08005 - cathodic disbonding of epoxy coatings - effect ...

Paper No.

08005

CATHODIC DISBONDING OF EPOXY COATINGS – EFFECT OF TEST PARAMETERS Ole Øystein Knudsen SINTEF Materials and Chemistry 7465 Trondheim, Norway [email protected] Jan Ivar Skar Hydro Aluminium Research Centre PO Box 2561 3908 Porsgrunn, Norway

ABSTRACT A large number of standardized test methods for cathodic disbonding are available. Parameters like voltage, electrolyte composition, temperature, sample configuration etc vary over a wide range between the various tests. A round robin test recently organized by NACE showed that one can expect significant variation in result between test labs, in spite of identically coated samples and test procedures. This paper presents a review of published literature on the effect of various parameters on cathodic disbonding, and results from a study on the effect of oxygen concentration, dry film thickness, applied potential and electrolyte composition on cathodic disbonding. The objective with the paper is to discuss the importance of various test parameters on cathodic disbonding results: Electrolyte composition, presence of hypochlorite in the electrolyte, applied potential, dry film thickness, and oxygen concentration in the electrolyte. Keywords: Organic coatings, cathodic disbonding, test parameters

Copyright Government work published by NACE International with permission of the author(s). The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

1 Ole Knudsen - Invoice INV-204089-AHEQZL, downloaded on 3/30/2009 9:18:59 AM - Single-user license only, copying and networking prohibited.

INTRODUCTION The mechanism of cathodic disbonding is not known in every detail. Researchers have generally agreed that the cathodic reaction forms an alkaline water film under the coating that causes the disbonding. However, the exact consequence of the alkalinity is not known. The transport of reactants for the cathodic reaction has also been subject to some discussion, as the review below will show. Several authors have suggested mechanisms for cathodic disbonding. Watts has sorted these mechanisms in three groups, depending on their locus of failure:1 in the coating near the interface2-6, at the interface7,8 or in the oxide on the steel surface.9,10 Common for all these mechanisms is the assumption that the cathodic reaction under the coating forms a high pH aqueous film at the interface, which is responsible for the disbonding. Both oxygen reduction (Eq. 1) and hydrogen evolution (Eq. 2) are possible cathodic reactions that will increase the pH at the cathode: O2 + 2 H2O + 4e− = 4 OH−

(1)

2 H2O + 2e− = H2 + 2 OH−

(2)

According to Leidheiser, the two reactions have the same rate on bare steel in seawater when polarized to about -1000 mV SCE.11 At higher potentials the oxygen reduction dominates, while the hydrogen evolution dominates at lower potentials. Leidheiser assumed that the resistance between the anode and the cathode is so high that the potential at the cathode under the coating is too high for hydrogen evolution. Haji-Ghassemi et al. have shown that the sample must be polarized to potentials well below -1200 mV SCE to get significant hydrogen evolution under the coating.12 The oxygen reaction will therefore be the dominating cathodic reaction under the coating, both at the free corrosion potential and under cathodic polarisation with zinc or aluminium anodes. The oxygen reaction requires supply of oxygen and water, but also cations to balance the negatively charged hydroxide on the product side. Coatings are usually quite permeable to both oxygen and water. Researchers have therefore generally assumed that oxygen and water are transported through the coating.1,10,13,14 The transport of cations has been subject to more discussions. When cathodic disbonding take place under free corrosion Leidheiser et al. and Stratmann et al. have concluded that the cations are transported in the aqueous film under the disbonded coating.10,15 However, for cathodically polarized samples authors have found indications for transport of cations through the coating. Leidheiser and Wang found that the disbonding rate of a 10 – 20 µm thick polybutadiene film depended on both cation mobility and film thickness, so they concluded that the cations go through the film.16 Parks and Leidheiser found that polarization of a painted sample increased the uptake of ions in the coatings three to twenty eight times.17 They then assumed that the flux of ions through the coating was sufficient to account for the observed disbonding rates. Leidheiser and his group have mainly studied polybutadiene coatings with film thickness less than 50 µm. Jin et al. found that the disbonding rate of chlorinated rubber decreased with increasing film thickness up to 50 µm, but were constant for film thickness above 50 µm. This was attributed to a change in the rate limiting step from transport through the film for thin coatings to transport under the disbonded film for thick coatings. LITERATURE SURVEY OF FACTORS AFFECTING CATHODIC DISBONDING Oxygen Concentration in the Electrolyte Stratmann et al. found that no disbonding occurs in the absence of oxygen under free corrosion potentials.15 The disbonding of an unpigmented alkyd resin was investigated in argon-atmosphere with


