Coorrision Protect by Organic Coating

Electrochimica Acta 45 (2000) 25152533

Corrosion protection by organic coatings: electrochemicalmechanism and novel methods of investigation

G. Grundmeier a,* , W. Schmidt b,1, M. Stratmann b

a Thyssen Krupp Stahl AG, Forschung, Zentrales Qualitats- und Prufwesen, Eberhardstr. 12, 44120 Dortmund, Germanyb Uni6ersitat Erlangen-Nurnberg, Lehrstuhl fur Korrosion und Oberflachentechnik, Martensstrasse 7, 91058 Erlangen, Germany

Papers received in Newcastle, 20 December 1999

Abstract

The application of electrochemical techniques for corrosion studies of organic coatings on reactive metals isconsidered from the analytical and mechanistic standpoint. Techniques such as electrochemical impedance spec-troscopy (EIS), scanning vibrating electrode and scanning Kelvinprobe (SKP) are powerful tools to better understandthe fundamental processes of corrosion at defects and underneath coatings. In the first part of this paper these threetechniques are discussed in more detail as they present a very complementary approach to understand the ensembleof coating degradation, processes in defects and corrosion underneath coatings, respectively. The second part of thispaper focuses on the two important mechanisms of cathodic delamination and filiform corrosion (FFC). Since bothforms of corrosion are characterised by certain electrochemical reactions underneath coatings and are localised innature, the discussion focuses on the application of the SKP to give new insights in these corrosion phenomena. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Corrosion protection; Organic coatings; Electrochemical impedance spectroscopy; Kelvinprobe

www.elsevier.nl:locate:electacta

1. Principal aspects of corrosion protection by organiccoatings

Protecting reactive metals by covering their surfacewith organic coatings is a smart way to take advantageof mechanical properties of metals such as steel oraluminium while preventing them from corrosion andintroducing one or multiple requested surface proper-ties in one step. These properties might be colour, wearresistance, formability, noise reduction and electronicinsulation.

Organic coatings consist of a binder or vehicle, pigments, and

additives such as dryers, hardening agents, stabilisingagents, surface activating compounds, and dispersionagents.The vehicle, usually polymers of relatively low molec-

ular weight, determines the basic physical and chemicalproperties of the coating. However, additional pigmentscan significantly influence the properties of the coating.Their role is to provide colour, act as a barrier forcorrosive species and to serve as actively corrosioninhibiting species. Nowadays steel is usually covered bymetallic coatings, which are in most cases zinc or zincalloys. An inorganic conversion layer is deposited ontop of the metallic coating to generate a corrosionresistant interface and to provide a link to the organicprimer. For automotive applications usually a cathodicelectrocoat is used as a corrosion resistant organicprimer. The topcoat gives the system its appearanceand acts as a barrier between the corrosive medium andthe inner layers. Both the primer and the topcoatcontain corrosion active pigments.

* Corresponding author.E-mail address: [email protected] (G. Grund-

meier)1 Present address: Max-Planck-Institut fur Eisenforschung

GmbH, Max-Planck-Strasse 1, 40237 Dusseldorf, Germany.

0013-4686:00:$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S0013 -4686 (00 )00348 -0

G. Grundmeier et al. : Electrochimica Acta 45 (2000) 251525332516

It is obvious from the combination of these multiplelayers that the mechanism of corrosion for technicalsystems especially under conditions of natural weather-ing is a very complex one. However, it is the aim tounderstand the fundamental processes of coatingsdegradation as a tool to optimise new coating systemswith improved corrosion behaviour. Concerning thedetails of organic coating technology the reader isreferred to the numerous reviews and monographs [15]. An overview of the different corrosion phenomenaon polymer-coated metals can be found in Leidheisersreview [6]. Some of the methods of investigating poly-mer coatings on metals are outlined in Refs. [79].

While more and more functionality is introduced intocoatings, still the aspect of corrosion protection espe-cially for steel and aluminium are of great interest inresearch and development. This is due to the fact that hazardous compounds such as Cr(VI) which nowa-

days guarantee excellent corrosion protection prop-erties have to be replaced with alternativeenvironmentally friendly compounds;

the introduction of new light metals such as magne-sium with specific corrosion behaviour require spe-cially adopted coatings;

use of water based or 100% solvent free coatings willreplace solvent based coatings;

application of new curing technologies such as UVor electron beam (EB) curing might lead to newspecific reactions at the metal:polymer interface; and

the trend to sell pre-coated steel sheet to the automo-tive industry to omit secondary corrosion protectionprocedures and to reduce the costs caused by expen-

sive paint shops raises new demands for thin organiccoatings.The corrosion protection properties of an organic

coating most often result less from the barrier proper-ties but more from the maintenance of adhesion to thesubstrate under chemical and electrochemical condi-tions imposed by the environment. It could be shownthat for typical organic coatings used for corrosionprotection the diffusion rate of H2O and O2 far exceedsthe diffusion limited value for oxygen reduction [1012]. However, ion solubility within the coating is typi-cally very small due to the low dielectric constant ofcommon coatings [13].The following roles of organiccoatings provide corrosion protection: Barrier for ions leading to an extended diffusive

double layer. Adhesion of the coating. Blocking of ionic paths between local anodes and

cathodes along the metal:polymer interface. Vehicle of corrosion active pigments and inhibitors

which are released in the case of coating damage.Fig. 1 schematically shows the influence of an or-

ganic coating on the electrode potential and the size ofthe double layer in front of the metal surface.

Fast metal dissolution due to the large electric field(107 V:cm) leads to a negative electrode potential in thedefect area in contrast to the polymer-coated areawhere metal dissolution is strongly inhibited and adiffuse double layer is observed.

