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
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
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.
3. Close-up views of one of the
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
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.
4. SEM image of the spot surface
showing the nature of corrosion.
5. SEM/EDX of the oxide layer formed
at the spots.
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).
7. Cross-sections of the tube wall
showing different degrees of localized
8. Tube material microstructure has
equiaxed grains, typical of annealed Ti,
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
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
10. SEM/EDX analysis of the layers
formed on the condenser thinned
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
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.
1 ASTM B338-09,
Standard Specifications for Seamless and Welded Titanium
and Titanium Alloy Tubes for Condensers and Heat
2 ASM Handbook, Vol. 13, Corrosion, ASM
3 http://www.azom.com/, May 16, 2010.
4 Donachie, Jr., M. J., Titanium: A Technical
Guide, ASM International, 2000.