Engineering, 2010, 2, 580-584
doi:10.4236/eng.2010.28074 Published Online August 2010 (http://www.SciRP.org/journal/eng).
Electrochemical Generation of Zn-Chitosan Composite
Coating on Mild Steel and its Corrosion Studies
Kanagalasara Vathsala, Thimmappa Venkatarangaiah Venkatesha, Beekanahalli Mokshanatha
Praveen, Kudlur Onkarappa Nayana
Department of Studies in Chemistry, School of Chemical Sciences, Kuvempu University, Shankaraghatta, India
Received December 16, 2009; revised February 26, 2010; accepted March 6, 2010
Abstract
A Zinc-Chitosan composite coating was generated on mild steel from zinc sulphate-sodium chloride electro-
lyte by electrodeposition. The electrolyte constituents were optimized for good composite coating. The cor-
rosion resistance behavior of the composite was examined by weight loss, polarization and impedance
methods using 3.5 wt% NaCl neutral solution as medium. Separate polarization profiles were recorded for
composite coating and compared with zinc coated sample. SEM images of coatings were recorded for the
pure and composite coating.
Keywords: Composite coating, Chitosan, SEM, Impedance, Electrodeposition
1. Introduction
Zinc electroplating is an industrial process and is widely
used to coat on steel for enhancing its service life. As
zinc is electrochemically more active than steel and
hence it sacrificially protect the steel from corrosion.
However zinc itself undergoes corrosion leading to the
formation of zinc compounds called white rust on its
surface. This tendency of formation of white rust reduces
the life of the coating from the expected period. There-
fore to enhance the life span of the zinc coating and to
avoid the white rust formation the alternative methods
like surface modification is adopted. The earlier modifi-
cation methods are associated with chromate based for-
mulations and the procedure is very simple to generate
passive chromate films on corroding zinc coatings. The
use of chromate passivation is prohibited because of
pollution hazards. An alternate to this chromation is to
generate surface films or surface barriers with specific
organic molecules or with certain addition agents [1-6].
Also the service life of zinc coating is enhanced by in-
cluding the inert materials in its coating. The inclusion is
done by codeposition of these materials with zinc and
thus generating composite coating. These zinc composite
coatings exhibit better corrosion resistance property.
Nowadays the nanosized materials are codeposited to get
better zinc composite with better corrosion resistance
[7-10].
A survey of literature reveals that the conducting
polymers were used for anticorrosive coatings and as
inhibitor for steel [11-13]. However limited information
connected to zinc - polymer composite coatings on steel
is available in the literature and especially with zinc -
biopolymer composites.
The chitosan is one such biopolymer used in corrosion
inhibition of mild steel without causing environmental
problems. Chitosan possess good biocompatibility, che-
mical resistance, mechanical strength, antimicrobial pro-
perties and thermal stability and have been utilized suc-
cessfully in biotechnology, for different applications.
The hydroxyl apatite chitosan nanocomposite was ob-
tained on stainless steel to provide better corrosion pro-
tection [14,15]. Chitosan is widely used in industry due
to its film forming and gelation characteristics. In dilute
solutions it is a linear polycation with high charge den-
sity. This electrochemical property was utilized in the
present work to get the zinc chitosan composite film on
mild steel from electrolysis and its corrosion resistance
property was tested.
2. Experimental
2.1. Plating Process
Zinc and Zn-chitosan coatings were electrically depos-
ited from sulphate-chloride bath. The constituents of the
bath were 250g/L ZnSO4·7H2O, 40 g/L NaCl, 30g/L
H3BO3 and 0.g/L chitosan (88% deacetylated). In all the

K. VATHSALA ET AL. 581
experiments distilled water and analytical grade reagents
were used. The pH of the bath solution was adjusted to
2.5-3 by adding dil.H2SO4 and NaHCO3. The bath was
stirred for few hours before subjecting it into plating ex-
periments. The cathode was mild steel and anode was
zinc (99.99%). The mild steel surface was polished me-
chanically, and degreased with trichloroethylene in de-
greased plant followed by water wash. Before each ex-
periment the zinc surface was activated by dipping in
10% HCl for few seconds and was washed with water.
Equal area of anode and cathode was selected for elec-
trode position process. The bath temperature was at 300
K. The deposition process was carried at 4 A/dm2 and
under mechanical stirring.
