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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

INFLUENCE ON THE DECANTAION WATER ON THE CORROSION

RESISTANCE OF AN ORGANIC COATING APPLIED TO THE

INTERNAL PARTS OF THE CRUDE OIL STORAGE TANKS

Nadia HAMMOUDA1,

*, Kamel Belmokre1

1,*

Department of Chemistry, Faculty of Sciences, 20 Août 1955 University

Route d’El-Hadaiek, B.P.26, Skikda, Algeria. E-mail: n.hammouda@univ-skikda.dz ,

hammoudanad@yahoo.fr

Abstract. This study focuses on the evaluation of corrosion protection behaviors/performances of zinc

rich epoxy polyamide primer (ZRP), containing an amine group on the sandblasted carbon steel

surface in decanting crude oil by Electrochemical Impedance Spectroscopy (EIS), corrosion potential

(Ecorr) measurements and EIS of the sandblasted carbon steel, and therefore also the electrical models

(equivalent electrical circuits) used to explain the impedance results. The electrochemical behavior of

painted surface was estimated by EIS parameters that contained paint film resistance, paint film

capacitance and element Z which represents the electrochemical process at the metal/solution

interface. This was further confirmed by the decrease of polarization resistance (Rf) and OCP towards

more negative value was also the indicator of the deterioration of paint film protectiveness. The

electrochemical technique is an effective method for studying corrosion performance. We have

proposed an interpretation of the deterioration with time of the capability of the zinc particles in the

paint to provide cathodic protection to the carbon steel. Key words: Carbon Steel Zinc Rich Epoxy

Polyamide Primer, Corrosion Protection, Electrochemical Impedance Spectroscopy (ZRP),

Polarization Resistance.

1. Introduction Zinc-rich primers technology has been used for years, finding application in marine industry, civil

infrastructure and military vehicles. Zinc-rich layer is used as a single coat or as a component of the

paint system. Coating systems consisting of zinc-rich primer and topcoat offer decorative and barrier

as well as anti-corrosive protection to the steel substrate [1–8]. Sacrificial pigments such as zinc

pigments require zinc in large quantities in order to enable flow of electric current [9–16]. At the

initial stage the zinc dust film being in electrical contact with the steel surface plays the role of a

sacrificial anode providing cathodic protection. At a later period zinc corrosion products such as zinc

oxide or/and zinc hydroxide are formed, thus barrier protection begins to dominate [9, 10]. The greater

the amount of zinc corrosion products which form on the zinc particles surface, the higher the

probability of losing electrical contact between zinc particles themselves or zinc particles and the steel

substrate. Thus it may lead to reduction and in the end complete loss of protection based on the

cathodic mechanism. High electrical conductivity of coating is required for providing the first period

of protection to the steel substrate [10–12]. The protection mechanism of zinc-rich coatings and their

electrochemical performance have been thoroughly studied using mainly potential measurements and

electrochemical impedance spectroscopy (EIS) [4, 6–8, 11, 12]. The impedance behavior of zinc-rich

paints is different compared to typical barrier coatings. Performance barrier coatings possess

protective properties when their impedance modulus exceeds 106–10

7 Ω.cm

2 in the low frequency

range [13]. On the contrary zinc-rich paints exhibit low value (102–10

3 Ω.cm

2) of impedance modulus

during the first period of protection. The electrical conductivity of these paints is related to percolation

phenomena determining the intensity of sacrificial action of zinc particles [12]. It is stated that in order

to obtain continuous electrical contact between zinc particles (i.e. exceed the percolation threshold) a

zinc content above 80–86% (depending on type of binder) by weight in dry paint is very important to

create percolation paths through the coating [14]. On the other hand, the porosity of the coating

strongly affects the accessibility and wetting of zinc particles. In this case the ratio of pigment volume

concentration (PVC) to critical pigment volume concentration (CPVC) should be greater than one to

enable aqueous electrolyte to penetrate and allow sacrificial action of zinc particles [15, 16].It seems

