Volume 2 Paper 19
An Electrochemical Impedance Spectroscopy Analysis of Protective Behaviour of Final Coatings on Naval Steel
E.C. Bucharsky, E.B. Castro and S.G. Real
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JCSE Volume 2 Paper 19
Submitted 13th September 1999
An Electrochemical Impedance Spectroscopy Analysis of Protective Behaviour
of Final Coatings on Naval Steel.
E.C. Bucharsky#, E.B. Castro and S.G. Real
Instituto de Investigaciones Fisicoqu�micas Te�ricas y Aplicadas (INIFTA),
Suc. 4, C.C.16 (1900) La Plata, Argentina.Email:mailto2('bcastro','inifta.unlp.edu.ar')
#Universidad Nacional de Quilmes, Roque Saenz Pe�a 180, (1876)
The protective performance
of zinc rich paints(ZRP)/top-coating systems has been investigated using
electrochemical impedance spectroscopy (EIS) and corrosion potential
measurement, at different immersion times. ZRP formulations included zinc
pigment content close to 78% and different organic binders, such as alkyd
resins and epoxypolyamide, an alkyd type resin was used as a top-coating.
Experimental impedance diagrams were described in terms of transfer function
analysis using non-linear fit routines. Changes in the dielectric properties of
the top-coating, monitored by EIS, were used to evaluate the water uptake.
§2 Key words: organic coatings,
electrochemical impedance, cathodic protection.
§3 Coatings or linings with organic and inorganic compounds
are a widespread method to provide corrosion protection in order to improve the
durability of engineering structures. Transport properties of water and
corrosion species are very important factors in the coating formulation design.
§4 In the development of coatings it is therefore necessary
to optimize the permeability of the coating for water and corrosive species.
Water, becomes the major cause of swelling, loss of adhesion, deterioration of
the mechanical properties and the start of the corrosion process. Modern
electrochemical techniques, such as electrochemical impedance spectroscopy, has
been used for the investigation of the protective coating properties on metals
[1-5]. In the present work, changes of coating properties have been monitored
employing electrochemical impedance spectroscopy to evaluate the water
transport and protective behaviour in final coating system containing an
alkyd-type-top and either sprayed zinc or zinc rich paint (ZRP) based on
different organic binders such as epoxy and alkyd. For this purpose, a short description of the theory
related to the capacitance method of evaluating water up-take is presented.
§5 Water is transported through the coating by two different
and simultaneous mechanisms: convection through pores and imperfections and
diffusion through the polymer matrix.
§6 The convection of the electrolyte through the coating
pores is responsible for the initiation of the corrosion process at relatively
short times, (t � 1h),
allowing the electrical contact between Zn particles in the ZRP and with the
diffusion through the top-coating layer, is a much slower process, resulting in
an increase of the permittivity of the polymer with immersion time.
Accordingly, higher values of the film capacitance, C, may be determined from
impedance measurements recorded at increasing immersion times. C being related
to the polymer permittivity, ep, by:
eo is the permittivity of vacuum, A is the area
of the polymer film and l is the thickness.
§8 In this way impedance measurements may be used as a method to monitor the diffusion of water in the coating.
empirical equation of Brasher and Kinsbury , accounts for the relationship
between the coating capacitance and the volume fraction of water in the coating
layer, as already described by de Witt et al :
Where f is the volume fraction of water in the coating, C(t) is the capacitance
at time t and C(0) is the capacitance at t = 0, 80 is the relative permittivity
related to water diffusion in the paint film, such as the diffusion coefficient
D, the solubility S and the permeation coefficient P, may be derived from
impedance data recorded at different immersion times. For this purpose the
transport equation of water in the coating must be solved with the adequate
§11 As a
first approximation water diffusion may be described by Fick�s laws. The
solution of Fick�s second law , for a free film taking up water from the
environment, was given by Crank and Park [8,9], considering as boundary
For t � 0,����� The concentration of water
in the coating is uniform (or zero).
For t > 0, ���� The surface
concentrations of water at x = 0 and x = l are equal and constant. The existence of water , is assumed at the
interface due to the fast convective transport through the pores, as indicated
by EIS data described below.
§12 The flux of
water, at x=l/2 ( half thickness of free film) is Jw = 0.
into account eq.(2), the following expression may be derived,
valid at long immersion times:
simulation of experimental C(t) values, in terms of eq. (3) allows the
determination of the diffusion coefficient, D, if the polymer layer exhibits
ideal Fickean behavior even at long immersion times.
procedure requires good estimated values of the initial coating capacitance, C0,
and of the capacitance at infinite time, C�.
solution of Fick�s laws indicates a linear dependence of C(t) vs t1/2,
for t � 0 , so C0 may be calculated from the extrapolation of
this plot at t = 0.
