Novel Pretreatments of Metals for Corrosion Protection by Coatings

Chapter 26

Novel Pretreatments of Metals for Corrosion Protection by Coatings: Part I, Plasma Polymerized Hexamethyldisiloxane on Cold-Rolled Steel

Downloaded by CORNELL UNIV on September 6, 2016 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch026

W. J. vanOoijand K. D. Conners 1

Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012 In an attempt to replace metal pretreatments that produce toxic by -products with more environmentally friendly treatments without sacrificing the degree of corrosion protection, the use of plasma pretreatments and plasma-polymerfilmshas been investigated. ColdRolled Steel (CRS) panels were treated in a plasma of oxygen or a mixture of argon and hydrogen, or both sequentially. A film of plasma -polymerizedhexamethyldisiloxane (PPHMDS) was then deposited in the same plasma reactor. The panels subsequently had a cathodic E -coat applied over the PPHMDS. These panels, a phosphated CRS panel, and an untreated CRS panel, were then subjected to a cyclic accelerated environmental corrosion test, after which they were tested using Electrochemical Impedance Spectroscopy (EIS). The EIS tests were also conducted over an extended period of time, during which the panels were continuously exposed to a 1.0 M solution of NaCl. The results indicate that under certain pretreatment conditions, PPHMDS, when used as a primer for Ε-coating, can perform as well or better than standard phosphating.

Current technology for the corrosion protection of cold-rolled steel (CRS) involves the incorporation of a phosphate layer on a solvent cleaned surface, followed by a chromate rinse. This treatment is used to ensure good adhesion of the cathodic electrocoat primer (E-coat) to the metal substrate. Due to the toxic wastes produced by this process, pressure from EPA has prompted research efforts focusing on the replacement of this system with a more environmentally friendly system. Any new system must also provide a comparable degree of corrosion protection achieved by the phosphating/chromating system. One possible replacement system being investigated is the use of a plasma cleaning procedure followed by the application of a plasma polymerfilmin the same reactor.

322

©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


323 A plasma treatment using a non-reactive gas, such as oxygen, has been found to be effective in removing carbon contaminationfromthe surface of CRS [1]. The carbon contamination is thought to be a major contributor in the lack of adhesion between metals and polymer coatings. Plasma polymers have properties ideally suited for use as a corrosion protection coating [2]. If the CRS is first plasma treated, and a plasma polymer film is applied over this clean surface in the same reactor, this film can then act as a corrosion protection coating, and also as a primer coating for subsequent coatings, such as the E-coat and the topcoat. The authors have shown that these plasma polymer films do provide corrosion protection in an aggressive environment [3]. It has also been stated that these films are conductive enough to allow the application of an E-coat [4]. Therefore, the entire system of the plasma cleaned surface, covered by a plasma polymer coating and an E-coat should provide the same adhesion and corrosion protection characteristics of the phosphated/chromated surface covered with an E-coat. Electrochemical Impedance Spectroscopy (EIS) has gained popularity as the technique of choice for investigating the corrosion protection capabilities of polymer coatings on metals [5,6]. This technique, which measures the current response to a fluctuating applied potential as a function of the ACfrequency,can provide a quick assessment of the corrosion protection capabilities of a coating in a laboratory or insitu environment. This is best achieved by fitting the collected EIS data to the performance of an Equivalent Circuit Model (ECM) subjected to the same current andfrequencyas used in the test of the coated CRS panel. The difficulty lies in the design of the ECM. Only if the ECM chosen for comparison contains elements that are analogous to the physical properties of the coating/metal system can the analysis be taken as representative of the performance of the system. As well as the pre-assessment of a coating system, the deterioration of a coating can also be investigated using EIS [7]. This is again best achieved by fitting the collected EIS data to the performance of the ECM, and monitoring the changes in the values of the ECM elements with time. Correlation of the EIS analysis with data from environmental corrosion performance tests can be used for verification of the conclusions drawnfromthe EIS analysis. Historically, EIS has been used to investigate the properties or the deterioration of polymer coatings. However, if the parameters of the plasma polymerization and the Ε-coating are held constant, any differences in the performance of the prepared samples should be attributable to the plasma treatment. This work shows that the system using plasma treatments followed by the application of a plasma polymer coating provides corrosion protection of CRS that is comparable to the phosphating/chromating system. It also shows that EIS can be used to investigate the effectiveness of the pretreatments applied to CRS prior to the application of polymer coatings. Experimental Materials. The CRS panels used were automotive grade, type 1010 low carbon sheet material obtainedfromArmco Research & Technology. The gases used in the plasma cleaning, (oxygen, argon, and hydrogen), were obtainedfromWright Bros.,


