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SG71026.FR
Copy No. 41
\ CORROSION INDUCED ADHESION'LOSS-LOW OLATILE ORGANIC CONTENT (VOC) COATINGS
FINAL REPORT
May 1,1990 through April 30,1992
CONTRACT NO. N00014-90-C-0110
Prepared for:
Scientific Officer Materials Interfaces Division
Office of Naval Research 800 North Quincy Street
Arlington, VA 22217-5000
Attn: A. John Sedriks, Code 1131M
Prepared by:
M. Kendig, Principal Investigator H.-S Ryang, T. Liao, M. Cunningham and S. Jeanjaquet
MARCH 1993
Approved for Public Release - Distribution Unlimited.
Rockwell International @A!!! Science Center
‘ I
The Contractor, Rockwell International Corporation Science Center, hereby certifies that, t o the best of i ts knowledge and belief, the technical data delivered herewith under Contract No. ~ ~ ~ ~ ~ 5 ' - % - * G l / d is complete, accurate, and complies with all requirements of the contract.
, AQENCY USE ONLY (leave E!ank) 2. REPORT DATE 03/08/93
a. REPORT TYPE AND DATES COVERED FR 05/01/90 through 04/30/92
O f f i c e of Naval Research Arlington. VA 2221 7-5000
I I. SUPPLEMENTARY NOTES
. TITLE AND SUBTITLE
Corrosion Induced Adhesion Loss-Low Volatile Organic Content (VOC) Coatings
ALfTHOR(S)
M. Kendig, H.-S Ryang, T. Liao. M. Cunningham and S. Jeanjaquet
PERFORMING ORGANIZATION NAME(S) AND ADORESS(ES1
Rockwell International Science Center 1049 Camino Dos Rios Thousand Oaks, CA 91360
5. FUNDING NUMBERS
7. PERFORMING ORGANIZATION REPORT NUMBER
SC71026.FR
SPONSORING I MONITORING AGENCY NAME(S) AN0 ADDRESS(ES)
Materials Interfaces Division 9. SPONSORING I MONITORING
AGENCY REPORT NUMBER
h. OISTRIBLfTINGIAVAILABlLlTY STATEMENT 12b. DISTRIBUTION CODE
4. SUBJECTTERMS 15. NUMBEROFPAGES 52
16. PRICECODE
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Table' of Contents
Page
Introduction ......................................................................................... . , . , .
Experimental ....................................................................................... 1 . Coating Formulation ......................................................... 2 . Steel Substrate Preparation ............................................. 3 . SCF C02 Coating ............................................................. 4 . Electrochemical Testing ...................................................
Data Analysis ...................................................................................... Results ................................................................................................ Discussion of Results ......................................................................... Conclusions ........................................................................................ Recommended Future Work ................................................................ References .......................................................................................... List of Symbols ................................................................................... APPENDIX I - Electrochemical Impedance and the Identification of Environmentally Compliant Protective Coatings ......................................
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List of Figures
Figure Page 1
2
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10
11
12
13
Schematic for SCF-C& coating spray apparatus ..................................... 5
Schematic of the electrochemical impedance cell. The coated sample (w) is the working electrode (w) .................................................................... 6
(a) Electrical schematic for a steeVcoatindelectrolyte interface. (b) Schematic for the impedance after subtracting elements due to the coating.
(a) Log impedance (ohm cm2) vs log frequency (rads) and (b) phase angle vs log frequency (rads) for the 9 pm SCF-COZ coated steel after 24 h exposure to 0.5 M NaCI. (c) and (d) show calculated and observed spectra obtained using the model in Fig. 3c in place of Rpo The schematic depicts the model for the simulation .................................................... 9,lO
(a) Log impedance (ohm cm2) vs log frequency (rads) and (b) phase angle vs log frequency (rads) for the 19 pm SCF-CO2 coated steel after 24 h
(a) Log impedance (ohm cm2) vs log frequency (rads) and (b) phase angle vs log frequency (rads) for the 12 pm SCF-COZ coated steel after 24 h exposure to 0.5 M NaCI. The coating contained Molywhite ZNP
(a) Log impedance (ohm cm2) vs log frequency (rads) and (b) phase angle vs log frequency (rads) for the 18 pn SCF-CO2 coated steel after 24 h exposure to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-Williams ZnO/Zn Molybdate corrosion inhibiting pigment) ............. 13
Rpo vs time of exposure to 0.5 M NaCl for several SCF C02 spray- coated steel coupons ...................................................................... 14
Calculated volume fraction of water vs time of exposure to 0.5 M NaCl for several SCF C02 spray-coated steel coupons ......................................... 14
Cd vs time of exposure to 0.5 M NaCl for several SCF C02 spray- coated steel coupons ...................................................................... 15
&or vs time of exposure to 0.5 M NaCI for several SCF C02 spray coated steel coupons ...................................................................... 15
Open circuit potential, bar. vs time of exposure to 0.5 M NaCl for several SCF Co;? spray coated steel coupons .................................................. 16
Time dependence for bor (a)& and c d (b) and Rpo and R,, (c) as a result of the exposure of the 9 pm SCF-Ce coated steel coupon to 0.5 M NaCl ................................................................................ 18
(c) Model for the initial impedance for a defect ....................................... 7
exposure to 0.5 M NaCl ....................................................... :. ........ 11
(Sherwin-Williams ZnO/Zn Molybdate corrosion inhibiting pigment) ............. 12
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15
16
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18
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21
22
List of Figures
Figure
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Time dependence for br (a) C, and Cj (b) and Rpo and (c) as a result of the exposure of the 15 pm SCF-CO2 coated steel coupon to
Time dependence for kor (a) C, and Cj (b) and R p and Rcor (c) as a result of the exposure of the 19 pni SCF-CO2 coated steel coupon to 0.5 M NaCl ............................................... : ................................ Time dependence for qor (a) C, and Cd (b) and Rpo and &or (c) as a result of the exposure of the 8 pm SCF-CO2 coated steel coupon to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-
Time dependence for qOr (a), C, and Cd (b) and Rp and &or (c) as a result of the exposure of the 12 ptm SCF-CO2 coated steel coupon to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-
Time dependence for bor (a) C, and Q (b) and Rp and Kor (c) as a result of the exposure of the 18 pm SCF-CO2 coated steel coupon to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin- Williams ZnOfZn Molybdate corrosion inhibiting pigment). ........................ (a) Photographs for the pigment-free SCF-COz coated steel coupons after more than 300 h exposure to 0.5 M NaCI. (b) Photographs for the pigmented SCF-CO2 coated steel coupons after more than 300 h exposure to 0.5 M NaCl ................................................................. 25.26
Calculated volume fraction of water for the pigmented and unpigmented
Rpo as a function of coating thickness. Data from all experiments .................. Photographs of typical coating defects: (a) pinhole or crater (b) apparent
0.5 MNaCl ................................................................................ 19
20
Williams ZnOfZn Molybdate corrosion inhibiting pigment). ........................ 21
Williams ZnO/zn Molybdate corrosion inhibiting pigment) ......................... 22
23
coatings .................................................................................... 21
30
foaming ................................................................................... .30,3 1
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List of Tables
Table P a g e
1 Coating Formulation ...................................................................... 4
2
3
4
Samples Used in the Study ....... ................... ............ ........................ Summary of Pigment Dispersion Tests .... .................... ............ .... .. ... ... (a) Fractional Changes in Film Capacitance and Conductivity. (b) Impedance Parameters Compared to Area Disbonded ............................ 24,28
Log Correlation of EIS Parameters to Disbond Area .... . . . .. ..... . . . .. .. . ... . . .. . ... 28
6
17
5
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Introduction
Environmental concerns demand a greater reduction and ultimate elimination of the volatile organic compounds VOCs that are released from coating and cleaning solvents used in producing corrosion protective metal finishes. A number of approaches have been reviewed.'-2 The approaches most often considered include technologies associated with 1) high solids coatings, 2) water-based coatings. 3) powder coatings. 4) non-aqueous dispersions (NAD)3 and 5) spray coating using supercritical fluid CO2 (SCF CO2) dilution.4 In addition, water-based electrophoretic deposition of coatings can be considered, but this approach has limited application for field or depot repair and maintenance.