§17 C� may
be determined from capacitance values at very long immersion times, provided
that no swelling of the coating takes place, ie C(t) is constant for t � �.
§18 SAE 1020 (UNS G10200) steel plates 20 x 8 x 0.2 cm were
used as metallic substrate. Metal surfaces were initially sandblasted to AS 2 1/2−3 degree (SIS Standard 05 59 00/ 67), degreased with vapor toluene, and
finally coated with different ZRP and top-coating alkyd resins; in all the
cases panels were prepared in duplicate and stored seven days for curing at 20�2
�C before beginning the tests. The primer
compositions and dry film thickness are assembled in Table 1. ZRP formulations
included spherical zinc as unique pigment, whose physical properties are given
in Table 2. For the sake of comparison, some steel samples were covered with
sprayed Zn (sample S3 in Table 1).
§19 Table 1.� ZRP and Top Coat Compositions
Zn/Total Solids Ratio
Zn/Total Solids Ratio
§20 Table 2. Zinc Pigment Physical
Density (g cm-3)
Apparent density (g cm-3)
Oil Absorption (g/100 g)
§21 The ZRP/Topcoat dry film thickness was measured with
an electromagnetic gauge employing bare sanded plates and standards of known
thickness as reference. Potentials were measured and referred to in the text
against a saturated calomel electrode (SCE). Electrochemical impedance
Spectroscopy (EIS) measurements in the 3 mHz �
65 kHz frequency range were performed in the potentiostatic mode at the
corresponding corrosion potential attained after different exposure times in
artificial sea water using a frequency response analyzer and an electrochemical
interface (Solartron, FRA 1250 and EI 1186, respectively) integrated with a PC
system. The exposed geometrical area of samples was 16 cm2. For
impedance measurements, an activated Pt probe was coupled to the SCE through a
10 �F capacitor to reduce phase shift errors at high frequencies. Artificial
sea water was prepared according to the ASTM Standard D 1141-90. Detailed
descriptions of the experimental setup and data processing have been described
§22 The standarized procedures ASTM B 117-85 (Salt Spray
Chamber) and ASTM D 4541-85 (Pull-Off Adhesion) were also performed on the
painted steel samples for comparing their results with the electrochemical
ones. After these tests, the painted panels were assessed with the ASTM
Standards D 610-68 and D 714-87 in order to evaluate the degree of rusting and
of blistering, respectively, in an attempt to correlate visual observations and
§23 The dependence of the corrosion potential Ecorr
on immersion time (Fig. 1) in artificial sea water illustrates changes in the
protective mechanisms supplied to the steel substrates by the studied final
coating. At the beginning of the exposure
the oscillations of the open circuit potential indicate that Ecorr
remains undefined due to an initial barrier protective mechanism.
§24 With increasing immersion time, (t > 1h) Ecorr
attains a potential that lies in the range of the corrosion potential of pure
Zn electrodes in sea water . This fact indicating that water has reached
the ZRP layer, giving rise to the electric contact among the Zn particles and
with the steel substrate. In this way the cathodic protection mechanism period
is established, at relatively short immersion times.
§25 Figure 1.������� Ecorr dependence on exposure time
in artificial seawater for the tested samples
§26 The fast initial transport of water into the ZRP layer
must occur by convection through the pores and imperfections of the top-coating
polymer. Diffusion of water through the polymer matrix is a much slower
process, producing changes in the top-coating capacitance, C, at longer times,
as described above.
§27 After prolonged exposure time in the electrolyte, t > 120 days, Ecorr
attains values corresponding to the corrosion potential range of steel in sea
water (-0.65V), indicating that the cathodic protection is no longer active.
§28 The comparison of Ecorr vs t plots, of ZRP (of
the same Zn content) without top-coating, previously published, with the
results depicted in fig.1, shows a longer period of cathodic protection for the
samples with top-coating. This fact can be interpreted considering that the
top-coat acts as a barrier for the transport of both water and oxygen.
§29 Impedance spectra at different immersion times for sample
S1, S2 and S3 are depicted in Figure 2. Impedance data display similar features
as those reported for ZRP without the top-coating [1,10], indicating the
presence of water in the ZRP layer at short immersion times, as described
above. Nevertheless significant diferences are evident: Higher impedance values
are determined at comparable immersion times, and a continuous decrease of the
modulus lZl is observed, in contradiction with EIS data for ZRP without
Top-coat. For t > 30 days relatively constant values of lZl are attained.
These experimental facts indicate a continuous increment of the exposed active Zn area, due to a much
longer wetting period, in comparison with the data of ZRP without Top-coat. The
whole corrosion process of active Zn is also hindered by the smallest rate in
the oxygen diffusion process.
§30 Figure 2������� Nyquist plots for samples
S1, S2 and S3 at different immersion time.