Corrosion of a polymer-coated metal requires thepenetration of water molecules, ions and oxygen to theinterface. Even visibly non-damaged films show an

Fig. 1. Schematic of the shape of the double layer for electrolyte covered metal (top) and polymer-coated metal (bottom).

Ms.LinhHighlight

G. Grundmeier et al. : Electrochimica Acta 45 (2000) 25152533 2517

onset of corrosion after a certain time. Thus the diffu-sion of these species probably follows small pores orpathways within the polymer that facilitate this trans-port. However, the most severe corrosion processesstart at local defects within the coating. These might beintroduced during production e.g. at cut edges, or aregenerated during the lifetime of the coated material, forexample caused by stone chipping or scratching. Suchdefects lead to the contact between the corrosive elec-trolyte and the bare metal. Subsequently, coated sys-tems corrode normally along the polymer:metalinterface. The corrosion mechanisms are controlled bythe metal substrate or its metallic coating. For thereactive metals steel, zinc-coated steel and aluminiumthe perdominant mechanisms are the cathodic delami-nation and filiform corrosion (FFC), respectively. Theexistence of these two different corrosion mechanismsand the fact that iron can show both mechanismsdepending on the composition of the environment,verify that the mechanism of corrosion depends on themetallic substrate and the composition of its passivelayer and corrosion products as well as on transportphenomena. Moreover, the fact that corrosion starts atthe metal surface and that progress of corrosion hap-pens at the interface between the metal and the polymermakes electrochemical methods the most suitable toolsto understand the corrosion of organic coatings onreactive metals.

In the following first part of this contribution thoseelectrochemical methods which are most interesting andpopular for polymer-coated metals are described inshort. The second part presents the current knowledgeof corrosion mechanisms for polymer-coated steel andaluminium.

2. Electrochemical methods of investigation

High ohmic resistance of organic coatings impedesthe use of dc-type electrochemical measurements, sinceduring a potentiostatic polarisation of a polymer-coated metal the potential drop across the polymericlayer is orders of magnitude larger than the potentialdrop across the metal:polymer interface. This has led tothe development of a number of advanced electrochem-ical techniques [14]. Electrochemical impedance spec-troscopy (EIS) and the scanning Kelvinprobe (SKP)have found widespread application for electrochemicalstudies of the degradation of polymer-coated metals.New electrochemical scanning techniques such as thescanning vibrating electrode technique (SVET) and lo-calised impedance spectroscopy (LEIS) recently havefound application for assigning integral corrosion be-haviour to local defects in the coating and for measur-ing corrosion processes within the defect as a functionof the adjacent coating. While the galvanostatic pulse

method, relaxation voltammetry, EIS and electrochemi-cal noise method are valuable techniques for studyingdielectric properties, onset of defect formation andprocesses of coating degradation, recent scanning refer-ence electrodes techniques such as SVET and SKPenable localisation of defects (SVET and SKP) withhigh spatial resolution of a few tens of micrometers.The SKP moreover allows study of localised corrosionprocesses underneath insulating coatings [14].

In this contribution the electrochemical techniquesare ordered according to the following principal pro-cesses relevant for the corrosion of polymer-coatedmetals: swelling and ion incorporation into the coating and

formation of intrinsic defects such as pin holes; onset of localised corrosion in defects; and spreading of corrosion underneath the coating which

can be caused by on anodic or cathodic reactions.

2.1. Measuring the deterioration of organic coatings byelectrochemical impedance spectroscopy

Practically speaking EIS provides a measure of theresistance of the organic coating to aqueous and ionictransport. The technique is based on the measurementof the current response on small sinusoidal perturba-tions of the electrode potential as a function of thefrequency of the perturbation [15,16]. In the followingwe will focus on the possibility of detecting the onset ofcorrosion and the progress of corrosion underneath thepolymer coating.

Different models have been proposed to analyse EISmeasurements obtained for model systems and techni-cal coatings. These models have been applied for indus-trial screening of organic coatings on bare andphosphated steel in 0.5 M NaCl solution [17]. Fig. 2shows a typical impedance spectrum of a quasi-idealcoating which does not exhibit any indication of corro-sion attack even after exposure time of up to half ayear.

The Bode plot shows a pure capacitative behaviourover a wide frequency range and the polarisation resis-tance at low frequencies is in the order of 1011 ohmcm2. This is the simple case of a homogeneous 3-D filmwhich is well illustrated in Fig. 3.

The impedance of the coated electrode is describedby a parallel combination of the capacitance, CL; andthe resistance, RL, of the layer

ZL( jv)RL

1 jvRLCL(1)

The coating capacitance is given as

Ccoo0A

d(2)


Fig. 2. Impedance of a quasi-perfect coating on steel. Experi-mental data () and optimum fit () using the EC in Fig. 3 [18].

partially damaged coating which thereby caused cor-rosive undermining of the coating.The first case has been studied recently by van West-

ing et al. [18]. Measurements were carried out forpigmented and unpigmented high resistance epoxycoatings on cold rolled steel. The authors fitted theimpedance of the coating using a constant phase ele-ment (CPE). Fig. 4 shows the plots of the CPE parame-ters Y0 and n during a long term exposure in 3% NaClsolution. The parameter Y0 reflects the total polarisabil-ity and n is a measure of the interaction between thepolarisable groups [19].

The first steep change in both parameters is due tothe swelling and ion incorporation of the polymercoating. The CPE-constant Y0 increases to a stationaryvalue due to saturation of the coating but shows a bendaround 400 h. The corresponding features were ob-served for the CPE-power n. An observation of themetal surface with a stereo microscope after 400 hshowed the existence of small corrosion spots under-neath the coating. The same type of behaviour, preser-vation of high impedance with changingCPE-parameters, was found also for pigmentedcoatings.

