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Volume 2 Paper 9


The behaviour of epoxy powder coatings on mild steel under alkali conditions

Darwin, A.B. and Scantlebury, J.D.

Corrosion and Protection Centre, UMIST PO Box 88, Manchester M60 1QD, UK
E-Mail Address:

Abstract

A clear commercial epoxy-powder lacquer, has been applied by electrostatic spray onto mild steel panels and cured at 235°C at a film thickness of 100 μm. Coatings were tested in alkaline environments of mainly pH 13.2. Assessment was by exposure at open circuit where delamination rates were measured as a function of time. Variations in cation and cation concentration and the presence or absence of chloride were studied. Delamination mechanisms have been proposed.

Keywords: Epoxy lacquers, power coatings, mild steel, alkali environments, adhesion,

Introduction

This paper is part of a study of epoxy-coated rebar steel in concrete. In continuation of previous work [1], investigations were made at open circuit, using a clear epoxy powder on mild steel panels in various alkali environments. Variations in specific cation, pH and presence of chloride were examined. The mechanisms of adhesion loss are investigated.

Experimental

Cold-rolled mild steel Q-Panels, 1 mm thick, were degreased by washing in detergent solution, rinsing well in de-ionised water and then spraying with ethanol. Pigment-free epoxy powder was supplied by Akzo Nobel Powder Coatings. The panels were heated in an oven, such that when they were removed and the epoxy powder sprayed onto them they would be in the temperature range 235-240oC, a requirement of the coating for its curing.. A Gema Volstatic manual fluidised bed powder system and PG1 powder gun was used to achieve a powder coating thickness within the range 100 � 10 �m. Panels with obvious defects in the powder coating were not used. A 3.0mm diameter holiday was introduced into the epoxy coating by means of a flat-head drill bit such that the Q-panel steel was exposed. As an alternative to the 3mm holiday, a pinhole was introduced to some specimens.

Test cells were then created by adhering a perspex tube of 5 cm diameter and 6 cm nominal height to the epoxy-coated panel, such that the artificial holiday was at its centre. Adhesion was achieved with the sparing use of silicone sealant.

The test solutions used are shown in Table 1. The composition of the simulated pore solution was 0.3N KOH + 0.05N NaOH + 0.02N Ca(OH)2, suggested by Schiessl and Reuter [2]. Testing was at open circuit with a minimum of three panels used for each exposure condition. The pHs of the solutions were measured daily and adjusted as necessary. Delamination rate (apparent by a lighter or occasionally darker halo of delaminated coating) and half-cell potential measurements were made.

Electrolyte

pH

Electrolyte

pH

 

KOH(aq)

12.0

KOH + 0.6M KCl

13.2

13.2

NaOH+ 0.6M NaCl

13.2

14.0

LiOH + 0.6M LiCl

13.2

NaOH(aq)

13.2

Sim. Pore Soltn.+ 0.6M NaCl

13.2

Sim. Pore Soltn.

13.5

Table 1: Exposure Solutions for Powder-Coated Panels

Results and Discussion

Figure 1: Typical Potential Responses of FBE-Panels Exposed to Test Solutions

Typical potential/time responses of FBE-coated panels with a 3mm holiday exposed to the different electrolytes are shown in Figure 1:

All the FBE-coated panels in solution with no chloride have potentials suggesting their passivity or near-passivity. Exposed steel remained bright and untarnished, and no corrosion products were visible (Figure 2).

 

Figure 2: Appearance of FBE-Panel Exposed to Test Solution without Chloride

Figure 3: Appearance of FBE-Panel Exposed to Test Solution Containing Chloride

The effect of the addition of 0.6M chloride to the solutions that FBE-coated panels were exposed to is shown in Figure 3. The potentials of all the panels became more negative, in the region of –400 to –550mV. The exposed substrate at the artificial holiday tended to remain bright and untarnished. Blue deposits began to appear underneath the coating adjacent to the holiday. At the edges of the artificial holidays, red deposits appeared, these observations strongly suggesting that corrosion was occurring. The presence of a region of bright visibly uncorroded substrate adjacent to and at the delamination front suggests that delamination of the FBE coating from its substrate was not being caused by anodic undermining. Closer examination by SEM confirmed that there was no visual evidence of substrate degradation there in either passive or corroding systems (Figures 4 and 5).

Figure 4: Morphology of Q-Panel Substrate Beneath Delaminated Coating (Passive System)

Figure 5: Morphology of Q-Panel Substrate Beneath Delaminated Coating (Corroding System)

The presence of a cathode region at the coating/substrate disbondment front has been widely reported previously by many authors [3-12], and is often an implicit and sometimes explicitly stated part of their postulated coating delamination mechanism. Potential/time data shows that corrosion of the FBE-coated panels initiates soon after they are placed in the test solutions containing chloride ions. The corrosion process does not take place on the exposed mild steel substrate of the holiday, but is seen to develop at its edge. A Tafel plot of substrate potential/current response in pore solution can be seen to be virtually identical in comparison with pore solution with chloride (Figure 6). Only when the bond between the mild steel test specimen and its encapsulating epoxy araldite resin is poor does evidence of crevice corrosion appear. It is suggested that corrosion initiates in a crevice between the FBE-coating and mild steel substrate, caused by the passive current density on the steel.