2.2. Weight Loss Measurements
The coating thickness prepared for corrosion tests was in
the range of 10–15 µm. The corrosion rate by weight loss
measurements were performed for mild steel samples
coated with pure zinc and Zn-chitosan composite. The
electrolyte was 3.5 wt% NaCl solution and the test sam-
ples were immersed vertically in the solution which was
maintained at room temperature. The difference in
weight was measured once in every 24 hours for a period
of 15 days. In each weight loss measurement the cor-
roded samples were rinsed in alcohol, dried with hot air,
and then the weight was noted. The weight loss evalu-
ated was used for estimating the corrosion rate.
2.3. Salt Spray Test
The salt spray test as per (ASTM B 117) was carried out
in a closed chamber. The deposited plates were freely
hanged inside the chamber and subjected to continuous
spray of neutral 5 wt% NaCl vapors. The specimens
were observed periodically and the duration of the time
for the formation of the white rust was noted.
2.4. Electrochemical Measurements
A conventional 3-electrode cell was used for polarization
studies. The zinc coated or Zn-chitosan composite coated
specimen with surface area of 1 cm2 was used as working
electrode. Saturated calomel and platinum foil were em-
ployed as reference and counter electrodes respectively.
The electrolyte was 3.5 wt% NaCl solution. The corro-
sion resistance property of these specimens was evalu-
ated from the anodic polarization curves.
The electrochemical impedance measurements were
performed using AUTOLAB from Eco-chemie made in
Netherlands. The steel specimens and their dimensions
were same as that of polarization experiment. The EIS
was recorded in the frequency range from 100 kHz to 10
MHz with ± 5 mV AC amplitude sine wave generated by
a frequency response analyzer.
The surface morphology of the coatings was examined
using a JEOL-JEM-1200-EX II scanning electron mi-
croscope
3. Results and Discussion
3.1. Corrosion Rate Result
The zinc and composite coatings was generated on sepa-
rate mild steel plates having the thickness of about
10-15µm. The steel panels were immersed completely in
3.5 wt% NaCl solution for different time intervals and
the weight loss values were used to calculate the corro-
sion rate. Figure 1 represents the corrosion rate (wt loss/
hour) profiles with respect to number of hours. The cor-
rosion rates of both composite and zinc coatings were
very high in the beginning and decrease exponentially in
the middle and it becomes constant after 200 and 150 hrs
for zinc and composite coatings respectively. At any
given time the rate of corrosion for composite was al-
ways less than that of zinc coating. This suggests that the
composite coating possess higher corrosion resistance
property. This property was due to the presence of chito-
san in the zinc matrix.
3.2. Salt Spray Test Result
The industrial method of testing the corrosion behavior
of zinc-plated objects is salt spray test. The test was
conducted by spraying 5 wt% NaCl solution in a cham-
ber. The NaCl drops accumulated on the surface of the
coated specimens facilitate the corrosion resulting in zinc
salts called white rust. The time taken for the formation
of white rust was the indication of the corrosion rate. The
higher corrosion resistance delays the production of
white rust. In the present case the pure zinc produced the
white rust after 19 hrs and the Zn-chitosan composite
050100 150 200 250 300 350 400
2
3
4
5
6
7
8
9
zinc coating
composite coating
Corrosive velocity (10-5kg/m2.h)
Time (h)
Figure 1. Variation of the corrosion rate with immersion
time for zinc and composite coated samples in 3.5 wt.%
NaCl solution.

K. VATHSALA ET AL. 582
produced the white rust after 28 hrs. This test confirms
the enhancement of corrosion resistance of zinc in the
presence of chitosan in its matrix.
3.3. Electrochemical Result
Figure 2 shows anodic polarization profile of zinc and
Zn-chitosan coated sample in 3.5 wt% NaCl solution.
The linear variation was observed in the beginning up to
-1.01 V and afterwards there was gradual increase in
current indicating electrochemical oxidation of zinc.
However in the case of composite coating, the potential
was always more positive for any given current density.
This indicates that the composite requires extra potential
to bring anodic reaction. Thus the composite possess
higher resistance to corrosion process on its surface.
The Nyquist plots for zinc and Zn-chitosan coatings
are shown in Figure 3. The larger loop was produced by
Zn-chitosan coatings whereas smaller semicircle was
obtained for pure zinc. It can be easily observed from the
figure that Rp values are higher for composite coating
than zinc coating. This indicates that composite coating
is more corrosion resistant than zinc coating.
3.4. Surface morphology
The SEM images at lower and higher magnification were
represented in Figure 4. Also the SEM images of cor-
roded surface of zinc and composite are given in Figure
5 and Figure 6. The SEM images show the practical
evidences on the corrosion protection ability of compos-
ite coating.
