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The impact of microorganisms on the corrosion protection by
self-
assembled layers of phosphonic acids in natural waters
Ekatarina KRISTAN MIOČ1, Helena OTMAČIĆ ĆURKOVIĆ
1
1Faculty of Chemical Engineering and Technology, University of
Zagreb, Zagreb, Croatia,
[email protected] [email protected]
Abstract
The microbiologically influenced corrosion can cause problems
where ever natural water is present
(e.g., in cooling towers, in drinking water pipelines, in sewage
systems as well as in the food industry,
on ship halls, and in oil production). Although highly resistant
alloys are used for designing these
plants, MIC has been shown to occur on a number of materials,
including conventionally used
structural materials such as stainless steels and cupronickel
alloy. It is therefore of great importance to
develop antibacterial coatings that are stable, environmentally
benign, and provide good corrosion
resistance to improve alloys performance under corrosive
condition. Since many commercial corrosion
inhibitors are ecologically unsuitable due to their toxic impact
on the ecosystem, the application of
self-assembled mono- and multilayers presents potential
ecologically suitable solution for increasing a
corrosion resistance of alloys. The aim of this work is to
investigate the possibility of stainless steel
and cupronickel alloy protection by self-assembled layers of
octadecylphosphonic acid prepared by
spraying method. Behavior of unprotected metals in natural
medium, Sava river water and seawater,
and in tentatively sterilized water by membrane filtration
method was followed simultaneously in
order to differentiate to which extend the corrosion is
influenced by microorganisms. In the same
conditions the behaviour of samples modified with phosphonic
acid was studied. The protective
properties of such formed layers are examined during a time
period of 40 days by linear polarization
resistance method while the examination of the metal surfaces is
conducted by optical and scanning
electron microscopy.
Keywords: natural water; microbiologically influenced corrosion;
biofilm; long-chain phosphonic
acid; self-assembled layers.
mailto:[email protected]:[email protected]
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Introduction
Corrosion of materials by bacteria has been a long-recognized
phenomenon. Numerous
industries have reported microbiologically influenced corrosion
(MIC) problems [1]. The
cooling industry is particularly susceptible to MIC because of
the increased use of non-
traditional water sources. Generally, the toxicity of copper to
microorganisms has led to the
belief that MIC of copper is insignificant. But the fact is that
sulphate reducing bacteria (SRB)
are one of the prevalent causes of corrosion of cooling system
materials [2]. It is therefore of
great importance to develop antibacterial coatings that are
stable, environmentally benign, and
provide good corrosion resistance to improve metals performance
under corrosive conditions.
Marine and river water environment is sensitive to toxic
compounds and many commercial
corrosion inhibitors are ecologically unsuitable for natural
water application. Recently, the
application of self-assembled mono- and multilayers (SAMs) has
been introduced as potential
ecologically suitable solution for increasing corrosion
resistance of different metals and their
alloys [3-6]. SAMs have been also proposed as an efficient
method for biofouling control [7].
SAMs are thin assemblies of dense and well defined structure
that block the active spots on
the metal surface, presenting a barrier to electron transfer and
ion penetration and change its
wettability. Compared to other methods for surface modification,
SAMs have many
advantages – only a small amount of organic compounds is needed
to cover the metal surface,
they can be easily prepared and they form through the
chemisorption of molecules on the
solid substrate. The most commonly studied SAM systems are
thiols on the non-oxidized
metal surfaces and silanes on oxide surfaces. However, the major
disadvantages of these two
systems are their oxidation in time or instability in aqueous
and biological media [8, 9]. The
promising replacement for thiols and silanes are long-chain
organic acids, especially
phosphonate acids since they are relatively stable and can be
attached to a wide range of oxide
surfaces [10-12]. The other advantage of long-chain phosphonic
acids is that they are non-
toxic. Although dip-coating method is the most common used for
SAMs preparation, the aim
of this work is to investigate the possibility of stainless
steel and cupronickel alloy protection
by self-assembled layers of octadecylphosphonic acid (ODPA)
prepared by spraying method
because it is more practical than the usually used method.
Behaviour of unprotected metals
and samples modified with phosphonic acid in natural medium,
Sava river water and Adriatic
seawater, and in tentatively sterilized water was followed
simultaneously during a time period
of 40 days in order to differentiate to which extend the
corrosion is influenced by
microorganisms.
