The corrosion performance of organosilane based pre-treatments for coatings on galvanised steel

Progress in Organic Coatings 38 (2000) 17 26 The corrosion performance of organosilane based pre-treatments for coatings on galvanised steel M.F. Montemor a, A.M. Simões a,, M.G.S. Ferreira a, B. Williams b, H. Edwards b a DEQ, IST, Av.Rovisco Pais, 1096 Lisboa codex, Portugal b British Steel, Welsh Tech. Centre, Port Talbot, West Glamorgan, SA13, 2NG, UK Received 31 May 1999; accepted 1 October 1999 Abstract The work aims at the search of environmentally friendly pre-treatment technologies for improved coating adhesion and corrosion resistance of coil coatings. Hot-dip galvanised steel panels were treated with a zirconium nitrate/organosilane solution. Three different treatments based on organofunctional silanes were investigated. The analytical characterisation of the substrates was made by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The corrosion behaviour of the painted systems under immersion in sodium chloride solution was investigated by means of electrochemical impedance spectroscopy. The corrosion resistance of the painted panels is interpreted based on the nature of the organosilane used in the pre-treatment and on its distribution over the substrate. 2000 Elsevier Science S.A. All rights reserved. Keywords: Hot-dip galvanised steel; Organosilane; Electrochemical impedance; Auger electron spectroscopy; X-ray photoelectron spectroscopy 1. Introduction Surface treatments currently used to promote adhesion on coil coatings are frequently based on chromium chemistry. Though efficient as both adhesion promoters and corrosion inhibitors [1] the toxicological properties of chromates render their continued use unfeasible, and alternative systems, with lower environmental burden, must be sought. The actual tendency is therefore to develop new low toxicity systems that may replace the traditional technologies involving chromates. These new systems should be able to improve the protection against corrosion and also to provide good adhesion characteristics. In this respect, organosilane technology seems to be promising for the pre-treatment of metal surfaces prior to organic coatings application [2 5]. Various pre-treatments based on the use of organosilanes were examined. The organosilane was applied following a zirconium nitrate treatment. The pre-treated surfaces were characterised by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), since both the distribution of the silane on the surface and the chemical state of the various elements should be a determinant factor for coating adhesion. The analytical experiments were also per- Corresponding author. Tel.: 351-1-8417234; fax: 351-1-8404589. E-mail address: pcsimoes@alfa.ist.utl.pt (A.M. Simões). formed on the hot-dip galvanised substrates (HDG) both as-received and zirconium nitrate-treated. The corrosion behaviour of the finished painted samples was studied by electrochemical impedance spectroscopy (EIS) during immersion in sodium chloride solution. This technique has proved to be effective to measure the corrosion resistance of painted systems [2,6]. The results were compared with those of the conventional chromium based pre-treatment. 2. Experimental procedure 2.1. Preparation of samples and solutions Industrially produced smooth hot-dip galvanised panels were degreased as substrate (zinc coating with 5% Al, 275gm 2 ) were cleaned in Gardolene 187 U, washed with tap water, drip-dried and immersed in a zirconium nitrate solution [0.05 M Zr (NO 3 ) 4 ] for 2 3 s. After immersion, the excess solution was removed and the panels were dried in aovenat150 C for 30 s. The panels were then immersed in organosilane solution (5% v/v) for 30 s and then dried to remove the excess solution and finally oven dried at 80 C for approximately 20 min. A number of panels thus prepared were used for the analytical experiments, whereas others were painted with a polyester-based primer and then submitted to corrosion experiments. Primer-coated samples 0300-9440/00/$ see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0300-9440(99)00080-6

