J. BATELI] et al.: THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION ANALYSIS
317–323
THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION
ANALYSIS
ANALIZA KOROZIJE NA KONDENZACIJSKI CEVI
V TERMOELEKTRARNI
Jakov Bateli}
1
, Vedrana [pada
2
, Marko Kr{ulja
3*
, Sanja Martinez
4
1
HEP Production 52234 Plomin luka, Plomin luka 50, Sector for Thermal Power Plants, Plomin Power Plant Operation, Croatia
2
Istrian Polytechnic, METRIS Materials Research Centre of the Region of Istria, Zagreba~ka 30, 52100 Pula, Croatia
3
University Juraj Dobrila of Pula, Faculty of Engineering, Zagreba~ka 30, 52100 Pula, Croatia
4
University of Zagreb, Faculty of Chemical Engineering and Technology, Department of Electrochemistry, Maruli}ev trg 19,
10 000 Zagreb, Croatia
Prejem rokopisa – received: 2023-01-14; sprejem za objavo – accepted for publication: 2023-05-15
doi:10.17222/mit.2023.747
The thermal-power-plant condenser tubes, conducting cooling seawater, experienced frequent leakages, prompting the hereby
presented failure analysis. Visual inspection of the corroded/ruptured tube segments revealed that localized corrosion may be
linked to seawater sediments and thick layers of corrosion products at the tube water side. Sediments were found along the tube
bottom and thick corrosion products were found adjacent to the point of contact between the baffle plates and the exterior tube
walls. Underdeposit corrosion, accelerated due to sulphate-reducing bacteria, was diagnosed based on the excess sulphur found
at the pit bottom using EDX, the FT-IR spectrum indicating the presence of biofilm and SEM determining the featuring corro-
sion products based on the biofilm. It was proposed that the corrosion initiating deposits were linked to the condenser shutdown
and startup periods. Results showed that pitting corrosion occurred under the influence of biofilm microorganisms tolerant to
copper. The main goal of this research was achieved by defining a systematic procedure for avoiding a shutdown of the thermal
power plant.
Keywords: corrosion, pitting, thermal power plant, condenser
Kondenzacijske cevi analiziranega primera, ki se nahajajo v termoelektrarni Plomin, Hrva{ka prena{ajo hladilno morsko vodo.
Pri tem pogosto pride zaradi korozije do zaznavanja nenadnega pu{~anja cevi in potrebno je takoj{nje ukrepanje oziroma
odprava netesnosti. V pri~ujo~em ~lanku avtorji opisujejo analizo po{kodb v enem od tak{nih primerov. Vizualni pregled
segmentov korodirane in po{kodovane cevi je pokazal, da je lokalna korozija morda povezana z usedlinami iz morske vode in
debelimi plastmi korozijskih produktov na vodni strani cevi. Usedline so avtorji na{li vzdol`no na dnu cevi in debele plasti
korozijskih produktov so se nahajale tudi v bli`ini stika med odbojnimi plo{~ami in zunanjimi stenami cevi. Korozija pod
usedlino je bila pospe{ena zaradi delovanja sulfatnih redukcijskih bakterij, kar so avtorji potrdili s prisotnostjo prebitne
vsebnosti `vepla na dnu korozijske jamice. Karakterizacijo mest na katerih je pri{lo do korozijskih po{kodb so avtorji izvedli z
rentgensko energijsko difrakcijsko spektroskopijo (EDX) in infrarde~o spektroskopijo s Fourierjevo transformacijo (FTIR).
FTIR spektrum je potrdil prisotnost bio filma in s pomo~jo vrsti~ne selektronske spektroskopije (SEM) so le tega skupaj s
korozijskimi usedlinami morfolo{ko opredelili oziroma ovrednotili. Avtorji na osnovi analiz ugotavljajo, da je za~etek
nastajanja korozijskih oblog posledica zagonov in ustavitev obratovanja termoelektrarne. Rezultati analiz so pokazli tudi, da je
pri{lo do jami~aste korozije zaradi mikroorganizmov, ki se nahajajo v biofilmu in so odporni na baker. Glavna prednost in
spoznanje pridobljeno s temi analizami je definiranje postopkovnega sistema s katerim naj bi se ~imbolj izognili ustavitvam in
ponovnim zagonom obratovanja termoelektrarne.
