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Investigation: Failure of a surface condensertitanium tube04.01.2011 | Al­Meshari, A., Saudi Basic Industriese Corp., Jubail, Saudi Arabia; Al­Enazi, S. , SABICTechnology Centre, Jubail Industrial City, Saudi Arabia; Diab, M. , SABIC Technology Centre, Jubail Industrial City,Saudi Arabia

Advanced analytic methods determine contributing factors in heat exchanger corrosion problem

Keywords:

This case history focuses on the investigation of localized thinning of titanium (Ti) tubes in a surface condenser inan ammonia unit. Several characterization techniques were applied, including stereomicroscopy, optical microscopyand other methods. Detailed analyses showed that the tube thinning is attributed to iron­induced crevice corrosion.Possible root causes for failure involved the presence of high concentrations of iron (Fe) particles and chloride (Cl)ions in the steam condensate, which can accelerate the corrosion process. Another factor was tube flow­inducedvibration that may have occurred at high processing flowrates, leading to a “localized” Fe deposition on the tubesurface. This case history outlines the sources for the failures as well as the recommendation to prevent future events.

Background.

Localized thinning was observed on Ti tubes of a surface condenser for an ammonia unit. The condenser is ahorizontal exchanger using straight tubes with two passes. The tube thinning was detected by eddy current testingperformed on 34% of the exchanger tubes. External wall loss was located in the middle of the top two rows of tubes,i.e., between baffles 5 and 7 (Fig. 1). The surface condenser had been in service for about 16.5 years.

Fig. 1. Schematic of the surface condenser showing thesteam condensate and seawater flow direction, as well asthe location of the severe tube thinning.

In the condenser, steam condensate flows into the shell side, whereas seawater is introduced in the tube side. Thematerials of tubes, tube sheets, and shell are B338 Gr.2 welded (Ti), SB265 Gr.2 Ti clad on SA516­70 carbon steel, andA516­70 carbon steel, respectively. The tubes are 7­m long, 0.7­mm thick and have 19­mm outer diameter. Table 1lists the steam condenser design and operating conditions.

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Visual examination. One tube sample, approximately 75­cm long, was submitted for analysis (see Fig. 2a). Thesample was deformed by the tube pulling process. Rounded, button­like, dark spots were observed at the 12 o’clockposition on the tube (Fig. 2b–2c). The spots were perfectly rounded and equally spaced, having a diameter of about 8mm. The distance between the centers of adjacent spots is approximately 13 mm. Fig. 3 is a close up photo of thespots. Stereomicroscopic examination of the spot surfaces revealed significant thinning that produced smoothgrooves covered with blackish layers.

Fig. 2. Rounded, button­like, dark spots observed at the12 o’clock position on the tube external surface.

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Fig. 3. Close­up views of one of the dark spots observed on the tube.

Chemical analysis. The chemical composition of the tube material was determined using X­ray fluorescence(XRF) spectrometry and C/S analyzer (Table 2). The material conforms to the chemical requirement for B338 Gr.2

(Ti).1

Surface analysis. The sample was examined under Scanning Electron Microscope/Energy Dispersive X­ray(SEM/EDX). The metal loss at the rounded spots produced a smooth, grooved surface (Fig. 4). EDX of the blackishlayer formed at the spot showed that it is composed mainly of Ti and iron oxides (Fig. 5). Some Na, Si, Cl and P were

also detected in the layer.b A thicker layer, containing higher concentrations of iron oxides, was noticed in the spot(Fig. 6). Interestingly, no Ti was found in that layer.

Fig. 4. SEM image of the spot surface showing the nature of corrosion.

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Fig. 5. SEM/EDX of the oxide layer formed at the spots.

Fig. 6. Thick oxide layer observed at the spot.

Metallographic examination. Cross­sections from the tube sample were prepared for metallographicexamination. Two cross­sections of the thinned areas are shown in Fig. 7. Severe thinning occurred in some areas(Fig. 7a), whereas milder thinning was observed in others (Fig. 7b). The minimum thickness measured wasapproximately 0.12 mm. The tube material microstructure possesses equiaxed grains, typical of annealed Ti type 2(Fig. 8). EDX of the oxide layer formed at reaction front confirmed the presence of high Fe concentrations (Figs. 9and 10).

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Fig. 7. Cross­sections of the tube wall showing different degrees of localized thinning.

Fig. 8. Tube material microstructure has equiaxed grains, typical of annealed Ti, as etched.

Fig. 9. SEM/EDX analysis of the layers

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formed at the affected areas.

Discussion.