a scanning Kelvin probe. In Ar-atmosphere no galvanic element was formed and no disbonding was observed. When the Ar-atmosphere was replaced by O2 the disbonding started. Leidheiser and Wang found that little cathodic disbonding occurs when the oxygen is removed from the electrolyte.16 The samples were polarized to -1.35 V SCE. In a later paper they investigated the disbonding of an epoxy coating in various electrolytes with different oxygen solubility. It was then found that the disbonding rate decreased when the oxygen concentration decreased.10 Electrolyte Type and Concentration Several studies have dealt with the effects of the cation type and concentration on the disbonding rate. Leidheiser et al. reported that the disbonding rate increased with the cation type in the following order: CaCl2, LiCl, NaCl, KCl, CsCl. This order agrees with the mobility of the cations in water.10 The disbonding in solutions of divalent cations has been reported to be very slow or not occurring at all.16,18,19 This is attributed to the low solubility of the various hydroxides, which gives a lower pH compared to the alkali metals. Leidheiser et al. have reported that cathodic disbonding is independent of the type of anion in the solution. The disbonding rate was found to be almost the same in NaCl, NaBr and NaF solutions.16 Leidheiser and Wang have studied cathodic disbonding of a polybutadiene film as function of electrolyte concentration.16 They found that the rate of disbonding decreased with electrolyte concentration. They attributed this to the water activity in the electrolyte. When the electrolyte concentration increases the water activity decreases, and the water concentration in the polymer film also decreases. Then both the transport of water through the coating and the ionic conductivity of the film decrease. Increasing the electrolyte concentration also decreases the oxygen solubility, which also may decrease the disbonding rate. Applied Potential It seems to be generally accepted that the disbonding rate increases with decreasing electrode potentials. Jin et al. have reported that they found a linear relationship between applied potential and the disbonding rate.20 Kendig et al. found that the disbonded distance increased with decreasing potential down to about -1000 mV (Ag/AgCl), but that the effect of potential was smaller below -1000 mV (Ag/AgCl).21. The potential on the metal exposed in the coating defect is approximately equal to the applied potential. Due to the resistance in the electrolyte under the disbonded coating, the potential increases with distance from the initial coating defect. This means that in the disbonding front at the potential is higher than the applied potential. Stratmann et al. have measured the potential under the disbonded coating with a Scanning Kelvin Probe.15 The holiday in the coating was at the free corrosion potential, about -750 mV SCE. The potential under the coating increased with distance from the holiday. When the disbonded distance was about 6 mm, the potential in the disbonding front was about –350 mV SCE. Steinsmo has investigated the effect of the potential on charge transport through continuous coatings.22 The charge transport was found to increase with decreasing potential. Polarisation curves were obtained for some of the test specimens, and showed an accurate linear relationship between potential and current density. Hence, the coating behaved as an ohmic resistance. Dry Film Thickness Jin et al. have reported that the disbonding rate decreased linearly when the DFT increased up to 100 µm for a chlorinated rubber lacquer, but that the disbonding rate was independent of DFT above 100 µm.20 However, he had very few data points above 100 µm. Leidheiser et al. have reported that the rate