Impedance spectra for defect containing polymer-coated metals exposed to corrosive electrolytes can befitted according to the electronic circuit (EC) shown inFig. 5. Rpo has been called the pore resistance in [20],which it is considered to be due to the formation ofionically conducting paths in the polymer. Rde is thepolarisation resistance at the metal surface in contactwith the ionically conducting paths and Cdl is thecorresponding capacitance. The resulting impedanceplot is schematically shown in Fig. 6.

Scully and Hensley calculated the theoretical effect ofvarious percentages of defect area on the impedance[21]. The impedance parameters were obtained from aniterative procedure, whereby synthetic spectra werecompared with experimental spectra and updated to agood correspondence. In this procedure, pore resistance(here: Rd), charge transfer resistance (here: Rt) anddouble layer capacitance were held constant. The defectarea was varied. The resulting synthetic EIS data asshown in Fig. 7 were strikingly similar to that observedduring the degradation of actual coatings. The high (fh)and low (fl) frequency breakpoints increase linearlywith the defect area with fl saturating above 0.001%defect area.

In the case of spreading corrosion underneath thecoating, a transmission line model as schematicallyshown in Fig. 8 represents the system as described byKendig et al. [22].

Resistance values Rs(i) characterise the ohmic resis-tance between the defect and the respective active partunderneath the coating. Only if the ohmic resistanceRs(i) is sufficiently low the effective capacitance Cdl

where o is relative dielectric constant, o0 is dielectricconstant in vacuum, A is coating area, and d is coatingthickness.

Thus the capacitance measurement by EIS canprovide information on the water uptake, since thisincorporation of polar molecules leads to an increase inthe dielectric constant of the coating.

Three cases might now be distinguished: corrosion underneath the coating without any defect

in the coating itself, partially damaged coating with cracks reaching the

metal surface, and

Fig. 3. Impedance model of a defect free organic coating on ametal surface in contact with an electrolyte.


Fig. 4. CPE-parameters Y0 (left axis) and n (right axis) vs. time during a long time exposure in 3% NaCl solution (Reprinted withpermission from [18]. 1994 Elsevier Science).

which is equal to the sum of the capacitance within thepore and those underneath the coating, does indeedscale with the disbonded area as assumed by manyauthors.

Theoretically, it is obvious that the determination ofcharge-transfer resistance and double layer capacitanceis possible only if the time constants of the interfacialreaction and the polymer film are clearly separated suchthat the impedance of the coating is smaller or equal tothe impedance of the interfacial reaction. This might bethe case for very thin polymers and highly inhibitedinterfaces. If the impedance of the interfacial reaction issignificantly smaller than the impedance of the polymerfilm, it is unlikely that the impedance of the interfacecan be deduced from the overall impedance of thepolymer-coated metal. To circumvent the problem ofthe high coating impedance between the reference elec-trode and the metal polymer interface, Feser and Strat-mann developed a set-up where the reference electrodeis directly placed at the interface [11].

This experimental set up is schematically shown inFig. 9 where one reference electrode is located at themetal:polymer interface and a second one is positionedin front of the coating showing a significant differencedue to the large impedance of the coating. Both spectraexhibit just one time constant, however, solely spectrumB depends strongly on a change of the oxygen activityin the electrolyte as shown in Fig. 10. The resistance atlow frequencies increases with decreasing oxygen activ-ity [11]. Thus spectrum A is determined mainly by theproperties of the polymer, whereas spectrum B shows

the impedance of the interface, which depends on theactivity of oxygen. While this set-up is appropriate forfundamental studies, it is too sophisticated to be ap-plied to the evaluation of technical systems. Moreover,there is still no spatial resolution in this measurement.

However, there is a need to measure the corrosionproperties of polymer-coated metals locally due to thefact that mostly only a very small part of the coating isdamaged, which dominates the electrochemical be-haviour of the whole exposed surface. In this casescanning reference electrodes should help to assigndefects and to separate anodic and cathodic areas ofgalvanic couples.

Fig. 5. Impedance model of a defect containing organic coat-ing on a metal surface in contact with an electrolyte.


Fig. 6. Schematic impedance spectrum of a defect containingpolymer on a metal.

Fig. 8. Equivalent circuit schematics for an organic coating incase of a coating with a disbond starting from a scratch [22].

2.2. Detection of local defects by means of scanningelectrode techniques

Defects in organic coatings may originate from theproduction process (e.g. cut edges, forming induceddefects) or from mechanical impact (e.g. stone chip-ping). However, coatings which may possess ionic con-ductive pathways or ionic residuals are located at theinterface so that corrosion starts at sites of the coatingwhich are not visibly damaged. Since EIS is a priori anintegral method it can detect the existence of such

defects but it can not assign them to certain points onthe examined surface. The idea of a local measurementis to examine areas separately which differ in theiractivity on one sample.

Fig. 7. Bode magnitude plot showing the theoretical effect of various percentages of defect area on the simulated impedancebehaviour of polymer-coated steel with a total area of 10 cm2. Hypothetical defect areas are indicated using both area% and theASTM D610 scale [21].


Fig. 9. Experimental set-up used for the impedance analysiswith a reference electrode at the metal:polymer interface [11].

the influence of inhibitors and pigments on the activityof these defects.

Close to the metal surface, current lines follow radialcurves into the solution. Assuming that the specificresistance r of the electrolyte is constant, the currentlines lead to hemispheres of constant potential. Thecurrent lines intersect these hemispheres perpendicu-larly. The values of the potential are a function of thecurrent, the specific resistance of the electrolyte r, andthe distance of the hemisphere to the current source orsink.

