(Type I corrosion and Type II corrosion) produced by Na2SO4. The mechanism of dierent types of hot corrosion is

discussed. Finally, the cause of hot corrosion is discussed including the origin of Na,V, S and the formation of Na2SO4.

furnaces had been commissioned for about 1 year with naphtha feed material. However, since 2002, four of

the furnaces used hydrogenation residual oil replacing naphtha as feed material. These four furnaces saw

* Corresponding author. Tel.: +86 21 6425 3055; fax: +86 21 6425 3810.

E-mail address: guankaishu@ecust.edu.cn (K.S. Guan).

Engineering Failure Analysis 12 (2005) 112

www.elsevier.com/locate/engfailanal1350-6307/$ - see front matter 2004 Elsevier Ltd. All rights reserved.The inuence of temperature, composition and microstructure of materials on hot corrosion is discussed. To avoid such

hot corrosion, precautions should be taken, including increasing Cr content in weldments, decreasing protrusion in

weldments, washing internal surfaces of tubes with hot steam regularly.

2004 Elsevier Ltd. All rights reserved.

Keywords: Hot corrosion; Pitting corrosion; Tube failures; Chemical-plant failures; Welded fabrications

1. Introduction

The tube weld joints of an ethylene cracking furnace saw severe pitting after a short time of service. TheK.S. Guan a,*, Fubiao. Xu b, Z.W. Wang a, Hong Xu a

a Research Institute of Process Equipment, East China University of Science and Technology, 130 Meilong Road,

200237 Shanghai, PR Chinab Shanghai Petrochemical Company Ltd., PR China

Received 25 May 2004; accepted 30 June 2004

Available online 2 September 2004

Abstract

Hot corrosion often takes place at elevated temperatures in gas turbines and other equipment. However, hot corro-

sion in ethylene cracking tubes is seldom seen. In this case, an examination of pitting in ethylene cracking tubes made of

HP 40 steel (25Cr35Ni) which failed in weldments (Ni-base alloy) has been conducted to identify the cause of failure.

Results show that these failures had been caused by hot corrosion produced by Na2SO4. A molten state was noticed

from the morphology. Analysis revealed that pitting in tube weldment was caused by high-temperature hot corrosionFailure analysis of hot corrosion of weldments inethylene cracking tubesdoi:10.1016/j.engfailanal.2004.06.003

pitting failures after 700900 h run with the new feed material. One of the failed furnaces was repaired by

welding. However, after 1 month of service, pitting was again detected in the tube weld zone of this repaired

furnace. By contrast, other furnaces which did not change feed materials have been running continuously

without any problems. So the failure is associated with hydrogen residual oil.

Pitting takes place at number of tube weld joints in the bottom portion of the inlet tubes in the radiantchambers. The tube is made of HP40 steel (26Cr, 35Ni, 0.4C), and weldment is made of nickel-based alloys.

on these specimens did not show any appreciable damage in OD (outer diameter) and the cross-section

2 K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112of the tube was found to be of uniform thickness. Outer surface was as usual brownish in colour and

thickness of external oxide scale was normal.

The failed tubes have no bulge, carburisation, deformation or wall thinning. There was no evidence

of any localized overheating on the outer and inner surfaces of the tubes. But the pitting is caused by

molten metal that indicated that low melting point compounds formed which results from mutual ac-

tion of media and weld metal. In general, failure of furnace tubes can occur in a variety of modes.

However, based on the above examinations, only the following modes may be deduced to cause the

failure of the tube welds: (1) high temperature sulde corrosion, for example, gas sulde, H2S, SO2 re-acts with nickel to form low melting point compounds; (2) hot corrosion produced by molten salt; (3)

weldment chemical components cannot meet the requirement. To identify the failure reason, chemical

components, metallography and corrosion products and corrosion media analysis should be carried

Table 1

Impurity components of hydrogenation residual oil and naphtha

Feed materials S (wt%) Fe (mg/kg) Ni (mg/kg) V (mg/kg) Cu (mg/kg) Na (mg/kg)

Naptha 0.045

pittingK.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112 3out by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diraction

(XRD) and Infrared spectroscopy (IR).

3. Results

3.1. Chemical composition of weld joint

Chemical compositions of weldment and weld wire are displayed in Table 2. The composition of the weld

joint is in agreement with the requirements of the specications for nickel-based metal which can satisfy

elevated temperature operation.

pitting

pitting

Fig. 1. Schematic diagram of failed tube.

Fig. 2. Pitting form and position in cracking tubes: (a,b) pitting at weld joint in bottom of input tube.