324


Inc. The HMDS was obtained from Aldrich Chemical Co.. The E-coat used was an ED-5000 type obtained from PPG Inc. Sample Preparation. The CRS panels were ultrasonically cleaned in acetone for 20 minutes, and placed in a parallel plate DC plasma reactor. In this reactor, the substrate to be treated or coated is the cathode, and is placed between two anodes. This allows both sides of the sample to be treated or coated simultaneously. The size of the anodes, approximately 22 cm by 22 cm, is larger than the size of the cathode, which in this work was 10 cm by 15 cm. This electrode arrangement is shown schematically in Figure 1. After placement in the reactor, these panels were first cleaned using a plasma of oxygen, and/or a plasma of an argon/hydrogen mixture. The type of plasma cleaning used in this research was based on results of previous work by the authors [1, 3, 8]. The details of this step are given in Table I. After plasma cleaning, a thin film of plasma-polymerized hexamethyldisiloxane (PPHMDS) was deposited on both sides of the substrate. During the deposition, the chamber pressure was held constant at 125 mtorr, the monomer flow rate was 2.5 seem, and the power was 20 mA and 1200 volts. The plasma cleaned/plasma polymer coated samples then had a cathodic electrocoat primer applied by immersing in a bath of the paint at a voltage of 180 V for 3 minutes. The panels were then removed from the bath, rinsed using DI water, and then cured at 175°C for 30 minutes. A plain CRS panel, and a phosphated CRS panel were similarly Ε-coated to be used as a baseline in this study. Analysis/Characterization. EIS was carried out using the CMS300 system from Gamry Inc., with a Stanford Research Systems model SR810 Lock-In Amplifier.

Figure 1. Plasma Reactor Electrode Schematic.


325


Table I.

Plasma Cleaning Conditions

Sample #

Plasma Cleaning Process

CRS Phosphated CRS 912-D 914-A 923-C

NA NA None 20 minutes O2, 10 minutes 10 minutes ΑΓ/Ή2,10 minutes

The sample was connected to the equipment using a three electrode cell. The counter electrode was a screen mesh of platinum, whose surface area is much greater than the working electrode surface area of 1.78 cm , which is the coated metal being tested. A reference electrode (Ag/AgCl) is used to measure the polarization potential of the system. The electrolyte used was a 1 M NaCl solution. The samples were then subjected to a modified version of a cyclic accelerated environmental exposure test (GM-scab test). In this test, the samples are first scribed diagonally across the face of the sample through the Ε-coat and the plasma polymer to the metal. They are then placed in racks so that the orientation of the scribed face of the sample is approximately 45° to horizontal, and exposed to 60°C and 85% RH for 23 hours in a humidity/temperature chamber. Once removed from the chamber, they are placed in a solution of 5% NaCl for 15 minutes. After soaking, they are removedfromthe solution and allowed to air dry for 45 minutes. This procedure defines one cycle. The samples are put through a minimum of 25 cycles. The samples are analyzed by measuring the amount of delamination of the coating away from the scribe as indicted by the "bubbling up" of the coating as a result of corrosion product build up at the plasma-polymer/metal interface. Usually, six measurements are taken along the length of the scribe, and the average of these measurements is given as the average delamination value. If the samples show no major difference in the average delamination value after 25 cycles, additional cycles may be used. After the GM-scab test was completed, the samples were exposed to a 1M NaCl solution for an extended period of time. This long term exposure test was conducted to investigate the deterioration of the coating, or the deterioration of the adhesion between the coating and the CRS. The samples were exposed to the salt solution using a glass joint approximately 15 cm long. A three electrode cell was used, with the counter electrode being a graphite rod, and a Ag/AgCl reference electrode. EIS data was collected daily for the first week, and weekly or bi-weekly thereafter. For all the EIS tests, data was collected over afrequencyrange of 0.001 Hz to 100 kHz. This data was then plotted in a Bode plot format, which plots the impedance and the phase angle as a function of thefrequency.This format allowed a simple graphical analysis of the data [6]. Additional analysis of the EIS data was also performed using Equivalent Circuit Modeling (ECM) (Figures 2-3). An ECM was designed based on the graphical analysis, as well as the possible relationship 2


326

between the Bode plots analysis and the physical characteristics and properties of the sample being tested. The arrangement of the ECM was determined by the shape, and the number of slope changes in the impedance modulus and the phase angle vs. frequency curves.


