Volume 9 Preprint 12
AC corrosion of mild steel in marine environments and the effects ofcathodic protection
Dae-Kyeong Kim, J.D.Scantlebury, Srinivasan Muralidharan, Tae-Hyun Ha, Jeong-Hyo Bae, Yoon-Cheol Ha and Hyun-Goo Lee
Keywords: AC corrosion, mild steel, cathodic protection, sea water
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Volume 9 Preprint 12
AC corrosion of mild steel in marine environments and the effects of
Dae-Kyeong Kim1, J.D.Scantlebury2, Srinivasan Muralidharan 1*,3, TaeHyun Ha1, Jeong-Hyo Bae1, Yoon-Cheol Ha1 and Hyun-Goo Lee1
Institute, 28-1, Seongju-dong, Changwon, 641-120, Republic of Korea.
Corrosion and Protection Centre, University of Manchester, M60 1QD,
Concrete Structures & Failure Analysis Group, Corrosion Protection
Division, Central Electrochemical Research Institute, Karaikudi– 630 006,
Tamilnadu, India. (e-mail:firstname.lastname@example.org)
The influence of alternating current (AC) corrosion of mild steel in natural
sea water was studied systematically under cathodic protection (CP)
condition. Electrochemical studies were carried out at the CP protection
potential namely -780 mVSCE. Corrosion rate determination at the different
applied AC current densities was carried out by conventional weight loss
method for the exposure period of 24 hrs. The pH of the test solutions for
the exposure period of 24 hrs was noted. The amount of leaching of iron
(Fe) into the solution at various AC current densities was done by using
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science
and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it
should not normally be referenced in published work. © UMIST 2004.
inductively coupled plasma spectrometry (ICP). Mild steel specimens were
subjected to surface examinations after treatment with various AC current
densities under CP condition. Optical electron microscopy was used for
analysing the surface of the mild steel. All the studies revealed that mild
steel tends to corrode when applying AC even though it is under CP
conditions. The corrosion rates are increased with increasing AC current
densities. Two to three fold increases in the corrosion rates was obtained at
100 A/m2 . Surface micrographs showed the spreading of red rust products on
the mild steel surface after 10 A/m2 . The concentration of Fe was also
found higher above 10 A/m 2. The electrochemical measurements couples
with surface examination and solution analysis proved to be a very effective
tool by means of characterizing the AC corrosion of mild steel in sea water
Keywords: AC corrosion, mild steel, cathodic protection, sea water
Corrosion caused by AC interference is of concern when high-voltage
transmission lines run parallel to buried pipelines over a long distance, when
catenary systems are close to concrete rebars in railway tunnels, when
concentric neutral copper wires in electrical cables are used for urban
distribution, in the presence of AC stray currents etc. In the last decade,
some leaks on high pressure gas pipelines were attributed to the influence
of induced AC, although the pipelines were properly cathodically protected
. Literature suggests that until the 1970’s AC corrosion would have been
only a fraction or a few percent of the amount of corrosion that would have
been caused by direct current (DC) of the same value [2-4]. The influence
of AC corrosion of Fe, Pb, Cu, Al and other metals are reported [5-7]. Most
of the literatures concern the influence of AC in acidic and sulphate
solutions [8-14]. However, more researches are essentially needed for the
influence of AC on chloride environments. This investigation emphasis
studies on the influence of AC corrosion on cathodically protected mild steel
in sea water medium. A simple electrochemical approach is adopted for the
evaluation of AC corrosion of mild steel in natural sea water.
Mild steel obtained from Dongil Industries Co., Ltd., South Korea was used
and the composition in wt% is follows: C (0.43), Si (0.22), Mn (0.72), P
(0.013), S (0.015), Ni (0.05), Cr (0.10), Cu (0.12) and Fe balance. Mild steel
specimen of size 2 cm (l) X 2 cm (b) x 0.5 cm (thickness) was used and the
total exposed surface area was 11 cm2. They were given mechanical
polishing and then successively with finer SiC papers (240, 400, 600, 800,
1200 grit). Before and after each immersion experiment, individual
specimens were cleaned with 50% (v/v) hydrochloric acid, followed by a
rinse with de-ionized water and acetone and dried under stream of hot air.
Each specimen had one tapped end so that it could be mounted on a
threaded specimen holder. Natural sea water (Jinhae Sea, South Korea) was
used as aggressive solution and the composition is given in Table-1.
2.2.1.Weight loss experiments with and without AC under CP condition
Immersion experiments were carried out to quantify the corrosion rate of
mild steel in natural sea water under the influence of various AC current
densities. The weight of the mild steel samples was taken before and after
immersion using Ohaus Explorer 4-digit electronic balance for gravimetric
weight loss measurements. The AC signal was applied between the working
electrode (mild steel) and the counter electrode (platinum gauge). Solartron
1480 Multistat electrochemical measurement unit coupled with Solartron
1255-B Frequency Response Analyzer and multi media computer was used to
supply the required AC waveform. An oscilloscope was used to adjust as well
as to monitor the AC signal applied. Experiments are carried out on the basis
of protection potential namely -780 mVSCE as follows: (i).to find out the
actual CP current at the protection potential in natural sea water in the
absence of AC sources and (ii). to find out the corrosion behaviour of mild
steel in natural sea water under CP in the presence of various AC current
densities. For the above, firstly experiments were carried out by applying
constant protection potential namely -780 mV SCE on mild steel working
electrode in natural sea water and the protection current (DC current) was
monitored for an exposure period of 24 hrs using a computer controlled data
logger. Actually, the current was monitored by using a Keithley 2701
Ethernet Multimeter data acquisition system controlled by computer. So the
program was made to record one value at the end of one hour. Like that 24
readings are recorded for the total exposure period. The values obtained are
reasonable and reproducible from the triplicate set of experiments.