Klju~ne besede: jami~asta korozija, termoelektrarna, kondenzator
1 INTRODUCTION
Tubes from the shell and tube condenser that use
cooling seawater in a coastal thermal power plant that
encountered frequent leakages, were investigated in or-
der to determine the cause of corrosion. The thermal
power plant TE Plomin 2 Block B has a nominal load of
217 MW, a high-pressure steam boiler of 670 t/h
(147.4 bar/535 °C) using coal as fuel. The efficiency of
the boiler is 92.9 % with a thermal power of 544 MW.
The condenser with 12,000 tubes of the steam turbine is
cooled by sea water in one pass, and the intake of cool-
ing water is located at the Plomin Bay at a depth of
24 m. The amount of steam is 116.36 kg/s and the
pressure is 0.046 bar with a dryness of 91 % while the
cooling water flow is 86,677 kg/s; the intake temperature
is 20 °C and the outlet temperature is 27.5 °C. A pH
value of 8.83 and oxygen concentration of 79 mg/L in
February proved that the cooling water taken from deep
water intake is saturated with oxygen, having the pH
value in the normal range. A test of the sea water showed
that NH
3
< 0.1 mg/L, Cu < 2 mg/L, Fe < 2 mg/L, and sil-
icon dioxide average is 0.0021 mg/L. The average salin-
ity of the Adriatic Sea is around 38.3 ‰ while the aver-
age salinity of the ocean is 35 ‰. The tubes were made
of CuZn20Al2As alloy and were declared to conform to
the EN 12451 and VGB-R 106e standard guidelines.
1
The length of a tube is 11,920 mm. The operating envi-
ronment conditions within the tube condenser can cause
Materiali in tehnologije / Materials and technology 57 (2023) 4, 317–323 317
UDK 620.193.472/.479:621.311.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(4)317(2023)
*Corresponding author's e-mail:
mkrsulja@unipu.hr

premature failure of tubing and piping. In some cases
these include an inadequate ferrous sulphate addition and
a high level of chlorination.
2
In other cases, the failure of
condenser tubes is microbially influenced by a biofilm
formation of copper-tolerant microorganisms.
3,4
A failure
of the condenser brass tube in thermal power units is
caused by a failure of the condenser cooling tube, which
is an occurring problem.
2–5
The tube interior includes the
water side of the condenser, and the velocity of seawater
inlet at an unstable or high degree of turbulence can also
lead to corrosion of copper alloys. The corrosion rate
was evaluated and the copper loss was 60 mg/L at a cool-
ing water flow of 8677 kg/s. The flow velocity varies
from 5.5 ms
–1
to 8.5 ms
–1
and the calculated flow rate
equals 1.89 m
3
s
–1
, which is lower than the maximum al-
lowable flow rate of 2 m
3
s
–1
, specified by the tube manu-
facturer. The system operates in the once-through mode.
The seawater inlet temperature varies from 12 °C to
21 °C and the outlet temperature varies from 18 °C to
28 °C. In order to execute the overhaul, the power plant
is shut down for an average of 30 days per year when the
condenser tubes are emptied of the seawater. The cause
and type of corrosion are investigated to avoid the need
for a tube replacement and to lessen the intensity of fail-
ure of the power plant that cause high financial losses.
2 EXPERIMENTAL PART
For the investigation of a tube specimen made from
CuZn20Al2As, several methods and types of equipment
were used. A GDS500A LECO optical emission spec-
trometer was used for determining the chemical compo-
sitions of metals and alloys, together with the incandes-
cent discharge method and 99.999 % argon.
A microhardness test was carried out according to
HRN EN ISO 6507-1: 2018.
6
A metallographic analysis,
a light microscope analysis, was performed with an
Olympus BX51 metallographic microscope. Scanning
electron microscopy (SEM) and energy dispersive X-ray
spectroscopy (EDS) that allow a targeted analysis of
sample surfaces were conducted with a QUANTA FEG
250 SEM FEI scanning electron microscope with field
emission and OXFORD PENTA FET EDS detector.
