Engineering Failure Analysis 18 (2011) 1108–1114

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Engineering Failure Analysis

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Short communication

Failure analysis of heat exchanger tubes of four gas coolers

S.R. Allahkaram a,⇑, P. Zakersafaee b, S.A.M. Haghgoo b

a Center of Excellence in High Performance Ultra Fine Materials, School of Metallurgy and Materials Engineering, University College of Engineering,University of Tehran, P.O. Box 11155-4563, Tehran, Iranb School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 4 September 2010Received in revised form 15 November 2010Accepted 28 November 2010Available online 4 December 2010

Keywords:Heat exchangerGas coolerFailure analysisCrevice corrosion

1350-6307/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.engfailanal.2010.11.015

⇑ Corresponding author. Tel./fax: +98 216111410E-mail address: akaram@ut.ac.ir (S.R. Allahkaram

a b s t r a c t

A Number of leaks occurred on four heat exchangers used on an off-shore platform in thesouth of Iran. As a result heat exchanger tubes made of Inconel 625 failed after only twoyears in operation. The failure was caused by pitting corrosion in two contact regions, tubesand baffles as well as in tube sheet and shell contact regions in spite of sufficiently corro-sion resistance of Inconel 625 to sea water. X-ray diffraction analysis was conducted onresidual corrosion products, while micro structures of propagated pits were studied usingscanning electron microscope and also examination of susceptibility of Inconel 625 to cre-vice corrosion was performed by multiple crevice assembly and anodic polarization in cre-vice solution. Investigation of failed exchanger tubes revealed that leaks in the tubes weredue to the phenomenon of crevice corrosion.

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1. Introduction

There are four gas coolers located on a platform in seawater in the south of Iran. The gas cooler is a shell and tube heatexchanger with the gas flowing inside the tubes (inlet temperature = 66 �C and outlet temperature = 45 �C) and the seawateris driven through the shell side (inlet temperature = 31 �C and outlet temperature = 38 �C). The tubes, tube sheet and bafflesare made of Inconel 625. The first indication of the leakage in the tubes was observed only 6 months after commencement ofoperation. After one year, plant faced shut down due to low efficiency of gas coolers during production. At this time the gascooler exchangers were dismantled. It was found out that a number of tubes had failed due to corrosion.

One of the most common failure mechanisms of heat exchanger tubes is usually due to crevice corrosion that it encoun-tered in tube ends and at tube-to-tube sheet joints [1]. Crevice corrosion is a localized form of corrosion that occurs withincrevices or at shielded surfaces, where stagnant solution is present. Degradation of materials due to crevice corrosion maycause leakage or loss of critical tolerances which may critically affect the performance [2].

Ni–Cr–Mo alloys (Inconel 625) are used in marine environments, where corrosion resistance is essential. This class ofalloys generally has excellent pitting resistance in marine service conditions. However, exposure studies have shown thatnickel super alloys are susceptible to crevice corrosion in marine environments [3]. Oldfield and Sutton have refined crevicecorrosion model mathematically and conceptually by describing the progression of four stages:

Stage 1: Depletion of oxygen within the crevice.Stage 2: Increase in acidity and chloride concentration of the crevice solution.Stage 3: Permanent breakdown of the passive film, andStage 4: Propagation of crevice corrosion.

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Oldfield–Sutton model defines the initiation stage of crevice corrosion in terms of the time required to form a de-passivating critical crevice solution (CCS). The CCS chemistry is argued to develop in crevices as a consequence of a sequenceof events involving passive dissolution, crevice de-aeration, metal cation hydrolysis, and mass transport. In this model, thepassive current density (ipass) provides metal ion hydrolysis. This anodic reaction also promotes oxygen (O2) depletion. Metalcations (e.g. Cr3+, Mo3+) hydrolysis within the crevice and the migration of chloride (Cl�) ions into the crevice account foracidification. Acidification, in turn, leads to breakdown of the passive film and enhanced anodic dissolution [4–6].

In this investigation various tests were performed to determine the causes of failure and to investigate crevice corrosionsusceptibility of alloy 625 under operational condition.

2. Experimental procedure and results

2.1. Visual inspection

During one of the overhauls, tubes were removed for inspection. Fig. 1 shows inside the gas cooler after the shell has beenremoved. It can be seen that they fouled by marine growth and corrosion deposits.

Visual examination of the failed tubes revealed that leaks had been found on several regions of the gas coolers. The leakswere located in confined areas where the tubes were in contact with the baffles (Fig. 2).

Furthermore, dye penetrate testing (DPT) was carried out on tubes and sheet. The DPT revealed that damages to the tubesheet were confined to the back face of tube sheet region (Fig. 3).

