Interest in the electrochemical noise method (ENM) as a technique for assessment of anti-corrosive prop-erties of organic coating is constantly increasing. This is because it has advantages such as quickness, anon-intrusive nature and ease in gathering and interpretation of data. There are a number of availablearrangements of the ENM that can be used to perform noise measurements. Two of them, called suc-cessively single substrate (“SS”) and no connection to the substrate (“NOCS”) have been examined andcompared with DC resistance method in order to determine whether the ENM is attractive and accurateenough to be accepted as a convenient way to test organic coatings in site situations. Results have showngood correlation between these two ENM arrangements and the DC resistance method. As the mostpromising arrangement, the NOCS has been considered in more detail. NOCS involves taking measure-ments from three areas on the coated substrate without the need of electrical connection to the substrate.The aim of this part was to check if the individual values of resistance can be computed by varying theway the cells are connected to the measuring equipment. The results obtained allow the suggestion tobe made that the result of a single measurement of NOCS is dominated by the cell which acts in thisconfiguration as a working electrode 1 (WE1). In this work also an attempt has been made to deal with
the practical considerations involved in making measurement in the field. Dismountable cells to act astemporary connectors to the substrate, in the form of “copper pads”, have been developed and are dis-cussed. These “copper pads” have indicated their potential usefulness in conducting ENM measurementsin site situations. To make ENM usable in the field, good reproducibility of obtained results is required.Therefore, experiments have been conducted to consider this aspect as well. It has been proven that ENMresults are reproducible.
. Introduction
In present times coating by organic paints is the most commonethod of anti-corrosion protection for various objects. Improve-ents are being introduced continuously for better performance of
rganic coatings. Because of the constantly developing activity inhis area, it is also essential to find the most convenient method fornvestigation of properties of these coatings (especially assessingheir protectiveness). It is desirable for these methods to be non-ntrusive, quick, easy to operate and that interpretation of gatheredata will not be complicated.
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Taking into consideration all of these requirements, the elec-rochemical noise method (ENM) seems to be promising. ENM ishe best way of making a measurement that does not affect the
system. The method does not produce any voltage levels acrossthe metal-paint interface (it is non-destructive). ENM is also easilyautomated. Measurements can be performed very quickly and theobtained data is easy to interpret [1].
The principal aim of ENM when applied to assessment oforganic coatings is to obtain a single result: the noise resistance(Rn). Electrochemical noise can be described as naturally occurringfluctuations in potential and current around a mean value in elec-trochemical cell [2]. From these fluctuations the derived parametervoltage noise (standard deviation of the potential data series �v)and current noise (standard deviation of the current data series �i)can be obtained. These parameters are used in an Ohms Law rela-tionship to calculate the desired value—the noise resistance [3].
Rn = �v
�i(1)
lectrochemical noise method (ENM) to make it more practical fordoi.org/10.1016/j.electacta.2013.09.067
Therefore ENM is an electrically non-intrusive method becauseduring the test only natural fluctuations of current and potentialare measured. ENM does not need any activation signal. It is also
2 D. Mills et al. / Electrochimica Acta xxx (2013) xxx– xxx
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The experimental work can be divided into two main parts, eachpart includes two sections:
NOCS work:
Fig. 1. “Salt bridge” arrangement of ENM.
ery sensitive method [4]. Another advantage is the relatively shortime of a single measurement (only a few minutes).