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

that in order to increase electrical contact between the particles themselves and between the zinc

particles and the steel substrate, an attempt needs to be made to fill the space between the standard

zinc particles with conductive substituents. Considering paint containing spherical zinc particles, it is

possible to find that electrical contact between zinc particles is obtained in single points only in the

best case, Fig. 1a [17]. In reality contact between particles can be the critical factor in ensuring

cathodic protection action of zinc-rich paint containing spherical particles. This statement can be

confirmed by SEM micrograph of standard zinc-rich paint containing 92 wt. %, Fig. 1b. It can be seen

that there is limited electrical contact between zinc particles [17]. The assessment of the role played by

Zn pigments in the resistance of the epoxy ZRP/epoxy/Polyamide coating is the main objective of the

present investigation. The electrochemical impedance spectroscopy (EIS) and corrosion potential

(Ecorr) measurements of the sandblasted carbon steel, and therefore also the electrical models

(equivalent electrical circuits) used to explain the impedance results. The dispersion of the zinc

pigments in the coating and the corrosion products were characterized by scanning electron

microscopy (SEM), optical micrograph through morphology and Raman spectra through phase

identification.

2. Exponential details

2.1. Sample Material and Preparation The metallic substrate was A283C carbon steel (according to NF10027 standard) in conformity with

the norm API (American Petroleum Industry), used in the tanker, the chemical composition of the

tested carbon steel is given in Table 1. Before coating application, the metallic substrate was

sandblasted to Sa 2.5 (Swedish Standard SIS 0559 00/67) (roughness Ra 6.2 μm) (Fig. 2a) or polished

with emery paper up to G 400.

Commercial zinc rich epoxy polyamide primer was immediately applied onto steel panels using a

brush or a roller (Fig. 2b). Once cured, the samples were packed, identified, and stocked in a

desiccator until the moment of testing. The coating thickness was measured using an Elcometer gauge

and was found around 70±5 μm for all panels, the composition of the coating is proprietary

information. Coated panels were cut out (100 × 60 × 4 mm) and an electrical wire was added in order

to allow electrochemical measurements. With the aim to achieve the electrochemical measures in the

best conditions it has been suited that the areas of about 15 cm2 exposed to the electrolytic solution

were sufficient. It seemed necessary to use a surface of paint relatively big in contact with electrolytic

solution in order to compensate the insulating role of the sample as the thickness of the film grows.

Mansfeld reports in a technical document [18, 19] a study of Kendig and Scully suggesting the use of

samples covered with a ratio area/thickness of the coating of at least 104 to assure satisfactory

electrochemical measurement. Samples were exposed under open circuit potential conditions in sea

water, for electrochemical impedance.

Fig.1a. Electrical contacts between zinc microparticles

tightly packed in zinc-rich coating. Fig.1b. SEM image of the zinc-rich coating

with embedded zinc particles.

Electrical contact

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

Table 1. Chemical composition of carbon steel (A283C) (% in weight).

Figs.2. (a) Cross-section SEM micrograph on sandblasted carbon steel (Sa 2.5) (b) Cross-section

optical micrograph of the studied epoxy polyamide paint. The observed white particles are due to the

spherical zinc particles.

3. Results and discussion

The corrosion rate was monitored using in situ electrochemical methods such potentiodynamic

polarization, linear polarization resistance, and electrochemical impedance spectroscopy (EIS)

methods.

3.1 Influence of the surface state on the electrochemical properties of the A283C carbon

steel in decanting crude oil

a- Open circuit potential: The OCP measurement is a simple additional tool that provides

complementary information to EIS results regarding the corrosion undergone by the steel substrate

after exposure to aggressive solutions. The variation of OCP over time for sandblasted carbon steel is

shown in Fig.3.

The evolution of the curve is clearly related to differences in the surface characteristics, there is a

displacement of the potential to more negative values. The gradual decrease in OCP after 30 min may

be produced by the diffusion of water. The potential is noted by immersing the sandblasted surface of

the steel in decanting crude oil, and then every minute until the stability is almost total. In the analysis

of Fig. 3, we note more electronegative free potential values, they tend towards -0.699 V vs. Ag/AgCl,

and this is due to a continuous degradation of the surface of the sand-blasted steel where we are

witnessing dissolution of the metal. This degradation could be attributed to the roughness of the

surface, because it is sandblasted steel. So we are witnessing a change in the condition of the metal

surface where we observe corrosion products. These corrosion products more or less adherent causes a

relative stability of the free potential.