The high solids and water-based coatings still emit VOCs and do not provide sufficient protection due to poor wetting and mechanical properties for the former and poor water resistance in the latter. In addition, the high solids approach generally uses low MW resins, which form highly cross-linked films that are too brittle. Low molecular weight resins also have a higher degree of functionality and hence higher surface tension leading to a large number of defects associated with poor wetting of the substrate.5 By their nature, water-based coatings contain residual hydrophilic functionality which tends to enhance their ionic conductivity, a disadvantageous property for corrosion protection. The inferior performance of these coatings is illustrated in results previously obtained in this program and contained in Appendix 1.6 Both the wafer-based coating and the high solids coating show a more rapid decrease in the coating resistance, Rpo, as a function of the time of exposure as compared to the conventional high VOC coating. This is also reflected in the appearance of corrosion.
NADs are another form of high solids technologies where higher molecular weight resins are dispersed in a high concentration in a solvent. NADs eliminate the problems of high solids, specifically the uses of low MW resins, but still contain solvents.
Powder coatings contain virtually no solvents, but require electrostatic spray and often require complicated cure processing. Other problems associated with powder coatings include difficulty in covering recesses in complicated assembled parts and the development of high gloss finishes.7
SCF C02 dilution has been explored by Union Carbide4 and a consortium of automobile manufacturers. Their process uses the SCF COz as the primary viscosity-reducing solvent in a
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formulation that still contains secondary solvents for maintaining flow in the pre-cured film. Nevertheless, advantages of SCF coating include ease of application (only slightly modified conventional spray equipment is necessary), replacement of the highly volatile primary solvent with a totally benign material, and higher atomization and better control of the spray. Furthermore, conventional high-performance resins can be used with SCF C& dilution.
Preliminary work in this laboratory showed6 (see Appendix I) that solvents can cause more problems than the benefits they provide in terms of the corrosion protective properties of organic coatings. Indeed, slowly removed solvents have caused shrinkage, thereby producing internal stresses8; and solvents produce regions of high ionic conductivity9 in addition to the adverse environmental effect. Coatings can retain considerable solvent when cured.8 As stated by Miller et al.: “...solvents are used only as a means of preparing the solids for effective application to the surface. It would be most desirable if the materials could be applied as solids without dilution because then the expense of solvents could be saved and environmental pollution problems resulting from exhausted solvents would be avoided.”’O In short, solvents of any form are neither inherently beneficial for corrosion protection nor for the environment. Conventional technology requires them only for converting a resin from a pre-processed solid powder to a thin continuous film. The time is right for finding other methods of accomplishing film formation such as the use
of SCF 0 2 as the application medium.
Advances in the use of SCF C Q as a diluent and the development of newer, less viscous epoxy resins” demand closer examination of using SCF C&, but without the secondary solvents. Rather. a hybrid approach entailing the marriage of low viscosity (high solids) epoxy resins with the SCF C& deposition has been considered. Note that some of the newer low viscosity epoxy resins skirt the problem of producing highly cross-linked and brittle filnis.11-12
In this study an epoxy coating has been formulated to be compatible with supercritical CQ and has been spray coated onto’steel substrates that were subsequently tested for corrosion resistance. In some instances of this study, a Zn Molybdate/ZnO/Zn Phosphate (Sherwin-Williams Molywhite Z N P p pigment was used as a corrosion preventative pigment. This pigment was selected on the basis of its good performance as a non-toxic Zn chromate substitute as determined using a novel electrochemical screening test developed in this laboratory in a separate ~r0gram.l~
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Experimental
I . Coating Formulation. Non-VOC epoxy resin formulations for metal corrosion protective coatings consist of a low viscosity bisphenol F (Ciba-Geigy PY306), a cresyl glycidyl ether reactive diluent (Ciba-Geigy, DY023). and a 2-phenyl imidazole curing agent. The precise formulas for the material used in this study and the details of preparation appear in Table 1. The role of the reactive diluent was two-fold 1) further reduction of the resin viscosity and 2) prevention of crystallization of PY306. This resin system was chosen since it exhibited good compatibility with SCF C02 and was stable enough to process at 50-60'C yet could be cured at room temperature if desired. Wetting agents were added to lower the surface tension of the coating sufficiently to minimize the fomiation of defects due to poor or insufficient wetting. Samples were oven cured for several hours at 60T.
Tests to ascertain conditions required to disperse a non-chromate bearing corrosion inhibiting pigment in the resin were performed by including different dispersion agents in an 8020 ratio of epoxide resin (Ciba Geigy PY 306) to reactive diluent (Ciba Geigy DY 023).
For one series of samples, -15% by weight of the corrosion inhibiting pigment, Molywhite ZNP@D. was included with the resin (see Table 1). The organotitanate was included in the formulation to disperse the pigment. The resin and pigment were ball-milled overnight before adding the curing agent and introducing to the SCF-Ca autoclave/spray apparatus (see Table 1).
2. Steel Substrate Preparation. Steel samples were sandblasted with 100 mesh silica at a pressure of 40 psi and wiped with acetone. The samples prepared in this manner were coated within 24 h.
3. SCF C02 Coating. Figure 1 shows the schematic for the SCF-CO2 coating apparatus. It included a airless air gun connected to an autoclave containing the SCF-CO2 solution of the coating components. The supercritical COS spraying required a modified airless air gun. The nozzle and body of the gun was modified so that it could be maintained at a temperature above 70'C (typically 75'C). A 0.01 1" nozzle with an 8" pattem was used.
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Table 1 Coating Formulation
part1
hbtcrial
1 2 3 4 Adhesion Promolcr (Dow-Coming 26040)
Modification for Pigment
Epoxidc Resin (Araldiu: PY 306 .Ciba-Gcigy) Cresyl glycidyl ether (Amldile. DY 023, Ciba-Gcigy) Leveler - Span 80 (IC0
Malerial
1 2
4 Adhcsion Promolcr (Dow-Coming 26040) 5 Pigment (Sherwin-Williams Molywhitc ZNP) 6 Dispcrsant (Kcnrich 2381)
Epoxidc Rcsin (Anlditc PY 306 .Ciba-Gcigy) Cresyl glycidyl elhcr (Anlditc DY 023, Ciba-Gcigy)
3 level^ - Span 80 (XI)
Typical % of Batch@ Rcsin
457.00 80 114.25 20
1.37 0.25-0.3 0.26 0.05
Typical 5% of Batch&) Rcsin
96.06 80 23.99 20 .219 0.24
0.068 0.056 18.023 15.2 1.198 1 .o
Part I!
I
Proccdurc (Non-Pigment):
89.8 gm of pan I arc added LO 3.147 g of crushcd 2pZ (Part ll)(3-4% in final mixturc) The solution is heated with conswnt stirring at 60 C for 1 h The solution is immcdiatcly coolcd, filtcrccl by passing lhrough a I00 mcsh scrccn and Placed in an autoclave at an approximatc ntio of 5:2 parts of resin to C02
Proccdurc (Pigment):
Pan 1 with the pigment is placed in a 250 plastic bottle conwining ccramic balls Part I is ball milled overnight (I6 h) at mom tcmpcrature Pan I is scpanted from the balls and 90.2 g is mixed with 2.68 g of crushed PnR I1 Thc mixture is stirred at 60 C for 1 h The solution is immediatcly cooled. passcd through n 100 mcsh scwn and Placed in an autoclave at a ratio of 5:2 parts of resin to C02
Curing Agent (CurczoI2pZ. Pacific Anchor Chcmical Co.) 100
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Temperature N Controller
T/C I II Q II' I I
U Fig. 1 Schematic for SCF-COz coating spray apparatus.
During the spraying operation the pressure was monitored; typically it was above the critical pressure. The autoclave temperature was maintained at 7SC. Table 2 contains information on the test samples that were prepared. The coating thickness is an average value determined by a magnetic induction thickness monitor (Checkline). Thickness variations for a given sample were 10-15%.
4. EIecrrochemicat Tesring. Coated samples of the steel were mounted in electrochemical cells (Fig. 2) for exposure to air-equilibrated 0.5 M NaCl and concomitant evaluation of the electrochemical impedance over a broad frequency range ( 1 0 4 Hz to IO3 Hz).
