Organic Coatings for Corrosion Control - American Chemical Society

Chapter 25

Defect Area Calculated from Electrochemical Noise and Impedance Measurements

Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch025

R. L. Twite and Gordon P. Bierwagen

Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105

The uses of electrochemical methods to estimate the defect area on steel substrates under an organic coating have been proposed in the literature. These citations suggest that the defect area can be estimated using electrochemical impedance spectroscopy (EIS) parameters from an equivalent circuit or extracted from the "raw" data of an electrochemical noise (ENM) test. In this investigation, these methods are applied to estimate the defect area on Al 2024-T3 substrate coated with a number of different aerospace primers. The results are compared with image analysis data from scanning electron micrographs (SEM). It is found that the method utilizing the ENM measurements agrees best with scanning electron microscopy / image analysis data. EIS methods only apply to systems when coating resistance values are low and coating failure is detectable. Although the different methods disagree with respect to the numerical values for defect areafractions,each predict a time dependent increase in defect area and can be used qualitatively to estimate the amount of system deterioration and its rate.

Electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM) are the two techniques currently used for relatively rapid evaluation of corrosion protection offered by coating systems [metal substrate/pretreatment/primer/topcoat]. Traditional tests such as salt spray (ASTM B117) and Prohesion™ test provide qualitative data and are used for ranking coatings systems (1,2,3,4). ENM and EIS can provide quantitative information on both the extent and mechanism of coating degradation due to corrosion. Furthermore, these methods can be used to estimate the defect area at the coating/metal substrate interface. Four methods currently used to estimate the defect area using data from either EIS or ENM are described briefly.

308

©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

309 The first three methods discussed are based on values obtained from electrochemical impedance spectroscopy. The equivalent circuit (EC) method of data analysis provides a method to estimate capacitance (C), resistance (R) and "pseudo" inductance (L) valuesfroman EIS spectrum. The most common EC used to represent a coated metal substrate is shown in Figure 1 (5). Cc


Cdl

Hh

Rs

Rpo

Rp

Figure 1. Equivalent circuit used for interpretation of EIS data. 2

2

2

R (Ω cm ) is the solution resistance, Rpo(Q cm ) the pore resistance, Cc (μΡ/αιι ) the coating capacitance, Rp(Q cm ) the polarization resistance and Cdi (μΈ/cm ) is the double layer capacitance. The corresponding Bode plots from such an EC are shown in Figure 2. s

2

2

a

Log Frequency (Hz) Figure 2. EIS phase angle (•) and log modulus ( · ) vs. logfrequencyplots for degraded coating on a metal substrate.


310 The pore resistance value is a measure of the resistance of microscopic and / or virtual pores in a coating. Virtual pores are local areas of low film resistance caused by effects such as low crosslink density or high pigment volume concentration (PVC) in the continuous film. A plot of R vs. immersion time gives a good indication of coating degradation. It is defined as po

R° L

* =^ PO

0)

R° =pd

(2)


where po

Ad is the defect area, ρ is the coating resistivity and d is the total coating thickness. The ingress of ions into coating defects is slower than water so that, unlike capacitance values, Rpo cannot be measured accurately at very short and long exposure times. Therefore, it is necessary to understand that R cannot be used independently to determine the defect area of a coating, but must be combined with capacitance values. The following method introduced by Hirayama and Haruyama combines both capacitance and resistance terms to calculate the defect area of a degrading coating. 6 The method proposed by Hirayama and Haruyama is the first of two techniques discussed that utilizes the break point frequency method as defined below. It is designated as BPF1. p0

Break Point Frequency (1) [BPF1]. The BPF method utilizes the break point frequency in the high frequency region of the EIS spectrum. The break point frequency is the frequency at which the phase angle (Φ) equals 45°. It is accepted as being the point when the coating's resistive and reactive impedances are equal and is defined by equation 3:

/

=

_ L _

1 =

( 3 )

where C =C° A

(4)

C ° = ^

(5)

C

C

Where ε is the coatings dielectric constant and ε is the permittivity of free space. The coating capacitance is also dependent on the total exposed area A of the sample. Equation 1 shows that R is dependent on the coating resistivity, p, which is likely to change with exposure time. Due to this, it has been argued that BPF1 falls short for a 0

po


311 good estimation of defect area since both ρ and ε change with immersion time (5). Mansfield and Tsai recognized the coating resistivity dependence and proposed a new calculation using the break point frequency but also being independent of ρ (7,8). Break Point Frequency (2) [BPF2]. BPF2 combines both resistance and capacitance terms in the calculation, but also incorporates a second frequency term. The second frequency term is the frequency at minimum phase angle, f j . Mansfeld et. al. found experimentally that f i could be defined as (7,8). m

m

n

n


1 ,2

fmvn

l

A^C C R J e

(6)

m

where (7) A

d

By taking the ratio between ft and f j , the defect area can be determined independent of the coating resistivity, as shown by equation 8. m

n

TM^NM^ ν

J mm

C

(8)

c '

where

A and

C° ^dl^ «3=7

c:

00)

It was mentioned that it is necessary to incorporate capacitance terms with resistance terms in order to obtain an accurate account for the defect area of a degrading coating, as was done with BPF1 and BPF2. The third method uses only pore resistance values to determine the defect area of a coating and will be used as a comparison with the previous two methods. Pore Resistance Method. The defect area due to pores in the film can be measured using Rp independent of the capacitance values (9). This procedure tends to give defect area values five orders of magnitude lower than the defect area values 0


312 obtainedfromcalculations involving capacitance terms (9). Another way of defining pore resistance is shown in equation 11.

ι

1

where κ is the solution conductivity (Ω' cm"), d is the total film thickness, Ν is the number of pores and A is the size of each pore assuming all pores are of the same dimension. The effective resistance of a layer of the electrolyte solution occupying the same area as that of the coating is defined as R«, where: Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch025

p o

*e ^7 =

( 1 2 )

κΝΑ The ratio of the R p to R« gives the porosity of the coating. 0

R.

Moo

(13) Rpo

Λ

Again, these calculations are based on a constant ρ value during the period of immersion. As mentioned earlier, Rpo only provides reasonable measure of defect area for intermediate exposure. R values, however, can vary by up to two orders of magnitude among "identical" test samples (9). It is also important to note that pore resistance values can increase during exposure time. Such an increase is due to the blockage of pores by corrosion products and / or other insoluble contaminants. An increase in pore resistance values is commonly detected for aluminum alloy substrates where it is common for the oxide layer thickness to increase with time. The final method discussed is based on data retrieved from electrochemical noise measurements. p o

Electrochemical Noise Method (ENM). Skerry and Eden proposed a method that uses the coupling currentfromENM data to assess the defect area of a coating on a metal substrate (10). The coupling current is the resultant current induced by corrosion or thermal effects occurring spontaneously in the paired sample set-up. They suggested that the total blister volume could be approximated by determining the total charge passed through the coating system during the immersion period (4). The total charge (Q) is calculated using the DC coupling current obtainedfromthe ENM measurement and can be written as:

|β|

= Σΐ^

14

()

where At is the time increment between measurements and |i| is the absolute value of the coupling current. The charge can also be related to equivalent OH* groups formed


313 in the galvanic cell. The following half-cell equations represent the reactions at the metal surface: 2H2O + O2 + 4e" = 4 OH"

(15)

2 +

M = M + 2e"

(16)

With the assumptions that 1) the ionic concentration in the blister is equal to that of the bulk solution (Kn(mho cm") = Kbiister(mho cm")) and 2) the blister assumes a hemispherical shape, the total blister volume (V) is calculated as: Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch025

1

1

V-—M—-2

3

n

"^iF^3

v

nr

T

i

( 1 7 )

where F is Faraday's constant and r is the cross sectional radius of the blister. The defect area is estimated by determining the value of r. Since the blisters are assumed to be hemispherical in shape the cross sectional area is that of a circle and the defect area is defined as 2

Α =πτ

(18)

ά

The ratio of defect area to total area is calculated by dividing equation 18 by the total exposed area (A). Resultsfromelectrochemical monitoring of three different organic coatings on A1-2024-T3 were used to estimate the defect areas after 9 days of immersion in dilute Harrison's solution (77). The results obtained from each technique were compared with each other and with data obtained by image analysis of SEM micrographs.

Experimental Coating Systems The coating systems used in this study were received from McDonnell Douglas Aerospace, St. Louis, MO. The representative coatings systems were a spray coat epoxy/polyamide solvent borne (MIL-C-23377), a spray coat epoxy/polyamide water borne (MIL-C-85582) containing chromated pigments and an epoxy / blocked isocyanate electrocoat containing no inhibitive pigments. Each primer system was applied to a 3 inch by 6-inch aluminum alloy 2024-T3 panel. The substrate was cleaned and degreased prior to paint application. No conversion coating was used for any of the coating systems investigated. The film thickness (primer only) rangedfrom10 to 20 μηι. Electrolyte Solutions Three different electrolyte solutions are commonly used in the laboratory to study corrosion inhibition of coating systems. These solutions are i) 5% NaCl, ii) 3% NaCl and iii) dilute Harrison's solution ((8.5 mM (NH4) S0 and 26 mM (NaCl)). The 5% NaCl solution is used by researchers to compare their results with the ASTM standard B-117 salt spray test. The 3% NaCl solution has 2


