ORIGINAL PAPER

Investigation of coating delamination on steels by surfacetopography and Volta potential difference

Tongyan Pan & Zhaoyang Wang

Received: 28 May 2012 /Revised: 17 November 2012 /Accepted: 9 December 2012 /Published online: 22 December 2012# Springer-Verlag Berlin Heidelberg 2012

Abstract Corrosion-induced delamination of an epoxy coat-ing on the AISI/SAE 1045 carbon steel was studied under ahumid atmospheric condition (temperature of 25 °C, onestandard atmospheric pressure, and relative humidity of90 %) by the technique of scanning Kelvin probe force mi-croscopy (SKPFM). Surface-polished 1045 samples were firstcold coatedwith the epoxy and then subject to the atmosphericcorrosion under the humid atmospheric condition. At speci-fied time intervals, surface Volta potential of the samples wasmeasured using the SKPFM over the dry surface of epoxycoating. The map of Volta potentials demonstrated high con-trasts among three characteristic zones: intact steel-epoxyinterface, delaminated interface, and interface with activecorrosion, which based on a rigorous calibration procedurewere then linked to the actual corrosion potential of the steel(measured using a potentiostat w.r.t. a saturated calomel elec-trode). The SKPFMwas found to be able to provide a mean ofdirect and nondestructive detection of early active corrosionand coating delamination of steels at a submicroscopic reso-lution, which outperformed the conventional electrochemicaltechniques for such purposes.

Keywords Carbon steel . Epoxy coating . Coatingdelamination . Scanning Kelvin probe force microscopy

Introduction

The atmospheric corrosion rate of carbon steels, usually asstructural steels, can be significantly reduced by organic or

polymer coatings, that is, by paint coatings or a modificationof steel surface with a thin-layer organic molecules. Exam-ples are the epoxies, lacquers, and other organic coatings[1–3] on general steel infrastructure, underground pipelines,off-shore platforms, and general ground vehicles working inatmospheric or marine conditions. Regarding the protectionmechanisms of organic/polymeric coatings, usually it is notthe direct barrier effect on the diffusion process that givesrise to the corrosion stability but the specific electrochemicalproperties of the metal-coating interface; in particular, theformation of an extended diffuse double layer [4, 5]. How-ever, in presence of defects, e.g., pores and pinholes thatmay penetrate through the coating, the diffusion barrier islowered and the delamination rate of coating at the defect isdetermined by the formation of galvanic cells [6]. In cases ofsteels, sometimes the local anode of the cell is the defect andthe local cathode is given by the delamination frontier. Thisprocess is known as cathodic delamination. In other cases,the delamination frontier is the local anode in the galvaniccell, which is known as anodic delamination, often occur-ring as filiform corrosion that also is common in aluminumand magnesium alloys in addition to steels, when coatedwith organic, inorganic, or metallic materials [6].

The presence of coating and the small size of corrosionsites, for example the 0.05- to 0.3-mm wide filaments infiliform corrosion of steels, have excluded the use of con-ventional electrochemical techniques in studying coatingdelamination. One example in this regard is the high resis-tance of organic coatings that can nullify the function ofelectrochemical impedance spectroscopy, which often failsto identify corrosion sites under coatings. Indeed, few con-ventional techniques can reach a spatial resolution to imageareas of 100 μm. The scanning Kelvin probe (SKP) hasbeen used for measuring the Volta potentials of various bareor coated metal surfaces. However, although spatial resolu-tion of SKP can be improved through tip size reduction and

T. Pan (*) : Z. WangThe Catholic University of America, 620 Michigan Avenue, N.E,Washington, DC 20064, USAe-mail: pan@cua.edu

Z. Wange-mail: wangz@cua.edu

J Solid State Electrochem (2013) 17:1109–1115DOI 10.1007/s10008-012-1972-4

better distance control, high spatial mapping has not beenfully achieved yet by SKP. Hence, in the state of knowledgea clear microscopic picture of local reaction sites, such asthe filament head or vicinity of the head in filiform corro-sion, is still missing.

One early experiment with SKP was carried out byRohwerder et al. [7] who studied the delamination ofpolymer-coated zinc and steel surfaces under cyclic humid-ity conditions. This experiment gives the first indicators onthe complex delamination mechanism, and demonstrated theimportance of anodic reactions on the stability of the inter-face. Williams and McMurray [8] used the SKP technique tostudy the influence of chromate (CrO4

2−) on the kinetics andmechanism of delamination processes of polyvinyl butyralcoatings on hot dip galvanized steel. With the help of SKP,Williams et al. [9] showed that the outward migration ofhydroxide ions in the electric field alongside the interfacestopped inward migration of cerium cations from the defect,while the cerium cations from the dispersed silica and ben-tonite in the PVB coating reached the active sites andeffectively inhibited coating delamination. Williams et al.also successfully applied SKP in the study of filiform cor-rosion [10].