The value of noise resistance allows us to assess the pro-ectiveness of paint coatings. Bacon et al. [5], doing extensive
easurements, found a relationship between the resistance valuend the protection afforded by the coating to the substrate. Lesshan 106 � cm2 indicates poor corrosion protection, more than08 � cm2—good corrosion protection, value between 106 � cm2
nd 108 � cm2 shows an intermediate level of corrosion protection.For ENM measurements three electrodes are required: two
orking electrodes (WE1 and WE2, between these the current isecorded by a zero resistance ammeter (ZRA) at regular intervals)nd one reference electrode RE. Potential is measured betweenE and the working electrode couple. Current and potential areegistered simultaneously [6]. The reference electrode must have
stable potential. Therefore SCE (saturated calomel electrode) isost commonly chosen in laboratory measurements.Testing of organic coatings with ENM started in the late eight-
es using a configuration of electrodes which is now called thestandard” or “salt bridge” arrangement (Skerry and Eden) [7].his standard arrangement needs two separate working elec-rodes (WE1 and WE2) which are two nominally identical coatedanels (sample 1 and sample 2), and one reference electrodeRE)—saturated calomel electrode SCE. A defined area of each sam-le is in contact with solution e.g. typically laboratory testing oneell is stuck on each coated sample and filled with electrolyte. WE1nd WE2 are joined to the substrates of samples 1 and sample 2,espectively, and the RE is put in to either of two cells. An elec-rolytic bridge (“salt bridge”) is necessary to provide the ionic pathetween cells (see Fig. 1).
This configuration of ENM suffers from one disadvantage—twoominally identical isolated substrates are required what makes itlmost impossible to be used in the field because, obviously, it isot easy to find two electrically isolated substrate elements in thetructure.
The desire to explore outdoor applicability of ENM and furtherntensive work led to the invention of new ENM arrangement calledingle substrate (“SS”) [8]. Mabbutt proposed a configuration oflectrodes, where the substrate is one complete unit carrying twoells stuck on the coating. These cells are filled with electrolyte.he substrate is attached as a reference electrode (RE) in this con-guration and replaces the salt bridge. Two standard laboratorylectrodes are put into each separate cell of electrolyte and act asorking electrode 1 (WE1) and working electrode 2 (WE2) (see
ig. 2). This arrangement is better than the standard one from theractical point of view. Here only one substrate is needed whichakes this technique more applicable in the field and enables the
NM to be used to examine coatings on conventional structures.An even more novel arrangement was invented by Woodcock
6]—No connection to the substrate (NOCS). This technique does
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assessment of anti-corrosive coatings, Electrochim. Acta (2013), http://dx.
ot require any electrical connection to the substrate which meanst has great potential to become an appropriate method used inhe field. In this set-up three cells are stuck on the same coated
Fig. 2. Single substrate arrangement of ENM.
substrate and filled with electrolyte. Three standard laboratoryelectrodes are required. Each of them is put into each cell withelectrolyte. One SCE acts as a reference electrode (RE) and two oth-ers are connected to the ZRA as working electrode 1 (WE1) andworking electrode 2 (WE2), respectively (see Fig. 3).
Because of the desire to use the ENM in site situations ratherthan only in the laboratory, this current work concentrates onthe NOCS arrangement. Results obtained using this set-up arecompared with results from the single substrate arrangement ofENM and also with DC resistance method. A considerable amountof previous work has been done comparing the single substratemethod with the standard (“bridge”) set up and comparing theNOCS method with the standard set up. Either very good or goodagreement has been obtained. These comparisons can be found aspart of the work in references [3,4,6,8,10,11,13]. Also there has beenprevious work comparing values with impedance (0.1 Hz value)and R (DC) with Rn (these can be found in references [3,6,11]).
The problem that may be encountered in case of NOCS regardingto its use in the field is that on the one substrate resistance of thecoating may be different in different places and this can cause thefinal result to be dominated by any one of cells. Thus the aim of thisproject was to check whether the individual values of resistancecan be computed by varying the way the cells are connected to themeasuring equipment.
This present work contains as well results from other experi-ments associated with making the ENM technique more useful inthe field. In this it builds on work conducted previously [1] and isconcerned about a new type of electrode incorporated into a dis-mountable cell (“copper pad”) as a way of temporary connection tothe substrate.
Also, as an important factor enabling a method to be com-monly used, the reproducibility of results obtained using ENM waschecked and reported.
2. Experimental
electrochemical noise method (ENM) to make it more practical fordoi.org/10.1016/j.electacta.2013.09.067
Fig. 3. No connection to the substrate arrangement of ENM.
situation from the field when in one measurement of NOCS, theremay be areas with different resistance values.
Designation of selected cells and their resistance values fromthe DC resistance method are presented Table 1.
Table 1Designation of selected cells and their resistance values from the DC resistancemethod.
