Initial Studies of Electrochemical Comparison of Coating Performance

Chapter 1 2

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Initial Studies of Electrochemical Comparison of Coating Performance in Flowing versus Stationary Electrolyte

Carol S. Jeffcoate and Gordon P. Bierwagen

Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105 Electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM) were used to evaluate the corrosion protection performance of 3 different organic marine coatings (combinations of an alkyd (A), a zinc silicate (Z) and a green epoxy (G) as primers and two top coats were a haze gray epoxy (H) and a silicon alkyd (S).) over mild steel in both static immersion andflowingelectrolyte conditions. The calculation of noise resistance(R )was made using 3 different set sizesfromthe accumulated raw data, where the sample set of 40 raw data points gave the best results. The flowing electrolyte has a marked effect upon the performance of a coating system. For the coating systems tested the R andlowfrequency impedance values were significantly lower in theflowingcells, when compared with the values obtained from similar panels in a 'stationary' electrolyte. n

n

Electrochemical methods have long been utilized to assess the corrosion protective performance of coatings. Although the earlier work was with DC resistance^), recent work has concentrated on more sophisticated techniques^, 3, 4). The methods used in this study are electrochemical noise measurement (ENM)(5, 6, 7) and electrochemical impedance spectroscopy (EIS)(£, 9). The primary advantage of ENM is that it is a non perturbing technique i.e. the spontaneous current and voltagefluctuationsare measured between two nominally identical panels, with no external potential or current applied(7). The method can be applied as a semi-continuous monitoring technique and with time gives an indication of the overall coating performance. EIS gives more detailed information about the coatings electrical properties but at the sacrifice of perturbing the electrochemical system by applying a scan of AC potentials over a range of frequencies(P). ©1998 American Chemical Society

151


152 This work has been designed to test typical marine coating systems for corrosion protection performance in a laboratory environment. The function of a marine coating is two fold. The primary requirementfora coating, of this type, is to provide corrosion resistance. The coating is the primary method for corrosion protection with cathodic protection being used as a secondary method(/0). Another function of marine coatings is to provide anti-fouling protection^/, 12). Fouling from animal and vegetable matter is a matter of considerable concern. The increased drag through water causes a drop in maximum speed and efficiency for a given vessel Environmental restrictions on chromate pretreatment and anti fouling chemicals has spurred on the search and developments of new environmentally compliant coating systems. All new coatings require extensive preliminary laboratory testing and, ultimately, very expensive marine exposure evaluations. Marine exposure, despite the extreme cost, is a very necessary part of thefinalevaluation. In an attempt to have a more 'realistic' laboratory assessment, the flowing electrolyte has been added to the testing scheme. The typical laboratory test set-up includes a conductive electrolyte (usually containing chloride) in 'stationary' contact with the test panels. This work investigates, via electrochemical testing the effect of a flowing electrolyte on the degradation of marine coatings.

Experimental Sample Preparation The test samples were three layer marine coatings on mild steel. The layers were combinations of three primers: an alkyd (A), a zinc silicate (Z) and a green epoxy (MIL-P 24441 type 1 F-150) (G). The two top coats were a haze gray epoxy (MH-P 24441 type F-151) (H) and a silicon alkyd (S). Each of the three layers were of approximate equal thickness and gave an average total coating thickness of 200±10umThe primer/topcoat combinations tested in this investigation were (from substrate inter&ce to electrolyte) AAS, ZAS and GHS. The GHS and AAS combinations were chosen as typical barrier type marine coating systems currently in service. The zinc silicate in the ZAS system works mainly by a sacrificial mechanism, and is non-common for this application. The electrical contact was made through wires which were attached to the test samples. The back and edges of each sample were coated with a colophony rosin / beeswax mixture (3/1) to act as an effective inert, high resistance protective coating. The exposed surface area of the test panel was 50 cm (0.005 m ). 2

2

Cell Design For both theflowingand the stationary electrochemical cell, a solution of 3% sodium chloride (NaCl, analytical grade) in distilled deionized water was used as the conductive electrolyte. Solutions in both cells were open to the laboratory air and changed monthly to minimize algae growth. The reference electrode for ENM was a saturated Calomel electrode, and for the EIS measurements a platinum secondary electrode was also used.


153

Figure 1 Schematic of theflowingelectrolyte electrochemical cell, showing the baffles andfluidflowpattern.

