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Accelerated testing of waterborne coatings
Progress in Organic Coatings 54 (2005) 211–215
Accelerated testing of waterborne coatings ´ Fekete ∗ , B´ela Lengyel Eva Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Received 7 February 2005; accepted 14 June 2005
Abstract According to environmental regulations, many of the traditionally used organic solventborne coatings should be replaced by low-toxicity and environmentally friendly alternatives (e.g. by waterborne paints). We report here effects of weathering on waterborne coatings. Three styrene–acrylate waterborne paint systems containing various types of inorganic pigments were studied on steel substrate; salt spray, humidity chamber and field exposure tests were carried out on them. The accelerated laboratory tests were performed both on coatings after 2 weeks of coating preparation (“fresh” coating) and on naturally aged ones, i.e. after field exposures of various durations ranging from 3 months to 2.5 years. We found that—for a certain time—the longer the exposure period, the better are the results of salt spray and humidity chamber tests. Additional experiments were carried out on samples with different pretreatments: in some cases the results of the accelerated tests after cyclic dry–wet or heat pretreatments are better than that of “fresh” coatings. © 2005 Elsevier B.V. All rights reserved. Keywords: Salt spray test; Humidity test; Dry–wet cyclic pretreatment; Heat pretreatment
1. Introduction Choosing a coating system to protect a metal construction against corrosion is important to harmonize the required durability of the construction and the expected lifetime of the coating, and naturally it is necessary to take economical and technical aspects into consideration. Reliable lifetime prediction of the coatings is an essential—although difficult—task. We may expect adequate performance estimates if the coating is tested in the same—or similar—environment as that of the actual application [1]; however, such a natural exposure test requires too long time. For reducing test time, accelerated natural exposures and laboratory tests have been developed. These methods have been discussed in many works, among them in ref. [1] by Applemann. Accelerated outdoor tests have additional disadvantages over the non-accelerated ones: these are of poor reproducibility because of uncertainties of weather (note that the “non-accelerated” outdoor tests are much less sensitive to ∗
Corresponding author. Fax: +36 1 3257892. ´ Fekete). E-mail address: [email protected] (E.
0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.06.007
weather changes, since the time scale of the test is larger than that of the weather changes). The advantages and drawbacks of various exposure tests (natural and accelerated natural) have been discussed by Johnson and McIntyre [2]. Although accelerated laboratory tests are, in general, the fastest and their conditions are reproducible, reliability of their results is sometimes inadequate. Unfortunately, during the accelerated tests not only the relevant corrosion processes are speeded up but other, unwanted processes are initiated as well [3–5]. According to the generally adopted view, an accelerated test is reliable and acceptable if, for a series of coatings, it yields the same ranking as that obtained by the natural exposure test. By the results of these tests and the field experiments, it is possible to predict the lifetime of a coating. Carlozzo and co-workers were seeking correlation between six accelerated test methods and nine geographically different exposure sites applying nine different coating systems [6,7]. Many groups follow corrosion protection properties by electrochemical methods like impedance spectroscopy [3,8–13] or noise analysis [14–16]. The salt spray chamber and humidity chamber tests are widely used accelerated laboratory procedures for predicting
212
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
corrosion performance of coatings [1] although these methods have often been criticized [1,3,4,17], especially in relation to waterborne paints [18], but good correlations were found between the results of salt spray and natural marine environment exposure tests [7,19]. Different procedures were developed modifying and varying these methods and there are researchers who suggest pretreatments before tests too, for example Wienbeck [20]. Other authors applied tests involving UV exposure and/or some type of heat treatments [13,17,21]. Fig. 1. Percentage of rusted area on the base metal as a function of natural exposure duration for samples covered with coating no. 2.
Fig. 2. Dependence of corroded area on natural exposure and on the salt spray chamber durations. Natural exposure durations are given as: (a) 2 weeks at room temperature and (b–h) 3, 6, 9, 12, 18, 24, 30 months of natural exposure, respectively.