Figure 3a shows the electrical circuit model for an organic coating on a steel surface. This model is generally accepted to account for the electrochemical impedance response for organic coated steel exposed to corrosive environments.15-16 although interpretation of the individual elements is subject to some dispute.17The elements include &, the coating capacitance (F/cm*), and the coating resistance Rp (ohm cm2). In series with rhe coating resistance is an element Rwr (ohm an2) and GP which are associated with faradaic and double layer charging processes at the interface. This impedance takes the form l/(jwY)n. wheT j2= -1 and w is the angular frequency in radlS. &p contains various responses including that due to complex adsorption kinetics (pseudo Capacitance) and effects due to surface roughness. [The necessity of invoking a Z C ~ may well
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Code
s3 s5
S7N1 S7N2 SI2 SI3 SI4 SI5 SI6 SI8
Table 2 Samples Used in the Study
thickness Rcssure
wn psi
Comments
15
14 20 17 9 I5 19 12 8
18
subcritical
1000-1200 400-600 1150-1325 1 150-1325 1350-1450 1225-1350 1125-1225 1400-1625 Contained Molywhitc ZNP 1250-1 375 Contained Molywhitc ZNP 1100-1200 Contained Molywhitc ZNP
1- Aux.
W I I
Eg. 2 Schematic of the electrochemical impedance cell. The coated sample (w) is the working electrode (w).
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cc
(b)
zpo = - coating -
substrate
CC
- ZPO
Fig. 3 (a) Electrical schematic for a steeVcoating/electrolyte interface. (b) Schematic for the impedance after subtracting elements due to the coating. (c) Model for the initial impedance for a defect.
acknowledge that the charge transfer and pseudocapacitance are inherently inseparable. Such considerations, however, are beyond the scope of this report.]. Rcor depends inversely on the corrosion rate at the base of defects and pores in the coating. Hence its normalization to the area of the sample, though conventional, remains somewhat meaningless.
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Electrochemical impedance measurements were obtained for samples placed in the cell shown in Fig. 2 using a PAR (Princeton Applied Research) Model 273 Potentiostar/ Galvanostat and a Solamon Model 1250 Frequency Response Analyzer. The system was controlled using a PC running the software provided by Princeton Applied Research. The data were stored on magnetic disk and wete analyzed using EQUIVCRT.1G19
Data Analysis
To extract the values for the impedance parameters in order to characterize the kinetics of the degradation of the coating, different frequency ranges of the impedance spectra are fit to portions of the model. At the very highest of frequencies, the impedance of C, is very low and shorts everything else except Rs , the solution resistance. Hence, at a sufficiently high frequency the observed impedance equals Rs. For high impedance coatings this frequency is generally inaccessible below 10 kHz, the nominal upper limit for the measurement system.
At intermediate frequencies where 2 appears to vary as I/f (where f is the frequency in Hz), the impedance can be modelled as a series circuit of Rs and C,. This frequency range is used to evaluate G.
The following tmnsfomiations on the observed impedance, 2, are then performed
The resulting reduced impedance, Zrz, is that shown in Fig. 3b. The complex impedance of Fig. 3b defines a semicircle in the complex plane (in-phase impedance vs negative out-of-phase impedance) the center of which falls below the real axis. The equation for this impedance takes the following form:
% = Rpo Rcor/(l -b (i%orcd)") (3)
Here c d is derived from Y and n parameters of the 2cp. element as:
Q = yVn &.Jl-n)/n (4)
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A nonlinear least-squares analysis using EQUIVCRT readily fits I h to this model by optimizing the parameters Y. Rcor. n for a minimum in a weighted sum of squared difference between simulation and observed
Results
Figures 4-7 show typical impedance spectra taken for the coated samples after 24 h exposure to the 0.5 M NaC1. The dotted lines show the observed spectra in terms of impedance magnitude (Figs. 4a, 5a 6a and 7a), and phase (Figs. 4b, 5b, 6b, and 7b) while the solid line shows the simulated spectra obtained by calculation from the fitted parameters. Generally there is relatively good agreement between the fitted spectra and the calculated spectra, particularly at the lower and upper range of frequencies. Few exceptions to this general observation were recorded.
A
u Y L cn
I v u
" 8 0 -
4 6 0 -
u - I ,-.J I ..,... 1 ,,...., I , ,,,-
to-' I O 0 10' I O 2 10' 10' 105 2sf (rad I s )
Fig. 4 (a) Log impedance (ohm cm2) vs log frequency (rads) and (b) phase angle vs log frequency (rads) for the 9 pm SCF-COz coated steel after 24 h exposure to 0.5 M NaCI. (c) and (d) show calculated and observed spectra obtained using the model in Fig. 3c in place of Rpo The schematic depicts the model for the simulation.
,
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Sample S12
0.5 ll NaCl
IO-' 10' IO' lo2 10' 10' 10' Zxff ( raals)
80 - Sample 512
60 -
40 - -
lo-' IO0 IO' lo2 10' I 0' I 0' Zxff (rad/$)
OS3 nFlcm2 It
p - z z z r - ~ 7.35 nFlem2 8 4 5 nFlom2 w 190 k o h om2 ~
2.2 Mohm em2 12 MQhm Cm2
Fig. 4 (Cont'd)
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';J los -; 1 0' IO-' loo 10' loz 10' IO' IO'
2sf (radlr)
2 80 - U U L
w I Y " c
6 0 -
0.5 tl NaCl - G, 4 0 - a
- I '4 ,,,,.,,,I ' I ,
lo-' too IO' to2 10' 10' 10' 2aff (radls)
Fig. 5 (a) Log impedance (ohm cm*) vs log frequency (rad/s) and (b) phase angle vs log frequency (rads) for the 19 pm SCF-COz coated steel after 24 h exposure to 0.5 M NaCI.
Figure 4, however, shows an example of significant deviation of the observed spectra from the fitted spectrum. The deviation occurs at intemiediate frequencies because an additional time constant must be present due to frequency dependence for the impedance of the electrolytic penetration. Most likely. a microscopic dewetted zone on the surface can require an additional time consGnt defined by the R-Zcpc circuit in series with the Rp, as illustrated in Fig. 3c. Including such an element for the coating in Fig. 4 provides a much better fit for this intermediate frequency range, as shown in Figs.4c-d. Note that the need for an additional time constant occurred only at the very initial stages of exposure. The element shorting the coating reverted to a frequency independent Rpo after longer times (>IO0 h) since one resistive path in the defect dominates (Fig. 3c). In general, however, the simpler model (Fig. ?a) described the impedance response for the coated metals throughout the exposure. This is confirmed by comparing the observed and fit spectra shown in Figs. 5-7.
I
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I 0' 1 0-' 10' IO' 10' lo3 IO' lo5
2sf (reels)
u n e c P
....- SI5 60
8O' 24 h
lo-' I O 0 10' 1 oa I o3 I o4 I o5 2sf (rad/$)
Fig. 6 (a) Log impedance (ohm cm2) vs log frequency (rads) and (b) phase angle vs log frequency (rads) for the 12 pi SCF-CO2 coated steel after 24 h exposure to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-Williams ZnOKn Molybdate corrosion inhibiting pigment).
Experimentally, two sets of samples were considered. The frst set was relatively thick and was produced as a preliminary run (samples S3, S5 ,S7N1 and S7N2 in Table 2). The second set
was prepared to have a range of thicknesses. Half of those of the second set contained a corrosion inhibiting pigment (SI2418 in Table 2).
Figure 8 shows a composite plot of the Rpo values observed for exposure (samples S3, S5, S7N1, S7N2) of coatings having different thicknesses. The figure shows a substantial increase in Rpo for coatings thicker than 14 pm. From the capacitance of the coatings the apparent volume fraction of water can be calculated using the often cited Brasher and Kingsbury approach?0
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N
0 .) Y Y L 01 " v I
Y Y
FI 5
c
a
90
80
2uf (radlr)
Fig. 7 (a) Log impedance (ohm cm2) vs log frequency (rad/s) and (b) phase angle vs log frequency (rads) for the 18 pm SCF-COz coated steel after 24 h exposure to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-Williams ZnOEn Molybdate corrosion inhibiting pigment).
where v i s the volume fraction of water and C, (t) and G(0) are the coating capacitances at timet and time 0 respectively. The calculated volume fraction of water taken up by the coatings appears in Fig. 9. The water uptake for the thicker coatings follows a similar time evolution, giving a steady state value of about 5% water after 100 h. Deviations at initial times for the 17 pn sample may be explained by either experimental error in determining the capacitance or the coupling of faradaic capacitances. Coupling of parallel Faradaic or double layer capacitances is most certainly the case for the 14 pn sample. which exhibits a very high apparent volume fraction of water. Note that Eq. (5 ) is valid only for low volume fractions of water in the coating. Anomalously high capacitances may result when defects contribute to the apparent coating capacitance as a result of low localized values for the coating resistance.
