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Failure analysis of cracks occurred on SS304L tubes of shell and tube heat exchanger during its first start up in phases 2&3 gas refinery of South Pars

Gas Complex

Askar Soltani1, Mohsen Najafi2, Reza Ghorbani3, Reza danaei4

1Inspection Department of South Pars Gas Complex, IRAN, askarsoltani824@gmail.com

2 1Inspection Department of South Pars Gas Complex, IRAN, m.najafi@gmail.com

3 1Inspection Department of South Pars Gas Complex, IRAN, R.ghorbani@gmail.com

41Inspection Department of South Pars Gas Complex, IRAN, r.danaei@gmail.com

Abstract

Stress corrosion cracking in austenitic stainless steels is one the predominant failure root causes in oil and gas refineries. In this paper failure analysis of SS304L tubes in a shell and tube heat exchanger within one of the gas refineries in South Pars Gas Complex (SPGC) has been investigated. Tube side medium in this heat exchanger was sour gas coming from high pressure separators and shell side medium was low pressure steam in order to heat the gas during winter to prevent hydrate formation. To make sure of hydrate prevention, there is also a continuous injection of mono-ethylene glycol (MEG) into the gas stream before introducing the gas heater. This MEG may contain high levels of chloride and calcium ions which may cause problems in injected lines and proceeding equipments. This gas heater failed after a few days of startup. In visual inspection many cracks observed on the tubes and also deposits found inside the tubes. Samples taken from the deposits and XRD analysis were performed. Also, specimens were cut from the cracked area of tubes and analysed by scanning electron microscope. Diffraction pattern of XRD analysis indicated remarkable content of complex phases of chromium and calcium elements in the composition of the deposit. SEM images from cracked area demonstrated branched intergranular cracks. High chloride content of injected MEG and chromium depletion around the grain boundaries in the presence of tensile stress due to tubes internal pressure concluded as root causes of failure which can caused CLSCC1 . Keywords: CLSCC; MEG; EDX; SEM; XRD

1 Stress Corrosion Cracking

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Introduction Chloride stress corrosion cracking in austenitic stainless steel material has been reported by many researchers in many publications [1-5]. Due to the formation of passive layers on Active-Passive type alloys such as Stainless Steels, they have suitable resistance in corrosive environments but in the case of destruction of this layer, localized corrosion will occur. Several conditions may cause rupture of passive films on Stainless Steels as following:

Chemical species in solution which can cause local breakdown of the passive film, particularly the presence of chloride ions in contact with stainless steels and other alloys

Imperfections in passive film like physical defects, surface scratches and inclusions Loss of oxidizing species in solution (e.g., dissolved oxygen, Fe3+ ions, NO2− ions,

etc). Stresses and the presence of environmental conditions incapable of immediate film

repair in the case of film breakdown. The rupture propagates into the underlying metal and is sustained by the concentration of stress at the leading edge of the crack and by corrosion mechanisms associated with a crevice. The result would be stress-corrosion cracking (SCC) [6]. Pits are initiated at preexisting conditions on a passive surface or as a consequence of local events such as physical or chemical damage to the passive surface. Pit propagation will not occur if conditions lead to immediate repassivation of the local region. Pitting is usually preceded by an induction time to activate the local region following which the pit propagates as an occluded cell [6]. The pits, generally associated with alloys forming very protective passive films, is restricted to localized penetration of a passive film, resulting in a sharply defined discontinuity in the surface with penetration into the metal, which may enlarge with depth. Pits of this type may be initiated on a surface that is macroscopically free of deposits. Pits may also form under deposits of inert material that restrict access of a cathodic reactant or under deposits containing microbial species generating local acidic environments [5]. Any substance affecting the passive layer will affect the resistance to corrosion and particularly ions like chloride, fluoride and other halogenides are apt to break down the steel’s protective oxide layer and thereby increase the corrosion rate significantly. When a pit appears on stainless steel surfaces if this passive film cannot be repaired the pit will grow due to autocatalytic behavior [8-12]. Internal pressure inside the tubes imposes tensile stresses on the pits sharp ends and creates cracks which will grow by this tensile stress. Pit can penetrate through the wall and lead to leakage without fracture or as soon as reaching the materials fracture toughness a wall through crack will happen .Stainless steels are particularly susceptible to SCC in chloride environments and high temperature and the presence of oxygen can aggravate this phenomenon. All austenitic grades, especially AISI304 and 316 are susceptible to some degree. Although some localized pitting or crevice corrosion probably precedes SCC, the amount of pitting or crevice attack may be as small as to be undetectable. Stress corrosion is difficult to detect while in progress and can lead to rapid catastrophic failures of pressurized equipment [7]. In gas refineries a heat exchanger is considered to heat the gas stream coming from high pressure separators in order to prevent hydrate formation during winter case and keep the gas

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temperature higher than 18 degree Celsius. Depending on the sea line arrival temperature during winter and requirement for sea-line depacking operations, the gas is heated by low pressure steam in the gas heater. A schematic of process flow diagram including gas heater has been demonstrated in Fig.1.