The recent advancements in the technology of scanningKelvin probe (SPM) based on the technique of atomic forcemicroscopy (AFM), a powerful tool for characterizing sur-faces with very high spatial resolution, have led to theinvention of scanning Kelvin probe microscopy (SKPFM)that has paved a new road for in-situ exploration of the sub-microscopic corrosion phenomena at a resolution that hasnot been achieved before [11–14]. SKPFM was successfullyused to produce maps of the Volta potential distributionacross surfaces of AA2024-T3 in air [15–18], which pro-vided clear evidence regarding shape, position, composi-tional inhomogeneities, as well as the local practicalnobility of intermetallic compounds at sub-micron resolu-tion. Potentials associated with cathodic and anodic reac-tions in filiform corrosion are expected to vary by several100 mV and could be sensed by SKPFM with very highlateral resolution. Senöz et al recently successfully used theSKPFM to monitor the propagation filament on a HMDSOfilm coated aluminum alloy subject to filiform corrosion at asatisfactory resolution to provide a more detailed map oflocal oxygen concentration cells [16]. The work principlesof SKPFM can be found from pioneering work conducted inliterature [19, 20].

Surface corrosion on steels beneath general organic orpolymeric coatings routinely requires the removal of thepaint and subsequent remediation of the corroded area.Frequently this process is time consuming, laborious, andproduces large quantities of waste material that must bedisposed of in an appropriate manner. Therefore, early de-tection of surface corrosion would significantly reduce the

required amount of coating to be removed and facilitate amore efficient maintenance schedule. Nevertheless, SKPFMmeasurement of Volta potential must be performed withcautions as the potential can be easily disturbed by factorsother than local electrochemistry. Such factors may includethe structure and composition of passive oxidative film thatcan be instantly formed in air, and certain charged speciesadsorbed on the sample surface. As such, a careful correla-tion much be established between the Volta potential and theactual corrosion potential with all the aforementioned dis-turbances effectively controlled, which today can be readilyachieved in an environment chamber such as a gas purifica-tion glovebox [15].

Material and methods

Carbon steel AISI/SAE 1045 is a structural quality carbonsteel produced to chemical composition requirements detailedin Table 1. AISI/SAE 1045 is composed of essentially acombination of two elements, iron and carbon, with otherelements present in quantities too small to affect the proper-ties. The Steel AISI/SAE 1045 contains a relatively highcontent of carbon but low contents in chromium that helpsminimize the general corrosion. Sample coupons of one cen-timeter in length and width, 0.3 cm thickness were cut from a1045 steel rod, which were then cold-mounted in epoxy resin.The coating was left in air for a week for curing, and then thesample was mechanically ground and polished to have one topsurface coated with epoxy and other with bare steel surface.

In this study, the coating epoxy was prepared by mixingprimary resin: EPON 1001-X-75 with a second agent: EPI-CURE 3115-X-70 in a 2:1 ratio. A coating of 300 nm inthickness was deposited on the sample using a rotating spin-coater, from which the coating thickness can be controlled byrotating speed. Before testing the samples were cleaned ultra-sonically with ethyl alcohol and dried in air 10 h. The coatingdelamination was initiated by immersing the coated sample ina 0.5 MNaCl solution (open to air) until rust was observed onthe bare edges of the samples. The sample was then kept in ahumidity chamber, where the relative humidity (RH) wasmaintained at 90 %. The SKPFM analyses were started whenfilaments were observed at the edge of the coating.

The epoxy-coated steel samples were tested by a BrukerSKPFM. The probe maps the surface Volta potential above

Table 1 Major alloying components of carbon steel AISI/SAE 1045

AISI/SAE designation Composition of alloying chemicals (wt.%)

C Mn P S

1045 0.45 0.75 0.03 0.04

1110 J Solid State Electrochem (2013) 17:1109–1115

the epoxy coating on the sample. The SKPFM probe used inthis study had a conductive Pt/Ir-coated silicon tip with aresistivity of 0.01–0.025 Ωcm. The cantilever had a reso-nant frequency of 75 kHz, and the tips had an apex radius of25 nm. These geometric characteristics were selected toensure a good resolution in potential measurement. TheVolta potential was measured at a randomly selected loca-tion (100×100 μm in size) on sample surface.

