evaluation of cathodic protection behavior of waterborne inorganic zinc‐rich silicates containing...

Evaluation of cathodic protection behaviorof waterborne inorganic zinc-rich silicates

containing various contents of MIO pigmentsMohammad Naser Kakaei and Iman Danaee

Technical Inspection Engineering Department, Shahid Tondgouyan Faculty of Petroleum Engineering,Petroleum University of Technology, Abadan, Iran, and

Davood ZaareiPolymer Engineering Department, Faculty of Technical and Engineering, Islamic Azad University – South Tehran Branch, Tehran, Iran

AbstractPurpose – The aim of this paper is to study the corrosion protection behavior of water-borne inorganic zinc-rich coatings based on potassium silicate/nanosilica developed with various zinc and micaceous iron oxide (MIO) contents during cathodic protection stage.Design/methodology/approach – The formulated coatings were applied on carbon steel panels and were subjected to electrochemical impedancespectroscopy (EIS) and free corrosion potential measurements for characterization of corrosion protection behavior. Also atomic force microscopy (AFM)and optical microscopy were used to investigate the surface topography of coatings.Findings – All of the coatings preserved the cathodic protection ability throughout 75 days of exposure to 3.5% NaCl solution. Supporting results ofelectrochemical tests and microscopic observations revealed that replacement of zinc by MIO particles reduced both the rate of reactivity and theduration of cathodic protection of inorganic zinc-rich coatings. It was observed that the coatings demonstrated a reactivation step after a dry-wet cycleimplying that cyclic immersion can change the overall duration of cathodic protection stage.Originality/value – The paper describes formulation and investigation of corrosion protection behavior of an environmentally friendly zero-VOCcoating as well as providing an insight into EIS of zinc-rich coatings.

Keywords Steel, Corrosion protection, Coatings technology, Cathodic protection, Inorganic zinc-rich, Micaceous iron oxide,Electrochemical impedance spectroscopy, Corrosion potential, Reactivity

Paper type Research paper

Introduction

During the past century, the use of inorganic zinc-rich coatings,either by themselves or as primers in multi-coat systems, hasbeen one of the efficient methods to prevent corrosion of steelstructures in corrosive atmospheres.Due to inherent porosity ofthese coatings, zinc corrosion products get locked into thesurface pores and their galvanic protection mechanism fadesinto barrier one as time goes on (Sørensen et al., 2009). Havingan inorganic and close-to-nature structure and also the active/barrier protection switch capability are the keys to these beingdurable andhigh performance anticorrosive coatings.However,a major drawback of classic solvent-based ethyl silicate coatingsis the emission of volatile organic compounds (VOC), whichcontribute to atmospheric pollution. Recently, with thereduction in VOC content of coatings, developments havereturned to nontoxic, environmentally friendly zinc rich silicatecoatings based on waterborne silicates (Thomas, 2009).Thestudyof effects causedby the incorporationof co-pigments

and extenders into the formulation of inorganic zinc-rich coatingshas been an important activity in this field (Akbarinezhad et al.,

2011; Falberg, 1996; Bastidas et al., 1991a, b; Morcillo et al.,

1998). The reason is that the percentage of zinc corroded during

the cathodic protection stage of zinc-rich coatings is relatively

small compared to the initialmetallic zinc content (Morcillo et al.,

1998). On the other hand the use of co-pigments can improve

mud cracking resistance and weldability and reduce topcoat

bubbling, risks of zinc release into the environment, and help to

reduce the relatively high cost of zinc dust (Hare, 1998).The influence of partial replacement of zinc bymanydifferent

conductive extenders (to maintain the electrical conductivity of

the coating film) has been investigated, amongst which only

di-iron phosphide and carbon black (to some extent) gave

satisfactory results (Akbarinezhad et al., 2011; Bastidas et al.,

1991a, b; Falberg, 1996; Morcillo et al., 1998; Parashar et al.,

2001; Feliu et al., 2001; Sørensen et al., 2009).Non-conductive extenders such as barytes, mica and talc also

are commonly used in the formulation of inorganic zinc-rich

coatings, for certain technical reasons (Parashar et al., 2001).