4. Discussion.
The experimental results of the present investigations
inferred that the chitosan can be included in the deposit
easily. It acquires a positive charge by protonation in
0
100
200
300
400
500
-1.04 -1.03 -1.02 -1.01 -1.00 -0.99 -0.98
zinc coating
composite coating
Potential (V)
Current density(cm-2)
Figure 2. Anode polarization curves for zinc and composite
coated samples in 3.5 wt.% NaCl solution.
5 1015202530354045
-2
0
2
4
6
8
10
12
14
16
-Z''(ohm)
Z'(ohm)
zinc coating
composite coating
Figure 3. Impedance diagrams for pure zinc coated and
Zn-chitosan coated samples in 3.5 wt.% NaCl solution.
Figure 4. SEM images for the two samples. (a) Zinc coating,
(b) composite coating.
Figure 5. SEM images for two samples after anodic polari-
zation for (a) zinc coated; (b) composite coated sample.
Figure 6. SEM images for two samples after 15 day’s weight
loss measurements. (a) zinc coated; (b) composite coated.
Acid solution [15].
233 2
chit NHHOchit NHHO

 
During electrodeposition naturally H2 evolution takes
place and there is increase inOH  ions at the close vi-
1mX 4000
a
1m X 4000
b

K. VATHSALA ET AL. 583
cinity of the cathode. TheseOH ions combine with pro-
ton of the protonated chitosan at the electrode and get
precipitated. This solid precipitate codeposited with the
zinc. Generally the composite coating of zinc with other
material is generated by dispersing the insoluble particles
in the electrolyte. Here the solid is dispersed in liquid
state. Even the same procedure is followed in polymer-
metal composite. However in chitosan-metal composite,
the chitosan was codeposited from electrolyte, where in
chitosan and electrolyte were in single phase. Chitosan
exists as polycation in acid solution and reaches the
cathode easily during electrodeposition and it will get
deposited on the cathodic site.
The results of corrosion rates for zinc and composite
coatings from chemical and electrochemical methods are
in agreement with each other. The composite coating
with chitosan provides higher corrosion resistance than
zinc coating. As these molecules possess higher molecu-
lar weight and larger molecular size, they cover the cor-
roding surface to larger extent through its cationic point
attached to cathodic site of the surface. Thus there may
be formation of barrier which prevents the direct contact
of corroding metal with the corrosive medium. There are
reports in the literature on the corrosion inhibition of
polymer molecule to metals [16-17]. The weight loss
method, impedance, salt spray test results of the present
study revealed higher corrosion resistance property of
composite coating. In all these methods, probably chito-
san hinders the anodic reaction and finally the corrosion
rate was decreased. The delayed white rust formation in
salt spray inferred that the inclusion of the chitosan
makes the composite coating to acquire more corrosion
resistant property. Also the higher RP value and more
positive potential of composite coating make the deposit
nobler than zinc coating. The corrosion rate and time
profiles indicate that the corrosion velocity (Figure 1) of
composite was always less than zinc coating.
Figure 4 shows the SEM image of the zinc and Zn-
chitosan composite coatings. Composite coated samples
have a ridge shaped grains on the surface which reveals
the inclusion of chitosan into zinc matrix.
The anodic polarization of zinc and composite (Figure
5(a) and 5(b) showed that zinc coating undergoes more
dissolution than composite. The crystals get dissolved
easily during corrosion (Figure 5(a)). This had not been
observed in composite coating (Figure 5(b)). The SEM
images of samples after 15 days of chemical corrosion
showed larger deep pits arising out of higher corrosion
rate for zinc coating (Figure 6(a)). The composite coat-
ing (Figure 6(b)) exhibited small pits which distributed
throughout the surface and resulted uniform corrosion
with lower rate. These experimental results revealed
higher corrosion resistance property of Zn-chitosan com-
posite coating compared to pure zinc coating.
5. Conclusions
Zn-chitosan composite was generated by electrodeposi-
tion from sulphate bath. The precipitated chitosan was
codeposited along with zinc. The performance of com-
posite coating was established from the results of weight
loss, polarization, impedance and salt spray test. In all
these studies Zn-chitosan composite exhibits better anti
corrosion performance. The SEM images of surface pro-
vide an evidence for the presence of chitosan in coating
and crystalline nature. The composite showed uniform
and lower corrosion rates than that of zinc coating.
6. Acknowledgements
The authors are grateful to University Grant Commission,
New Delhi, Govt. of India [Major Research Project
F.32-220/2006(SR)] for providing financial assistance.
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