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Volume 2 Paper 14


Silane Coatings for Replacement of Phosphate/Chromate Pretreatments of Automotive Metal Sheets

 W.J van Ooij* and Guru Prasad Sundararajan

Department of Materials Science and Engineering University of Cincinnati, Cincinnati, OH 45221-0012 USA (* Corresponding author; E-mail: )

Abstract

Silane coatings based on bis-amino and bis-polysulfur silanes have been shown to be an effective replacement for phosphating (including final chromate rinse) pretreatments of electrocoated metals. The performance of these silanes on automotive metals which includes cold-rolled steel (CRS), electrogalvanized steel (EGS), hot-dip galvanized steel (HDG), and aluminum alloys have been shown to outperform the current phosphating pretreatments. These silanes if applied at optimized conditions provide outstanding corrosion protection and adhesion with a standard electrocoat (ED-5000) in a variety of tests including GM scab test, EIS and Hot-salt immersion tests. These films have also been characterized using RAIR and AFM.

Keywords: Silanes, E-coat, HSS, EIS, NMP, bis-amino, bis-polysulfur, RAIR, AFM

Introduction

Organofunctional silanes are hybrid organic-inorganic compounds that can be used as coupling agents across the organic-inorganic interface. These functional silanes have the structure X3Si(CH2)nY, where X represents a hydrolyzable group such as methoxy or ethoxy and Y an organofunctional group. The functional group attaches itself to the paint polymer that is applied to the silane-coated metal. When the silane is symmetrical about the functional group Y, viz. if there are two hydrolyzable  (X3) groups about the functional group, then these molecules are called as bis-functional silanes. These bis-functional silanes have the structure X3Si(CH2)nY(CH2)nSi X3, where X represents a hydrolyzable group such as methoxy or ethoxy and Y an organofunctional group.

The environmentally toxic properties of phosphates and chromate pretreatments on automotive metals have been widely documented. Although some alternatives have been proposed [1,2], they have not yet been universally acceptable or cost-effective without loss of performance. In our laboratory we have developed an environmentally friendly as well as cost-effective replacement for these pretreatments by the use of novel organofunctional silanes on automotive alloys which includes electrogalvanized steel (EGS), hot-dipped galvanized steel (HDG), cold-rolled steel (CRS) and aluminum alloys (Al 6061and Al 6111).

 The typical silanes are bis-organofunctional silanes viz.  bis-(trimethoxysilylpropyl)amine and bis-(triethoxysilylpropyl)tetrasulfide. Earlier we had reported the performance of organofunctional and non-functional silanes on metals as a pretreatment for painting and demonstrated that the silane with the optimum performance is not the same for all the metals [3]. However, as a part of the ongoing work on automotive steels, we have now successfully reduced the choice of silanes to the above two silanes.

In this paper, we have shown that silane-based pretreatments using either of both of these silanes can outperform the currently used phosphating treatments of automotive metal sheets

After active hydrolysis with water, the bis-amino silane obtains the structure: (HO)3 � Si � (CH2)3 �NH - (CH2)3 � Si � (OH)3. The bis-amino silane is supplied as a methoxy ester while the bis-polysulfur one is supplied as an ethoxy ester. Similarly, the bis-polysulfur silane has the following structure after hydrolysis: (HO)3 � Si � (CH2)3 �S4 � (CH2)3 � Si � (OH)3

Experimental

The automotive materials of CRS, EGS, HDG, Al 6061 and Al 6111 were obtained from ACT laboratories, Hillsdale Michigan. The silanes were obtained from Witco Corp., Friendly, WV, as liquids of approximately 95% purity.

The bis-amino silane having an amine functional group could be completely hydrolyzed in water. However, the stable pH range (where condensation of silanols do not occur) was found to be between 4-9.5. Bis-polysulfur on the other hand required at least 50-vol% of solvent (preferably ethanol) and had a pH stability range of 3.5- 9.5 [4]. However, we have earlier shown [5] that amino silane hydrolysis at an acid pH protonates the group and hence not suitable for deposition on a zinc surface as the zinc oxide is stable within a pH of 8-12. So bis-amino was deposited at a pH of 8.5 on EGS, acetic acid being used for controlling the pH. The bis-polysulfur silane (sulfane) could be hydrolyzed without addition of acid at a natural pH of 4.5.

The silanes after application were in-place cured or preferentially baked at 100°C for 5 minutes and then electrocoated with the standard automotive E-coat ED-5000 (from PPG-Industries). After elecctrocoating, the panels were cured at 175°C for 20 minutes. The performances of the silanes were then reported for EIS, GM 9540P accelerated cyclic corrosion test.