The SKPFM used in this study is the latest version of theSPM force microscope (Multimode 8) made by Bruker. ThisSKPFM is particularly suitable for measuring high-resolution topography and Volta potential differences onconductive and semi-conductive surfaces, featuring a 100×100-μm scan field, a 1×1-Å lateral resolution, a potentialsensitivity of 1 eV, an exclusive peak force tapping technol-ogy that enables low tip-sample interaction forces, and alarge range of working temperatures from −35 to 250 °C.Specifically, the SKPFM used has a 2-nm probe tip size,which can readily reach the resolution of one angstrom orthe atomic resolution to clearly see the atoms of a samplesurface. At such a high lateral resolution, local groups ofatoms with higher Volta potential difference can be clearlydistinguished from others. More useful in corrosion science,a more accurately measured Volta potential map can be usedto pinpoint the critical potential locations where microcellsmight form in the complex metallic matrices.

The technique of SKPFM was first introduced by Non-nemacher et al. [11, 12] for microelectronic applications,and was applied to corrosion studies in the late 1990s byFrankel et al. [17, 18]. The AFM mode endows SKPFM thecapability to obtain quantitative information of the topogra-phy of a sample, which in corrosion studies can be used tomonitor the in-situ corrosion rate and coating delamination.The SKPFM has provided sufficient resolution for resolvingintermetallic particles on aluminum, iron or magnesiumalloys, a topic of significant and practical relevance[13–25]. In the state of knowledge related to corrosionscience, SKPFM is becoming more known for interpretingVolta potential contrasts as a measure for the formation ofgalvanic elements during corrosion, based on the originalwork by Schmutz and Frankel [17, 18]. In their work, a highcorrelation between corrosion potential of metals and Voltapotential differences measured by SKPFM was found; alsothe Volta potential differences measured on aluminum alloysurfaces seemed to correlate well with actual corrosionbehavior. These findings have inspired others, includingthe PI, to follow and in the recent years a number of metallicmaterials including magnesium, aluminum and iron alloyhave been characterized by SKPFM and relevance of thefindings for assessing the possible corrosion behavior wasclaimed [19–22]. Based on a correlation between measuredVolta potential difference across a surface and relative prac-tical nobility, SKPFM is useful for understanding the

interaction between different sub-nanoscale phases of aheterogeneous sample, such as the different types of inter-metallic oxides in the matrix of steels and aluminum alloys[17–22]. The shape, position, compositional inhomogenei-ties, and local nobility of these entities can be accuratelydetermined at the submicroscales.

When it was time to take the readings of surface Voltapotential, the sample was taken out of the NaCl solution,and surface dried for SKPFM analysis. To make theSKPFM-based technology practically useful, the measuredVolta potential need to be correlated to the corrosion poten-tial that usually is measured w.r.t. a standard electrode suchas the saturated calomel electrode (SCE). Since the applica-tion of SCE is limited only to macroscopic measurements(on objects usually of above-1-cm sizes), a commercialhigh-purity electrolytic-type iron (99.95 % minimum purity)was adopted for the purpose of correlation. SKPFM mea-sured surface potentials on a high-purity iron sample weresupposed to give small variations across a relatively smallsample surface (e.g., 100×100 μm) that contains no or littleheterogeneities and has a rather uniform surface passivationcondition. The potential measured at different locationsshould be close to the average potential or the corrosionpotential of the domain measured w.r.t. SCE. NaCl solutionsof nine different concentrations were prepared, from 0.1 to0.9 M and incremented in 0.1 M, with one iron sampletested in each solution using both SKPFM and SCE. Thesurface Volta potential map of the steel sample after 30-minute immersion in the NaCl solution was made first forthe 0.5 M NaCl solution.

The surface potential and corrosion potential of the othereight groups of iron samples, each with three duplicates andin one NaCl concentration, were also measured using theSKPFM and SCE respectively. The average SKPFM resultof each group was first converted to a relative surfacepotential (RSP) by subtracting the average potentialcorresponding to the 0.5 M NaCl solution off the averageSKPFM reading of the group. The high-purity iron sampleshows a relatively small range of surface potential, i.e., from−6 to +5 mV in terms of RSP (see Fig. 1a). The corrosionpotential of the same iron sample was measured to be−229 mV w.r.t. the SCE electrode.