Fig. 3. Dependence of corroded area on the natural exposure and on the humidity chamber durations. Natural exposure durations are given as: (a) 2 weeks at room temperature and (b–h) 3, 6, 9, 12, 18, 24, 30 months of natural exposure, respectively.
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
Some waterborne coatings exhibit poor performance in the salt spray test, whereas their durability in field exposure is quite good [17]. As an explanation, corrosion protection of waterborne coatings is assumed to improve upon (a certain period of) weathering. The accelerated tests are usually made within a few days or weeks after coating preparation (these samples will be hereafter referred to as “fresh” coatings), so the information on the behavior refers to the “fresh” coatings rather than to the continuously ageing ones. The aim of present work was to verify this assumption. In addition, we tried to find pretreatment methods after which the lifetime of a coating would be predicted more reliably than with the “fresh” ones.
2. Experimental Three paint systems (primer + topcoat) were studied. These are the followings: Coating no. 1: zinc phosphate pigmented primer + topcoat containing inorganic color pigment. Coating no. 2: zinc phosphate and zinc oxide pigmented primer + topcoat containing inorganic color pigment. Coating no. 3: zinc phosphate pigmented primer + topcoat containing inorganic color pigment and micaceous iron pigment. In all cases, the binder was a physically drying waterborne styrene–acrylate copolymer emulsion. The thickness of the films varied from 140 to 160 m, they were applied by spraying on 1 mm carbon steel plates of 10 cm × 20 cm size. Two types of laboratory accelerated tests were carried out: the salt spray chamber test (standard ASTM B 117-02) and the humidity chamber test (standard ISO 6270). The exposure periods in the chambers were 75, 200 and 500 h. Natural exposure (outdoor) experiments were carried out at a site in Budapest (having a mild continental climate) with exposure times ranging from 3 months to 2.5 years.
213
In this paper, we characterized the corrosion protection of the coatings only by inspecting the metal: after taking the samples from the chambers or from the outdoor exposure sites, the coatings were removed from metal plate and then the percentage of rusted area was quantified. For the evaluation of the degree of rusting on painted steel surfaces, a series of rusted samples—as standards—were used. The salt spray and humidity tests were carried out on the “fresh” coatings, and also on samples which underwent various procedures previously as follows: 1. outdoor exposure of 3, 6, 9, 12, 18, 24, 30 months of duration; 2. kept indoor for 1 year at room temperature; 3. other treatments as a. heat (500 h at 60 ◦ C); b. wet–dry cycles (1 day in distilled water, then 1 day in air, repeated ten times, temperature 25 ◦ C).
3. Results and discussion We observed no base metal corrosion on the samples of outdoor exposures during the 2.5 years on coatings nos. 1 and 3. On samples covered by coating no. 2, signs of base metal corrosion appeared after 1.5 years. In Fig. 1, we show the percentage of rusted area on the base metal as a function of natural exposure duration for samples covered with coating no. 2. We show the dependence of the portion of the corroded area on natural exposure and on the salt spray chamber durations in Fig. 2. In Fig. 3, the portion of the corroded area is plotted against the natural exposure and against the humidity chamber durations. All “fresh” coatings showed rather poor corrosion protection both in the salt spray and in the humidity tests (Figs. 2 and 3, respectively).
Fig. 4. Images of the samples with coating no. 3, after outdoor exposure, of duration indicated, followed by a 500 h salt spray chamber test and a final removal of the coating.
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´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
The outdoor exposure, however, improved corrosion resistance: samples with outdoor exposures showed a better performance in the accelerated tests than those with “fresh” coatings. Up to a certain time of exposures, the longer the exposure period had been, the better the tests results were. This optimum time of exposure—in what follows:
toptimum —depends on the coatings and on the type of tests (cf. Figs. 2 and 3). To illustrate this, in Fig. 4 we show the images of the samples with coating no. 3 after various times of outdoor exposure, followed by 500 h salt spray test and finally removal of the coating.
Fig. 5. Dependence of the portion of the corroded area on the different pretreatments and on the salt spray chamber durations. The pretreatments are as follows: (a) 2 weeks at room temperature, (b) 1 year at room temperature, (c) 1 year of natural exposure, (d) 10 wet–dry cycles and (e) 500 h at 60 ◦ C, respectively.