Designation of cell DC resistance value [� cm2]
A 6.40E + 09B 6.35E + 09
ARTICLEA-21284; No. of Pages 7
D. Mills et al. / Electrochi
• Measurements using this arrangement on one of the sam-ples and also in SS (single substrate) arrangement and by DCresistance method in order to compare NOCS with other arrange-ments/methods,
• Further experiments on NOCS and investigation of situationswhen one of the three involved areas in one NOCS measurementis different than the other two e.g. when its value of resistanceis higher or lower than the others; it was checked if it is possibleto identify a low one among two highs and high one among twolows by making different connections to the measuring equip-ment.
Experiments associated with making the ENM more useful in thefield:• Practical development—a new type of electrode “copper pad”,• Checking the reproducibility of the ENM.
Samples were prepared using cold-rolled mild steel test pan-ls (“Q-panels”) as a substrate material. Dimensions of each panelere: 100 mm × 150 mm and the thickness 0.8 mm. Each test
ample was washed in deionised water and then degreased insopropane-1-ol just before painting. Coatings were applied bypreader bar.
Types of coatings that were used to perform experiments weres follows (with the average coating thickness is given in brackets):
ALKYD (Pronto Paints Co.) Quick Drying, High Build Anti-CorrosivePrimer/Finish (105 ± 5 �m)—used in comparison measurementsof NOCS and SS arrangements of ENM and DC resistance method,POLYURETHANE (Bayer Co.) two component: (acrylicpart) Desmophen 670/(isocyanate part) Desmodur N100(102 ± 5 �m)—used in all other experiments.
In the NOCS work a number of cells made of PVC were attachedo the coated substrates using silicone sealant and then they werelled with 3% sodium chloride solution. The area exposed to elec-rolyte was 4 cm2 (each cell).
In the reproducibility experiments the whole area of the coated panel samples was totally immersed in a beaker also contain-
ng 3% NaCl solution. Backs and sides of the samples were coveredith beeswax to prevent the occurrence of corrosion in these areas.
here were immersed in solution for between 4 and 6 weeks beforeeasurements were made so their Rn values should be quite sta-
le. In these experiments copper pads were used as electrodes (asescribed later in this paper) with the area of 4 cm2 each.
ENM measurements were performed using an ACM GillACnstrument connected to a computer operating software providedy ACM Company. Electrochemical noise data was collected at 2 Hzampling rate for 512 data points per one measurement. It allowedesults to be gained from one experiment within about 4.5 min. Fre-uency of 2 Hz is the standard frequency used when ENM is appliedo organically coated substrates. A Faraday cage was not used in thisork because it was intended to develop the ENM for practical usehere incorporation of such a cage would be difficult.
Obtained noise data was treated using “ENANALIZ” software—annalytical programme created by Cottis. This programme isesigned to remove the DC drift from the current noise and poten-ial noise data. Each set of potential and current data recorded by theCM software is subsequently saved (potential and current sepa-ately) in single “txt” files, then each of these files is uploaded to theNANALIZ software and immediately after we obtain the requiredalue of standard deviation after trend removal from potential dataet or current data set. These are then combined to give the modi-
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ed version of Rn. We do not have information about what occursithin the ENANALIZ software. In order to better demonstrate theature and meaning of DC drift removal, an example of potentialata before and after removing the DC tendency is shown in Fig. 4.
Fig. 4. An example of potential data before and after removing the DC tendency.
Regarding the instrument to make DC resistance measurements,a Keithley 610 electrometer (a high impedance multimeter) wasused.
2.1. NOCS work
For the purpose of comparison of NOCS arrangement of ENMwith SS (single substrate) arrangement and with DC resistancemethod, all of the experiments were done on the same sample, per-forming each measurement twice and taking the average of twoobtained results. In NOCS arrangement three saturated calomelelectrodes (SCEs) were connected to the ACM box (as WE1, WE2and RE) and each of them was immersed into each of three cellsall filled with electrolyte and stuck onto one coated panel. In singlesubstrate arrangement the reference electrode (RE) was connectedto the substrate and two SCE were connected to WE1 and WE2and put into two cells filled with electrolyte on one coated panel.Set-up of DC resistance method in the laboratory was as follows:a reference electrode (also SCE) was put into a cell of electrolyteand connected to the high on the Keithley electrometer and thesubstrate was connected to the low.