The cell containing theflowingelectrolyte was constructedfroma 35x25x25 cm high density polyethylene (HDPE) tank with three, 26 cm baffles to obtain a total path length of approximately 95 cm, as shown in figure 1. A constant volume of 10 liters of electrolyte was maintained and a pump circulating at 35 liters/minute provided aflowrate of approximately 0.055ms* (3.33 mmin *), ensuring laminarflowover the surface of the samples. The stationary electrolyte electrochemical cell consisted of a circular glass dish 18 cm in diameter and 10 cm deep with sufficient electrolyte to cover the exposed surface of the samples. A slotted polystyrene sheet held the samples in position vertically (the pairs facing each other, with a gap of 2cm between them). For theflowingelectrolyte cell, the electrochemical measurements were taken after the pump was switched off andflowstopped. 1

Sample Pairing for Noise Studies Three nominally identical panels of each of the three types of paint system were immersed into theflowingcell By cross pairing sample the three panels of each type, it was possible to maximize the number of possible electrode pairs. By cross pairing the three single panels (A, B, C) this allowed three pairs of panels for the electrochemical noise (ENM) studies (AB, AC, BC) for statistical validity.

Experimental Methodology EIS and ENM measurements were takenrightafter immersion in the test cells and then at recorded intervals during the immersion time. Thefluidpump was switched off during measurements to ensure static conditions. ENM measurements on the nominally identical panels (paired by AB, AC, BC) were taken onfirstimmersion and at intervals of 15 to 20 days during the extended exposure. Sample acquisition rate for the simultaneous raw potential and raw current data was every 2 seconds (0.5 Hz). The noise resistance (Ro) was calculated using the

154 standard deviations of therawpotential (σν) and therawcurrent noise (σι) as in equation 1.


R

=

Calculations of the standard deviation were made by taking 10,40 and 128 raw data points and variations in the resultant Ro values recorded. For the EIS measurements thefrequencyrange wasfrom65500 to 0.02 Hz and the applied AC voltage was 10 mV R M S . Results and Discussion

Electrochemical Noise Method The variation of raw data sample size, Le. the number of raw data points used to calculate the standard deviation of the simultaneously measured current and potential values,forthe calculation of Ro gave varying results, an example of which is given in table I. The smaller sample size of 10 raw data points taken over a time period of 20 seconds gave, at several time intervals, a divide by zero error, Le. a steady state current was measured over a 20 second sampling period therefore producing a zero standard deviation. The variable Ro observedforthe 128 raw data point (taken over 256 seconds) sample size is worthy of caution because the σν and σι values may be artificially high due to any uncorrected DC drift occurring during the measurement time-span. Therefore a raw data sample size of 40 (taken over 80 seconds) data points was chosen for the calculation of Ro, to minimize the effect of DC drift and divide by zero errors. Table I Typical noise resistance values for AAS pair 1, calculated using the standard deviation of 10,40 and 128 raw data points. Days 0 4 10 25 42 66 76

Rn,

Rn,

oof 10 Data Points Divide by Zero Error 3.25 xlO 3.32 xlO 2.26 xlO Divide by Zero Error 7.09x10* 5.18 xlO 7

7

7

5

aof40 Data Points 1.22x10" 3.20 xlO 3.16 xlO 2.25 xlO 2.35 xlO 1.17 xlO 2.12 xlO 7

7

7

7

7

5

oof 128 Data Points 1.28x10" 3.19 xlO 2.90 xlO 2.17 xlO 2.83 xlO 1.37 xlO 2.19 x10 7

