Keywords: Organic coatings, Zirconia film, Adhesion, Pre-treatment.
|Series||Alkoxide Molarity (g/mole)||N� layers||Chelating agent||Temperature (�C)||Time (min)||Comment|
|7||0.3||3||Acetylacetone||250||30||Treated in boiling water|
|8||0.3||3||Acetylacetone||150||30||Treated in boiling water|
A multilayer polyester paint coating formed by a primer and a topcoat was applied in all cases. The average dry film thickness (DFT) was about 30 μm. This kind of paint is usually used for electrical appliances . A part of the coated samples were cross-scratched and tested evaluating the damage in Salt Fog Chamber (ASTM B117). In order to assess the adhesion promoted by the pre-treatment, quantitative measurements were made using a pull-off test (Sebastian IV) and measurements of the swelling of the film using N-methyl pyrrolidone . The FTIR analyses were performed by a Spectrometer BioRad FTS 165, coupled with a BioRad UMA 250 device. In addition, Electrochemical Impedance Spectroscopy was used in order to monitor the increase in wet area and some other interfacial phenomena. A three-electrode cell was used, constituted by: Working electrode: Pre-treated and painted samples Counter electrode: Platinum sheet Reference electrode: Ag/AgCl (207 mV vs SHE) The electrolyte was 0.3%wt. Na2SO4, because of its characteristics of conductivity and low aggressiveness. The equipment used was a EG&G PAR 273 potentiostat with a Schlumberger Solartron Frequency Response Analyser 1255. The results were registered with M398 software and treated with the EQUIVCRT software .
Figure 1. Spectra IR of zirconia samples 1 and 3.
The results obtained in Salt Fog Chamber test are shown in Table II. The detachment near the scratch was measured by gently tearing off the disbonded area with a knife. In some cases blisters were also observed. The disbondment was not uniform along the cross scratch, so the data reported in the table are average values. In Table II also the maximum and the minimum were shown, when the range was too wide. These data evidenciated the adhesion properties of zirconia films depend on several parameters. Time and temperature of thermal treatment appear to be the most important of them. As discussed above, the heating promotes the reduction of organic load, so that the presence of a very few residuals seems to be advantageous to the adhesion. In order to maintain low the amount of organic residuals when the thickness of the films increases, the time or the temperature of the treatment have to be increased too. On the other hand, the zirconia prepared from solution containing the acetylacetone showed the worst behaviour among the sol-gel samples, even when they underwent the same thermal treatment. The stronger chelating action of acetylacetone with respect acid and its very difficult hydrolysis could account for these findings. The acetylacetonato groups bonded to zirconium prevent the condensation and the formation of an oxide network . At the aim of to promote the hydrolysis, some of the samples were also hydrolysed in boiling acidic water, but the recorded improvement was too small and not sufficient to consider interesting the way. In fact that condition did not enhance the condensation accordingly, and the surface of the samples appeared dusty and not continuous, through observation with a stereoscopic microscope. Finally the microstructure of the samples obtained from solution containing the acid appeared more suitable to adhere to the organic coating.
|Series||Detachment after 7 days (mm)||Detachment after 21 days (mm)||Comments|
|1||0.3||0.7 (0.6 to 1)||No blistering|
|2||0.4||1.2 (1 to 1.3)||No blistering|
|3||0.5 (0.3 to 0.8)||2 (1.8 to 2.5)||Small blistering near the scratch|
|4||0.8 (0.5 to 1)||3.5 (3 to 4)||Total detachment after 21 days. There were large blisters all over the samples.|
|5||0.4||0.6 (0.5 to 1)||It was difficult to detach|
|6||1.5 (1 to 2)||---||After 7 days, all the paint was detached|
|7||0.6 (0.2 to 1)||1.8 (1 to 2)||Different detachment depending on the position in the cross scratch. Blistering|
|8||1.1 (0.5 to 1.8)||---||Disbonded area is brittle. Blistering|
|9||6.5||---||Blistering all over the sample|
|10||0||0.1 (0.1 to 0.2)||It was difficult to detach|
|11||0.4||2.5 (2 to 3)||There was blistering mainly near the cross-scratch|
It is remarkable that the series 5 along with series 1 resulted the best in promoting adhesion. Moreover, the series 5 resulted even better than the other at longer time of analysis. The use of a more concentrated solution of alkoxide seems to be positive for a good performance of the zirconia film. At first glance this result could appear odd. Anyway the presence of a great number of residuals, in the film prepared from a concentrated solution, means they are closer than in a layer obtained from a diluted one. That configuration is likely more favourable to give condensation reactions, resulting a dense and thick metallic oxide layer. A ranking of the adhesion behaviour in Salt Spray Fog of our samples could be done. As regard the reference samples, those without pre-treatment resulted obviously not good at all. The use of iron phosphate as a pre-treatment shows good results in short times of exposure (7 days), compared with the zirconia film in series 1, 5 and 2, but it is not a good option in long service life. Since now, tricationic phosphate looks the best option .