Figure 6:Tafel Plots Illustrating the Effect of Poor Polymer Adhesion

The delamination data derived for each exposure condition was averaged. These values are plotted in Figure 7, where the average radial delamination of the panel is plotted against the square root of the time of its exposure to the test solution. A linear relationship is found using these axes for both passive and corroding systems (Table 2). Data for panels exposed to pH 12.0 KOH solution is not plotted because no delamination could be found after 6 months testing.

This parabolic delamination shows that rate of FBE disbondment is greatest upon its initiation, decreasing with time until a theoretical cessation of disbondment at infinity. More importantly from a mechanistic viewpoint, this relationship indicates that the rate-determining process is along the coating / substrate interface from the coating holiday. This relationship has also been found for coated systems corroding at open circuit by other workers [10,11,14]. If transport of this delamination parameter was through the coating, then a linear relationship between radial delamination and time of immersion would be encountered.

Figure 7: Effect of Test Solution on Delamination of FBE-Panels

Referral to Table 2 informs us that delamination rates in corroding systems tend to be greater than those in passive. It is therefore likely that the rate-determining step in the delamination process of corroding and passive systems will be different.

The FBE-panels which were exposed to passivating solutions at open circuit underwent delamination. The only electrochemical reactions which took place on their substrates in these conditions would have been the small passive current derived from the formation and dissolution of the substrate surface oxide film. It is therefore unlikely that the cause of delamination was an electrochemical process.

When the delamination rate constants of the FBE-panels are plotted on a log10 scale against the respective pHs of the passivating test solutions that they were exposed to, a linear relationship is derived (Figure 8). Concentration of hydroxyl ions in solution is therefore directly proportional to the rate of FBE coating delamination, and the rate-controlling step in the delamination process is the passage of hydroxyl ions along the coating/substrate interface. This information indicates that it is these hydroxyl ions which are directly responsible for coating delamination. It is possible that this relationship is due to the increasingly alkaline environments affecting acid-base interactions between coating and substrate, and hence polymer adsorption and adhesion, as suggested by Fowkes and Mostafa [15].

Immersant Solution

Slope, Kd (mm/day)

Constant (mm)

Delay Time (days0.5)

KOH:
pH 13.2

0.165

-0.649

3.9

pH 14.0

1.427

-6.886

4.9

NaOH:
pH 13.2

0.227

-1.122

4.9

Pore Soltn:
pH 13.5

0.870

-4.502

5.2

Pinhole-
Pore Soltn:
pH 13.5

 

0.723

 

-2.225

 

3.1

KOH/KCl

2.083

-7.064

3.4

NaOH/NaCl

1.487

-5.427

3.6

LiOH/LiCl

0.549

-0.781

1.4

Pore/Cl

1.834

-7.947

4.3

Table 2: Calculated Equation of Line and Delay Time for FBE-Coated Panels at Open Circuit

Figure 8: Linear Relationship between Soltn. pH and Kd of Non-Corroding FBE-Panels

Extrapolation of the pH / Kd graph to the point of zero delamination rate constant correlates with a bulk solution pH of approximately 13.0. This value will be associated with the minimum pH required to cause disbondment in the FBE / substrate system. At pHs below 13.0 there should therefore be no delamination. It has been shown that there was no delamination of coating from an FBE-panel after 6 months of exposure to a pH 12.0 solution. These observations suggest that the disbondment mechanism is unlikely to be either the chemical disruption of the polymer / metal oxide bonds by the presence of polar water molecules or through creation of a hydrodynamical stress on the bonds, as suggested by Funke [16]. If the disbondment mechanism were related to hydrodynamical stress on the bonds, then increasing solution osmotic pressure would be expected to result in a decrease in delamination rate. It has been shown that delamination will occur at pHs greater than approximately 13.0; the proposed oxide dissolution delamination mechanism proposed by Ritter and his co-workers [6,17-19] is therefore unlikely to be the cause of this delamination because it will not be thermodynamically viable at a pH of 13.2 [20]. That the relationship between pH and Kd is linear is of interest, since this means that Kd is proportional to the logarithm of hydroxyl ion concentration in solution. The delamination mechanism is therefore not a simple one; hydroxyl ions are not involved in a {1 hydroxyl ion : 1 coating/substrate adhesive bond disrupted} relationship. It is therefore unlikely that delamination of the FBE coating ion is caused by its saponification.

