J. BATELI] et al.: INVESTIGATION OF PIPELINE FAILURE IN A THERMAL POWER PLANT’S PROCESS WASTEWATER ...
135–140
INVESTIGATION OF PIPELINE FAILURE IN A THERMAL
POWER PLANT’S PROCESS WASTEWATER
DISTRIBUTION SYSTEM
PREISKAVA PO[KODB CEVOVODA V SISTEMU ZA OBDELAVO
IN DISTRIBUCIJO ODPADNE VODE TERMOELEKTRARNE
Jakov Bateli}
1*
, Vedrana [pada
2
, Lovro Liveri}
3
, Sanja Martinez
4
1
HEP Proizvodnja d.o.o. TE Plomin, Plomin luka 50, 52234 Plomin, Croatia
2
METRIS Materials Research Centre of Region of Istria, Zagreba~ka 30, 52100 Pula, Croatia
3
University of Rijeka Faculty of Engineering, Vukovarska 58, 51000 Rijeka, Croatia
4
University of Zagreb Faculty of Chemical Engineering and Technology, Maruli}ev trg 19, 10000 Zagreb, Croatia
Prejem rokopisa – received: 2020-08-12; sprejem za objavo – accepted for publication: 2020-10-26
doi:10.17222/mit.2020.159
In this paper, abundant, compacted, dust-like, corrosion scales from the interior of a corrosion-damaged pipeline were analyzed
to determine the main reason for leakage and flow damping in the process-wastewater distribution system in the thermal power
plant Plomin. The chemical composition, morphology and microbiological activity of the corrosion deposits present on the inner
pipe wall were investigated and the physical and chemical analyses of the purified boiler water was made. The results show the
presence of iron-related bacteria (IRB) in the corrosion deposits as well as in the distribution system water. XRD analysis shows
exclusively magnetite and goethite with no calcium carbonate present in the layers, therefore indicating that no protective car-
bonate scales, which would protect the pipeline steel, had initially formed. It was concluded that the primary causes of intense
corrosion were iron-oxidizing bacteria, that through their metabolism, support the redox cycling process, the formation of large
tubercles as well as irregular thinning of the pipeline wall with separated anodic and cathodic areas.
Keywords: pipeline failure, microbiologically influenced corrosion (MIC), process wastewater distribution system, tubercles
V ~lanku avtorji opisujejo analizo obilne in kompaktne, prahu podobne korozijske usedline iz v notranjosti korozijsko
po{kodovanega cevovoda. Cilj analize je bil dolo~iti glavni razlog za pu{~anje in du{enje (zmanj{anje) pretoka v cevovodu
sistema za distribucijo odpadne vode termoelektrarne Plomin na Hrva{kem. Izvedli so kemijsko in morfolo{ko analizo ter
analizo mikrobiolo{ke aktivnosti prisotnih korozijskih usedlin nastalih na notranjih stenah cevi. Izvedli so tudi fizikalno in
kemijsko analizo o~i{~ene vode v bojlerju. Rezultati raziskav so pokazali prisotnost bakterij povezanih z `elezom (IRB, angl.:
iron-related bacteria) v korozijskih usedlinah, kakor tudi v sistemu za distribucijo vode. Rentgenska difrakcijska analiza (XRD)
je pokazala predvsem prisotnost magnetita in getita. V plasteh pa ni bil prisoten kalcijev karbonat, ki naj bi se tvoril na za~etku.
Avtorji zato ugotavljajo, da ni pri{lo do tvorbe za{~itne plasti, ki bi {~itila jekleni cevovod. V zaklju~ku avtorji poudarjajo, da je
primarni vzrok za intenzivno korozijo oksidacija `eleza zaradi prisotnih bakterij, ki preko svojega metabolizma podpirajo
redoks (redukcijsko-oksidacijski) cikel, tvorbo velikih tuberklov (v obliki nabreklin in gomoljev), kakor tudi nepravilno
tanj{anje sten cevi v med seboj lo~enih anodnih in katodnih podro~jih.
Klju~ne besede: po{kodbe in odpoved cevovoda, mikrobiolo{ko vplivana korozija, sistem za obdelavo in distribucijo odpadne
vode, tuberkli
1 INTRODUCTION
Process wastewater is generated as a consequence of
the technological process of electricity production.