The results previous discussed brought about the further study was restricted to samples of series 1 and 5. Different sheet were coated by zirconia and another polyester paint system (also used on domestic electric appliences). Moreover, in order to prove without any doubt the effect of the heat treatment, other samples have also been prepared, the features of which are reported in Table III, along with the results of Salt Fog Chamber test. The results were even better than those expected. All the samples with sol-gel films applied and hydrolysed without the thermal treatment showed a significant detachment in short-time exposure. Besides of the effect of the new paint, it is noteworthy the better performance of the sol-gel pre-treatment with respect the iron phosphate, even though not as good as the tricationic posphate. Moreover the behaviour of these new series maintains very good also after large time of exposure in Salt Fog Spray. The results of the test of reference samples had shown once more how important is the pre-treatment of the steel in promoting adhesion. The low corrosion and the negligible blistering phenomenon observed for the samples 5" and 5 proved the reactions of condensation take place fastly in a concentrated solution. So that a thicker and stiffer layer of gel could be obtained from those solutions, which can take more advantage from the thermal treatment already at low temperature.
|Series||Solution Molarity, Tmax and time of the thermal treatment||Loss of adhesion (detachment from the scratch in mm)||Comments*|
|7 days||13 days||20 days||27 days|
|1||0.3M, 250�C, 30||0||0||< 0.5||< 0.5||Few blisters and corrosion in the scratch|
|1||0.3M, no heat treatment||0||5||-||-||Completely covered with small blister|
|5||0.6M, 150�C, 30||0||0||< 0.5||0.5||Blisters and some points of rust in the scratch|
|5||0.6M, no heat treatment||0||2 3||-||-||Completely covered with small blister|
|5"||0.6M, 250�C, 30||0||0||< 0.5||< 0.5||Few small blisters and some points of rust in the scratch|
|9||Degreased steel||9 10||-||-||-||Large blisters near the cross-scratch|
|10||Tricationic phosphate||0||0||0||0||First blistering after 27 days (small blisters). Few points in the scratch|
|11||Iron phosphate||4||-||-||-||Large blisters near the cross-scratch.|
Van Ooij et al  have proposed the measure of the adhesion of the paint coating on the base of its swelling in N-methyl pyrrolidone (NMP). In fact the swelling produces stress, which could detach the paint from substrate. This test of adhesion of a film to metal depends on the thickness of film, the nature of the coating, and especially the pre-treatment on substrate surface. The results of this test are expressed as time of total detachment of the organic film in NMP. This parameter is named NMPRT (N-methyl pyrrolidone retention time). The test is based on the hypothesis that NMP acts on organic coating, disregarding substrate and pre-treatment. Actually the presence of organic residuals in the zirconia films of this study, even if negligible, could invalidate the assumption, so that the detachment could be due both to the stresses induced by swelling and a chemical-physical interaction between NMP and the zirconia films. In order to clarify this effect FTIR analyses on unpainted samples, before and after the immersion in NMP at 60�C, have been performed. In some cases (series 1) the organic residual part in the zirconia thin film was highly removed (Figure 2) by NMP, but in the series 5, the difference were not detectable before and after immersion (Figure 3), probably due to the greater thickness of these samples. It can also evidenciated the presence of the NMP adsorbed on the films, from a signal at about 1700 cm-1 referring to C=O bonds. So, it seems that the remotion of the organic components of the zirconia thin film by the NMP affects the results of this test.