The delamination rate constant Kd for corroding FBE-panels is plotted versus hydrated cation mobility in test solutions with chloride in Figure 9 . A trend is found showing that increasing hydrated cation mobility in the test solution results in an increased rate of coating delamination, an observation made by other workers [10,13,14,21,22].

Figure 9: Relationship Between Delamination Rate and Cation Mobility

When the delamination rate constants created with the relevant cations in the corrosion-inducing solutions are plotted on the best-line fit generated by delamination rates of FBE-coated panels in solutions of different pH (Figure 8), the corresponding pHs at the delamination front may be estimated (Figure 10). These pHs are based on the assumption that pH at the delamination front of passive panels is essentially the same as that of the bulk solution, and that delamination of an FBE coating from a corroding substrate is predominantly due to hydroxyl ions. The estimated pHs are shown in Table 3 below:

Figure 10: Estimation of pH at Delamination Front of Corroding Test Specimens

Cation

Estimated pH

Hydrated Cation Mobility, U (x 10m2s-1V-1) [22]

pH at Saturation[22]

K+

14.4

6.6

15.28

Na+

14.0

4.6

15.02

Li+

13.4

3.4

14.72

Table 3: Solution pHs at the Delamination Front for Different Cations in Corroding Non-Chromated FBE-Panels Estimated from pH-Induced Delamination Data

These pHs are less than those attainable by the cation hydroxides at saturation. The rate-controlling step in the delamination process of corroding FBE-coated panels is therefore the passage of the hydrated cations through the holiday and along the coating/substrate interface to the cathodic site. The cations are being transported to the cathode such that charge balance may be achieved.

It may noted from the derived linear relationship of Figure 9 that no delamination should take place if the cations present in solution have an absolute ionic mobility of or below approximately 1.6 x 108m2s-1V-1. An explanation for this observation may be made by consideration of the important factors responsible for maintenance of a high pH at the disbondment front. In the corroding underfilm system, the pH at the disbondment front will be controlled by the rate at which cations are transported to the cathode site and their maximum attainable pH, and the rate at which hydroxyl ions are transported away from it. From the pH / delamination rate constant data recorded for non-corroding non-chromated FBE-Panel systems it may be seen that below a given pH, delamination will not take place. It may be postulated from Figure 8 that this pH is also that which would be generated at the delamination front by a cation of absolute mobility 1.6 x 108m2s-1V-1. At cationic mobilities at or below this value the flux of hydroxyl ions away from the disbonding front would be too great for generation of a sufficiently high pH to cause delamination.

From SEM analysis it has been seen that disbondment does not leave visible coating on the steel panel substrate (Figures 4 and 5). The FBE-resin is already intimately mixed with its hardener in its powder form prior to spraying onto the heated panel, and its gel time occurs in a matter of seconds [24]. There is therefore no time for a separation of resin and hardener to take place, as has been suggested for solvent-based paint systems [25]. The FBE coating should therefore approximate the same chemical and structural composition from the outer limit of the polymer / substrate interphasial region to its external surface. In this situation, if the mechanism of adhesion loss was coating degradation and cohesive failure, then it would be equally likely to occur at any depth of the FBE coating on the passive specimens at open circuit. Failure always appeared to occur at or near the coating / substrate interface, i.e. in the interfacial or interphasial region, so these mechanisms were not the cause of disbondment.

From the radial delamination / square root of time graph a delay time before delamination begins to occur is seen, as found by other researchers including Leidheiser and co-workers [21,26] and Sharman, Sykes and Handyside [14]. This is independent of both hydrated cation mobility and solution pH (Figures 11 and 12).

Figure 11: Effect of Cationic Mobility on Delay Time of FBE-Panels in Passivating Solution

Figure 12: Effect of Solution pH on Delay Time of FBE-Panels in Passivating Solution

Leidheiser, Wang and Igetoft [21] linked delay time to ion ingress into the coating, and Wang [27] to water diffusion through it. The differing delay times of the corroding systems cannot be explained in terms of an interfacial attack process from the holiday at the coating / substrate interface. If this was the important determinant, then delay times should be very similar because the samples exposed to chloride started to corrode after approximately the same period of immersion. Delay time may therefore be related to a transport process through the FBE-coating. There is no evidence that it is related to cationic transport, so this delay time is probably a function of either water or oxygen transport. Mayne [28] has shown that oxygen transport through a coating is a relatively rapid process, and Cottis [29] has stated that a typical oxygen permeability of an epoxy resin is 5.2 x 10-12 mole.m-1s-1atm-1, so it is unlikely that this factor could account for the length of delay time recorded in these conditions (typically 3-4 days). It is therefore possible that the delay time of the corroding system at open circuit is related to water transport through the coating.

Conclusions

The following conclusions may be drawn from this investigation into delamination of FBE-coatings in alkaline environments:

Acknowledgements

The authors would like to thank the EPSRC, Allied Bar Coaters and in particular Mr. J. Hartley for supporting and encouraging this research.

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