Experimental
Materials and sample preparation
In order to prepare working electrodes, stainless steel rod,
AISI 316L, and cupronickel rod,
70Cu-30Ni, was cut-out in 0.5 cm thick samples and on their back
side a copper wire was
soldered. At the end they were embedded into epoxy resin and the
exposed surface of working
electrode of SS and CuNi was 1.13 and 1.33 cm2, respectively.
The electrodes were, prior to
all surface treatment, abraded with emery paper grade 800, 1200
and 2500, and polished with
α-Al2O3 particle size 0.1 μm, degreased with ethanol in
ultrasonic bath and rinsed with re-
distilled water. The SAM formation was carried out by the
following procedure: oxide
formation, 24 h at 80 °C for cupronickel and 24 h at room
temperature for stainless steel,
followed by spraying samples with ODPA/EtOH (concentration
10-2
M) solution, and final
drying step, 5 h at 80 °C. For comparison, blank samples were
prepared, on which native
oxide layer was formed during 24 h at 80 °C (CuNi) or room
temperature (SS).
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Electrochemical measurements and surface studies
The electrochemical investigations were conducted in a three
electrode cell, in a river water
and seawater, and in tentatively sterilized water
simultaneously. A platinum foil and saturated
calomel electrode were used as the counter and reference
electrode, respectively. All
potentials in the text are referenced against saturated calomel
electrode (SCE). The
polarization measurements were performed at the narrow (± 20 mV
vs. Eoc) potential range,
with a potential scan rate of 0.166 mV s−1
during a time period of 40 days. The
electrochemical measurements were performed using a Bio-Logic
SP-300 potentiostat. The
examination of the metal surfaces is conducted by optical and
scanning electron microscopy
after exposure to corrosive medium for 40 days. SEM morphology
analysis was performed
with VEGA 3 SEM TESCAN at an acceleration voltage of 5 kV. In
order to preserve the
biological specimens, before SEM analysis samples were
chemically fixed with
glutaraldehyde, dehydrated by passing the specimens through a
graded series of ethanol-water
mixtures to 100% ethanol, and then dried.
Results and discussion
River water
For application of SAMs in a corrosion protection it is not
sufficient to determine only the
initial protection level but also to verify if the protection
remains satisfactory in time. For that
reason, polarization DC measurements in narrow potential range
were conducted over longer
period of time. Also, after electrochemical testing, surface
sample analysis was performed.
The polarization resistance values (Rp) determined from the
polarization measurements are
given in Fig. 1.
(a)
0 5 10 15 20 25 30 35 40
100
1000
10000
Rp / k
cm
-2
CuNi - RV CuNi/ODPA - RV
CuNi - SRV CuNi/ODPA - SRV
t / day (b)
0 5 10 15 20 25 30 35 40
100
1000
10000
Rp / k
cm
-2
SS - RV SS/ODPA - RV
SS - SRV SS/ODPA - SRV
t / day
Fig. 1. Polarization resistance dependence on time of exposure
to natural (RV) and sterilized
(SRV) river water for blank and ODPA treated cupronickel (a) and
stainless steel samples (b).
By comparing the polarization resistance values of blank samples
in natural and sterilized
river water, more or less higher resistance value may be
observed for samples immersed in the
natural medium. Such behaviour could be due to biofilm formation
on the surface of the
substrate preventing the penetration of corrosive ions up to the
surface. Other possibility is
that filtration procedure results in better removal of mud and
other compounds that could
form deposits on metal surface. A similar trend can also be
observed in the resistance value
for the treated samples. Although polarization resistance of
treated samples decreased in time,
it remained at least 10 times higher than Rp of untreated
samples.
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After exposure to the natural and sterilized river water for 40
days, the optical microscopy and
scanning electron microscopy measurements were performed.
Results of surface analysis are
presented in Fig.2.-Fig 4.
CuNi
river water
CuNi
sterilized river water ODPA/CuNi
river water
ODPA/CuNi
sterilized river water
SS
river water
SS
sterilized river water ODPA/SS
river water
ODPA/SS
sterilized river water
Fig. 2. Optical microscopy images with 250x magnification of
samples after exposure to
natural and sterilized river water
Optical microscopy images show the formation of biofilms on
substrate surfaces immersed in
natural medium, which is in a good agreement with previous
assumption. Furthermore, it is
clear that the blank CuNi sample immersed in the sterilized
medium more heavily corroded
and less covered by deposits than the one immersed in natural
river water, which is in
accordance with the reduced value of the polarization resistance
due to the corrosion process.