C Cu P Si Mn S

0.24 0.20 0.035 0.4 0.9 0.04

(a

)

(a)

(b)

Spherical Zinc

Embedding resin

Coating

Carbon Steel

(a)

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

Fig.3. Variation of OCP with Time in decanting crude oil for sandblasted carbon steel.

b- Potentiodynamic polarization curves

The polarization curve of A283C sandblasted carbon steel electrode immersed in decanting

crude oil is presented in fig. 4. Electrochemical parameters like corrosion potential (Ecorr) and

corrosion current density (icorr) were determined (Table 2). The scan rate was 0.5 mV/s. The

curve given by Figure 4 relative to the behavior of our sandblasted carbon steel in decanting

crude oil, characterize the general shape of the polarization curves in the above experimental

conditions. These curves are plotted after potentiokinetic stability free corrosion potential.

Table 2. Electrochemical parameters

Medium Ecorr (V/Ag,AgCl ) icorr (µA/cm²) Rp (kΩ.cm²)

decanting crude oil -0.655 0.973 49.62

Fig.4. Polarization curve of sandblasted carbon steel immersed in decanting crude oil.

Figure 4 shows that cathodic curve of A283C carbon steel showed little change, although the anodic

curve was dependent on the surface state. According to the curve drawn in the decanting water, in the

vicinity of the corrosion potential a marked diffusion kinetic domain for sanded steel, its current

density of the order of 0.9731 μA/cm2 reflects a non-rapid attack of sanded steel. The sanded surface

0 5 10 15 20 25 30

-720

-700

-680

-660

-640

-620

-600

-580

-560

-540

-520

Po

ten

tial (m

V/A

g-A

gC

l)

Time (minutes)

-1,2 -1,0 -0,8 -0,6 -0,4 -0,2

-14

-12

-10

-8

-6

-4

-2

log

i (

mA

/cm

2)

Potential (V/Ag-AgCl)

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

has more microanodes. Mechanical surface treatments caused changes in both thermodynamic and

kinetic characteristics of the base material. Thermodynamic stability of the surface is represented by

the values of corrosion potentials Ecorr in our case. More negative value of this electrochemical

characteristic means that the surface is subjected to corrosion and thermodynamically is not stable.

The kinetics of the corrosion process is expressed by the corrosion current density icorr reflecting the

intensity of running corrosion processes in an electrolyte. This electrochemical characteristic has a

direct relation to the corrosion rate rcorr.

3.2 Electrochemical Properties of Sandblasted Carbon Steel in decanting crude oil

The sandblasting pretreatment generally helps to remove surface contaminants and corrosion products

from the surface and forms rougher surfaces suitable for further painting operations.

On the one hand, the quality and morphology of the sandblasted surface are responsible for rougher

and less uniform. The Electrochemical Impedance Spectroscopy (EIS) measurement is carried out in

decanting crude oil, using sandblasted carbon steel panels (A283C). The electrochemical impedance

diagram is shown in Fig. 5. The values of various parameters are summarized in Table 3. We note a

continuous deterioration of the sandblasted carbon steel to Sa 2.5 provoking a change in the state of

the metallic surface. It can for example, to cover of corrosion products, weakly adhesive which

provoke a stability of the free corrosion potential; the value was around –0.698 V vs. Ag/AgCl.

Table 3. Parameters values extracted from the fitting procedure.

Medium Ecorr

V/Ag,AgCl Re (Ω.cm

2)

Rct

(kΩ.cm2)

Cdl (µF.cm-2

)

decanting crude oil -0.698 1.364 335.8 47.39

Fig.5. Evolution of Nyquist diagrams as a function of immersion time in decanting crude oil for the

sandblasted carbon steel.

