Znd. Eng. Chem. Res. 1993,32, 818-825
818
Interfacial Chemistry of a Single-Step Phosphate/Paint System Chhiu-Tsu Lin,’ Ping Lin, and Fernando Quitian-Puello Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115-2862
The nature of chemical mechanisms, bondings, and structures in a single-step polyester-melamine enarnel/HsPOr system, in particular, at the interface of polymer/phosphate/steel,was investigated by using ac impedance analysis and 80° grazing angle reflectance Fourier transform infrared spectroscopy. An in-situ phosphatizing coating on “Q” panel was illustrated and shown to behave like a pure capacitance. The “simultaneous” chemical reaction of &PO4 with “Q” panel surface and polymer resin was evidenced to produce a corrosion protective barrier by the formation of interfacial layer of crystalline iron phosphate and to enhance coating adhesion by the covalent bondings of P-0-C linkage. The technological and environmental impacts of in-situ phosphating coating process as compared to those of state-of-the-art multistep painting techniques are discussed.
I. Introduction Phosphate treatment technology is used in industry for two primary reasons (Hall, 1978);to improve the adhesion of paint and to enhance the resistance to substrate corrosion thereby achieving a longer finish life. The adhesion of steel/phosphate/polymer systems has two origins: mechanical (Wake, 1976) and chemical (Kaelble, 1970). For phosphate conversion coating on “Q” panel, the formation of P-0-C primary bonds at the polymer/ phosphate interface and of P=O/Fe chemical complexes at the phosphate/steel interface should give strong coating adhesion. The tendency of a metallic substrate to corrode is a function of the following factors (Hare, 1978): the surface characteristics of the metal and of the metal/ protective film interface; the physical, electrical, and electrochemical properties of such film; and the nature of the environment in which the system is placed. A good corrosion protective barrier may be formed by a nonconductive interfacial layer of metal phosphate together with a good quality of paint film that behaves like a pure capacitance (Leidheiser, 1979). The quality of finish required by an industrial product determines the degree to which the surface pretreatment and phosphating are carried out in the multistep process. The present state-of-the-art phosphate conversion coating involves a multistep process, Le., phosphate chemistry and polymer chemistry proceed independently and separately, and considerable energy, material, labor, and control. Recently, a single-stepphosphate/paint system comprised of polyester-melamine enamels and H3P04 was successfully formulated in our laboratory (Lin et al., 1992b), where H3P04 is admixed and formulated in the paint formula to form a one-pack composition. The phosphate chemistry and polymer chemistry are expected to take place independently, but simultaneously. The “simultaneous” chemical reaction of in-situ phosphatizing reagent, HsP04, with metal substrate and polymer resin should form a superior surface coating. As is often the case in coexist system reactions the advantages of the combined system (i.e., a single-step process) are greater than the s u m of those of the systems’ components (i.e., a multistep process). Good formulations are the foundation of any coatings business. A full factorial experiment on the formulation of a single-step polyester-melamine enamel/HaPOr system has been conducted (Lin et al., 1992a) to obtain an optimal composition and processing parameter. The
* To whom correspondence should be addressed.
optimal factors and levels for the single-step unicoat paint film (i.e., referred to as formula 143-117)on “Q” panel are H3P04acid concentration = 1.53’% in weight percent based on the total weight of the paint formula, volatile organic compounds(VOCs) type = isopropyl alcohol, cross-linker/ vehicle (CLA/VEH)ratio = 0.22, dry film thickness (DFT) = 1.0 mil, and bake schedule = 325 OF for 10 min. The coating formula 143-117on “Q” panel was exposed (Meldrum and Lin, 1993) to the standard salt spray, humidity, and outdoor fence testings. The visually observed film depredation results were compared to those of multistep commercial coatings. Four “clear” bake enamel systems-water-borne polyurethane (referred to as UQ and UB), solvent-borne phenolic modified alkyd (referred to as PQ and PB), solvent-borne epoxy (referred to as EQ and EB), and water-reducible alkyd (referred to as AQ and AB)-were used for comparison. Q and B referred to paint coatings on “Q“ panel and Bonderite lo00 panel (Parker-Amchem),respectively. The testing modes considered were scribe corrosion, creep, delamination, wet adhesion, recovery, blistering, filiform corrosion, surface corrosion, fading, pitting, spotting, and any other effects described in the ASTM standards which were considered pertinent to the general performance. The results were ranked (Meldrum and Lin, 1993) as follows: for salt spray at 164 h, EB > UB > 143-117 > AB > AQ > PB > PQ > UQ > EQ; for humidity testing at 309 h, 143-117>UB >AB EB si PB>AQ>EQ>PQ>UQ; and for fence testing at 1600 h, UB > 143-117 si AB N PB > AQ > EB > UQ > PQ > EQ. In short, it was shown (Meldrum and Lin, 1993) that in each of the exposure cases the single-step system 143-117 performed with equivalent ranking to the Bonderite lo00 coated samples (i.e., a typical multistep process) and significantly better than the “Q” panel comparisons. The question is the fundamental understanding of chemical mechanisms, bondings, and structures of the insitu phosphating process, and their correlations to the “real-world” coating performance. The corrosion resistance and adhesion to the substrate of an in-situ phosphatizing paint coating on “Q”panel have been illustrated (Lin et al., 1992a,b) to depend on the amount of formulated, the thickness of paint film applied, and the time and temperature of thermal curing schedule. Thus, it was proposed (Lin et al., 1992a,b; Meldrum and Lin, 1993) that H3P04 in the unicoat system tends to diffuse to and react with the metal surface, providing a corrosionprotective barrier to the substrate and simultaneously making available the proper functionality to form chemical bondings with polymer resins. In this paper, we will
o a s a - 5 8 s 5 / 9 3 / 2 6 3 2 - Q 8 l a ~ ~ ~ .0~ l1993 o American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 819 examine the electrochemical properties of the in-situ phosphatizing coatings and show how the small variation of H3P04 in the unicoat system can change the coating performance. Corrosion parameters such as chargetransfer resistance (&) and coating parameters such as coating capacitance (C,) will be derived from electrochemical impedance spectroscopy (EIS)data by modeling the data with equivalent electrical circuit models. Evidence will be provided for (i) the elementary surface chemistry at the molecular level of metal phosphates on “Q” panel and (ii) the “simultaneous” chemical reaction of metal phosphates with polymer resins to form strong adhesive bondings of P-O-C linkage. The results will be used to demonstrate the benefits of a single-step selfphosphating process over those of multistep techniques.
11. Experimental Section Water-extendable polyester (Cargill resin no. 5778), cross-linking resin [hexakis(methoxymethyl)melamine, Resimene no. 745, referred to as HMMMI, 85%phosphoric acid, water, and organic solvents (e.g., isopropyl alcohol, butyl carbitol, etc.) were used to formulate a single-step phosphate/paint system. The optimal composition used in this investigation is referred to as 143-117. To verify the existence of P-0-C linkage between the metal phosphate interfacial layer and the polymer resin, polyester acid phosphate was also employed in the formulation of in-situ phosphatizing paints. The polyester acid phosphate was synthesized by one of our industrial collaborators; Chemical Products Division, Cargill, Inc. The uncoated mild steel panels [“Q” panel, SAE 1010: C 0.08-0.13%; Mn 0.3-0.6%, P(max) 0.04%, S(max) 0.05%3 were mechanically polished to a mirror-finished surface before coating. IR spectra were recorded using a Mattson Cygnus 25 FTIR spectrophotometer equipped with a Spectra Tech FT-80 grazing angle accessory. The absorbance or transmittance spectra of a thin coating are produced by ratioing the single-beamspectrum of the same substrate with the thin coating. The interferometer is purged with dry nitrogen. Water, carbon dioxide, and baseline corrections are necessary in most cases. To conduct electrochemical impedance spectroscopy (EIS) measurements, ac impedance data were obtained at the open-circuit potential using a PAR 273 potentiostat/ galvanostat and PAR 5210 lock-in amplifier. The coated “Q” panel was assembled on a Model KO 235 flat cell (EG&G Princeton Applied Research) which consists of a glass cylinder clamped horizontally between two end plates. One end plate houses the working electrode (i.e., the coated “Q” panel) and the other houses the counter electrode. The reference electrode is housed in a Luggin well, with a fixed Teflon Luggin capillary protruding from the bottom of the cell. The exposed area of the working electrode (Le., coated “Q” panel) to the cell solution (3% NaCl solution) is 1.13 cm2. This area was presoaked in the cell solution for 72 h before the measurement. The reference electrode is silver in silver chloride/saturated potassium chloride solution, and the counter electrode is platinum/rhodium (8cm2). A system software, Model 388 (EG&G Princeton Applied Research) was adopted to measure the response of an electrochemical system to ac excitations at frequencies from 100 kHz to 5 Hz using a 5 mV peak-to-peak single-sine technique, and from 11Hz to 10 mHz using some higher applied voltages of 5-30 mV multi-sine technique depending on the thickness of coated samples. The experimental data were acquired and stored using an IBM PS/2-30 computer and analyzed by means of Bode and Nyquist plots on a Epson FX-850 plotter.