Obviously, the use of these extenders leads to loss of electrical

contact between zinc particles and with the substrate. Hence, it

seems that the incorporation of non-conductive extenders is

reasonable only in the case that the reactivity of the zinc particle

network is higher than adequate and it is intended to control the

electrical conductivity, or the case that loss of cathodic

protection occurs sooner, but better adhesion, mechanical

The current issue and full text archive of this journal is available at

www.emeraldinsight.com/0003-5599.htm

Anti-Corrosion Methods and Materials

60/1 (2013) 37–44

q Emerald Group Publishing Limited [ISSN 0003-5599]

[DOI 10.1108/00035591311287438]

The authors wish to express their gratitude to Pars Zinc Dust and SormakMining Co. for their material support.

37

properties, or even barrier protection is achieved instead.

Micaceous iron oxide (MIO) is the most widely used lamellarpigment for anticorrosive barrier coatings and essentially is a

type of haematite (Fe2O3). Lamellar particles of MIO usuallyalign parallel to the substrate and impede the transport of

aggressive species (Sørensen et al., 2009).Although little attention has been paid to electrochemical

behavior of waterborne inorganic zinc-rich coatings, a look intothe results of other types of zinc-rich coatings can be beneficial.

Electrochemical impedance spectroscopy has been employedextensively to investigate the behavior of solvent borne zinc-richcoatings. Nevertheless, there are disagreements in the literature

concerning the modeling and interpretation of the impedancespectra. The electrochemical behavior has been interpreted by

means of a simple Randles-type circuit (Akbarinezhad et al.,2011; Armas et al., 1992; Hammouda et al., 2011; Pereira et al.,1990; Shi et al., 2011), by two Randles circuits connected inseries (Liu et al., 2011; Xie et al., 2003), by a Randles circuit

with additional capacitance and resistance componentsconnected in parallel (Akbarinezhad et al., 2011; Park et al.,2012; Shi et al., 2011), (where each equivalent circuitadditionally may consist of a Warburg diffusion elementassociated with mass transport phenomena), by transmission

line (TL)models considering the porous nature of the electrode(Abreu et al., 1996, 1999; Izquierdo et al., 1992; Real et al.,1993), and using other equivalent circuit designs(Ostanina et al., 2004; Vilche et al., 2002).The impedance spectra commonly show two capacitive

loops, perhaps containing overlapped contributions of different

elements over the same frequency range. The high (high/medium)-frequency capacitive loop has been attributed to the

contact impedance (Zm) between zinc particles (Abreu et al.,1996, 1999), the charge transfer associated with the corrosion

reaction of the zinc particles (Feliu et al., 2001; Pereira et al.,1990), the charge transfer of zinc dissolution during the initialstages followed by the contribution of insulating characteristics

of corrosion products after some time (Morcillo et al., 1998;Vilche et al., 2002), or (in general) to the integral paint film

performance (Akbarinezhad et al., 2011; Liu et al., 2011;Park et al., 2012; Selvaraj and Guruviah, 1997; Shi et al., 2011;Xie et al., 2003), or (ambiguously) to the combined effect ofcoating properties and charge transfer kinetics (Armas et al.,1992; Hammouda et al., 2011). The low-frequency loop hasbeen postulated to be related to the zinc dissolution process

(Abreu et al., 1996, 1999; Park et al., 2012; Xie et al., 2003),oxygen diffusion (Armas et al., 1992; Real et al., 1993;Vilche et al., 2002) or diffusion of zinc and iron corrosion

products (Shi et al., 2011).Despite of these huge disagreements in modeling and

interpretation of electrochemical behavior of zinc richcoatings, it is widely accepted that the accumulation of zinc

corrosion products in the pores of the coating at least partly isresponsible for the increase in the resistance or the length of

the chord of the high-frequency impedance loop (Abreu et al.,1992, 1996, 1999; Hammouda et al., 2011; Morcillo et al.,1998; Pereira et al., 1990; Selvaraj and Guruviah, 1997;Shi et al., 2011; Vilche et al., 2002; Xie et al., 2003).The aim of the present study was to formulate

environmentally friendly zinc-rich paints based on nanosilica-

modified potassium silicate as an inorganic binder. (Increasingthe silica/alkali molar ratio of silicate notably improves theperformance and properties of the inorganic zinc-rich coating

(Parashar et al., 2001).) Spherical zinc particles (zinc dust) were

employed as a primary pigment, and MIO as a co-pigment toinvestigate the electrochemical behavior of formulated andapplied paints in saline solution. Electrochemicalmeasurements were carried out during 75 days of continuousimmersion and subsequently after a week of drying/a day ofimmersion cycle.

