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rotective coatings are ap-plied to many industrial

steel structures for corrosion control.Polymeric coatings act as a barrier,separating the metal structure from acorrosive service environment. Theyreduce the access of corrosive speciesto the metal substrate and interruptthe flow of electric current requiredfor the steel corrosion processes. Akey attribute in the performance ofprotective coatings is, therefore, a lowpermeability to ions, water, gases, andother corrosive species of the serviceenvironment.

EIS is a technique well suited forevaluating coating permeability orbarrier properties based on the electri-cal resistance of the coating. The im-pedance of a coating is related to thenature of the polymer, its density, andits fillers. EIS has been used widely inthe laboratory for determining coatingperformance and for obtaining quanti-tative kinetic and mechanistic infor-mation on coating deterioration.1-14 The impedance of a coating is observedto decrease as a function of time of exposure to a corro-sive environment. The decrease in impedance is observedto be related to the loss of barrier properties and the on-set of under-film corrosion.5,6 In laboratory studies ofvarious generic types of coatings, the relative perfor-mance of different coatings could be determined well inadvance of the visible effects of deterioration; these stud-ies also showed a good relationship between coating im-pedance and coating field performance.8-13 A review ofthe application of EIS in the evaluation of coatings has

been published recently.14-16

In recent years, EIS has beenadapted to evaluate coatings that arein service on industrial structuresand equipment. Field measurementshave been made on car paint on sev-en-year-old automobiles and coatingson field structures;17 marine coatingson underwater structures;18 alkydsand chlorinated rubber coatings onsteel structures;19 ballast tank coat-ings inside U.S. Naval ships;20 epoxytank linings in oil production equip-ment and phenolic coatings in railcars;21 fusion-bonded epoxy (FBE)coatings on transmission pipelines;22

epoxy coatings systems on highwaybridges;23 and a variety of industrialstructures including protective coat-ings on aircraft.24

The objectives of this article areto describe the theory and methodol-ogy of the EIS technique, to presentan example of the use of EIS in a lab-oratory coating evaluation program,and to review recent progress in field

evaluation of coatings in industrial service.

Basis of EIS MeasurementsDefinition of ImpedanceThe protectiveness of a polymeric coating is related to itselectrical resistance. That is, protection increases as elec-trical resistance increases, where an electrical resistanceof at least 106 to 107 Ω cm2 is required for reasonable cor-rosion protection1,25 (Ω = ohm). Modern high-buildpolymeric coatings can have electrical resistances of 1011

Ω cm2 or greater.

EIS:Electrochemical

ImpedanceSpectroscopy

A Tool To Predict Remaining Coating Life?By Linda G.S. Gray, KTA-Tator (Canada), Inc., Edmonton, Alberta, Canada, and

Bernard R. Appleman, KTA-Tator Inc., Pittsburgh, Pennsylvania, USA

P

Fig. 1: Top: EIS measurement system showinginstrumentation and attached cell

Bottom: Two attached cells on test panel(Gamry Instruments)

The resistance across a polymeric coating film is mea-sured using DC electrical methods. From Ohms law, resis-tance R is defined as

R = V/Iwhere V is the applied DC voltage and I is the measuredDC current.The impedance across a coating film is measured usingAC electrical methods. The analogous equation in ACelectricity for impedance, Z, is

Z = Vac/Iacwhere Vac is the applied AC voltage and Iac is the mea-sured AC current.

Polymeric coatings and electrochemical corrosionprocesses both have an inherent resistance and capaci-tance character. AC methods can identify and quantita-tively measure the individual resistance and capacitanceelements, as described below in the discussion of equiva-lent circuits. DC methods measure the net resistance onlyof a coating. Therefore, AC electrical techniques normallyprovide considerably more information on coatings andunder-film corrosion processes than DC methods.

In EIS analysis, the coating film and under-film corro-sion processes are regarded as a system of resistors andcapacitors.2,5,6 The magnitude of the coating impedancevaries with frequency, depending upon the respective re-sistances and capacitances. Capacitors cause a phase shiftbetween input voltage and output current, an effect readi-ly measured by AC methods. Normally a simple inspec-tion of impedance data provides a good indication of thecoating’s barrier properties.