To establish the correlation between the SKPFM mea-sured surface electrical potential (in terms of RSP) and thecorrosion potential measured w.r.t. SCE, the nine RSPs werethen correlated to the SCE results as shown in Fig. 1b. Adefinite linear relationship with a high correlation coeffi-cient can be formulated between the RSPs and the SCEcorrosion potentials. Figure 1b suggests the high sensitivityand precision of using SKPFM measured surface potentialfor evaluating the corrosion behavior of steels. In this study,the Volta potential measured on coated sample will bereported on the RSP scale.

J Solid State Electrochem (2013) 17:1109–1115 1111

Results and discussions

The state of knowledge in corrosion-induced coating delam-ination of structural steels misses a physicochemical mecha-nism at the submicroscale. The SKPFM measured surfacetopology and Volta electrical potential is expected to shedlight on the electromotive mechanisms for the formation ofmicroscopic corrosion cells at the steel-coating interface. The30-min surface topology and Volta potential measured abovethe coated steel sample under the atmospheric condition(RH=90 %) are shown in Fig. 2, in which Fig. 2a clearlyshows a swollen region (being lighter in color) initiated at theleft-hand side that has a height above the other regions.Figure 2b shows the surface Volta potential of the samesample as scanned in Fig. 2a. The RSP at the swollen regionis significantly lower than other regions (being darker incolor). The RSP and corrosion potential of the coated steelcan be easily estimated in Fig. 2b with respect to Fig. 1.

The surface topology and Volta potential was measuredafter 2 h over the same coated steel sample exposed to thehumid atmospheric condition (RH=90%), as shown in Fig. 3.Figure 3a shows a trend of growth in the lighter-coloredregion, where the lighter color indicates a level of altitudehigher than the rest region of the same sample as measured by

the SKPFM in the AFMmode. Such higher topography mightbe caused by the corrosion products formed underneath thecoating. The worm-like region has a width about 5 μm, andmight be caused by an on-going filiform corrosion at the steel-epoxy interface. Figure 3b shows the surface Volta potential ofthe same sample scanned in Fig. 3a. The lower (darker) RSP atthe right-hand part of the scanned domain than the left-handpart (where the corrosion was initiated) clearly demonstratedthe direction of the active corrosion, that is, the head of afilament in filiform corrosion. As the corrosion moves to theinner part of the scanned domain, the tail of the filament tendsto delaminate off the steel substrate and demonstrates a higherVolta potential (lighter color). The filament tail moves along apath that goes in the same direction of corrosion occurrence.This tail part of the filament may serve as the cathode of theactive corrosion. However, the surface Volta potentials ofregions not showing delamination remained about the samelevel of Volta potential as that shown in Fig. 2b. Also, the RSPand corrosion potential of the coated steel can be estimated inFig. 3b with respect to Fig. 1.

Figure 4 shows the surface topology and Volta potentialmeasured by the SKPFM over the same coated steel sampleafter 10-h exposure to the humid atmospheric condition(RH=90 %). Figure 4a shows the continued growth of the

• -1

• -5

• -6

• +5

• +3

• -2

a b

• -3• +3

• +2

• +3

• +4

• -2• +4

• -2

• +2

• +2

• 0

• +1

• -4

• -1

Fig. 1 a Thirty-minute RSPmap (in millivolts) of the high-purity iron sample in 0.5 MNaCl solution (scope, 100×100 μm); b thirty-minute RSPvs. SCE measured corrosionpotential at different NaClconcentrations

+ 5 mV

-25 mV

-85 mV

-55 mV

30 nma b

20 nm

10 nm

0 nm

Fig. 2 Thirty-minute surface ofAISI/SAE 1045 steel scannedby SKPFM (RH=90 %): asurface topology and b surfaceVolta potential

1112 J Solid State Electrochem (2013) 17:1109–1115

filament in the filiform corrosion, with a slightly varyingwidth, towards the inner part of the SKPFM scanned do-main. Figure 4b gives the surface Volta potential of the samesample scanned in Fig. 4a. The lower RSP at the right-handpart of the swollen region than the left-hand part demon-strated the head of a major filament in filiform corrosion.Moreover, as the filiform corrosion moves to the inner partof the scanned domain, delamination at the tail portion ofthe major filament became larger, demonstrated by a higherVolta potential (lighter color) in the tail locations. It isequally noteworthy that morphology of the delaminationmay also occur in directions that deviate off the mainstreamheading direction of the filament shown in Figs. 3a and 4a,as shown by the enlarged area in Figs. 3b and 4b thatdemonstrates higher Volta potential in the tail part of thefilament. The surface Volta potentials of regions withoutdelamination remained about the same level of Volta poten-tial as that shown in Fig. 3b, and the RSP and corrosionpotential of the coated steel can be estimated in Fig. 4b withrespect to Fig. 1.