In order to screen the silanes for performance and to reduce the 1000 hr Salt Spray test program, we have introduced a new test called the Hot-Salt Soak test (HSS). This involves immersion of e-coated panels with two parallel scribes (scribed deep into the base metal) about 10 cm long into a 3% NaCl solution for 5 days at 55°C. The panels are then washed with DI water and a tape-pull off using an adhesive tape is done. Figure 1 shows the comparative performances of the silane-pretreated, phosphated and the blank EGS panels in the HSS test. The average creep from the scribe gives the extent of corrosion and adhesion performance of the e-coat with the metal. The HSS test has been found to be very useful for demonstrating the effect of corrosion and adhesion performance of functional silanes with the e-coat.

It has been reported earlier that the properties of the silane films on metals depends upon the metal cleaning procedure, concentration of the silane, pH of application and post-treatment [2-6]. All the automotive metals were degreased ultrasonically using acetone, hexane and then alkaline cleaned with alkaline cleaner for 5-10 minutes. They were then washed thoroughly with DI water until complete wettability was attained and then blown-dried. They were then dipped into the corresponding silane solution for at least 2 seconds and then in-place cured.  The bis-amino coated silane films were preferentially baked at 100°C for 5 minutes. The silane coated films were then cathodically electro coated with the paint ED-5000 (from PPG Inc.). The painted panels were then screened using HSS test. EIS in 3% NaCl provided the resistance of the e-coat with the silane. The experiment was carried out using CMS 300 EIS system from GAMRY Instruments. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was graphite and the area of exposure of the painted panels was about 5.25 cm2. The experiments were carried out at open circuit potential of the system and an AC voltage of 50 mV was applied as the input signal.  A conventional nested loop electrical model with two time constants was used for fitting the experimental data and the corrosion resistance Rcorr was obtained from the EIS analysis software obtained from GAMRY Inc. (Figure2)

The panels were also subjected to the GM Scab creep cyclic corrosion test which was performed by exposing them in a humidity chamber to conditions at 60C and 85% relative humidity followed by salt immersion, drying and exposure in a cycle for 4 weeks. The scribe creep was measured after a tape-pull off using a standard adhesive tape.

The chemical structures of the deposited silane films were studied by reflection-absorption infrared spectroscopy (RAIR). The RAIR spectra were obtained using a BIO-RAD FTS-40 FT-IR spectrometer equipped with a BIO-RAD variable angular reflectance attachment at an angle of 60°. The spectra were acquired for 500 scans using 4 cm-1 resolution. The spectra for the silane films were obtained by subtracting their corresponding background spectrum obtained with the uncoated polished substrates.

AFM images of polished pure zinc substrates with and without silane films were obtained on a Burleigh Instruments METRIS-2000 AFM system. Samples were analyzed at ambient conditions. The instrument was operated in the contact mode during imaging and the micromachined silicon nitride cantilever was in contact with the surface under a reference force of ~5.0 N. The topography of the surface was a measurement of the adjustment in the vertical direction necessary to keep the reference force constant. 

Results and Discussion

Hot-salt soak test and GM scab test

While either of the above two silanes deposited on EGS, HDG and Al 6061 could provide adhesion and corrosion performance on par with the phosphated system, for CRS it was found that the mixture of bis-amino and bis-polysulfur (2% prehydrolyzed concentration each, 9:1 mix ratio, at pH= 8), provided results comparable to the zinc phosphated CRS The Hot-Salt Soak test clearly shows the effect of pH on automotive substrates, specially on zinc surfaces (Table 1). While the bis-amino silane deposited between 8-9 showed the best performance, those bis- amino silanes deposited below pH=8 showed considerable delamination of the paint. Similarly the concentration of the silane was also critical as far as the e-coatability is concerned. Concentrations of bis-amino higher than 2% provided inferior performance. This could be attributed to the fact that silane films are not electronically conductive and hence beyond a critical concentration of 2% for a bis-amino film, the adhesion of the e-coat with the silane decreases during electrocoating due to poor conductivity of films of higher concentration [7]. The bis-sulfur films on the other hand, which would be more conductive, provide corrosion protection even at 5% concentration [7]. The electronic conductivity of a 2% bis-amino film on EGS is 8.81 Ohm-1 cm-1 and that of a 2% bis-sulfur film is 12.61 Ohm-1 cm-1. This correlated with the corrosion performance results obtained from GM test.  The requirement of an optimum concentration (2% in case of bis-amino) is also seen in the GM test results (Table. 2)

EIS of E-Coated Steels

EIS has been shown to be a scientific method to study the corrosion performance of polymer-coated metals in recent years. However, from the data shown (Table 3), it can be inferred that the quality of the paint can mask the performance of pretreatment. Hence in this case EIS testing at pH = 6 was not useful for studying the effect of pretreatment on metals. Also we can see that even an alkaline-cleaned panel with e-coat performs well in neutral 3% NaCl. The differences are seen only when the pH of the NaCl was decreased to an acidic value of 3. Also we can see that the correlation between EIS and GM Scab tests were poor as far as performance of bis-polysulfur silane on CRS is concerned. This is in agreement with our earlier results indicating a poor correlation between adhesion and corrosion results [8]

