Corrosion Science 50 (2008) 1637–1643

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Corrosion Science

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Evaluation of the corrosion performance of coil-coated steel sheet as studiedby scanning electrochemical microscopy

R.M. Souto *, Y. González-García, S. GonzálezDepartment of Physical Chemistry, University of La Laguna, E-38207 La Laguna, Tenerife, Canary Islands, 38207 Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 July 2007Accepted 12 February 2008Available online 10 March 2008

Keywords:A. Organic coatingsA. Coil-coated steelB. SECMC. DegradationC. Blistering

0010-938X/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.02.019

* Corresponding author. Tel.: +34 922 3180 67; faxE-mail address: rsouto@ull.es (R.M. Souto).

This work focuses on the investigation of the very early stages of the degradation of coil-coated gal-vanized steel sheet through swelling and blistering during exposure to chloride-containing aqueoussolutions and the possible effect of the zinc-based metallic coating on the degradation at these veryearly stages. Three types of coil-coated steel whose difference was in the zinc-based metallic coating,namely galvanized, galfan and aluzinc, were considered. Scanning electrochemical microscopy (SECM)operating in the feedback mode was employed to image topographic changes when the samples wereleft at their spontaneous open circuit potential. Swelling of the coating and nucleation of blisters wereobserved for all the samples when they were exposed to naturally aerated 0.1 M KCl solution within24 h exposure. Conversely, featureless and flat surfaces were found when the samples were eitherexposed to 0.1 M K2SO4 or to 10 mM KCl. For chloride concentrations of 0.1 M and above the chlorideions were observed to promote coating degradation nearly immediately upon immersion in theelectrolyte.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Metallic materials are often protected against corrosion by coat-ing them with polymeric films which act as a physical barrier toseparate the metal from the aggressive agents present in the envi-ronment to which they are exposed. In this way, the metal/coatingsystem can benefit of the good mechanical properties of the metalsand metallic alloys, whereas gaining effective anticorrosion resis-tance at the same time. Corrosion in coated metals usually resultsfrom the combination of various processes, namely the existence ofdefects in the coating, the penetration of the corrosive agents fromedges, losses of adherence and the corrosive attack to the metal.These processes usually initiate from small defects present in thecoating, such as holes or flaws produced during the handling ofthe material. They may occur as the result of minor accidental im-pacts, or even during the process of manufacture itself.

Also defect-free coatings are known to deteriorate due to thepenetration of water and aggressive agents from the environment,leading to the onset of blistering and/or delamination of the paintfilm. Organic coatings generally exhibit a rather high permeabilityto water [1–3] and to oxygen [4,5] despite their resistance to theflow of ions [6]. The exact mechanism for the penetration of spe-cies from the electrolyte and the transport phenomena inside thepolymeric matrix have not been ascertained yet, and are the objectof many investigations at present.

ll rights reserved.

: +34 922 3180 30.

A new route for the investigation of these effects has been openedby the employment of scanning electrochemical techniques for suchstudy. Both the scanning reference electrode technique (SRET) [7–9]and the scanning vibrating electrode technique (SVET) [10–13] arebecoming increasingly employed to monitor potential distributionsand ionic current flows in the vicinity of defects in coating films, thusallowing for the advancement of the corrosion reaction of the under-lying metal substrate to be investigated. Despite their applicability,they bear the limitation that they can exclusively be employed whenthe underlying metal is already corroding as the result of its directexposure to the electrolytic environment through defects in thecoating. Thus, they cannot assist in the investigation of the earlystages in the degradation at defect-free coatings.

We have recently reported on the suitability of an alternatemicroelectrochemical technique to overcome these limitations.Scanning microelectrochemical microscopy (SECM) allows for elec-trochemical reactions to be monitored in situ when a polarizablemicroelectrode is rastered over the coated substrate at a very shortdistance [14]. In this way, changes occurring in the coating inducedby specific anions in the electrolyte could be monitored long beforedefects could be produced through the coating. The distinctive ef-fect of chloride ions to promote blister formation in coil coatingsteels was thus detected at a very early stage of the degradationprocess [14].