Composition C Mn Si S P Cr Ni Nb WWeld wire ERNiCr-3(82FM) 0.037 3.09 0.18 0.010 0.010 20.15 71.16 2.47

Weld metal 0.45 1.03 1.47 0.007 0.025 26.80 34.50 1.24 0.06Fig. 3. As received tubes: (a) outer surface, (b) inner surface.

Table 2

Chemical composition of weld wire and weldment material (wt%)

4 K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 1123.2. SEM morphology of corrosion surface

Fig. 4 shows the SEM morphology of the molten surface. The corrosion morphology presents obviously

uxing and uid ow.

3.3. Transverse sections and compositions of corrosion scale

It is important to know the elements of corrosion product at the initial site of the pitting corrosion.

Unfortunately, the original corrosion site had been molten. However, analyzing the side of pitting imme-

diately near the inner surface can still play an important role to identify the attacking ionic and elemental

species. Fig. 5 shows the cross-section of the corrosion product and EDS (in Table 3) analysis near the inner

surface for one of the received samples. As can be seen in the photomicrograph, close to the inner surface a

layer type corrosion characterized by an uneven scale/base metal interface without any sulde depleted

zones was found. Corrosion products consist of two layers: a thick external porous layer and an internallayer which was relatively compact.

Fig. 6 illustrates the corrosion-section of the corrosion product immediately near the inner surface for

another as received weld joint. Layer type corrosion is found, though the layer type is not as obvious as

that in Fig. 5. The local internal layer, marked B in Fig. 6, intrudes deeply into the matrix metal. TheEDS analysis of the corrosion product is given in Table 4. These layerwise analyses revealed that there were

loose deposits of haematite (Fe2O3), which is a common end product of corrosion of iron, in the outer layer.

Adjacent to the metal substrate there were relatively condensed layers with rich in Cr.

SEM observations, as in Figs. 5 and 6, reveal that the interior layer is darker than the metallic matrix. Ade-alloying eect, due to heavy element precipitation could be responsible for the observed colour of the

interior layer. The EDS analysis of corrosion products in dierent regions shows that the corrosion prod-

K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112 5ucts present S and Na, especially Na up to 67% and S above 1% immediately near the inner surface. Fig. 7

gives the corrosion-section of corrosion products near the outer surface, a non-layer type corrosion product

was present, characterized by a smooth scalemetal interface and a continuous, uniform precipitate-

depleted zone of Cr beneath the oxide scale (see Table 5).

Fig. 5. Corroded region consisting of two layers: (1) thick external oxide layer (marked A1, A2), (2) internal de-alloyed layer(marked B).

Fig. 4. SEM surface morphology of corrosion product.

Table 3

EDX analysis of corrosion products corresponding to Fig. 5 (wt%)

Element O Na S Cr Ni Fe Nb

Location A1 12.78 1.52 0.3 21.85 46.54 13.57Location A2 40.97 0.9 0.2 3.56 52.68Location B 9.29 1.3 0.8 32.99 41.4 10.34 5.21

Table 4

EDX analysis of corrosion products corresponding to Fig. 6 (wt%)

Element O Na S Cr Ni Fe Nb

Location A 48.42 7.38 1.07 2.55 2.11 32.54Location B 39.38 6.73 1.47 29.56 13.81 7.07

6 K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112Fig. 6. Initial site of corrosion product: (1) external oxide layer (marked A), (2) internal de-alloyed layer (marked B).3.4. X-ray diraction analysis of corrosion products

In order to identify the structure of compounds in the corrosion products, X-ray diraction spectra for

corrosion products was carried out. The principal constituents in the deposits are Fe2O3, Cr2O3 and NiFe

arranged from higher to lower content. The elements, such as S and Na or their compounds were too small

for bulk detection by the less sensitive XRD. However, their presence in the scales was unequivocal and

played a very important role in making the weld materials melt.

Fig. 7. Non-layer type corrosion products near outer surface.

Table 5

EDX analysis of corrosion products (wt%)

Element O Na S Cr Ni Fe Nb

Location A 57.15 0.92 0.04 11.73 2.9 18.83 0.49Location B 10.95 66.34 19.19 3.52

K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112 73.5. IR analysis of corrosion products

XRD cannot detect compounds of S and Na. Of particular interest is whether or not sulde salts of so-

dium were formed. Fig. 8 shows the IR analysis result. SO24 ions were detected in the corrosion productand it is possible that Na2SO4 developed.