By measuring the potential differences DV betweenpoint A and point B, which are separated by a distance2d, local currents can be calculated according to

ilocal 1r

DV2d

(3)

Scanning reference electrodes enable the measure-ment of potentials as a function of the location. Theutilisation of glass capillaries in combination with refer-ence electrodes such as a calomel electrode enablemeasurement of corrosion potentials while pseudo-ref-erence electrodes such as Pt-wires are used to measurethe potential difference between two points in solution.Included in this latter category are closely spaced refer-ence electrodes that give a direct measure of the currentdensity from the potential gradient and distance be-tween them as shown in Fig. 11(a). Such electrodes areused for the scanning reference electrode technique(SRET).

The vibrating probe of the SVET gives a directmeasure of the electric field, or from Ohms law, thecomponent of current density at the point in the direc-tion in which the electrode vibrates (Fig. 11(b)). Thevibration may be parallel or perpendicular to the sur-face of the investigated sample.

The advantages with respect to the SRET are thehigher local resolution and sensitivity for small currentsbased on the applicable lock-in technology. As a stan-dard microprobe, a thin Pt-wire, which is isolated ex-cept at its tip, is used to map the surface and itspotential is measured versus a reference electrode,which is immersed into the same electrolyte but faraway from the sample surface. Usually, the Pt-tip isplatinised to reduce the interfacial impedance of theprobe.

The local galvanic current and the measured poten-tial difference perpendicular to the sample surface areagain correlated according to Eq. (3).

Although Eq. (3) gives a direct correlation of mea-sured potential difference and vertical current, usually acalibration of the reference electrode is done whichresults in a coefficient which takes into account theexperimental conditions such as the impedance of themicroelectrode, real amplitude of the vibration andresistance of the electrolyte. For the calibration process,

Measuring local electrode potentials by means ofscanning reference electrodes has a long history inelectrochemistry. Recently, the development of theSVET led to the utilisation of localised current densitymaps to detect local defects in organic coating afterforming or to measure the activity of cut edges incorrosive environments [2325]. The local measurementof currents can not overcome the inherent difficulty inmeasuring corrosion underneath high resistance coat-ings but helps to understand the origin of defects and

Fig. 10. Impedance spectra (Bode plots) on the coated ironsample after a change from N2 to O2-purged electrolyte (0.1 MNaCl). (a) Total impedance, (b) partial impedance; 1, N2, 0min; 2, O2, 15 min; 3, O2, 65 min; 4, O2, 110 min; 5: O2, 320min [11].


Fig. 11. Schematic of the two electrode probe configuration of the SRET (a) and the one electrode configuration of the SVET (b)[14].

usually a point current source of known current andsize is measured by the SVET. In recent years theapplication of the SRET and SVET covers also the fieldof polymer-coated metals.

Recently Zou et al. investigated the degradation ofcoil-coated galvanised steel at the cut edge [25]. Theauthors focused on the influence of the chromate con-tent in the coating on the active zinc dissolution at thecut edge. Chromate clearly led to a rapid diminishing ofanodic activity of the exposed zinc. The chromate freeprimer did not lead to the equivalent inhibition of thezinc dissolution (see Fig. 12). Not surprisingly, theinhibition of the Zn dissolution measured with theSVET was in accordance with a high corrosion resis-tance of the chromate containing primer.

Recently, local impedence spectroscopy (LEIS) basedon the SRET has been applied to study the local acsolution current density above polymer-coated metals[26,27]. The magnitude of the local impedence Z(v)local is derived from equation Eq. (4).

Z(v)localV(vapplied)

I(v)(4)

where V(v)applied is the magnitude of the appliedvoltage pesturbolon between the reference and workingelectrode.

Twenty-six carbon steel samples were polished,cleaned and then contaminated by dropping a smallamount of NaCl solution on the surface of the speci-men. After drying the sample was coated with an epoxyresin. Impedance measurements were done in a diluteNaCl solution. The authors revealed that even above avisible blister underneath the coating an impedancespectrum almost equal to that of the intact area ismeasured by LEIS as long as the coating itself is intact.The reason is the high impedance of the coating inseries with the low impedance of the interface in the

contaminated region. A smaller change observed di-rectly above the blister was assigned to a local changein the capacitance of the coating.

A new and promising way of LEIS measurementbased on the SVE technique was developed by Bayet etal. [28].

2.3. Measuring corrosion at the metal:polymerinterface by means of a scanning Kel6inprobe

The SKP allows the difficulty of conventional refer-ence electrode techniques which require a conductingpath between the reference electrode and the workingelectrode to be overcome. In principle the Kelvinprobemeasures the work-function of a sample using the vi-brating condenser method [29,30]. A schematic of theSKP is shown in Fig. 13. Under certain circumstancesthe work-function is determined by the electrode poten-tial and therefore the Kelvinprobe is able to measurelocal electrode or corrosion potentials. The major ad-vantage of the Kelvinprobe in comparison to conven-tional electrochemical devices such as electrochemicalreference electrodes is the fact that the Kelvinprobemeasures electrode potentials without touching the sur-face under investigation across a dielectric medium ofhigh resistance.

In principle the Kelvinprobe consists of a metallicreference electrode, which is separated from the sampleby a dielectric medium and connected to the sample bya metallic wire.

As after connection of the two metals, the electro-chemical potential of the electrons within both phaseswill be identical a charging of one sample with respectto the other (Volta-potential difference) will be ob-served. Therefore, for a given and constant work func-tion of the reference metal the work function of thesample can be determined by a measurement of the

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Fig. 12. The current density distribution over a cross section of hot-dip galvanised steel in 10 mM NaCl. The primer P4 (chromate containing) was applied on one side of thezinc while the primer P6 (chromate free) was applied on the other side. (a) 10 min in 10 mM NaCl, (b) 100 min in 10 mM NaCl, (c) contours of (a) [25].