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EIS Measurements: Cells, Instrumentation,and Data PresentationThe impedance across a coating film is measured with theuse of an electrochemical cell (Fig. 1). Two common celldesigns are shown schematically in Fig. 2. The “attachedcell” consists of a plastic tube cemented to a coated metalpanel. The attached tube is filled with an electrolyte (suchas 5% NaCl). A counter and reference electrode are insert-ed into the solution. The reference electrode, typically asilver/silver chloride electrode, is used to apply a con-trolled AC voltage across the coating film. The counterelectrode, typically platinum or another inert material, isused to measure the resulting AC current across the coat-ing film. The area inside the plastic tube is the area of thecoating for which impedance is measured. A picture of theattached cell and EIS instrumentation is shown in Fig. 1.

The attached cell is particularly convenient when coat-ing impedance is measured as a function of time underimmersion conditions. The cell can be placed in a convec-tion oven to obtain measurements at elevated tempera-ture. Non-flat surfaces can be accommodated. A variantof this is the clamp cell,12 which is similar to the attachedcell except that the cell is clamped on rather than cement-ed to the panel.

A second cell design (called the flat cell, EG&G Prince-ton Applied Research), also shown in Fig. 2, has the samecomponents as the attached cell but is more suited forpre- and post-run evaluation of coated panels that havebeen exposed to actual or simulated process conditions,e.g., noxious fluids at elevated temperature and pressure.Coated panels of a wide range of sizes and coating thick-nesses can be accommodated in either cell.

Data Output and InterpretationThe impedance of a coating is measured as a function offrequency from 105 Hz to 0.001 Hz or less. An AC voltageranging from 5 to 500 mV in amplitude (rms) is appliedto the coating. The resulting AC current (including phaseshifts) is measured, and the impedance derived. For coat-ings analysis, the most useful output format for the datais a Bode plot, consisting of a Log-Log plot of the imped-ance (Log Z) versus frequency (Log f) and a semi-Logplot of phase angle shift (Phase) versus frequency (Log f).Inspection of the Bode plot readily provides very usefulinformation on coating performance.

High-performance coatings with good barrier propertiesbehave as pure capacitors with a simple, readily recog-nized response. The equivalent circuit and impedance re-sponse is shown in Fig. 3. The circuit is a simple capaci-tor, Ccoat. The Log Z plot is a straight line with a slope of-1. The phase shift of a capacitor is 90°. At low frequen-cies in the range of 0.1 to 0.001 Hz, the coating imped-ance falls in the range of 109 to 1011 Ω cm2.

Fig. 2: EIS Cells Schematic, (top): attached cell. (bottom): flat cell(EG&G Princeton Applied Research)

Leads toinstrumentation

Reference andCounter electrodes

Coated Panel

Coated Panel Reference Electrodes

Counter Electrode

Permeable coatings, where the permeability is due ei-ther to inherent properties or deterioration, have a slight-ly different equivalent circuit. A resistance element, Rcoat,appears parallel to Ccoat (Fig. 3). The plot of impedanceversus frequency now has a region in which the imped-ance levels off (plateaus) at low frequency (i.e., 107 ohmcm2 at 0.1 Hz), as shown in Fig. 4. The phase shift variesbetween 0 and 90° in the resistance and capacitance ac-tive frequency ranges, respectively.

When corrosion and disbondment are occurring at thecoating/metal interface, an additional circuit element ap-pears in series with the coating resistance, Rcoat. Theequivalent circuit and response are shown in Fig. 5. The“corrosion” element consists of the polarization resis-tance, Rpol, which can be related to the corrosion rate ofthe base metal, and a double layer capacitance, Cdl (par-allel to Rpol), which results from the aqueous electrolytein contact with the metal surface.5,6,26,27

Information about the barrier properties (and relativepermeability) of a coating is obtained from the low fre-quency part of the Bode plot. To facilitate analysis of thedata and assessment of coating performance, Log Z at 0.1Hz is read from the plots by interpolation. The Log Z val-ue at 0.1 Hz is tabulated or plotted and used as the basisof comparison between coatings. It is also used for moni-toring the change in a coating’s impedance as a functionof exposure time to a test environment. Selection of Log Zat a frequency of 0.1 Hz is somewhat arbitrary but repre-sents a compromise between speed of analysis and selec-

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tion of a frequency at which the per-formance of different coatings can beeasily distinguished.