Secondly, CP condition was simulated in the electrochemical cell by
maintaining the cathodic DC current constant and varied the AC amplitudes
at various current densities at a fixed frequency of 60 Hz. Triplicate
experiments were carried out for each AC current density and the average
corrosion rate was made in mm/yr. Corrosion rate in mm/yr was made by
using difference in weight losses between initial and final weight of the
specimens for the exposure period of 24 hrs.
At the end of weight loss measurements the mild steel specimens were
taken out carefully and visually examined for any corrosion products like red
rust observed on the surface aided with magnifying lens .
For pH measurements, AC sources were applied between mild steel working
electrode and platinum counter electrode in natural sea water for the
exposure period of 24 hrs. At the end of the experiment the pH of the
solutions was determined. The pH for the blank system and at the various
AC applied current densities was measured. At the end of the exposure
period 50 cc of the solution was taken in a 100 ml beaker and the pH of the
solution was measured using a standard calibrated pH meter. The pH was
measured by using a portable ISTEK pH meter (Model 76P) with a relative
accuracy of ±0.002. Triplicate measurements were made for each system
and the average was plotted against the current density.
2.2.4.Surface examination by optical microscopy
Optical microscopy was used to examine the nature of corrosion on the mild
steel surface before and after immersion in various test solutions. The mild
steel specimens were immersed in test solutions for fixed duration of 24 hrs
under the CP condition with and without AC. At the end of 24 hrs the
specimens were taken out from the solution, thoroughly washed with
distilled water, cleaned with acetone and dried. Micrographs were recorded
for the blank and at the different applied AC current densities in natural sea
water. The microscopic examination was carried out using Olympus-GX 71
make computer controlled microscope.
2.2.5.Solution analysis by inductively coupled plasma (ICP) spectrometry
The solution analysis was carried out with an idea to quantify the leaching
characteristics of mild steel. It is expected that during the immersion
studies the metal dissolution expected releasing considerable amount of
metal ions from the material due to the AC source. The concentration of Fe
present in the test solution was determined under CP condition with and
without AC. The solution analysis was carried out by an inductively coupled
plasma-atomic emission spectrometry (ICP, Applied Research Laboratory,
USA). Triplicate experiments were carried out for each system and the
average values are noted.
3. Results and Discussion
3.1.Cathodic protection of mild steel in natural sea water without AC
Mild steel was subjected to perfect CP i.e.-780 mVSCE in natural sea water
and the respective cathodic DC current was measured for the exposure
period of 24 hrs in sea water. The cathodic DC current was -0.1498 mA. As
expected the system was in CP condition, no corrosion was noticed.
Specimens were examined visually at the end of exposure period and it was
found that no rust was formed on the mild steel surface. Its indicate that
the system was perfectly cathodically protected. This cathodic DC current is
utilized for the further studies with different AC current densities.
3.2.Determination of corrosion rate by weight loss measurements
Fig.1. shows the corrosion rate of mild steel in natural sea water under CP
with and without AC. It was observed that the corrosion rate for mild steel
was increased with increasing various AC current densities. The corrosion
rate at the perfect CP condition was almost negligible. On the other hand,
the corrosion rate was gradually increased upto the AC current density
namely 10 A/m 2. After that there was a drastic increase in the corrosion
rate was noticed. Weight loss experiments reveals that cathodically
protected specimen at -780 mVSCE with out AC showed no corrosion rate and
the corrosion rate abruptly increased at the higher AC current densities. The
later was due the anodic AC voltage peak go over the corrosion potential
when the AC current density was applied. This could be interpreted that AC
current flow causing charge transfer still being existed overwhelming
cathodic protection current. The higher magnitude of corrosion rate for mild
steel in sea water at the higher current densities may be due to the
destabilization of the passive layer on the mild steel surface. The growth of
the passive layer occurs as a result of electrochemical reaction and this
passive layer is tends to depassivate at the higher current densities and
accelerate the corrosion of mild steel.
Visual observations showed that no corrosion product was obtained in the
absence of AC sources under CP condition. More red rust products are seen
on the mild steel surface in natural sea water at higher AC current densities
even though under CP condition. Mostly, red rusts formed at the one place
rather spread towards the entire surface. At higher AC source, more than
90% area showed red rust products.