Boiler water is part of process wastewater distribution
system and is generated in an auxiliary boiler, stone
pulverizers, wash residue of the regenerative air heater,
condensate from the boiler area, the bilge pit and the pit
of the bunker tract. Given that wastewater is very uneven
in quality and quantity, with the purpose of a uniform
treatment process, the equalization of wastewater takes
place at the thermal power plant Plomin buffer pool with
a capacity of 1200 m
3
.
1–3
Further pH stabilization treat-
ments were performed with the help of lime milk by sift-
ing water from the neutralization tank to the sedimenta-
tion tank. Through the central pipeline, the wastewater
from the flocculation tank is decanted in the settling
tank.
4,5
The clarified wastewater leaves the precipitator
through the overflow gutter and goes to the pH control
tank where the pH is adjusted with hydrochloric acid and
filtered through a sand filter.
6–8
The average inflow of pu-
rified wastewater (chemical composition given in Ta-
ble 2) is approximately 5 m
3
/h. The purified water from
the sand filter is discharged into accumulation tank,
50 m
3
volume and through the overflow into the sur-
rounding channel. The main purpose of the accumulation
tank is to accumulate treated wastewater for rinsing the
droplet separator system during the technological pro-
cess of desulphurization of the flue gases.
7–9
The total av-
erage daily water needs for rinsing the droplet separator
system 55 m
3
, which means that rinsing can be done en-
tirely using only accumulated wastewater in the thermal
power plant Plomin. During daily operations over a pe-
riod of 6 years, the treated wastewater recovery system
Materiali in tehnologije / Materials and technology 55 (2021) 1, 135–140 135
UDK 620.1:66.026.2:62-776.6:621.311.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(1)135(2021)
*Corresponding author's e-mail:
jbatelic@gmail.com (Jakov Bateli})

was showing a continuous decrease in the consumption
of treated wastewater and an increase in the consumption
of raw water for rinsing the droplet separator. By analyz-
ing the values of pressures measured at the beginning
and end of the pipeline, it was found that the pressure
drop is almost 50 % higher than the constructed values.
Therefore, it was concluded that there is significant
damping disrupting the pipeline flow and possible leak-
age. That required more frequent corrective and mainte-
nance actions on the pressure pipeline. With a further vi-
sual examination in Figure 1. of the cut section of the
steel pressure pipeline (DN125, PN 16, total length 820
meters), it was found that deposits were formed on the
inner side of the pipe wall. The subjected section of the
pressure steel pipe was given a detailed analysis in order
to determine the cause of failure.
1–9
Iron-oxidizing bacteria, also known as metal-deposit-
ing microorganisms, is causing microbiologically influ-
enced corrosion (MIC) in many studies.
10–11
MIC is a
complex interaction between the environment, the micro-
bial population and the metal substrate.
11–13
It is esti-
mated that microbial activity is causing annually about
20 % of the corrosion damage of metals, of which a sig-
nificant part is due to aerobic corrosion influenced by
iron-oxidizing bacteria (IOB) and anaerobic corrosion
influenced by sulfate-reducing bacteria (SRB).
10–12
The
reaction of carbon-steel pipes is considered to be a com-
plicated process that is affected by bulk-water quality, in-
cluding temperature, pH, alkalinity, oxygen concentra-
tion and the presence of sulfate and chlorine in the
system, etc.
13
XX Little and Wagner
11
considered that the corrosion
occurred in the presence of iron bacteria followed the
crevice corrosion mechanism.
11
The role of iron-oxidiz-
ing bacteria lies in the formation of condensed oxygen
zones and the partition of the metal surface into small
anodic sites and large surrounding cathodic area.
11,12
Moreover, the deposit layers formed by iron-related
bacteria could also create condensed oxygen zones and
initiate crevice corrosion in water systems containing
corrosion inhibitors. The corrosion rate beneath the rust
deposits remains high due to the low penetrability of the
corrosion inhibitors and declining of the local oxygen
concentration where the site might turn anodic.
During the last decades in the cooling water system
of the production industry, SRB and IOB were the most
common groups of bacteria on tubercular corrosion and
induced microbiologically influenced corrosion in the
cooling circuit. They caused poor water quality and
equipment clogging, which included flow blockages due
to tubercle formation, choking of valves and strainers,
pipe punctures and high corrosion rates, resulting in the
serious pitting corrosion of carbon-steel equipment.