The present study is aimed at further exploring the behaviour ofcoil coating steels during exposure to electrolytic solutions usingscanning electrochemical microscopy. Surface mapping operatingin the feedback mode of the SECM gives detailed information, such

0 10 20 300.2

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1.0

I(L)

L

Fig. 1. Approach curve to a polyester-coated steel containing a galvanized layerduring its immersion in 0.1 M KCl solution. I = i/ilim is the dimensionless tip current,and L = d/a is the dimensionless distance between the sample and the tip. (h) Ex-perimental curve, and (––––) fitting with RG = 3.04. Tip potential: +0.5 V vs. Ag/AgCl/KCl (saturated) reference electrode.

Fig. 2. SECM images of polyester-coated steel containing a galvanized layer during its im(c) 9 h and (d) 24 h after immersion in the solution. The time origin corresponds to thedirections. Values of Z-axis: Current, nA. Scan rate: 50 lm s�1. Tip potential: +0.5 V vs. A

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as the nucleation and growth of microscopic blisters under thepolyester coating during its exposure to aqueous electrolytes atambient temperature. The effect of two relevant factors participat-ing in the system, namely the concentration of the aggressive ionand the composition of the metallic coating in the CCS, were thusinvestigated.

2. Experimental

The samples we used were coil-coated steel (CCS). They werecut from rolled panels to the desired size of 10 mm by 10 mm witha guillotine from the topcoat side downwards. The samples con-sisted of a 0.4 mm mild steel foil galvanized with a 20 lm thickmetallic coating. Next, a chromate layer was applied prior to paint-ing with a 25 lm polyester paint. This consisted of 5 lm thick pri-mer containing strontium chromate as inhibitor, and covered witha single 20 lm thick topcoat containing TiO2. Three differentmetallic coatings were considered in this work, which only variedin the composition of this layer:

� Galvanized: the metallic layer consisted of pure zinc.� Galfan: the metallic layer had a nominal composition of 95% zinc

and 5% aluminium.

mersion in 0.1 M KCl solution. The images were taken after (a) 16 min, (b) 75 min,initiation of the scan. The figures represent an area of 330 lm � 330 lm in X and Yg/AgCl/KCl (saturated) reference electrode. Tip-substrate distance: 15 lm.

R.M. Souto et al. / Corrosion Science 50 (2008) 1637–1643 1639

� Aluzinc: the metallic layer had a nominal composition of 55%zinc and 45% aluminium.

The testing electrolytes were made using 18 MXcm water (Milli-pore MilliQ* plus). Chemicals were all analytical grade quality.Solutions were prepared with a concentration of 0.1 M in one ofthe following salts: KCl and K2SO4. Additionally, some selectedexperiments were performed in a 0.01 M KCl solution. All experi-ments were performed at ambient temperature in a naturally aer-ated cell.

The sample was located at the bottom of a microflat cell [14],thus exposing the coated side upwards to the test solution. Plati-num microelectrodes (10 lm diameter) were employed. They wereprepared from 10 lm platinum wires sealed in glass. A videomicroscope was used to aid microelectrode positioning and samplelevelling using three thumbscrews arranged in a tripod configura-tion. An Ag/AgCl/saturated KCl reference electrode and a stainlesssteel wire used as counter electrode were also introduced in theelectrochemical microcell.

Fig. 3. SECM images of a polyester-coated steel containing a galfan layer during its imm(d) 24 h after immersion in solution. The figures represent 100 lm � 100 lm in X and Y dZ-axis: Current, nA. Scan rate: 20 lms�1. Tip potential: +0.5 V vs. Ag/AgCl/KCl (saturated

The SECM experiments were carried out using a CH900 scan-ning electrochemical microscope (CH Instruments, Austin, TX,USA), controlled with a personal computer. The electrochemicalinterface was a bipotentiostat, though the system was operatedin a three-electrode configuration since the coated sample was leftunbiased for the duration of the experiments. That is, the sampleswere at their corresponding open-circuit potential.

