Cathodic delamination Quantification of ionic transport rates along coating–steel interfaces

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OC-2310; No. of Pages 9

Progress in Organic Coatings xxx (2009) xxx–xxx

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Progress in Organic Coatings

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athodic delamination: Quantification of ionic transport rates alongoating–steel interfaces

.A. Sørensena, K. Dam-Johansena, C.E. Weinellb, S. Kiil a,∗

Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs., Lyngby, DenmarkHempel A/S, Lundtoftevej 150 DK-2800 Kgs., Lyngby, Denmark

r t i c l e i n f o

rticle history:eceived 2 June 2009eceived in revised form 4 August 2009ccepted 20 August 2009

eywords:orrosionisbondingisbondment

a b s t r a c t

So-called cathodic delamination is one of the major modes of failure for organic coatings immersed inelectrolyte solutions (e.g. seawater). Cathodic delamination occurs as a result of the electrochemical reac-tions, which takes place on a corroding steel surface. This means that reactants must continuously betransported from the bulk solution to the cathodic areas. The transport of sodium ions from a defect in thecoating to the cathodic areas is generally considered the rate-determining step for cathodic delaminationbecause the transport of oxygen and water through the coating is sufficient for the corrosion processes. Inthis work, a novel practical method, which allows direct estimation of the apparent diffusion coefficientof sodium ions in the ultrathin aqueous layer at the coating–steel interface, is described. The apparent

ccelerated testingrotective coatings

diffusion coefficients estimated are of similar magnitude as previously reported values and show anacceptable repeatability. The method was used to obtain the apparent diffusion coefficients of sodiumions in the coating–steel interface for three commercial inert-pigmented epoxy coatings. The delami-nation rates predicted using the apparent interfacial diffusion coefficients and Fick’s second law, underthe assumption of a transport controlled mechanism, show qualitative agreement with the observeddelamination rates in 0.5 M sodium chloride. This confirms that the rate-determining step of cathodic

port o
delamination is the trans
. Introduction

Organic barrier coatings are extensively used to protectmmersed steel structures, such as ballast tanks, ship hulls, windurbines and bridges, from corrosion [1]. When a part of the coat-ng system is damaged or mechanically removed, electrochemicaleactions will occur at the defect site because the bare steel is inirect contact with the surrounding environment. At the defect, aapid dissolution of the solid steel is possible, whereas the dissolu-ion of solid steel is strongly inhibited below the adherent coating.eawater and other reactants within the defect will slowly diffusento the intact coating–steel interface, where the ions will serves charge carriers and thereby enable oxygen to be reduced belowhe coating on the oxidized steel surface [2]. The reduction of oxy-en on steel surfaces is based on a complex mechanism consistingf many steps in which electrons are transferred from the oxide

Please cite this article in press as: P.A. Sørensen, et al., Cathodic delaminationProg. Org. Coat. (2009), doi:10.1016/j.porgcoat.2009.10.011

urface to the oxygen species. Several reactive intermediate prod-cts, such as superoxide and hydroxy radicals may be formed in thisrocess [3–5]. However, the main product is hydroxyl ions. In ordero preserve the local charge neutrality, cations must therefore be

∗ Corresponding author. Tel.: +45 45252827; fax: +45 45882258.E-mail address: [email protected] (S. Kiil).

300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2009.10.011

f sodium ions along the coating–steel interface.© 2009 Elsevier B.V. All rights reserved.

transported from the defect to the cathodic areas to neutralize thecharge of the hydroxyl ions generated by the cathodic reaction [6].The presence of positively charged alkali metal ions, such as sodiumand potassium ions, and negatively charged hydroxyl ions results ina highly alkaline environment within the thin aqueous layer at thecoating–steel interface and pH values up to 14 have been reported[7–9]. The reactive intermediate products and the alkaline environ-ment will adversely affect the adhesion between the anticorrosivecoating and the steel, resulting in partly or complete delaminationof the coating as shown in Fig. 1.

The mechanism of cathodic delamination of organic coatingson steel surfaces has been studied extensively for several decades[6,10–16]. Experimental and theoretic studies have shown thatthe amount of water and oxygen consumed by the cathodic reac-tion underneath a delaminated coating is less than the amountof water and oxygen that diffuses through a typical anticorro-sive coating [2,17,18]. This means that the transport of oxygenand water cannot control the rate of cathodic delamination. Therate-determining step of cathodic delamination of inert-pigmented

: Quantification of ionic transport rates along coating–steel interfaces,

epoxy coatings appears to be the transport of positively chargedhydrated ions to the cathodic areas. Several possible routes for thecations have been suggested [6,14,15]. However, cathodic delami-nation of well-formulated epoxy coatings appear to be controlledby the interfacial transport of hydrated cations in a thin aqueous

dx.doi.org/10.1016/j.porgcoat.2009.10.011

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ARTICLE IN PRESSG Model

POC-2310; No. of Pages 9

2 P.A. Sørensen et al. / Progress in Organic Coatings xxx (2009) xxx–xxx

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ig. 1. Idealized sketch of cathodic delamination. Solid iron is transformed into fnderneath the delaminated coating. Sodium ions must migrate along the coatinglkaline environment and reactive intermediate products weakens the adhesion be

ayer at the coating–steel interface from the defect to the delami-ation front in most naturally encountered environments [18–21].