Results and Discussion In previous work by the authors, various combinations of plasma cleaning were investigated [9]. From that investigation, the samples that ranked the best, the worst, and intermediate in corrosion performance in EIS tests and in the GM-scab test were chosen, along with plain CRS and phosphated CRS samples, to undergo the long term exposure test. The impedance modulus of the EIS data taken at the initiation, after 15 days, and again at 107 days of the solution exposure are shown in Figures 4-8. A comparison of the maximum value of the modulus shows that the samples that were plasma cleaned with an oxygen treatment have better corrosion protection parameters than all other samples in the set. This value can be related to the total resistance of the system. This resistance can further be correlated with the corrosion protection properties of the coating system. Realizing that the deposition of the PPHMDS and the Ε-coating were both carried out under identical condition for all samples, this comparison is then a comparison of the pretreatment. It can thus be

Figure 2. Bode Plot of plasma cleaned/plasma-polymer coated/E-coated CRS.



Figure 3. Equivalent Circuit Model used to analyze EIS data of plasma cleaned/plasma-polymer coated/EOcoated CRS.

Δ 24 hour exposure D 1

a

0

° ° • ο • ο

α

α

£

15 days exposure

Λ

ο 107 days exposure

ύ

'·············.°::

5ί •

α

•

D

• D

α

α

•

Log Freq (Hz)

Figure 4. Modulus of Impedance for Phosphated CRS.


328

Û

Δ 24 hour exposure

û


α 15 days exposure ο 106 days exposure

-+-2.0

-1.0

0.0

1.0

2.0

4.0

3.0

5.0

Log Freq (Hz)

Figure 5. Modulus of Impedance for CRS.

1.0E+10 Δ 24 hour exposure

l.OE+9

• 15 days exposure l.OE+8 Q,

1.0E+7

3

1.0E+6--


•o ο S

1.0E+5 -f 1.0E+4 1.0E+3 -2.0

-+-1.0

0.0

1.0

2.0

3.0

4.0

Log Freq (Hz)

Figure 6. Modulus of Impedance for Sample 12-D.


5.0

329

l.OE+10 Δ 24 hour exposure

1.0E+9 >° ο ο ο ο 8

• 15 days exposure Θ



1.0E+8 S

1.0E+7

3

β

3

1.0E+6

^

1.0E+5

•a ο

Ê â

a â

â â

â â

â

1.0E+4

Â

1

1.0E+3 -2.0

I

-1.0

I

I

0.0

1

1

1.0

I

2.0

A â

»

3.0

4.0

A 5.0

Log Freq (Hz)

Figure 7. Modulus of Impedance for Sample 14-A.

1.0E+10 Δ 24 hour exposure • 15 days exposure

l.OE+9

ο 107 days exposure *g

l.OE+8

JB

Q,

1.0E+7

3

1.0E+6

Ό Ο

S

1.0E+5 1.0E+4 l.OE+3 -2.0

-1.0

0.0

1.0

2.0

3.0

4.0

Log Freq (Hz)

Figure 8. Modulus of Impedance for Sample 23-C.