Fig. 6. Dependence of the corroded area on the different pretreatments and on the humidity chamber durations. The pretreatments are as follows: (a) 2 weeks at room temperature, (b) 1 year at room temperature, (c) 1 year of natural exposure, (d) 10 wet–dry cycles and (e) 500 h at 60 ◦ C, respectively.
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
toptimum was found to be the shortest on the samples with coating no. 2 (12 months before the salt spray and 9 months before the humidity test). This coating showed the worst performance during outdoor exposure, i.e. this was the coating underneath which signs of base metal corrosion appeared the earliest (after 18 months). toptimum was found to be the longest in case of samples with coating no. 3 and 1 with salt spray and humidity tests, respectively. Till now, we could not rank coatings nos. 1 and 3 based upon the results of outdoor exposure. The samples, which were kept indoor at room temperature for 1 year prior to laboratory accelerated tests, showed better performance than those with “fresh” coatings (cf. Figs. 5 and 6 for the two different tests). However, these samples were worse than the ones after 1 year outdoor exposure (with the exception of samples of coating no. 2 in the humidity chamber test). This finding made us to conclude that the improvement of the coating’s corrosion protection properties during outdoor exposure are affected by various factors like UV illumination, temperature, humidity, their combinations and variations, etc. From the infinite combinations of these factors, we chose two simple cases. The results are as follows. Some improvement and definite improvement could be observed when heat treatments and dry–wet cyclings were applied, respectively (cf. Figs. 5 and 6). During the outdoor exposure, the three coatings were found to be different: corrosion spots appeared with coating no. 2, whereas the other coatings were found undamaged even after 2.5 years. Still, the salt spray tests did not reveal any difference between samples with “fresh” coatings. Differences between the coatings increased not only during the outdoor exposure, but also during the dry–wet cyclings and heat treatments: in all cases coating nos. 2 and 3 appeared to be the worst and the best, respectively, according to the results of the salt spray tests. The same order is found, if the topimum values are considered.
4. Summary and conclusions With “fresh” coatings, the salt spray and humidity tests yielded very poor results. By testing the corrosion protection of coatings in salt spray and humidity chambers, we have observed that the performance improved if coatings were exposed outdoor for months prior to the test. This justifies the assumption that corrosion protection of waterborne coatings is improved upon weathering. By increasing the length of the period of outdoor exposure, the results of the tests proved to become better but better till a certain point. This time period corresponding to the improving behavior was the shortest for the coating (no. 2), which showed the worst performance during outdoor exposure, i.e.
215
this was the coating underneath which signs of base metal corrosion appeared first. The dry–wet cycling and heat treatment also improved corrosion performance, as determined by salt spray and humidity chamber tests. Based on these findings we conclude, that the salt spray and humidity chamber tests would be appropriate for characterizing corrosion protection of waterborne coatings if these tests proceeded by treatments of some type of wet–dry cycling, may be along with additional heat treatments. Wienbeck also found appropriate the salt spray and humidity chamber tests if combined with certain treatments [20]. It seems to be instructive to extend these studies to other paints containing different binders and pigments.
