Environmentally Friendly Anti-Corrosion Primers for Hot-Dip Galvanized (HDG) Steel

Transcription

1 Environmentally Friendly Anti-Corrosion Primers for Hot-Dip Galvanized (HDG) Steel C. Shivane, P. Puomi and W.J. van Ooij University of Cincinnati, Dept. of Chemical and Materials Eng., Cincinnati, OH , USA. to Abstract An effective anti-corrosive chromate-free, low-voc, waterborne, direct-to- primer for hot-dip galvanized steel (HDG) has been developed. This coating is made of a waterborne dispersion containing organofunctional silanes, waterdispersed resins, cross-linkers, minor binders and chromatefree anti-corrosion inhibitors. Typical systems are based on epoxy resins. These primers are unique as they can be applied onto the directly after alkaline cleaning. We will discuss the optimization of the superprimer coating for HDG steel. The selection of silanes, resin combinations, cross-linkers and minor binders was systematically optimized for the primers by using a L9 orthogonal array consisting of four factors each having three levels. The corrosion resistance of the coatings was evaluated using electrochemical methods and performance tests. During the study cured superprimer components and also complete coatings were characterized using sophisticated characterization methods. The results provided information on an optimized coating formulation on HDG steel that survives 2000 hours of ASTM B-117 salt spray testing. This coating contains intermediate amounts of epoxy, acrylate and silane, but no polyurethane. Introduction The objective of this study is to develop a one-step, chromatefree, low-voc, water-borne silane-containing primer for hotdip galvanized (HDG) steel. The idea of the superprimer is to replace chromate-containing pretreatments and primers with a 2-in-1 step primer that has low volatile organic content (VOC). Replacements for chromate in both steps (pretreatment and priming) are studied, because of the wellknown toxicity of chromate compounds [1]. The schematic of the superprimer coating system, which we call the future system, is shown in Figure 1 on the right [2]. The concept of superprimers basically involves any paint formulation containing some combination of bis-silanes, resins, crosslinkers and pigments, which improve the corrosion resistance of s [3]. The use of silanes in coatings has been explored more widely for applications that are not necessarily for the corrosion protection of s [4,5] or then the work falls more into the category of pretreatments or prepaint treatments of s [6, 7]. Before this optimization work, experiments with various coating components were made in separate studies on HDG Current System primer + Cr pigment chromate film oxide New System primer + Cr-free pigment silane film Future System superprimer Figure 1: Schematic of the future superprimer coating system and the new system consisting of a silane-based pretreatment and a chromate-free primer compared with the current system (Van Ooij et al. 2004) steel [8]. Based on the results of the component experiments, a set of chemicals which showed potential was identified under each component, i.e., major binder, minor binder, cross-linker and silane. The aim was to investigate what the concentration of each component should be in order to achieve optimized coating performance on HDG steel. Experimental The experimental set-up was planned using the Taguchimethod, which is a design of experiment (DOE) technique, wherein orthogonal matrices are used in the test set-ups and results are obtained in the form of response tables or optimization plots. The method gives information on factors affecting a certain property [9]. The coating combinations were prepared according to a L Taguchi-matrix, entailing four factors each having three levels. Table 1 shows the selected factors and levels. The matrix formed based on choices made in Table 1 is shown in Table 2. The components and the levels (Table 1) were chosen based on previous experiments. All nine formulations contained 5 wt.-% of zinc phosphate (ZP) and 5 wt.-% of titania. The two epoxies and the silane were first mixed with ZP and titania. The crosslinker, polyurethane and polyacrylate were then added and the ingredients were mixed to a homogeneous formulation. The formulations were applied on cleaned HDG panels using a draw-down bar, which produces a wet film