As to the mechanism of coating delamination on steels,the studied AISI/SAE 1045 steel under an epoxy coatingrevealed a clear separation of local active anodes in the headand cathodes at tail locations at a higher potential. Theanodic activity however is not restricted in the mainstreamheading direction of the filament but also exists at a fewother boundary locations of the darker-colored anodic area(see Fig. 3b), which is in good agreement to the existingstudies by other researchers [21, 22, 26]. The thin darker

lines at the borders of the tail however could be artifactsproduced in SKPFM scanning. In general, the catholic ac-tivity occurs in the close vicinity of anodes and is responsi-ble for coating delamination. The morphology of thefilaments is worm like, and the anodes and cathodes werewell distinguished in Volta potential. The interconnectedvoids or channels under delamination along the filament,as a result of a loss of adherence of the coating, are potentialsupplier pipeline that provide access of oxygen to the anodichead. This proposed mechanism is supported by viewing theapparently un-delaminated region with respect to thedelaminated region as well as considering the rather thickepoxy coating; oxygen supply from such regions reasonablyis negligible. It seems that oxygen transported via suchvoids/channels was responsible for the coating delamina-tion, as no apparent damage were observed elsewhere on thecoated surface based on the topology and Volta potentialmaps. The Volta potential of the steel surface just beneaththe epoxy coating also may be dependent on the concentra-tion of the chloride ion in the NaCl solution (as shown inFigs. 2b, 3b, and 4b) as well as the oxygen content, becausethe chloride ion can possibly migrate into and be concen-trated in the head of the filament through the interconnectedvoids or channels even if the concentration of the chlorideion is constant in the bulk solution.

Regarding the novelty of this work with respect to theprevious works, the SKPFM analysis was conducted usingdifferent procedures and under a different atmospheric con-dition. Also the Volta potential was measured at different

+ 5 mV

-25 mV

-85 mV

-55 mV

30 nma b

20 nm

10 nm

0 nm

Fig. 3 Two-hour surface ofAISI/SAE 1045 steel scannedby SKPFM (RH=90 %): asurface topology and b surfaceVolta potential

a b+ 5 mV

-25 mV

-85 mV

-55 mV

30 nm

20 nm

10 nm

0 nm

Fig. 4 Ten-hour surface ofAISI/SAE 1045 steel scannedby SKPFM (RH=90 %): asurface topology and b surfaceVolta potential

J Solid State Electrochem (2013) 17:1109–1115 1113

times from all existing work. Specifically, an epoxy layerwas coated to the surface of a carbon steel. The coatedsamples were first immersed in a 0.5 M NaCl solution (opento air) until rust was observed on the bare edges of thesamples, and then studied under a humid atmospheric con-dition (temperature of 25 °C, one standard atmosphericpressure, and relative humidity of 90 %) by the techniqueof SKPFM. Surface-polished 1045 samples were first coldcoated with epoxy and then subject to the corrosion. Atspecified time intervals (30 min, 2 h, and 10 h), surfaceVolta potential of the samples was measured using theSKPFM over the dry surface of epoxy coating.

Metallurgically, any specific type of steel is an alloy ofiron and certain alloying elements. Carbon is the mostimportant alloying element in steels, and the other alloyingelements include metals such as chromium, nickel, manga-nese, vanadium and tungsten, and nonmetallic elements,such as nitrogen, sulfur, phosphorous, and silicon. Steelsin their service conditions each have a unique metallurgicalstructure that consists of multiple crystalline grains of ironembedded with various crystalline defects such as vacan-cies, impurities and dislocations inside the grains and/or ontheir boundaries. With a different chemical composition,internal structure, grain size, and morphology that dependon the cooling temperature and rate of cooling, each metal-lurgical phase possesses a different set of electrochemicalproperties such as the resistance to oxidation.