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TIME (h)
Fig. 8 Rpo vs time of exposure to 0.5 M NaCl for several SCF COz spray-coated steel coupons.
0.2
-0- SCF c02 (20um) 0.1 --t SCF C02 ( 17um)
-B- C02(1411m)
300 0.0
0 100 200
TINE (h)
Fig. 9 Calculated volume fraction of water vs time of exposure to 0.5 M NaCl for several SCF COz spray-coated steel coupons.
For the thinner coatings, the interfacial impedance could be evaluated giving values for cd and Rcor. The time dependence for these values appears in Figs. 10 and 11 respectively for the preliminarily prepared samples. Low values of R,, correspond with high values of cd . Since c d includes effects due to double layer charging, charging of a pseudo-capacitance dependent on the electrochemical kinetics at the interface, and transmission line response related to disbonding, the
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irameters is not surprising. The coating with the lowest inverse com.. capacitance has the highest &or. and the coating with the highest capacitance exhibits the lowest &. Note that the interfacial impedance for the thick 20 pm coating could not be determined since it exhibited virtually an infinite Rpo (Rpo >loo0 M ohm cm2).
-a- w2(1411m)
0 100 200 300 TIME (h)
Fig. 10 Cd vs time of exposure to 0.5 M NaCl for several SCF C02 spray-coated steel coupons.
300 ."
0 100 200
TIME (hl Fig. 11 br vs time of exposure to 0.5 M NaCl for several SCF COz spray coated steel coupons.
The open circuit potentials for the samples whose impedance parameters have appeared in Figs. 8-10 are plotted as a function of time in Fig. 12. The thinner samples that show significant defects exhibit open circuit potentials clustered around -550 to -600 mV vs SCE, whereas the 20 pm sample that showed an Rpo well above 500 Mohm cm2 exhibited Ecor in excess of -200 mV vs SCE. Clearly in the highly porous samples, anodic processes at the base of defects dominate the
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corrosion process; and for the thicker coating with superior barrier properties as evidenced by the high RW, the cathodic reaction becomes much more significant relative to the more polarized anodic reaction. As a result, the corrosion potential is significantly higher for the thickest sample.
0 IO0 200 300
TIME (h) Fig. 12 Open circuit potential, Ecor. vs time of exposure to 0.5 M NaCl for several SCF C02
spray coated steel coupons.
The second series of experinients compared samples with and without the non-chromate molybdate-based pigment (Molywhite@ ZNP). Before the pigmented coatings were formulated and applied to the sandblasted steel substrates, a study was performed to identify a suitable dispersing agent. A number of different dispersing agents having hydrophile-lyophile balance numbers (HLB numbers)21-23 ranging from 4.3-15 were evaluated. The HLB number depends on the relative number of hydrophilic and lyophilic groups in an organic surfactant. Surfactants containing a large number of hydrophilic groups such as sulfonate salts tend to have high HLB numbers, but a large proportion of aliphatic functionality contributes to lower HLB nunibers.21-22 An HLB number can be assigned to any given two-phase system. The results show that HLB numbers of 15 or above are required to disperse the ZNP pignient in the coating resin. Subsequent experimental work showed an organotitanate provided superior dispersion (Table 3).
The time evolution of the fitted parameters for coatings with and without the pigment on steel appear in Figs. 13-18. A few trends can be described. In general, the capacitance for the coating changes very little on a log scale. This behavior is consistent with the interpretation of C, being the dielectric capacitance for the relatively inactive organic coating. The coating capacitance remains relatively constant with time of exposure regardless of whether the coating contains or does not contain the pigment (compare data for C, in Figs. 13b, 14b, 15b with comparable data in Figs. 16b, 17b, 18b).
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Additivca
1.0% Tween 8d 0.8% Twecn 80D.2% Span 80 0.6% Tween 80/0.4% S m 80 0.4% Tween 80/0.6% Span 80 02% Twccn 80/0.8% Span 80 1.0% Span 80 A T U ~ (Z w/o of Pigment) A T U ~ (2 w/o ofPigmcnt)c A T U ~ (I w/o of Pigment) A T U ~ ( I w/o of mixture$ Lica 38 (2 w/o of pigmcnt)c Lica 38 (2 w/o of pigmcnt)c Kcnrich 238F (2 w/o of pigmcnt)c Kcnrich 238P (2 w/o of pigmcnt)c Kcnrich 238F (1 w/o of Kenrich 238Je ( I w/o of pigmenc)d B u s p e r ~ e 4 7 ~ (2 w/o of pigmcnt)c B u s p e r ~ c 4 7 ~ (2 w/o of pigmcnt)c
Table 3 Summary of Pigment Dispersion Tests
Process Scttling time. Scttling time. cmrse line (says) (days)
stir stir stir stir stir stir stir ball mill stir ball mill stir ball mill stir ball mill stir ball mill stir ball mill
3 3 3 3 3 3 <I < I <I <I <I < I 9 10 7 7 6 5
9 9 8 8 7 7 <I <I < I < I < I <1 >I% >I28 >tog >lo6 7 5
HLB Numbcr
15 12.9 10.7 8.6 6.4 4.3 unk. unk. unk. unk. unk. unk. unk. unk. unk. unk. unk. unk.
Notes:
[a,] Base material is rcsin (80% Ciba Ccigy. Aralditc PY 306. 20% Ciba Gcigy DY 023 [rcactive dilucnt. Cresyl gllicidyl elher] ) plus 15 wlo pigmenl (Sherwin Williams, Molywhite ZNP). % Addiiivc is for aclivc ingrcdicnl. [b.] ATU is Mli- Terra-U (BYK-CHcmie). [c. 1 Additive addcd as 10% in alcohol. Alcohol evaporatcd. Id. 1 Additive mixed with resin and pigmcnl. [e.] Ken-react. Kcnrich Pcuochcmicalr Inc. If.] Buckman Laboratories. [g.l gelled at 14 days. [h.] Sorbim Monooleate (IC1 Americas. Inc.). ti.) Polysorbate 80 (IC1 Americas. Inc.).
Contrary to the time evolution of &, Cd. Rp and Rcorgenerally show significant changes (sometimes over several orders of magnitude) with time of exposure. The relative changes in Rp (expressed as dopdop., where op is the inverse of Rp at the initial time and dop is the change observed at the end of the test), Rcor (expressed as docor/Omr. where Ocor is the inverse of &or),
and Cd (expressed as dCdCd) over the more than 300 h exposure, are tabulated in Table 4a. Note that the fractional changes in all of these parameters for the pigmented coating generally exceed comparable parameters for the pigment-free coating. The increase in Q exhibits the largest changes as compared to the others. The largest changes in acor occur for the thicker coatings. For the unpigmented coating the fractional change in op is substantially less compared to that for the pig- mented film. In the unpigmented film opo remains condtant or doubles, whereas op for the pig- mented coating exhibits a 10,000-fold increase (Table 4a). Although opo for the unpigmented
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coating shows little change, cd for the unpigmented coatings increases by 10-500 times of the ini- tial (Table 4a). Likewise, Ocor for the uhpigmented coating also increases significantly (Table 4a).
(a) ' 0 4
Gi - 2 0 0 - 0 v) M -400- >
> Y E -600 L
8 - 8 0 0 - w
-1ooo+
Sample S 12
1 1 I I 1 Y
- -
--- -
I I I I I .ti
10000 p (b) I I I Y
0.01 b I I I 0 100 200 300 400
Time (hours)
I I I I
0.001 k 1 I I I I 0 50 100 150 200 250 300
Time (hours)
Fig. 13 Time dependence for Ecor (a),Cc and c d (b) and R,, and RcOr (c) as a result of the exposure of the 9 pm SCF-C02 coated steel coupon to 0.5 M NaCI.
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0 4 (4 I I I 0
6. - 2 0 0 - - 0 v1 M - 4 0 0 - >
- > E - 6 O o - L - e * - v L 8 - E i O O - - w
- 1 0 0 0 3 I I I *
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0.01 L I I I 4 0 100 200 300 400
Time (hours)
N 5 100 E c
p 10 0 u c - c " u LI
.C n l
0.1 0 100 200 300 400
Time (hours)
Fig. 14 Time dependence for Ecor (a) C, and cd (b) and Rpo and Rcor (c) as a result of the exposure of the 15 pm SCF-CO2 coated steel coupon to 0.5 M NaC1.