Anticipated performance of a coat-ing based on Log Z at 0.1 Hz isshown in Fig. 6. This summary figureis based on the large literature of EISlaboratory and fieldwork on coatings.

EIS in the Laboratory: Evaluationof Liquid Pipe Coatings

BackgroundProtective coatings, in conjunctionwith cathodic protection, are used tocontrol external corrosion on oil andgas transmission pipelines. In newconstruction, lengths of pipe that areexternally coated in a shop are weldedtogether in the field. The resultinggirth welds and related ancillary com-ponents such as flanges, risers, andtees are coated in the field with liquidcoating products. Pipe coating tech-nology has been rapidly advancing

and improving, with continual introduction of products. Many pipeline owners conduct laboratory test pro-

grams as the first step in the evaluation and qualificationof new products for use on their pipelines. The test pro-grams normally include an assessment of cathodic dis-bondment resistance, wet adhesion, impact resistance,flexibility, and hardness, among other tests specific to theowner’s line. In our laboratory, EIS measurements havebeen included in many of these test programs as an addi-tional means of assessing coating performance. The fol-lowing is an example of the EIS measurements made insuch a test program and their use in the selection of thebest pipe coating.

Laboratory MethodsSix liquid pipeline coatings, assigned name codes from Ato F, were evaluated. Coated test panels, 4 x 4 x 0.25inches, were prepared by the respective coating manufac-turers and submitted to our laboratory for testing. The liq-uid products were a mix of 99 to 100% solids epoxies andpolyurethanes.

EIS measurements were made using the attached cellmethod. Acrylic tubes, 1.5 in. (3.7 cm) in diameter, werecemented to the coated panels and filled with 5% NaClsolution. The tubes were closed with stoppers and placedin a convection oven at 65 C (149 F). This temperaturewas somewhat higher than the service temperature andaccelerated the test. At intervals of 1, 4, 7, 14, and 28days, the panels were temporarily removed from the oven

Fig. 3: Equivalent Circuit and Bode plot, representing a high-performance coating with good barrierproperties and no deterioration. Ccoat = 100 pF

coating. Over the duration of the mea-surements, little subsequent changewas observed, suggesting that the bar-rier properties were stable and that lit-tle to no change or deterioration wasoccurring at the test conditions.

The behavior of coating D (Fig. 8)was quite different from coating A. Af-ter one day, the Bode plot for coating Dwas very similar to that for coating A.However, at subsequent intervals, theimpedance decreased significantly. Inthe low frequency range, the impedancereached a limiting value, which was al-most independent of frequency. Thiscoating was acting like a resistor andcapacitor in parallel, like the model inFig. 4. The data suggest that this coat-ing was becoming increasingly perme-able with time, with a significant loss ofbarrier properties.

To observe the change in impedancefor each coating as a function of time,Log Z at 0.1 Hz was read from the Bodeplots. These data are plotted as shown

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and the impedance of the coating withinthe area of the attached tube was deter-mined. A silver/silver chloride referenceelectrode and platinum counter electrodewere used. Measurements were made induplicate for each coating.

Results and DiscussionThe Bode plots obtained at the varioustime intervals are shown in Figs. 7 and 8for coatings A and D, respectively. Notethat in the raw data, the impedance val-ues have not been corrected for the cellarea of 11.4 cm2.

The Bode plot obtained for coating Aafter one day was characteristic of ahigh-performance coating with very lowpermeability. The coating behaved likea capacitor, generating a straight linewith a slope of -1. After 4 days, therewas a small drop in the coating imped-ance at low frequency, producing somecurvature in the Bode plot. Based on si-multaneous gravimetric and EISstudies,28 this initial drop in impedanceis attributed to the uptake of water,which reduces the impedance of the

Fig. 4: Equivalent Circuit and Bode plot, representing a high-performance coating after deterioration andloss of barrier properties. Rcoat = 107 ohms, Ccoat = 100 pF

Fig. 5: Equivalent Circuit and Bode plot, representing a coating after deterioration and onsetof under-film corrosion. Ccoat = 1 nF, Cdl = 100 nF, Rcoat = 104 ohms, Rp = 106 ohms

in Fig. 9. The impedance measurements were corrected toa 1 cm2 area.