In order to deal with the second section of “NOCS work”, theresistance of the coatings was initially measured by DC resistancemethod and when it was found that the resistance value was stablefor each cell (after about 2 weeks from filling the cells with thesolution), six of them were selected and the NOCS measurementswere done. As it turned out, even when prepared under laboratoryconditions, a coating showed different resistance values at differentplaces on the same sample. This is typical of single coat samples ofcross linking organic coatings—even at the same thickness they canshow considerable variation in resistance values. Cells to conductthe NOCS experiments were chosen in such a way to simulate the
lectrochemical noise method (ENM) to make it more practical fordoi.org/10.1016/j.electacta.2013.09.067
Table 2Explanation of the sequence of measurements for A–B–C configuration.
A–B–C (high–high–high)
RE WE1 WE2NOCS 1 A B CNOCS 2 A C BNOCS 3 B A CNOCS 4 B C A
cis
•••
•
fbc
nc
2t
wc“deea
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NOCS 5 C A BNOCS 6 C B A
It was decided to perform NOCS measurements in four differentonfigurations of cells (as it was stated earlier NOCS arrangementnvolves three areas on one coated substrate to perform one mea-urement):
Three cells with high resistance (A–B–C),Three areas with low resistance (X–Y–Z),One area with low resistance and two with high resistance(Z–C–A),One area with high resistance and two with low resistance(Z–Y–A).
Measurements in each configuration were performed in six dif-erent combinations of the way of connecting electrodes to the ACMox, i.e. different cells were connected to WE1, WE2 and RE in eachombination.
Table 2 gives an explanation of how the electrodes were con-ected to the measuring device in the configuration where threeells had high resistance (cells A, B, C).
.2. Experiments associated with making the ENM more useful inhe field
In this part practical aspects of outdoor application of the ENMere considered and developed. Here instead of using SCEs and
ells stuck on samples filled with electrolyte a new type of electrodecopper pad” was used (see Fig. 5). The current work introduces aevelopment of this type of electrode which was reported in anarlier work by Mills, Broster and Razaq [1]. Also samples werexposed to the 3% NaCl solution differently—by total immersion in
beaker.In order to examine the coated surface, a defined area must
e in contact with the solution. An idea of applying the wafer-ike electrodes, such as the “copper pad”, comes from the needo create electrodes that allow measurements to be made in theeld, because it would be impractical to stick on cells and fill themith a solution in a field situation. The “copper pad” has advan-
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assessment of anti-corrosive coatings, Electrochim. Acta (2013), http://dx.
ages because it is flexible, relatively inexpensive and not difficulto make and can be connected to the ACM GillAC box. It is alsocceptable as a reference electrode under field conditions [1,9].
Fig. 5. Design of “copper pad” and its application.
PRESScta xxx (2013) xxx– xxx
The “copper pad” is a piece of copper sheet (2 cm × 2 cm) to theback of which a wire is soldered so that the pad can be connected tothe ACM box. Several layers of filter paper are attached to the bot-tom (inner face) of the copper. Silicone sealant is used to cover theback (outer face) and sides of the “copper pad”. Before the exper-iment, the filter paper has to be soaked in whichever solution isrequired [1] (in this work 3% NaCl). The silicone was also used toattach pads to the surface. It allows the exact areas to be specifiedand keeps the pads on the surface.
This new design of electrode was used to perform reproducibil-ity experiments and the measurements were conducted in singlesubstrate arrangement of ENM. Although NOCS arrangement is verypromising, because of the obvious advantage of no requirement ofelectrical connection to the substrate and some work has been donein the laboratory confirming the usefulness of this arrangement [6],it is not yet a fully verified method for application in the field [1].Therefore the single substrate arrangement was chosen to performreproducibility experiments as it is likely to have good applicationas an “in the field” method confirmed by previous works done on arange of samples of various resistances [1,8,10].