Experimental

Components and formulations

All the raw materials were of commercial grade and providedfrom the domesticmarket. A 40 percent (w/w) aqueous solutionof potassium silicate with silica/alkali molar ratio of 3.29/1was supplied from Iran silicate industries. In order to increasethe silica/alkali ratio, a 30 percent (w/w) acidic (pH ¼ 4)colloidal solution of nanosilica with particle size of 10-20 nmwas supplied from Sharif Nano Pigment. Specific amounts ofthe two solutions weremixed gently and fully to obtain a binderwith 4/1 SiO2/K2O molar ratio. Zinc dust with an averageparticle size between 4 and 6mm(fine fromPars ZincDust) andMIO with more than 85 percent of particles having dimensionsless than 44mm (2325 mesh from Sormak Mining Co.) wereused. In order to find the content of MIO lamellar particles, atest was conducted according to ASTM D 5532-94 and it wasfound that more than 65 percent of particles were of lamellarshape. Paints were formulated with different contents of zincand MIO particles. Compositions based on the weightpercentage for different formulations are shown in Table I.

Preparation of coated steel panels

Panelswith dimensions of 15 £ 8 cm2were cut from2mm thickSAE 1010 steel sheets blasted to Sa 21

2(SIS 055900-1967) with

profile of 25mm. Prior to application of the coatings, the testpanels were degreased with acetone. Components of the paintwere fully mixed and coatings were applied by means of anadjustable film applicator. Thicknessmeasurementsweremadeusing an Elcometer gauge and dry film thicknesses were foundto be 100 ^ 10mm for all panels. To ensure film curing, thepanels were kept in a laboratory atmosphere for ten days beforebeginning the tests. A-100 formulation was applied induplicate. X-cuts with a scribe width of 0.5mm were made onone of them (hereafter denoted by A-100s) to monitor thehealing or bridging ability of the coating.

Laboratory tests

Atomic force microscopy (AFM) was carried out using anEasyscan 2 (Nanosurf AG, Switzerland) AFM with associatedsoftware to examine the surface topography of coated panels.Prior to electrochemical tests, polyvinyl chloride tubes were

attached to the painted specimens using cyanoacrylateadhesive. The geometrical area exposed to 3.5% NaCl

Table I Characteristics of zinc rich paints

Paint

designation

Zinc

dust MIO

Binder

(solid matter)

Zinc dust/total

pigment(%)

A-70 61.6 26.4 12 70

A-80 70.4 17.6 12 80

A-90 79.2 8.8 12 90

A-100 88 – 12 100

B-90 81 9 10 90

Note: Percentage by weight of different components

Cathodic protection behavior of waterborne zinc-rich silicates

Mohammad Naser Kakaei, Iman Danaee, Davood Zaarei


Volume 60 · Number 1 · 2013 · 37–44

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(from Merck) solution as the electrolyte was 2 cm2. In astandard three-electrode-cell, coated specimens were used asthe working electrode while a platinum rod and a saturatedAg/AgCl were used as counter and the reference electrodes,respectively. The EIS measurements were carried out using anAutolab PGSTAT 302N Potentiostat/Galvanostat and FRA2frequency response analyzer (Eco Chemie B.V.) at opencircuit potential with an AC amplitude of 10mV over afrequency range of 100 kHz-10mHz. The corrosion potentialmeasurements were made using the same cell and devices asfor the impedance measurements. After 75 days of exposure,the electrolyte solutions were emptied from polyvinyl chloridetubes, but non-adhering corrosion products were allowed tosettle and precipitate on the bottom. After a drying period of aweek, tubes were filled with fresh 3.5% NaCl solution and

electrochemical measurements were made 24 h later. Whenthe electrochemical measurements had been completed,cross-sections made perpendicular to exposed andunexposed coated surfaces were observed by opticalmicroscope.

Results

Surface morphology

Figure 1 shows a typical top view micrograph of an A-80sample. Spherical zinc and plate-like MIO particles aredistributed randomly in the structure. Figure 2 shows the topand 3D views of a typical topographic AFM image of thesame specimen, in which pigment particles and pores areclearly visible. This high surface roughness is due to theshortage of resin for full wetting of the pigments.