Optical microscopy images of treated samples reveal the opposite
situation where in sterilized
medium much more deposits are observed.
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Natural river water Sterilized river water B
lan
k C
uN
i sa
mp
le
OD
PA
/Cu
Ni
trea
ted
sa
mp
le
Fig. 3. SEM images of blank CuNi alloy and ODPA films after
exposure to natural and
sterilized river water
SEM images of blank CuNi alloy and ODPA films after exposure to
natural and sterilized
river water show that the entire surface of the untreated sample
in natural medium is covered
with larger aggregates which are not only corrosion products. It
can be seen at 5000x
magnification that the structures present on the surface can
attributed to the microorganisms,
while the fibrous structure corresponds to the appearance of
extracellular polymer material.
Although copper ions are toxic to numerous microorganisms, they
formed colonies not
directly on the metal surface, but on the top of the aggregates.
It is supposed that
mucopolysaccharide matrix protects them from toxic copper ions.
It can also be seen the
presence of localized corrosion- pits, which are often
associated with the corrosive influence
of microorganisms. The pitting is significantly more harmful to
the mechanical properties of
the material than uniform surfaced corrosion, which can be seen
at the surface of blank
sample after exposure to sterilized medium. SEM images of
protected samples in natural
medium show that after 40 days of exposure to corrosive media,
the layers are almost
completely homogeneous and almost without the cracks in the
film. Since the surface is
hydrophobic, it has prevented the formation of larger biofilm
deposits on the surface of the
film.
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Natural river water Sterilized river water B
lan
k S
S s
am
ple
OD
PA
/SS
tre
ate
d s
am
ple
Fig. 4. SEM images of blank SS alloy and ODPA films after
exposure to natural and sterilized
river water
SEM images of blank SS alloy and ODPA films after exposure to
natural and sterilized river
water show similar results as previous samples: the formation of
biofilm on the surface of the
untreated sample and pitting corrosion caused by microorganisms,
and the surface of the
treated sample almost completely homogeneous, without cracks in
the film, and without any
microorganisms.
Seawater
After investigation of protective properties of ODPA films in
the river water, the corrosion
behaviour of protected and blank cupronickel and stainless steel
samples was also examined
in natural and sterilized seawater during 40 days of continuous
exposure. Polarization
resistance evolution in time is presented in Fig.5.
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(a)
0 5 10 15 20 25 30 35 40
0
100
200
300
400
500
600
700
800
Rp / k
cm
-2
t / day
CuNi - MV CuNi/ODPA - MV
CuNi - SMV CuNi/ODPA - SMV
(b)
0 5 10 15 20 25 30 35 40
1
10
100
1000
10000
100000
Rp / k
cm
-2
SS - MV SS/ODPA - MV
SS - SMV SS/ODPA - SMV
t / day
Fig. 5. Polarization resistance dependence on time of exposure
to natural (MV) and sterilized
seawater (SMV) for blank and ODPA treated cupronickel (a) and
stainless steel samples
It can be observed that Rp values of all treated samples were
higher than for the bare untreated
samples in natural and sterilized seawater. During the entire
period of measurement they
remained at least two times and 100 times higher for ODPA films
on CuNi and SS samples,
respectively. It is interesting to note the differences in the
corrosion behaviour of untreated SS
samples in natural and sterilized media. In a natural medium the
deterioration of the oxide
film can be clearly see which is certainly caused by the
activity of microorganisms. After
exposure to the natural and sterilized seawater for 40 days, the
optical microscopy and
scanning electron microscopy analysis were performed, and
results are given in Fig.6.-Fig 8.
CuNi
seawater
CuNi
sterilized seawater ODPA/CuNi
seawater
ODPA/CuNi
sterilized seawater
SS
seawater
SS
sterilized seawater ODPA/SS
seawater
ODPA/SS
sterilized seawater
Fig. 6. Optical microscopy images with 250x magnification of
samples after exposure to
natural and sterilized seawater
Optical microscopy images doesn’t show the formation of biofilms
on substrate surfaces
immersed in natural medium, but it is obvious that all samples,
treated and untreated,
immersed in the natural seawater are more heavily corroded than
samples from sterilized
medium. Electrochemical measurements showed significant
difference in Rp for blank
stainless steel samples in two examined media. Optical
microscopy confirmed that in natural
seawater steel surface gets more damaged than in sterilized
medium. Although the protective
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film in natural medium is damaged on the treated CuNi and SS
samples, no significant drop in
Rp values was observed in the previous results.