Before the measurements were started the samples remained inthe solution of 3% NaCl for 2 weeks, to let them reach a stable state.Measurements were performed during 5 days, every morning andevery afternoon. Before each measurement the samples were takenout from the solution, washed with deionised water and dried. Thentwo “copper pads” were attached to the coated substrate in areasdefined by silicone sealant. Connections were made to them fromWE1 and WE2. RE was attached to the substrate. Every time thetests were repeated seven times one after another.
Research has been conducted in an earlier work to determinethe time that the pad needs to remain in contact with the samplebefore the measurement [1]. This time is required to reach a levelof equilibrium by an organic coating when in contact with the testsolution (“time to settle”). Former studies has shown that the “timeto settle” of between 30 min and 45 min would typically be requiredusing 3% NaCl as the test solution. Therefore, it was decided that inthese experiments the time would be 40 min.
Once measurement was completed, samples were put back tothe solution and copper pads were washed with deionised waterand dried.
3. Results and discussion
3.1. Comparison between NOCS, SS and DC resistance method
Results from the initial part of the experimental work are pre-sented in Fig. 6. This figure shows very good compatibility ofresults from NOCS (no connection to the substrate) and SS (singlesubstrate) arrangements of ENM and DC resistance method for anALKYD paint. DC resistance values are presented for three sepa-rated cells—all of them were used in NOCS tests and two of themin SS measurements. Rn values obtained in both arrangements ofENM and also DC resistance values are all within a factor of twowhich is a typical level of variation in ENM and indicates goodagreement between them (average resistance R = 2.42 × 108 andstandard deviation of R = [±3.55 × 107], S.D. log R = [±0.06]). Onlyone set of results is presented here, however this experiment wasrepeated on several different samples with similar results, theseare reported in [14].
3.2. NOCS arrangement investigation
electrochemical noise method (ENM) to make it more practical fordoi.org/10.1016/j.electacta.2013.09.067
In previous works (e.g. [6]) and also earlier in this current work,the NOCS arrangement has shown promising results, but it waswhen all three cells had a well known and the same or similar
D. Mills et al. / Electrochimica Acta xxx (2013) xxx– xxx 5
1.00 E+00
1.00 E+01
1.00 E+02
1.00 E+03
1.00 E+04
1.00 E+05
1.00 E+06
1.00 E+07
1.00 E+08
1.00 E+09R n
/ohm
cm
2
NOCS SS DC Resistanc e 1 DC Re sista nce 2 DC Resistanc e 3
Fig. 6. Comparison between two arrangements of ENM (NOCS and SS) and DC resis-tance method for ALKYD paint.
Table 3Sequence of measurements +Rn values for each single NOCS measurement in A–B–Cconfiguration.
A–B–C
RE WE1 WE2 Rn [� cm2]NOCS 1 A B C 2.24E + 09NOCS 2 A C B 2.52E + 09NOCS 3 B A C 1.12E + 09NOCS 4 B C A 1.08E + 09NOCS 5 C A B 1.13E + 09NOCS 6 C B A 1.40E + 09
Table 4Sequence of measurements +Rn values for each single NOCS measurement in X–Y–Zconfiguration.
X–Y–Z
RE WE1 WE2 Rn [� cm2]NOCS 1 X Y Z 3.03E + 07NOCS 2 X Z Y 2.47E + 07NOCS 3 Y X Z 7.48E + 07NOCS 4 Y Z X 1.31E + 07
romotioitt
TSc
Table 6Sequence of measurements +Rn values for each single NOCS measurement in Z–Y–Aconfiguration.