of cathodic disbonding of polybutadiene films decrease with increasing film thickness.10 The films they studied were less than 50 µm thick. Time Dependence of Disbonding Several researchers have found that the disbonded area increases linearly with time.10,15,20,21 Leidheiser et al. have indicated that this relationship can be derived from Fick's 1st law by making the proper assumptions.10 If the transport of cations is rate limiting for the disbonding, the cations must then be transported by diffusion in the aqueous film under the disbonded coating. If the cations mainly are transported by electromigration under the disbonded film, following Ohm's law, the relation between disbonded area and time may be different. Both migration and diffusion may also occur at the same time. This means that Ohm's law or Fick's law can not be used alone for deriving a mathematical expression for disbonded area as a function of time. When plotting the disbonded area as function of time, it is usually found that the intercept with the time axis is somewhat later than the start of the experiment. This time is called the delay time and is usually attributed to the ingress of water, oxygen and ions (reactants) into the coating.10 After the reactants have reached the substrate under the coating the cathodic reaction and the disbonding process can start. Pre-treatment of the Substrate and Surface Roughness Jin et al. 20 studied the disbonding rate as a function of surface roughness. The coating was a 20 µm chlorinated rubber lacquer and the surface roughness varied around 1 µm. The disbonding rate was found to decrease linearly with increasing surface roughness. This was believed to be due to the increased contact area between the coating and the substrate when the roughness increased. Watts et al. have also measured cathodic disbonding as a function of substrate roughness.8 They studied substrates with a larger span in roughness (Ra between 0.1 and 3.8 µm). They also found that the rate of disbonding decreased with increasing surface roughness, but not linearly. The effect was largest up to Ra = 2. They explained the effect of the surface roughness by increased diffusion path length for sodium ions. If the sodium ions are transported by diffusion along the metal surface, increasing the surface roughness would increase the diffusion path length. Leidheiser et al. have studied the effect of different surface pre-treatment on cathodic disbonding.16 Various ways of cleaning the steel surface was tested. Phosphating the surface decreased the disbonding significantly. Harun et al. investigated surface pre-treatment with certain types of silanes, and found that 3-glycidoxypropyltrimethoxysilane (3-GPS) reduced the disbonding rate of an unpigmented epoxy lacquer (60 µm) by a factor of three. No effect was found with an unpigmented alkyd.23

Effect of hypochlorite Hypochlorite is formed at the counter electrode when polarizing a sample cathodically with a potentiostat in chloride containing solutions (Eq. 3). Hypochlorite is also formed at the counter electrode in impressed current systems for cathodic protection. Polarization with sacrificial anodes does not produce hypochlorite, since the anodic reaction then is the corrosion of the sacrificial anode. During testing of cathodic disbonding in small electrolyte volumes, a significant concentration of hypochlorite can develop in the electrolyte. The formation is directly proportional to the polarization current. Hence, the amount will increase with number of samples and holiday size on the samples. Cl-(aq) + 2OH-(aq) = ClO-(aq) + H2O(l) + 2e-

(3)


Hypochlorite is an aggressive species and may attack the coating chemically. This is an artificial effect, since it will only appear in laboratory testing. According to Kehr the effect can be significant if the coating is thin and the test temperature is high.24 The effect can be avoided by placing the counter electrode in a separate electrolyte, connected to the test electrolyte with a salt bridge or a membrane.

MATERIALS AND EXPERIMENTAL METHODS

Materials The steel specimens were prepared from hot rolled steel with composition 0.10% C, 0.10% Si, 0.52% Mn, 0.14% P, 0.014% S, 0.23% Cr and Fe to balance. Two pre-treatments were used. Either the panels were blast cleaned with steel grit G24 to Sa 2½ and medium roughness (Ry ca 70 µm),25 or they were ground (Ry ca 5 µm) and degreased. Two model coatings, prepared with an amide cured epoxy binder, were used. One was pigmented with an extender only (Epoxy 0), while the other also contained 5% (wt.) aluminum flakes (Epoxy 5), see TABLE 1. In addition a commercial coal tar epoxy and a commercial vinyl tar were included in some of the tests. On blast cleaned substrate the coating films were applied in two coats by airless spraying to an average dry film thickness of about 300 µm. On ground steel the coatings were applied by a slide applicator in two coats to a total dry film thickness of about 200 µm. The coatings are thinner than normally recommended for immersion service, but sufficiently thick for the short term tests performed in this study. TABLE 1 Composition of model epoxy coatings used for studying cathodic disbonding. Epoxy 0

