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Volume 2 Extended Abstract 13

Submitted 26th August 1999

Effect of Barrier Pigments on Cathodic Disbonding

Ole Øystein Knudsen and Unni Steinsmo 
Postal Address: SINTEF Materials Technology, 7465 Trondheim, Norway
E-mail Adrdress: ,

  Keywords: Cathodic disbonding, aluminium pigments, glass flake pigments

Introduction

The purpose with this paper is to examine the effect of flake shaped glass- and aluminium pigments on cathodic disbonding. Model coatings with various contents of glass and aluminium pigments have been prepared and tested for cathodic disbonding. We have found that both glass and aluminium pigments improve the resistance of the coatings against cathodic disbonding. The effect was found with two different binders, which indicate that this is a general effect, independent of the binder. We have outlined two possible mechanisms for the effect. The first possibility is that the flake shaped pigments improve the barrier properties of the coating. Cathodic disbonding depends on transport of reactants through the coating. The barrier pigments may then impede the transport, which may slow down the cathodic disbonding. The other possibility we have looked at is buffering reactions with the pigments. Leidheiser et al. have suggested that the aluminium may reduce the pH at the coating/substrate interface and thereby decrease cathodic disbonding .
Al2O3 + 2 OH- = 2 AlO2- + H2O
(1)
Aluminium pigments have been widely used in engineering paints and protective coatings for many years. The aluminium pigments are shaped as flakes in different sizes. The pigments we have used are about 20 - 40 µm wide and 1 µm thick. This geometry, along with their metallic nature, is responsible for most of their properties in protective paints. The aluminium pigments improve the barrier properties of protective paints . Glass flake pigments are also used in protective coatings, but not to the same extent as aluminium flake pigments. They are produced by breaking thin glass (about 5 µm thick) into small flakes, about 100 - 400 µm wide. Their relatively large size restricts them to be used in very thick coatings only. The shape and impermeability to water and oxygen suggest that glass flakes will improve the barrier properties of the coating.

Experimental

We have used model coatings with two different binders: an amide cured epoxy and an amine cured epoxy modified with a hydrocarbon resin (epoxy mastic). The epoxy binder was pigmented with different concentrations of aluminium flake pigments. The epoxy mastic binder was pigmented with either aluminium flake pigments or glass flake pigments. The aluminium pigments we have used are about 20 - 40 µm wide and 1 µm thick. The glass flake pigments were up to 400 µm wide and 10 µm thick. Cathodic disbonding for all nine model coatings was tested in a long term test. The substrate was 200 x 200 x 5 mm steel plates cut from mild steel. They were blast cleaned with steel grit to Sa 2½ and medium roughness (Ry = 76 ± 7 µm). The paints were applied by airless spraying in two coats. The dry film thickness was between 250 and 350 µm. The samples were given a circular holiday in the middle, 6 mm in diameter, to start the cathodic disbonding process. They were exposed in continuously renewed natural seawater at 8-13°C. Galvanic coupling to zinc anodes gave cathodic polarisation to about -1050 mV SCE in the coating holiday. The experimental setup is shown in Figure 1.  

Figure 1. Test setup for cathodic disbonding.

Results and Discussion

Figure 2 shows the disbonded distance in the long term test as a function of time for the amide cured epoxy coatings with aluminium pigments. 15 samples of each model paint were prepared, and cathodic disbonding was measured during a two years period. With increasing time intervals, 1-3 samples was removed and analysed. The aluminium pigments were highly effective in improving the resistance of the coating towards cathodic disbonding. The coatings with 10 and 20% aluminium had not disbonded at all after 18 months of exposure. After two years the 10% aluminium coating had some disbonding, and about 3 mm of the coating around the holiday could be removed. The 5% aluminium coating had disbonded about 20 mm and the coating without aluminium about 65 mm in the same period of time.   Figure 2. Cathodic disbonding of epoxy coating with various concentrations of aluminium pigments. 