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(4 0 %
- 2 0 0 - 0 v)
g -400 -
I I I 4
- -
d a- E -600 - -
Y L 0
- ~ - 8 0 0 - -
-10003 I I I +I
N (a,
5 100 E c
E. 10 u U c c a
n .- - 1
8 0.1
0 100 200 300 400
Time (hours)
Fig. 15 Time dependence for Ecor (a) C, and C,j (b) and Rpo and Rcor (c) as a result of the exposure of the 19 pm SCF-C02 coated steel coupon to 0.5 M NaCI.
10000 4 1 I I 4
* CC -0- cd 5 1000 - N
(b) -
%
c u U c
100 - -
- 10:7 ; a 1 c .C " -
a e-- - e 8 0.1 - - 0.01 3 I I I +i
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G- - 2 0 0 - 0
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-
(a) O C I I I I 4J
v,
> M -400 > v E - 6 0 0 - -
- 8 0 0 - -
-
L
-1000 + I I I +I
Fig. 16 Time _.
1
E
E
n u
c P u u c 0 n n u K
.C
1 oo A - - 1 1 - I - I - I
0 50 100 150 200 250 300
- - I
Time (hours)
0.01 b I I I il 0 IO0 200 300 400
Time (hours)
pen-me for Ecor (a) Cc and cd (b) and R,, and Rcor (c) as a result of the exposure of the 8 pm SCF-CO2 coated steel coupon to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-Williams ZnO/Zn Molybdate corrosion inhibiting pigment).
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0 4 (4 g - 2 0 0 - VI
* > v, E -400 - 6 0 0 - v L 0 4 - 8 0 0 -
-1000f
I I I Y
- - - JI'------. -
- 1 I I .h
1000 n
100 E c E 10
0.01
I r k ] + Rcor
b I I 1 0 100 200 300 400
Time (hours)
Fig. 17 Time dependence for Ecor (a), Cc and C,j (b) and Rpo and Rcor (c) as a result of the exposure of the 12 pm SCF-CO2 coated steel coupon to 0.5 M NaCI. The coating contained Molywhite ZNP (Sherwin-Williams ZnO/Zn Molybdate corrosion inhibiting pigment).
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+ Rcor
ct I 1 - *I t
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Sample S18
- 1 o o o k I I I 0 100 200 300 400
Time (hours)
N
5 . L c
10' 1 1 oo
A - T I - I - 1 - I
0 50 100 150 200 250 300
- - - - Time (hours)
E 1000 N
U ' E = 100 E u e a c n c n
u 10
1 2
0.1
Time lhniirc\
I Fig. 18 Time dependence for kor (a) C, and Q (b) and RP and Rcor (c) as a result of the expo- sure of the 18 p n SCF-CO2 coated steel coupon to 0.5 M NaC1. The coating contained Molywhite ZNP (Sherwin-Williams ZnO/zn Molybdate corrosion inhibiting pigment).
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Table 4a Fractional Changes in Film Capacitance
and Conductivity
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S 14 19 10 2 SI3 15 500 1 SI2 9 100 1 SI8 18 3000 104 S15 12 3 105 104 S 16 8 104 104
The corrosion potentials for the pigmented coatings differ significantly from the corrosion potentials for the unpigmented coatings. Typically E,,,, for the pigmented coated steel exceeds -500 mV vs SCE (Figs. 16a, 17a, 183). whereas the potentials remain in the -550 to -600 mV region for the unpigmented coatings on steel (Figs. 13a, 14a, 15a). This suggests that the molybdenum based pigments in the coatings interact to a degree with the substrate.
After exposure (-300 h), the samples were removed from the impedance cells and were photographed under identical lighting conditions. Figures 19a and 19b show photographs of the samples coated with the pigment-free (19a) and pigment coatings (19b).
These photographs were digitized using a Hewlett-Packard Deskscan desktop scanner. The resulting negative images for each sample were then processed using Image, Version 1.4124 to evaluate the area disbonded. Note that negative images were used to evaluate the disbonding since the disbonding appears as a light region due to light scattering, whereas the corrosion spots tend to absorb light. Accordingly, using Image 1.41, the negative images were sharpened and contrast was enhanced. A density slice was then set to select the disbonded regions, which were subsequently evaluated in terms of total area. This area defines Ad, the apparent area disbonded. These results are tabulated in Table 4.
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Fig. 19 (a) Photographs for the pigment-free SCF-CO2 coated steel coupons aftermore than 300 h exposure to 0.5 M NaC1. (b) Photographs for the pigmented SCF-CO2 coated steel coupons after more than 300 h exposure to 0.5 M NaC1.
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Fig. 19 (Cont’d)
Discussion of Results
In time, Rp decreases for both the pigmented and unpigmented coatings (Figs. 13c, I&, 1.5~. 16c, 17c, 18c). However, RW decreases much more for the pigmented coatings (Figs. I&, 17c, 18c). This suggests that the pigmented coatings tend to “open up” with exposure to the aqueous environment. This may also be viewed as a tendency for hydrolysis of the pigmentkoating bond. Another manifestation of this appears in Fig. 20 in terms of the apparent water uptake. The apparent water uptake for the pigment-containing coatings exceeds that for the unpigmented coatings. Note that volume fractions above 0.1 challenge the validity of Eq. (5). Nevertheless, the increased calculated volume fraction of water for the pigmented coatings
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c 0 U u a L U
.. h)
E a 0 > c
0 50 100 150 200 250 300
Time (hours)
Fig. 20 Calculated volume fraction of water for the pigmented and unpigmented coatings.
confirms the trend in increased water uptake for the pigmented coatings in quality if not in quantitative detail.
The area disbonded for the pigmented coatings exceeds that for the unpigmented coatings (Table 4a). Recently, considerable controversy as to the relationship of disbonding to impedance parameters has occurred.*7 Haruyama et al.25 maintain that Rp is inversely proportional to the extent of disbonding. Hence, the breakpoint frequency, fb = lBp0 Q, is proportional to the area of disbonding since C, is relatively constant.
Table 4b compares final values for several key impedance parameters with Ad as determined by photographichnage analysis. To establish the functional correlation of the parameters, p. with Ad, the data were fitted to the general fomi
p = kp Ad" (6)
where k and n are constant and Ad is the disbond area. This analysis forces the data to a linear log- log relationship. The results appear in Table 5. The c d , doppdopo. and dCd/Cd parameters all exhibit rather complicated nonlinear dependence (In1 > 1) on Ad, whereas Rp varies roughly with the inverse square of Ad. This does not agree with an inverse relationship of Ad with Rpo There is no apriori reason to believe that Rp should vary inversely with area if R,, is assumed to represent the ionic resistance across the coating, which may 47 modified as a result of damage to the coating resulting from corrosion. The rate of corrosion does, of course, depend on the area disbonded.
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Hence, a correlation between It-$,, and Ad may exist, but it may or may not be proportional to Ad in all cases. On the other hand, &m, which represents the corrosion resistance at the coating metal interface, depends inversely on Ad (Table 1). This is to be expected based on the proposed impedance model since &or depends on the area of active interface. However, the complicated nonlinear dependence of Cd on Ad is problematic since GJ should also depend on the active area. That cd has been derived from a constant phase element having an exponential value near 0.5 suggests that Cd is highly dependent on the volumetic geometry of the disbond zone as well as on the total active area, Ad. While this explanation is not entirely satisfactory, a detailed understanding of the complicated relationship between impedance parameters and disbonded area requires further study.
Table 4b Impedance Parameters Compared to Area Disbonded
S 14
S13
s12
S18
s15
S16
Parameter
19 0.45 0.50
15 1.23 0.79
9 1.23 1.46
18 3.23 4.10
12 6.45 5.72
8 5.48 5.72
RS,,
Mohm cm2
6.42
2.42
0.68
0.08
0.04
0.04
Cdf Rior RC,,x A'& R;,,
nF/cm2 Mohm cm2 Mohm cm2 Mohm cm2
10.8 10 0.10 1.61
246 3 0.09 1.24
106 3 0.06 4.41
45539 3 0.24 37.5
49539 1 0.12 25
48507 I 0.12 25
Table 5 Log Correlation of EIS Parameters to Disbond Area
fi/cm2 5.0 f 0.3 3.5 f 0.5 - 4.2 f 0.7 4.3 f 1.0
MR*cm2 -1.5 f 0.2 -2.1 * 0.2 - 4.8 f 0.3 3.4 f 0.5
MR.cm2 0.1 f 0.1 -0.8 f 0.2
x 2
0.815 4.796 0.24 1.12 0.15
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The observed Ad can be used to renormalize Reor to the disbonded area by the transformation
= RLr Ad56.3 cmz renormalized k o d (7)
Here Rior is the final value for br. The total sample area is 56.3 cm2. As shown in Table 4b, the renormalized br values for the pigmented coatings are slightly greater than the values for coatings of comparable thickness but having no pigment. This suggests some inhibiting role provided by the pigment. Table 4b also shows the value of REor/Rko. Since R, relates directly to the porosity rather than to Ad, dividing RLr by RLo may represent a more relevant normalization of the corrosion rate since the anodic dissolution concentrates at the base of the pores. Using this re-scaling of R[,, the pigment is predicted to decrease the corrosion rate by about an order of magnitude.(Table 4b). Further work is required to definitively establish a true active area and hence valid renormalization of Rmr.