Several studies have considered the effects of the type andoncentration of the ions in the electrolyte solution on the delam-nation rate. The type of anion appears to have little influence onhe rate of delamination [10,11,13] whereas the delamination rates nearly proportional with the mobility of the alkali metal cations.urthermore, the delamination rate is proportional with the con-entration of the cations [11,13]. The delamination of coated metaln solutions of divalent cations has been reported to be very slowr not occurring at all, which has been attributed to the low sol-bility of the various hydroxides [10,22,23]. The typical apparenteaction order of 0.5 [24] for cathodic delamination (correspondingo a process obeying the laws of Fick) supports the hypothesis of arocess controlled by the interfacial transport of hydrated cations.

Despite the fact that the transport of sodium ions along theoating–steel interface appears to be the rate-determining step forathodic delamination, attempts to quantify the interfacial rates ofransport are scarce [25–28]. The interfacial diffusion coefficientsf various cations have recently been estimated by application ofhe Scanning Kelvin Probe technique [27]. Other authors [25] applyhe product of the diffusion coefficient and the coating–steel inter-ace height as a measure for the interfacial transport rate. The


pplication of such a setup is limited by the fact that the heightf the coating–steel interface is not readily estimated or in anyay well defined. Consequently, a reliable estimate of the inter-

acial diffusion coefficient is not readily achieved. Other reportedalues for diffusion coefficients of the hydrated cations along the

ig. 2. Idealized time-lag response for a concentration perturbation experiment.he dotted line illustrates the time-lag, � (time required to obtain a constant rate ofiffusion) resulting from diffusion through a semi-permeable membrane.

s iron at the defect and oxygen is reduced to hydroxyl ions at the cathodic sitesl interface to neutralize the charge of the hydroxyl ions. The presence of a highlythe coating and the substrate, resulting in delamination of the coating.

coating–steel interface have been estimated using mathematicalmodels of cathodic delamination by fitting the predicted delamina-tion rate to the observed delamination rate by varying the diffusioncoefficient [25,29,30]. However, there may be significant uncer-tainties in the diffusion coefficients estimated from mathematicalmodels of cathodic delamination because the detailed mechanismfor cathodic delamination is not completely understood [31]. Asa consequence of this, new methods capable of quantifying thetransport of cations along the coating–steel interface may provideimportant knowledge about the detailed mechanism of cathodicdelamination and assist in the development of novel anticorrosivecoatings. Quantification of the interfacial transport may also be auseful input to the present mathematical models of cathodic delam-ination [32,33], which are restricted by the lack of information onionic interfacial diffusion. Hence, a simple method capable of esti-mating the interfacial diffusion coefficients of the cations along thecoating–steel interface is needed.

This article addresses the development of a novel practicalmethod, which can be applied for estimation of the diffusion coef-ficients of hydrated sodium ions along a coating–steel interface.

2. Theory

The so-called time-lag method has proven to be an effectivemethod for obtaining the diffusion and permeability coefficients ofideal systems [34]. Since its origins in 1920, interest in the time-lagmethod has expanded because of its value in characterising sim-ple permeation processes and also complex systems of diffusionwith simultaneous adsorption and surface diffusion. The method


has historically been applied to diffusion and adsorption in porousmembranes and diffusion in polymer membranes and has recentlybeen applied for estimation of the ionic permeability of intactcoatings [35]. The simple nature of the time-lag method enablestransport parameters to be directly obtained from experimental

Fig. 3. Principle sketch of coating–steel interface prepared by insertion of nails intothe liquid coating.


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P.A. Sørensen et al. / Progress in

ata when the diffusion coefficient of the species considered doesot change with time. Hence, avoiding the intensive mathematicalreatment required by other techniques.

The basis of time-lag methods is a concentration step pertur-ation, which is performed in one chamber of a two-chamberermeation cell, which is divided by a semi-permeable membrane.he time dependent diffusion response may be followed by con-inuous monitoring of the concentration in the non-pertubatedhamber. This procedure is known as the time-lag method, becausehere is a time-lag between the perturbation and the onsetf the change in the concentration on the other side of theemi-permeable membrane. During a time-lag experiment, theoncentration in the pertubated chamber remains almost constantnd increases slightly in the non-pertubated chamber. This meanshe concentration gradient is approximately constant. After someime, the mass transfer between the two chambers stabilizes andteady state conditions are achieved. A typical time-lag responseor a semi-permeable membrane is shown in Fig. 2.