Z–Y–A
RE WE1 WE2 Rn [� cm2]NOCS 1 Z Y A 1.91E + 07NOCS 2 Z A Y 1.01E + 09NOCS 3 Y Z A 1.87E + 07NOCS 4 Y A Z 1.09E + 09
NOCS 5 Z X Y 7.46E + 07NOCS 6 Z Y X 2.29E + 07
esistance. More challenging becomes the use of this arrangementf ENM in a site situation, when we have to deal with measure-ents on unknown painted surfaces, which can differ in the values
f resistance, and the final result can be dominated by any one ofhe cells. Therefore, the aim of NOCS measurements was to checkf there is any way to find out what determines the value of Rn
btained by NOCS. To this end tests were done as it was describedn experimental part and results are presented in Tables 3–6. These
Please cite this article in press as: D. Mills, et al., Developments in the eassessment of anti-corrosive coatings, Electrochim. Acta (2013), http://dx.
ables give an explanation of how the electrodes were connectedo the measuring device and also contain the obtained Rn values.
able 5equence of measurements +Rn values for each single NOCS measurement in Z–C–Aonfiguration.
Z–C–A
RE WE1 WE2 Rn [� cm2]NOCS 1 Z C A 1.70E + 09NOCS 2 Z A C 1.25E + 09NOCS 3 C Z A 1.63E + 07NOCS 4 C A Z 1.06E + 09NOCS 5 A Z C 1.82E + 07NOCS 6 A C Z 1.30E + 09
NOCS 5 A Z Y 3.88E + 07NOCS 6 A Y Z 1.48E + 07
From the results presented in Tables 3 and 4 it can be stated thatin case of NOCS measurements, when the three involved cells havea similar resistance values, all of the results from the six arrange-ments (NOCS 1–NOCS 6) are similar to each other and correlate wellwith the values obtained in DC resistance method (see Table 1).Thus, in this case it is possible to evaluate successfully the anti-corrosive properties of coatings by NOCS. However, this does notanswer the question as to which factor determines the results ofNOCS tests and what happens if the resistance of three cells is notthe same or similar.
In an attempt to answer these questions, further research onNOCS (see Tables 5 and 6) has been done with three cells where twoof the resistances are the same and one is different. It can be seenthat in the case of involving one cell with low resistance and twocells with high resistance then with the NOCS3 and NOCS5 arrange-ments, the value of Rn is significantly lower (about two orders ofmagnitude) than in the other four cases. This indicates that NOCS3and NOCS5 is definitely dominated by cell Z (low). The four otherresults are dominated either by cell A or C. In the case when twocells have low resistance and one has high resistance, again twoof six NOCS results are significantly different than the other four,but here NOCS2 and NOCS4 are much greater than NOCS1, NOCS3,NOCS5 and NOCS6. This indicates a domination of cell A (high) inNOCS2 and NOCS4. The NOCS1, NOCS3, NOCS5 and NOCS6 resultsare dominated either by Z or Y.
After analyzing the results and combining them with six variousconfigurations of NOCS an interesting conclusion has been reached:
Taking into account both of the arrangements of cells it canbe seen that they differ only in “central” cell (Z–C–A and Z–Y–A).Therefore, the set-up for cells Z and A is the same in both cases.In case of configuration Z–C–A the final result is dominated by cellZ (low) in NOCS3 and NOCS5, when the Z cell acts as a workingelectrode 1 (WE1). In the arrangement of Z–Y–A, the A (high) celldominates the results in NOCS2 and NOCS4, when it acts as WE1as well. This analysis allows us to suggest that the result of a sin-gle measurement of NOCS is dominated by cell which acts in thisconfiguration as a working electrode 1 (WE1). An explanation ofthis could be that because in the Gill ZRA, WE2 is connected to thepositive input of an operational amplifier, arranged as an invertingamplifier with a feedback resistor connecting the output to the neg-ative input and the positive input also connected to ground. WE1 isconnected to the negative input (see Fig. 7). The result of this con-figuration is that WE2 and WE1 attempt to stay at the same valueas each other, and it follows that WE1 will dominate the result. Ifthis is true, measurements of NOCS would be limited to three singletests in order to determine the resistance value of each individualtest cell. However, further work needs to be done in order to verifythe above suggestion.
3.3. Results of reproducibility experiments using “copper pads”
lectrochemical noise method (ENM) to make it more practical fordoi.org/10.1016/j.electacta.2013.09.067
In order to check the reproducibility of the ENM, experimentswere performed as described in experimental part. An example ofthe results obtained for one of the samples is presented in Fig. 8.