111. Results and Discussion A. An Equivalent Electrical Circuit Model for the In-Situ Phosphatizing Coatings. Electrochemical impedance spectroscopy (EIS) has been proven (Strivens and Taylor, 1982;Scantleburg, 1981)to be a powerful tool for the study of coating performance and undercoated metallic corrosion. Corrosion parameters such as chargetransfer resistance (or faradiac corrosion resistance, &) and coating parameters such as coating capacitance (C,) can be derived from EIS data by modeling the data with equivalent electrical circuit models (EECM) (Gerrish and Dugger, 1981;Bard and Faulkner, 1980; Scully, 1986). Of the EECM reported in the literature, three models have been used to generalize the majority of the EIS data gathered on coated metals: (1)ageneral impedance model for coatings that behave like pure capacitance (Utrilla and Morcillo, 1983), (2) a general impedance model for coated metals (Kendiget al., 1983),and (3)a diffusion impedance, or Warburg, model (Walter, 1986). Factorial designs of a single-step polyester-melamine enamel/H~POrsystem manifested (Lin et al., 1992a) that %H3P04 has the largest main effect on the in-situ phosphatizing coating performance. Presumably, the best way to illustrate the chemical principle (i.e., the sensitive of in-situ phosphatizing reagent, H3P04) of a single-step coating technique is to model with EECM of the EIS data obtained for an in-situ phosphatizing paint composition with % H3PO4 at, below, and above its optimal value. 1. Pure Capacitance Model (Utrilla and Morcillo, 1983): Paint Formula 143-117 with H3P04 = 1.5 wt 7% (an OptimalValue). In EECM, the fundamental relation of interest for a pure capacitance is well-known (Utrilla and Morcillo, 1983). Briefly, a frequency-dependent impedance can be described mathematically ~ ( w= ) l4e4
(1) where 1 4 in ohmcm2 = X, = (wC)-l, C is capacitance, w (frequency in radian per second) = 27rf (f = frequency in hertz), and 8 is the phase angle. A plot of both the 4 and 8 versus, the logarithm of frequency logarithm of 1 is required for a proper description of the impedance, i.e.,
log 121= log(1/27rC) - log f (2) For a “perfect capacitance”, the Bode-magnitude diagram 4 versus log f ) should show a straight line with a (log 1 slope of -1 and the Bode-phase diagram (8in degrees versus log f ) should have a phase angle of -9OO. Figure 1shows the Bode-magnitude diagram (graph a) and Bode-phase diagram (graph b) for a single-step polyester-melamine enamel/l.5% H3P04 coating on “Q”panel, which display the characteristics of a pure capacitance coating. The coating capacitance C, can be determined by following the straight line in the Bode-magnitude diagram of Figure 1to the frequency where log w = 0 (Le., f = 0.16 Hz). The result gives C, = 2.2 X 10-lo F/cm2. Essentially, parameters which characterize the electrolyte/coating/metal system can be determined from measurements of the frequency dependence of the impedance, Z(w). A common way to determine these parameters is via a Cole-Cole (or Nyquist) plot in which-Z”(w) (complex component of impedance) is plotted versus Z’(w) (real component of impedance) for each frequency where Z(o) was measured. It is evidenced that the Nyquist plot in Figure ICshows only the presence of a high-frequency response indicative of a purely capacitive dielectric for the unicoat paint formula, 143-117,a t an acid concentration = 1.5%. The EECM analysis of the EIS data obtained for the in-situ phosphatizing paint formula 143-117 at acid
820 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
C1.l
riu
3 -2
,
I
I
I
I
X I
I
5
1 og Freqcimcv Wz)
f"
x
I,, ,
2'
* Rc
X
I
I
,
,
,
8 . EIUBEIO
X
Zf
0
0
(Ohlid
Figure 1. EIS measurementsand EECM analysis of 143-117system with 1.5% on "Q" panel. (a) Bode magnitude;(b) Bode phase; (c) Nyquist plot.