Epoxy 5

Amide cured epoxy resin Extenders Solvent Additives Aluminium paste, non-leafing

36.0 26.7 35.9 1.4 -

34.4 25.3 33.7 1.3 5.3

Density of wet paint [g/cm3] PVC (%) Vol % solids

1.24 21.6 51

1.25 24.0 51

Testing of Cathodic Disbonding The following parameters were studied in this investigation: Electrolyte composition, presence of hypochlorite in the electrolyte, applied potential, dry film thickness, and oxygen concentration in the electrolyte. Samples for cathodic disbonding were given a circular holiday in the middle, 6 mm in diameter, to start cathodic disbonding. The samples were polarized by a potentiostat. Test conditions and duration varied from test to test and is given for each experiment in the Results and Discussion section. The disbonding was measured destructively by gently lifting off the disbonded film with a scalpel. The diameter of the disbonded circle was measured in four directions and the disbonded distance was calculated from the average diameter. Unless stated otherwise, three parallels were


tested. Testing of cathodic disbonding in pure nitrogen or oxygen atmosphere was performed in a glass loop described in FIGURE 1.

W C

5 R

7

6

10

3

A

1

4

2 1 Electrolyte reservoar 2 Pump 3 Oxygen selective electrode 4 Celles for exposing samples 5 Potentiostat

9 8

4

6 Refecence cell (SCE) 7 Gas inlet 8 Coated steel samples 9 Pt counter electrode 10 Keithley mod. 617 Electrometer

FIGURE 1 - Apparatus for exposing samples in electrolytes with controlled oxygen concentration.

Resistance Measurements on Free Films Free films were mounted between two cells filled with electrolyte. Resistance measurements were performed with a Keithley Model 617 Programmable Electrometer connected to two saturated calomel electrodes. The electrodes were connected to the electrolytes through liquid junctions. FIGURE 2 shows a schematic drawing of the cells and the connections. The resistance was measured at 1.00 V constant voltage. The instrument was certified to have 57 TΩ internal resistance.

5 Output

Input

Hi Lo

Hi Lo

1 Paint film 2 Silicon rubber gasket 3 Saturated Calomel Electrode 4 Glass cell 5 Kiethley Model 617 Electrometer 6 Faraday cage 6

3

1

4

2

FIGURE 2 - Schematic drawing of cells and connections for measuring the resistivity in free paint films.


RESULTS AND DISCUSSION

Effect of electrolyte composition on cathodic disbonding FIGURE 3 shows cathodic disbonding of Epoxy 5 in LiCl, NaCl and CsCl as a function of the molar conductivity26 of the cations in water. The disbonded area is a linear function of the molar conductivity of the cation. Skar has earlier shown that the disbonding of Epoxy 0 is a function of cation mobility.27 In the literature this is usually said to mean that the transport of cations is rate limiting.17,18

Disbonded area [mm2]

600 500

Cs+

400 300 Na+

200 100

Li+

0 3

4

5

6

7 2

8

-1

Molar conductivity [mS m mol ] FIGURE 3 – Correlation between molar conductivity of the cation and cathodic disbonding of Epoxy 5. Conditions: 250 µm dry film thickness, 25°C, -1050 mV SCE, 0.25 M electrolytes. FIGURE 4 shows cathodic disbonding results for three different coatings in three different electrolytes. Each result in the figure is an average of three parallel samples. The ASTM-G8 electrolyte consists of 1% by weight of NaCl, Na2CO3 and Na2SO4. Testing in the ASTM-G8 electrolyte was performed both with and without the counter electrode (CE) inside the test electrolyte. When counter electrode was separated from the test electrolyte, the samples were not exposed to hypochlorite.