5.0

330

concluded that the oxygen treated samples show corrosion protection properties that are superior to the two baseline samples, plain CRS and phosphated CRS. These trends also carried through the long term exposure testing of the samples. However, we can also see from this preliminary analysis that not plasma treating the CRS prior to depositing the PPHMDS coating is actually detrimental to the corrosion protection properties of the coating system. An alternative method for analyzing this data graphically is to note the change in the maximum modulus of impedance value for each sample. We can see that the phosphated CRS shows the greatest change in the maximum modulus, with a change of more the 1 Λ decades during the length of the test. The plasma treated samples, as well as the CRS sample, show a change of less than 1 order of magnitude. This would seem to indicate that there is less deterioration of the coating and/or the interface in these samples than with the phosphated sample. An exception to this trend is seen in the sample with no plasma treatment, and a plasma polymer coating (sample 12-D). With this sample, it seems that the deterioration of the interface was more or less complete prior to the solution exposure test. This deterioration was due to the exposure in the humidity chamber. This resulted in very low values of a maximum impedance modulus, which stayed more or less constant during the entire period of the test. A more detailed analysis of impedance data involves the use of an Equivalent Circuit Model, or ECM. In this approach, a simple electrical model is chosen based on the graphical analysis of the impedance data presented in the form of a Bode plot, in conjunction with the known properties of the system being studied. This approach has been used previously by Mansfeld et al. in the investigation of polybutadiene coated metals [10, 11], and Hirayama and Haruyama in the general investigation of degraded coated steel with pores [12]. Figure 2 shows a representative plot of the data from all the samples in this set prior to the GM-scab test. Although the values of the slopes and the position of the breaks in the curves vary from sample to sample, the general shape of the curve is the same for all samples tested. This plot shows that there are essentially three time constants in the EIS data gathered. The highfrequencyportion of the plot represents the resistance and capacitance of the top polymer layer, the Ε-coat in this system. The mid frequency range of data represents the resistance and capacitance of the intermediate polymer layer, the plasma polymer in this system. The low frequency portion of the data represents the corrosion reactions taking place a the plasma-polymer/metal interface. Based on this information, the ECM that best represents the plasma cleaned/plasma polymer coated/E-coated CRS is shown in Figure 3. The parameter R represents the resistance of the electrolyte solution. R and C represent the pore resistance and capacitance of the cathodic Ε-coat. The pore resistance represents the integrity of the coating, i.e., the density of pinholes and defects over the area tested. The value of the C is dependent on the thickness of the coating and the degree of solution saturation of the coating. The two parameters, R i and C i, represent properties similar to the C and R , only in this case they are representative of the plasma-polymer coating. R represents the polarization resistance, which is a property of the polymer/metal interface. Cdi represents the double layer capacitance present at the interface of a sa turated polymer and a metal


ι

s

po

c

c

p

p

c

po

p


331 substrate. The parameters C i and R i were also used in the data analysis of the two baseline samples so that a comparison of the parameter values of the entire sample set could be achieved. In the phosphated CRS sample, these values would represent the phosphate layer of the coating system. It is not quite clear what the physical correlation of these parameters represent in the CRS sample. It is possible, and probable, that they represent the inherent oxide layer found on CRS. In all cases, including the baseline samples, the fit of the experimental data to the model data using this third time constant was exceptional. In this work, the pretreatment of the CRS samples was investigated. Therefore, the values of R and Cdi, which are related to electrochemical processes that occur at the metal/plasma-polymer interface, were used for comparison of the corrosion protection properties of the coating system [10, 11]. This is the area that would be most significantly affected by the pretreatment. The double layer capacitance, Cdi, is established once the aqueous solution has penetrated the coating to the plasmapolymer/metal interface. For the aqueous solution to spread at the interface, there must be some delamination of the plasma-polymer from the metal. This delamination is caused by the corrosion of the metal and causes a Faradic charge transfer process to occur, which can be measured as R . The changes in these values over the length of the exposure test are shown graphically in Figures 9 and 10, and listed in Table II. In Figure 9, we see that the initial value of R was virtually identical for all samples except 12-D, which received no plasma treatment prior to plasma-polymer deposition. Over the length of the solution exposure test, the value of R dropped for all samples. We see in Table II that the phosphated CRS sample had the largest drop in R , while the CRS sample and 23-C had drops of approximately 33% of the phosphated sample. Sample 14-A showed a significant drop in R , but not quite as great as the phosphated CRS sample. In Figure 10 we see that values of C i range between 10' to 10" farads, with sample 14-A having the lowest, or best value, and the phosphated CRS having the highest, or worst value. The change in these values is listed in Table II. As with the values of R , we see that the phosphated CRS had the greatest change, while sample 23-C had the least change. The changes in both values discussed above indicate a deterioration of the adhesion of the plasma-polymer to the metal substrate. This causes a decrease in the R and an increase in Cdi. As water penetrates through the Ε-coat and the plasmap

p


p

p

p

p

p

p

7

9

d

p

p

Table II.