The value of the electrolyte resistance Re is very weak, of the order of 1.364 Ω .cm², what shows that

the medium is conductive. It is characterized by a single capacitive loop. As arc is not centered on the

real axis Z for the calculation of Rp was extrapolated "low frequency" at zero frequency portions. The

value of the charge transfer resistor Rct which is of the order of 335.8 kΩ.cm², could be due to the

penetration of the electrolyte to our sanded surface [20] which explains the film formation of corrosion

products that grows with the immersion time [21, 22] as rust layers consist mainly of hydroxides such

as Fe(OH)3 or an iron oxide Fe2O3. In contrast, the capacity of double layer Cdl is of the order of 47.39

μF/cm². This value is attributed to the porous nature of the film of the corrosion products formed

abandonment and present at the metal interface.

3.3 Potentiodynamic polarization study (Polarization curves)

0 10 20 30 40 50 60 70 80

0

10

20

30

40

50

60

70

80

-Zi (K

oh

m.c

m2)

Zr (Kohm.cm2)

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

A- Behavior of zinc-rich epoxy polyamide primer in decanting crude oil The polarization curves of our primer electrode immersed in the decanting crude oil are presented in

fig. 6. Electrochemical parameters like corrosion potential (Ecorr), corrosion current density (icorr) and

polarization resistance (RP) were determined (Table 4). The scan rate was 0.5 mV/s. As presented in

fig. 8 and table 4, the values of Ecorr and icorr were obtained from the extrapolation of anodic and

cathodic Tafel lines located next to the linearized current regions. The values of polarization resistance

(Rp) for our coated electrode were calculated as reported in previous work [23] corrosion as follows:

[

( )] …………………………… (1)

…………………… (2)

Where, k is a constant that defines the units for the corrosion rate (=3272 mm/(amp.cm.year)), EW the

equivalent weight in grams/equivalent of iron alloy (EW = 27.93 grams/equivalent), d the density in

gcm−3

(=7.86), and A the area of the exposed surface of the electrode in cm2. According to the values

summarized in Table 4, which concerns the electrochemical behavior of our epoxy polyamide primer

in decanting crude oil, we observe from the first hours of immersion and from the value of the

polarization resistance Rp (3.17 kΩ.cm²) and the low value of the corrosion current density which is of

the order of 21.2175 μA/cm², that our primer has good resistance to corrosion and it played a good

barrier effect between our metal substrate and the study medium, but after two days of immersion we

observe a decrease in the polarization resistance Rp (1.97 kΩ.cm²) with an increase in the corrosion

current density which is of the order of 32.1203 μA/cm², but after eight days of immersion the value of

the polarization resistance Rp increases (3.31 kΩ.cm²).

This is due to penetration of electrolyte (water and oxygen molecules) and destructive ions into the

ZRP and formation of nonconductive zinc corrosion products (e.g., ZnO) which tend to seal the

coatings pores (result of removing solvent during curing process), the coating resistance increases.

Accumulation of corrosion products in the pores of our primer is believed to be responsible at least to

some extent for this behavior. We observe a cyclic phenomenon when the evolution of the film

resistance and the corrosion current is due to the presence of the zinc pigments in our paint.

Fig.6. Polarization curves obtained for an epoxy polyamide primer

as a function of immersion time in decanting crude oil.

Table 4. The obtained results of polarization experiment

Parameters

Time (d)

Ecorr

(V/Ag-AgCl)

icorr

(µA/cm²)

Rp

(kΩ.cm²)

1 -0.959 21.217 3.17

-2,0 -1,8 -1,6 -1,4 -1,2 -1,0 -0,8 -0,6

-7,5

-7,0

-6,5

-6,0

-5,5

-5,0

-4,5

-4,0

-3,5

-3,0

Cu

rren

t (µ

A/c

m²)

Potential (V/Ag-AgCl)

1 d

2 d

4 d

8 d

10 d

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

2

4

8

10

-1.066

-1.085

-1.075

-1.061

32.120

29.654

21.753

30.198

1.97

1.90

3.31

2.90

After 10 days of immersion, the value of the corrosion current density increases with a value of

30.1982 μA/cm2 and contrary to this we note a decrease in the resistance of the paint film: 2.90

kΩ.cm², we attribute this state of clogging pores of the paint by the corrosion products formed, due to

the attack of the material through the pores of our primer paint. But unfortunately these products are

unstable and come off after a few days; this is confirmed by the decrease in the value of the

polarization resistance. According to Abreu et al. [11], the evolution of the free corrosion potential