14
Despite a large number of research papers reported
on the MIC of carbon steels, the quantitative data and in-
formation about the real role of the IOB in the MIC pro-
cess are still not clear, due to the lack of information and
evidence of the mechanism describing MIC in water-dis-
tribution systems,
10–15
because most of these studies were
made in the laboratory. In the present paper, the focus is
on understanding the real role of IOB in the MIC-pro-
cess on the wastewater distribution system of thermal
power plant Plomin.
2 MATERIALS AND METHODS
2.1 Materials
The corrosion-product samples were taken from a
section of damaged steel pressure pipeline (total length
820 m, DN125, PN 16). Two main sample categories
were visually identified and investigated. One sample
unite was named "the black sediment" because it was vi-
sually black, and was taken from the bottom and central
mass of corrosion scales that was attached to the inner
wall of the pipeline. Another sample that was visually
brown, was named "the brown sediment" and was taken
from the surface of the corrosion scales turned towards
the pipeline center. Basic information of the field condi-
tions, as well as the material and year of installation in
the power plant, is shown in Table 1. The tested waste-
water sample came from the same water piping.
J. BATELI] et al.: INVESTIGATION OF PIPELINE FAILURE IN A THERMAL POWER PLANT’S PROCESS WASTEWATER ...
136 Materiali in tehnologije / Materials and technology 55 (2021) 1, 135–140
Figure 1: Appearance of the interior of the investigated steel pressure pipeline

Table 1: Basic data on the sample of the steel pressure pipeline
User: HEP-Proizvodnja d.o.o
Plant location: TE PLOMIN 2, Plomin Luka
Sample dimension: Pipe ø125*3,6
Part of installations: Boiler water pipeline
Working pressure: 7-10 bar
Overheating temperature: 20 °C
Year of construction: 2010.
Pipeline material: EN St 35.8 (1.0061), DIN 17175
2.2 Methods
2.2.1 Microbiological analysis of wastewater distribu-
tion systems sediment and water
The microbiological test was performed on boiler
water taken at the outlet of the system and on the corro-
sion-products sediment taken from the surface of the
pipe. Sulphate-reducing bacteria (SRB) were tested us-
ing the Dalynn Biologicals kit, and iron-related bacteria
(IRB) using the BTI Products kit. Sargent Welch lead ac-
etate Test Paper and HCl p.a. by Kemika were used for
the sulfide presence testing of the bottom, central and top
layers of corrosion products.
2.2.2 XRD analysis of wastewater distribution systems
sediment
Black and brown corrosion products were subjected
to X-ray diffraction analysis (PXRD) on a Shimadzu
XRD-6000 diffractometer with Cu–K
 radiation, with a
voltage acceleration of 40 kV and current 30 mA, in the
range 3–83° 2 with a step of 0.02° 2 and a retention
time of 0.6 s. The results are graphically presented as a
function of the intensity of diffraction maxima of the dif-
fraction angle on the images appeared.
2.2.3 Chemical analysis of water with the calculation of
corrosivity indices
Chemical analysis of the wastewater sample was per-
formed in an accredited laboratory for water analysis.
Calculation of the Corrosivity Indices was done accord-
ing to the literature.
16
2.2.4 Macro cut of a grinded section of the pipe wall
A macro cut was made from the sample of the steel
pipe, on which the corroded wall thickness and its ap-
pearance was inspected visually.
3 RESULTS AND DISCUSSION
3.1 Microbiological analysis of sediments and waste-
water
The control results of IRB and SRB-bearing samples
are shown in Figures 2a and 2b, respectively. The waste-
water and corrosion sediments tests yielded identical re-
sults shown in Figures 3a and 3b. Positive results were
obtained for IRB and negative for SRB.
The control results of the sulfide analysis for the sul-
fide-free and sulfide-bearing samples are shown in Fig-
ures 4a and 4b, respectively. These were used for a vi-
sual assessment of the tested samples.
The results of the sulfide analysis on the central, bot-
tom and top layers of the sediment are shown in Fig-
ure 5.