The heat exchanger tubes were examined with a binocular. In effect, the defects were very large. They were several mil-limetres in depth and in length. Fig. 4 Indicates that the corroded area is about 7 mm long and 2 mm deep.

2.2. Chemical analysis

The alloy composition was confirmed by optical emission spectroscopy method (Quantometry analysis). Table 1 gives thecomposition which corresponds to alloy 625, a high nickel alloy containing 9% molybdenum.

2.3. XRD analysis

Deposits scrapped from the tubes and shell in the gas cooler was analysed using X-ray diffraction method (XRD). Theanalyses were as follows:

(1) Scale in the tubes: compound made of elemental sulphur crystals.(2) Deposits inside the shell: 60% carbonate compounds, 30% of iron oxides FeO(OH), Fe3O4 and the rest was minerals and

NaCl.

Fig. 1. (a) Fouling by marine growth on heat exchanger external tubes and (b) formation of deposit on tubes.

Fig. 2. Photographs showing damages on tubes.

Fig. 3. Photograph showing damage on back face of tube sheet.

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2.4. SEM and EDX analysis

Fig. 5 shows scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses of corroded area. As it isobserved, the EDX analysis from surface of the specimen away from the damaged regions shows short peaks of Cr, Mo, Feand Ni elements, whereas in the corroded regions, these elements have been depleted due to corrosion action in this region.The presence of Cl� ions shows that in these regions electrolyte solution is quite aggressive.

2.5. Corrosion testing

Since degradation on heat exchanger tubes was confirmed to the regions where crevice corrosion had occurred, hence thecorrosion behavior of alloy 625 was investigated using the multiple crevice assembly, electrochemical potential measure-ment and anodic polarization.

Fig. 4. Close-up view of one of the crevice area showing (a) length (b) depth.

Table 1Chemical composition of Inconel 625 in wt.%.

Element specimen

Cr Mo Ni Al Mn Si Cu Nb Ti C P

Tube and tube sheet 21.2 7.95 61.01 0.12 0.146 0.368 0.156 3.48 0.251 0.016 0.005

Fig. 5. SEM and EDX analyses of corroded area.

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Fig. 6. Multiple crevice assembly.

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Samples for corrosion testing were selected from the tube sheet due to the flat nature of this rejoin of gas cooler. Corro-sion specimens prepared in square sections with a nominal dimension of 50 � 50 � 3 mm. Specimens were dry-abraded to800 grit using silicon carbide (SiC) metallurgical paper and then washed in acetone to remove any surface grease prior tocorrosion testing. The temperature for all seawater corrosion experiments was fixed at 45 �C ± 2 �C.

2.5.1. Crevice corrosion test using multiple crevice assemblyCrevice corrosion tests were carried out on the specimens using the multiple crevice corrosion assembly shown in Fig. 6.

The multiple crevice washers were bolted to both sides of each specimen using Teflon bolts and nuts.The crevice assemblies were tightened to a torque of 8 N m. Periodic visual inspections were performed to determine the

crevice attack. The examination looked for signs of damages under the multiple crevice washers. Alloy 625 specimens wereremoved after 20, 50, 80 and 120 days from seawater.

After exposure of the specimens in Persian Gulf seawater at 45 �C for 15, 30, 45 and 60 days the specimens were removedfrom the solution and cleaned with acetone, alloy 625 exhibited discoloration as shown in Fig. 7.

2.5.2. Electrochemical potential measurementFor investigating the potential changes of alloy 625, the specimens were exposed to seawater for 60 days and the open

circuit potential (OCP) was monitored until steady state potential (SSP) was reached. All the potential measurements in this

Fig. 7. Optical image of alloy 625 after crevice corrosion test.

Fig. 8. Open circuit potential vs. time curve for alloy 625.

S.R. Allahkaram et al. / Engineering Failure Analysis 18 (2011) 1108–1114 1113

study have been carried out vs. saturated calomel electrode (SCE). Fig. 8 shows the OCP for alloy 625 exposed to seawater for60 days.

2.5.3. Anodic polarization testThe anodic polarization curves were generated on crevice free specimens exposed to a simulated crevice solution with

low pH (acidified to a pH � 0.25), high [Cl�] (saturated with NaCl), oxygen free (de-aerated with nitrogen). This crevice solu-tion chemistry was chosen based on experimental studies conducted by Oldfield and Sutton on pH measurements of lessthan or equal to 8.2 for the 625 specimens exposed to natural seawater [7].

The polarization curves were generated potentiodynamically at a scan rate of 1 mV s�1 and in a range of �400 mV to+800 mV. The anodic polarization behavior of alloy 625 in the simulated crevice environment and seawater is illustratedin Fig. 9.