The amide cured epoxy has low commercial interest due to the high content of solvents. The epoxy mastic binder was therefore included in the test to represent a more commercially interesting binder. The disbonding distances for the epoxy mastic coatings are given in Figure 3. Pre-treatment, application, curing and exposure were similar to the long term test of the amide cured epoxy. The unpigmented epoxy mastic gave about 8 mm disbonding in four months, but then the coating started to blister randomly all over the sample. After that, cathodic disbonding spread from the blisters, and it became impossible to measure the disbonded distance from the holiday. After about one year the whole sample had disbonded. We would like to emphasise that this is not a general problem with epoxy mastics. We have tested several commercial epoxy mastics which have perform much better than this. The blistering of the epoxymastic without barrier pigments was independent of the initial coating holiday, and was even found on the back of the samples. Inside the blisters we found a highly alkaline liquid, which means that the blistering was caused by the cathodic reaction under the coating. The blistering was therefore also a kind of cathodic disbonding. It then seems that there are two different disbonding mechanisms, one mechanism associated with a mechanical damage in the coating, and another mechanism independent of any mechanical damage. This is in accordance with Type I and Type II failure described by Watts and Castle . Type I failure was associated with downward diffusion of the cation via conductive pathways in the film, corresponding to the random blistering of our coatings. Type II failure was associated with transport of the cation under the disbonded coating from a mechanical damage in the film. For the epoxy mastic coatings the aluminium pigments had a positive effect on both types of failure.  

Figure 3. Cathodic disbonding of epoxy mastic coating with various concentrations of aluminium pigments.

As for the amide cured epoxies, the epoxy mastics with 10% and 20% aluminium pigments had no cathodic disbonding at all after 18 months exposure. No blistering or other signs of coating deterioration were observed either. Due to the low content of solvent, the concentration of aluminium in the dry film was lower than for the corresponding amide cured epoxies. Since the unpigmented coating performed so badly, the aluminium pigments had a tremendous effect on both random blistering and cathodic disbonding.

Due to spark hazard, the classification companies have set a limit to the concentration of aluminium in marine paints to maximum 10% by weight in the dry film . Glass flake pigments were therefore also tested with the epoxy mastic, as an alternative barrier pigment. The disbonding of these coatings is also shown in Figure 3. The glass flakes inhibited the disbonding compared to the unpigmented coating, but not as much as the aluminium pigments. After 4 months some disbonding was found on the epoxymastic with 10% glass. For the 20% glass coating disbonding was first found after a year of exposure. After 9 – 10 months disbonding started from random points all over the coating with 10% glass, and after one year it was impossible to measure the disbonded distance from the holiday. After 18 months about 70% of the coating had disbonded. For the 20% glass coating some random initiation of disbonding was first found after 18 months.

The random initiation of cathodic disbonding of the epoxymastics with 10 and 20% glass flake pigments was probably due to opening of conductive pathways through the coating, as for the blistering of the unpigmented epoxymastic. The reason why the coatings with glass flake pigments did not develop blisters may have been a reinforcing effect of the pigments. The random initiation of disbonding for the two films was therefore also a Type I failure, in the terminology by Watts and Castle . Since the glass flake pigments delayed this type of failure, the pigments may have delayed the process of opening conductive pathways through the film. We have measured the transport parameters for oxygen, water and ions through the amide cured epoxy as a function of aluminium pigment concentration. Table 1 shows the transport parameters for water and oxygen in the four coatings. 

Table 1. Transport of reactants through epoxy coatings with various aluminium concentrations.

 

Water

Oxygen

Al pigment conc.
[Wt.%]

Uptake
[Vol.%]

D
[10-10 cm2/s]

D
[10-9 cm2/s]

P
[10-9 cm2/s]

0

3.3 ± 0.07

5.5 ± 0.4

32 ± 2.5

1.1

5

3.3 ± 0.04

4.3 ± 0.5

10 ± 0.6

-

10

3.2 ± 0.03

4.2 ± 0.7

5 ± 0.7

0.7

20

3.8 ± 0.22

2.4 ± 0.7

5 ± 0.8

0.6

  A barrier mechanism for the effect of aluminium pigments on cathodic disbonding implies that the pigments decrease the transport of at least one of the reactants, and that this reactant affects the rate of disbonding. The rate of cathodic disbonding should then correlate with the transport rate of this species. Figure 4 shows the transport properties and disbonding rate for the films as function of aluminium pigment concentration. The responses are normalised to the results for the epoxy coating without aluminium pigments.

 

Figure 4. Cathodic disbonding and transport properties of the coatings as function of aluminium pigment concentration. The responses are normalised to the result for the coating without aluminium pigments.