Acknowledgements The paints were supplied by their manufacturer Magyarlakk Ltd.,their help and advice are highly appreciated. The financial help of the Hungarian Research Foundation OTKA (contract no. T 029727 and no. T 42452) is acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
B.R. Applemann, J. Coat. Technol. 62 (787) (1990) 57. B.W. Johnson, R. McIntyre, Prog. Org. Coat. 27 (1–4) (1996) 95. F. Deflorian, L. Fedrizzi, S. Rossi, Corrosion 54 (8) (1998) 598. B.S. Skerry, C.H. Simpson, Corrosion 49 (8) (1993) 663. F.X. Perrin, M. Irigoyen, E. Aragon, J.L. Vernet, Polym. Degrad. Stab. 72 (2001) 115. J. Andrews, F. Anwari, B.J. Carlozzo, M. Dilorenzo, R. Glover, S. Grossman, et al., J. Coat. Technol. 66 (837) (1994) 49. B.J. Carlozzo, J. Andrews, F. Anwari, M. Dilorenzo, R. Glover, S. Grossman, et al., J. Coat. Technol. 68 (858) (1996) 47. A. Amirudin, D. Thierry, Prog. Org. Coat. 26 (1995) 1. F. Mansfeld, J. Appl. Electrochem. 25 (1995) 187. J.M. McIntyre, H.Q. Pham, Prog. Org. Coat. 27 (1996) 201. M. Del Grosso Destreri, J. Vogelsang, L. Fedrizzi, F. Deflorian, Prog. Org. Coat. 37 (1–2) (1999) 69. D.H. van der Weijde, E.P.M. van Westing, J.H.W. de Wit, Mater. Sci. Forum 289–292 (1998) 237. G.P. Bierwagen, L. He, J. Li, L. Ellingson, D.E. Tallman, Prog. Org. Coat. 39 (2000) 67. J. Mojica, E. Garc´ıa, F.J. Rodr´ıguez, J. Genesc´a, Prog. Org. Coat. 42 (2001) 218. H. Greisiger, T. Schauer, Prog. Org. Coat. 39 (2000) 31. G.P. Bierwagen, C.S. Jeffcoate, J.P. Li, S. Balbyshev, D.E. Tallman, D.J. Mills, Prog. Org. Coat. 29 (1–4) (1996) 21. R.W. Flynn, R.E. Telan, Resin Rev. 39 (2) (1989) 11. E. Spengler, F.L. Fragata, I.C.P. Margarit, O.R. Mattos, Prog. Org. Coat. 30 (1997) 51. E. Almeida, D. Santos, J. Uruchurtu, Prog. Org. Coat. 37 (1999) 131. H. Wienbeck, Farbe Lack 107 (2) (2001) 112. A. Miszczyk, K. Darowicki, Corr. Sci. 43 (2001) 1337.
Accelerated testing of waterborne coatings ´ Fekete ∗ , B´ela Lengyel Eva Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Received 7 February 2005; accepted 14 June 2005
Abstract According to environmental regulations, many of the traditionally used organic solventborne coatings should be replaced by low-toxicity and environmentally friendly alternatives (e.g. by waterborne paints). We report here effects of weathering on waterborne coatings. Three styrene–acrylate waterborne paint systems containing various types of inorganic pigments were studied on steel substrate; salt spray, humidity chamber and field exposure tests were carried out on them. The accelerated laboratory tests were performed both on coatings after 2 weeks of coating preparation (“fresh” coating) and on naturally aged ones, i.e. after field exposures of various durations ranging from 3 months to 2.5 years. We found that—for a certain time—the longer the exposure period, the better are the results of salt spray and humidity chamber tests. Additional experiments were carried out on samples with different pretreatments: in some cases the results of the accelerated tests after cyclic dry–wet or heat pretreatments are better than that of “fresh” coatings. © 2005 Elsevier B.V. All rights reserved. Keywords: Salt spray test; Humidity test; Dry–wet cyclic pretreatment; Heat pretreatment
1. Introduction Choosing a coating system to protect a metal construction against corrosion is important to harmonize the required durability of the construction and the expected lifetime of the coating, and naturally it is necessary to take economical and technical aspects into consideration. Reliable lifetime prediction of the coatings is an essential—although difficult—task. We may expect adequate performance estimates if the coating is tested in the same—or similar—environment as that of the actual application [1]; however, such a natural exposure test requires too long time. For reducing test time, accelerated natural exposures and laboratory tests have been developed. These methods have been discussed in many works, among them in ref. [1] by Applemann. Accelerated outdoor tests have additional disadvantages over the non-accelerated ones: these are of poor reproducibility because of uncertainties of weather (note that the “non-accelerated” outdoor tests are much less sensitive to ∗
Corresponding author. Fax: +36 1 3257892. ´ Fekete). E-mail address: [email protected] (E.