Figure 2. FTIR spectra of series 1 before (A) and after immersion in NMP (B).
Figure 3. FTIR spectra of series 5 before (A) and after immersion in NMP (B).
There was no total detachment of the coating on the samples pre-treated with phosphate, but after 2 hours we could appreciate the formation of blisters reaching a 4M and 7M degree (according to ASTM D 714), for the iron and tricationic phosphates, respectively. Anyway the results showed again that the zirconia films, even if are not as good as tricationic phosphate, are better than the degreased steel.
The tests carried out with a pull-off device (Sebastian IV) have not been useful in this research, because of the type of failure. The failure mode was decohesive in the topcoat, and we did not see the primer. As a result we just can say that the mechanical features of the coating are poor or inferior to the substrate adhesion.
Impedance measurements were performed in order to follow the progressive loss of adhesion between the coating and the pretreatment or substrate. Because of the high quality of the organic paint, for all the samples only a capacitive behaviour could be recorded, so that the coating system do not allow us to evaluate the differences existing among the pre-treatments. At the aim of overtaking the impasse, some defects have been introduced on the organic coating by using a metallic pin. The size of the defects was almost the same in all samples (≈150 μm diameter), according to the measurements through microscopical observations. That value was also confirmed by the analyses of the first loop in the impedance diagrams (Figure 4), which allow to determine the area of the defect. Generally the whole Nyquist diagrams show two semicircles at the beginning of the experiment, and a third one after few hours (Figure 5). The fitting of the impedance results was performed by Equivcrt software . In the high frequency semicircle, the simulation have yielded values, which could be associated with the properties of the coating as well as the size of the defect. Anyway in absence of a model, the interpretation of the rest of the diagram is not easy and still in progress. As an example, the semicircle with a maximum frequency at 3.9 Hz in Figure 5 can be deconvolved in two components. Provoking larger defects (300 μm) on the coated surface, the impedance diagrams became simpler, comparing same period (Figure 6). It can be assumed the presence of corrosion products occludes the defect when it is small, hiding the progress of the detachment of the coating. Nevertheless, increasing the size of defect moves the response due to the coating out of the experimental frequency window. As a a consequence the size of the defect was shown a relevant and important parameter in the corrosion processes and further efforts have to be addressed to explain both small and large defects affect impedance response.
Figure 4. Detail (high frequency zone) of the Nyquist diagram of a coated zirconia samples with an artificial defect (150 μm); the whole diagram is showed in Figure 5.
Figure 5. Nyquist diagram of a coated zirconia sample with an artificial defect (150 μm).
Figure 6. Nyquist diagram of the coated zirconia sample with an artificial defect (300 μm).
2. F. Deflorian, L. Fedrizzi, J. Adhesion Sci. Technol. 13 (5), 629-645 (1999).
3. L.Benedetti, P.L. Bonora, M. Sassoli, Proceedings of Eurocoat 94 (Volume II), Sitges, Spain, 1994, p. 31-49.
4. Werther Neri, Introduzione alla Verniciatura delle Superfici Metalliche, 3a. Edizione, Ed. Tecniche Nuove, Milano, 1990.
5. M. Guglielmi, J. Sol-Gel Sci. Technol. 8, 443-449 (1997).
6. R. Di Maggio, S. Rossi, L. Fedrizzi, P. Scardi, Surface & Coatings Technol. 89, 292-298 (1997).
7. C.J. Brinker and G.W. Scherer, Sol-gel Science, Academic Press, San Diego, 1990.
8. D. Peter, T.S. Ertel and H. Bertagnolli, J. Sol-Gel Sci. Technol. 5, 5-14 (1995).
9. M.Chatry, M. Henry, M. In, C. Sanchez and J. Livage, J. Sol-Gel Sci. Technol. 1, 233-240 (1994).
10. R. Di Maggio, R. Campostrini, G. Guella, Chem. Mater., 10 (12) 3839-3847 (1998).
11.W.J.Van Ooij, R.A.Edwards, A.Sabata, J.Zappia, J.Adhesion Sci. Technol. 7 (8), 897-917 (1993).
12.B. Boukamp, Solid State Ionics, 20, p. 31 (1986).