Chapter 27
Novel Pretreatments of Metals for Corrosion Protection by Coatings: Part II, Plasma Polymerized Hexamethyldisiloxane Films on Galvalume
1
1
2
W. J. van Ooij , N. Tang , S.-E. Hörnström , and J. Karlsson 1
Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012 Dalarna University, S-78188 Borlänge, Sweden 2
Thin films of plasma-polymerized hexamethyldisiloxane were deposited on Galvalume substrates. The films were analyzed by AES. The corrosion performances of the coil- painted plasma samples were tested in the new Volvo indoor corrosion test. The test results were compared with those of the samples with standard chromate and zirconium pretreatments. It was found that a plasma cleaning step prior to deposition of the films had a marked effect on the corrosion performance of the painted system. The plasma deposition conditions had a significant effect on the composition and thickness of the films but their effect on the corrosion performance of the painted samples was minor. The optimized plasma coating had a performance comparable to that of the chromate pretreatment. Galvalume is a trade name of steel strip hot-dip-coated with an Al-43.4Zn-l.6Si alloy. The metal-coated steel strip can be coil-coated to achieve a decorative and corrosion-resistant material. It is a general experience that painted Galvalume provides a better corrosion protection than painted hot dip-galvanized steci, but that, initially, it is more sensitive to edge corrosion propagation (edge creep) (J, 2). The coil-painted sheet is given its final shape by a forming process after painting. The industrial coil paint process today usually uses chromate in pretreatment and in primer in order to achieve a high degree of adhesion and corrosion performance (3). However, the chromate solution is a potential health hazard during handling and also undesirablefroman environmental point view. In recent years, plasma polymerization of organic monomers has been proven to be a useful method for surface modification of a variety of materials (4). Plasma-polymerized thin films deposited on metal surfaces are highly crosslinked, pinhole-free, thermally stable, well adhered to substrates and compatible with paints (5). It was reported that plasma polymerization of organosilicon monomers can be ©1998 American Chemical Society
335
336 an effective pretreatment of cold-rolled steel (CRS) giving corrosion protection comparable to that obtained by a fine-grained automotive phosphate (6,7). The objective of the research described in this paper was to utilize plasmapolymerized hexamethyldisiloxane (HMDS) films as a pretreatment for Galvalume panels prior to painting. This work is part of our ongoing efforts to develop novel, environmentally compliant pretreatments of metals to replace chromâtes and phosphates. Another pretreatment of Galvalume prior to painting on the coil line, namely by organofunctional silanes, is also being studied in our laboratory (#). In this paper, we report results of our study of the effects of plasma cleaning and plasma film deposition conditions on the composition of the plasma film as well as the corrosion performance of the painted metal systems. The corrosion performance of the plasma pretreated samples were compared to those of two standard pretreatments: a conventional chromaterinseand a commercial zirconium treatment that has been proposed as a replacement of the chromate. Experimental HMDS (99.3%) was purchased from United Chemical Technologies. The 55% AlZn-coated steel panels were supplied by SSAB Tunnplât AB (Borlânge, Sweden). The metal substrates were first cleaned with acetone in an ultrasonic cleaner for 15 minutes and were dried in air. The plasma polymerization of HDMS on Galvalume panels was conducted in a custom-built DC reactor which is shown in Figure 1. Rotary and diffusion pumps were used for pumping down the pressure. The flow rates of gases were controlled by three Brooks 5850 mass flow controllers. In this experiment, the Galvalume panels were placed between two stainless steel plates. The Galvalume substrate was either a cathode with two stainless steel (SS) panels placed as anodes on both sides of the cathode, or, alternatively, was disconnected electrically, i.e., was floating. In that case, one SS plate was the cathode and the other one was the anode. The sample to be coated was then placed in the full glow of the plasma. Before deposition, the metal surface was cleaned with a plasma of oxygen or a mixture of hydrogen and argon. The plasma cleaning and deposition conditions are listed in Tables I and II, respectively. Two pretreatments from Chemetall GmbH were used for comparison; a standard chromate pretreatment (9) used in coil painting lines and a pretreatment based on alkaline oxidation and an after rinse in a hexafluozirconic acid based solution (10). The panels were finally painted with a chromium-free polyester coil-line primer and a polyester topcoat. A Perkin-Elmer PHI 660 Scanning Auger Microprobe was used to record elemental depth profiles of the samples. A 200 nA electron beam accelerated to 10 keV was used for excitation. Sputtering was performed with argon ions accelerated to 1 keV. The sputtering rate was calibrated using a Ta Os film of known thickness. Reflection-Absorption IR (RAIR) spectra of PP-HMDS films were acquired on a Bio-RAD FTS-40 FTIR Spectrometer with a deuterated triglycine sulfate detector, using a resolution of 4 cm" and were averaged over 128 scans. A new cyclic accelerated indoor corrosion test designed by Volvo was used to test the performance of the samples after painting. One test cycle included one hour of spraying with a 1% NaCl solution followed by a cycling of the relative humidity between 90 and 45% during 3V days. The length of the periods at 90% 2
1
2
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jas feed Tubes
-Electrodes
Feed-Through 'Collar
Exhaust Mechanical Valve To Vacuum Pump/Control
Figure 1. Schematic diagram of the DC plasma reactor.
338 Table I. Sample ID PPF1 PPF2
Plasma Cleaning Conditions of the Galvalume Panels*
o
H seem
Ar seem
seem
Ρ Pa
V volts
mA
Time min.
-
-
-
-
-
-
-
2
4
8
700
80
30
16
530
80
30
2
PPF3
2
7.5
PPF4
2
4
8
600
80
30
PPF5
2
4
8
1100
80
30
*all panels were connected to the cathode
Table II.
Deposition Conditions of HMDS on Galvalume Panels
Sample HMDS ID seem PPFl 2.5 a
Ρ Pa 26
On Pulse OffPulse μ*
/
V volts 500
mA 20
Time min. 10
a
2.5
13
331
331
700
20
10
a
2.5
13
331
331
630
20
10
a
2.5
13
331
331
700
20
10
b
2.5
13
331
331
900
20
10
PPF2
PPF3 PPF4
PPF5
a: the sample was connected as a cathode in between two stainless steel anodes. b: the sample was floating in the plasma between the anode and cathode.
relative humidity was seven hours and the length of the periods at 45% relative humidity, including the transition times, was five hours. The temperature was 35°C throughout the test. The samples were tested for 19 weeks (38 cycles). A 5 cm vertical scribe down to the steel base was applied to each panel. The performance in the corrosion test was evaluated by measuring the average edge and scribe creep after removing loose paint by an industrial adhesive tape.
Results and Discussion The AES depth profiles of the films PPF1-5 are shown in Figure 2. The profiles show that the different cleaning and deposition conditions had a pronounced influence on the chemical composition of the plasmafilms.PPF4 had a well-defined film containing Si, Ο, Ν and C. The O/Si ratio was approximately 2.5 and fairly
339
100
PPF 1 80 C Ο
Al.....—-
•
60 ...... C 40
-
0
;
ο