The data in Fig. 9 show that coatings A, C, and F havesimilar behavior and outstanding barrier properties underthe test conditions. The Log Z impedances are in therange of 10 to 11. There is no discernible downwardtrend, suggesting the coatings all have very low perme-ability, low water uptake, and very little deteriorationwith time. Based solely on impedance measurements,these coatings would be anticipated to be highly protec-

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tive, particularly under the milder service condi-tions of many pipelines (e.g., moist soil at 5 to 15C [41 to 59 F]).

Coating B had a somewhat low impedanceinitially, which dropped further to Log Z = 8over seven days of immersion. With a Log Z of8, this coating has an appreciable permeabilityto water and electrolyte but is still in the imped-ance range in which corrosion protection is pro-vided. Of significance is that the impedance sta-bilized after seven days with little furtherchange, suggesting that after the initial hydrationand related chemical changes, little furtherchange or deterioration occurred.

Coatings D and E had relatively high initial im-pedances, with Log Z > 10. Their impedancedropped significantly, however, over the first seven

days of immersion. The data in Fig. 9 show that the im-pedance continued to decrease slowly with time andwould be anticipated to eventually approach the range ofLog Z = 6 to 7, below which significant corrosion protec-tion is lost. Based on EIS data only, these coatings wouldbe anticipated to have the poorest long-term field perfor-mance of the products evaluated.

Correlation of EIS data with field performance is notavailable at this time because of the newness of the prod-ucts. However, data are being collected and will be pre-sented at a future time.

In service, the impedance of the pipe coatings would beanticipated to change with time, probably in the manner ofthe laboratory test as shown in Fig. 9. The magnitude andrate of change in impedance with time might be used as ameans to assess the rate of coating deterioration and re-maining coating life.

EIS Field MeasurementsEIS has been used primarily as a laboratory technique. Inrecent years, however, the use of EIS as a field techniquehas been under development, with EIS measurements

EIS TestingLaboratory panels - new coatingTest separatorProduced water tank

ResultsLog Z = 8.8–9.2Log Z = 7.6–8.4Log Z = 4.4–5.6

Table 1: Summary of EIS Test Data From OilProduction Facility

Fig. 6: The key for interpretation of the impedance data

Fig. 7: Pipe coating A: Bode Plots. 5% NaCl at 65 C. Impedance (Z)uncorrected for cell area of 11.4 cm2

Increasing Corrosion Protection

Coating Impedance, Log Z (Z in ohms cm2 @ 0.1 Hz)

Excellent

Evaluation of Tank Liningsin Oil Field Production Vessels

BackgroundEIS measurements were made on a test separator and amanway cover from a produced water storage tank locat-ed in a conventional oil production facility in Alberta.21

The units were coated internally with an epoxy tank lin-ing and had been in service for two years. The objectivewas to assess the condition of the epoxy coating, forwhich there was relatively little field experience.

Impedance of the Epoxy Coating before ServiceThe impedance of the new coating was measured on lab-oratory panels. When the coating was hydrated in 5%NaCl solution at 23 C (73 F) for 24 hours before measure-ment, the impedance was Log Z = 9.2 (0.1 Hz). Whenthe panel was hydrated at 60 C (140 F) in 5% NaCl forseven days, the impedance dropped to Log Z = 8.8 (0.1Hz). These values were relatively low for a new epoxytank lining that had not been in service, compared to oth-er products with which we are familiar.