The permeability coefficient can be obtained from the steadytate part of the experiment, whereas the time-lag is related tohe diffusion of the species considered across the semi-permeable

embrane.Reliable quantification of the interfacial transport of sodium

ons by application of the time-lag method, requires that a givenodel interface separating the two solutions bears a high degree

f resemblance to a real interface between an organic coating andts substrate. Furthermore, the experimental time needed for esti-

ation of the interfacial diffusion coefficient should be minimized.ere, it is proposed that the coating–steel model interface can bestablished by insertion of nails consisting of thoroughly cleanedold rolled low-alloy carbon steel into the liquid coating and thathe experimental time can be controlled by adjusting the thick-ess of the wet coating. The principle of preparing a coating–steel

nterface by insertion of nails into a liquid coating is illustrated inig. 3.

. Mathematical model

The mathematical model is analyzed under the simplifyingssumption that only diffusion determines the transport of cationslong the coating–steel interface. This is a simplifying assumptionecause anions are excluded from the all transport processes duringathodic delamination, suggesting that the electrical field cratedy the corrosion process controls the ionic transport along theoating–steel interface. The effect from the corrosion process andiffusion may be lumped into an apparent diffusion coefficient. Thiseans that the formal diffusion coefficient derived in mathemati-

al analysis may be considered as an apparent diffusion coefficient,hich has been modified by the ionic mobility and an electrical field

radient [11]. Thus, Fick’s second law can be used to describe theransient one-dimensional diffusion of ions along in the ultrathinqueous layer at the coating–steel interface.

∂ci

∂t= − ∂

∂�

(−Di

∂ci

∂�

)(1)

here ci is the concentration of component i, t the diffusion time,the distance from the reference point and Di the diffusion coeffi-

ient of the species considered.The boundary conditions may be identified by considering the

implified schematic view of a coating specimen separating the


wo solutions is shown in Fig. 4. Furthermore, the concentrationradient across the coating sample can be assumed to be constantecause the concentrations in the pertubated and non-pertubatedhambers only changes slightly with time. This means that theoundary conditions are given by concentrations of sodium ions

Fig. 4. Cross-section view of simplified coating sample without nails separating thesodium chloride solution and the distilled water. � = 0 is the boundary between theconcentration perturbed chamber and the coating sample and � = �0 is the boundarybetween the coating sample and the ion free solution.

in these chambers. The concentration of sodium ions in the per-tubated chamber is ci,u, while no sodium ions are present in thenon-pertubated chamber. The initial condition is given by assum-ing that the coating sample is free of penetrant at the start of theexperiment.

c(�, 0) = 0 (2)

c(0, t) = ci,u (3)

c(�, t) = 0 (4)

The rate at which the ions emerges from the coating sampleto the non-pertubated chamber is given by D dc

d�

∣∣�=�0

. The total

amount of ions transported to the non-pertubated chamber can beobtained by integration of equation describing the ions emergesfrom the coating sample to the non-pertubated chamber withrespect to the time, t. The full analytical solution to Fick’s secondlaw in this specific case, is given by [36]

Qi,t

� · ci,u= Di · t

�2− 1

6− 2

�2

∞∑n=1

(−1)2

n2exp

(−Di · n2�2t

�2

)(5)

where Qi,t is the amount of ions transported across the sample, �the thickness of the membrane, ci,u the initial concentration on thefeed side and n is an integer.

Low order approximations of the analytical solution to Fick’slaw will converge rapidly towards the full analytical solution forthe conditions relevant for this study (e.g. n = 3).

The exponential term can be neglected when t → ∞. This meansthat the analytical solution to Fick’s second law may be approxi-mated by the straight line given in Eq. (6).

Qi,t = Di · ci,u

�

(t − �2

6Di

)(6)

By isolating t in Eq. (3), it can be seen that extrapolation of thelinear part of a plot of Qi,t/�·ci,u versus time to intercept with theabscissa yields a time-lag, which is related to the diffusion coeffi-cient by Eq. (7).

� = �2

6Di(7)

4. Experimental

The design of the two-chamber diffusion cell, illustrated in Fig. 5,used for estimation of the diffusion coefficient of hydrated sodiumions in the coating–steel interface is similar to the design previously


used by Carneiro et al. [35] for measuring the permeability of intactcoatings towards chloride ions.