0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.06.007
weather changes, since the time scale of the test is larger than that of the weather changes). The advantages and drawbacks of various exposure tests (natural and accelerated natural) have been discussed by Johnson and McIntyre [2]. Although accelerated laboratory tests are, in general, the fastest and their conditions are reproducible, reliability of their results is sometimes inadequate. Unfortunately, during the accelerated tests not only the relevant corrosion processes are speeded up but other, unwanted processes are initiated as well [3–5]. According to the generally adopted view, an accelerated test is reliable and acceptable if, for a series of coatings, it yields the same ranking as that obtained by the natural exposure test. By the results of these tests and the field experiments, it is possible to predict the lifetime of a coating. Carlozzo and co-workers were seeking correlation between six accelerated test methods and nine geographically different exposure sites applying nine different coating systems [6,7]. Many groups follow corrosion protection properties by electrochemical methods like impedance spectroscopy [3,8–13] or noise analysis [14–16]. The salt spray chamber and humidity chamber tests are widely used accelerated laboratory procedures for predicting
212
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
corrosion performance of coatings [1] although these methods have often been criticized [1,3,4,17], especially in relation to waterborne paints [18], but good correlations were found between the results of salt spray and natural marine environment exposure tests [7,19]. Different procedures were developed modifying and varying these methods and there are researchers who suggest pretreatments before tests too, for example Wienbeck [20]. Other authors applied tests involving UV exposure and/or some type of heat treatments [13,17,21]. Fig. 1. Percentage of rusted area on the base metal as a function of natural exposure duration for samples covered with coating no. 2.
Fig. 2. Dependence of corroded area on natural exposure and on the salt spray chamber durations. Natural exposure durations are given as: (a) 2 weeks at room temperature and (b–h) 3, 6, 9, 12, 18, 24, 30 months of natural exposure, respectively.
Fig. 3. Dependence of corroded area on the natural exposure and on the humidity chamber durations. Natural exposure durations are given as: (a) 2 weeks at room temperature and (b–h) 3, 6, 9, 12, 18, 24, 30 months of natural exposure, respectively.
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
Some waterborne coatings exhibit poor performance in the salt spray test, whereas their durability in field exposure is quite good [17]. As an explanation, corrosion protection of waterborne coatings is assumed to improve upon (a certain period of) weathering. The accelerated tests are usually made within a few days or weeks after coating preparation (these samples will be hereafter referred to as “fresh” coatings), so the information on the behavior refers to the “fresh” coatings rather than to the continuously ageing ones. The aim of present work was to verify this assumption. In addition, we tried to find pretreatment methods after which the lifetime of a coating would be predicted more reliably than with the “fresh” ones.
2. Experimental Three paint systems (primer + topcoat) were studied. These are the followings: Coating no. 1: zinc phosphate pigmented primer + topcoat containing inorganic color pigment. Coating no. 2: zinc phosphate and zinc oxide pigmented primer + topcoat containing inorganic color pigment. Coating no. 3: zinc phosphate pigmented primer + topcoat containing inorganic color pigment and micaceous iron pigment. In all cases, the binder was a physically drying waterborne styrene–acrylate copolymer emulsion. The thickness of the films varied from 140 to 160 m, they were applied by spraying on 1 mm carbon steel plates of 10 cm × 20 cm size. Two types of laboratory accelerated tests were carried out: the salt spray chamber test (standard ASTM B 117-02) and the humidity chamber test (standard ISO 6270). The exposure periods in the chambers were 75, 200 and 500 h. Natural exposure (outdoor) experiments were carried out at a site in Budapest (having a mild continental climate) with exposure times ranging from 3 months to 2.5 years.
213
In this paper, we characterized the corrosion protection of the coatings only by inspecting the metal: after taking the samples from the chambers or from the outdoor exposure sites, the coatings were removed from metal plate and then the percentage of rusted area was quantified. For the evaluation of the degree of rusting on painted steel surfaces, a series of rusted samples—as standards—were used. The salt spray and humidity tests were carried out on the “fresh” coatings, and also on samples which underwent various procedures previously as follows: 1. outdoor exposure of 3, 6, 9, 12, 18, 24, 30 months of duration; 2. kept indoor for 1 year at room temperature; 3. other treatments as a. heat (500 h at 60 ◦ C); b. wet–dry cycles (1 day in distilled water, then 1 day in air, repeated ten times, temperature 25 ◦ C).