An EDS microanalysis of the chemical compositions
of solid materials was carried out using FE SEM (SEM),
a HighVacumm secondary electron detector with 20 kV
and a spot size of 5 μm. A FT-IR spectroscopy analysis
of the chemical composition was carried out with IR
spectroscopy, and KBr tablets were made using a Tensor
27 FT-IR spectroscope. The spectra were the result of the
mean value of 64 recorded spectra for a single sample,
and the recording resolution was 4 cm
–1
. The values of
the obtained bands are marked on the spectrum.
3 RESULTS
3.1. Visual and optical microscopy analysis
Two tube segments were examined visually and mi-
croscopically, one with a rupture and one without any
rupture. Photographs of both tube segments are shown in
Figures 1a and 1b, the condenser is shown in Figure 1c
and the positioning of tubes in the condenser is shown in
Figure 1d. In general, the entire steam side of the inves-
tigated tube segments was covered with a uniform,
brown, thin and well-adhering layer of copper patina ex-
cept for the areas with a partially faded oxide band,
showing that the tubes were in direct contact with the
baffle plate. Also, the ruptured-tube shape indicated a
tube bending over the baffle plate. No other corrosion or
erosion defects were found at the steam side of the tubes.
The rupture was situated within the faded oxide band;
the thickness of the tube was around 1 mm. The steam
J. BATELI] et al.: THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION ANALYSIS
318 Materiali in tehnologije / Materials and technology 57 (2023) 4, 317–323
Figure 1: Photographs of: a) the ruptured tube segment, b) the interior of the non-ruptured tube segment, c) condenser, d) tubes in the condenser

side of the tube in the immediate area of rupture is
shown enlarged in Figure 2a. At the water side of the
tube, the same rupture was observed within the localized
pit of an irregular shape (Figure 2b), while the rest of
the surface in the immediate vicinity was covered by a
cracked light-brown layer of corrosion products, through
which red and black sublayers were visible. The bottom
of the pit was black.
At the water side of the tubes, adjacent to the sup-
posed contact with the baffle plates, the areas of red and
black discolouration were visible below the golden-
brown cracked layer (Figure 1b). The circumference po-
sition of these thick corrosion-product deposits varied
depending on the baffle-plate contact spot. On the sup-
posedly bottom part of the tube, from approximately
5:00–7:00 hours, a reddish-brown layer extended along
the tube. Shallow pitting damage was evident within that
area in the form of individual pits or clusters of smaller
pits.
The rest of the water side was covered with a uni-
form, thin, golden-brown layer. No significant corrosion
damage was observed in that area and it could be con-
cluded that the layer protected the metal well. A similar
layer was observed
7
on copper alloys under the influence
of flowing (1.5 ms
–1
) aerated seawater at a temperature
> 40 °C. At temperatures between 20 °C and 30 °C, this
layer is usually darker, brown and partially adherent, and
a dark-brown, well adhered layer is formed at tempera-
tures below 10 °C.
3.2. Chemical analysis
Table 1 shows the chemical composition of the
CuZn20Al2As alloy. Except for arsenic (As), all the ele-
ments of the alloy are within the permissible limits given
by EN 12451. The sum of bismuth (Bi), silicon (Si) and
sulphur (S) contents does not exceed 0.3 %, which satis-
fies the requirement that the metals, other than those
with prescribed percentages, should not be present in an
amount greater than 0.3 %. The percentage of arsenic
(0.0865 %) is slightly higher than its limit (0.06 %). Ar-
senic (As) is added to brass to reduce corrosion and de-
zincification, so its slightly higher concentration proba-
bly does not adversely affect the corrosion properties of
the test alloy.
3.3. Metallographic, microhardness and Fourier trans-
form infrared analysis
A metallographic sample was cold cut, ground and
polished in several steps to achieve a flat surface,
cleaned and etched with the ASTM No. 30 reagent for 30
seconds and then dried before measuring. A
metallographic analysis of the tube material has shown
that the investigated brass consisted solely of the phase
(Figure 3 a), which is a solid solution of zinc and other
alloying elements in copper. No aggregates or precipi-
tates were observed. At a magnification of 200× anneal-
ing twins were observed, indicating that the metal was
heat treated
8
. The grain size was in a range of 10–50 μm
as prescribed by the HRN EN ISO 12451 standard for
the CuZn20Al2As alloy.