4. Discussion

4.1. Failure mode

4.1.1. Over-heating

Chemical component analysis shows that the weld joint is a nickel-based alloy which meets the require-

ments of elevated temperature operation. No local temperature excursions, grain growth or extensive creep

at the location of failures could be observed. No evidence of creep or carburisation are found at the failed

location, nor any grain growth observed in the attacked regions. Hence, an over-temperature event for the

local surface of the tube could be ruled out. Measurement of furnace temperature has also conrmed that

the operating temperatures were within design (680900 C).

Fig. 8. IR analysis result of corrosion products.

Then, the only two failure modes that could explain the pitting type failure with molten corrosion scale

are suldation caused by sulde or hot corrosion caused by molten salt.

4.1.2. Suldation

Nickel based metal is susceptive to suldation corrosion. It is common that a relatively high content of Sin the feed gas or condensed to the failed sites would be required to cause suldation failure. Frequently,

sulde layers or sulde particles are observed in the corrosion products or between corrosion scale and me-

tal matrix (internal sulphide). However, no evidence of suldation could be detected in the attacked loca-

tion. Furthermore, if suldation was responsible for pitting failure, it is dicult to explain the absence of

failures in other furnace tubes using naphtha as feed materials which contain more S than hydro residual

oil. Suldation causing pitting failure could also be eliminated as a cause of failure in view of prior

experience.

4.1.3. Hot corrosion

This is the only mode of failures that could explain pitting with a molten surface in the presence of a

relatively low content of S. The Na2SO4 induced accelerated oxidation of numerous nickel-base alloys

has been observed [24]. The morphology and general characteristics of the failed location near the outer

surface are typical of hot-corrosion attack of Type I, i.e., high-temperature attack by sodium sulphate, and

near the inner surface Type II hot corrosion, i.e., low-temperature attack [5,6]. Corrosion product analysis

revealed that it is in agreement with the low-temperature hot corrosion characterization with little or no

chromium sulphides or alloy depletion in the metal ahead of the corrosion front [7]. As a summary: (1)the major products of hot corroded weld joints are Fe2O3 and Cr2O3, (2) the alloy/scale interface is very

irregular near the inner surface, and even near the outer surface.

4.2. Molten salt-induced hot corrosion

Several mechanisms have been suggested to explain the process of hot corrosion. The initiation of hot

corrosion is often attributed to failure of the protective oxide layer, which allows the molten salt to access

directly the substrate metal. This failure may result from corrosion, chemical reactions, etc., [811].The mechanisms proposed for the hot corrosion propagation stage are the salt uxing and electrochem-

ical mechanisms [1215]. The salt uxing mechanism was originally proposed by Goebel and Pettit [4].

According to this mode, the protection eciency of the surface oxide layer might be lost as the result of

uxing of this layer in the molten salt. The uxing can be caused either by the combination of oxides with

O2 to form anions (i.e., basic uxing), or by decomposition of oxides into the corresponding cations andO2 (i.e., acidic uxing). Acidic uxing takes place when the O2 activity in the molten salt is markedlylowered; it leads to a much more severe oxidation compared with basic uxing. As opposed to basic uxing,

acidic uxing can be self-sustaining, since displacement from stoichiometry does not become progressivelymore dicult as the reaction proceeds. A negative solubility gradient was proposed as a general criterion

for continuing hot corrosion attack [12]. However, some hot corrosion cases take place which do not have a

negative solubility gradient.

Because fused Na2SO4 is a dominant ionic (Na+) conductor, the overall corrosion mechanism must be

electrochemical, as for aqueous corrosion, and many supporting experimental studies have been done [16

19]. In salt lms containing substantial concentrations of multivalent transition metal ions (e.g., Fe2+/Fe3+,

V4+/V5+, etc.), the oxidation reaction at the substrate/salt interface and the reduction reaction at the salt/gas

interface could involve these species (in preference to SO3 reduction), with a countercurrent diusion ofthese ions carrying the current through the salt lm. Thus, in many aspects, the electrochemical reaction

not only certainly occurs, but also seriously aects the acidbase chemistry of the salt lm, and the asso-

8 K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112ciated gradients.

4.3. Identication of the failure mechanism for the tube weld joints

From above analysis, we can deduce the corrosion procedure and corrosion mechanism as follows. Ini-

tially, Type II corrosion is dominant, due to a relatively low temperature at the inner surface, characterized

by pitting attack. Since the weld joints protrude at the inner surface, the salt or S, Na, V elements can de-posit without diculty. When the accumulated sodium sulde reaches a certain concentration at the initial

nickeTh

(1)

salt, and localized corrosion can take place and propagate. Due to the area of initially molten site being

betwe

ized c

to hig

K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112 9weld joint due to its low Cr content and the high potential in the base metal due to its high Cr content, the

pitting is conned to the weld joint. It is easy to understand that hot corrosion takes place only in weld

joints by pitting.