3. Results and discussion We observed no base metal corrosion on the samples of outdoor exposures during the 2.5 years on coatings nos. 1 and 3. On samples covered by coating no. 2, signs of base metal corrosion appeared after 1.5 years. In Fig. 1, we show the percentage of rusted area on the base metal as a function of natural exposure duration for samples covered with coating no. 2. We show the dependence of the portion of the corroded area on natural exposure and on the salt spray chamber durations in Fig. 2. In Fig. 3, the portion of the corroded area is plotted against the natural exposure and against the humidity chamber durations. All “fresh” coatings showed rather poor corrosion protection both in the salt spray and in the humidity tests (Figs. 2 and 3, respectively).
Fig. 4. Images of the samples with coating no. 3, after outdoor exposure, of duration indicated, followed by a 500 h salt spray chamber test and a final removal of the coating.
214
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
The outdoor exposure, however, improved corrosion resistance: samples with outdoor exposures showed a better performance in the accelerated tests than those with “fresh” coatings. Up to a certain time of exposures, the longer the exposure period had been, the better the tests results were. This optimum time of exposure—in what follows:
toptimum —depends on the coatings and on the type of tests (cf. Figs. 2 and 3). To illustrate this, in Fig. 4 we show the images of the samples with coating no. 3 after various times of outdoor exposure, followed by 500 h salt spray test and finally removal of the coating.
Fig. 5. Dependence of the portion of the corroded area on the different pretreatments and on the salt spray chamber durations. The pretreatments are as follows: (a) 2 weeks at room temperature, (b) 1 year at room temperature, (c) 1 year of natural exposure, (d) 10 wet–dry cycles and (e) 500 h at 60 ◦ C, respectively.
Fig. 6. Dependence of the corroded area on the different pretreatments and on the humidity chamber durations. The pretreatments are as follows: (a) 2 weeks at room temperature, (b) 1 year at room temperature, (c) 1 year of natural exposure, (d) 10 wet–dry cycles and (e) 500 h at 60 ◦ C, respectively.
´ Fekete, B. Lengyel / Progress in Organic Coatings 54 (2005) 211–215 E.
toptimum was found to be the shortest on the samples with coating no. 2 (12 months before the salt spray and 9 months before the humidity test). This coating showed the worst performance during outdoor exposure, i.e. this was the coating underneath which signs of base metal corrosion appeared the earliest (after 18 months). toptimum was found to be the longest in case of samples with coating no. 3 and 1 with salt spray and humidity tests, respectively. Till now, we could not rank coatings nos. 1 and 3 based upon the results of outdoor exposure. The samples, which were kept indoor at room temperature for 1 year prior to laboratory accelerated tests, showed better performance than those with “fresh” coatings (cf. Figs. 5 and 6 for the two different tests). However, these samples were worse than the ones after 1 year outdoor exposure (with the exception of samples of coating no. 2 in the humidity chamber test). This finding made us to conclude that the improvement of the coating’s corrosion protection properties during outdoor exposure are affected by various factors like UV illumination, temperature, humidity, their combinations and variations, etc. From the infinite combinations of these factors, we chose two simple cases. The results are as follows. Some improvement and definite improvement could be observed when heat treatments and dry–wet cyclings were applied, respectively (cf. Figs. 5 and 6). During the outdoor exposure, the three coatings were found to be different: corrosion spots appeared with coating no. 2, whereas the other coatings were found undamaged even after 2.5 years. Still, the salt spray tests did not reveal any difference between samples with “fresh” coatings. Differences between the coatings increased not only during the outdoor exposure, but also during the dry–wet cyclings and heat treatments: in all cases coating nos. 2 and 3 appeared to be the worst and the best, respectively, according to the results of the salt spray tests. The same order is found, if the topimum values are considered.