9
This type of brass micro-
structure was proven to be the most resistant to corro-
sion.
10
The microhardness of the tube material,
CuZn20Al2As brass, was determined according to EN
ISO 6507-1 and equalled 107.4 ± 0.5 HV . The specified
value was <110 HV , therefore in accordance with the re-
quirement of EN 12451. The sample of corrosion prod-
ucts for the FT-IR analysis was taken from the steam side
J. BATELI] et al.: THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION ANALYSIS
Materiali in tehnologije / Materials and technology 57 (2023) 4, 317–323 319
Figure 2: Microscopic images of the investigated rupture at: a) the steam side of the tube and b) the water side, magnification of 50×
Table 1: Mean values of the measured mass fractions (w/%) of individual elements in the sample expressed as percentage (%)
Zn Al Fe Ni Si Bi As S Cu
Average 21.41 2.201 0.0199 0.0492 0.0398 0.0226 0.0865 0.0026 76.17

of the ruptured-tube segment, from around the rupture
site (Figure 3 b).
The spectrum largely corresponds to the illite mineral
(clay material) with a composition of (K
2
H
3
O)
(Al
2
Mg
2
Fe)
2
(Si
2
Al)
4
O
10
[(OH)
2
,(H
2
O)], which probably
originates from the seawater. The bands at (798, 875,
1427, 1797) cm
–1
and the bands in the range of
2800–3000 cm
–1
coincide with the spectrum of calcium
carbonate. The bands at (713, 875, 1027, 1271, 1427,
1617 and 1637) cm
–1
correspond to the spectrum of the
standard biofilm containing polysaccharides and
proteins
11
.
J. BATELI] et al.: THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION ANALYSIS
320 Materiali in tehnologije / Materials and technology 57 (2023) 4, 317–323
Figure 3: a) Microscopic image of the metallographic sample, b) the FT-IR spectrum of corrosion products
Chemical composition, w/%
#1 #2 #3 #4 #5 #6 #7 #8
Cu 87.95 34.39 52.98 77.58 41.30 73.45 44.66 27.81
Zn 2.46 12.87 12.73 6.68 6.40 14.14 6.86
C 4.97 26.81 6.48 21.50 14.64 28.51
O 3.18 25.92 36.77 3.21 16.15 1.47 32.56 21.78
Al 0.24 3.81 0.73 1.13 3.33 2.36
Si 0.21 3.13 1.02 1.49 2.22 4.06
S 0.35 3.31 9.09 1.41 3.08 3.85
Fe 0.64 2.80 2.97
Ca 0.40 0.55
Mg 0.67
K 0.59
Cl 0.32
Figure 4: Composition obtained with the EDS microanalysis of the tube water side
Chemical composition, w/%
#1 #2 #3 #4 #5 #6 #7 #8
Cu 55.44 32.97 56.93 60.13 33.79 54.98 62.00 35.02
Zn 10.74 4.11 11.96 15.48 7.09 2.66 3.73
C 21.25 33.48 21.57 16.82 51.10 19.50 26.47 36.00
O 8.55 19.89 6.84 5.68 7.21 15.69 9.97 14.70
Al 0.87 1.87 0.67 1.52 0.29 1.57
Si 0.21 3.06 0.56 0.12 0.26 2.95
S 0.16 0.25 0.21 0.19 0.24
Fe 1.25 0.74 0.38 5.64 0.30 1.28
Ca 0.24 1.83 0.16 2.95
Mg 0.29 1.15 0.52 0.56
K 0.29 0.28 0.38
Cl 0.36 0.86 0.36 0.18 0.43 0.66 0.51
Figure 5: Composition obtained with the EDS microanalysis of the tube steam side

3.4 SEM/EDS analysis
SEM micrographs and EDS chemical composition of
the immediate vicinity of the rupture site for the water
side are shown in Figure 4, and for the steam side, they
are shown in Figure 5. Eight measuring points were
added to better observe the difference in the chemical
composition of different areas of the surface and obtain
the mean values. The most common elements found with
the EDS analysis were copper, zinc, carbon and oxygen
(Cu, Zn, C and O), indicating the composition of the
substrate, and probably the formation of oxides and car-
bonates of copper. There is also Al, which probably orig-
inates from the substrate and is known to be embedded
into the protective brass patina
7
in the form of Al
2
O
3
. Sil-
icon (Si) and sulphur (S) are also components of the al-
loy, but both of them, in particular S, occurred in the
EDS analysis in excess compared to their alloy content
(Table 1). Therefore, these elements are also likely to
originate from the environment, along with calcium (Ca),
magnesium (Mg), potassium (K) and chlorine (Cl). Iron
may originate from the previous treatments with ferrous
sulphate that forms a protective layer of lepidocrocite
( – FeOOH) on the copper surface.