(2) Anodic and cathodic reactions. The reactions can be written as:

Oxidation : M M2 2e 1

Reduction : O2 4e 2O2 2Another reducing species may be S ions.

S2O27 2e SO24 SO2 O2 3

The analysis shows that Fe is rich in the corrosion product. So Fe ions are the main reducing species.

Fe3 e Fe2 4Anodic oxidation of the matrix metal can occur in the environment not only if it contacts directly with

molten salt, but also if it is covered by an oxide layer. In the former case, anodic ions dissolve directly into

the molten salt, in the latter, anodic ions diuse outward through the oxide layer and dissolve into the mol-

ten salt or produce an oxide layer by reacting with O2.(3) Formation of corrosion products. The following reactions may be expected to occur according to Eqs.

(1)(4) and the result of the corrosion product analysis.en the site beneath the molten salt and the substrate metal beneath the oxide layer, just like the local-

orrosion commonly observed in aqueous media. This is because that there is a potential dierence due

h cathode:anode area ratios. The schematic model is shown in Fig. 9. Due to the low potential in thevery small, other substrate metal is still covered by the protective oxide layer. A local galvanic cell is formedPitting attack. As explained above, once the initial site is molten, the metal can access directly moltencic deductions are as follows:l level which is of benet for both forms of hot corrosion.e electrochemical mechanism is introduced to explain the hot corrosion of the tube weld joints. Spe-site, it can react with nickel to form low-melting eutectics of Na2SO4NiSO4 which cause the failure of the

protective oxide layer and allow the molten salt to access directly the substrate metal. During pitting prop-

agation to the outer surface through the wall, the temperature increases, and Type II corrosion changes to

Type I corrosion.

As far as the base metal (HP40) is concerned, the inner surface is smooth, and salt is dicult to deposit

on the base metal surface. On the other hand, HP40 alloys contain a relatively high chromium level and lowFig. 9. Model of localized hot corrosion in weld fusion zone.

At the interface of metal/molten salt, metal is dissolved:

Fe Fe2 2e 5

Ni Ni2 2e 6

Cr Cr3 3e 7Further oxidation can occur for Fe2+:

Fe2 Fe3 e 8O2 enters into the crystal lattice of oxides and reacts with Fe to produce Fe2O3.

O increases and results in the deposition of these oxides. Due to forming in the molten salt, the oxide is

loose

Ni2SO

conte

hot c

scale.

10 K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112Na2SO4- NiSO4 meltFe2O3

Cr2O3 orrosion is predominant, with a continuous, uniform precipitate-depleted zone of Cr beneath the oxide

The surface scales consist of non-protective chromium and iron oxides.A schematic representation of the corrosion products, is provided in Fig. 10. The essential factures of the

corrosion products associated with dierent types of corrosion are as follows: (1) In the low temperature

form of layer type hot corrosion, the oxides of chromium and nickel are increasingly agglomerated into

large interconnecting oxide networks. The outer scale contains predominantly the non-protective oxide

of iron, and the inner layer is non-protective Cr-rich oxides. (2) With increasing temperature, non-layer4Na2SO4 which is blow away in high speed gas. Some Ni ions form NiFe alloy particles. So the

nt of Ni is low in the products.As far as Ni ions is concerned, following reaction is expected to take place. Some Ni2+ ions is in molten2+and non-protective.3O2 2Fe3 Fe2O3 9The binding energy for Cr3+ and O2 is stronger than that for Fe and O2. So the Cr3+ can replace Fe

ions in the oxides and allow Fe ions to diuse to the outside. In this way, the oxides in the Fe-rich outer

surface layer and Cr-rich inner surface layer grow continuously into the matrix metal. Due to the low activ-

ity of O2 at the salt/metal interface, Fe2+ dissolved into the molten salt is thermodynamically stable in oxi-des. However, these Fe2+ ions diuse out to the salt/gas interface and produce oxide:

Fe2 O2 Fe32O2 10Furthermore, loose Fe2O3 layer formed at the outer surface.