In general, Ti alloys exhibit excellent corrosion resistance in many environments. They have always been one of thebest choices for such applications as surface­condenser tubes. Titanium owes its corrosion resistance to theformation of a protective, passive titanium oxide (TiO) scale. Nevertheless, Ti is not completely immune to corrosion.Indeed, Ti may readily corrode in certain conditions. For instance, the thinning observed on the subject surface­condenser tube appears to have been caused by a special type of crevice corrosion, often referred to as iron­inducedcrevice corrosion. As its name implies, iron­induced crevice corrosion occurs when Fe particles deposit on or aresmeared into the Ti surface forming crevices, thus leading to disruption of the protective TiO scale.2,3 As aconsequence, a galvanic cell is established between Ti (cathodic) and Fe (anodic), where Fe particles corrodepreferentially. The anodic dissolution of the Fe generates Fe ions that combine with Cl ions in the condensate to form

iron chloride that in turn reacts with water to produce hydrochloric acid (HCl) and metal hydroxide (MOH):4

MCl + H2O r HCl + MOH

Obviously, the formation of HCl results in a significant reduction in the solution pH at the crevice and that preventsthe reformation of the passive TiO film. Inevitably, the reaction will proceed until the tube is perforated. The attackcaused by Fe­induced crevice corrosion manifests itself by a very characteristic circular pit morphology. Iron­inducedcrevice corrosion is known to be catalyzed by temperature rise and/or high Cl concentration in the condensate.Therefore, the increase in the surface­condenser shell­side­inlet temperature would have aggravated the attack. Ironcarried over in the steam may have originated from corrosion and/or erosion of steel pipes and other components(e.g., impingement plate). Further, the surface­condenser tubes at the steam­condensate inlet were probably subject tosome vibration induced by the above­design flowrates in both tubes and shell sides. It is suggested that the Feparticles carried over in the steam hit, deposited and accumulated on the tube surface. The tube vibration led toredistribution of the Fe particles on the tube surface, such that Fe accumulation occurred at equally spaced areas,inducing the localized thinning.

However, it cannot be ruled out that the Fe particles could have been smeared over the tube surface during fabricationand installation processes. Localized corrosion of Ti tubing has been attributed to scratches in which traces of Fe

were detected.4 It is interesting to note that the surface condenser had been in service for about 16.5 years without anyfailures (or thinning), implying that the Fe particles were most likely carried over in the steam condensate, than ratherbeing smeared onto the tube surface during fabrication. This may be supported as the external tube­wall loss waslocated in the middle of the top two rows of tubes.

Fig. 10. SEM/EDX analysis of the layers formed on the condenser thinned tube.

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Conclusions.

The steam condenser tube thinning is attributed to Fe­induced crevice corrosion. Presence of high concentrations ofFe particles and Cl ions in the condensate accelerates the Fe­induced crevice corrosion. Tube flow­induced vibrationmay have occurred due to the above­design flowrates.

Recommendations.

The study generated several recommendations for the facility:• Surface condenser operating conditions should be kept within the design conditions.• Concentrations of Fe and Cl in the condensate must be monitored and controlled.• Source of Fe particles should be identified and eliminated to avoid formation of crevices. HP

LITERATURE CITED

1 ASTM B338­09, Standard Specifications for Seamless and Welded Titanium and Titanium Alloy Tubes forCondensers and Heat Exchangers.2 ASM Handbook, Vol. 13, Corrosion, ASM International, 1993.3 http://www.azom.com/, May 16, 2010.4 Donachie, Jr., M. J., Titanium: A Technical Guide, ASM International, 2000.

Nomenclature

Al AluminumC CarbonCa CalciumCl ChlorideCu CopperCr ChromiumFe IronK PotassiumMn MagnesiumNi NickelN NitrogenO OxygenP PhosphorusSi SiliconNa SodiumTi Titanium

The authors

Abdulaziz Al­Meshari is a failure analyst at SABIC Technology Centre­Jubail, Saudi Arabia. He hasa PhD degree in material science and metallurgy from the University of Cambridge (UK) and MSdegree in corrosion science and engineering from UMIST (UK). He has been a NACE and ASMmember since 2000.

Mohammad Diab is a failure analyst at SABIC Technology Centre­Jubail, Saudi Arabia. He has a BSdegree in mechanical engineering from King Fahd University of Petroleum & Minerals (KSA).

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Saad Al­Enazi is a failure analyst at SABIC Technology Centre­Jubail, Saudi Arabia. He has a MSdegree in manufacturing management and BS degree in mechanical engineering from the University ofToledo.

S. K.Mandal06.01.2012

Sir,We are facing iron carry over from air cooled condenser of our 2x150MW power plant due to which we aremaintaining high pH and blow down is more. DM water is reached more than 3%. Pleaas advice for solution.

Regards,(S.K.Mandal)