Corrosion potential evolution

Corrosion potential measurements were made using an Ag/AgCl reference electrode. However, with an acceptabletolerance, it is feasible to obtain potential values versus SCEby subtraction of 45mV from readings obtained with an Ag/AgCl reference electrode, which was done for convenienceand comparability. Figure 3 shows the evolution of corrosionpotential of specimens with immersion time. As expected bythe incorporation of MIO into the coatings formulation,electrical connection between some zinc particles was lost.

Thus, as shown in Figure 3, the more the MIO in the

formulation, the higher was the corrosion potential.Figure 3 shows also that the potential gradually shifted

toward more positive values with time, which was a general

trend for all samples. It is widely accepted that the decrease inzinc-to-steel area ratio, as a result of zinc corrosion and loss of

electrical contact between zinc particles due to accumulationof corrosion products in the pores, is responsible for the

potential increase. B-90, having a high level of zinc and a highporosity (due to lower content of binder), was least affected

during the 75 day exposure, as shown in Figure 3.The A-100 and A-100s samples were identical, except that

the latter was scribed. A-100s exhibited a more negativecorrosion potential than did A-100 during the first hour of

exposure but had a less negative potential during the rest of

test period.Awidely accepted empirical criterion for cathodic protection

is to ensure a minimal negative potential of 2780mV vs SCE(Bardal, 2003). None of the coatings lost cathodic protection

ability during the exposure time, with respect to this criterion.Since no indication of iron corrosion was observed on exposed

areas of specimens at the end of tests, it is believed that thiscriterion is satisfactory.Potential values at (75 þ 1st) days of exposure

(corresponding to the free potential value obtained after

24 hours from renewal of the electrolyte for specimens thathad passed a week of drying before re-immersion) showed

great changes toward more negative values, compared to

those obtained on the 75th day.

Electrochemical impedance spectroscopy

Typical Nyquist plots of EIS spectra for coated specimens

obtained at immersion times of 4, 75 and 75 þ 1 days areshown in Figures 4-6. Despite failing to account for the

porous nature of the coatings, the equivalent circuit modelsshown in Figure 7(a) and (b) provide a fairly good description

of the experimental impedance diagrams. A simple Randlescircuit was used to model the behavior of coated panels at 4th

day of exposure (Figure 7(a)). A compound circuit composedof two Randles circuits connected in series, as shown in

Figure 7(b), was used to interpret the behavior of coated

panels in the at 75th and 75 þ 1st days of exposure time. Toaccount for roughness effects, compositional and structural

heterogeneities, due to a random distribution of pigments(which lead to variations in the time constants), constant

phase elements (Q) were used instead of ideal capacitors. Rs,Rct, Cdl, Rc and Cc represent the solution resistance, the

double layer capacitance, the charge transfer resistance,the coating resistance and the coating capacitance in the

equivalent circuits of Figure 7, respectively.Typical Nyquist impedance spectra for A-70 as a function of

immersion time are shown in Figure 8. In the final stages of

exposure (75th and75 þ 1st days), all the coated panels showedtwodistinct loops, exceptA-100,whichwas thought to have two

overlapped loops (Figures 5 and 6). Tables II and III summarizethe electrochemical parameters values obtained from curve

fitting of EIS spectra. According to the results shown inTable II,Rc generally increased with exposure time (until the 75th day).

Accumulation of corrosion products in the pores is believed tobe responsible at least to some extent for this behavior. Rct is

inversely the measure of reactivity of the coating and the valuedecreased with increase in the level of zinc loading (in

formulations with the same total pigment weight percentage).

Figure 1 Typical top view micrograph of A-80 showing spherical zincand plate-like MIO particles




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This orderwasmore or less preserved during the exposure time.

In general, Rct increased with time, implying that the reactivity

of the coatings decreased. After renewal of the electrolyte

(75 þ 1st day of exposure), Rc, Rct andQdl generally decreased.

Figure 9 shows the exposed surface areas of B-90 as the most

reactive (a) and A-70 as the least reactive (b) specimens at the

end of electrochemical tests, signified by the amount of the

white rust covering the exposed surfaces. Figure 9(c) and (d)

show the exposed surfaces of B-90 and A-100s after removal

Figure 2 Typical topographic AFM images of A-80 showing surface roughness and porous nature of the coating

Figure 3 Evolution of corrosion potentials of different samples withexposure time

Figure 4 Impedance spectra of different samples after four days ofexposure to saline solution

0

550

1,100

1,650

2,200

0 800 1,600 2,400 3,200

Zre (Ω.cm^2)