Comparison with control samples shows that all three
samples were negative, i.e., sulphide-free. Since
sulphides are formed as a result of SRB metabolism,
17
this result confirms the previous observation that SRB
J. BATELI] et al.: INVESTIGATION OF PIPELINE FAILURE IN A THERMAL POWER PLANT’S PROCESS WASTEWATER ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 135–140 137
Figure 2: Control results of: a) IRB-bearing and b) SRB-bearing sam-
ples
Figure 4: Control results of a lead acetate paper test on a sulfide-free
and a sulfide-bearing sample
Figure 3: Results of positive IRB test and negative SRB test, obtained
in both wastewater and corrosion product samples

are not detectable. The microbiological analysis of the
sediment samples shows the presence of IRB.
3.2 X-ray diffraction analysis
XRD analysis was performed on sediment samples
named according to their color, i.e., the black and brown
samples. The results are shown in Figures 6 and 7 as a
function of the intensity of the diffraction maxima about
the diffraction angle.
The black sediment sample consists of very well
crystallized magnetite, Fe
3
O
4
, ICDD PDF (International
Center for Diffraction of the Powder Diffraction File) #
65-3107. No additional diffraction maxima are observed
on the diffractogram, thus it can be determined that mag-
netite is the only crystalline phase in the sample. More-
over, no amorphous halo is observed, so it can be deter-
mined that there is no amorphous phase in a larger share.
The diffraction maxima of the brown sediment sam-
ple are significantly lower and wider, indicating finer
crystallites. No pronounced amorphous halo is observed,
although the visual impression may be the opposite due
to the low intensities. In this sample, the main crystalline
phase is goethite, FeO (OH), ICDD PDF # 81-463, while
the secondary phase is magnetite, Fe
3
O
4
, ICDD PDF #
65-3107. It is not excluded that another crystalline phase
is present in a very small proportion in the sample.
The corrosion products are mainly magnetite and to a
lesser extent goethite, which is found in a thin surface
layer. There is no sulphate-reducing bacteria and
sulphides in the composition of the sediment, and it is
supported by the appearance of corrosion that indicates
this result. Sulphate-reducing bacteria cause very local-
ized damage and the tested sample shows the appearance
of general corrosion over the entire surface of the pipe
wall.
18,19
The positive test on iron-related bacteria, proba-
bly indicates the the presence of iron oxidizing bacteria
which oxidize Fe
2+
, formed by dissolution of the pipe
material, into Fe
3+
.F e
3+
is an oxidant that significantly
promotes the further corrosion of steel. Moreover, redox
cyclization probably takes place so that the corrosion
process continues. The formation of dense layers of
products begins above the anodic site through the accu-
mulation of deposits on the surface.
20
The electrostatic
forces gradually strengthen at the location and the sur-
face of the accumulated deposits gradually passes into
the cathode. This creates the optimal conditions for the
formation of large tubercles.
21
3.4 Macro cut of a grinded section of the pipe wall
A section of the pressure steel pipeline was cut out,
freed from loose corrosion products, ground along the
profile, to make a macroscopic sample adequate for in-
spection of the steel pipe thickness, as seen in Figure 8.
Figure 9 shows the inner surface of the pipeline after
removal of loose corrosion products. Distinctive, anodic
(depressed) and cathodic areas are clearly visible at the
surface.
Through a visual inspection it was concluded that the
surface was extremely damaged and irregularly thinned.
Since the entire surface is equally damaged by corrosion,
more and more leaks can be expected to occur on the
pipeline over time.
J. BATELI] et al.: INVESTIGATION OF PIPELINE FAILURE IN A THERMAL POWER PLANT’S PROCESS WASTEWATER ...
138 Materiali in tehnologije / Materials and technology 55 (2021) 1, 135–140
Figure 7: XRD analysis of brown powder residue
Figure 5: Results of three sulphate test made on central, bottom and
top layers of the pipeline wall sediment
Figure 6: XRD analysis of black powder residue

3.5 Physical and chemical analysis of wastewater with
calculation of the corrosivity index
The physicochemical characteristics of the tested
wastewater are shown in Table 2 and the ionic composi-
tion is shown in Table 3.
The Langelier saturation index (LSI) is defined as:
LSI=pH–pH
sat
(1)
where pH
sat
is calculated according to:
pH
sat
=(9.3+A+B)–(C+D) (2)
In Equation (2), A depends on the TDS, B depends
on the temperature of the water, C depends on the water
hardness and D depends on the water alkalinity.
The Larson Skold Index is defined as:
L&Skl =
([Cl ] [SO ])
([HCO ] [CO ])
4
2
33
2
−−
−−
+
+
(3)
where the anion concentrations are presented in Table 3.