Fig. 13. Schematic of the SKP.

2.3.1. System metalpolymerhumid airFor non-highly-oriented polymers with a rather small

dipole potential is, the following result is obtained:

ECorr!Wref

FxPolo1:2

ref"DCPolref (8)where Wref is the work function of the reference metal,xPol the surface dipole potential of the polymer and E1:2

ref

is the half-cell potential of the reference metal.Similar to an electrolyte covered metal surface, an

electrode potential of the inner interface would bemeasured. However, the physical meaning of this elec-trode potential is not as obvious, as it cannot beinterpreted by conventional electrochemical kinetics.The electrode potential may in the absence of anyfaradaic current be determined by dipole orientation ofsegments of the polymer chain. If, however, faradaiccurrents like the reduction of oxygen are possible at theinner interface, then the interface will be polarised untilthe rate of the oxygen reduction is negligible. This istrue e.g. for polymer-coated gold surfaces.

2.3.2. System metaloxidepolymerhumid airThe Volta-potential difference is given by

DCPolref DFOxMeDFOxDFPolOx 1Fm e

MeWref

FxPol

(9)

and if the Volta-potential drop across the oxide layer issubstituted by the corresponding change in chemicalcomposition, then for an iron substrate, DC refpol is:

DCPolref DmFe3:Fe30

FDFPolOx

WrefF

xPol

RT

FlnFe3

Fe2n

(10)

Thus the Volta-potential difference represents theoxidation level within the oxide scale at the metal:poly-mer interface [39].

2.3.3. System metal:metaloxideelectrolytepolymerhumid air

This system is typical for a delaminated interfacewhere an electrolyte layer is existent between the sub-strate and the polymer. This potential drop is called themembrane or Donnan-potential and is directly associ-ated with the preferential incorporation of ions into thepolymeric matrix [38,40,41]. As a result:

ECorrWref

FxPolo1:2

ref DFDDCPolref (11)

Therefore the Volta-potential difference DC refPol allowsthe corrosion potential at the inner metal:electrolyteinterface buried below the polymeric coating to bemeasured only if the Donnan-potential is known orsmall. Usually the Donnan-potential is of specific im-

Volta-potential difference. DC refsample is usually measuredby the vibrating condenser method of the Kelvinprobe[3032]. In this technique, sample and reference elec-trode form a condenser and the reference electrode isforced to vibrate in a mean distance to the sample d( byan amplitude Dd. Then the change of the capacitance Cis given by:

Coo0A

d( Dd sin(vt)(5)

where o is dielectric constant of the medium, o0 iselectric field constant, v is frequency of vibration forthe reference metal, and A is surface area of the refer-ence plate; and therefore a current results in the exter-nal circuit which is given by:

iDC samplerefdCdt

(6)

If an external voltage U is switched into the externalcircuit (see Fig. 13), then:

i (DCU)dCdt

(7)

with i0 for DCU.Therefore, in a conventional approach, the voltage U

is changed until the current i vanishes, and for thiscondition (nulling technique) DC refsample is measured.

The voltage between sample and probe necessary toreduce the AC-signal to zero is regarded as the Volta-potential difference DC refsample. During measurement thechamber of the SKP is kept at very high relativehumidity (typically \95%) in order to keep the electro-chemical reactions running.

A relation between the Volta-potential differenceDC refsample and the corrosion potential Ecorr of the cor-roding interface exists, which must be derived for differ-ent interfaces of interest. For polymer-coated metals theKelvinprobe allows to measure the potential distribu-tion at the inner buried interface [3338].


Fig. 14. Top and middle sketch: Delamination mechanisms onpolymer covered steel and zinc; bottom: FFC on aluminiumalloys.

reference material. The calibration procedure hastherefore to be performed frequently and typically onlywith a simple metal:metal cation system like Cu:CuSO4.

3. Corrosion mechanisms of polymer-coated steel,galvanised steel and aluminium

As far as the substrate is concerned, the instability ofa metal polymer interface is governed by three crucialproperties: the electron-transfer properties at the inter-face, the redox properties of the oxide between themetal and the polymer and the chemical stability of theinterface with respect to those species, which areformed during the electron transfer reaction (ETR). The rate of ETRs is strongly influenced by the

surface composition of the metal. As most materialsare covered by oxides their electronic properties willdetermine the rate of ETR. Therefore, metals whichare covered by electron conducting or semiconduct-ing oxides such as iron or zinc will show a differentreactivity at the substrate:polymer interface in com-parison to those materials which form highly insulat-ing oxides such as aluminium.

Certain oxides are characterised by a fixed ratio ofanions and cations (e.g. Al2O3) whereas others havea strongly potential dependant composition such asiron-oxides due to the flipping of valence states(Fe2, Fe3) in the cation sublattice. Any change ofthe electrode potential is reflected in a change of thevalence states and this will change the semiconduct-ing properties, as e.g. Fe2-states can be regarded asdonors of the n-type semiconductor [45,46]. Further-more, during reduction of the oxide the base mate-rial will be oxidised and this redox reaction mayreduce the adhesion of the coating.

During the electron transfer very reactive intermedi-ates and reaction products are formed which willchemically react with the material itself. It is shownbelow that major reaction products are OH ionswhich are generated during the reduction of molecu-lar oxygen. Certain metals such as iron are verystable under those conditions whereas others likealuminium or zinc are covered by oxides which arehighly soluble in alkaline electrolytes [47].Accordingly, it must be expected that steel, zinc-

coated steel, and aluminium will behave rather differ-ently due to the different electronic, redox and chemicalproperties of the interface. In this paper the delamina-tion reactions on three materials will be compared,which symbolise limiting cases (Fig. 14): Iron, with highly electron conducting oxides, which

are stable in alkaline media. Zinc, with semiconducting oxides, which are not

stable in alkaline media.

portance for polymers with a high density of fixedcharges (ion exchange membranes), as polymers withfixed cationic functional groups will exchange anionsexclusively and vice versa [41,42]. Lacquers used forcorrosion protection may have some fixed ionic groups;however their concentration is orders of magnitudeslower than the one of typical ion exchange membranes.