–Zim

g (Ω

.cm

^2)

A-70A-80A-90A-100A-100 sB-90

Figure 5 Impedance spectra of different samples after 75 days ofexposure to saline solution

0

1,000

2,000

3,000

4,000

5,000

0 4,000 8,000 12,000 16,000Zre (Ω.cm^2)

–Zim

g (Ω

.cm

^2)

A-70A-80A-90A-100A-100 sB-90

Figure 6 Impedance spectra of different samples after 75 þ 1 days ofexposure to saline solution

0

1,000

2,000

3,000

4,000

0 3,000 6,000 9,000 12,000

Zre (Ω.cm^2)

–Zim

g (Ω

.cm

^2)

A-80A-90A-100A-100 sB-90




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of white rust, respectively. While trying to remove the rust

from the surface by means of a blade, the A-100s coating

detached easily from the substrate in three regions around the

scribe marks.

Cross-sectional microscopy

Figure 10 shows optical micrographs of the cross-sections of

exposed and unexposed areas of the A-70 and B-90 specimens.

As is obvious in this figure, the B-90 coating still had plentiful

intact zinc particles after 76 days of exposure to saline solution,

but the A-70 sample showed a greater degree of consumption.

This was in agreement with the evolution of the potential values

of the two specimens, shown in Figure 3, where the potential

value of the B-90 coating showed less change from initial value

by the end of the 75th day. This implies that although B-90 had

had a larger corrosion rate than did A-70 over the exposure

period (confirmedby the lowerRct value and the greater amount

of white rust visible in Figure 9(a)), it still possessed sufficient

unreacted, interconnected zinc particles to provide effective

cathodic protection. In general, both the corrosion rate of the

zinc-rich coating and the duration of cathodic protection

decreased with increase in the amount of MIO particles.

Discussion

The scribe marks that were made on the A-100s specimen,

upon immersion, allowed the electrolyte to penetrate through

the coating and readily wet the exposed steel surface. Due to

better electrolyte penetration, activation of zinc particles

(Abreu et al., 1996) occurred faster for the A-100s specimen,

which was consistent with its lower corrosion potential, as

measured during the first hour of immersion. Zinc particles in

the vicinity of scribe mark corroded faster and the

accumulation of corrosion products healed or bridged the

scratches. Higher values of corrosion potential of the A-100s

(except for the first hour) were the result of a loss of electrical

contact between zinc particles in the areas adjacent to the side

surfaces of scratches, and areas around this, together with

filling of the scratches by corrosion products.The single capacitive loop visible in Figure 4, which probably

containing overlapped contributions of different elements over

the same frequency range, was attributed to the zinc dissolution

process where Rct values of different specimens show a

meaningful order. This attribution was consistent with the

results of Novoa et al. (1989), who observed that during initial

stages of immersion of an inorganic zinc-rich coating where

zinc-to-iron area ratio is more than 10:1, the impedance spectra

are mainly affected by zinc. Abreu et al. (1996, 1999) arguedthat since the zinc surface is several times greater than that

necessary for effective galvanic protection and whereas the

anodic and cathodic reactions represent parallel paths,

Figure 7 Equivalent circuits used to model electrochemical behavior ofcoated specimens in the (a) initial and (b) final stages of exposure tosaline solution

(a)

(b)

Figure 8 Impedance spectra for A-70 as a function of immersion time

0

600

1,200

1,800

2,400

3,000

3,600

0 2,000 4,000 6,000 8,000 10,000Zre (Ω.cm^2)

–Zim

g (Ω

.cm

^2)

1 d2 d4 d8 d16 d25 d39 d55 d75 d

Table III Capacitance values obtained from the fittings of EIS spectra shown in Figures 4-6

4 75 75 1 1

Time (days) Qc (F cm22) Qdl (F cm

22) Qc (F cm22) Qdl (F cm

22) Qc (F cm22) Qdl (F cm

22)

A-70 – 9.2 £ 1024 8.29 £ 1026 3.91 £ 1024 – –

A-80 – 1.02 £ 1023 2.64 £ 1025 7.051024 6.06 £ 1025 2.47 £ 1024

A-90 – 1.03 £ 1023 3.35 £ 1026 9.01 £ 1024 4.01 £ 1026 2.74 £ 1024

A-100 – 1.30 £ 1023 4.75 £ 1025 5.40 £ 1024 4.62 £ 1025 5.4 £ 1024

A-100s – 1.21 £ 1023 7.57 £ 1026 5.25 £ 1024 1.56 £ 1025 2.33 £ 1024

B-90 – 3.49 £ 1023 9.31 £ 1026 1.49 £ 1023 1.87 £ 1026 2.98 £ 1024

Table II Resistance values obtained from the fittings of EIS spectrashown in Figures 4-6