Field MeasurementsThe test separator is used to measure the volumes of oil,water, and gas being produced from one or more oil wells.It is therefore exposed to raw, unprocessed petroleum fluids.The maximum operating temperature of the vessel was60 C (140 F). Based on visual inspection, the coating insidethe test separator was in good condition. The average im-pedance of the coating in the oil phase was Log Z = 8.4and in the water phase was 7.6. The impedance in the wa-ter phase was lower than in the oil phase, as expected, be-cause water is typically the primary source of coating degra-dation. The field measurements suggested that although thecoating in the separator was still protective, considerabledeterioration had occurred over the two years of service, in

made on coatings in service. The benefits of field EIS mea-surements are anticipated to be• an assessment of the current protectiveness of a coatingin service;• an assessment of the extent of coating deterioration as aresult of service conditions;• a determination of coating integrity in warranty inspections; • an identification of local areas of deterioration due tohot spots, concentration of chemicals, or the presence ofan aggressive phase within a vessel;• an assessment of the rate of coating deterioration, ob-tained from benchmarks and by collecting field measure-ments during inspections over an extended period of time(the data could also assist in determining inspectionschedules); and • a determination of the remaining service life.

Two examples of EIS field measurements are presentedhere. They consist of measurements on an epoxy tank liningin an oil field application21 and onFBE coatings on transmissionpipelines.22 Modifications of labora-tory equipment and methods weremade to facilitate the field measure-ments. This involved development offield-useable cells, extended cabling,a portable power supply, and devel-opment of measurement verificationtechniques.21 Bode plots were record-ed, and the impedance (Z) at 0.1 Hzwas read from the plots and reportedas Log Z. The field data were com-pared to samples of new coatings onlaboratory panels to assess the pro-tectiveness and degree of deteriora-tion.

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Fig. 8: Pipe coating D: Bode Plots. 5% NaCl at 65 C. Impedance (Z) uncorrectedfor cell area of 11.4 cm2

Fig. 9: Summary of impedance, Log Z (Z @ 0.1 Hz in Ω •cm2) as a function of immersion time at 65 Cin 5% NaCl for liquid pipe coatings.

AnalysisThe EIS measurements on the epoxy tank lining, summa-rized in Table 1, suggested that it was not providing ahigh degree of protection initially and that it was deterio-rating relatively rapidly in view of the mild service. TheEIS results were later found to be consistent with fieldexperience. The coating was subsequently found to havepoor performance in other vessels at other facilities andwas gradually replaced with other epoxy coatings.

Pipeline Coating Field MeasurementsBackground

EIS measurements were made on four operating pipelinesthat were coated with FBE.22 The FBE coatings rangedfrom 5 to 20 years in age in a range of environments. Theobjective of the EIS measurements was to determine theintegrity of the FBE and the extent of deterioration sinceinstallation. Integrity of the FBE, in conjunction with aneffective cathodic protection system, is considered essen-tial to preventing corrosion and stress corrosion crackingon transmission pipelines.

ObservationsField measurements were made based on the availability ofpipelines through scheduled digs. Details regarding thepipeline, environment, age, and condition of the coatings areshown in Table 2. The site locations, all within Canada, wereas follows: #1 in the Northern Prairies; #2 and #3 in moun-

spite of relatively mild conditions, i.e., the impedance haddropped from Log Z = 8.8 (new coating hydrated at 60 C[140 F]) to 7.6 (2 years in service in the water phase).Therefore, the coating would not be anticipated to have alengthy service life.

EIS measurements were also to be made on a producewater tank with the same epoxy lining. Access to the tankcould not be obtained because of scheduling delays, and,subsequently, EIS measurements were made on a manwaycover from the tank. The cover was blistered, with thesize and density of the blisters ranging from #2F to #2M(ASTM D71429). Many of the blisters were open, withstar-shaped or crescent-shaped cracks. The blistering hadoccurred at the coating/metal interface, and the base met-al was covered with black corrosion products, presumablyiron sulfide from reaction with hydrogen sulfide in theproduction fluids.

Impedance measurements were taken on non-blisteredand blistered areas. In either type of area, the impedanceof the coating was very low, with Log Z ranging from 4.4to 5.6. These impedances are well below the range pro-viding corrosion protection and are consistent with a verydeteriorated coating. For comparison, the impedance ofbare steel in a 5% NaCl solution ranges from Log Z = 2to 3 (i.e., 100 to 1,000 Ω cm2), depending on the build-upof corrosion products. Inspection of the produced watertank, carried out later, confirmed that blistering was alsopresent on the coating in the tank.

