This case history focuses on the investigation of localized thinning of titanium (Ti) tubes in a surface condenser in an ammonia unit. Several characterization techniques were applied, including stereomicroscopy, optical microscopy and other methods. Detailed analyses showed that the tube thinning is attributed to iron-induced 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 flow-induced vibration that may have occurred at high processing flowrates, leading to a localized Fe deposition on the tube surface. This case history outlines the sources for the failures as well as the recommendation to prevent future events.
Localized thinning was observed on Ti tubes of a surface condenser for an ammonia unit. The condenser is a horizontal exchanger using straight tubes with two passes. The tube thinning was detected by eddy current testing performed 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 the steam condensate and seawater flow direction, as well as the location of the severe tube thinning. |
In the condenser, steam condensate flows into the shell side, whereas seawater is introduced in the tube side. The materials of tubes, tube sheets, and shell are B338 Gr.2 welded (Ti), SB265 Gr.2 Ti clad on SA516-70 carbon steel, and A516-70 carbon steel, respectively. The tubes are 7-m long, 0.7-mm thick and have 19-mm outer diameter. Table 1 lists the steam condenser design and operating conditions.
Visual examination. One tube sample, approximately 75-cm long, was submitted for analysis (see Fig. 2a). The sample was deformed by the tube pulling process. Rounded, button-like, dark spots were observed at the 12 oclock position on the tube (Fig. 2b2c). The spots were perfectly rounded and equally spaced, having a diameter of about 8 mm. The distance between the centers of adjacent spots is approximately 13 mm. Fig. 3 is a close up photo of the spots. Stereomicroscopic examination of the spot surfaces revealed significant thinning that produced smooth grooves covered with blackish layers.
| Fig. 2. Rounded, button-like, dark spots observed at the 12 oclock position on the tube external surface. |
| Fig. 3. Close-up views of one of the dark |
spots observed on the tube.
Chemical analysis. The chemical composition of the tube material was determined using X-ray 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 X-ray (SEM/EDX). The metal loss at the rounded spots produced a smooth, grooved surface (Fig. 4). EDX of the blackish layer 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.
| Fig. 5. SEM/EDX of the oxide layer formed |
at the spots.
| Fig. 6. Thick oxide layer observed at |
Metallographic examination. Cross-sections from the tube sample were prepared for metallographic examination. Two cross-sections 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 was approximately 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. 9 and 10).
| Fig. 7. Cross-sections of the tube wall |
showing different degrees of localized
| Fig. 8. Tube material microstructure has |
equiaxed grains, typical of annealed Ti,
| Fig. 9. SEM/EDX analysis of the layers |
formed at the affected areas.
In general, Ti alloys exhibit excellent corrosion resistance in many environments. They have always been one of the best choices for such applications as surface-condenser tubes. Titanium owes its corrosion resistance to the formation 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 surface-condenser tube appears to have been caused by a special type of crevice corrosion, often referred to as iron-induced crevice corrosion. As its name implies, iron-induced crevice corrosion occurs when Fe particles deposit on or are smeared into the Ti surface forming crevices, thus leading to disruption of the protective TiO scale.2,3 As a consequence, a galvanic cell is established between Ti (cathodic) and Fe (anodic), where Fe particles corrode preferentially. 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 prevents the reformation of the passive TiO film. Inevitably, the reaction will proceed until the tube is perforated. The attack caused by Fe-induced crevice corrosion manifests itself by a very characteristic circular pit morphology. Iron-induced crevice corrosion is known to be catalyzed by temperature rise and/or high Cl concentration in the condensate. Therefore, the increase in the surface-condenser shell-side-inlet temperature would have aggravated the attack. Iron carried over in the steam may have originated from corrosion and/or erosion of steel pipes and other components (e.g., impingement plate). Further, the surface-condenser tubes at the steam-condensate inlet were probably subject to some vibration induced by the above-design flowrates in both tubes and shell sides. It is suggested that the Fe particles carried over in the steam hit, deposited and accumulated on the tube surface. The tube vibration led to redistribution 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 fabrication and 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 any failures (or thinning), implying that the Fe particles were most likely carried over in the steam condensate, than rather being smeared onto the tube surface during fabrication. This may be supported as the external tube-wall loss was located in the middle of the top two rows of tubes.
| Fig. 10. SEM/EDX analysis of the layers |
formed on the condenser thinned tube.
The steam condenser tube thinning is attributed to Fe-induced crevice corrosion. Presence of high concentrations of Fe particles and Cl ions in the condensate accelerates the Fe-induced crevice corrosion. Tube flow-induced vibration may have occurred due to the above-design flowrates.
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
1 ASTM B338-09, Standard Specifications for Seamless and Welded Titanium and Titanium Alloy Tubes for Condensers 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.