Disbonded distance (mm)

25 20

Epoxy-0

15 Vinyl tar

10 5

Epoxy tar

0 3% NaCl

Artificial seawater

ASTM-G8 incl HClO

ASTM-G8, separate CE

FIGURE 4 – Disbonded distance measured in different electrolytes. The ASTM-G8 test was performed both with counter electrode inside the test electrolyte and separated from the test electrolyte. The latter will eliminate hypochlorite from the test. Conditions: 230 µm dry film thickness, 25°C, -1400 mV SCE, 21 days The figure shows that there was little or no effect of electrolyte composition on cathodic disbonding. Testing of Epoxy-0 in ASTM-G8 solution including hypochlorite gave less disbonding than without hypochlorite. This effect was observed for only this coating and is difficult to explain by any theory, so we assume that this difference is due to other experimental variation. Since the coatings tested were rather thick, the test only lasted for 21 days and the test was performed at room temperature, no significant degradation of the epoxy due to hypochlorite was expected. FIGURE 3 shows that cathodic disbonding is dependent on the mobility of the cations in the electrolyte. The three electrolytes tested in the study shown in FIGURE 4 mainly vary in anion composition, while sodium is the main cation in all three electrolytes. The concentration of sodium is also quite similar. Other studies have shown that the anions in the electrolyte have very little effect on the disbonding rate.28 Another factor that may affect the disbonding rate is the amount of oxygen in the electrolyte. Also with this respect the electrolytes studied are quite similar. The oxygen solubility is calculated to be 6.9 ppm in 3% NaCl, 6.8 ppm in artificial seawater and 6.7 ppm in the ASTM-G8 solution. The oxygen solubility was calculated by the method suggested by Hale.29 Another question that has been put forward is how formation of calcareous deposits in the coating holiday will affect the disbonding rate when tests are performed in natural or artificial seawater. The deposits may act as a resistance against transport of ions, resulting in an Ohmic loss and a higher potential on the steel surface. As shown in FIGURE 6 the disbonding rate decreases when the potential increases (becomes less negative). FIGURE 5 shows cathodic current density for a sample exposed in natural seawater at -1050 mV SCE. A calcareous deposit developed in about 30 days, decreasing the cathodic current from ca 300 mA/cm² to ca 50 mA/cm². FIGURE 4, however, shows little difference between testing in artificial seawater and 3% NaCl, indicating that the effect is negligible. The amount of test data is too small to make general conclusions, though. The test duration was 21 days, and much of the calcareous deposits should have been formed during this period. For longer tests the effect may become more important though, when more of the test is conducted with a fully developed deposit.


Current density (mA/cm²)

400 350 300 250 200 150 100 50 0 0

20

40

60

80

Days

FIGURE 5 – Cathodic current density on bare steel exposed in natural seawater at ca 10°C and -1050 mV SCE. Effect of applied potential Disbonding rate as function of applied potential for Epoxy 0 is given in FIGURE 6, showing an increase in disbonding rate with decreasing potential. A linear relationship is indicated in the figure, though another relationship can not be ruled out, since the variation between parallels was rather high. A linear relationship is in agreement with results published by Jin et al.20 The correlation coefficient for the straight line is 0.72. A linear relationship between applied potential and disbonding rate indicates that the process is controlled by an Ohmic resistance. 4.5 Disbonding rate (mm²/h)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -1500

-1300

-1100

-900

-700

-500

Potential mV SCE

FIGURE 6 – Disbonding rate for Epoxy 0 as function of applied potential. Test conditions: 0.5% NaCl, 21 days exposure, 25°C