The pH of the sea water at the end of the exposure period of 24 hrs with
respect to various AC current densities is given in Fig.2. The pH of the sea
water used was 8.34. On the other hand there was a small increase in the
pH values with increasing AC current densities. The pH at the 100 A/m2 was
9.0. It reveals that the degree of alkalization is increased in the solutions.
The degree of alkalization of the solutions close to a metal is believed to be
playing a major role in the corrosion process. The alkalization arises from
influence of either the anodic or cathodic DC current along with AC sources
which electrochemically reduces water into OH-.
2H2 O + O2 + 4e- ----------> 4OH-
or 2H2O + 2e- -----------> H2 + 2OH-
The accumulation of OH- and consequent pH change would be the result if a
mass balance for OH- in the solution. The combination of elevated pH and a
vibration of the DC potential within this area caused by the AC may induce
corrosion either by a destabilization of the passive layer or in the case of
extreme alkalization by entering the general corrosion area (HFeO2 stabilization) at high pH. The AC also indirectly contributes to the
alkalization process. When AC imposed on a coupon placed in a chloride
solution will lead to a depolarization of the electrochemical kinetics and AC
accelerates the DC current and consequently the rate by which OH - is
produced under cathodic polarization. In general production of OH- ions
would increase the conductivity of the solution adjacent to the metal.
3.5.Surface examination by optical microscopy
The optical micrographs for mild steel in natural sea water at the protection
potential -780 mV SCE with varied AC current densities are shown in Fig.3.
Micrographs recorded at the CP condition without AC is shown in Fig.3(a).
No rusts are seen from the micrograph. But the surface morphology was
changed when the AC current density is increased from 1 A/m 2 to 100 A/m2 .
Figs.3(b) and (c) showed the micrographs at the 20 and 100 A/m 2 AC current
density respectively. Here due to higher acceleration of corrosion by the AC
source the entire surface showed red rust products. The changes in the
surface morphology of mild steel by the AC source
is indicating that AC
source have a strong influence on the corrosion kinetic parameter i.e.
corrosion current density. This observation was agreed with literature
which AC affects the kinetic parameters such as Tafel slopes and exchange
current densities for carbon steel, copper and zinc in 1 M sulphate solutions
. The initiation of corrosion is due to the metal solution interface could
be altered by the AC signal and thereby affecting the corrosion kinetics. In
this case both reversibility and faradaic rectification aspects have to be
considered to understand the AC induced corrosion. Here the irreversibility
of the chemical reactions occurring at the interface causes a change in the
double layer compositions and a modification of the metal surface. A
definite trend was observed between AC corrosion immersion studies and
surface morphology examinations.
3.6.Solution analysis by inductively coupled plasma spectrometry
The concentration of iron in test solution at the -780 mVSCE CP with varied
AC current densities is given in Fig.4. The concentration of iron is increased
while increasing AC current densities. The concentration of Fe in test
solutions upto 10 A/m 2 is comparatively smaller than higher AC current
densities. The leaching of Fe was almost doubled at the higher current
densities. For example the amount of iron in test solution at 10 A/m 2 was in
the range from 10 ~12 ppm, but it showed 22~25 ppm at 100 A/m 2. The
same observation was noticed in weight loss measurement also. So a
definite correlation was observed between weight loss measurements and
solution analysis studies.
All these data’s clearly indicated that AC sources having definite influence
on the corrosion of mild steel in natural sea water under CP conditions. Most
probably, both reversibility and faradaic rectification aspects have to be
considered to understand the AC induced corrosion. The irreversibility of the
chemical reactions occurring at the interface causes a change on the double
layer composition and a modification of the metal surface.
The following conclusions can be drawn from this investigation:
1. AC sources able to accelerate the corrosion of mild steel even though
they are cathodically protected in natural sea water.
2. AC current density upto 10 A/m2 showed lesser corrosion rates. But two
to three fold increases in the corrosion rate was obtained at the higher
AC current densities.
3. Visual observations reveal that 90% area of the mild steel was affected
by red rust products at the higher AC current density region.
4. The degree of alkalization was found increase with increase of AC
5. Micrographs showed the spreading of red rust products on the mild steel
surface. Red rusts are visually seen when the AC source was increased
after 10 A/m2. The concentration of Fe in the solution also directly
proportional to the AC current density.
6. A good agreement was observed between weight loss measurements
coupled with surface morphological studies and solution analysis.
Acknowledgements: One of the authors (S.M.) thanks CSIR and CECRI, India
for the permission to pursue a PDF at KERI, South Korea. S.M. also thanks
KOFST, South Korea for the financial assistance through Brain Pool Program.
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Table 1 : Chemical compositions of sea water
Bi carbonate (HCO 3) as CaCO 3
Total hardness (as CaCO3)
Corrosion rate (mm/yr)
AC current density (A/m )
Fig.1. Corrosion rate for mild steel in sea water under CP
with and without AC current densities
AC current density (A/m )
Fig.2. pH of sea water under CP with and without AC
Fig.3.Micrographs of mild steel in sea water under CP
(a) blank (b) 20 A/m2 (c) 100 A/m2
Concentration of Fe (ppm)
AC current density (A/m2 )
Fig.4.Concentration of Fe in sea water under CP with and without
AC current densities