18 M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 submitted to industrial chromate pre-treatment were also tested. The organosilane solutions were prepared with deionised water. Three organosilanes were tested: A: CH 2 OCHOCH 2 CH 2 CH 2 Si(OCH 3 ) 3 B: CH 2 =CHC 6 H 4 CH 2 NHCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 HCl C: NH 2 CH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 2.2. Analytical experiments AES depth profiles, Auger mapping and XPS analysis were made on the bare and pre-treated substrates under investigation, before the application of the primer coating. The analytical experiments were carried using a 310 F Microlab (VG Scientific) equipped with a field emission type electron gun, a concentric hemispherical analyser and a differentially pumped ion gun. Auger spectra were taken using a 10 kev, 40 na primary electron beam that made an angle of 30 with the surface normal. Spectra were run in constant retard ratio mode (CRR = 10 ev) with an ener- getic resolution of 0.2%. Calibration of the analyser was made according to the following peak energies: Cu LMM at 918.62 ev; Ag MNN at 357.80 ev and Au NVV at 70.10 ev. Ion etching was performed at a pressure of 1 10 7 mbar using high purity argon (etching current of 1 Amm 2 ). The etched area was a crater with a diameter of 0.5 mm and the electron beam had a spatial resolution of 100 nm. XPS experiments were performed on the sample surface with a non-monochromatic Mg anode (K = 1253.6 ev). Spectra were obtained in constant analyser energy mode (CAE = 30 ev), and corrected for C1s at 285.0 ev. Although it was preferred to work with etching times in all the profiles presented, an approximate etching rate of 1 Å s 1 can be assumed under the working conditions. 2.3. Electrochemical experiments The corrosion behaviour of the primer-coated substrates was investigated by EIS, using a three electrode arrangement: the working electrode, the reference electrode (saturated calomel electrode) and a Pt counter electrode. A 0.15% w/w NaCl solution was used at room temperature and Fig. 1. Auger depth profiles for the different substrates before primer-coating application.

the experiments were carried at the open circuit potential for different immersion times. Measurements were made using a 1255 Solartron FRA (50 khz 1 mhz) and a 1286 Solartron Electrochemical Interface. A 10 mv (rms) sine wave was applied. All tests were made in duplicate. M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 19 3. Results 3.1. Analytical experiments The Auger depth profiles obtained on the different samples are depicted in Fig. 1. The profile obtained on the as-received sample shows a strong signal for the oxygen at the surface with an atomic ratio O/Zn 2. For the sample treated with Zr(NO 3 ) 4, the Zr signal is observed in the outermost layers, being extinguished after 400 s of etching time. In the samples treated with the silanes, the Si peak was observed at the surface, with atomic percentages ranging from 10 20%. The Si profile extends over 100 s of etching time for the sample treated with silane A, being thicker for the samples treated with silanes B and C. The Si content decreases with the etching, showing that the silane is concentrated on the surface. A similar evolution of the Si profile has been reported by Hörnstrom et al. [7]. The evolution of the atomic ratio Si/Zr with the etching time obtained from the AES depth profiles is depicted in Fig. 2. For silanes A and C, the silane layer seems to have a low concentration, in contrast with silane B that reveals a higher concentration of Si near the surface. For silane C, however, the silane layer seems to extend over a larger depth when compared to the other treatments. The atomic ratios Si/Zn and Si/Zr obtained from the XPS analysis are depicted in Fig. 3. The sample treated with silane B revealed an Si/Zn atomic ratio 4 5 times higher than that observed on the sample treated with silane A. On the other hand, the sample treated with silane C revealed a Si/Zn atomic ratio of approximately 1.5, which is about three times higher than sample A. The highest Si/Zr atomic ratio was observed on samples treated with silanes B and C, in accordance with the results depicted in Fig. 2. Fig. 2. Evolution of the atomic ratio Si/Zr with the etching time for the different pre-treatments. Fig. 3. Si/Zn and Si/Zr atomic ratios obtained from the XPS analysis. Figs. 4 6 depict the XPS spectra obtained on the samples treated with the different silanes. The binding energies and the widths of the different XPS peaks are summarised in Table 1. The Si 2p spectra show two peaks, at 102.0 and 102.9 ev, due to the fact that Si is bound to carbon (C Si O) and to oxygen (O Si O). The width is in the range 1.9 2.2 ev for the two peaks. On sample B, however, the peak at lower binding energy (C Si O) becomes broadened to 2.90 ev, possibly due to the presence of the halogen atom. The Zr 3d 5/2 peak obtained on the sample treated with silane A corresponds to ZrO 2 (182.2 ev) [8] but the peak position slightly changes in the samples treated with silanes B and C, where the peak was detected at 182.6 ev. This energy is about 0.4 ev higher than that expected for the oxide, suggesting a bond with a more electronegative group. The width of the peak does not change significantly, being around 2.20 2.30 ev for all the peaks. On the sample treated with silane A, the Zn 2p 3/2 peak was observed at 1022.3 ev. This energy corresponds [8] to ZnO (1022.2 ev) and/or Zn(OH) 2. However, for the samples treated with silanes B and C, the broadening of the spectrum, as well as its asymmetry suggests the existence of a second peak at higher energy. Although the origin of this second peak is not totally understood, it may be due to the presence of the halogen atom (Cl) or the nitrogen atom in the B and C silane molecules, respectively. Thus, in silane B, the Cl group may react with the metallic substrate leading to the presence of a new Zn peak at higher binding energies. The presence of the Cl group may also be responsible for the shift observed in the Zr 3d 5/2 peak. For the sample treated