Ecorr allows to follow the electrochemical activity of the zinc-rich paints (ZRPs). It is believed that the

electrochemical processes occurring in such systems are the oxidation of zinc particles ( ) and the reduction of dissolved oxygen (

) respectively [24]. The

authors reported that the Ecorr evolution for liquid ZRP coated samples is in close relationship with the

ratio of active areas (zinc/steel) and allows to define the cathodic protection (CP) duration which is the

period where Ecorr remains lower than −0.86 V vs. Ag/AgCl [25, 11]. The ratio of the surface area of

active zinc to the steel determines the Ecorr of ZRP coated panels. The increase in this potential

corresponds to the decrease of the electroactive zinc area which means the decrease of the cathodic

protection intensity. This is generally attributed to the isolation of the zinc particles by the zinc

corrosion products in the coating.

The overall reaction can also be expressed as (2Zn + O2 + 2H2O → 2Zn (OH)2) → 2ZnO + 2H2O) [11,

26]. Zinc corrosion product (ZnO) is a semi-conductive material. Consequently as the result of these

reactions, effective surface area of active zinc particles decreases and hence increasing the OCP values

of our coated panels, Fig.7a shows a typical top view micrograph of an epoxy polyamide paint,

spherical zinc are distributed randomly in the structure in Fig.7b optical micrograph of the studied

epoxy polyamide paint the white region corresponds to zinc corrosion products appeared on the

surface of ZRP coating.

Fig.7.Cross-section optical micrograph of the studied epoxy polyamide paint containing spherical zinc

(55% Zn). (a) Before exposure: The observed white particles are due to the spherical zinc particles (b)

after exposure: Areas highlighted by red circles indicate zinc corrosion products.

Lower OCP values result that the zinc particles in our primer are electrochemically active enough for

CP during ten days immersion, and the zinc particles are in proper galvanic contact to the steel

substrate. In our ZRP the coating was electrochemically active with a high number of electrical

contacts between zinc particles. This means that a higher part of the zinc particles was involved in a

percolation process [27].

GX5 GX5

Zn corrosion products

(a) (b)

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

B. Corrosion potential (Ecorr) measurements

The evolution of the free corrosion potential (Ecorr) of coated panels allows following the

electrochemical activity of the ZRPs [24, 28, 11]. Ecorr evolution allows defining the duration of

cathodic protection for the coated samples. The free corrosion potential for samples is plotted versus

immersion time and presented in Fig. 8. It must be noted that the variation of Ecorr values depends on

the ratio of zinc to steel (active areas) [29]. According to the commonly accepted criterion to provide

cathodic protection, Ecorr value should remain lower than −0.735 V (Ag/AgCl) equivalent to -0.780 V

(Ag/AgCl) [7]. As shown in Figure 10, It can be seen that Ecorr was cathodic between −1.061 and

−0.959 V vs. Ag/AgCl during the ten days of entire immersion, Ecorr values stay at the same level close

to −1 V (Ag/AgCl) for almost 240 h, This indicates that there is a galvanic contact between zinc

particles ensuring good cathodic protection to steel substrate.

This shows that the zinc particles in the primers were electrochemically active with a high number of

electrical contacts between zinc particles. This high percolation means that zinc pigments improve a

good electrical contact which implies that the steel substrate was under a good cathodic protection

(CP). That means that a higher part of the zinc particles was involved in a percolation process. When

electrolyte diffuses into the coating, the ratio of active zinc powder to steel substrate will increase and

OCP will decrease to a value close to free corrosion potential of zinc in galvanic connection with steel

which is about −1.16 V (Ag/AgCl) in corrosive sodium chloride solution [30, 26].

It is also noticeable that the OCP values for our tested primer is always more active (more negative),

these data allow concluding that the selected liquid probably promotes the activation of the zinc

pigments extending the duration of the cathodic anticorrosive protection.

Fig.8. Variations in corrosion potential with time for zinc-rich primer.