4

314 been widely used as a simple electrolyte representative of marine salt water. Dilute Harrison's solution has been used by Skerry and others as the closest electrolyte representative of outdoor atmospheric conditions (72). This solution is also used in the ASTM standard G85-00 Annex A5 Prohesion™ Weatherometer test. Two of the primers used in this study are current military specification primers and the third (ecoat) is an experimental aerospace primer, therefore dilute Harrison's solution was the electrolyte solution used to reproduce conditions specific to an aircraft fleet. The specific conductance of the solution was measured using a Labcraft™ Digital Conductivity Meter with an accuracy of ±0.3%. The specific conductance values of the individual solution obtained at the time of measurement ranged from approximately 7 Q^cm" to 14.5 Q^cm" . 8


1

1

Prohesion™. Replicates of three of each sample were placed in a Q-Fog Prohesion™ Weatherometer for 2000 hours following ASTM standard G85-00 Annex A5(2). Tests consisted of cycling between dry conditions at 35 °C and wet (with electrolyte solution) conditions at ambient temperature. The cycle was repeated every 2 hours. Electrochemical Analysis. ENM and EIS measurements were conducted using a Gamry Potentiostat controlled by Gamry CMS 100 software. A Schlumberger® 1250 frequency response analyzer was used forfrequencycontrol during impedance measurements. The details of the analysis can be found in reference (75). Scanning Electron Microscopy. SEM samples of the coated panels were mounted on regular stubs, examined and photographed on a JEOL, JSM-6300V scanning electron microscope. Scanning areas were 600 μηι by 300 urn in most cases and micrographs with magnification of 1000 X were taken. Digital Image Analysis. SEM pictures are analyzed using Optimus® version 5.0 digital image analysis package on a P-90 workstation. Defect area estimations were based on the contrast between thefilledpores and continuous resin portions.

Results and Discussion The corrosion protection behavior of a solvent borne and a water borne epoxy/polyamide spray coat along with an epoxy / blocked isocyanate electrocoat (ecoat) primer was monitored using EIS and ENM. Figure 3 is the log modulus vs. log frequency plot for the three coatings after 9 days of exposure in dilute Harrison's solution. It is evident from Figure 3 that the e-coat system maintains capacitive behavior and displays no break pointfrequency.As observed by Tsai and Mansfeld, neither BPF1 nor BPF2 offer good prediction for high resistance coatings (> 10 Ω) (7). For this reason, defect area calculations for the e-coat were not made using BPF1 or BPF2. Table I lists the values obtained from ENM and EIS analysis that were used for calculation of the defect area. Table II lists the equations and the percent defect area for each sample. It must be noted that ENM measurements are done on two panels to obtain one value, which is an average of the two panels. Each ENM estimation will 7



315

Figure 3. Log Modulus vs. Logfrequencyfor water borne ( Δ ) , solvent borne (•) and e-coat (o) primer systems after 9 days exposure in dilute Harrison's solution. have two corresponding EIS values. Also included in Tables I and II are values and equations used for the R method. The standard deviations of the coupling currentfromENM measurements are shown in Figure 4. The values displayed are on the average 2 orders of magnitude lower than the actual coupling current values. The higher values for the solvent borne spray coat are indicative of a more pronounced extent of corrosion taking place in the system. p o

The standard deviations are used to highlight the trend displayed from ENM measurements. According to the results presented in Figure 4, the defect areas increase in the order of solvent borne > water borne > e-coat. The results displayed in Table 2 also show the above trend when using ENM values for defect area estimations. The trend is not the same when using EIS data to estimate the defect areas. In the cases where the BPF techniques are used for calculations, the e-coat could not be evaluated and the water borne system displayed a higher defect area with respect to the solvent borne system. The defect area estimations from the ENM and the BPF techniques give values, which varyfromslightly under 1% up to 8%. Values from the Rpo method are very low compared to the previous estimations. These low values quantitatively suggest that no delamination is occurring. As was mentioned earlier, R cannot be used alone to measure the defect area because the coatings display both capacitive and resistive properties. All of the estimations obtained from ENM, BPF1 and BPF2 lie within one order of magnitude. Several aspects may be addressed to explain the slight ambiguity in the numbers. First the differences between estimations from the BPF methods and the ENM method may be accounted for by analyzing what is actually measured when using EIS compared to ENM. When using ENM, as described in this paper, the resistive and capacitive components of a coating system cannot be separated, but instead are measured as a whole. Therefore the ambiguity can only be solved when these componentsfroman ENM measurement can actually be separated and the data re-evaluated. Secondly, the ENM calculations are based on several assumptions. To reiterate what was mentioned earlier, these p o


316

Table I. EIS and ENM parameters extracted from electrochemical analysis after 9 days immersion. Parameter / File ΜΩ)