Fig.1 Process flow diagram of gas heating unit

The type of this gas heater is a shell and tube heat exchanger. The material of tubes is according to ASTM A312 grade 304L. Process conditions of this gas heater during winter case have been illustrated in table 1.

Table 1 Process conditions of gas heater

Operating Temperature (C0)

Operating Pressure

(bar a)

in out in out

Shell side 155 147.9 4.5 4

Tube side 12.2 25 73.5 72.8

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This gas heater may be by-passed during summer time, except during packing and depacking operations. There is considered three mono-ethylene glycol (MEG) injection points on the raw gas stream after separation in slug catcher and before introducing into the gas heater tubes. MEG injection shall be injected continuously in these three points for hydrate inhibition and corrosion prevention. LP steam condensate from gas heater is recovered in steam condensate surge drums. The flashed steam is cooled down through air coolers and the produced LP condensate is routed to condensate recovery unit. Intermittent checking from this surge drum is done in order to monitor the purity of steam and check the probability of gas leakage from tubes into the steam side.

The aim of this paper is to investigate the root causes of failure occurred in AISI304L stainless steel tubes of a shell and tube heat exchanger in one of the gas refineries of South Pars Gas Complex in south of IRAN.

Examination

Visual examination

After a few days of startup of this heat exchanger, the laboratory results from the steam condensate showed hydrocarbon contamination. So, the exchanger put out of service and hydrostatic pressure test performed. Pressure drop during hydrostatic test indicated that there is leakage from some tubes. Visual examination was done after pulling out the tube bundle. Since the fluid in shell side was pure low pressure steam, no corrosion deposit observed on external surface of the tubes. Dye penetrant examination on accessible tubes showed that there are tubes with cracks and small pits on their external surfaces. Some of the failed tubes which marked during PT examination were cut to conduct metallographic analysis. Fig.2 shows circumferential cracks and pits observed on these tubes external surfaces.

Fig.2 Pitting and cracks observed on the tubes after liquid penetrant test

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One of the failed tubes was cut longitudinally to investigate its internal surface conditions. As illustrated in Fig.3, sticky and hard deposit observed inside the tube after cutting.

Fig.3 longitudinal cut of cracked tube and observing deposit inside the tube

After removing the sticky and thick deposits, so many pits revealed on the tubes internal surface as depicted in Fig.4.

Fig.4 Pitting inside the tube near the cracks

Sample taking

A sample was taken from the deposits inside the tubes to conduct XRD analysis in order to analyze its chemical composition. Also, a specimen was cut from cracked area for microscopic examination. In order to investigate the cracks morphology, the surface of the specimen investigated under light and scanning electron microscopes after surface preparation.

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Results

Laboratory analysis

The results of laboratory analysis from the deposits formed inside the tubes have been illustrated in table 2.

Table 2 Laboratory results of chemical analysis of deposits inside the tubes

Component Atomic %

Ca 87.9

Mg 3.5

Fe 3.9

Laboratory analysis from the injected MEG revealed high levels of chloride and calcium ions which were due to low efficiency in MEG regeneration unit.

XRD analysis

In order to conduct a more precise analysis, XRD examination was done on the sample which was taken from deposits inside the tubes. The analysis was done with Philips X’Pert diffractometer with CuKα radiation and diffraction pattern was taken from two-theta value of 8 to 110 degrees. Diffraction pattern has been shown in Fig.5. As it is obvious, the deposit consists of calcium complexes with high intensities.

Fig.5 Diffraction pattern of deposits inside the

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Light microscopic examination

To find the cracks initiation point, the specimen which was taken from cracked zone, was examined under light microscope. After finding the crack’s end it was selected to conduct microscopic analysis. Surface preparation was done by grinding with SiC papers of grit 120 to 1500 and final polishing was done with alumina abrasive particles. Etching the samples was first done by Nital 2% solution but no clear image from the grain boundaries obtained. Second tested solution was standard Kalling solution (containing HCl 33%, ethanol 33 cc, distilled water 33cc and CuCl2 1.5 gr) which usually apply for etching high strength steels and some grades of stainless steels, but it couldn't appear the grain boundaries. Efforts for etching continued by solutions containing HNO3 10 cc and HCl 20 cc but no obvious grain boundary appeared. Finally, electrochemical etching in Nitric acid solution by using an aluminum plate as cathode and applying DC voltage with the magnitude of 3 volts for 20 seconds revealed the grain boundaries under light microscope, as illustrated in Fig.6.

Fig.6 Light microscopic image from the etched surface at magnification of 500X

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One of the surface cracks was selected and microscopic pictures taken from the path of its growth. Fig.7 shows the cracks end with the magnification of 500X. As illustrated in this figure, the main crack has width of about 20 micron and highly branched cracking appearance is obvious.

Fig.7 Images of cracks end with highly branched appearance with the magnification of 500X

Since before getting to this branched appearance zone, the crack is relatively wide, we cannot talk about the cracks propagation route in this region confidently and investigations shall be pursued on the branched zone. However optical image in Fig.7 indicates intergranular corrosion morphology but for more precise investigation it was decided to continue investigations by applying SEM microscope.