3O2 2Fe3 Fe2O3 11At the molten salt/gas interface, Cr3+ ions in the molten salt oxidise and O2 is reduced. So the activity of

2Fig. 10. Schematic representation of corrosion products.

known that Ni alloy is very susceptible to this form of corrosion and chromium is the most eective alloyingelement to combat hot corrosion [7,12]. Many of these aspects of alloy chemistry have relevance to theproblem of corrosion. The early alloys owed their corrosion resistance to the development of a protective

Cr2O3 oxide scale. However, this oxide will not form on nickel-base alloys if the chromium content falls

below 1015%. Based on experience, weld wire containing the same alloy compositions as the base metalThe reductive eect of carbon in the oxidized protective layer has been demonstrated in [20]. The reduc-

ing species is CO, allowing a fast reduction at the gas/salt interface. So CO plays a part in the acceleratedattack.

5. Prevention approaches

Several approaches should be employed to control hot corrosion of the tube weld joints. These ap-

proaches includes proper selection of alloys of weld wire and joint structure, and washing the tube inner

surface with hot vapor.

5.1. Alloy of weld wire selection

The resistance of superalloys to hot corrosion is directly related to the chemical composition of the alloy

and its thermomechanical history. Unfortunately, many alloying elements have an adverse eect on the

mechanical properties of the superalloy at high temperature and its resistance to hot corrosion [13]. It is

(a) (b)

Fig. 11. Alternative V-groove joint type which ensures the inner surface is smooth: (a) original joint type, (b) new joint type.

K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112 11(HP40 alloy) should be used instead of the original Ni-base alloy weld wire.

5.2. Structure of weld joints

In order to ensure complete joint penetration, a V-groove preparation was used originally in the jointshowed in Fig. 11. This joint type presents an uneven surface at the inner surface which allows salt to de-

posit easily. So one should design another V-groove joint type to ensure that the inner surface is smooth.

The salt does not then accumulate at the joint sites.

5.3. Wash with hot vapor

A proven approach to minimize hot corrosion is to wash the tube using hot vapor. Hot vapor can dis-

solve and carry away deposited salt and prevent the initiation of hot corrosion. Specic washing proceduresare usually covered in detail in the relevant maintenance manual for the tubes concerned.

6. Conclusions and recommendations

Pitting failure took place in the tube weld joints of the cracking furnace. Molten state and ow is obvious

on the failed surface. Corrosion products analysis shows that a layer type corrosion characterized by an

uneven scale/base metal interface without any sulde depleted zones is found close to the inner surface.A non-layer type corrosion product is present, characterized by a smooth scalemetal interface near the

[15] Kish JR, Ives MB, Rodda JR. Corrosion mechanism of nickel in hot, concentrated H2SO4. J Electrochem Soc

2000;147(10):363746.

12 K.S. Guan et al. / Engineering Failure Analysis 12 (2005) 112[16] Rahmel A. Electrochemical aspects of molten-salt-enhanced corrosion. Mater Sci Eng 1987;87:34552.

[17] Rapp Robert A. Hot corrosion of materials: a uxing mechanism?. Corros Sci 2002;44:20921.

[18] Kim JJ, Cho SM. Eect of galvanostatic treatments on hot corrosion of Ni. J Mater Sci Lett 1994;13(21):15736.

[19] Hara M, Shinata Y. Electrochemical studies on hot corrosion of NiCrAl alloys in molten Na2SO4NaCl. Mater Trans JIM

1992;33(8):75868.

[20] Otero E, Pardo A, Perez FJ, Alvarez JF, Utrilla MV. The eect of dierent surface treatment on the molten salt hot corrosion of

In-657 superalloy. Surf Coat Technol 1996;85:15662.outer surface. The principal constituents in the deposits are Fe2O3, Cr2O3, NiFe compounds and SO24 ions.

Tube weld joint failure is attributed to hot corrosion (Type II low-temperature corrosion at the initial

stage, and Type I high-temperature corrosion at the propagation stage), produced by sodium sulde.

Several approaches should be employed to control hot corrosion of tube weld joints. These approaches

include: (1) weld wire containing the same alloy compositions as base metal (HP40 alloy) should be used

instead of the original Ni-base alloy weld wire; (2) a proper joint structures should be introduced to ensure

the inner surface is smooth; (3) the tube inner surfaces should be washed with hot vapor.

References

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Failure analysis of hot corrosion of weldments in ethylene cracking tubesIntroductionVisual observations and experimentalResultsChemical composition of weld jointSEM morphology of corrosion surfaceTransverse sections and compositions of corrosion scaleX-ray diffraction analysis of corrosion productsIR analysis of corrosion products

DiscussionFailure modeOver-heatingSulfidationHot corrosion

Molten salt-induced hot corrosionIdentification of the failure mechanism for the tube weld joints

Prevention approachesAlloy of weld wire selectionStructure of weld jointsWash with hot vapor

Conclusions and recommendationsReferences