Fig. 7. Schematic representation of an operational amplifier—the basis for the zeroresistance ammeter (ZRA) with an indication of connection to WE1 and WE2 [12].
1
1
1
1
1
1
1
1
1
1
1
R n/o
hm c
m2
1.00 E+00
1.00 E+01
1.00 E+02
1.00 E+03
1.00 E+04
1.00 E+05
1.00 E+06
1.00 E+07
1.00 E+08
1.00 E+09
1.00 E+10
set 1 set 2 set 3 set 4 sset 5 sset 6 seet 7
Fi
T2a
brHottaog
ig. 8. Rn values in log scale showing repeatability of seven data sets (day 2, morn-ng).
his figure shows the repeatability of the seven data sets from day, taken in the morning. The average resistance value is 1.13 × 109
nd standard deviation of R = [±3.50 × 108], S.D. log R = [±0.14].Generally, in results obtained from each day, good agreement
etween each of the seven data sets could be observed. All of theesistance values were close to 109 � cm2, within a factor of two.owever, in some cases, there was a scatter of results (up to a factorf four). There is a lot of random external factors that may influencehe electrochemical noise, and such factors may cause scatter inhe results, but, for use in the field, it is not significant enough to
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assessment of anti-corrosive coatings, Electrochim. Acta (2013), http://dx.
ffect the sufficiently good assessment of anticorrosive propertiesf coatings. Thus, it can be said that the obtained results indicateood repeatability of the technique.
Fig. 9. Rn values in log scale showing the reproducibility of results.
PRESScta xxx (2013) xxx– xxx
Subsequently, to see the performance of results within the fol-lowing days, the average of every seven data sets was taken andpresented versus time. Fig. 9 shows what was obtained. After twoweeks of the exposure in the solution, the coating should haveachieved a state of equilibrium and the resistance value shouldremain constant.
Fig. 9 confirms this assumption (the average resistance valueis 1.99 × 109 and standard deviation of R = [±1.24 × 109], S.D.log R = [±0.26]) and also indicates that this technique has not onlygood repeatability when measurements are taken immediately oneafter another, but it has also good reproducibility when measure-ments are conducted at longer intervals.
4. Conclusions
Results of the research done for the purpose of this work haveshown that the electrochemical noise method is a promising toolwhich can be used for assessing the anticorrosive properties oforganic coatings in site situations.
The most novel arrangement of ENM–NOCS – has been inves-tigated and it has been proven that results of measurementsdone in this configuration correlate nicely with another arrange-ment of ENM – single substrate – and also with DC resistance,a method which has been used for many years in labora-tories as a technique to determine protectiveness of organiccoatings.
NOCS arrangement has been also investigated in terms ofthe possibility to determine the resistance value for individ-ual examined areas. The analysis of results has allowed us tomake the suggestion that the result of a single measurementof NOCS is dominated by the cell which acts in this configu-ration as working electrode 1 (WE1). Although further work isrequired to verify the truth of this assumption, it holds out thepromise that even when all three areas in NOCS have differ-ent resistances, it may be possible to calculate their individualvalues.
Also the good reproducibility of the ENM method has beenproven. The use of electrodes in the form of the “copper pads” isa development which will enable ENM to be applied in site situa-tions. “copper pads” appear to be a satisfactory way of connectingtemporarily to the coated substrate.
Acknowledgements
Thanks are due to the University of Northampton for provid-ing laboratory facilities and allowing the experiments to be carriedout for the purpose of this work. The authors thank Dr. Steve Mab-butt for helpful discussions. Acknowledgement is also made to Dr.Phillip Munn, the head of Midland Corrosion Services Ltd., for takinga strong interest of this work.
References
[1] D.J. Mills, M. Broster, I. Razaq, Continuing work to enable electrochemical meth-ods to be used to monitor the performance of organic coatings in the field, Prog.Org. Coat. 63 (2008) 267–271.
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[3] D.J. Mills, Comparison of ENM, EIS and DC resistance for assessing and moni-toring anti-corrosive coatings, J. Corros. Sci. Eng. 8 (12) (2004) 2–10.
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