The coatings formed by the totally non-VOC, SCF-CO2 process exhibit corrosion protection. Furthermore, corrosion inhibiting pigments can be dispersed and co-deposited with the resin. However, the formulation is far from optimized. Below about 17 pm in thickness, the coating exhibits a significant number of defects as shown by the phoiographs in Fig. 21, which presents R, for all samples plotted as a function of the coating thickness. The dependence of R, on thickness is highly nonlinear and suggests a threshold of about 17 pm, below which the coatings have significant porosity. In fact, macroscopic defects have been observed such as those shown in Fig. 22. Here two types of defects are present. The first type of defect, a single pore (A in Fig. 22), has been outlined with a pen. This type of defect is characterized by a mounding up of resin around the central void (as determined by profilonietry). The second type of defect (B in Fig. 22) is characterized by numerous small holes. This latter type of defect may result from the release of entrapped gas. The addition minor quantities of wetting and defoaming agents should greatly minimize such phenomena. The first defect (A) may relate u, the presence of solid particulates due to resin inhomogeneity. Better homogenization of the curing agent and resin could eliminate such defects.
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One Week Exposure (0.5 H NaCl) . s C F Coated Steel
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5 10 15 20 25
Coating Thickness, pm
Fig. 21 Rpo as a function of coating thickness. Data from all experiments.
0.1 cm
(a)
Fig. 22 Photographs of typical coating defects: (a) pinhole or crater (b) apparent foaming.
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H 0.1 cm
(b)
Fig. 22 (Cont’d)
Conclusions
It is feasible to spraycoat carbon steel with a non-VOC protective organic coatings using supercritical fluid carbon dioxide (SCF-(202). A formulation based on an epoxy with a reactive diluent produces a corrosion-protective film on steel by this process.
The formulated film appears to be susceptible to formation of visible defects (Fig. 22) which result in a highly nonlinear dependence of coating pore resistance on coating thickness (Fig. 21).
A non-chromate corrosion inhibiting pigment containing ZnO and Zn molybdate can be dispersed in the formulated resin. The HLB number for the pigment is estimated to be above 15 (Table 3). An organotitanate (Kenrich 2385) produced the best dispersion of all of the dispersants examined (Table 3).
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Incorporation of the pigment in the coating degraded the barrier properties of the film, as evidenced by a general decrease in R p and increase in calculated volume fraction of water taken up by the film over more than 300 h of exposure to 0.5 M NaCI.
Normalization of the calculated corrosion resistance, Rcor. to the disbonded area, Ad,
demonstrates that the ZnO/Zn molybdate inhibiting pigment reduces the corrosion rate of the steel at defects. If b r i s normalized with respect to the porosity expressed in terms of R-Ip, then the pigment is calculated to reduce the corrosion by a factor of ten.
Recommended Future Work
While the work presented in this report clearly demonstrates the feasibility of applying SCF-C02 coating technology to eliminate VOC hexavalent Cr in corrosion-protective coatings on steel, the formulated coating remains to be optimized.
First, the coating must be modified by developing formulation procedures to improve homogenization of the coating. In addition improved wetting, flow and defoaming must be obtained by the use of appropriate additives. In all cases the additives must be compatible with the SCF-CO2 medium. A number of silane material are now under active consideration to improve wetting and flow. Dispersion of rheological control agents and proper adjustment and homogenization of the resin will also be considered.
Second, the dispersion of the non-chromate corrosion protective pigment must be improved. A finer grind of the pigment will be used, and a variety of different dispersants having HLB numbers in the range of 20-30 will be investigated for this purpose.
Third, an optimum coating should be applied to test coupons using state-of-the-an SCF- C02 spray coating equipment developed by Union Carbide and Nordson. Panels so coated can then be subject to EIS and long-term atmospheric exposure. In all cases, EIS will be used to evaluate the details of the coatin; degradation, particularly as a function of the coating composition.
/ I
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References
1 . David E Pulley, “Compliant Coatings for Aerospace Applications”, Report, Naval Air Development Center, Warminster, PA.
J. Larry Jameson, J . Coal. Technol.. 57 (728). 101( 1985).
Mohinder S. Chattha and Joseph C. Cassatta, J. Coat. Technol., 57 (730). 41 (1985).
K.A. Nielson, D.C. Busby, C.W. Glancy, K.L. Hoy, A.C. Kuo and C. Lee, “Supercritical Fluid Spray Application Technology: A Pollution Prevention Technology for the Future.”, Union Carbide Chemicals and Plastics Company Inc., South Charleston, WV. Proceedings of the 17th Water-Borne and Higher Solids Coatings Symposium, New Orleans (1990).
K.A. Nielson, D.C. Busby, C.W. Glancy, K.L. Hoy, A.C. Kuo, and C. Lee, “Supercritical Fluid Spray Application Technology: A Pollution Prevention Technology for the Future”, presented at the 17th Water-Borne and Higher Solids Coatings Symposium, New OrIeans, LA (1990).
Robert E. Sievers, Brian N. Hansen, “Chemical Deposition Methods Using Supercritical Fluid Solutions”, US Patent #4,970,093 (1990).
2.
3.
4a.
4b.
4c.
4d.
4e.
Union Carbide, “Unicarb System Supply Units”, brochure, Union Carbide.
Anon., “Supercritical C02 As a Solvent: Update on Union Carbide’s Process”, Modern Paint and Coatings, 56, June 1991.
5.
6.
S. Peter Pappas, J. Coat Technol., G1( 774). 51 (1989).
M. Kendig and S. Jeanjaquet, “Electrochemical Impedance and the Identification of Environmentally Compliant Protective Coatings”. paper #92 465 Corrosion ’92, Nashville, TN (1992). NACE.
E. Brodnar. “Utilization de la Peinture en Poudre dans I’industrie automobile” 7.
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Masaki Shimbo, Mitsukazu Ochi, and Katsumasa Arai, J . Coat. Technol., 57 (728). 93 (1985).
J.E.O. Mayne and D. Mills, JOCCA, 58,155 (1975).
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Emory P. Miller, “Methods of Application of Coatings”, American Chemical Society (1985).
David Helfand, “Epoxy Resins for Coatings in the 1990’s’’. paper #476, Corrosion ’90, Las Vegas (1990).
R. Brady, Journal of Protective Coatings and Linings, April 1989.49 (1989).
Sherwin-Williams, “Moly-white Chromate Replacement Pigments for Metallic Surfaces”. P.O. Box 1028, Coffeville, KS.
M. Kendig, S. Gordon and S. Jeanjaquet, “Rapid Electrochemical Screening of Non- chromate Pigments for Paint”, to be presented at the 183rd Meeting of the Electrochemical Society, (1993).
F. Mansfeld and M. Kendig, “Electrochemical Impedance Tests for Protective Coatings”, in ‘Laboratory Corrosion Tests and Standards’, ASTM STP 866, G.S. Haynes and R. Baboian, eds., ASTM, Philadelphia, PA, 1985.
M. Kendig and J. Scully, Corrosion, 46 (l), 22, 1990.
M. Kendig, F. Mansfeld and S. Tsai, “Discussions on the Relationship of Breakpoint Frequencies to Delamination”, Corrosion, 47 (12). 964 (1992).
B. Boukamp, “EQUIVCRT” University of Twente, P.O. Box 217, 7500 Enschede, Netherlands, 1988; and B. Boukamp, in Computer Aided Acquisition and Analysis of Corrosion Data,’M. Kendig ed., PV 85-3, Electrochemical Society Proceedings Volume, 1985.
B. Boukamp, “EQUIVCRT Users Manual”, University of Twente, Enschede. Netherlands, 1989.
D. Brasher and A.H. Kingsbury, J. Appl. Chem., 4,62 (1954).
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A.W. Adamson, “Physical Chemistry of Surfaces”, pp 475476, J. Wiley, 4th edition, 1982.