Effect of Film Thickness on Cathodic Disbonding The effect of film thickness on cathodic disbonding was investigated for the two model epoxy coatings Epoxy 0 and Epoxy 5. The coatings were applied in thickness between 100 and 500 µm. The test conditions were not the same for the two coatings. The Epoxy 5 samples were exposed in substitute seawater at 25°C and polarized to -1050 mV SCE, While Epoxy 0 was exposed in 1.5% NaCl at 20°C and polarized to -700 mV SCE. Less aggressive conditions were used for the Epoxy 0 samples in order to avoid blistering of the coating, since the film thickness was very low for some of the samples. FIGURE 7 shows the results. Epoxy 0 showed a linear decrease in cathodic disbonding with increasing film thickness. Increasing the film thickness usually improves the barrier properties of the film, which may explain the effect. The film thickness did not seem to have any effect on the disbonding rate for Epoxy 5. The test conditions were quite different for the two coatings, and the polarization potential in particular. However, this is probably not the reason for the different behavior. Epoxy 5 was pigmented with aluminum flakes, which has been shown to have a significant effect on cathodic disbonding. The effect of aluminum is related to chemical effects at the steel/coating interface. As long as the aluminum pigmented coat is applied directly on the steel surface, the barrier properties of the intermediate or topcoats will have little effect on the disbonding rate.30 This was probably also the case in this experiment. The ruling factor is probably a reaction of aluminum with hydroxyl ions. Hence, the effect of film thickness on disbonding rate will vary between coating products.

Disbonded distance (mm)

30 25 Epoxy 0 20 15 10 Epoxy 5

5 0 0

100

200

300

400

500

600

Dry film thickness (µm)

FIGURE 7 – Effect of dry film thickness on cathodic disbonding. Experimental conditions for Epoxy 5: substitute seawater, 25°C, -1050 mV SCE, blast cleaned steel substrate. Experimental conditions for Epoxy 0: 1.5% NaCl, 20°C, -700 mV SCE, blast cleaned steel substrate.

Effect of Oxygen Concentration in the Electrolyte The effect of oxygen concentration in the electrolyte was tested on both Epoxy 0 and Epoxy 5. The samples were exposed in substitute seawater saturated with pure oxygen or air, polarized to –1050 mV SCE by a potentiostat. The dry film thickness was 171 ± 13 µm for Epoxy 5 and 167 ±13 µm for Epoxy 0. The oxygen concentration in the electrolyte was monitored continuously. The experiments in pure


oxygen atmosphere were performed in the apparatus shown in Chapter 5. In the air saturated electrolyte the oxygen concentration was about 6 ppm, while in the oxygen saturated electrolyte the concentration was above 20 ppm, which was the upper limit of our instrument. If we assume that the concentration is proportional to the partial pressure in the gas (Henry’s law), then the oxygen concentration was about 30 ppm. FIGURE 8 shows the cathodic disbonding of Epoxy 0 and Epoxy 5 in oxygen saturated and air saturated electrolytes. Both coatings had significantly more cathodic disbonding in the oxygen saturated electrolyte. In the literature, the time dependence of cathodic disbonding is usually described with an initial “delay time” followed by a linear increase in disbonded area with time.10 The delay time is usually attributed to ingress of reactants into the coating. Increasing the oxygen concentration in the electrolyte may then have two effects on the rate of cathodic disbonding. It may increase the slope of the disbonded area/time curve or it may shorten the “delay time.” The delay time is found by extrapolating the area/time curve to intercept with the time axis. The delay time values found in FIGURE 8 have high standard deviations and are therefore difficult to discuss. For Epoxy 0 the slope of the area/time curve was higher in pure oxygen than in air, and the difference is statistically significant. In oxygen the slope was 13.1 ± 0.8 mm2/h and in air the slope was 4.2 ± 0.2 mm2/h. For Epoxy 5 the regression line for pure oxygen is not statistically significant, and it is impossible to say whether the increase in disbonding is due to an increased slope of the curve or a decreased delay time.