RA-IR

The bis-amino and bis-polysulfur silane films were also characterized by RA-IR. The IR spectrum (figure 3) of bis-amino shows the presence of strong Si-O-Si  peak  present at 1144 cm-1 and IR (figure 4) of bis-polysulfur shows S-S peaks at 460 cm-1 [9]. The presence of broad peak around 3340 cm-1 indicates the presence of  a large amount of OH groups that are hydrogen bonded (due to Si � OH groups or water present in the film). The peak present at 881 cm-1 indicates the presence of Si-O-C2H5 groups present in the film, which shows that the silane cannot be completely hydrolyzed even after four days of hydrolysis.  It is interesting to note the IR spectrum of mixture of bis-amino and bis-polysulfur films (mixing ratio 3:1) is simply the addition of the individual spectra (figure 5) Further, it can be seen there is no peak present around 881 cm-1 when the bis-sulfur silane is mixed with bis-amino silane in the ratio 1:3, which indicates that the amino-silane aids in hydrolyzing the bis-sulfur silane almost completely.

AFM

Both the bis-functional silanes were analyzed using AFM.  Figure 6 shows the 3-dimensional surface topography of a blank polished Zn surface. Figures 7 and 8 show the surface scan of films of a 2% bis-amino and a 5% bis-polysulfur films respectively on Zn. These figures show that both silane films formed under the conditions studied were continuous and it can be seen that the surface topography of bis-polysulfur film differs from that of the bis-amino silane. The former appears to be smoother than the amino-film.  However, both the films appear to be smoother than the uncoated Zn substrate.

Conclusions

A process consisting of coating thin films of organofunctional silanes on automotive substrates have been proposed to replace the conventional phosphating pretreatments. EIS, HSS and GM Tests have been successfully used to demonstrate the effect of deposition parameters on metal. RA-IR and AFM has been used for characterizing these silane films.  EIS was found to be a useful tool, only the when the corrosive medium (3% NaCl) is made aggressive by making the pH acidic. Also poor correlation is seen between the EIS and GM test results that agree with our earlier findings.  For EGS, HDG and Al 6061 alloys, one of the bis-functional silanes when applied at an optimum pH and concentration, provided excellent adhesion and corrosion protection with the electrocoat ED-5000. For CRS, an optimized mixture of these two silanes provided a performance on par with the phosphated systems. An extensive characterization of electrical properties of these films and its structure will be reported elsewhere.

References

1. W.J. van Ooij and K.D. Conners, J. Electrochem. Soc., Vol. 95-13, 229 (1995)

2. T.J. Lin, J.A. Antonelli, D.J.Yang, H.K. Yasuda, F.T.Wang, Prog.Org.Coat. 31, 351 (1997)

3. W.J. van Ooij, Chunbin Zang, Jun Q. Zhang and Wei Yuan, International Symposium on Advances in Corrosion Protection by Organic Coatings- ACPOC 127 (1997)

4. Thomas van Schaftinghen, Thesis, Vrije Universiteit Brussels, Belgium, 1999.

5. W. Yuan and W.J. van Ooij, J. Coll. Interface Sci., 185 (1997).

6. Chunbin Zhang, Ph.D. Thesis University of Cincinnati, Department of Materials Science and Engineering, 1997.

7. Guru Prasad. S, MS Thesis, Department of Materials Science and Engineering, University of Cincinnati, 2000.

8. N. Tang, W.J. van Ooij, G. Gorecki; Prog.Org.Coat. 30, 255 (1997)

9. Socrates.G, Infrared Characteristic Group Frequencies, 2nd Ed., John Wiley  (1994)

 

Table1: Hot Salt Soak Test Results
(Immersion of Scribed panels in 3% NaCl for 5 days at 55 � 5oC).