72 JPCL February 2003 www.paintsquare.com

#4>108.7 ohms cm2 (intact)>107.6 ohms cm2 (blister)>106.4 ohms cm2 (blister)

Intermediate to good

21 years

12NPS36At base of 5 m rise;clay/sand, stonyExtensive cathodic disbond-ment and blistering, pooradherence to pipe surface,superficial corrosion undercoating

Table 2: EIS Field Measurements on FBE-Coated Pipelines (Condensed from Ref. 22)

SiteImpedance

Assessment basedon EISAge of line, years

DFT, milsPipe sizeSite characteristics

Visual appearanceof coating

#1>109.7 ohms cm2 (atthe measuring limit ofthe instrument)

Excellent

5

15NPS20Clay/sand in slightlyhilly, dry areaExcellent applicationand condition; no blis-tering or disbondment

#2>109.5 ohms cm2 (on pipeat the measuring limit ofthe instrument)1010.4 ohms cm2

(free film)Excellent

19

13NPS36Mountain slope with gooddrainage; clay soil, rockSomewhat rough applica-tion but excellent condition;no blistering; poor adhe-sion adjacent to girth weld

#3>106.9 ohms cm2 (intact)>105.2 ohms cm2 (un-perfo-rated blister)>103.3 ohms cm2

(perforated blister)Intermediate to poor

Pipe coated in 1971,exposed to UV for 22 yearsbefore installation in 1993Not determinedNPS30Mountain valley on 30-degree slope; wet clayCathodic disbondment andblistering in the 9 to 12o’clock quadrant; some blisters perforated; showingcorrosion

JPCL February 2003 73

tainous terrain; and #4 in the Rocky Mountain foothills. The five-year-old FBE at Site #1 was in excellent condi-

tion visually with no blistering, cathodic disbondment, orother signs of deterioration. The impedance was Log Z ≥8.9, the measuring limit of the instrumentation. In com-parison, the impedance of new FBE on a lab panel wasLog Z = 9.7 (after 24 hours of hydration in 5% NaCl at23 C [73 F].

Similarly the 19-year-old FBE at Site #2 was in excellentcondition, with no blistering, cathodic disbondment, orsoil abrasion. The finish was slightly rough and uneven,presumably because of the coating application methods of19 year ago. The impedance of the FBE was Log Z ≥ Log8.7, the measuring limit of the instrumentation. The im-pedance of the new, current-day version of the FBE wasLog Z = 9.7.

At site #2, a portion of the FBE adjacent to the girthweld had poor adhesion. Small sheets of coating severalinches in size could be pried from the pipe surface with aknife. No metal loss or corrosion under the poorly bondedcoating was visible. The impedance of a free film of thedisbonded FBE was measured in the laboratory and ob-served to be Log Z = 9.6, almost identical to newly ap-plied FBE. Based solely on impedance measurements andvisual observations, it was concluded that the barrierproperties of the FBE had deteriorated little, if at all, in 19years of service. The free film measurements showed thatthe barrier properties were retained even when the adhe-sion was low. The good performance is probably due tothe mild environment, i.e., an up-slope with gooddrainage.

At site #3, the FBE had been in service for 8 years afterbeing stored outdoors for 22 years. The FBE showed ex-tensive blistering at the 9 to 12 o’clock position. The im-pedance of intact areas of the coating was Log Z = 6.9.The impedance of areas with intact blisters was Log Z =5.2. Therefore, this coating had moderate to poor barrierproperties, which were attributed to UV deterioration fromoutdoor storage prior to installation.

The 21-year-old FBE at site #4 was blistering extensivelyover all of the pipe surface. The FBE had poor adhesion,and superficial corrosion was observed on the pipe exterior.The impedance of intact areas of the coating was Log Z =8.4. The impedance of blistered areas ranged from Log Z =6.2 to 7.4. In spite of the blistering, the coating had moder-ate to relatively good barrier properties. The extensive blis-tering was attributed to inferior application procedures.

ConclusionsThis article has described several uses of EIS for evaluat-ing critical properties (barrier properties and permeability)of protective coatings under laboratory or field conditions.In particular, the manner in which coating impedance de-

creased with time was used as a means to monitor coat-ing protectiveness and deterioration.