2 thickness of about 28 μm. The coated panels were step cured at room temperature (RT) for 14 days followed by a short high temperature cure at 135 ºC for 30 minutes. The additional high temperature cure was used based on earlier results, which showed that such a cure significantly improves the barrier properties of the coatings [10]. Table 1: Selected factors and levels 3. Polyurethane dispersion Neorez R Bis-benzene silane (BB) The levels of each factor g of the epoxies g crosslinker g of the epoxies g crosslinker g of the epoxies g crosslinker g acrylate g acrylate g acrylate g polyurethane g polyurethane g polyurethane g BB g BB g BB Table 2: The matrix formed by the selected factors and levels Two epoxies 1:1 + crosslinker Polyure -thane Factors 1. Resin combination; two epoxies in ratio 1:1 + the crosslinker; Ancarez Ar 550: EPI- REZ 6520-WH-53 1:1 + EPI-KURE 6870-W Acrylate dispersion ECO-CRYL 9790 Sample Acrylate Bisbenzene silane HDG steel panels were purchased from Stillwater Steel Supply, Stillwater, OK. The EPI-REZ 6520-WH-53 epoxy, the EPI-KURE 6870-W-53 crosslinker and the ECO-CRYL 9790 polyacrylic resin were all from Hexion Chemicals, Houston, TX. The other epoxy, Ancarez Ar 550 was from Air Products Inc., Allentown, PA and the polyurethane resin, NeoRez R-972 was from DSM Neoresins Inc., Wilmington, MA. The 1, 4-bis (trimethoxysilylethyl) benzene, i.e. the bisbenzene silane was obtained from Gelest Inc., Morrisville, PA. The painted panels were studied by using the following methods: ASTM D 714 salt immersion test, ASTM D 4752 MEK double rub test, Machu test, ASTM D tape adhesion test and Electrochemical impedance spectroscopy (EIS). Salt water immersion test was done for 30 days in 3.5 wt.-% NaCl solution. In the Machu test the painted panels are cross-scribed, and then immersed in a solution of 5% NaCl + 0.6% H 2 O 2 at 37 C for two days. On the second day 0.6% H 2 O 2 is added. After 2 days of immersion, the panels are taken out and adhesive tape is used to pull off the delaminated paint. For the tape adhesion test the primer coated panels were ed with a military polyurethane, Desothane HS. The ed panels were immersed in deionized water for 7 days before testing wet adhesion. The EIS lowfrequency impedance of the primer coated panels was measured before and after immersion in 3.5 wt.-% NaCl for 30 days. The experimental setup for EIS has been described elsewhere [11]. During the entire study cured superprimer components and also complete coatings were characterized using FTIR, solid state 13 C-NMR and SEM/EDX techniques, which provide information of the chemical composition of the components and the coatings. After the study the optimized result formulation with and without was compared with two chromate controls by using the ASTM B117 salt spray test and Ford APGE test, which is a cyclic corrosion test used in the automotive industry [12]. Results and Discussion After salt immersion for 30 days none of the panels showed any scribe corrosion or deterioration. The same occurred with the MEK double rub test. All nine samples gave values exceeding 300 rubs. After the 2-day exposure to the Machu test all nine samples looked fine. Thus, the exposure time was extended to three days, after which none of the samples showed substantial scribe corrosion, creep or delamination. Therefore, the test was continued for four days. However, all the formulations deteriorated badly by the end of day 4 of the test. The aggressive conditions of the test may have caused partial delamination of the coatings from the substrate already during the first three days of immersion, but evidently no sign of this was detectable by mere observation of the panels. On the fourth day the deterioration process must have progressed very rapidly in all samples resulting in a similar result. Hence, three of the tests used were not able to differentiate between the performances of the samples. The tape adhesion test differentiated one sample from the others. The first sample of Table 2 had poor adhesion. This sample is compared to one of the well-performing samples in Figure 2. As can be seen from the figure, sample 2 shows an adhesion of 2B (~25% delamination) and some blistering on the coating surface. Delamination had occurred both at the primer- and primer- interfaces. None of the other 8 samples showed any delamination (adhesion 5B). A bar plot showing the EIS low frequency impedance values of the nine coatings is shown in Figure 3. The EIS test results (Figure 3) were analyzed using an optimization plot as shown in Figure 4. The EIS values were normalized to a scale of 100. A value of 100 was given to the low-frequency impedance value of sample 6, as it had the highest value after 30 days and the other impedance values were scaled accordingly. The same level for a factor is present

3 in three formulations of the matrix (Table 2). This gives rise to three normalized values for each factor level. The average of these three EIS test values for a level was calculated and this was the average response of the factor level. Then plots were drawn showing the variation of these average EIS response values with the level of each factor. Based on the plots shown in Figure 4 the optimum level for each factor was chosen. The combination of the optimum level of each factor forms the optimum formulation shown in Table 3. inhibitor based on Ca, Zn, P, Si, O) and 1 wt.-% of sodium metavanadate showed the best corrosion resistance [8]. Table 3: Optimized formulation levels for the factors defined in Table 1 Factor Best level Best amount in grams The two epoxies + crosslinker Acrylate Polyurethane 1 0 Bis-benzene silane Low freq. impedance (Mohm) Sample 2 Sample 6 Figure 2: Topcoat adhesion test results. Samples 2 and 6 from the matrix shown in Table _1 2_2 2_3 2_4 2_5 2_6 2_7 2_8 2_9 Formulation Day 1 impedance Day 30 impedance Figure 3: EIS low-frequency impedance values of the coatings shown in Table 2, on day 1 and day 30 of exposure in 3.5 wt.- % salt solution An important observation for the matrix is that the optimized level of polyurethane is zero as shown in Table 3 for the optimized formulation, i.e., the acrylate and the polyurethane are antagonistic and the acrylate aids performance better than polyurethane. After optimization the formulation shown in Table 3 was prepared and compared to two chromate controls in the ASTM B117 and the Ford APGE test. The optimized coating was tested with and without in these two tests and during the tests EIS measurements were run every week on the samples by a hand-held corrosion sensor. By this time SEM/EDX results of pigment combinations had shown that a pigment combination of 10 wt.-% of Corrostain 228 (an Av. low freq. impedance Two epoxies 1:1 + crosslinker Polyurethane Levels Acrylate Bis-benzene silane Figure 4: Optimization plot based on EIS results for the matrix shown in Table 2 Therefore, this combination was inserted into the optimized formulation (Table 3). The chromate controls were abbreviated BPT and CPT. BPT stands for bare HDG steel + solvent-borne chromate-containing epoxy primer + Desothane HS polyurethane. CPT is an abbreviation of chromated HDG steel + solvent-borne chromate-containing epoxy primer + Desothane HS polyurethane. The ASTM B117 salt spray test results of these three samples are presented in Figure 5 after 2000 hrs of testing. As can be seen from Figure 5 the performance of the optimized coating is similar to the chromate controls, even if the optimized sample is completely chromate-free. This result is rare, because usually chromate-free samples perform inferior to chromate-containing samples. The sample also did