7

7

7

7

s

155 The noise resistance (Rn) vs. time (130 days) plots for all sample pairs immersed in theflowingelectrolyte is shown infigures2 and 3. The trend of Rn for the three samples of each coating type is shown to be very reproducible. The alkyd (AAS) samples show an initial drop of Rn followed by a reasonably steady state reading for the majority of the exposure period. A slight, further, decrease is seen in all the pairs after 96 days. Epoxy sample pairs showed a similar trend. An initial very high value of Rn sharply decreasedfroml.lxlO to 6.8x10 Qcm after 10 days of immersion, followed by a relatively constant Rn value for 60 days. The rapid decrease in Rn observed in all the sample systems, on initial immersion, is due to water uptake and consequent swelling of the films. The last of the test systems, ZAS, exhibited slightly different behavior. The initial, immersion, value of Rn (4.8x10 Qcm ) was lower with a sharp decrease to Rn values close to values expected for a bare steel electrode after 10 days of immersion. A similar effect is observed with the samples immersed in the stationary electrolyte. Average Rn values vs. time plots for samples in the stationary electrolyte is shown infigure4. On immersion in the stationary electrolyte all the coating types exhibited high values of Rn, in the range of 10* - 10 Qcm . The AAS samples maintained a high Rn value, above 10 Qcm , for the duration of the test. The GHS also maintained the average Rn value above 10 Qcm for the monitoring time. ZAS again proved to be the exception, showing a large drop in R« value after only 10 days of immersion. After the 70 day immersion time the Rn value dropped further to 10 Qcm , similar to that of theflowingelectrolyte case.


10

s

2

7

u

8

2

2

2

8

2

5

2

Figure 2 Ribbon graph of noise resistance vs. time, showing the reproducibility between the pairs of panels with identical coatings


156

——AAS 1

- « - AAS 2

—·—AAS 3

GHS 1

GHS 2

-•••-GHS 3

ZAS 2

-

— Z A S

G

1

—

-

ZAS 3

AAS

1Ε+08

ZAS 25 42 T i m e / Days

Figure 3 Line graph of noise resistance vs. time, showing the reproducibility between the pairs of panels with identical coatings

G

1E+08

• — ~»

AAS GSH ZAS

10 Time, Days

Figure 4 Noise resistance vs. time plot, for the stationary electrolyte electrochemical cell, showing the poor performance of the ZAS compared to the performance of the AAS and GHS panels


157 It is seen that although the initial (on immersion) Rn values were similar for all three coating systems, for both types of test, the Rn value after a period of 1 month was found to be consistently lower for all samples in theflowingenvironment. Figure 5 shows a direct comparison of the two cases. Of the three coating systems investigated the most marked difference is seen with the AAS coating system A variation of nearly three orders of magnitude between theflowingand the stationary Rn values was observed. The GHS samples in theflowingcell showed a drop of Rn values by two orders over the values obtainedfromstationary electrolyte samples. This reduction in the Rn value for a 'good' coating in the stationary cell re-classifies the coating to 'firir' when theflowingsystem is considered. The resultsfromboth of the ZAS sample sets indicate a presence of conductive pathways to the metal surface. Rn values for both theflowingand stationary electrolyte are shown in table Π. 1E+12

Figure 5 Ribbon graphs showing the difference in noise resistance values for the flowing and the stationary electrolyte electrochemical cell Table Π Average noise resistance values, with time, obtainedfromtheflowingand the stationary electrolyte tests. AAS R . , Qcm Stationary on Immersion Stationary after 22 days Stationary after 70 days Flowing on Immersion Flowing after 25 days Flowing after 76 days

GHS Rn, Qcm

2

2

9

6.11 xlO

6.91 xlO

9

4.09x10"

2.69 xlO

8

3.77x10"

2.79 xlO

10

1.07 xlO

10

4.82 xlO

7

2.53 xlO

7

4.29 xlO

6

3.4 xlO

2.97x10" 5.79 xlO

1.33 xlO 4.08 xlO

ZAS Ro, Qcm

2

3.42 xlO

2.03 xlO

9

6

7

5

7

6

4

1.31 xlO

158


Electrochemical Impedance Spectroscopy The impedance spectra were viewed with regard to the shape (one or two time constants) and the value for the maximum stable impedance modulus ( | Z | m B x ) . The IZjmax value gives an indication of the barrier properties of thefilm,the higher this value, the better the barrier type protection for a given coating system Monitoring the |Z|max value over an extended period of time gives observations similar to noise data. Samples in theflowingelectrolyte gave values of |Ζ|η»χ, after an immersion of one month, significantly lower than those obtainedfromthe samples immersed in the stationary electrolyte. Table ΙΠ lists the |Z| max values obtained from the EIS measurement for theflowingand the stationary cells. Table ΙΠ : Comparison of EIS Data obtainedfromStationary and Flowing Electrolytes. AAS |Z|max » & C m Stationary, on Immersion Stationary, after 28 days Stationary, after 96 days Flowing, on Immersion Flowing, after 35 days Flowing, after 110 days