1200

900

1000

2

800 600 400

Epoxy 5

800

Oxygen

Disbonded area [mm ]

2

Disbonded area [mm ]

Epoxy 0

Air

200

700 600 500

Oxygen

400 300 Air

200 100

0

0

0

25

50

75

100

0

Hours

100

200

300

Hours

FIGURE 8 – Cathodic disbonding in different oxygen concentrations. Experimental conditions: Substitute seawater, 25°C, -1050 mV SCE. We also tested cathodic disbonding in a N2 purged electrolyte. Blast cleaned steel samples were coated with Epoxy 0, five parallels with 271 ± 14 µm dry film thickness. The samples were exposed in substitute seawater at 25°C, and polarized to -1450 mV SCE by a potentiostat and aluminum counter electrodes. The oxygen concentration was continuously monitored with an oxygen selective electrode, and was found to be less than 4 ppb during the whole experiment. The cathodic reaction could therefore not be oxygen reduction. Some cathodic disbonding was found in the oxygen free electrolyte, and the disbonding rate was calculated to be 18 ± 1 mm2/day. FIGURE 4 shows cathodic disbonding for the same coating (Epoxy 0) under similar conditions (0.5M NaCl, -1400 mV SCE, 25°C), but in an air saturated electrolyte. The disbonding rate was then 57 mm2/day (20 mm disbonded distance in 21


days). Hence, cathodic disbonding occurred in the oxygen free electrolyte, but the rate was significantly lower than in the air saturated electrolyte. The pH under a disbonded coating has been reported to be about 14,18,31 and the equilibrium potential for the hydrogen reaction at pH 14 is -1070 mV SCE. I.e. the potential under the coating must then be lower than -1070 mV SCE for the reaction to occur at pH 14. For samples polarized to -1050 mV SCE very little or no disbonding should be expected in oxygen free solutions. Transport of Reactants Skar has outlined three different routes for transport of cations to the disbonding front, as FIGURE 9 illustrates.32

Under disbonded film

Through disbonded film

Through adhering film

Steel Disbonding front FIGURE 9 – Possible transport pathways for cations to the disbonding front on a coated specimen with a defect.32 The resistivity of a coating is normally too high to allow significant transport of ions through the film. The highly alkaline environment under the disbonded film may degrade the film by e.g. breaking crosslinking bonds. The resistivity of the film may then decrease, which will increase the probability for ionic transport through the film after cathodic disbonding. However, the amount of hydroxide produced under the coating is probably not sufficient to degrade the bulk of the coating. To test this, free coating films were prepared by cathodic disbonding. Epoxy 0 was applied on ground steel to 190 µm dry film thickness and exposed in substitute seawater at 25 °C polarized to -1050 mV by a potentiostat. The disbonded films were carefully cut and peeled off the substrate and mounted between the cells, as shown in FIGURE 2. The resistivity of the disbonded films was about 4·1013 Ωcm at 23°C in substitute seawater. For an adhering films of the same coating product we measured resistivities in the range of 1·1013 to 1·1014 Ωcm. Hence, the disbonding process therefore can not have degraded the films significantly. Cross sections of a coated sample after cathodic disbonding were also studied in a Scanning Electron Microscope. The film was a commercially available epoxy mastic exposed in natural seawater for two years, polarized to –1050 mV SCE by zinc anodes. The sample was embedded in epoxy, cut with an emery disk and ground at least 2 mm down from the cut. The gap between the steel and the disbonded film was probably not affected by the sample preparation. The film had disbonded about 8 mm from the holiday. The gap between the steel and the disbonded paint film was about 10 µm. In the front the gap varied between 1 and 5 µm.