E-coat #2

13

10

2.75E+05 3.93E+04 1.00E+06 5.50E+06 n/a

n/a

Solvent Borne #2 101

Water Borne #2 73

Water Borne #1 77

C (F)

3.18E-06

2.97E-06

5.00E-05

5.31E-05

n/a

n/a

Q (F)

3.04E-08

2.83E-08

2.18E-08

5.10E-08

2.50E-09

2.80E-09

Re(n>

1.58E+04 2.50E+03 4.50E+05 4.40E+05 2.08E+09 2.20E+09

fb (Hz)

335.00

3257.00

f«in(Hz)

32.57

V U

10.29

f


E-coat #1

Solvent Borne #1 101

11.00

12.90

n/a

n/a

258.00

0.26

0.26

n/a

n/a

12.62

42.64

50.00

n/a

n/a

17.70

n/a

n/a

3.86E+03 8.59E+03 6.63E+05 7.66E+05 n/a

n/a

24.57

27.70

Φ Rc at t=0 (Ω)

/ A = 1 e-4) at t=0 (Ω cm ) 4.323

9.621

2

R° ( c

17.40

742.560

857.920

n/a

n/a

5.30E-06

4.17E-06

1.00E-05

3.00E-05

n/a

n/a

C° i (A / A = le-4) at t=0 (F/cm ) 4.73E-03

3.72E-03

8.93E-03

2.68E-02

n/a

n/a

Cdi at t=0 (F) 2

d

d

C (t=0)(F) c

2

C° = C (t=0) * A (F/cm ) c

c

2.5E-08

1.92E-08

2.47E-08

2.33E-08

n/a

n/a

2.8E-07

2.15E-07

2.77E-07

2.61E-07

n/a

n/a

1.80E-03

1.80E-03

7.50E-04

7.50E-04

2.10E-03

2.10E-03

9.60E-03

9.60E-03

9.60E-03

9.60E-03

9.60E-03

9.60E-03

A,otai (cm )

11.20

11.20

11.20

11.20

11.20

11.20

IQI (C/cm )

0.05

0.05

0.03

0.03

0.016

0.016

d

(cm)

κ (mho/cm) 2

2

Table II. Defect area calculations and percent defect area for each coating system after 9 days immersion. Parameter/file d/(R *K)

= NA (cm )

d/KA=R«

(Ω/cm )

c

ncgrn2b

1.06E-06 6.70E-06

1.55E-08

1.59E-08 9.39E-12 8.88E-12

d

2

0.017

0.007

NA /A

0.007

1.38E-09

1.42E-09 8.38E-13 7.93E-13

ncylw2b

0.017

9.46E-08 5.98E-07

d

ncgrylb

ncgry2b

ncgrnlb

ncylwlb 2

0.020

0.020

1.55E-08

1.59E-08 9.39E-12 8.88E-12

0.003

0.042

0.014

0.018

n/a

Α*/Α, υ = (f /f „) *C 7C (BPF2) 0.006

0.009

0.056

0.024

n/a

n/a

% Defect Area ENM

8

8

5

5

2

2

% Defect Area BPF1

0.3

4.2

1.4

1.8

n/a

n/a

% Defect Area BPF2

0.6

0.9

5.6

2.4

n/a

n/a

% Defect Area

9x ΙΟ

6x lO.j

1 χ 10

1.06E-06 6.70E-06 e

0

A d / A ^ = fb*(2itC Rc ) (BPF1) c

2

οϋ

b

nu

0

c

dJ

-6

7

1 χ 10

7

8x 1 0


n/a

n

8x 10""

317

estimations are based on the fact that Ko = Kbiister, the shape of the defect is hemispherical, the only cathodic reaction is the reduction of water, and the reactions involved are irreversible. When dealing with blisters, the conductivity of the solution inside the blister is usually different from that of the solution. Blisters are caused by impurities or corrosion product at the coating/metal interface. When the impurity is ionic orfroma corrosion product the conductivity of the blister solution is higher than the bulk solution. When the impurity isfroman organic species, the conductivity of the blister solution is lower than that of the bulk. Two of these assumptions can be eliminated by assuming that pores or breaks in the film rather than blisters are the major source of film defects. Thus one can say that KQ Kp and the volume is estimated by using the volume of a cylinder with a hemispherical shaped defect onto the substrate (where r(cylinder) = r(hemisphere)). This does not account for outward spread of the defect or a pit deeper than the radius of the pore. The values obtained using this assumption were within 0.5% of the estimations made using a hemispherical shape. Thirdly, the two break pointfrequencymethods also are based on several assumptions. BPF1 assumes constant film thickness, d, and coating resistivity, p. Mansfeld and Tsai's calculations correct the coating resistivity by defining an equation that is independent of p, but BPF2 is also dependent on C

Organic Coatings for Corrosion Control - American Chemical Society

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