C -Electrochemical Impedance Spectroscopy Behavior of Epoxy polyamide primer

EIS was applied to give more detailed information on electrochemical performance and the

mechanism of protection provided by the investigated zinc-rich paints. Typical Nyquist plots of EIS

spectra for API A283C coated carbon steel electrode by a primer layer (epoxy polyamide) at his open-

circuit potential (-1.160 V vs. Ag/AgCl) after their immersion in decanting crude oil for 48 hours are

shown in Fig. 9. The spectra represented in these figures were analyzed by best fitting to the

equivalent circuit model shown in Fig. 11. The EIS parameters obtained by fitting this circuit are listed

in Table 5. The main features of these Nyquist diagrams involve a capacitive semicircle in the high-

frequency (HF) region (one constant of time) become smaller with increasing immersion time

followed in the low frequency part (LF) by a linear straight line inclined to 45° to the real axis, this is a

characteristic of the process of diffusion (WARBURG diffusion) [31].

2 4 6 8 10-1,10

-1,08

-1,06

-1,04

-1,02

-1,00

-0,98

-0,96

-0,94

EO

CP, V

vs A

g/A

gC

l

Immersion, days

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

Fig. 9. Evolution of Nyquist diagrams as a function of immersion

time in decanting crude oil for coated steel. (Z', Z" Ω.cm2).

Table 5. Parameters values extracted from the fitting procedure.

Time

(days)

Ecorr

(V/Ag, AgCl)

Re

(Ω.cm2)

Rp°

(kΩ.cm2)

Rd

(kΩ.cm2)

Cf

(μF.cm2)

1

2

3

-0.937

-0.667

-0.597

223

562.8

623

14.38

3.67

1.75

0.705

0.414

0.307

17.48

2.163

1.015

It is reported in literature that the two capacitive loops obtained for impedance spectra on ZRPs

correspond to the charge transfer (high frequency loop) and oxygen diffusion process (low frequency

loop) [30]. This indicates that the semicircle relates to dissolution of zinc dust contained in the coating

and the diffusion impedance indicates the limitations of mass transfer for this reaction inside the

coating. As the coating continued to be exposed diffusion impedance taking the shape of the Warburg

impedance begins to dominate. It might be an effect of decreasing the active surface area due to zinc

consumption and increasing amount of zinc corrosion products [17]. The capacitive contribution

merges as a slight distorted loop as a consequence of the porous nature of the electrode [32, 33]

Deviation from the ideal RC behavior in terms of a distribution of time constants due to surface

inhomogeneities, roughness effects, and variations in properties and composition of surface coatings

can be considered in a dispersion formula for the corresponding transfer function [34]. According to

the results shown in Table 5, Rp° decreased with exposure time (14.38 kΩ.cm2

to 1.75 kΩ.cm2) allow

us to conclude that the film of corrosion products put on the interface metal/solution, through the pores

is becoming unstable and that the coating is very porous, or even discontinuous [35]. The existence of

a diffusionnel control is an indicator of the defective coating; this is confirmed by the decrease in the

values of the diffusion resistance (Rd): 0.705 kΩ.cm2, 0.414 kΩ.cm

2 and 0.324 kΩ.cm

2. The increase

or decrease in Rd, is due to a phenomenon of clogging and unclogging or creation of new pores in the

paint. During the 72 hours of immersion the capacity of our film Cf decreases (0.705 μF.cm-2

to 0.307

μF.cm-2

).

In the case of our coated samples the OCP increases from −0.937 V vs. Ag/AgCl to more positive

value −0.597 V vs.Ag/AgCl while immersion time increases, reaching a common level that lies

intermediate between zinc and steel corrosion potentials in our study environment. The stationary

value of Ecorr ~ −0.600 V vs.Ag/AgCl correspond to a situation where the zinc galvanic action is

probably suppressed due to the partial stabilization of corrosion products on the pigment surface.

Under these circumstances, attack of the substrate (red rust) can be visually observed at several

localized spots on the epoxy polyamide samples [36]. This rapid increase in Ecorr towards less negative

0 2 4 6 8 10 12 140

1

2

3

4

5

6

-Zi (k

oh

m.c

m2)

Zr (kohm.cm2)

24 h

48 h

72 h

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

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potential values typical of steel and close to −0.597 V vs. Ag/AgCl, the increase in this potential

corresponds to the decrease of the electroactive zinc area which means the decrease of the cathodic

protection intensity. This is generally attributed to the isolation of the zinc particles by the zinc

corrosion products in the coating or that zinc pigments were consumed figs .10, the electrical contact

between them was lost and they were unable to provide cathodic protection of the steel substrate [17].