The SEM micrograph of the black corrosion-product
layer on the water side, shown in Figure 6, features
blurred areas that probably correspond to the biofilm.
The EDS elemental composition recorded at four sites
over a larger area than that shown by the micrograph has
again demonstrated high sulphur concentrations.
The ruptured sample was further subjected to clean-
ing in 10 % citric acid and then in 2 M HCl. The residual
black deposits at the bottom of the previously investi-
gated pit (Figure 7 a) were further analysed with
SEM/EDS (Figure 7 b). The deposits consisted of po-
lygonal crystals and their composition is presented in
Table 2.
Table 2: Mean values of the measured mass fractions (w/%) of indi-
vidual elements in the sample expressed as percentage (%)
Cu Zn C O Al Si S
63.52 8.46 8.35 5.44 1.08 0.86 12.29
The absence of all other elements, except the ele-
ments present in the substrate and the extremely high
percentage of sulphur, however, indicates the likely pres-
ence of sulphide at the bottom of the pit.
12
4 DISCUSSION
The corrosion and rupture of the condenser tubes
considered in this study were of a strictly localized char-
acter and initiated at the water side. In this study, no
J. BATELI] et al.: THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION ANALYSIS
Materiali in tehnologije / Materials and technology 57 (2023) 4, 317–323 321
Chemical composition, as measured, %
#1 #2 #3 #4
Cu 6.39 39.66 27.25 3.48
Zn 24.57 14.08 13.64 3.01
C 23.27 22.34 28.30 25.09
O 27.81 10.96 18.03 39.64
Al 4.07 2.05 2.46 4.97
Si 3.98 1.46 1.75 10.15
S 0.61 6.99 4.01 0.57
Fe 4.87 0.89 0.99 3.11
Ca 0.49 1.54 5.12
Mg 3.94 1.14 1.26 1.47
K 0.31 1.58
Cl 0.44 0.46 0.63
Figure 6: Black corrosion-product layer on the water side
Figure 7: Sample cleaned a) in 10 % citric acid, b) in2MH C l

traces of erosion or corrosion-erosion damage, flow
damage due to turbulence or jet-impact damage were
found.
However, we did find cavities or holes assumed to be
pit holes that contained corrosion products. Therefore, it
was assumed that the corrosion damage was pitting cor-
rosion because, as observed through the pit, it had a
somewhat elliptical shape. Pitting can be initiated by
localised chemical or mechanical damage made to the
protective oxide film by accident. It can also be caused
by localised damage made to a coating or the presence of
non-metallic inclusions in the metal structure. Pitting is a
result of an abnormal anodic area surrounded by a nor-
mal surface that acts as a cathode that influences the
localised loss in the thickness. The thickness loss can be
narrow and deep, shallow and wide, elliptical, vertical
grain attack, subsurface, undercutting or horizontal grain
attack.
The shallow pitting at the bottom of the tubes was as-
sumed to be initiated under the water where sediments
were retained after the draining of cooling seawater due
to periodic emptying of the system. However, the thick-
est corrosion products and the most intensive corrosion
attack were concentrated at the water side of the tubes,
adjacent to the contact between the tubes and the baffle
plates. This observation indicated that pitting corrosion
was preferentially influenced by the local conditions at
that site.