EIS was used to monitor the barrier properties of sixpipe coatings in laboratory performance testing and to as-sist in the evaluation and selection of pipe coatings forfield use. The behavior of the six coatings varied widely,ranging from little to no change in impedance with time(best performance), to a continual decrease in impedancewith time (poorest performance). Since similar changes inimpedance would be anticipated to occur in service, im-pedance might be used as a means to assess the rate ofcoating deterioration and remaining coating life.

Several examples were also presented where EIS wasused to evaluate the protectiveness and extent of deterio-ration of coatings that were in service in industrial appli-cations, i.e., oil field production equipment and transmis-sion pipelines externals.

Some additional possible examples of applications areas follows:• determining the degree of water permeation of a coat-ing subject to accelerated laboratory testing (to assess thevalidity of the accelerated test and to establish practicaltimes for test duration),• comparison of the degree of permeation or film break-down in accelerated and natural exposures, and• comparison of the increase in permeability of coatingsapplied to rusted or previously painted surfaces.

Ultimately, the value of this technique will be judged onits capability to provide better and earlier information oncoating lifetimes. This information could assist manufac-turers, specifiers, and users in providing more cost-effec-tive protection to the myriad of structures affected by cor-rosion.

AcknowledgementsThe authors gratefully acknowledge the cooperation andassistance of facility owners in providing both data andaccess to their facilities, and coating manufacturers in pro-viding samples of their products.

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2. H. Leidheiser, Editor, “Corrosion Control by OrganicCoatings” NACE (1981).

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4. F. Mansfeld, S.L. Jeanjaquet, and M.W. Kendig, “AnElectrochemical Impedance Spectroscopy Study of Re-actions at the Metal/Coating Interface,” in CorrosionProtection by Organic Coatings, M.W. Kendig and H.

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7. F.M. Greenen, J.H.W. DeWit, and E.P.M. Van Westing,“An Impedance Spectroscopy Study of the DegradationMechanism for a Model Epoxy Coating on Mild Steel,”Progress in Organic Coatings 18: 299–312 (1990).

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10. J.N. Murray and H.P. Hack, “Testing Organic Architectur-al Coatings in ASTM Synthetic Seawater Immersion Con-ditions Using EIS,” Corrosion 48(8): 671–685 (1992).

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13. H.P. Hack and J.R. Scully, “Defect Area Determinationof Organic Coated Steels in Seawater Using the BreakPoint Frequency Method,” J. Electrochem. Soc. 138(1):33 (1991).

14. J.N. Murray, “Electrochemical Test Methods for Evalu-ating Organic Coatings on Metals: An Update. Part I.Introduction and Generalities Regarding Electrochemi-cal Testing of Organic Coatings,” Progress in OrganicCoatings 30: 225–233 (1997).

15. J.N. Murray, “Electrochemical Test Methods for Evalu-ating Organic Coatings on Metals: An Update. Part II.Single Test Parameter Measurements,” Progress inOrganic Coatings 31: 225–264 (1997).

16. J.N. Murray, “Electrochemical Test Methods for Evalu-ating Organic Coatings on Metals: An Update. Part III.Multiple Test Parameter Measurements,” Progress inOrganic Coatings 31: 375–391 (1997).

17. K. Homma, H. Kihira, and S. Ito, “Application of Alter-nating Current Impedance Method to Evaluation ofCoating Deterioration,” NACE Corrosion 91, Paper No.483, 9 pages (1991).

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tion of Electrochemical Impedance Techniques to Esti-mate Corrosion Damage of Steel Infrastructures,” Cor-rosion Forms and Control for Infrastructure, ASTM STP1137, V. Chaker, Ed. ASTM (1992).

19. Lj. Dukic and I. Stern, “The Measurement of ElectricalImpedance of Organic Coating on Steel Structures,”Proceedings, 8th International Congress on MetallicCorrosion, 7th Congress of the European Federation ofCorrosion (111 Event), Mainz, Germany, October, pp.1055–1060 (1981).

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Editor’s Note: This article is based on a paper given atSSPC 2002, November 3-6, 2002, in Tampa, FL, andpublished in the conference Proceedings, SSPC publicationno. 02-16.

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