Although SKPFM as a promising technique for studyingthe corrosion phenomena and coating durability has beenexplored on different types of metallic materials, such workhowever has been limited to a few groups, mainly theFrankel’s and Stratmann’s. As such, the authors of the studyhence wanted to test the reproducibility of SKPFM forstudying the corrosion phenomena and coating durability(by using different test conditions, different atmosphericcondition of the filiform corrosion, and different time forthe Volta potential measurement, etc.). More importantly,the authors wanted to test the validity of its use at a higherresolution to study the role of crystalline defects. If such useis feasible, the very fundamental mechanisms of corrosionsuch as the role of crystalline defects in the initiation ofcoating delamination can be readily studies. For a briefsummary, the most noticeable differences of this study in-clude (1) The calibration of RSP vs. SCE using differentconcentration of NaCl solution, and (2) the in situ monitor-ing of the propagation of the filiform corrosion of the epoxy-coated 1045 steel by measuring the Volta potential at differ-ent time 30 min, 2 h, and 10 h in sequence, which are shorteras compared with the work by Leblanc and Frankel [21]. (3)Although the SKPFM was reported in a similar in situapplication by Senoz and Turcu [22], the application wasmade on aluminum alloy instead of steel as was used in thisstudy.

In comparison, the previous work was conducted byexposing the coated sample to fumes of concentrated HClfor 10 min, and then kept in a humidity chamber where aconstant relative humidity of 93 % was maintained [21]. Assuch the finding from this study can supplement informationfor verifying the claimed findings in previous work. Thedifferent experimental conditions of SKPFM in this workfrom that of existing work was intended to provide moreinformation and clearer visualization of the phenomenon offiliform corrosion on epoxy-coated steel which was notsufficiently done in previous works [21].

This study acquired a surface topology map and a Voltapotential map of coated steel surface at a higher lateralresolution (1 nm), which was an obvious improvement overthe existing work in the study of the filiform corrosion ofsteels. The map of Volta potentials measured in the studydemonstrated higher contrasts among three characteristiczones: intact steel-epoxy interface, delaminated interface,and interface with active corrosion, which based on a rigor-ous calibration procedure were then linked to the actualcorrosion potential of the steel. Based on such a high reso-lution, SKPFM was found to be able to offer a high-accuracy prediction of delamination or early filiform corro-sion of steels or other metals and alloys. In addition, theVolta potential by SKPFM points to a promising directionfor studying the fundamental mechanisms of general corro-sion related problems.

Moreover, in this work a large number of NaCl solutions,i.e., nine different concentrations, were prepared to conductthe study of corrosion-induced delamination of an epoxycoating on a steel. The average SKPFM result of each groupwas first converted to a RSP which is useful to indicate oranalyze the evolution of filiform corrosion. In this study, thesurface potential and corrosion potential of a total of ninegroups of iron samples were measured. With such a data-base, the correlation between the SKPFM measured surfaceelectrical potential and the corrosion potential measuredw.r.t. SCE was established at a higher reliability, whichwas intended by the author to make the SKPFM techniquemore applicable in practice.

Summary and conclusions

Towards a fundamental understanding of coating delamina-tion of the steel AISI/SAE 1045, the SKPFM was used todirectly capture the Volta potential differences across thesurface of a coated area. The surface potential of the steelsamples was measured in the humid atmospheric condition,and the delamination behavior of the coated steel was stud-ied by comparing the surface Volta potential under threedifferent zones: intact steel-epoxy interface, delaminatedinterface, and interface with active corrosion, which based

1114 J Solid State Electrochem (2013) 17:1109–1115

on a rigorous calibration procedure. Based on the analysis ofthe results from this experimental study, a series of findingsare summarized as follows.

1. SKPFM is capable of providing a surface Volta potentialmap of coated steel surface at a sufficiently high spatialresolution and sensitivity to electrical potential, whichoutperforms the existing conventional electrochemicaltechniques in studying the phenomena of coatingdelamination.

2. The surface Volta potential measured by SKPFM oncoating can be linearly correlated to the actual electrodeor corrosion potential on steel substrate, and thereforemay be used as a high-accuracy indicator to predictdelamination or early filiform corrosion of steels orother metals and alloys.

3. This SKPFM based study proves the previouslyreported observations that anodic activity is not restrict-ed to the mainstream direction of the filament growthbut also exists at other locations on the boundaries ofthe large anodic area, and that interconnected voids orchannels in filament tail portion might be the pipeline tosupply oxygen to the anodic head under rather thickcoatings (≥300 nm epoxy).

4. As today’s AFM technology can easily reach theatomic-scale resolution, the AFM-based SKPFM pointsto a promising direction for studying the fundamentalmechanisms of general corrosion related problems.

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