox ¼ �

ffiffiffiffiffiffiffiffiffi2c0

Eoxn

sð3Þ

where eox is the critical strain energy for scale failure and c0 is thesurface energy of the fracture surface. Thus, it can be inferred thatthe critical strain energy of the spallation of the graphite scale onthe Sample 1 of batch 3 is much higher than the samples of batch1 which indicates that the spallation did not primarily take placein the graphite layer. Further evidence can be obtained by the sur-face observation of the carburization samples. Compared with theSample 2, many bright areas were found on the surface of the Sam-ple 1 (Fig. 10a). EDS analysis demonstrates that the bright area isthe matrix (Table. 5), so scale detachment occurred between theoxide scale and matrix interface, which further accelerated the cok-ing and carburizing process during the experiment. The cross-sec-tion image of the carburization alloy confirmed this presumption.It was observed that severe cracking and spalling of the scale oc-curred on the surface of Sample 1, leading to the severe internal car-burizing (Fig. 11a).

Compared with the samples of batch 2, the similar carburiza-tion tendency of Sample 2 of batch 3 confirmed that the yttriumaddition ensured the integrity of oxide scale during the cyclic car-burization experiment, as shown in Fig. 9. A uniform graphite layerwas observed on the surface of Sample 1 of batch 3 (Fig. 10b). Inaddition, an integrity chromia layer was detected on the cross sec-tion profile of the alloy, accompanied with internal oxide devel-oped at the interface between oxide scale and matrix, as shownin Fig. 11b. Changing the scale rupture mechanism was consideredto improve the binding strength of the scale in our previous work[23]. As we have discussed above, the protective chromia layerinhibited the ion diffusion, and then retarded the development ofthe coke layer on the surface of the alloy. Internal carburizingwas also inhibited (Fig. 11b).

4. Conclusions

(a) The rapid coking speed of the cast sample induced a largethermal stress during the cyclic carburizing experiment. Itfacilitated the spalling of the out scale and led to exposureof the substrate. Although the spinel oxide (Mn1.5Cr1.5O4)developed in the environment with relatively low oxygenpartial pressure, the discontinuous spinel layer did not inhi-bit ion diffusion. As a result, internal carburization tookplace on the alloy during the cyclic carburizing. For compar-ison, the integrity chromia layer effectively protected thematrix from coking and carburizing attack.

(b) By inhibiting the outward diffusion of Ni and Fe, the forma-tion of filamentary coke was inhibited on the samples withan integrity oxide scale. Thus, free radical coke controlledthe coking process, and resulted in the formation of spheri-cal coke. However, the crack within the oxide and the cokelayer helped the outward diffusion of Ni and Fe, which facil-itated the formation of the filamentary coke.

(c) Scale with weak binding strength reduced carburizing resis-tance. However, yttrium facilitated the formation of internaloxide that improved the binding strength of the oxide scale.Thus, cyclic coking and carburization resistance wereimproved.

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