 Figure 4 shows that the decrease in water diffusivity, oxygen diffusivity and oxygen permeability was relatively small compared to the effect of aluminium on cathodic disbonding. The transport of cations through the film increased with the aluminium content, i.e. opposite to the effect on cathodic disbonding. Neither of the transport parameters correlates well with the disbonding rate. This indicates that the mechanism of the effect of the aluminium pigments on cathodic disbonding is something else than the barrier effect. The aluminium pigments are covered with oxide. According to the supplier the pigments contain 1-3% aluminium oxide. Leidheiser et al . have suggested that the oxide on the pigments may act as a buffer that decreases the pH beneath the film. Hydroxide generated by the cathodic reaction at the steel/coating interface is generally accepted as being responsible for cathodic disbonding. If hydroxide is removed from the interface by buffering reactions from the film, disbonding may also decrease. According to the Pourbaix-diagram for aluminium , solid aluminium oxide is not stabile above pH 9-10 and will react with hydroxide (1). A simple experiment was performed to test the buffer theory. The epoxy coatings with 0 and 10% aluminium pigments were applied in two-coat films in the four possible combinations of the two paints. The application and coating thickness is illustrated in Figure 5. If there is a buffer effect from the aluminium pigments, it will be important to have the aluminium pigments in the first coat. Then only system 3 and 4 will show decreased cathodic disbonding. If the effect of the aluminium pigments on cathodic disbonding is a general barrier effect, then system 4 will perform best, system 2 and 3 will perform equally and system 1 will perform worst. The disbonding rates are given in Figure 6.

Figure 5. Application of epoxies with 0 and 10% aluminium in two-coat films.

Figure 6. Cathodic disbonding for two coat films with and without aluminium pigments applied on top of each other. Test conditions: Substitute seawater, -1050 mV SCE, 25°C, 4 parallels. 

Figure 6 shows that the disbonding rate was low and almost the same for system 3 and 4. System 1 and 2 also had nearly the same disbonding rate, but much higher than system 3 and 4. This experiment clearly demonstrates the importance of applying the aluminium pigmented coat directly on the steel substrate. This also favours the buffer theory over the barrier theory. System 2 and 3 should have the same barrier properties, but the disbonding of system 3 is much slower than for system 2. In addition system 2 has only slightly less disbonding than system 1, which should have been a better barrier. The same is the case with system 3 compared to system 4. These results demonstrate that the positive effect of the aluminium pigments is linked to their presence at the steel/coating interface.

Conclusions

Aluminium and glass barrier pigments decreased cathodic disbonding substantially for two different epoxy coatings. Glass flake pigments were less effective than aluminium pigments in decreasing cathodic disbonding, but still they improved the performance of the coating significantly. The aluminium pigments had little effect on cathodic disbonding unless the aluminium containing film was applied directly on the steel surface. The effect was therefore connected to their presence near the steel/coating interface. This indicates that the aluminium pigments had an additional effect on cathodic disbonding besides any barrier effects. It has been suggested that buffering reactions with the oxide on the surface of the pigments may decrease cathodic disbonding.

References

1.   H. Leidheiser, W. Wang and L. Igetoft, "The Mechanism for the Cathodic Delamination of Organic Coatings From a Metal Surface", Prog. Org. Coat., Vol: 11, No: 1, p. 19-40 (1983).

2.   C. H. Hare, "Protective Coatings - Fundamentals of Chemistry and Composition", Technology Publishing Company, Pittsburgh (1994).

3.   W. Funke, "Towards Environmentally Acceptable Corrosion Protection by Organic Coatings - Problems and Realisation", Journal of Coatings Technology, Vol: 55, No: 705, p. 31-38 (1983).

4.   J. F. Watts and J. E. Castle, "The application of X-ray photoelectron spectroscopy to the study of polymer-to-metal adhesion. Part 1 Polybutadiene coated mild steel", J. Mat. Sci., Vol: 18, p. 2987-3003 (1983).

5.   DNV, "Guidlines No. 8, Corrosion Protection of Ships", Det Norske Veritas, Oslo (1996).

6.   M. Pourbaix, "Atlas of Electrochemical Equilibria in Aqueous Solutions", Pergamon Press, London (1966).

 

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