The calculated values of the Langelier saturation in-
dex (LSI) and Larson-Skold index (L&Ski), calculated
according to Equations(1-3, as well as their corrosivity
assessment are shown in Table 4.
According to the Langelier index, the water is
non-corrosive and tends to precipitate calcium carbonate.
XRD analysis has shown that calcium carbonate is not
present in the deposits, therefore indicating that no pro-
tective carbonate scales were formed at the pipeline sur-
J. BATELI] et al.: INVESTIGATION OF PIPELINE FAILURE IN A THERMAL POWER PLANT’S PROCESS WASTEWATER ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 135–140 139
Figure 8: Marco cut of the damaged pipe wall after removal of loose
corrosion products
Figure 9: Inner surface of the pipe wall after removal of loose corrosion products
Table 2: Physical and chemical analyses of purified wastewater
Properties Units
Boiler water –
output
Appearance Descriptive Transparent
Color Descriptive Colorless
Smell Descriptive No smell
Temperature °C 17.4
pH – 7.4
Electric conductivity 25°C, μS/cm 3079
TDS mg/L 443.3
Total hardness (Ca +
Mg)
mg CaCO
3
mg/L 1665
Water alkalinity (p-, m-) mg CaCO
3
mg/L 112
Total mineralization mg/L 2089
Salinity G NaCl/L 1.33
Table 3: Ionic composition of purified wastewater
Ion
Concentra-
tion / μL/L
Molar mass
/g mol
–1
Charge
Concentra-
tion / meq
Na
+
25.00 22.99 1 1.0874
K
+
3.70 39.83 1 0.0929
Ca
2+
339.00 40.08 2 16.9170
Mg
2+
199.00 24.31 2 16.3752
NH
4+
0.58 18.04 1 0.0322
Fe
2+
0.01 55.85 2 0.0003
toMn
2+
0.04 54.93 2 0.0014
Sum of cation concentrations 34.4728
Cl
–
804.00 35.45 –1 22.6779
HCO3
–
137.00 61.02 –1 2.2453
SO4
2–
581.00 96.06 –2 12.0966
Sum of anion concentrations 37.0798
Difference in anion and cation concentration 2.54
Percentage deviation 3.56
Table 4: Corrosivity indices of the purified wastewater
Index Value Interval Corrosivity assessment
LSI 0.21 LSI > 0
The water is supersaturated with
calcium carbonate and there is a
possibility of deposit formation.
L&Skl 15.49
L&Skl >
0.8
Water tends to cause localized
corrosion.

face. The Larson Skold Index indicates that wastewater
is capable of causing serious localized corrosion. How-
ever, from the occurrence of corrosion along the entire
wall of the pipeline, it can be concluded that sulphates
and chlorides are not decisive corrosion agents, but only
corrosion accelerators. The primary causes of the corro-
sion are likely the iron-oxidizing bacteria, which, in
combination with sulphides and chlorides in the
wastewater, cause intense corrosion, the formation of
abundant, compacted dust-like corrosion products and ir-
regular but general thinning of the pipe wall.
4 CONCLUSIONS
The following conclusions can be drawn:
• Corrosion products are made mostly of compacted
dust-like magnetite with a thin surface layer of
goethite.
• The presence of IRB was found in the sediment and
the distribution system wastewater.
• According to the Langelier index, water should pre-
cipitate calcium carbonate, but it has not been identi-
fied in the corrosion sediment layer.
• Due to the general appearance of corrosion along the
pipeline axes and circumference, sulphates and chlo-
rides, despite the high Larsoon Skold index, are prob-
ably not decisive corrosion agents, but only corrosion
accelerators.
• The primary causes of corrosion are likely the
iron-oxidizing bacteria, which in combination with
sulphates and chlorides in the water, cause intense
corrosion and the formation of abundant, compacted
dust-like corrosion products that are clogging the
pipeline.
• The macro sanding of the cut pipe wall revealed that
the entire pipe surface, longitudinally and circumfer-
entially, was equally damaged by corrosion, showing
irregularities in the form of overlapping wide pits so
that over time, more and more leaks can be expected
on the pipeline.
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J. BATELI] et al.: INVESTIGATION OF PIPELINE FAILURE IN A THERMAL POWER PLANT’S PROCESS WASTEWATER ...
140 Materiali in tehnologije / Materials and technology 55 (2021) 1, 135–140