4 75 75 1 1

Time

(days)

Rc(V cm2)

Rct(V cm2)

Rc(V cm2)

Rct(V cm2)

Rc(V cm2)

Rct(V cm2)

A-70 – 1.2 £ 104 5.34 £ 103 6.49 £ 104 – –

A-80 – 5.96 £ 103 9.65 £ 103 6.23 £ 104 7.04 £ 103 2.36 £ 104

A-90 – 1.85 £ 103 9.02 £ 103 5.05 £ 104 6.03 £ 103 2.44 £ 104

A-100 – 7.34 £ 102 6.51 £ 103 5.74 £ 103 5.75 £ 103 6.472 £ 103

A-100s – 1.12 £ 104 1.11 £ 104 1.09 £ 105 4.07 £ 103 1.52 £ 104

B-90 – 5.99 £ 102 6.89 £ 103 2.01 £ 104 6.23 £ 102 1.17 £ 104




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the impedance spectra of the whole system in the low frequency

domain correspond to the impedance of the anodic process

(dissolution of zinc).In the impedance diagrams of Figure 8, as time went on, a

second capacitive loop in the high frequency domain started to

develop, which is believed to be due to some extent to

accumulation of corrosion products in the pores. An obvious

inverse relationship was observed between the diameter of the

low-frequency loop in the impedance spectra and the absolute

value of the potential of coated samples at different immersion

times, which supports the previous claim that attributed the

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

frequency loop developing after awhile, and being progressively

affected by the accumulation of corrosion products, was here

generally ascribed to the coating properties.No single reason was found to fully explain the changes in

electrochemical parameters simultaneous with the decrease in

potential values after renewal of electrolyte (75 þ 1st day) and

further investigations need to be carried out on this issue.

However, as a justification for Rct and potential decrease, it can

be stated that renewal of electrolyte led to reactivation of the

zinc particles (that had been partially passivated theretofore as a

result of local pH increase due to cathodic reaction (Abreu et al.,

1996)), which in turn increased the zinc-to-steel active area

ratio. In any case, it is worth noting that after the dry-wet cycle

(drying period followed by renewal of electrolyte), all of the

coatings demonstrated an activation step, implying that cyclic

immersion can change the overall duration of the cathodicprotection stage.It is believed that higher reactivity of scribed areas on the

A-100s sample (Figure 9) created micro and macro cracks inthe coating structure, as the corrosion products occupy morevolume than pristine zinc. Furthermore, preferentialcorrosion in the vicinity of the scribe marks helped thestructure to be weakened. On the other hand, the B-90sample, being under uniform attack and having higherporosity (which accommodated higher amounts of salts andfacilitated migration of corrosion products and escape ofhydrogen outward the coating) was less deteriorated.

Conclusions

Water-borne inorganic zinc-rich silicate paints with differentcontents of zinc dust an MIO were applied on steel panels andcharacterized using electrochemical tests in saline solution. Thechanges in impedance parameters with exposure time, and withvariation of MIO content of coatings, were investigated. Noneof the coatings lost the cathodic protection ability during75daysof exposure.However, replacement of zinc by a non-conductiveco-pigment was revealed to reduce both the reactivity and theduration of cathodic protection of inorganic zinc-rich coatings.Since the effective barrier action of this type of coatings is notachieved in continuous immersion conditions, the influenceof incorporation ofMIO to inorganic zinc-rich paints on barrierproperties should be investigated using other techniques.

Figure 9 Exposed areas of (a) B-90 and (b) A-70 before removal of white rust and (c) B-90 and (d) A-100s after removal of white rust by means ofa blade

(a) (b)

(c) (d)




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After passing a dry-wet cycle, all the coatings demonstrated anactivation step, implying that the cyclic immersion can change

the overall duration of cathodic protection.

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Figure 10 Cross-sectional micrographs showing (a) unexposed area of B-90 (b) exposed area of B-90 (c) unexposed area of A-70 and (d) exposed areaof A-70




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Corresponding author

Iman Danaee can be contacted at: [email protected]




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