IC1 America, ‘“The HLB System”, 1984.
C. Hare, J. frorecrive Coorings and Linings, 9 (1 1). 66 (1992).
21.
22.
23.
24.
25.
W. Rosband, “Image-version 1.41”. NIH. Research Services Branch, NIMH.
S. Haruyama, M. Asari, T. Tsuru, “Impedance Characteristics During Degradation of Coated Steel” in ‘Proceedings of the Symposium on Corrosion Protection by Organic Coatings’, M. Kendig and H. Leidherser, Jr., eds, PV 87-2, Electrochem. SOC. (1987).
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List of Symbols
angular frequency in radls
corrosion resistance at coatinghetal interface (normalized to sample area)
coating resistance (normalized to sample area)
coating capacitance (normalized to sample area)
frequency in Hz
interfacial capacitance (normalized to sample area)
solution resistance (normalized to sample area)
Open-circuit or corrosion potential
constant for the constant phase element impedance (normalized to sample area)
volume fraction of water in the coating
time of exposure
supercritical C@
impedance of the coating metal interface, corrected for the solution resistance
impedance for the path shorting the coating
disbond area
total sample area
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APPENDIX I
Electrochemical Impedance and the Identification of Environmentally Compliant Protective Coatings
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~ ~ C K H E M I C A L IWEDANCE AND THE IDENTJFICATION OF ENVIRONMENTALLY
COMPLIANT PROTECTIVE COATINGS
M. Kendig and S. Jeanjaquet Rockwell International Science Center
1049 Camino dos Rios. Thousand Oaks, Califomia 91360
ABSTRACT
A number of corrosion-protective coatings containing reduced volatile organic content (VOC) do not compare favorably to a conventional high VOC coating. The evaluation of model solvent and non-solvent epoxy films using electrochemical impedance specfroscopy of samples exposed to 0.5 M NaCl demonstrated, however, that elimination of solvent entirely can be beneficial to the corrosion resistance of organic films.
Keywords: Low VOC coatings, organic coatings, electrochemical impedance.
INTRODUCTION
Within the next few years a number of organic and inorganic protective coatings for aluminum and steel that are either high in volatile organic compounds (VOCs) or contain hexavalent chromium (Cr(V1)) will have to be replaced by coatings which emit reduced VOCs. For manufacturers and end users. it will become critical to be able to rapidly evaluate whether a new environmentally benign product will be able to replace the currently used coating system. A rapid means for evaluating proposed "drop-in" replacements is needed.
In the area of VOCs, a fundamental question arises as to whether the new high solids technology or coatings made with active diluents will in theory perform to the same standards as do the high solvent based films. There has been some doubt cast on whether these coatings will perfom as
Electrochemical impedance spectroscopy has been suggested as an important method for the early evaluation of organic coatings23 and has been used 1) to compare conventional solvent-based
well.' I
I
coatings with low VOC materials, and 2) to address the question as to the fundamental benefiudeficit of eliminating the solvent in the process of forming an organic coating on steel for corrosion protection.
-
EXPERIMENTAL
Fully-formulated Aerospace Primers
Three different aerospace coatings were evaluated. They are described in detail in Table 1 and include a high solvent (VOC = 647 gL). a high solids (VOC = 340 gjL ), and a water-based coating. Carbon steel Q-panels (6" x 6") were treated with a nitric acid etch and then coated. The high solids and conventional coatings were applied by either an electrostatic spray (Es) or airless-air spray (S) to give a nominal 2 mil dry-film coating thickness as determined by gravimetric analysis. The water based system could not be applied by electrostatic spray SO it was applied to the steel substrates either by airless-air spray or by a doctor blade.
Model Coatings
To evaluate the role of solvent in determining the corrosion protective properties of organic coatings, two model coating systems were used. They are described in Table 2. The objective of using these model systems was to compare coatings of nominally the same polymeric material and same thickness in which one coating system would not contain a solvent while the other contained nominally 50% solvent. These coatings were drawn onto steel coupons using wire-wound bars. The steel coupons were wiped with acetone and rinsed in hexane before drawing on the respective coating. The samples were then cured for 30 min. at 100°C.
Elecmhemical Impedance Measurement
For the thick fully-formulated aerospace coatings, a 150 cm2 portion of the surface was examined using a gasketed Plexiglass cell containing the 0.5 M NaCl electrolyte and a platinized titanium mesh counter electrode. A SCE reference electrode contained in a glass tube with a VycorO tip which sewed as a Luggin capillary. A gold wire, insulated except for the tip, was capacitively coupled to the reference electrode and sewed as a high-frequency pseudo-reference electrode. The electrochemical impedance of the system was measured using the PAR 273 Electrochemical System. The same apparatus was used for the model coating systems except that the surface area examined was nominally 20 cm2.
RESULTS
Comparison of Conventional Coatings with Environmentally Compliant Coatings
The impedance model assumed for the coatings is quite well established.2.3 The model comprises a coating capacitor C, due to the coating dielectric. A resistance RP due to ionic transport shorts the capacitor. In some instances an interfacial impedance comprising, for example, a parallel combination of a corrosion resistance Rcorr in parallel with a double layer capacitance, Cd, can be determined from the impedance spectrum.
Figures 1-4 summarize the results for the coatings of Table 1. Figure 1 shows a log linear plot of the coating resistance RPq as a function of the time of exposure of the coated sample to 0.5 M NaCI. The water-based coating whether applied by spray (S) or doctor blade (DB) exhibits poor protection of the steel yielding a RP of 0.001 M ohm cm2 or less in the very fust hours of exposure. On the other hand the electrostatic sprayed (ES), high-solids coating provides significantly better protection as compared to the water-based coatings, but remains significantly inferior to the conventional, solvent-based, electrostatically-sprayed (ES) primer as evidenced by the relative values
. The conventional high-solvent coating maintains a Rp on the order of 100 Mohm cm2 for Phekst 300 hours of exposure.
The Rpo results are consistent with the long-term visual observations quantified by the ASTM D610 rating (10 = no corrosion, 1 = 50% corrosion) as shown by Figure. 2. The conventional coating maintains a value of 9 or above for the first 400 hr of exposure. The high solids material provides protection to a level of 8 for the first 200 hr and then drops to 6, but the water-based materials give a rating of 5 within the first 100 hours.
The corrosion potentials for the better performing conventional and high solids coatings remain high for 400 and 200 h, respectively; whereas the corrosion potentials for the water-based coatings immediately drop to -650 mV vs SCE, which is close to that for bare steel in an aerated NaCl electrolyte (see Figure 3).
Water uptake by the coating produces an increase in coating capacitance, C,. As shown in Figure. 4, C, for the high solids coating increases to a greater extent with time of exposure as compared to the conventional solvent-based coating, No data exist for C, for the water-based coatings. R o's for these materials were so low as to move the frequency range required for evaluation of%& beyond the accessible range of the instrument.
All of the evaluated coatings contain active corrosion-preventive chromate pigments. There was significantly more release of pigment from the high-solids coating as evidenced by the yellowing of the electrolyte. Slightly acidic (pH 5) blisters formed on the high solids coating, and an orange corrosion product formed on the base of the coating underneath the blister. For the conventional coating the corrosion product that formed under the coating at the few blisters was black, a more reduced iron oxide. Figure. 5a shows the conventional coating after 400 h of exposure to the 0.5 M NaCI. Only a few corrosion spots are visible. On the other hand the water-based coating showed 50% degradation (Figure 5b) while the high solids coating showed considerable blistering and corrosion (Figure 5c).
The results show that the candidates for high solids coating that have been examined, while capable of forming a coating film, do not provide the same level of corrosion protection as the conventional coating. It was also noted that the high solids material was significantly more brittle as compared to the conventional coating following exposure. This may be a direct result of the chemistry of the resin system used. As is often the case with high solids coatings applied from a formulation containing a solvent, some retention of solvent results due to the rapid gelling and curing of the resin. This will lead to a porous and poorly adherent coating. Another possible explanation is that the sprayed resin viscosity is too high to induce proper surface wetting resulting in the formation of gas pockets or voids in the surface depression of the adherend!
Comparison of Model Solvent and No Solvent Coatings
The results from the previous section showed that at least the low VOC materials considered as replacements for the conventional material do not compare favorably to the conventional primer. The question then is whether there is something inherently detrimental when a coating is applied with a low solvent or perhaps none at all. This would not seem to be the case in light of the recently published work by MayneS who concluded that "solvents interfere with the process of cross-linking and that films of improved protective value arise when they are eliminated from the system".