That attributed the low-frequency loop to the zinc dissolution process. This value corresponds to a

situation where the zinc galvanic action is probably suppressed due to the partial stabilization of

corrosion products on the pigment surface. Under these circumstances, attack of the substrate (red

rust) can be visually observed at several localized spots on the epoxy polyamide samples that

attributed the low-frequency loop to the zinc dissolution process. The high frequency loop developing

after a while, and being progressively affected by the accumulation of corrosion products, was here

generally ascribed to the coating properties. It can be concluded that our tested primer have the lowest

barrier properties due to the porosity of the coating.

Fig.10. Cross-section optical micrograph of the studied epoxy polyamide paint/carbon steel interface.

(a) before exposure (b) after exposure.

D. Equivalent Circuit for the EIS Simulation

The EIS data are fitted with a corresponding electrical circuit model using Zview version, where Rs

represents the ohmic resistance between reference and working electrodes, followed by the coating

capacitance (Cf) in parallel with a coating resistance (Rf) and finally an element Z which represents the

electrochemical process at the metal/solution interface (Fig.11).

Fig.11. General equivalent electrical circuit for organic coated metals.

3.4. Raman spectroscopy analysis

In order to understand the behavior of our zinc-rich epoxy polyamide paint, complementary analyzes

were carried out. Raman spectroscopy analyzes were performed, after immersion in our study

environment. This technique allows to identify locally zinc corrosion products inside the coatings.

Representative spectra obtained for sample is shown in Fig. 12. The main Raman band related to the

Magnetite (Fe3O4) is found around 667 cm-1

[7, 37]. The band centered at 543 cm−1

was attributed to a

non-stoichiometric oxide Zn1+xO [7, 38]. We detected additional peaks at 240.5 attributed to

GX110 GX110

Carbon Steel

Embedding resin

Zn corrosion products

Carbon Steel

Coating

Embedding resin

(a) (b)

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The Eighth Jordan International Chemical Engineering Conference (JIChEC 2017)

November 7-9, 2017

simonkolleϊte [4Zn (OH)2. ZnCl2. H2O] and the peak detected at a band of 409.7 cm–1

is attributed to

the presence of iron oxide FeO [7, 39-40], we detected also additional peak at 1340 cm–1

. The oxidized

forms were first observed at the solution/coating interface and progressed towards the steel substrate

as the immersion duration increased.

Fig. 12. Raman spectra of zinc-rich epoxy polyamide paint after immersion in decanting crude oil.

4. Conclusion

The influence of the surface state and the zinc pigments on the electrochemical behavior of zinc-rich

epoxy polyamide coatings were studied using electrochemical impedance spectroscopy (EIS). This

technique can provide much useful information about the protection mechanisms of ZRP. Open

Circuit Potential (OCP) of paint film was observed less negative than rusted one, the evolution of

these values were clearly related to differences in the surface characteristics. In this research the EIS

has provided information on the loss of the protection resistance, specially coating resistance (Rf), and

coating capacitance Cc which directly correlated with adhesion of the paint film to the surface.

Impedance data can be discussed in terms of a transfer function model corresponding to the equivalent

circuit for the steel/ZRP coating system involving the contributions of a parallel Rf -Cc element at high

frequencies for the paint film electrical properties and a Warburg like behavior at low frequencies. The

relative influence of diffusion processes through the paint film depends on exposure time of the ZRP

in the aggressive media. Red rust spots and voluminous white rust corrosion products on epoxy

polyamide samples were found. Corrosion of ZRP-coated steel involves defect formation, penetration

of aggressive media, loss of adhesion, and attack of the substrate. EIS data can be used to detect and

evaluate the early degradation of ZRP coatings on steel. Once the polarization time has ended, Zn

pigments oxidation takes place, leading to the cathodic protection of the metallic substrate.

5. Acknowledgements

Thanks are due to Sonatrach (Direction Régionale de Skikda, Algeria) for providing ZRP coated

panels.

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