20 M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 Fig. 4. XPS spectra obtained on the samples treated with silane A. with silane C, the peak at higher energy can be the result of a bond between the amino group of the silane and the metallic substrate. In this sample the deconvolution of the N1s spectrum showed the presence of two peaks: at 400.3 and 401.6 ev. The peak at lower energies can be attributed to the R NH 2 group (R is the organic chain), whereas the peak at higher energies may be correlated with the presence of another bond. In the literature [3,7] it is suggested that the action of the aminosilane with the metallic surface involves a protonated reactive group ( NH + 3 ). This group endows the metal surface with an excess of positive charge that can be responsible for the shifts towards higher binding energies observed on the Zr 3d 5/2 peak and on the Zn 2p 3/2 peak. Fig. 5. XPS spectra obtained on the samples treated with silane B. The XPS results obtained on the samples treated with silanes B and C suggest that the organosilane interaction with the surface involve not only the Si atoms, but also the halogen group (Cl in silane B) and the nitrogen group (silane C). These extra bonds can improve the silane deposition process, resulting in a higher Si/Zr atomic ratio as observed in Figs. 2 and 3. The spatial distribution of the different elements on the surface was examined by scanning Auger mapping (SAM). Figs. 7 9 depict the Auger maps obtained on the surface of the samples treated with silanes A, B and C, respectively. In these pictures the white areas correspond to a stronger signal from the element under investigation, whereas the darker areas, correspond to a lower content of the element. The

M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 21 Fig. 6. XPS spectra obtained on the samples treated with silane C. oxygen signal is very strong in all the samples, as expected due to the presence of the oxides. The zinc maps reveal only few areas where the Zn is present, since the surface is essentially covered by ZrO 2 and by the silane. The most interesting features of the Auger maps are related to the presence of Si and Zr. For the samples treated with silanes A and C the Si and the Zr maps are very similar. In both samples, Si is in low concentration, but spreads over the entire surface, with a reasonably homogeneous distribution. In contrast, the maps obtained on the sample treated with silane B reveal the presence of small islands with diameters of 1 5 m. These islands are rich in Zr, whereas the surface surround- ing them is rich in Si and impoverished in Zr. In spite of the largest Si/Zr atomic ratio observed at the surface of the sample treated with silane B (Figs. 2 and 3), it is plausible that the non-uniform distribution of Si over the surface (Fig. 8) may also influence the further behaviour of the painted panels. 3.2. Electrochemical experiments Figs. 10 13 show the spectra obtained on the samples coated with a primer, immediately after immersion and after 2, 15 and 30 days of immersion, respectively. At the begin- Table 1 Binding energies of the different species observed in the XPS spectra Silane Designation Si 2p Zr 3d 5/2 Zn2p 3/2 N1s Binding FWHM a Binding FWHM Binding FWHM Binding FWHM energy (ev) energy (ev) energy (ev) energy (ev) CH 2 OCHO(CH 2 ) 3 Si(OCH 3 ) 3 A 102.0 1.95 182.3 2.20 1022.3 2.30 102.9 2.04 CH 2 =CHC 6 H 4 (NHCH 2 ) 2 Si(OCH 3 ) 3 HCl B 102.0 2.90 182.6 2.20 1022.3 2.70 192.9 2.20 1023.3 2.30 NH 2 (CH 2 ) 2 NH(CH 2 ) 3 Si(OCH 3 ) 3 C 101.9 2.00 182.6 2.30 1022.3 2.53 401.6 2.00 102.9 2.20 1023.6 2.16 400.3 2.09 a Full width at half maximum.