The possibility that a better as well as more environmentally attractive coating can be developed by using no solvent at all is indeed an attractive possibility. A test of the hypothesis that total elimination of solvent can produce superior protective films was made by evaluating two nominally identical polymer coatings, one containing 50% solvent and the other formed from reactive diluents and containing no solvent (see Table 2). Coatings formed from these materials were applied using a wire-wound bar to degreased carbon steel coupons and exposed to 0.5 M NaCl during which time the electrochemical impedance was measured. The results appear in Figures 6-9.
Figure 6 shows the time dependence for Rpo for the solvent and solventless coatings. There is nearly a 4 order of magnitude difference of the Rpo between the two coatings after steady state is
.
achieved. The solvent based coating appears to be considerably defective giving an R, on the order of lOOK Q-cm2 after 400 hr as compared to the 200 M Q-cm2 for the solvent-free coating.
The capacitance which reflects water uptake by the coating rises rapidly after 200 hr to values above 2 nF/cm2 for the solvent containing formulation, whereas the coating with no solvent rapidly equilibrates at a constant value below 1 nF/cm2 after 1 day of exposure to the 0.5 M NaCl electrolyte. This result is graphically presented in Figure 7.
The corrosion potential for the solvent containing sample decreases to values in the vicinity of the freely corroding substrate after 200 hr while the solvent-free sample gives a stable corrosion potential near -100 mV vs SCE (Figure 8).
All of the above quantitative evaluations of the state of the coatings are consistent with the visual observations quantified on the basis of ASTM D610 and shown in Figure 9. The solvent-based coating shows a decrease to a corrosion rating of 3 after 500 hr of exposure to the 0.5 M NaCl whereas the solvent-free system gives a rating that does not drop below 7 after the same exposure.
DISCUSSION OF THE RESULTS
The first observation that can be made from the results presented in the previous section is that the newer technologies that do not entirely eliminate the solvent, but rather reduce it using high solids or water dispersion, do not provide "drop in" replacements for conventional systems. This is the case for those systems examined. None of the replacements considered achieved the same corrosion resistance as did the fully formulated high VOC primer as shown by the results of Figures 1-5.
Despite these observations for the fully-formulated, sprayable, low-VOC coatings, entirely solvent-free systems can, in principle, give superior corrosion resistance when directly compared to virtually identical solvent containing coatings as shown in Figures 6-9. The coating with no solvent showed superior corrosion resistance and water uptake as measured by electrochemical impedance spectroscopy.
The reason for the variation in the model solvent-free and solvent-containing coatings may depend less on the molecular structure and more on the type of defect introduced by the formation from the solvent. From positron annihilation spectroscopy performed by R. Granata? there seems to be little difference in the free volume in the resulting resin films for the two coatings. The solvent- free system showed only a slightly greater free volume (6% greater) as compared to the solvent- based system. The fact that the ionic resistance for the solvent-containing system is substantially less and the water uptake for this coating appears substantially higher as compared to the coating formed from a 50% solution must be attributed to defects on a scale somewhat larger than molecular. Near macroscopic voids produced by the solvent release process might provide the explanation for the poorer performance of the solvent -containing coating. While these results are consistent with the observation of Mayne that a solvent-free system is inherently more protective (less conducive to ionic and water transport), attributing the difference entirely to molecular structure (e.g., crosslinking) is not at all clear.
CONCLUSION
The following conclusions can be drawn from this work:
1. The fully-formulated low VOC replacement primers that were specifically examined were significantly less protective of steel substrates as compared to the conventional primer that was examined.
Removal of solvent from an organic coating process has no fundanmtally adverse effect on the corrosion protection provide by the coating to the substrate. To the contrary, removal of
2.
3.
1.
2.
3.
4.
5.
6.
the solvent has a beneficial effect on the transport of ions and water by the coatings as is consistent with observations by others.5
Since solvent removal has no adverse influence on protective properties. the key to finding a suitable non-VOC substitute lies in developing a suitable processing scheme which eschews volatile organic solvent.
REFERENCES
R. Brady. Journal of Protective Coatings and Linings. April 1989.49 (1989).
F. Mansfeld and M. Kendig. “Electrochemical Impedance Tests for Protective Coatings,” in Laboratow Corms ion Tests and Standa rds, ASTM STP 866, G.S. Haynes and R. Baboian, eds.. ASTM. Philadelphia, PA, 1985.
M. Kendig and J. Scully. Corrosion, g(1). 22,1990.
R.K. Meyers. C.J. Knauss. “Applied Polymer Science.” R.W. Tess. G.W. Poehlein, eds.; American Chemical Society. Washington, DC. 750 (1985).
J.E.O. Mayne, “The Cross-Linking and Adhesion to Mild Steel of Epoxy-Polyamine Film.” in 1, r ani atin 9, D. Scantlebury and M. Kendig eds., Electrochemical Society. Pennington. NJ. 1989.
The authors acknowledge the help and personal communication of data obtained by Dr. Richard Granata of Lehigh University, Bethlehem, PA, 1991.
ACKNOWLEDGEMENT
This work was performed with the support of the Office of Naval Research under contract “14-90-C-0110. The authors gratefully acknowledge initial guiflance from Dr. John Sedriks of the Office of Naval Research and Richard Johnson of the Space Systems Division of Rockwell Intemational for supplying some of the samples and for useful discussions.
FIGURE 4 - The time dependence of the coating FIGURE 1 - Coating resistance, R 0. as a capacitance for the indicated coatings exposed to function of the time of exposure to a 0. M NaCl air-equilibratd 0.5 M NaCI. equilibrated with the air at room temperature.
P
Time lhounl
FIGURE 2 - ASTM D610 corrosion rating (10 = n o corrosion, 1= 50% degradation) as a function of the time of exposure to a 0.5 M NaCl equilibrated with the air at room temperature.
- maawum --o- "noman 4 " * W O 4 " A I M I P O )
FIGURE 3 - The time dependence for the open- FIGURE 5 - Photographs of the samples after Circuit or comsion potential (Ec) for steel coated exposure: a) conventional high VOC coating, b) with the indicated coatings. water-based primer, c) high solids primer.
DI
Fig. 5(c)
-c NoSolvenl - Soivenl
u) r - - No soivenl
HGURE 8 - & as a function of time for the model coatings exposed to air-equilibrated 0.5 M NaCI.
12 ,
- No solvent - Solvent
4 - -.
0 m, a0 600 8m I m o Time. h
FIGURE 6 - Coating resistance. Rpo, as a function of time for the model coatings exposed 0 am 4oa 600 800 IWO
2 1
to &-equilibrated 0.5 M NaCI. Time.h
J u
Tfmm. h
FIGURE 7 - Coating capacitance. Cc. as a function of time for the model coatings exposed to air-equilibrated 0.5 M NacI.
J?IGURE 9 - ASTM D610 corrosion rating for the model coatings exposed to air-equilibrated 0.5 M NaCI.
TABLE I AEROSPACE COATINGS '
Conventional High Solids Water-based Primer Primer Qtegw
Primer
Induction time 30 min 30 min none Resin Epoxy/Amine Epoxy/poly amide Epoxy/poly amide %Solids 58-62 75.5 35.7
647 340 340 None MIL-P-23377F MLP85582
VOc(g/L) Mil. Spec
Type I, Class 2
Active Pigment Ca Chromate Sr chromate ' Sr chromate Conc wt. % 35-39 52 r w 30
i I
c " _
TABLE 2 MODEL COATING
No Solvent Film
Resin: AlarlditeCB 9513 Hardener: HY837CB (1) Span 80@ (sorbitan mono- oleate) (2) Xylene (solvent for Span SO)
Cure: 2 days at RT or 30 min at 1 W C Pot life: 30 minutes
100 32
0.25 2.25
Solvent Containing Film
Resin: Epon 100lCB (50% in sovent*) (3) 100 15 Hardener: HY 537(50% in solvent**) (1)
MultiflowO (4) 2
Solvents
Epon solvent* 2 ethoxyethanol methyl isobutylketone n-propanol
107g 4% 8g
Hardener solvent** 2 ethyoxyethanol isopropanol 4og xylene 8g
(1) Ciba-Geigy (2) IC1 America, Inc. (3) Shell Chemical (4) Monsanto
I
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