The upper chamber of the diffusion cell is designed to containan aqueous solution of sodium chloride while the lower cham-ber contains distilled water. The upper chamber has a volume of


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4 P.A. Sørensen et al. / Progress in Organ

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6. Results and discussion

The penetration of electrolytes through epoxy coatings is avery slow process compared to the diffusion of electrolytes alongthe coating–steel interface. The reported diffusion coefficients for

Table 1Coating composition (wt%) of liquid paint.

Compound Coatings

I II III

Amide cured epoxy resin 30.7 30.9 26.3

Fig. 5. Cross-section scheme of diffusion cell and coating sample with nails.

10 cm3. This is nearly three times greater than the volume of theower chamber, which has a volume of 75 cm3. Thus, it is reason-ble to assume that the ionic concentration in the upper chamberemains constant throughout the duration of the experiment. Thewo chambers are separated by a coating sample supported by7.0 ± 3.7 �m thick Kraft paper with the coating surface down-ards. Kraft paper is manufactured from wood pulp by a modified

ulphate pulping process. Kraft paper is cheep, well-known for itsigh strength and does not provide any significant resistance toass transfer (see Section 6) and has previously been used to sup-

ort coatings in permeation measurements [35]. In order to preventevelopment of osmotic pressure gradients, both the upper and

ower solutions are in contact with the atmosphere through smalloles in the upper part of the permeation cell.

During the experiment, both sodium and chloride ions will dif-use from the upper chamber to the lower chamber while waterill diffuse from the lower chamber to the upper chamber. There-

ore, a small reservoir with distilled water is connected to the lowerhamber to maintain a constant volume in the lower chamber. Thepper chamber must have a free volume, which can accommodatehe amount of water transported across the coating sample to allowransport of water from the lower to the upper chamber. A mag-etic bar is placed in the lower chamber to homogenize the lowerolution. The concentration of sodium ions in the lower chamber isontinuously monitored by a sodium-selective polymer membranelectrode from Metrohm with a measuring range of 5 × 10−6 M toM. A double-junction Ag/AgCl reference electrode from Metrohmas inserted in the cell and placed at a fixed position.

. Experimental procedure and preparation of coatingamples

The coating samples were produced by application of a liquidpoxy coating on a sheet of Kraft paper by an automatic coatingpplicator – Coatmaster 509 MC-III. Subsequently, the Kraft paperith the liquid coating was transferred to an easily penetrable sup-ort consisting of polystyrene. Cold rolled low-alloy carbon steelails with a diameter of 1.2 mm were manually pushed into theoating and the Kraft paper and to some extent the polystyrene.rior to insertion into the liquid coating, the nails were thoroughlyleaned with a water based alkaline detergent – Hempel Lightlean, rinsed with ultra pure water, abrasive grinded with 3 M abra-ive paper P360, rinsed with ultra pure water and immersed in


thanol to reduce the amount of residual water on the nails. Thisrocedure removes contaminants and prevents significant corro-ion of the cleaned nails, which must be stored carefully prior tosage.

PRESSic Coatings xxx (2009) xxx–xxx

The roughness of the surface of the nails was measured by aHandysurf E-35A from Zeiss. The measured arithmetic mean ofmaximum peak-to-valley height of five adjoining single samplinglengths, Rz, was 0.52 ± 0.20 �m. This indicates that the roughnessof the nails will not influence the magnitude of the estimated diffu-sion coefficients significantly [19]. Hence, the diffusion coefficientsestimated will not have to be corrected by a tortuosity factor.

The coating samples were allowed to cure for 7 days before theywere gently removed from the polystyrene support. Using a pairof sharp scissors, circular test specimens with a diameter of 9 cmwere cut from the samples. The dry film thickness of the coatingspecimens were measured using a magnetic gauge instrument –Elcometer 355 Top, which was calibrated prior to initialization ofmeasurements. The coating specimens were allowed to cure foranother 3 weeks at ambient temperature before they were used inthe permeation cell.

The coating sample supported by the Kraft paper was placedin the diffusion cell, which must be tightly closed. A magnetic barwas placed in the lower chamber before distilled water was fed tothe lower chamber using a syringe. The amount of water used forfilling the lower chamber was estimated by weighing the syringeprior and after injection of water. The sodium-selective polymermembrane electrode was then inserted and the permeation cellwas placed horizontally so that the upper chamber could be filledwith a known volume of 2.5 M sodium chloride solution.

The reference electrode and the sodium-selective electrode wasconnected to a 781 pH/Ion Meter from Metrohm with a measuringaccuracy of ±0.4%. The unit was controlled by a computer, whichwas connected to the pH/ion Meter by a RS 232 interface. Verifi-cation support for validation software from Metrohm was used tocontrol the entire unit and for data acquisition. The entire setup wasplaced in a thermostatically controlled room with a temperaturebetween 20 and 23 ◦C.