22 M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 Fig. 7. Auger maps obtained on the sample treated with silane A. ning of the immersion period the impedance spectra revealed a capacitive response, with a phase angle approaching 90 (Fig. 10). The capacitance was in the range 1 10 nf cm 2 and corresponds to the dielectric response of the primer coating. After 2 days of immersion, the samples treated with silanes A and C kept the same type of response, whereas a decrease in impedance occurred in the sample treated with silane B, due to the development of a resistive response in the low frequency part of the spectrum (Fig. 11). The sample treated with chromates revealed an impedance response similar to that obtained on samples treated with silanes B and C. After 2 weeks of immersion (Fig. 12) all the samples had become resistive at the low frequencies, revealing degradation of the coating. Meanwhile, another time constant developed at the low frequencies, suggesting the presence of a Warburg impedance or transmission lines associated with the pores of the coating [9,10]. This time constant is not completely defined in the range of frequencies studied. The resistance is in the range 10 6 10 7 cm 2 for samples treated Fig. 8. Auger maps obtained on the sample treated with silane B.

M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 23 Fig. 9. Auger maps obtained on the sample treated with silane C. with silanes A and C and for the Cr-treated samples, whereas in sample B it was only 10 5 cm 2. For the latter sample after 2 days of immersion, the measured resistance was >10 7 cm 2 ; thus a drop of two orders of magnitude occurred in the resistance, which indicated significant degradation of the coating. At this stage, no signs of corrosion were visible on any of the primer-coated samples. After 1 month of immersion (Fig. 13) treatment B was still the one with lowest impedance. In the samples with treatments A and B a low frequency process had become well defined. This process corresponded to an apparent capacitance of 0.5 Fcm 2, which can be associated to a corrosion process occurring underneath the coating, in a small fraction of the area. Both samples A and B revealed small blisters at this stage, whereas samples with treatment C and Cr-treated samples were intact to the eye. The sample treated with silane C revealed an impedance spectrum very close to that observed for the Cr-treated. 4. Discussion The process of corrosion in a coating depends upon several factors, namely the diffusion of water, ions and oxygen, the adhesion to the substrate and the rate of adhesion loss. Surface pre-treatments thus come as an important step in Fig. 10. Impedance spectra obtained on primer-coated samples immediately after immersion in NaCl, 0.15%: ( ) silane A; ( ) silane B; (+) silane C; ( ) chromate.

24 M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 Fig. 11. Impedance spectra obtained on primer-coated samples after 2 days of immersion in NaCl, 0.15%: ( ) silane A; ( ) silane B; (+) silane C; ( ) chromate. Fig. 12. Impedance spectra obtained on primer-coated samples after 14 days of immersion in NaCl, 0.15%: ( ) silane A; ( ) silane B; (+) silane C; ( ) chromate. Fig. 13. Impedance spectra obtained on primer-coated samples after 30 days of immersion in NaCl, 0.15%: ( ) silane A; ( ) silane B; (+) silane C; ( ) chromate.