The observed form of corrosion could be character-
ized as the corrosion beneath the deposits. The mecha-
nism of this corrosion is based on the emergence of the
so-called differential aeration cell.
12
Below the deposit,
which can be of inorganic (mud, sand) or organic
(biofilm) origin, oxygen depletion occurs due to the
physical sheltering of that space and oxygen consump-
tion in the corrosion reaction or by aerobic bacteria in
the biofilm. At the oxygen-depleted site, the metal be-
gins to dissolve predominantly (anode surface), and at
the sites where oxygen is more readily available, a cath-
odic corrosion reaction of oxygen reduction (cathode
surface) predominantly occurs. Additionally, at the pit
bottom, anaerobic bacteria may become active. The phe-
nomenon of underdeposit corrosion can be especially
pronounced during a shutdown when inorganic and or-
ganic deposits are likely to form. After emptying the sys-
tem, the cooling water may not be completely drained
and may reside at the bottom of the tubes supporting
high-humidity conditions. Under the same conditions,
water droplets or areas of the moistened layer of deposits
may also exist at the top of the tubes for a long time.
Such moist, stagnant areas would be suitable for the de-
velopment of microorganisms.
In the presence of anaerobic sulphate-reducing bacte-
ria, corrosion can be further accelerated
12
. These anaero-
bic bacteria are active in the niches where anaerobic con-
ditions prevail. As a product of metabolism of these
bacteria, sulphide ions are formed which react with cop-
per and black copper sulphide is produced. Sulphides are
highly corrosive to copper and copper alloys.
7
Sulphides
can also be present in seawater as a product of industrial
waste
13
or metabolism of marine cultures.
14
With the
presence of sulphides in very small amounts of a few
tens of μg/L, in solutions containing chlorides, the corro-
sion products formed on pure copper and copper alloys
are not compact and do not provide good protection.
15
It
should be noted that the cooling circuits of seawa-
ter-cooled power plants act as incubators for microorgan-
isms because they represent an aquatic environment of
an appropriate temperature (usually between 30 °C and
60 °C) and a pH of 6 to 9, supporting microorganism
growth and providing a continuous source of organic and
inorganic nutrients.
6
The location of thick corrosion products at the water
side just below the buffer plate may point to the specific
influence of the temperature on the formation of depos-
its. This phenomenon is referred to in the literature as
hot-spot corrosion.
16
This corrosion occurs at a site of the
overheating of the wall due to the high local temperature
on the steam side, i.e., due to the increased heat transfer
in the area of contact between the tube wall and the sup-
porting wall. Overheating and even distortion of a tube,
which was observed on the ruptured tube, may occur
when re-starting the system after a shutdown period, if
the condenser is steam-filled and there is not enough
cooling water in the system. Therefore, when starting the
system after a shutdown, special care must be taken to
avoid the overheating of the tube wall. One-time sudden
heating can trigger the cracking of the protective layer,
leading to the development of corrosion during the sys-
tem operation. Support flanges of the condenser were
made from S235JR (1.0038) EN 10025-2: 2004, old des-
ignation: 1.0037 material RSt 37-2, DIN 17120. The de-
gree of expansion of the flanges and piping material on
the condenser was not investigated.
5 CONCLUSIONS
The main cause for the shutdown of the above ther-
mal power plant was leakage of the cooling-system con-
denser tubes due to the corrosion caused by the pitting
mechanism. Pitting was caused by biofilm microorgan-
isms, chemically damaging the CuZn20Al2As tubes.
Any inhomogeneity of the condenser tube walls
could be a potential site for the initiation of corrosion.
The tube water-side areas adjacent to the contact with the
buffer plates as well as the bottom area of the tubes were
found to be the predominant sites of corrosion and
niches for bacterial growth. Deposits in both critical ar-
eas were assumed to be linked to the shutdown periods
of the condenser when it was emptied of the seawater, as
well as to the system startup after the shutdown period.
Underdeposit corrosion that occurs trough differential
aeration cells aggravated by the action of bacteria was
diagnosed.
J. BATELI] et al.: THERMAL-POWER-PLANT CONDENSER-TUBE CORROSION ANALYSIS
322 Materiali in tehnologije / Materials and technology 57 (2023) 4, 317–323

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