The time-lag method was used for estimation of the interfacialdiffusion coefficient of sodium ions for three commercial coatings.The applied coatings were all inert-pigmented, based on diglycidylether of bisphenol A and cured by an amide-adduct. Informationabout composition of coatings is given in Table 1.

Microscopic cross-cut investigation of the coating samplesshowed that the penetration of the applied coatings into theKraft paper was negligible. Estimations of the interfacial diffu-sion coefficients were performed three times with different coatingspecimens for evaluation of the precision. The potential differencebetween the reference electrode and the sodium-selective poly-mer electrode in the lower chamber was continuously recordedand converted into concentration data by a calibration curve. Anexample of the concentration history of sodium ions in the lowerchamber of the permeation cell is shown in Fig. 6.


Plasticizer 7.9 8.0 6.9Pigments 7.9 6.9 5.1Extenders 33.5 33.8 37.7Solvents 18.6 18.9 21.3Additives 1.5 1.6 2.7


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8. Estimation of interfacial length

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ig. 6. Example of sodium ion concentration history in lower chamber for commer-ial magnesium silicate and titanium dioxide pigmented epoxy-amide coating.

odium ions in the coating–steel interface is several orders of mag-itude greater than the diffusion coefficient of sodium ions in bulkpoxy coatings (see Table 2). This suggests that ionic transporthrough the bulk epoxy coating will not interfere with the measure-

ents of the interfacial diffusion coefficients and therefore may beeglected.

It is also clear, that there is a significant scatter in the val-es of the reported diffusion coefficients for sodium ions at theoating–steel interfaces. This may be partly explained by the facthat the applied coating systems have different properties andhat some of diffusion coefficients reported has been indirectlystimated from delamination rates. Furthermore, neither of the ref-rences addresses important parameters such as the temperaturend the tortuosity of the coating–steel interface, which are knowno influence to magnitude of the observed diffusion coefficients.

. Verification of fundamental principles

There are several issues, which must be addressed before, it haseen verified that a change in the concentration of sodium ions

n the lower chamber is due to transport along the coating–steelnterface.

Most commercial coatings contain extenders or pigments withmpurities of sodium ions, which may leach during testing. Theeaching of sodium ions from the different types of coating speci-

ens supported by Kraft paper were investigated by immersion ofhe coating specimens in ultra pure water in sealed containers for 3

onths. Analysis of the concentration of sodium ions by a sodium-elective electrode at the start and termination of the experiment


howed that the concentration of sodium ions in the ultra pureater remained very low throughout the measuring period and

ould be neglected. Hence, the leaching of sodium ions from theoating specimens can be neglected.

able 2epresentative interfacial and bulk epoxy diffusion coefficients of Na+ and Cl− for various

Reference Coating system

Glass and Smith [37] Epoxy-polyamideGlass and Smith [37] Pigmented epoxy-polyamideGlass and Smith [38] Pigmented epoxyHu et al. [39,40] Pigmented epoxy-polyamideWapner et al. [27] Heat-curing two-component epoxyHernandez et al. [30] Amine-modified epoxyLeng et al. [11] Polyamidoamines-epoxy-ester

a The values have been estimated by fitting a simulated delamination rate to a measure

PRESSic Coatings xxx (2009) xxx–xxx 5

Diffusion measurements on coating specimens without insertednails with dry film thickness identical with the coating specimenscontaining nails showed that no sodium ions penetrated the bulkcoating during the duration of the experiments. This means that thepenetration of sodium ions through the bulk epoxy coatings in theregions without nails can be neglected and that the sodium ions inthe non-pertubated chamber the must have been transported alongthe coating–steel interface. Measurements performed with nailsof various lengths showed that the apparent diffusion coefficientsestimated were not affected by the area of the nails, which werein contact with the 2.5 M solution of sodium chloride. This showsthat the transport of ions along the coating–steel interface was notaltered by a lager corroding area. Thus, the small differences in thecorroding areas amongst the samples will not interfere with theestimation of the apparent diffusion coefficients.

Verification of the negligible resistance towards mass transferprovided by the Kraft paper was found by measuring the diffusioncoefficient of sodium ions through intact Kraft paper by the time-lag method. The estimated value of 2.1 × 10−11 ± 1.3 × 10−11 m2 s−1

is significantly greater than the reported values for the diffusioncoefficient of sodium ions along the coating–steel interface. Fur-thermore, the value is similar magnitude as the diffusion coefficientof sodium ions at both infinite dilution (1.334 × 10−11 m2 s−1 [41])and concentrations up to 2.25 M (1.11 × 10−11 m2 s−1 [42]). Thisconfirms that the resistance provided by the Kraft paper can beneglected.