4 well in the Ford APGE test. A substantial drop could, however, be detected in the EIS low frequency impedance values of the optimized coating during exposure to the tests compared to the chromate controls. Low frequency impedance is mainly an indicator of the electrolyte absorption tendency of the coating. As the optimized coating was made from a waterbased formulation, it is evident that this sample is more hydrophilic than the control coatings prepared from solventborne coatings. The degrading effects of water absorption like swelling and blistering in coatings made from water-borne solutions can be reduced by incorporating silanes and minor binders into the coating that subjugate the negative effects. of the hydrophilicity of the coating formed but yet is waterresistant enough to withstand negative effects caused by water absorption. The structure of the coating also allows inhibitors of the coating to leach out on demand and protect the at scribes and defects. Acknowledgments The authors gratefully acknowledge SERDP (Strategic Environmental Research and Development Program) for funding, Dr. Guy Davis for corrosion testing, Dr. Tammy Metroke for NMR analysis expertise and Sumeet for the SEM/EDX analysis. Bibliography Figure 5: The optimized sample a) after 2000 hrs of salt spray testing compared to the chromate controls b) and c) tested also for 2000 hrs. However, the water penetration can also be turned into an advantage by the use of suitable inhibitors, provided an appropriate water-resistant film is still present in the coating. SEM/EDX studies of coatings exposed to 30 days of salt immersion testing have shown that due to the hydrophilic nature of the superprimer coating the chromate-free inhibitors of the coating are able to leach out on demand and protect the scribes of the coating in corrosive environments. The EIS and performance test results of the optimized coating indicates the phenomenon described, i.e., the low-frequency impedance value of the optimized coating drops at the beginning of the tests but the scribes on the panel remain nearly clean and no blistering and delamination is observed. The FTIR and NMR results of this coating have shown that the epoxy groups of the resin react with the amines of the crosslinker. The silanol groups of the hydrolyzed silane react with each other forming siloxane Si-O-Si and the OH groups of the resin forming C-O- Si. The minor binders also react with the other ingredients and are incorporated into the network, i.e., an interpenetrating water-resistant network is present in the coating, which assures the degradation resistance of the coating upon water penetration [8, 10]. [1] U.S. Public Health Service, Agency for Toxic Substances (1989) Toxicological Profile for Chromium, Report No. ATSDR/TP-88/10, July. [2] Van Ooij, W.J., Seth, A., Mugada, T., Pan, G. and Schaefer, D.W. (2004) A Novel Self-Priming Coating for Corrosion Protection Proc 3rd Int. Surface Eng. Congress, Orlando, FL, August 2-4 (on CD). [3] Seth, A., Van Ooij, W. J., (2004) J. Mat. Eng. Perf., 13, 292. [4] Bauer, F. and Mehnert, R., (2005) J. Polym. Research 12, [5] Neuder, H., Sizemore, C., Kolody, M., Chiang, R., and Lin, C. T., (2003) Prog. Org. Coat., 47, [6] K. Bittner, H. Domes, C. Jung, N. Maurus, T. Kolberg, M. Schinzel, H. Wietzoreck and T. Shimakura, (2003) US Patent 2003/ , 2 October. [7] I.J. Rangwalla, J.E. Wyma, P.J.J Merlin, M.S. Nanavati and L. Gallez, (2002) US Patent B1, 9 July. [8] Shivane, C., Puomi, P., Van Ooij, W.J., (2006) Novel one-step chromate-free primers for improved corrosion control of HDG steel, submitted for publication. [9] Taguchi G. and Konishi S. (1987) Orthogonal Arrays and Linear Graphs, ASI press Dearborn, MI. [10] Shivane, C., (2006) M.S. Thesis, University of Cincinnati, Chemical and Materials Engineering. [11] Zhu, D., and Van Ooij, W.J., (2002) J. Adh. Sci. Technol, 16, [12] Ford Laboratory Test Method, BI-23-1 (1981) April 30. Keywords: silane, primer, chromate-free, water-based, HDG steel Conclusions An optimized coating for HDG steel has been developed and discussed in this paper. The chemistry of the coating prepared from a water-based formulation is unique as it takes advantage

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