Natural seawater Sterilized seawater
Bla
nk
Cu
Ni
sam
ple
OD
PA
/Cu
Ni
trea
ted
sa
mp
le
Fig. 7. SEM images of blank CuNi alloy and ODPA films after
exposure to natural and
sterilized seawater
The entire surface of the blank CuNi samples is covered with
cracks in the oxide film, which
are largely present in the natural medium compared to sterilized
water (Fig. 7). The
deterioration of the protective oxide layer, which normally
protects the surface of copper and
copper alloys, may be due to increased concentration of chloride
or other aggressive anions
and compounds which cause corrosion, and are produced by
microorganisms. The SEM
images of protected samples in natural medium show that after 40
days of exposure to
corrosive media, the layers have cracks in the film, which
cannot be seen in the sterilized
medium.
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Natural seawater Sterilized seawater B
lan
k S
S s
am
ple
OD
PA
/SS
tre
ate
d s
am
ple
Fig. 8. SEM images of blank SS alloy and ODPA films after
exposure to natural and sterilized
seawater
Although at the 1000x magnification all the samples immersed in
natural and sterilized media
look similar, yet at a 5000x magnification it is clear that
pitting corrosion occurred on the
surface of blank sample and ODPA films exposed to natural water
while in sterilized water
corrosion attack is observed over complete surface. Similar to
what was observed in river
water, blank surfaces are much more covered by microbes and
their products than the samples
protected by ODPA films.
Conclusion
In this work the possibility of stainless steel and cupronickel
alloy protection by self-
assembled layers of octadecylphosphonic acid prepared by
spraying method in natural river
and seawater was examined. Results of electrochemical
investigation and surface analysis of
the samples after exposure to natural river and seawater show
that phosphonic acid are
relatively stable during 40 days of measurements and provide
good corrosion resistance under
corrosive condition. While on the blank surfaces the formation
of biofilms and pitting
corrosion caused by microorganisms was observed, protective ODPA
layers are almost
completely homogeneous and without cracks in the film,
especially in river water which is
less corrosive medium compared with seawater.
Acknowledgments: The research leading to these results has
received funding from Croatian
Science Foundation under grant agreement 9.01/253.
The financial support of the Foundation of the Croatian Academy
of Sciences and Arts, for
projects “Corrosion protection of metals in natural waters” is
gratefully acknowledged.
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References
1. Q. Zhao, Y. Liu, C. Wang, Appl. Surf. Sci., 252 (2005)
1620.
2. T.S. Rao, A.J. Kora, B. Anupkumar, S.V. Narasimhan, R. Feser,
Corros. Sci., 47 (2005)
1071.
3. S. Ghareba, S. Omanovic, Corros. Sci., 52 (2010) 2104.
4. C. Hao, R.H. Yin, Z.Y. Wan, Q.J. Xu, G.D. Zhou, Corros. Sci.,
50 (2008) 3527.
5. I. Felhosi, E. Kalman, Corros. Sci., 47 (2005) 695.
6. Q.Q. Liao, Z.W. Yue, D. Yang, Z.H. Wang, Z.H. Li, H.H. Ge,
Y.J. Li, Corros. Sci., 53
(2011) 1999.
7. K.M. Kruszewski, E.R. Renk, E.S. Gawalt, Thin Solid Films,
520 (2012) 4326.
8. N.T. Flynn, T.N.T. Tran, M.J. Cima, R. Langer, Langmuir, 19
(2003) 10909.
9. I.H. Sung, D.E. Kim, Tribol. Lett., 17 (2004) 835.
10. B.M. Silverman, K.A. Wieghaus, J. Schwartz, Langmuir, 21
(2005) 225.
11.Q. Quinones, A. Raman, E.S. Gawalt, Thin Solid Films, 516
(2008) 8774.
12. M. Dubey, T. Weidner, L.J. Gamble, D.G. Castner, Langmuir,
26 (2010) 14747.