In most equations of this section the Volta-potentialdifference DC is related to the electrode or corrosionpotential in a linear manner, however a calibrationconstant is needed in order to calculate the electrodepotential from a Volta-potential measurement. Foraqueous electrolytes the calibration constant is easilyobtained by measuring the Volta-potential of a metalelectrode which is exposed to an electrolyte containingthe metal cation in a defined concentration [43,44].Then the electrode potential of the metal:metal cationsystem is known from the Nernst equation and thecalibration constant is obtained e.g. by plotting theknown electrode potential versus the Volta-potentialfor different metal:metal cation systems. However, itshould be kept in mind, that the calibration constantcontains the work function of the reference metal anddepends strongly on the actual surface condition of the


Aluminium, with insulating oxides, which are notstable in alkaline media.

3.1. The delamination of organic coatings on iron

In the presence of oxygen the electrode potential ofthe metal:polymer interface changes in a well definedmanner with increasing distance from the defect: closeto the defect the potential is negative, whereas far awayfrom the defect rather anodic potentials are observed(Fig. 15). For most coatings the steep increase of theelectrode potential also marks the delamination fron-tier, the region of anodic potentials representing theintact interface. The physical origin of the suddenchange of the electrode potential however is given bythe migration of ions from the defect into the interfacewhich results in the polarisation of the highly polaris-able interface to the potential of the non-polarisabledefect [34]. This is clearly seen in potential profilesmeasured in the presence of oxygen, which are similarto the ones shown in Fig. 15. However, in the presenceof oxygen no steady potential increase is observedwithin the already delaminated zone (Fig. 16).

As a result the potential step in Fig. 16 only marksthe incorporation of ions into the interface and delami-nation is a subsequent reaction. Three regions areclearly seen in Fig. 15 which should be discussedseparately:

The intact interface is characterised by an anodicpotential plateau. This plateau results from the highelectronic conductivity of the oxide covered iron surfacewhich allows ETR but no ion transfer reactions. There-fore, oxygen will be reduced at this interface and thisreaction is balanced by the anodic oxidation of theoxide. As the electrode potential of the oxide is givenby the activity of Fe2 and Fe3 states, any oxidationresults in an anodic potential shift which is accompa-nied by a steady decrease of the donor density andtherefore by a decreasing rate of the ETR. Above acertain anodic potential the rate of the oxygen reduc-tion is extremely small and no further anodic potentialshift is observed. This is the final potential as measuredby the Kelvinprobe. The transient of the anodic poten-tial shift therefore marks the capability of the surfacetowards electron transfer. It has been shown, that aproper surface treatment decreases the rate of thisanodic potential shift dramatically to a point wherealmost no anodic shift is observed due to a completelyblocked interface (Fig. 17).

The sudden potential step marks the most interestingposition, as here reactions will occur which are respon-sible for the loss of adhesion. As discussed before, thepotential step is caused by the incorporation of ionsinto the interface and the galvanic coupling of theinterface to the defect. The cathodic potential step alsomarks the reduction of the previously oxidised oxide

Fig. 15. Typical potential distribution of a polymer coverediron substrate in humid air for different delamination times(times as indicated, electrolyte in the defect: 0.5 M KCl)(Reprinted with permission from [36]. 1999 Elsevier Sci-ence).

and the increase in its donor density. Obviously, thismust result in an increase in the rate of the electrontransfer. Surface analysis reveals no anodic activity inthis area as besides a passive film no thick oxide layersare found [35,37]. The anodic counter reaction of theoxygen reduction therefore must be the dissolution ofthe base material within the defect. Indeed a galvaniccurrent has been measured between both sites [36] andoxygen is reduced within the zone marked by the

Fig. 16. Potential profiles for different delamination times asindicated. The defect was covered by 0.5 M NaCl. The atmo-sphere in the SKP was water saturated argon [36].


Fig. 17. Transient of the potential relaxation in air aftercathodic polarisation at 1 V SHE in borate buffer surfacetreatment of the steel substrate as indicated.

potential shift instead of an anodic one. Experimentshave indeed proven that the anodic potential shift islinked to the galvanic current between defect and thefrontier of incorporation of ions and is measured onlyif oxygen will be reduced within the latter zone [3436].

The electrochemical situation is summarised in Fig.19. The galvanic element is dominated by the electronicproperties of the oxides being present at the interface asany shift in the electrode potential reflects a corre-sponding shift in the electronic properties and thereforein the rate of the oxygen reduction at the interface.During oxygen reduction a strongly alkaline electrolyteis formed which stabilises the oxide on the metal.Therefore anodic metal dissolution is never observedwithin the zones described above. As the galvanic ele-ment does obviously not destroy the metallic substratethe delamination of the organic coating is only causedby bond breaking within the adjacent organic layer. Ithas indeed been proven that intermediate radicalsformed during oxygen reduction are responsible for anoxidative destruction of the interface and therefore oniron the instability of the substrate:polymer interface isdirectly linked to the rate of oxygen reduction.

3.2. The delamination of organic coatings on zinc

The situation described for iron and steel is alsotypical for a zinc:polymer interface (Fig. 20) [48]. Zincis also covered by electron conducting oxides and there-fore allows oxygen to be reduced. Again at an intactinterface the anodic partial reaction is missing andtherefore the oxygen reduction leads to an oxidation ofthe oxide scale until the electronic properties are suchthat no further electron transfer is possible. This is truefor potentials which are approximately 500 mV morenegative than those of oxide covered iron.