3. Discussion

Ni–Cr–Mo alloys are practically immune to pitting corrosion because the alloying elements Cr and Mo provide resistanceto localized corrosion such as pitting and crevice corrosion in chloride containing solutions, but they may suffer due to cre-vice attack under aggressive environmental conditions [8,9]. Alloy 625 has been found to be prone to crevice corrosion inchlorinated ocean water, when exposed to certain crevice solution chemistries that are possible only in very tight crevicescharacterized by a narrow gap and extreme depth [10].

Crevice corrosion can occur under fouling, especially under hard-shell fouling in ambient temperature seawater. Crevicecorrosion may also occur under-deposits formed by heating seawater. Significant deposition could occur, when the seawatertemperature exceeds 55–60 �C if the seawater is not chemically treated [11].

Fig. 9. Anodic polarization curves for alloy 625.

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In this heat exchanger after washing tubes from fouling, close inspection revealed that there were not damages or cor-rosion under-deposits. Therefore, it is true to say that alloy 625 is quite resistant to under-deposit corrosion, under theseconditions. XRD analyses of deposits do not bring any new information. They are merely correlated with the fact that thegas flowing in the tubes contains H2S and as a result sulphur is present. On the shell side, the deposit results from the cor-rosion of alloys containing iron and contains salts from seawater.

OCP behavior of alloy 625 shows a more nobel behavior after 60 days of exposure to the seawater but it is susceptible tocrevice corrosion in location, where crevices are present. The EDX analysis from surface of the specimen away from the dam-age regions shows short peaks of Cr, Mo, Fe and Ni elements because in the corroded regions, these elements have been de-pleted due to corrosion action in this region. Presence of Cl� ions in these regions shows that electrolyte solution becomesaggressive. As a result, material resistance is reduced. Behavior of alloy 625 in these regions has been investigated in criticalcrevice solution. As it is shown in anodic polarization, there are two distinct regions of passivity for alloy 625 in seawater.This is due to the fact that this alloy is self-repassivating in seawater. This phenomenon has also been observed by Shaw et al.[12] when studying alloy 625 behavior in seawater using anodic polarization measurements. Here, too, the passive regionshave been split in two sections, which have been separated by a transpassive dissolution zone. It has been suggested that thesecond passive region occurring in upper section of the anodic polarization is common in nickel and nickel based alloys. Fur-thermore at much higher potentials (above 700 mV vs. SCE) transpassive dissolution is observed which corresponds to oxi-dation reaction of Cr. It has also been stated that the increase in current density at higher potentials to the transpassivedissolution and not to the localized Cl� ion indicate film break down, since at saturated NaCl concentration the potentialdid not decrease while the increase in current densities had been observed and no localized break down was noted.

Regarding anodic polarization curve for simulated crevice solution (acidified to pH of 0.25 and saturated with NaCl) thefirst passive region in the lower section of the curve is much smaller than the second passive zone occurring after a trans-passive separation zone. The passive behavior in this situation is different as compared to the neutral pH artificial seawater.In both cases the passive current densities were potential dependent. The passive current density in the simulated crevicesolution was found to be 1.5–2 orders of magnitude higher than that of seawater. The aggressive solution within the crevicehas changed the anodic polarization behavior of the alloy 625 inside the crevice.

According the low pH, high chloride concentration is responsible for increasing ipass, shifting the passivating potentialpositively and decreasing breakdown potential.

Decreasing the crevice gap increases the electrical potential along the crevice, increases the electrical conductivity of thesolution, and increases the severity of the crevice solution composition [7]. Oldfield’s modeling procedure has demonstratedthat tightness of the crevice is critical, showing that alloy 625 requires a much tighter crevice (<0.1 lm) than stainless steels(SS) (1–10 lm). Surface finish of the specimen six compressibility of the crevice former and sealing pressure on the creviceinterface each influence the crevice gap [3,13,14].

4. Conclusion

Under very tight crevice conditions, Inconel 625 can be severely attacked. Formation of deposits between tubes and baffleprovide tight crevices, which can entrap small amount of solutions that lead to enhanced corrosion of alloy 625 these regionsunder stagnant condition. Therefore, periodic cleaning of the heat exchanger from deposits is very necessary in order to pre-vent precipitations. Under such conditions, highly alloyed materials such as Inconel 686 and C-276 due to their inherentsuperiorities and corrosion resistance can be considered as suitable replacement for alloy 625. They have been shown to pos-sess far greater crevice corrosion resistance than Inconel 625, under similar laboratory condition.

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