If we consider a 1 mm2 area in the disbonding front, we can estimate the resistance between this area and the anode, both under the coating and through the coating. The potential difference between the anode and the disbonding front is the same for both routes. The resistance in the paint film and in the aqueous phase under the disbonded film will then determine which route is most important for electro migration. If we assume that the film is 100 µm thick and that the resistivity is 1011 Ωcm, then the resistance through the 1 mm2 area of the film will be 1011 Ω. If we then assume that the pH under the film is 14 (1M NaOH) the resistivity of the aqueous film will be about 10 Ωcm. If the aqueous film is 1 µm thick and that the disbonded distance is 10 mm, then the resistance in a 1 mm wide section under the disbonded film will be only 107 Ω. Based on the electro migration, it then seems quite evident that the cations are transported under the disbonded paint film. To test the assumptions further a test was performed where the holiday in the coating was sealed after the disbonding had started. If the transport of ions goes through the disbonded film sealing the holiday should not affect the disbonding rate (provided that oxygen and water are transported through the film). Nine samples of Epoxy 5 on ground steel were prepared and exposed in substitute seawater at 25°C and polarized to -1050 mV. After three days, three samples were evaluated, on three other samples the coating holiday was sealed with epoxy glue, and three samples were left unchanged. After eight days we measured the disbonding on the unchanged samples and the samples with sealed holidays. FIGURE 10 shows the results. Sealing the holiday seemed to stop the disbonding. Evidently cathodic disbonding depended on transport of reactants through the coating holiday. Based on the calculations above, it seems reasonable to assume that it is the cations that are transported by this route.

Disbonded area [mm2]

200 Unchanged

160 120 80

Sealed holiday

40 0 0

2

4

6

8

10

Days

FIGURE 10 – Effect of sealing the coating holiday on cathodic disbonding of Epoxy 5. Conditions: Substitute seawater, 25°C, -1050 mV SCE. Cathodic disbonding was also tested when the coating was sealed. An aluminum foil was glued to the surface of Epoxy 0 to prevent any transport through the coating. Samples were prepared by applying 200 µm DFT Epoxy 0 on ground steel. Four samples were then coated with 3 µm aluminum foil glued to the coating surface. All samples were then tested for cathodic disbonding under the same conditions as above. The unsealed coatings disbonded about 6 mm in 2 days, while the sealed films had less than 2 mm disbonding in 8 days. The 2 mm disbonding of the sealed film can be due to transport of all reactants along the interface or diffusion of NaOH from the coating holiday. This shows that cathodic disbonding also depended on transport of some reactant through the coating. Since coatings are


permeable to water and oxygen, it seems reasonable to assume that they are transported through the coating.

CONCLUSIONS Effect of various test parameters has been studied for two model epoxy coatings. •

• • • •

• •

The rate of disbonding depended on the type of cation in the electrolyte. However, testing in three different electrolytes gave almost identical results, probably because the electrolytes mainly differed in anion composition. The anions normally have little effect on the disbonding rate. Formation of hypochlorite in the electrolyte during testing had no effect on the disbonding rate. The effect was only studied for rather thick epoxy coatings at room temperature. For thinner coatings, higher temperatures and longer tests there may be an effect. Sealing the coating surface with an aluminum foil decreased the disbonding significantly, which means that cathodic disbonding also depended on transport of some reactant through the coating, probably oxygen. The disbonding rate increased with decreasing potential from -700 mV to -1400 mV (SCE). Disbonding was also studied as function of oxygen concentration in the electrolyte. The disbonding rate was low in nitrogen purged electrolytes, but not zero. By polarizing the samples to -1450 mV SCE some disbonding was obtained, but significantly less than in an air saturated electrolyte. The disbonding was faster in electrolytes saturated with oxygen than in electrolytes saturated with air. Thus, both the oxygen concentration in the electrolyte affects the disbonding rate. For one of the coatings the disbonding rate depended on the film thickness, while for the other film thickness did not affect the disbonding rate. Hence, effect of film thickness depends on properties of the coating. The resistivity of free paint films was higher than 1013 Ωcm, also for films prepared by cathodic disbonding. The resistance between the disbonding front and the anode was calculated to be much higher through the coating than in the aqueous film under the disbonded film. Sealing the coating holiday stopped the disbonding. The transport of charge therefore most likely takes place under the disbonded coating.

ACKNOWLEDGEMENTS Thanks to Statoil, Hydro, Carboline, International, Jotun, Aker, Kværner Rosenberg and the Research Council of Norway for financial support to the project.

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