The rate of which the sodium ions emerge from the coating spec-imens must be directly proportional to the area (e.g. perimeter ofthe nails) available for diffusion according to Fick’s first law. Scal-ing of the interfacial transport rates by changing the area availablefor diffusion may therefore be used to see whether the diffusion ofsodium ions from the upper chamber to the lower chamber obeythe laws of Fick. This is because the interactions leading to adhesionbetween the coatings and the nails should not depend on the totalperimeter provided that the nails are placed sufficiently far apart.Here, it is implied that the concentration gradient across the coatingsample is constant (e.g. thickness of the coating sample is constant).The total perimeter of the nails in the coating samples was variedby changing the number of nails in the coating specimens. Fig. 7shows that the state rates of transport of sodium ions from theupper chamber to the lower chamber are directly proportional tothe total perimeter of the nails inserted in the coating. This meansthat the laws of Fick are obeyed and that the total perimeter of thenails may be scaled without affecting the apparent rate at whichthe hydrated sodium ions are transported along the coating–steelinterface.


In addition to the total perimeter of the nails inserted into thecoating, the rate of transport across the coating specimen is propor-tional to the concentration gradient across the coating specimen

pigmented and non-pigmented epoxy coatings.

Bulk polymer diffusion coefficients [m2 s−1] Interfacial diffusioncoefficients [m2 s−1]

Na+ Cl− Na+

3.2 × 10−15 4.7 × 10−16 –9.2 × 10−17 6.3 × 10−17 –8.7 × 10−17 5.8 × 10−17 –– 4.6 × 10−16 –– – 1.5 × 10−11

– – 2.25 × 10−14a

– – 6.4 × 10−11a

d delamination rate using mathematical models.




6 P.A. Sørensen et al. / Progress in Organic Coatings xxx (2009) xxx–xxx

Fig. 7. Rate of transport of sodium ions across the coating sample versus the totalpiui

(abTlndacb

notrttsti

mt

Fcb

erimeter length of the nails inserted into the coating. The thickness of the coatings identical for all coating samples and the vertical lines show the experimentalncertainty on the measured rate of transport of sodium ions along the coating–steel

nterface.

e.g. the thickness of the coating in proximity of the nails). Liquidnticorrosive coatings have low surface tensions and viscositiesecause they are designed for maximal wetting of the substrate.his means that the coating will creep up along edges and that theength of the coating–steel interface is greater than the actual thick-ess of the dry coating as shown in Fig. 8. Measurements on severalifferent coatings showed the distance a coating will creep up alongnail and thereby exact length of the coating–steel interface is a

haracteristic for the specific type of coating and therefore has toe evaluated for each coating sample.

Optically microscopic investigations showed that the thick-ess of the coating which has crept up along the nails may bef similar order of magnitude as the interfacial distance alonghe coating–steel interface. These observations confirm that it iseasonable to assume that the diffusion of sodium ions throughhe bulk coating to the coating–steel interface does not affecthe measurements. This is because the time required for diffu-ion of sodium ions through the bulk coating is significantly largerhan the time of diffusion of sodium ions along the coating–steel


nterface.The length of the coating–steel interface was estimated by

icroscopically investigations of the uncorred region of the nailshat had been placed in the coating sample because the uncorred

ig. 8. Microscopic analysis of nail with attached coating. The length of theoating–steel interface is significantly greater than the thickness of the coatingecause the liquid coating creeps up along edges.

Fig. 9. Microscopic analysis of nails after exposure. The vertical lines show thelength of the coating–steel interface.

distance was identical to the distance of the nails covered with coat-ing prior to exposure. As illustrated in Fig. 9, the border betweenthe corroded and un-corroded steel is distinct.

9. Apparent diffusivity of sodium ions and cathodicdelamination

The apparent diffusion coefficients of sodium ions at thecoating–steel interface estimated, which are presented in Table 3demonstrate that the method has an acceptable repeatability.Furthermore, apparent interfacial diffusion coefficients of sodiumions are of similar magnitude as the values tabulated in Table 2.The apparent interfacial diffusion coefficients estimated are about3 orders of magnitude smaller than the diffusion coefficient ofsodium ions in a bulk electrolyte at infinite dilution [41]. On theother hand, the apparent interfacial diffusion coefficients are muchlager than the diffusion coefficients of ions in bulk epoxy coat-ings. This shows that the estimated apparent diffusion coefficientsare within a reliable range for the diffusion of hydrated ions in acase where the motion of ions is limited to the free volumes atthe coating–steel interface. Thus, the coating–steel interface is aregion where hydrated cations can be transported fast comparedto transport in the bulk coating.