M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 25 order to improve adhesion. The aim of this work was the characterisation and the comparison of different pre-treatments based on organosilanes on that process. Comparison of the performance of the three systems, corresponding to different silane treatments, placed treatment with silane B in the worst position, as far as exposure in full immersion is considered. The surface analysis had shown that treatment B lead to the highest Si/Zr atomic ratio on the surface. This increase of the Si/Zr ratio may be due to the formation of bonds between the halogen group (Cl) of the silane and the metallic substrate. However, the spatial distribution of Si was very heterogeneous, since it was essentially located around Zr islands that grew up over the surface. In spite of the higher Si/Zr atomic ratio, this uneven distribution corresponded to a poor performance of the coated system, with the lowest impedance of the three systems. This degradation of the primer coating occurred during the first 2 days of immersion. Treatments with silanes A and C resulted in better corrosion performances. In spite of the lower Si/Zr atomic ratio, these samples revealed a homogeneous spatial distribution of Si on the surface. This suggests an influence of the spatial distribution of the silane on the performance of the pre-treatment. Of these, system C was the only one with no signs of blistering after 1 month of immersion, giving results identical to those obtained on the Cr-treated samples, under the test conditions. The samples treated with silane C revealed a higher Si/Zr atomic ratio, suggesting a more efficient coverage of the surface by the silanes. This can be explained by the more linear structure of this molecule when compared to the silane with the epoxy group (silane A). A more voluminous group, such as the epoxy group, can be more difficult to arrange on the surface. The aminosilane (silane C), in contrast, has a linear chain and can thus be expected to give a better coverage of the surface. In fact, according to the literature [11 13] linear chain silanes seem to give better efficiency concerning adhesion. The mechanism by which the silane molecule interacts with the metallic substrate can be described according to: R Si(OCH 3 ) + OH M R Si(OCH 3 ) 2 O M + CH 3 OH The organic chain, R, may carry substituent groups, such as the amino group, which can react with the oxide substrate leading to improved deposition of the silane. Selector et al. [14,15] observed that when applied to aluminium, some aminosilanes could inhibit corrosion and help the formation of an hydroxide layer on the modified aluminium surface. Other authors [16,17] suggest the formation of chelate complexes, in which both the Si and the amino groups become bound to the substrate. The results suggest an interaction between the amino group and the metal oxide. Thus, the formation of the following complex between the metal and the silane may be assumed: This kind of interaction will result in a more uniform distribution of the silane over the surface and consequently in improved adhesion and better corrosion performances of the primer-coated samples. In fact, XPS results suggested the presence of bonds between the amino group and the metallic substrates, that may enhance the amount of silane over the surface as well as its spatial distribution as confirmed by Auger mapping that revealed an uniform distribution of Si over the surface. Moreover, the samples treated with the aminosilane revealed higher impedance, close to the values observed on the Cr-treated samples. 5. Conclusions A two-stage (zirconium + organosilane) pre-treatment for galvanised steel was tested. It was shown that the distribution of the silane over the surface and the content of Si on the surface, namely the ratio Si/Zr and Si/Zn, depended on the type of silane used. The pre-treatment that led to a higher Si/Zr atomic ratio was the one that revealed the worst corrosion performance. This is explained by a heterogeneous distribution of the silane, which concentrates around Zr oxide rich islands along the surface. The electrochemical impedance results have shown that the treatment involving the aminosilane gives the best corrosion performance, comparable to that observed with the chromate treatment. The difference in the behaviour of the different pre-treatments was associated with the degree of surface coverage by the silane, rather than its content on the surface. Acknowledgements Authors are grateful to ECSC (Contract no. 7210-PR 066 and Mel Chemicals (Dr. Peter Moles) for supplying the zirconium nitrate. References [1] E.V. Schmid (Ed.), Exterior Durability of Organic Coatings, FMJ International Publications, 1998. [2] A. Sabata, W.J. Van Ooij, R.J. Koch, J. Adhesion Sci. Technol. 7 (1993) 1153. [3] V.A. Ogarev, S.L. Selector, Prog. Org. Coat. 21 (1992) 135. [4] A. Sabata, W.J. Van Ooij, US Patent, 5326, 1994, p. 594. [5] V. Subramanian, W.J. Van Ooij, Corrosion 45 (1998) 204. [6] N. Tang, W.J. Van Ooij, G. Gorecky, Prog. Org. Coat. 30 (1997) 255.

26 M.F. Montemor et al. / Progress in Organic Coatings 38 (2000) 17 26 [7] S.E. Hörnstrom, U. Bexell, W.J. Van Ooij, J. Zhang, in: I. Olefjord, L. Nyborg, D. Briggs (Eds.), Proceedings of the European Conference on Application of Surface and Interface Analysis, Gotebörg, 1997, p. 987. [8] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis: Auger and X-ray Photoelectron Spectroscopy, Wiley, Cichester, 1990. [9] G.R.T. Schueller, S.R. Taylor, in: J.R. Scully, D.C. Silverman, M.W. Kendig (Eds.), Electrochemical Impedance: Analysis and Interpretation, ASTM, USA, 1993, p. 328. [10] M.W. Kendig, S. Jeanjaquet, J. Lumsden, in: J.R. Scully, D.C. Silverman, M.W. Kendig (Eds.), Electrochemical Impedance: Analysis and Interpretation, ASTM, USA, 1993, p. 407. [11] A.J. Mikhalski, Itogi Nauki Tekh., Khim. Tekhnol. Vysokomol. Soedin. 19 (1984) 151. [12] H. Ishida, in: H. Ishida, G. Kumar (Eds.), Molecular Characterisation of Composites Interfaces, Plenum Press, New York, 1985, p. 38. [13] H. Ishida, in: K.L. Mittal (Ed.), Adhesive Aspects of Polymeric Coatings, Plenum Press, New York, 1983, p. 45. [14] S.L. Selector, V.V. Arslanov, V.A. Ogarev, Int. J. Adhes. Adhesiv. 10 (1990) 99. [15] S.L. Selector, V.V. Arslanov, V.A. Ogarev, Zashch. Met. 26 (1990) 583. [16] E.P. Plueddemann, J. Adhes. 2 (1970) 184. [17] E.P. Plueddemann (Ed.), Interfaces in Polymeric Composites, Mir, Moscow, 1978, p. 181.