Specimen

Creep (mm)

 

 

EGS

 

Phosphated

1.2 � 0.9

Blank

22.6 � 10.1

Bis-amino (0.5%, pH=8.5, baked*)

5.1 � 2.1

Bis-amino (2.0%, pH=8.5, baked)

Negligible**

Bis-amino (3.0%, pH=8.5, baked)

3.9 � 1.1

Bis-amino (4.0%, pH=8.5, baked)

6.2 � 4.3

Sulfane (5%, natural pH, as-cured)

1 � 0.8

Sulfane (2%, natural pH, as-cured)

12.1 � 6.6

 

 

HDG

 

Blank

Complete Delamination

Phosphated

Negligible Delamination

Sulfane (5%, nat. pH, as-cured)

Negligible Delamination

Bis-amino (2%, pH=8, baked)

Negligible Delamination

 

 

CRS

 

Blank

6.8 � 1.7

Phosphated

Negligible delamination

Bis-amino (2%, pH=8, baked)

4.7 � 2.5

Bis-amino (2%, pH=5, baked)

5 � 1.7

Bis-polysulfur (5%, nat. pH, as-cured)

Complete delamination

Bis-polysulfur + Bis-amino (2% conc. Each prehydrolysed & vol. ratio of mixing is 1:3), pH=8, as-cured

1.1 � 0.4

Bis-polysulfur + Bis-amino (2% conc. Each prehydrolysed & vol. ratio of mixing is 1:9), pH=8, as-cured

3.5 � 2.0

Bis-polysulfur + Bis-amino (2% conc. Each prehydrolysed & vol. ratio of mixing is 1:1), pH=8, as-cured

6.1 � 1.8

 

 

Al alloys (AL6061 & AL 6111)

 

Phosphated

Negligible delamination

Bis-amino (2%, pH=8, baked)

Negligible Delamination

Bis-amino (2%, pH=8, as-cured)

Negligible Delamination

*   cured at 100oC for 5 minutes

** Average Creep < 0.5 mm

Table 2: GM SCAB Creep Test Results
(4 weeks of exposure to 60oC & 85%R.H)

Specimen

Creep (mm)

EGS

 

Blank

6.0 � 1.5

Phosphated

Negligible delamination*

Bis-amino (2.0%, pH=8,baked)

Negligible delamination

Bis-amino (2.0%, pH=5,baked)

2.8 � 2.2

Bis-polysulfur (5%, natural pH, as-cured)

Negligible delamination

 

 

HDG

 

Blank

4.7 � 2.6

Phosphated

Negligible delamination

Bis-amino (2%, pH=8, as-cured)

Negligible delamination

Bis-amino (2%, pH=8, baked)

Negligible delamination

Bis-polysulfur (nat. pH, in-place cured)

Negligible delamination

 

 

CRS

 

Blank

27.0 � 10.1

Phosphated

3.5 � 0.7

Bis-amino (2%, pH=8, baked)

18.6 � 5.4

Bis-polysulfur (5%, nat. pH, as-cured)

10.5 � 3.6

Bis-amino + bis-polysulfur mix. (2%, 9:1 vol.ratio, pH=8, as-cured)

6.5 � 1.7

 

 

Al alloys (Al 6061 & Al 6111)

 

Blank

Negligible delamination

Phosphated

Negligible delamination

bis-amino (2%, pH=8, as-cured)

Negligible delamination

Bis-amino (2%, pH=8, baked)

Negligible delamination

* Average Creep < 0.5 mm

Table 3: EIS of Electrocoated Automotive Steels

AUTOMOTIVE SYSTEM

CORROSION RESISTANCE (Rcorr in Ohms)- After 8 weeks in neutral 3% NaCl.

CORROSION RESISTANCE (Rcorr in Ohms)- After 8 weeks in 3% NaCl (pH=3).

EGS

 

 

Alkaline-Cleaned only

1.19 x 109

9.96 x 106

Phosphated

3.31 x 1010

3.25 x 105

Bis-amino (2%, pH=8, baked)

1.29 x 1010

5.69 x 109

 

 

 

HDG

 

 

Alkaline-Cleaned only

6.78 x 109

9.7 x 1010

Phosphated

6.69 x 109

8.75 x 105

Bis-amino (2%) coated

3.60 x 1010

2.75 x 1011

 

 

 

CRS

 

 

Alkaline-Cleaned only

4.63 x 109

2.75 x 108

Phosphated

5.59 x 109

1.44 x 106

5% bis-polysulfur (nat.pH, as-cured)

1.47 x 1011

2.63 x 109

 

 

 

Al6061

 

 

Alkaline-Cleaned only

1.37 x 1011

9.8 x 107

Phosphated

3.48 x 1010

3.45 x 1010

Bis-amino coated (2%)

5.34 x 109

3.23 x 1011

Figure 1: Hot salt soak test of e-coated EGS

Figure 2: Model for a corroding automotive metal with an electrocoat

Fig. 3: IR Spectrum of bis-amino (2%, pH= 8.5) on polished Chrome Steel

Fig. 4: IR spectrum of bis-polysulfur (5%, natural pH) on polished stainless steel

Fig. 5: IR spectrum of bis-amino and bis-sulfur mixture (3:1) on polished stainless steel

Figure 6: AFM image of a bis-polysulfur silane coated Zn surface

Figure 7: AFM image of a bis-amino silane coated Zn surface

Figure 8: AFM image of a bis-polysulfur silane coated Zn surface