3.9 xlO

10

9

7.5 xlO

2

IZLx.Qcm 9

6

9

4xl0

7

2xl0

8

7xl0

10

2.7 xlO lxlO

6

5

3.5 xlO

2

5.5 xlO

6xl0

2.0 xlO

8xl0

IZLx.Qcm

7

2.2 xlO

2.4 xlO

ZAS

GHS 2

5

10

2.8 xlO 3xl0

7

6

1.4 xlO

10

6

4

4.2 xlO

The initial IZImax values for all the panels were, within an order of magnitude as compared with R„ values obtainedfromthe noise measurements. After immersion in their respective environments for three months the |Z|,n« values resultsfromthe flowing cell were significantly lower than the values for the stationary electrolyte, shown infigure6. An exception to this is where the ZAS system, |Z| and Rn values show the same trend in both environments. The ZAS coating system protects the steel substrate by acting in a sacrificial manner. Sacrificial coatings are usually designed and formulated above the critical pigment volume concentration (CPVC) and would therefore be porous in nature. This type of formulation leads to a direct electrolyte connection through the open porous structure of the coating system. As expected, low Rn and |Z|max values were observed and extensive blistering occurred in both the flowing and stationary electrolyte. In all there is a good correlation (within an order of magnitude) between the R„ and the |Z|m« values with the exception of the stationary AAS which produced higher R„ values. Theflowingenvironment, although still maintaining laminarflow,is much more aggressive than the stationary cell.


159

Figure 6 Ribbon graphs showing the difference in impedance modulus values for the flowing and the stationary electrolyte electrochemical cells. As in the previous figure note the large difference in |Z|,nax values between theflowingand the stationary electrolyte for the AAS and the GHS systems

Conclusions When calculating R„fromraw potential and raw current data, the number of data points in the sample size needs to be chosen carefully. An uncompensated DC drift in the potential data results in an artificially high R„ value, conversely a drift in the current signal produces a depressed value of R«.. Too small a sample size, especially for good coating systems, produces divide by zero errors periodically, reducing the effective data size and possibly leads to inaccurate estimations of the coating performance. The introduction of aflowingelectrolyte into an electrochemical experimental set up has a marked effect upon the performance of a coating system. For the coating systems tested the Rn and |Z|m« value were significantly lower in theflowingcells, when compared with the values obtainedfromsimilar panels in a 'stationary' electrolyte. The reduction in effectiveness of these barrier type coatings reduces the ranking of both the AAS and the GHSfrom'good' to fair'(7) coatings. The |Z|max values obtainedfromEIS measurements show a good correlation to within one order of magnitude, with the R„, values obtainedfromthe ENM method. c

Acknowledgments This work was carried out under the auspices of the Office of Naval Research, Grant No. N00014-95-1-507, and the National Science Foundation - Industry / University Coatings Research Center.

160


References 1. 2. 3. 4. 5.

Bacon R.C., Smith J. J., Rugg F. M., Ind. Eng. Chem. 1948, 40, pp. 161-167 Skerry B.S., Eden D.A, Prog.Org. Coatings 1987,15,pp. 269-285 Skerry B.S., Eden D.A., Prog. Org. Coatings 1991, 19, pp. 379-396 Chen C.T., Skerry B.S., Corrosion1991, 47, pp. 598-611 Mills D.J., Bierwagen G.P., Skerry B.S., Tallman D.E., Proc. of the 12th

international Corrosion Congress, 1993, Vol 1 pp. 184 6. Bierwagen G. P., Mills D. J., Tallman D. E., Skerry B. S., Proc. of Symposium on

Corrosion and Corrosion Prevention in Seawater Environments, E.C.S., O 1993, Abstract No 81 7. Bierwagen G. P., J. ElectrochemicalSoc.,1994,141,pp. L155-157 8. Electrochemical Impedance, Scully J.R.; Silverman D.C.; Kendig M.W., Eds.; Special Tech. Publ. 1188, ASTM, Philadelphia, PA, 1993 9. Impedance Spectroscopy, MacDonald J.R., Ed.; Wiley-Interscience, New York, NY, 1987

10. Callow, M. Chemistry and Industry, 1990, pp 123 11. Martin, Β. Α., Materials Performance 1994, 33, pp 12 12. Coulson, K.E.W., Barlo, T.J., Werner, D.P., Oil & Gas Journal, 1991, 89, pp 80

Initial Studies of Electrochemical Comparison of Coating Performance

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