TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler
Kitab ı
81518. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC
2016
Comparison of Microbiologically Induced Corrosion Behavior of
316L Stainless Steel and Galvanized Steel as Cooling Tower
Materials
Tuba Ünsal¹, Simge Arkan¹, Esra İlhan-Sungur¹, Nurhan
Cansever²
¹İstanbul University, ²Yıldız Technical University - Türkiye
Abstract The aim of the study was to compare corrosion behavior
of 316L stainless steel (SS) and galvanized steel (GS) in the
presence of Desulfovibrio sp. by gravimetric and potentiodynamic
polarization methods. The growth curves were obtained by the
enumeration of Desulfovibrio sp.. Biofilm formation and corrosion
products on the metal surfaces were investigated by scanning
electron microscopy (SEM) and energy dispersive X-ray spectrometry
(EDS) analyses. SEM images showed that Desulfovibrio sp. in the
biofilm clustered into patches on the SS surface in contrast to
that on the GS surfaces. It was determined that Desulfovibrio sp.
leads to corrosion both of SS and GS. However, Desulfovibrio sp.
showed more corrosive effect on the GS than SS. 1. Introduction
Cooling tower is a heat exchanger which provides a water stream at
lower temperatures to industrial systems through the release of
waste heat to atmosphere [1]. The bacteria, fungi and algae that
entry into the cooling systems by the water and the air in contact
with water, led to the biofilm formation on the internal surfaces
of cooling towers. The biofilm formation causes corrosion of
various metal structures seriously in cooling towers such as
circulation pipes, water basin and heat exchangers [2]. In order to
prevent the damage of microorganisms to the coling tower material,
corrosion-resistant metals are preferred. Especially, 316L
stainless steel (SS) and galvanized steel (GS) are popular
construction materials for cooling tower systems. SS is generally
prefered in areas where water accumulates, such as the cold-water
basin, owing to their higher corrosion resistances. The improved
level of corrosion resistance in SS relates to the formation of a
chromium oxide film layer (passivation layer) on the surface of the
metal due to the chromium and nickel content, and the presence of
molybdenum [3]. Galvanized steel is frequently used in the
construction of cooling towers and tanks because of its resistance
to corrosion and biofouling. The corrosion resistance of this metal
has been attributed to the formation of a protective layer of
Zn(OH)2 [4]. However, cooling water conditions and the microbial
activity of sulfate reducing bacteria (SRB) frequently isolated
from the cooling water may cause severe corrosion resulting in
structural failures of the SS and GS systems [5].
The type of metals can affect activities of microorganisms and
hence corrosion behavior. To investigate this aspect of
microbiologically induced corrosion (MIC), corrosion behaviors of
SS and GS in the presence of Desulfovibrio sp. were investigated by
gravimetric and potentiodynamic polarization methods. The growth
curves were obtained by the enumeration of Desulfovibrio sp. using
Postgate B medium over 720 h. Biofilm formation and corrosion
products on the metal surfaces were investigated by scanning
electron microscopy (SEM) and energy dispersive X-ray spectrometry
(EDS) analysis. 2. Experimental Procedure 2.1. Microorganism This
study was performed using pure cultures of the strain Desulfovibrio
sp. isolated from cooling tower water by Ilhan-Sungur and Cotuk
[6]. Pure cultures of Desulfovibrio sp. was grown in Postgate’s
medium B (PB medium) at 30°C [7].
2.2. Preparation of the SS and GS Coupons The nominal elemental
composition (wt%) of the SS coupons used for the experiment was C
0.022, Cr 16.02, Ni 11.44, Mo 1.95, Mn 0.984, Si 0.382, P 0.035 and
S 0.010. The coupons with dimensions of 25 x 25 x 1 mm were abraded
through 240, 320, 400, 600 and 800-grit silicon carbide paper,
polished with aluminum oxide, washed with sterile distilled water,
degreased using acetone, and dried in a Pasteur oven at 70°C. The
coupons were weighed, and the total surface area of each coupon was
measured and then kept in a desiccator until use. The thickness of
the zinc coating was 5 μm in the GS coupons used for the
experiment. The dimensions of the coupons were 25 x 25 x 1 mm. The
total surface area of each coupon was determined. The cut areas of
all the coupons were coated with epoxy zinc phosphate primer
(Moravia, Turkey) (grey) and then covered with epoxy finish coating
(Moravia, Turkey) (black) to avoid the initiation of corrosion at
these disturbed areas. The coupons were weighed and then kept in a
desiccator until use. 2.3. Experimental conditions of lab-scaled
test and control systems The experiments were carried out in two
different systems such as lab-scaled test and control systems.