Changes in R and C i for Solution Exposure Test p

Sample # Phos CRS CRS 12d 14a 23c

d

AR 1.81 0.61 0.64 1.38 0.70

D

AC , 1.54 1.18 -0.05 1.79 0.73 d



1.0E-2 -X--12-D

1.0Έ-3

• · • - -PHCRS - ώ - - 23-C

1.0E-4

— o — CRS -

1.0E-10

Η

1

1

1

1

1

1

1

1

1

1

1

1

o- - 14-A

1

1

Time (days)

Figure 10. Change in C i with time for solution exposure test. p


h

333 polymer, it attacks the chemical species responsible for the adhesion of the plasmapolymer to the metal, thereby affecting the adhesion of the plasma-polymer to the metal substrate. It is postulated that this breakdown in the adhesion is due to hydrolytically unstable species at the plasma-polymer interface. Exactly what causes the instability, and what the exact form of the unstable species are is still unclear. This phenomenon, and the chemical nature of the interface which produces it is currently being investigated. Sample 12-D, which showed very little change in both Rp and Cdi, was the exception to the trends mentioned above. It is seen, however, that the absolute values of both R and Cdi are so much poorer than all the other samples, that we can only conclude, as we did with the graphical analysis, that the interface of this sample had already significantly deteriorated prior to the start of the solution exposure test. This deterioration was due to the exposure in the humidity chamber, and indicates that this sample had very poor adhesion of the plasma-polymer to the metal substrate prior to any testing. The results of the GM-scab test show that the oxygen treated sample performed comparably to the CRS and the phosphated CRS. These results also confirm that the results of the preliminary EIS data analysis can be used to assess the corrosion performance of these plasma cleaned/plasma polymer coated/E-coated samples, without the need for any long term environmental exposure tests. It must be remembered, however, that the EIS was performed on samples with no defects, while the GM-scab tests were performed on samples with a three inch scribe. It would thus be difficult to compare the results of the two tests, and make any general conclusions based on these comparisons.


p

Conclusions 1. Plasma cleaning by an oxygen plasma and the application of a plasma polymer on CRS prior to Ε-coating provides corrosion protection properties comparable to that of phosphating/chromating treatments. 2. The important step in this environmentally friendly treatment is the plasma cleaning, and in fact, deposition of a plasma-polymer without a plasma pretreatment is actually detrimental to the corrosion performance. 3. EIS and Equivalent Circuit Modeling of the EIS data can be used as an assessment of a system's corrosion protection properties, as well as for long term exposure testing, as long as the proper ECM is chosen for the EIS data analysis. References 1. K.D. Conners, W.J. van Ooij, S.J. Clarson, and A. Sabata, Journ. Appl. Polym. Sci.: Appl. Polym. Symp., 54, 167 (1994) 2. N.M. Morosoff, in, R. D'Agostino, ed., "Plasma Deposition, Treatment, and Etching of Polymers, Academic Press, San Diego, CA (1990) 3. K.D. Conners and W.J. van Ooij, CORROSION '95 Proceedings, Paper #379, ΝACE International, Houston, TX (1995)



334 4. T.J. Lin, B.H. Chun, H.K. Yasuda, D.J. Yang and J.A. Antonelli, J. Adhesion Sci. Technol. 5, 893 (1991) 5. H.P. Hack, and J.R. Scully, J. of Electrochem.Soc.,138, (1), 33 (1991) 6. M. Kendig and J. Scully, Corrosion 46, 22 (1990) 7. E.P.M. van Westing, G.M Ferrari and J.H.W. de Wit, Corrosion Science, 36, 979 (1994) 8. W.J. van Ooij and K.D. Conners, in D. Scantlebury and M. Kendig, eds., "Advances in Corrosion Protection by Organic CoatingsII",95229, The Electrochemical Society, Pennington, ΝJ (1995) 9. W.J. van Ooij, K.D. Conners and P.J Barto, Paper submitted for Presentation at ICAST '95, Amsterdam, The Netherlands, Oct. 16-21, 10. F. Mansfeld, M.W. Kendig, and S. Tsai, Corrosion, 38, 478 (1982) 11. F. Mansfeld and C.H. Tsai, Corrosion, 47, 958 (1991) 12. R. Hirayama, and S. Haruyama, Corrosion, 47, 952 (1991)


Novel Pretreatments of Metals for Corrosion Protection by Coatings

Recommend Documents