SEM image analysis

Scanning electron microscopy was performed on crack containing region by Vega Tescan microscope and SE detector in different magnifications and the path of cracks pursued till getting the branched zone. Fig.8 illustrates the SEM image from this zone with the magnification of 1000X in which corrosion around the grain boundaries is obvious.

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Fig.8 SEM image from branched area of cracks with magnification of 1000X

SEM image with the magnification of 5000X from one individual grain has been illustrated in Fig.9, which shows the zone around the grain has been corroded significantly. This corrosion can be related to the breakdown of passive layer around the grains boundary due to chromium depletion in this area and the presence of aggressive ions like chloride which can cause pitting corrosion. At the next step the pits can grow and crack initiation will be happen and propagated through the tube wall until the fracture occurrence.

Fig.9 SEM image from an individual grain with magnification of 5000X

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Conclusion

High levels of chloride ion in the stream of injected MEG can deteriorate the passive layer inside the tubes and create pitting corrosion. If the passive layer cannot be repaired it can lead to cracks in the presence of imposed tensile stress inside the tubes [8-9].

The observed XRD pattern of deposits which was formed inside the tubes indicated high peaks for complexes of calcium and chromium ( CaCr2O7 and CaCrO4 ). Since the standard molar Gibbs free energy of formation ( ∆Gf

0 ) for CaCrO4 is about -1277.38 [Kj/mol] and for Cr2O3 is about -1053.11 [Kj/mol], so there is more thermodynamic affinity for chromium to form complexes with calcium ions in compare with chromium oxide formation and after destruction of passive layer in the presence of chloride and calcium ions, these complex phases could be formed and precipitate inside the tubes [13].

SEM images from individual grains indicate that grain boundaries had been corroded severely and in austenitic stainless steels this can be aggregated in chloride containing media due to the chromium depletion in grain boundaries and breakdown of the passive layer.

Pits which were observed inside the tubes after removal of deposits indicated that the passive layer of the stainless steel had been damaged and deposit formation could aggregate this situation.

Microscopic examination by light microscope and scanning electron microscope revealed cracks with highly branched appearance. Examination with higher magnifications revealed intergranular corrosion. High tensile strength due to tubes internal pressure besides localized corrosion can cause IGSCC2 and presence of high levels of chloride in injected MEG seems to cause CLSCC3 in stainless steel tubes. By considering all mentioned process conditions it was recommended to change the material of the tubes from stainless steel to carbon steel.

2 Intergranular Stress Corrosion Cracking

3 Chloride Stress Corrosion Cracking

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References

[1] Bijayani Panda, M. Sujata, M. Madan, S.K. Bhaumik “ Stress Corrosion Cracking in 316L Stainless Steel Bellows of a Pressure Safety Valve”, Journal of Engineering Failure Analysis, 36 (2014), pp. 379-389.

[2] Shugen Xu, Chong Wang, Weiqiang Wang, "Failure Analysis of Corrosion in heat exchanger tubes during startup operation", Journal of Engineering Failure Analysis, 51 (2015), pp. 1-8.

[3] S.H. Khodamorad, N. Alinezhad, D.Haghshenas Fatmehsari, K.Ghahtan, "Stress corrosion cracking in Type. 316 plates of a heat exchanger", Journal of case histories in Engineering Failure Analysis, 5-6 (2016), pp. 59-66.

[4] R. Parrot & H. Pitts, "Chloride Stress Corrosion Cracking in Austenitic Stainless Steel", Health and safety laboratory Executive, 2011

[5] Thomas D.Traubert, Tim Abjure, "Metallurgical Analysis to Evaluate Cracking in a 316L Grade Stainless Steel Spiral Heat Exchanger", Journal of Failure Analysis and prevention, 12 (2016), pp. 198-203

[6] E.E. Stansbury and R.A. Buchanan, "Fundamentals of electrochemical Corrosion", 2000, pp. 273-282.

[7] ASM handbook Vol.13, "Corrosion", ASM International 1987, Page 1349.

[8] API Recommended Practice 571, "Damage Mechanism Affecting Fixed Equipment in the Refining Industry ", First Edition, December 2003, Sec. 4.5.1.

[9] Khelfa. A. Esaklul, “Handbook of Case Histories in Failure Analysis", ASM International Failure Analysis Community, 2 (1993), pp. 126-129.

[10] J.Neshati, B. Adib, A. Sardashti, "Stress Corrosion Cracking Detection of stainless Steel 304 in Chloride Media by Using Electrochemical Impedance Spectroscopy", Journal of Petroleum Science and Technology, 2 (2012), pp. 55-60.

[11] Claus Qvist Jessen, "Stainless Steel Corrosion", First Edition, 2011, pp. 78-83.

[12] H.S. Khatak, Baldev Raj, "Corrosion of Austenitic Stainless Steels, Mechanism, Mitigation and Monitoring", Woodhead Publishing Limited & Alpha Science International, 2002, pp. 73.75.

[13] Dean, John A. Lange’s, “Handbook of Chemistry”, 11th ed., McGraw-Hill, New York, 1979; pp. 9:4-9:128.