Ferrocene–methanol of concentration 0.5 mM was added to theelectrolyte solutions to act as electrochemical mediator at the tip.The oxidation of ferrocene–methanol at the microelectrode wasused to establish the height of the tip over the coated surface, andthen to image the surface in situ. The potential of the tip was heldat +0.5 V vs. the Ag/AgCl/saturated KCl reference electrode to enablethe diffusion-limited oxidation of ferrocene–methanol at the tip.

The sample to tip distance was set by slowly approaching thesurface and simultaneously recording the approach curve as themeasured current at the microelectrode vs. Z displacement. Aftermeasuring the approach curve, and taking Z = 0 the displacementat which a 75% reduction of the diffusion-limited current at the

ersion in 0.1 M KCl. The images were taken after (a) 30 min, (b) 66 min, (c) 9 h andirections. Given time indications were taken at the beginning of the scans. Values of) reference electrode. Tip-substrate distance: 15 lm.

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microelectrode was produced. The approach curves were modelledassuming that the reduction of the mediator at the surface was un-der kinetic control [15]. In this way, the true distance between thetip and the substrate was determined to be 5 lm. Next the micro-electrode was separated at a distance of 15 lm, which was taken asthe operating distance for SECM mapping. Images were acquiredby moving the microelectrode in a raster-type motion. Line scanswere made using a step/acquire scheme where the step size was1 lm. Line scans were acquired in the same scan direction. Thescan rate was 20 lms�1. The SECM images were drawn using thedimensionless current (I = i/ilim), where ilim is the limiting currentwhen the tip is far away from the surface. Analogously, the ap-proach curves were plotted using the dimensionless current (I)and the dimensionless distance between the tip and the surface(L = d/a), where a is the radius of the tip (12.5 lm), and d is the dis-tance between the tip and the surface of the sample.

3. Results and discussion

The experiments consisted in the detection of the changesoccurring in the topography of the CCS surface during its exposureto the test solution. The distance between the tip and the samplewas determined from the measured approach curve as depictedin Fig. 1. The data fitted well the theoretical model, and the tip-substrate distance could be determined accurately. Once the tip

Fig. 4. SECM images of polyester-coated steel containing an aluzinc layer during its immeand (d) 24h after immersion in the solution. The time origin corresponds to the initiationValues of Z-axis: Current, nA. Scan rate: 50 lms�1. Tip potential: +0.5 V vs. Ag/AgCl/KCl

was placed at 15 lm from the surface, it scanned an arbitrary areaof the sample for different exposure times. The changes in the cur-rent values measured at the microelectrode for the electrooxida-tion of ferrocene–methanol were related with the distancebetween the tip and the surface directly below it, thus respondingto topographic features of the coating film discovered as the micro-electrode passed above them.

3.1. Evolution of coil-coated samples during their exposure to 0.1 Mpotassium chloride solution

Three metal-coating systems were explored in this investiga-tion. The first material was the galvanized system, in which themetallic coating contains pure zinc. The other two systems con-tained aluminium alloyed with zinc, namely 95% Zn + 5% Al (galfansample), or 55% Zn + 45% Al (aluzinc sample). These three systemswere chosen as to cover a wide variety of compositions for themetallic coating in the coil coated steel.

Fig. 2 shows a sequence of SECM images recorded for the galva-nized sample exposed to the chloride-containing solution for 24 h.From the inspection of the map given in Fig. 2a, obtained after16 min immersion, it can be clearly seen that the coating alreadyexhibited a significant amount of swelling. Furthermore, a bigbulge can also be distinguished at the right of the image, whichseems to extend towards the centre of the scanned surface. The

rsion in 0.1 M KCl solution. The images were taken after (a) 18 min, (b) 2 h, (c) 10 hof the scan. The figures represent an area of 330 lm � 330 lm in X and Y directions.(saturated) reference electrode. Tip-substrate distance: 15 lm.