If however the atmosphere is changed to an oxygenfree one, then a rapid decrease of the electrode poten-tial is observed also for the intact interface (Fig. 21),whereas for iron the potential is stable for rather longtimes. This different behaviour is not well understoodup to now but may be caused by the rather differentdonor densities of both oxides. In order to reduce theiron oxide at the metal:polymer interface a large frac-tion of Fe3 states have to be transformed into Fe2.In-situ Mobauer studies have shown Fe2 concentra-tions up to 30% for electrode potentials of 0.3VH [31]. This limits any cathodic potential shift at a stableiron:polymer interface significantly. As oxides of com-parably mixed valence-states are unknown for zinc, acathodic potential shift may be possible for a rathernegligible charge.

As soon as ions are incorporated into the interface agalvanic element is set up, the potential of the interfaceis polarised cathodically and oxygen will be reduced.However, zinc oxides are not stable within the induced

potential increase. In order to compensate the chargecations will migrate to the zone of oxygen reduction.This is confirmed by spatially resolved ESCA-measure-ments (Fig. 18).

Between the defect and the steep potential increase asteady potential increase to more anodic values is ob-served. This steady potential increase is observed onlyin the presence of oxygen and therefore is linked to theoxygen reduction below the organic coating. The rea-son for the potential increase may be twofold: either theanodic potential shift marks a different chemistry at theinterface or it results from an ohmic potential dropcaused by the galvanic current. The change of thechemical composition at the interface can be ruled out,as during the oxygen reduction an alkaline pH isformed and the open circuit potential of a passive ironsurface in an alkaline medium would show a cathodic

Fig. 18. Substrate: iron. ESCA-concentration profile of themetal surface after the mechanical removal of the polymerlayer [70].


alkaline environment. AES sputter profiles measuredwithin the delaminated zone prove the significantgrowth of the oxide scale which indicates anodic reac-tions. This behaviour is also reflected in potential profi-les measured with the Kelvinprobe after a suddenremoval of oxygen from the atmosphere (Fig. 22). Forzinc potential profiles are measured which show astrong cathodic potential shift within the zone, whichhad been assigned to the locus of the oxygen reductionbefore. The potential shift even inverts the potentialdifference between the defect and the frontier of ion-in-

corporation: now the potential in the latter position is400 mV more negative than the potential within thedefect. This observation is not caused by an inversionof the galvanic element as in the absence of oxygen nogalvanic current flows between the defect and the inter-face. The electrode potentials are only defined by thethermodynamic equilibrium potential. Within the defectthe equilibrium is given by the Zn:Zn2 couplewhereas at the metal:polymer interface the couple Zn:Zn(OH)4

2 will prevail and this explains the observedpotential inversion. In the absence of oxygen the Kelv-

Fig. 19. Principle corrosion model explaining the formation of a galvanic element. Upper part: cross section through a metalpolymer interface with a defect in the polymer coating. Central part: overview of the polarisation curves at the defect (left side), theintact interface (right side) and the situation after galvanic coupling of the parts (lower graph).


Fig. 20. Typical 2-dimensional potential profiles on a polymercovered zinc substrate as measured with the SKP in air (95%r.h.) for different corrosion times (as indicated) with 0.5 MNaCl at the defect (Reprinted with permission from [48].2000 Elsevier Science).

inprobe is therefore a tool to measure a local pH at theburied interface and the observed time dependence ofthe potential profiles are dominated by the diffusion ofOH along the interface.

Interestingly, zinc:polymer interfaces may thereforealso be destroyed by an anodic metal dissolution trig-gered by the cathodic oxygen reduction, but the combi-nation of both reactions will buffer the pH andtherefore limit any destruction of the interface due toextremely high OH concentrations. For iron the op-posite is true.

In comparison to zinc, galvanised steel is of consider-ably higher technological interest. As long as the defectonly penetrates the coating and zinc is still present atthe defect, the situation is identical to the one of purezinc. If, however, the scratch also penetrates the gal-vanised zone and iron is exposed to the electrolyte atthe defect, the galvanic element will change and nowthe defect will be the anode and zinc at the interfacewill be the cathode and will protect the defect. Now thegalvanic element between defect and interface is indeedinverted but this is possible only if the zinc:polymerinterface has been delaminated before. Careful studieshave shown that indeed also for the zinc:iron couple ina first step the zinc:polymer interface is destroyed bycathodic delamination and only then zinc starts toprotect iron while acting as a sacrificial anode [48].

3.3. Filiform corrosion (FFC) of polymer-coatedaluminium

FFC of metals is characterised by a thread-like un-dermining of a coating [4952]. Two different regionsof the progressing filaments can be observed: the liquidfilled active head and a tail of corrosion products.Various metals such as Al, Fe and Mg show this kindof corrosion underneath a (polymer) coating. An ex-tended literature review of FFC investigations on alu-minium is given by Bautista [53], earlier reviewscombined with new experiments have been made byHahin [54], Hoch [55] and Ruggeri and Beck [56]. Somerecent results on FFC on aluminium alloys can befound in Refs. [5764].

The systems polymer-coated Fe and Zn which werediscussed before are characterised by a substrate, whichis covered by electron conducting oxides and thereforeallow electrochemical reactions to occur at any place onthe surface. Delamination is caused by the incorpora-tion of ions into the interface and the subsequentlyformed reaction products of the oxygen reduction,which will cut bonds at the interface and thereforeextend the delaminated zone. As the incorporation ofions into an intact interface may be difficult due to thelow dielectric constant a reaction zone will be estab-lished between the frontier of ion incorporation and thechemical destruction of the interface. Due to the limited

Fig. 21. 2-dimensional potential map of a polymer-coated zincsurface; defect covered with 0.5 M NaCl; top: 1640 minexposure in air; bottom: 21 h after exchange of atmospherefrom air to N2. (Reprinted with permission from [48]. 2000Elsevier Science).