4. Summary and conclusions With “fresh” coatings, the salt spray and humidity tests yielded very poor results. By testing the corrosion protection of coatings in salt spray and humidity chambers, we have observed that the performance improved if coatings were exposed outdoor for months prior to the test. This justifies the assumption that corrosion protection of waterborne coatings is improved upon weathering. By increasing the length of the period of outdoor exposure, the results of the tests proved to become better but better till a certain point. This time period corresponding to the improving behavior was the shortest for the coating (no. 2), which showed the worst performance during outdoor exposure, i.e.
215
this was the coating underneath which signs of base metal corrosion appeared first. The dry–wet cycling and heat treatment also improved corrosion performance, as determined by salt spray and humidity chamber tests. Based on these findings we conclude, that the salt spray and humidity chamber tests would be appropriate for characterizing corrosion protection of waterborne coatings if these tests proceeded by treatments of some type of wet–dry cycling, may be along with additional heat treatments. Wienbeck also found appropriate the salt spray and humidity chamber tests if combined with certain treatments [20]. It seems to be instructive to extend these studies to other paints containing different binders and pigments.
Acknowledgements The paints were supplied by their manufacturer Magyarlakk Ltd.,their help and advice are highly appreciated. The financial help of the Hungarian Research Foundation OTKA (contract no. T 029727 and no. T 42452) is acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
B.R. Applemann, J. Coat. Technol. 62 (787) (1990) 57. B.W. Johnson, R. McIntyre, Prog. Org. Coat. 27 (1–4) (1996) 95. F. Deflorian, L. Fedrizzi, S. Rossi, Corrosion 54 (8) (1998) 598. B.S. Skerry, C.H. Simpson, Corrosion 49 (8) (1993) 663. F.X. Perrin, M. Irigoyen, E. Aragon, J.L. Vernet, Polym. Degrad. Stab. 72 (2001) 115. J. Andrews, F. Anwari, B.J. Carlozzo, M. Dilorenzo, R. Glover, S. Grossman, et al., J. Coat. Technol. 66 (837) (1994) 49. B.J. Carlozzo, J. Andrews, F. Anwari, M. Dilorenzo, R. Glover, S. Grossman, et al., J. Coat. Technol. 68 (858) (1996) 47. A. Amirudin, D. Thierry, Prog. Org. Coat. 26 (1995) 1. F. Mansfeld, J. Appl. Electrochem. 25 (1995) 187. J.M. McIntyre, H.Q. Pham, Prog. Org. Coat. 27 (1996) 201. M. Del Grosso Destreri, J. Vogelsang, L. Fedrizzi, F. Deflorian, Prog. Org. Coat. 37 (1–2) (1999) 69. D.H. van der Weijde, E.P.M. van Westing, J.H.W. de Wit, Mater. Sci. Forum 289–292 (1998) 237. G.P. Bierwagen, L. He, J. Li, L. Ellingson, D.E. Tallman, Prog. Org. Coat. 39 (2000) 67. J. Mojica, E. Garc´ıa, F.J. Rodr´ıguez, J. Genesc´a, Prog. Org. Coat. 42 (2001) 218. H. Greisiger, T. Schauer, Prog. Org. Coat. 39 (2000) 31. G.P. Bierwagen, C.S. Jeffcoate, J.P. Li, S. Balbyshev, D.E. Tallman, D.J. Mills, Prog. Org. Coat. 29 (1–4) (1996) 21. R.W. Flynn, R.E. Telan, Resin Rev. 39 (2) (1989) 11. E. Spengler, F.L. Fragata, I.C.P. Margarit, O.R. Mattos, Prog. Org. Coat. 30 (1997) 51. E. Almeida, D. Santos, J. Uruchurtu, Prog. Org. Coat. 37 (1999) 131. H. Wienbeck, Farbe Lack 107 (2) (2001) 112. A. Miszczyk, K. Darowicki, Corr. Sci. 43 (2001) 1337.