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next image displays the condition of the coating surface after abouthalf an hour exposure, and it depicts the bulge located now at thecentre of the scanned region (cf. Fig. 2b). Longer exposures resultedin a more homogeneous distribution of swelling, which occurredsimultaneously to a further growth of the bulge that remains atthe back of the graphs (Fig. 2c and d).

These results support that the presence of chloride ions in theelectrolyte gives rise to surface heterogeneities in the form ofbulges in the polyester-coated metal samples, and they show upshortly after immersion in the aqueous environment. The detectedprotrusions can be attributed to the development of microscopicblisters either at the primer/topcoat interface or under the coating.But these blisters are neither stable nor fixed in specific sites underthe coating at such early stages of coating degradation, thus theyseem not to have reached the irreversible stage where corrosionprocesses occur at the metal/coating interface.

The topographic changes that occurred on the surface of a galfansample when exposed to 0.1 M KCl were also monitored by SECM,and the sequence of maps shown in Fig. 3 was obtained. Though aflat and featureless image was recorded when the surface wasscanned within the first minutes of immersion (cf. Fig. 3a), signif-icant changes could already be observed in the following scan de-picted in Fig. 3b. Thus, it was possible to detect inhomogeneousswelling of the coating within the first hour of exposure, whichled to the development of one bulge that covers slightly less thanhalf of the surface rastered by the tip (see Fig. 3c). Though the

Fig. 5. Images generated by SECM of polyester-coated steel with a galfan layer immersedafter immersion in the solution. The time origin corresponds to the initiation of the scan.axis: Current, nA. Scan rate: 20 lms�1. Tip potential: +0.5 V vs. Ag/AgCl/KCl (saturated)

bulge remained apparently unchanged in the next image, whichwas obtained at the third hour of immersion, the following mapsclearly demonstrate that the bulge continued growing. Greaterportions of the rastered surface were affected by the growing bulgeas time elapsed. Nevertheless, swelling of the film does not occur ata given location, but rather seem to be shifting over the surfacewith time quite randomly. It is only after 9 h immersion that swell-ing seems to stop moving on the surface, which may be an indica-tion of the nucleation of a blister on the right of Fig. 3d. This featureis then observed to grow consistently for the remaining of theexperiment which lasted for 24 h.

Finally, the measurements performed on an aluzinc sample dur-ing its exposure to the chloride-containing solution during one dayare next displayed in Fig. 4. Similarly to what happened for bothgalvanized and galfan samples, the polyester coating applied onthe aluzinc sample was found to exhibit a certain amount of swell-ing already in the first image. This was recorded just a few minutesafter immersion in the aggressive environment (see Fig. 4a). After53 min the swelling became more evident, and small bulgesappearing at the left side of the scanned surface could be observed.A later scan showed that one of the protrusions spread along thesurface in the direction of the X-axis (cf. Fig. 4b). It is after 3 h thatthe bulges merged to form a bigger one which extended over halfof the scanned surface as it remained seen during the followinghours of immersion in the chloride containing solution almostwithout changes (Fig. 4c). At the end of the experiment, this

in 10 mM KCl solution. The images were taken at: (a) 6, (b) 48, (c) 90 and (d) 195 minThe figures represent an area of 100 lm � 100 lm in X and Y directions. Values of Z-reference electrode. Tip-substrate distance: 15 lm.

Fig. 6. SECM images of a galfan CCS immersed in 0.1 M K2SO4 solution for one day. The images were taken after: (a) 32 min, (b) 3 h, (c) 11 h and (d) 24 h of exposure at thesolution. The time origin corresponds to the initiation of the scan. The figures represent an area of 100 lm � 100 lm in X and Y directions. Values of Z-axis: Current, nA. Scanrate: 25 lms�1. Tip potential: +0.5 V vs. Ag/AgCl/KCl (saturated) reference electrode. Tip-substrate distance: 15 lm.

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protrusion was found to split into two smaller bulges after 24 hexposure (Fig. 4d).