Fig. 22. Potential profiles as measured with the SKP on partly delaminated polymer-coated zinc sample after a change of theatmosphere from air to pure nitrogen; corrosion time before the change: 2710 min; corrosion times since change as indicated.(Reprinted with permission from [48]. 2000 Elsevier Science).

spatial resolution of the Kelvinprobe (up to a few tensof a micrometer), little information exists about thespatial extension of this reaction zone. Considering thebarrier properties of most coatings the reaction zonemay be in the order of some 10 nm and thereforecathodic delamination is possible only for the surface ofhomogeneous electronic properties.

For Al based alloys this is definitely not the case, asAl is covered by an insulating oxide and all ETR arelimited to highly localised reactive spots, which can beidentified as intermetallic inclusions of various compo-sitions, depending on the alloying elements and theimpurities. Therefore, Al-based alloys electrochemicallymay be considered as partially blocked electrodes andthe reactivity is determined by the surface area fractionof the inclusions and the conductivity of their specificoxide layers. If the spacing between the inclusions(some mm) is larger then the dimension of the reactionzone then the reaction sequence as discussed before isinterrupted and cathodic delamination should not beobserved anymore. This is indeed true and under com-parable experimental conditions blistering and anodicundermining is observed instead of cathodicdelamination.

If, however, the relative humidity in the storagechamber is reduced to values of approximately 85%another type of destruction of the metal:polymer inter-face is observed: FFC (Fig. 23).

This highly localised attack is not only observed forAl-alloys but also for steel under comparable experi-mental conditions. Obviously, cathodic delaminationrequires a high water activity besides an electron con-ducting surface, probably in order to allow ion migra-

tion along the interface. On iron, cathodic delaminationfails with decreasing humidity due to the lack of ionmigration into the interface whereas on Al cathodicdelamination is not observed even at high humiditiesdue to the lack of electronic conductivity.

There is agreement in the literature on FFC, that twodifferent areas of a corrosion filament can be distin-guished: the small, so-called active head (concentratedelectrolyte solution) at the front and the tail of solidcorrosion products.

The corrosion filaments start to grow from a scratch(containing an initiating electrolyte like NaCl solution)in the polymer coating in perpendicular direction to thescratch. At their very front, the active head, they carryan acidic solution of the metal cations and the initiatinganions [56]. The filaments do not cross each other[5052,65], their width increases with increasing RH[5052] and with thicker coatings [50,51]. The lattertwo effects have only been investigated for iron andsteel.

The driving force for the directed growth process isbelieved to be an oxygen concentration cell between thefront and the back of the active head [50,51,56]. There-fore, FFC is closely related to crevice and pittingcorrosion. Ruggeri and Beck [56] calculated transportrates of oxygen through the polymer coating andthrough the tail of corrosion products (FFC on iron)and thereafter proposed that diffusion through the tailis dominating. Furthermore, in FFC underneath metalplatings [50], the transport of oxygen through the coat-ing is impossible.

However, some authors speculated about FFC alsobeing a type of cathodic delamination, i.e. the cathodebeing the propagating reaction site [6668].


Fig. 23. Photograph of FFC attack on epoxy-coated aluminium alloy AA2024-T3. Filaments start to grow from the artificial defect(scratch) and from the samples edge.

Slabaugh [52] was the first to attempt measurementof the corrosion potentials of different parts of the FFCtrack to detect anodic and cathodic areas. He did thisby cutting the polymer along the FFC track open,lifting the organic coating off the surface and immedi-ately measured the electrode potentials by insertingmicro reference electrodes. This method however meansa major disturbance to the fragile FFC system, and theobtained results may therefore be doubted.

By using the SKP, one can overcome these difficultiesby measuring the corrosion potential in-situ without

destroying or even touching the surface of the polymercoating above the filament. Therefore, experimentalevidence can be provided for the long proposed, butnot yet proven theory of the electrochemical mechanismof FFC by measuring the potential distribution.

The different mechanism of FFC (anodic undermin-ing instead of cathodic delamination) is reflected inrather different electrode potentials around thefilaments head. Whereas for cathodic delamination thedelamination front is positively polarised with respectto the already delaminated zone the head of the filiform

Fig. 24. Photograph (a) and corrosion potential distribution (b) of a FFC sample (AA2024-T3). The potential scale is 300 mV fromblack (low potential) to white (hig potential) (Reprinted with permission from [69]. 2000 Elsevier Science).


shows a negative potential with respect to the tail (Fig.24) [69]. Therefore, the head can be identified as thelocal anode and the local cathode is situated behind theanode within the tail. Nisancioglu [58] was the first whoconsidered the role of the intermetallic particles as localcathodes during FFC of Al alloys.

The expansion of the disbonded zone is therefore notcaused by a chemical destruction of the organic phaseby the intermediates of the reaction products of theoxygen reduction but by a non-electrochemical attack.It is believed that the reason for delamination is thepressure of the highly concentrated electrolyte withinthe filiform head which is separated from the tail by adense membrane. This pressure causes a mechanicaldisbonding of the interface as it was proposed by vander Berg et al. [65] for iron FFC, and therefore allowsthe electrochemical reaction front to jump from onereactive site to the next one without the need of anyelectrochemical activity in between.

Scheck [64] reported a correlation between the extentof FFC and the glass transition temperature (Tg) of thepolymer coating. He observed a sharp rise in the extentof FFC attack, if the storage temperature exceeded Tg,which provides further evidence for a purely mechanicaldeadhesion mechanism.

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