§31 An increase of the whole system impedance is observed
after t > 100 days, as the formation of Zn corrosion products reduce the
active area, at long exposure times.
§32 The set of impedance diagrams contains valuable
information related to the characteristic coating parameters as well as to the
kinetics of corrosion process. Impedance spectra were analyzed using a non
linear fit routine according to the following transfer function:
§33 The transfer function described by eqs. (4) and (5)
corresponds to the dynamic behaviour of the equivalent circuit:
Where Rs is the electrolyte resistence, C is the top-paint
film capacitance and Rp the pore resistance in the top-coat. Zi
corresponds to the impedance related to the whole corrosion process
taking place in the ZRP layer, RC is related to the charge
transfer resistance of the oxygen reduction reaction and RA �accounts for the charge transfer resistance
of zinc dissolution reaction. Cdl is the capacitance associated with
the Zn/electrolyte interface. A finite diffusion impedance, Zd, was considered
in order to account for the transport process involved in the cathodic partial
Zd=RD0 (jS)-1/2 tanh(jS)�,
Do being the diffusion length and diffusion coefficient of oxygen.
§34 Figure 3���� Experimental and simulated
Bode and Nyquist plots, for the samples S1, S2 and S3 at different
§35 Figure 3 shows a fairly good agreement between
experimental and fitted results obtained according to the transfer function
described by equations (4) and (5).
§36 The C values obtained from the fitting procedures
were used to determine the diffusion coefficient of water through the coating
according to the theoretical analysis presented above. Equation (3) is valid
when the transport of water is properly described in terms of Fick�s laws. If
this is the case, two limiting conditions must be observed: a linear dependence of C vs t1/2
as t � 0, and a plateau in the C vs t curve as t � �, indicating the value of C�.
§37 The plot C �vs t 1/2 corresponding to the system S2,
is depicted in Figure 4.
comment(38)Figure 4.���� Dependence of
C on� t1/2 ��for sample S2.
§39 �From the
extrapolation to t = 0, a value of C0 = 3x10−9 F cm−2,
may be obtained. A value of C� = 4x10−7 F cm−2,
corresponding to the experimental capacitance after 28 days of immersion, was
considered as the capacitance at infinite time.
§40 Figure 5 depicts experimental and theoretical
[log(C(t)/C0)/log(C�/C0)] vs t plots
§41 Figure 5.����� Experimental and
theoretical [log(C(t)/Co) / log (C�/Co)] vs t plot, corresponding
to sample S2. The theoretical curve was
calculated in terms of Eq.3.
theoretical curve was calculated in terms of eq.(3), taking into account seven
terms of the series and D = 2x10−11 cm2s−1
for the diffusion coefficient of water in the polymer layer. Good agreement
between experimental and calculated data may be observed indicating that the
assumption of Fickean behaviour can be considered as a good approximation for
the diffusion of water through the polymers presented in this work.
§43 The dependence of fit parameters on immersion time is
assembled in Figure 6 for the different ZRP formulations.
§44 In the absence of Top-coat, the continuous decrease of
the capacitance Cdl as well as the increase of the resistance RA and
RC can be assigned to a progressive decrease of the
electrochemically active area due to the accumulation of� corrosion products in the ZRP.
§45 In the present case the existence of a top-coat introduces
modifications in this behaviour: An increase in the Zn active area, with
immersion time, is deduced by the decrease of the parameters RA and
RC and the increse of Cdl . Higher values of RDO
are also related to higher values of the diffusion length for oxygen transport,
to the presence of the top-layer.
§46 Figure 6.���
Dependence of fitting parameters C,� Cdl , RA� and� Rc on exposure time for samples S1, S2 and S3.Conclusions
system analysis employing small signal perturbation allows the determination of
specific parameters of the system, which characterize the protective
performance of the final paint scheme with increasing immersion time in sea
§48 The dependence of Ecorr, and of the
fitting parameters with immersion time indicates that the top-coating acts as a
barrier, hindering both water and oxygen transport. As a consequence the
wetting period of the ZRP layer is much longer, and the oxygen diffusion
transport is slower. In this way Zn particles provide active cathodic
protection for longer exposure times than ZRP without top-coating.
§49 EIS has proved to be a powerful tool allowing to monitor
changes in the coating properties, with immersion time as well as providing
information related to both water transport and corrosion process taking place
at the metal/coating interface.
§50 This research project was financially supported by Consejo
de Investigaciones Ci�ntificas y T�cnicas (CONICET), Comisi�n de
Investigaciones Ci�ntificas de la Provincia de Buenos Aires (CIC), and
Fundaci�n Antorchas. Part of the equipment used in this work was provided by
the DAAD and the Alexander von Humboldt-Stifung.
§51 ������� E.C.
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