From the foregoing it can be observed that galvanized, galfanand aluzinc samples show the same common trends when exposed

Fig. 7. SECM image of a galfan CCS immersed in 0.1 M K2SO4 solution for one day.The image was generated after recording the map depicted in Fig. 6d, and depictsanother portion of the exposed surface of the sample. The image represents 200lm � 200 lm in X and Y directions. Values of Z-axis: Current, nA. Scan rate: 25lms�1. Tip potential: +0.5 V vs. Ag/AgCl/KCl (saturated) reference electrode. Tip-substrate distance: 15 lm.

to 0.1 M KCl solution as discovered by using the SECM. This is anindication that the composition of the metallic coating of thepainted sample does not have a major effect on the early stagesof degradation of the coil-coated steels. It has been demonstratedthat both swelling of the coating and the formation of protrusionsoccurred even from the first minutes of exposure of the samples to0.1 M potassium chloride solution. These bulges can be relatedwith the initiation of blistering processes under the coating. Addi-tionally, these early bulges were found not to nucleate at fixedpositions in the coated sample, as it is clearly observable fromthe comparison of Figs. 2 and 4. In fact, the sites at which localaccumulation of electrolyte under the coating occurred were ob-served to shift location on the surface during the first 24 h ofimmersion in the chloride-containing solution.

3.2. Influence of the concentration of the aggressive environment onthe behaviour of polyester-coated coil coating steel. Evolution ofpolyester-coated CCS sample during immersion in 0.01 M KCl solution

The osmotic pressure of the electrolyte is regarded to deter-mine the extent of water permeability through the polymericmatrix of the coating. Consequently, the concentration of theaggressive environment is expected to play a major role in theinitiation of coating degradation processes. But it remains to beaddressed whether such effect can also be observed in the earlystages of the degradation processes as monitored by SECM. Thatis, whether the osmotic pressure affects the initiation of blisterformation under the coating.

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Next, a series of experiments were performed which consistedin monitoring the coating topography of the samples during theirexposure to a more diluted solution containing chloride ions,namely 0.01 M KCl. In this way, the sequence of images given inFig. 5 was obtained. It corresponds to a coil coating steel samplecontaining a galfan metallic coating exposed to the test solutionduring 3 h.

From the inspection of the maps it can be concluded that therastered surface remained flat and featureless during the wholeperiod, and there were no evidences of the development of singulartopographic features on the coating. These results are indeed verysurprising, because the same material developed swelling and blis-tering processes within the same interval of time when it was im-mersed in the more concentrated solution (compare Figs. 3 and 5).It can then be concluded that the concentration of the aggressivespecies (i.e., chloride ions) determines the onset of blistering. Thisis a new evidence supporting that the osmotic pressure of the elec-trolyte must be regarded as a key effect responsible for the accu-mulation of electrolyte under the organic film.

3.3. Evolution of coil-coated samples during their exposure to 0.1 Mpotassium sulphate solution

Coil coated samples exposed to 0.1 M potassium sulphate donot show any changes in the topography of the material duringthe first hour of immersion. That is, no bulges neither any singulartopographic features were observed to develop on the surface withtime, but the samples remained flat. This behaviour was indeedfound to persist when the exposure period extended over 24 h inthe same test solution. The polyester coating remained flat and fea-tureless throughout all the experiment. A sequence of images re-corded on a galfan sample by continuously scanning the samearea of the tested sample for one day is presented in Fig. 6. Thesame behaviour was also exhibited by the other coated materials,though they are not shown here for the sake of simplicity.

The featureless topography of the overall surface of the coatedsample exposed to the potassium sulphate solution was demon-strated by rastering the microelectrode over different zones ofthe sample. Fig. 7 shows an image of an area located far from thatsequentially monitored in the previous figure, and it was obtainedafter the specimen was immersed in the electrolyte for more than24 h.

It is clear from these results that the sulphate anion causes novisible damage to the coating after much longer immersion timeas compared to chloride ions. This implies that either sulphate ionsdo not permeate the coating at all, or that they cause no damage tothe interface when they do permeate.