UCTEA Chamber of Metallurgical & Materials Engineers
Proceedings Book
816 IMMC 2016 | 18th International Metallurgy & Materials
Congress
Postgate’s medium C (PC medium) was used in both systems [7]. SS
and GS coupons were exposed to Desulfovibrio sp. cultures in
separately during 8, 24, 72, 96, 168, 360 and 720 h in the
lab-scaled test systems. Control systems containing sterile medium
were set to work simultaneously with the test systems. The
lab-scaled test systems were set up PC medium (1440 ml) was
inoculated by appropriate volume (10%) of 1-day-old Desulfovibrio
sp. culture resulting in an initial concentration of 25x106 per ml
in the presence of SS and GS coupons. The culture in test systems
were mixed with magnetic stirrer (150 rpm) and the experiments were
carried out under anaerobic conditions in the glove box system at
30°C. The test coupons were removed at each sampling time for the
enumeration of Desulfovibrio sp. and the determination of corrosion
rate by the weight loss measurement method. In the control systems,
weight loss measurement was carried out simultaneously with test
systems. Biofilm formation and corrosion products on the metal
surfaces were investigated by scanning electron microscopy (SEM)
and energy dispersive X-ray spectrometry (EDS) analysis. 2.4.
Enumeration of Desulfovibrio sp. Enumerations of the number of
sessile and planktonic Desulfovibrio sp. were performed at each
sampling time. Sessile and planktonic Desulfovibrio sp. counts were
determined by the most-probable-number (MPN) technique using PB
medium. MPN tubes were incubated in the dark at 30°C. In each
inoculated tube, the growth of sulphate reducers was indicated by
the formation of a black FeS precipitate and turbidity [7]. 2.5.
Weight loss measurement The weight loss measurement was carried out
with three coupons removed at each sampling time with SS and GS
separately. Biofilm on the SS coupons were removed with sterile
cotton swabs. Then corrosion products were wiped out completely by
immersing the coupons 67% HNO3 and 0.6% HNO3 in an ultrasonic bath
for 2-5 min and 10 s for SS and GS coupons, respectively. The
surfaces of the exposed coupons were finally rinsed with distilled
water, cleaned in 100% ethanol and dried in Pasteur oven at 70°C
[8]. The difference between the initial and final weight was
reported as weight loss both type of metals. The values of the
corrosion rate were determined according to the ASTM standard G1-81
[9]. 2.6. Electrochemical measurements The electrochemical
corrosion tests were performed with a computer-controlled testing
device (Gamry-Interface1000, USA) by measuring the potentiodynamic
polarization method. All electrochemical tests were carried out in
a conventional electrochemical cell, with a carbon rod as the
counter electrode, a saturated calomel electrode (SCE) as a
reference electrode, and the prepared samples as the
working electrode. The working solutions were mixed with a
magnetic stirrer (150 rpm) and all of the measurements were
performed at 30°C. Square-shaped SS and GS coupons with a top
surface area of 1 cm2 were exposed to the Desulfovibrio sp. culture
and sterile PC medium. While the potentiodynamic polarization
curves of SS were obtained at a scanning rate of 1 mV/s between
-800 mV and +1700 mV, the potentiodynamic polarization curves of GS
were obtained at a scan rate of 1mV/s within ±500 mV compared to
the corrosion potential (Ecorr). Electrochemical measurements were
carried out at certain time intervals (8, 48, 96 and 168 h) during
168 h of exposure. 2.7. Surface analyses The biofilm formations and
corrosion products on the coupons were analyzed by SEM and EDS at
the end of the each sampling time. Coupons were fixed with 2.5%
glutaraldehyde, followed by dehydration in a graded series of
ethanol and air drying [10]. The dried samples were coated with a
gold layer (30 nm) and imaged with an electron microscope (ZEISS
(LS-10)). 3. Results and Discussion SS and GS coupons were exposed
to Desulfovibrio sp. culture over 720 h in the lab-scaled test
systems. Heterogenous, gelatinous and easily removable biofilm
layers on the surfaces of the coupons were observed (Figure 1).
However SEM images showed that Desulfovibrio sp. in the biofilm
clustered into patches on the SS surface.
Figure 1. SEM micrographs of the biofilm formed on the SS (a)
and GS (b) surfaces after 720 h of exposure in the Desulfovibrio
sp. culture.
UCTEA Chamber of Metallurgical & Materials Engineers
Proceedings Book
818 IMMC 2016 | 18th International Metallurgy & Materials
Congress
surface was very likely composed of a ZnS deposit. It is
possible that H2S enhances anodic dissolution of the zinc,
resulting in insoluble ZnS corrosion products and cathodic hydrogen
evolution [14].
Figure 4. Polarization curves of the SS (a) and GS (b) coupons
exposed to the sterile PC medium and Desulfovibrio sp. culture.
Figure 5. SEM micrographs and EDS analysis of the corrosion
products formed on the SS (a) and GS (b) coupons exposed to
Desulfovibrio sp. after 720 h.
4. Conclusions The experimental results are summarized as
follows:
1. The biofilm formed by Desulfovibrio sp. showed
different morphology and polarization resistance according to
the metals’ type.
2. The durations of the growth phases of sessile Desulfovibrio
sp. were similar on the SS and GS surfaces.
3. Desulfovibrio sp. accelerated corrosion both of the SS and
GS. However, the results indicate that GS is suffered corrosion by
Desulfovibrio sp. more than SS.
4. The corrosion rates both of SS and GS were not related with
the Desulfovibrio sp. count.
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Environmental
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