In the absence of an oxygen rich atmosphere or a galvanic cell,both sodium and chloride ions diffuse into the coating–steel inter-face. However, once a galvanic cell is established only positivelycharged ions are transported along the coating–steel interface,where they will serve as charge carriers [11,43]. This is becausehydrated cations are attracted by the excess of negative chargein the coating–steel interface. Increased local interfacial concen-trations of hydroxyl ions resulting from the reduction of oxygenwill thus accelerate the cations transport due to the presence ofelectrostatic field. The selective ion transport on corroding steel


surfaces can be assigned to the electrostatic field resulting from thecorrosion process (e.g. potential gradient between the anode andcathode). Experimental results suggest that, the diffusion of cationsappears may be insignificant in comparison with contribution from

Table 3Apparent interfacial diffusion coefficients of sodium ions at the coating–steel inter-face for three commercial epoxy coatings.

Coating Interfacial diffusion coefficient [m2 s−1]

I 1.4 × 10−13 ± 0.1 × 10−13

II 2.2 × 10−13 ± 0.2 × 10−13

III 3.0 × 10−13 ± 0.2 × 10−13




P.A. Sørensen et al. / Progress in Organic Coatings xxx (2009) xxx–xxx 7

Fig. 10. R relationship between the extent cathodic delamination rate ofinert-pigmented coatings and apparent interfacial diffusion coefficient of thecisc

m[

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of sodium ions was investigated by addition of 2 wt% N-[3-

oating–steel interface towards sodium ions. The vertical lines illustrate the exper-mental uncertainty related to the delamination rate while the horizontal lineshow the experimental uncertainty on the estimation of the interfacial diffusionoefficient.

igration (transport of ions under the influence of a potential field)44].

The transport of sodium ions is principally described by theernst–Planck equation, which takes the effects of both diffusionnd migration into account. However, the Nernst-Planck equationnd diffusion coefficients estimated by the time-lag method can-ot be used in combination for a quantitatively description of theransport of sodium ions along coating–steel interfaces. This isecause effects from migration and diffusion have not been mea-ured individually but have been lumped into an apparent diffusionoefficient. Regardless of the transport mechanism, the apparentnterfacial diffusion coefficients estimated may be used as a mea-ure for the delamination rate when the rate-determining step inhe process of cathodic delamination is the transport of cationslong the coating–steel interface.

i = −Di,e ·(

dci

d�+ R · T

zi · F· ci · d�

d�

)(8)

here Ji is the flux, Di,e the effective interfacial diffusion coefficientt infinite dilution, R the universal gas constant, T the absolute tem-erature, F Faradays constant, zi the charge of the considered speciend � the electrical potential created by the ionic species.

The true interfacial diffusion coefficient of sodium ions along theoating–steel interface can be estimated when the steel is not polar-zed (e.g. no corrosion occurs). This type of measurements could beerformed with the described experimental setup by purging thepper and lower solutions with an inert gas (e.g. nitrogen) and plac-

ng the entire diffusion cell in an inert atmosphere. The subsequent


reatment of the obtained data is already described in Section 2.The importance of interfacial transport of sodium ions for

athodic delamination was confirmed by comparison of thepparent interfacial diffusion coefficients with measured of the

Fig. 11. Hydrolysis of organic functional silane and subsequ

Fig. 12. Chemical structure of N-[3-(trimethoxysilyl)propyl]ethylenediamine.

delamination rates of the applied coatings. The extent of cathodicdelamination of the applied coatings on cold rolled low-alloy car-bon steel was removing the delaminated coating after exposure toa 0.5 M aerated solution of sodium chloride. Fig. 9 shows a directproportionality between the estimated apparent interfacial diffu-sion coefficients of sodium ions in the coating–steel interface andthe observed delamination rate. This demonstrates that the delam-ination rates predicted from Fick’s second law using the interfacialdiffusion coefficients, under the assumption of a transport con-trolled mechanism, show qualitative agreement with the observeddelamination rates in 0.5 M sodium chloride (e.g. similar condi-tions as seawater). This confirms that the cathodic delaminationof inert-pigmented coatings, under the conditions studied, is con-trolled by the interfacial transport of sodium ions from the defectto the delamination front (Fig. 10).

10. Influence of adhesion promoters on diffusivity andcathodic delamination

The adhesion of organic coatings to steel is weak due to thenature of the steel surface, which mainly consisted of oxidized steel.Epoxy coatings interact with the oxidized steel surface throughacid–base interactions, typically hydrogen bonds. Superior adhe-sion can be achieved by the formation of covalent bonds betweenthe coating and the steel surface.

One way of improving the adhesion between the organic coat-ings and the steel is to stabilize their connection by additionof coupling agents (e.g. adhesion promoters) [45,46]. Adhesionpromoters are often reported to improve adhesion and reduceinterfacial de-adhesion failures between the coating and the steelduring exposure to corrosive environments [47–50]. Silanes reactwith water in aqueous solutions to form hydrolyzed silanes, whichwill react with the surface of the substrate. The individual bondswill subsequently polymerize and build up layers outward fromthe substrate with the organic functionality oriented towards thecoating [51]. This process is shown in Fig. 11.