4. Conclusions

� The scanning electrochemical microscope (SECM) is a powerfultechnique for the in situ investigation of both the transport pro-cesses that occur through the organic coatings, and the conse-quent corrosion reactions at the metal/electrolyte interfaceduring their exposure to aqueous electrolytes.

� Inhomogeneous swelling of polyester-coated samples giving riseto small bulges was observed at very short immersion times in0.1 M KCl aqueous solution. At this stage it is not possible to

establish whether the reported local swelling is due to blisterformation at either the primer/topcoat interface or at themetal/polymer interface. The latest would be the early stagesof degradation processes under the coating, leading to themicroscopic nucleation of blisters. Nevertheless, the experimen-tal observation that chloride ions have an effect and not sul-phate anions seems to suggest that corrosion may play a role.Further research on this will be carried at our laboratory.

� Inhomogeneous swelling occurs during exposure of the coatedmetals to the chloride-containing solution regardless the com-position of their metallic coating. Therefore, the nature of thislayer does not significantly influence the processes occurringduring the early stages of polymer-coating degradation.

� The surface of the coated metals remains flat and there isno indication of bulge formation when the samples are ex-posed to either a dilute chloride solution, or to a chloride-freesolution.

� Local swelling results from the transport of chloride anionsthrough the matrix, which already occurs at the beginning ofimmersion. Therefore, the development of blisters below thecoating is caused by the permeation through the polymericmatrix of various species present in the electrolytic phase,namely water and chloride ions, and their subsequent accumu-lation at the coating-metal interface.

� The inhomogeneous swelling detected by SECM do not mergeand get located in fixed positions at these early stages of coatingdegradation. Though blistering nucleation is an early stage ofcoating breakdown, not every nucleated bulge is expected tolead to macroscopic failure of the coating at later stages.

Acknowledgements

We gratefully acknowledge the financial support of this workby the Ministerio de Educación y Ciencia (Madrid, Spain) in theframework of Project CTQ2005-06446/BQU, and by the Gobiernode Canarias (Project No. PI2004/075). The award by the Universityof La Laguna of a research fellowship to Y.G.-G. is gratefullyacknowledged.

References

[1] C.H. Hare, J. Prot. Coat. Linings 59 (February) (1989).[2] R.A. Dickie, A.G. Smith, Chem. Technol. 31 (January) (1980).[3] T. Nguyen, D. Bentz, E. Byrd, J. Coat. Technol. 66 (1994) 39.[4] R. Feser, M. Stratmann, Werkst. Korros. 42 (1991) 187.[5] W.W. Kittelberger, A.C. Elm, Ind. Eng. Chem. 38 (1946) 695.[6] J.E.O. Mayne, in: L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, third

ed., vol. 2, Butterworths-Heinemann, Oxford, 1994.[7] W. Fürbeth, M. Stratmann, Prog. Org. Coat. 39 (2000) 23.[8] G. Williams, N. McMurray, J. Electrochem. Soc. 148 (2001) 377.[9] M. Rohwerder, E. Hornung, M. Stratmann, Electrochim. Acta 48 (2003)

1235.[10] J.H.W. de Wit, Electrochim. Acta 49 (2004) 2841.[11] A.P. Nazarov, D. Thierry, Electrochim. Acta 49 (2004) 2955.[12] B. Reddy, M.J. Doherty, J.M. Sykes, Electrochim. Acta 49 (2004) 2965.[13] L. Changjian, Z. Xiangdong, D. Ronggui, F. Zude, in: Proceedings of the 7th

International Symposium on Electrochemical Methods in Corrosion Research,Paper 178, Budapest (2000).

[14] R.M. Souto, Y. González-García, S. González, G.T. Burstein, Corros. Sci. 46(2004) 2621.

[15] A.J. Bard, F.-R. Fan, M.V. Mirkin, in: A.J. Bard, M.V. Mirkin (Eds.), ScanningElectrochemical Microscopy, Marcel Dekker, New York, 2001, p. 243.