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Engineering Failure Analysis 18 (2011) 11081114

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Engineering Failure Analysisjournal homepage: www.elsevier.com/locate/engfailanal

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Failure analysis of heat exchanger tubes of four gas coolersS.R. Allahkaram a,, P. Zakersafaee b, S.A.M. Haghgoo ba 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, Iran b School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

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a b s t r a c tA Number of leaks occurred on four heat exchangers used on an off-shore platform in the south of Iran. As a result heat exchanger tubes made of Inconel 625 failed after only two years in operation. The failure was caused by pitting corrosion in two contact regions, tubes and bafes as well as in tube sheet and shell contact regions in spite of sufciently corrosion resistance of Inconel 625 to sea water. X-ray diffraction analysis was conducted on residual corrosion products, while micro structures of propagated pits were studied using scanning electron microscope and also examination of susceptibility of Inconel 625 to crevice corrosion was performed by multiple crevice assembly and anodic polarization in crevice solution. Investigation of failed exchanger tubes revealed that leaks in the tubes were due to the phenomenon of crevice corrosion. 2010 Elsevier Ltd. All rights reserved.

Article history: Received 4 September 2010 Received in revised form 15 November 2010 Accepted 28 November 2010 Available online 4 December 2010 Keywords: Heat exchanger Gas cooler Failure analysis Crevice corrosion

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 heat exchanger with the gas owing inside the tubes (inlet temperature = 66 C and outlet temperature = 45 C) and the seawater is driven through the shell side (inlet temperature = 31 C and outlet temperature = 38 C). The tubes, tube sheet and bafes are made of Inconel 625. The rst indication of the leakage in the tubes was observed only 6 months after commencement of operation. After one year, plant faced shut down due to low efciency of gas coolers during production. At this time the gas cooler 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 encountered in tube ends and at tube-to-tube sheet joints [1]. Crevice corrosion is a localized form of corrosion that occurs within crevices or at shielded surfaces, where stagnant solution is present. Degradation of materials due to crevice corrosion may cause leakage or loss of critical tolerances which may critically affect the performance [2]. NiCrMo alloys (Inconel 625) are used in marine environments, where corrosion resistance is essential. This class of alloys generally has excellent pitting resistance in marine service conditions. However, exposure studies have shown that nickel super alloys are susceptible to crevice corrosion in marine environments [3]. Oldeld and Sutton have rened crevice corrosion model mathematically and conceptually by describing the progression of four stages: Stage Stage Stage Stage 1: 2: 3: 4: Depletion of oxygen within the crevice. Increase in acidity and chloride concentration of the crevice solution. Permanent breakdown of the passive lm, and Propagation of crevice corrosion.

Corresponding author. Tel./fax: +98 2161114108.E-mail address: akaram@ut.ac.ir (S.R. Allahkaram). 1350-6307/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2010.11.015

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OldeldSutton model denes the initiation stage of crevice corrosion in terms of the time required to form a depassivating critical crevice solution (CCS). The CCS chemistry is argued to develop in crevices as a consequence of a sequence of events involving passive dissolution, crevice de-aeration, metal cation hydrolysis, and mass transport. In this model, the passive current density (ipass) provides metal ion hydrolysis. This anodic reaction also promotes oxygen (O2) depletion. Metal cations (e.g. Cr3+, Mo3+) hydrolysis within the crevice and the migration of chloride (Cl) ions into the crevice account for acidication. Acidication, in turn, leads to breakdown of the passive lm and enhanced anodic dissolution [46]. In this investigation various tests were performed to determine the causes of failure and to investigate crevice corrosion susceptibility 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 been removed. 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 leaks were located in conned areas where the tubes were in contact with the bafes (Fig. 2). Furthermore, dye penetrate testing (DPT) was carried out on tubes and sheet. The DPT revealed that damages to the tube sheet were conned 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 millimetres 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 conrmed by optical emission spectroscopy method (Quantometry analysis). Table 1 gives the composition 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). The analyses 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.

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Fig. 2. Photographs showing damages on tubes.

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

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 is observed, the EDX analysis from surface of the specimen away from the damaged regions shows short peaks of Cr, Mo, Fe and 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 conrmed to the regions where crevice corrosion had occurred, hence the corrosion behavior of alloy 625 was investigated using the multiple crevice assembly, electrochemical potential measurement and anodic polarization.

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

Table 1 Chemical composition of Inconel 625 in wt.%. Element specimen Cr Tube and tube sheet 21.2 Mo 7.95 Ni 61.01 Al 0.12 Mn 0.146 Si 0.368 Cu 0.156 Nb 3.48 Ti 0.251 C 0.016 P 0.005

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

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Fig. 6. Multiple crevice assembly.

Samples for corrosion testing were selected from the tube sheet due to the at nature of this rejoin of gas cooler. Corrosion specimens prepared in square sections with a nominal dimension of 50 50 3 mm. Specimens were dry-abraded to 800 grit using silicon carbide (SiC) metallurgical paper and then washed in acetone to remove any surface grease prior to corrosion testing. The temperature for all seawater corrosion experiments was xed at 45 C 2 C. 2.5.1. Crevice corrosion test using multiple crevice assembly Crevice 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 Teon 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 were removed 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 removed from the solution and cleaned with acetone, alloy 625 exhibited discoloration as shown in Fig. 7. 2.5.2. Electrochemical potential measurement For 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.

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

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