Amongst the group of adhesion promoters amino and epoxyfunctional silanes have found the most widespread usage forepoxy coatings because of their high degree of compatibil-ity with the binder. The effect of an amino functional silaneon the rate of cathodic delamination and interfacial transport


(trimethoxysilyl)propyl]ethylenediamine, see Fig. 12, to coating III.The addition of the amino functional silane to the coating

reduced the rate of delamination significantly. However, the addi-tion of amino functional silane had insignificant influence on the

ent polymerization of the silane on a metal substrate.


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pparent interfacial diffusion coefficient estimated. Therefore, thenterfacial transport of hydrated sodium ions does not seem to behe rate-determining step of the cathodic delamination in the silane

odified coating system.The improved durability of the silane coating compared to the

nmodified coating system is probably a result of the improvedtability of the coating–steel interface due to the dense silane mod-fied coating–steel interface. It has been suggested that the stableoating–steel interface will lead to a more diffuse double layertructure when hydrated sodium ions are present at the interface27,52]. The diffuse double layer will reduce the rate of all electro-hemical reactions at the coating–steel interface. In combinationith the blocking of active adsorption sites for oxygen on the oxi-ized steel surface by adsorbed amino-silanes, which bind to thexide surface via silanol groups, this leads to the significant slowown of cathodic delamination.

1. Prospective and limitations

From a practical point of view, the method may be usedo predict the extent of cathodic delamination of organic coat-ngs on metal surfaces. Although the experimental procedure

as established using cold rolled low-alloy carbon steel, the coldolled low-alloy carbon steel may be substituted by other metalshich experience cathodic delamination. The method can easily be

pplied for estimation of the diffusion coefficients of various otherations, such as calcium and potassium ions, at the coating–steelnterface by replacement of the sodium-selective electrode by auitable ion selective electrode. However, the experimental proce-ure is limited to predict the delamination rate for inert-pigmentedoatings. This is because the delamination process of non-inert-igmented coatings may involve chemical reactions, which theathematical model for diffusion along the coating–steel interface

oes not consider. An example of such a coating could be aluminumigments coatings.

In comparison with the Scanning Kelvin Probe technique, whicheasures the surface work function difference between conduct-

ng, coated or semi-conducting materials and a metallic probe,his new experimental design requires larger measurement times.urthermore, the work function can be directly correlated to theurface condition which means that the Scanning Kelvin Probean provide information regarding interfacial phenomena such aselamination. However, the perturbation method does not rely onostly equipment, the experimental procedure for preparation ofoating samples is simple and the method require little analysis ofhe collected data. Therefore, the perturbation method is a sturdylternative to the Scanning Kelvin Probe when the interfacial dif-usion coefficient is to be estimated.

2. Conclusions

A novel practical method, which allows direct estimation ofhe apparent diffusion coefficients of sodium ions along theoating–steel interface, was successfully designed. The setup waspplied in the characterization of the coating–steel interface ofhree commercial epoxy coatings and the estimated diffusion coef-cients of sodium ions along the coating–steel interface shown acceptable repeatability. The apparent diffusion coefficientsstimated are of similar magnitude as previous estimated valuesor the apparent diffusion coefficients of sodium ions along the


oating–steel interface. The delamination rates predicted using thepparent interfacial diffusion coefficients and Fick’s second law,nder the assumption of a transport controlled mechanism, showualitative agreement with the observed delamination rates in.5 M sodium chloride. This confirms that the cathodic delami-

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PRESSic Coatings xxx (2009) xxx–xxx

nation of inert-pigmented coatings, under the conditions studied,is controlled by the interfacial transport of sodium ions from thedefect to the delamination front.

Nomenclatureci concentration of component i in bulk solution (mol m−3)ci,u concentration of component i in the upper chamber

(mol m−3)Di molecular diffusivity of component i in the coating–steel

interface (m2 s−1)De,i apparent effective diffusivity of component i in the

coating–steel interface (m2 s−1)F Faradays constant (J V−1 mol−1)Ji flux of component i (mol m2 s−1)� length of coating–steel interface (m)�0 position of interface between coating and lower solution

(m)n integerR universal gas constant (J K−1 mol−1)Qi,t amount of component i transported across the coating

sample (mol)t time (s)T absolute temperature (K)zi charge number of component i

Greek symbols� time-lag (s)� electrical potential (V)

Acknowledgement

Financial support by J.C. Hempel’s Foundation and The TechnicalUniversity of Denmark is gratefully acknowledged. The authors aregrateful to Fie Wilbek and Solveig Thorsteinsson for their assistancein some of the experiments.

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Cathodic delamination Quantification of ionic transport rates along coating–steel interfaces

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