concentration = 1.5%, DFT = 1.0 mil, and bake schedule = 325 OF for 10 min, on "Q" panel seems to display a good quality of paint film and also have a nonconductive interfacial layer of metal phosphate as a good corrosion protective barrier. This analysis correlates well with the results of "real-world" performance (Meldrum and Lin, 1993). It is important to note that phosphate coatings prepared in the multistep processes are quite porous and begin to degenerate shortly after application if not quickly recoated with a suitable primer. In less than 4 days the unprotected iron phosphate film will deteriorate to a point where there is little corrosion protection (Hare, 1978). In a single-step in-situ phosphating process, the problem is avoided because the phosphate chemistry and the polymer chemistry in the unicoat system proceed independently and simultaneously. 2. Coated-Metal Model (Kendig et al., 1983): Paint Formula 143-117 with H3P04= 3 wt %. To completely characterize the impedance behavior of organic-coated metals (Epelbion et al., 1981;Mansfield and Kendig, 1984) under corroding conditions, a broad bandwidth frequency (e.g., 100 kHz to 10 mHz) modulating measurement must be performed. In general, information about the coating is obtained a t the higher frequencies, whereas corrosion process information is obtained at the lower frequencies. The insert EECM in Figure 2c for a coated metal electrode
Figure 2. EIS measurementsand EECM analysisof 143-117system with 3 % on "Q" panel. (a) Bode magnitude;(b)Bode phase; (c) Nyquist plot.
includes the coating resistance, R,, coating capacitance C,, as well as the faradaic impedance, Zf. The &represents impedance components associated with the electrochemistry at the coating/metal interface. Besides the paint film capacitance, e.g., C, calculated in the section above, another parameter of importance that is normally generated by impedance testing is the paint film resistance. The film capacitance is a function of the coating dielectric whereas the film resistance is a function of the insulating properties of the paint. A pure resistor should give a frequency-independent horizontal line in the Bode-magnitude diagram and a phase angle at Oo in the Bode-phase diagram. Figure 2 shows the Bode-magnitude plot (graph a) and Bode-phase plot (graph b) for a single-step polyester-melamine enamel/3% HsP04 paint film on "Q" panel. The plots are quite similar to the corresponding diagrams in Figure 1,except in the low-frequency range, i.e., f = 5CF10 mHz. For f < 50 mHz, 8 goes from -90° to Oo in Figure 2b and log 1 4becomes frequency independent in Figure 2a. At 50 mHz, the Bode-magnitude plot of Figure 2a gives
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 821
14 = R, + R,
= l/wC, = 2.8 X lo9ohm-cm2 (3) where Rn = solution resistance which is normally less than 500 ohm (Tait, 19891, R, N 2.8 X lo9 ohm.cm2, and C, is calculated as 1.1 X le9F/cm2. The faradaic impedance, Zf, may be considered as a parallel circuit of double-layer capacitance c d and a chargetransfer resistance Ret. Thus, the calculated capacitance of 1.1X le9 F/cm2should include the contributions from both coating capacitance C,and double-layer capacitance c d . Since the double-layer capacitance for a metal in an aqueous medium is on the order of 4.0 X lP F/cm2, the c d contribution to the calculated capacitance value seems to be quite small. In the same fashion, the reported resistance value of 2.8 X lo9 ohm*cm2should include the contributions form the coating resistance R, as well as from the charge-transfer resistance, Rd. The fact that C, (obtained from Figure 2a) > C, (obtained from Figure la) suggests a possible electrolyte penetration in the paint films of 143-117system with 3% H3P04 as resulting from the entrapping of an excess amount of H3P04 admixed in the paint formulation. For the coated-metal model (Epelbion et al., 19811, the impedance of this general EECM can have the following real (2% or 2') and imaginary (21, or 2") components:
The combination of ZR, and 2 1, in eq 4 by the elimination of w will give a circular equation of the form
A plot of 21, versus ZR, of eq 5 is characterized by a circle centered a t 2% = Rn + Rd2 and 21, = 0, and having a radius of RJ2. Experimentally, the semicircular form of eq 5 has been reproduced in a Nyquist plot using the data obtained from EIS measurements of a single-step polyester-melamine enamel/3 % H3P04 coating on "&" panel. This is displayed in Figure 2c. It is noted that 21, in the Nyquist plot of Figure 2c comes mainly from C, (coating capacitance). Its contribution falls to zero at high frequencies because it offers no impedance (i.e., 21, a 0-l). All of the current is charging current, and the impedance it sees is the ohmic resistance. As the frequency drops, the finite impedance of C, manifests itself as a significant 21,= l/wC,. At very low frequencies, the capacitance C, offers a high impedance and the current flow passes mostly through R, and Rn. The imaginary component, ZIm, falls off again. On the other hand, ZR, in the Nyquist plot of Figure 2c shows that R, + Rn = 3 X lo9 ohm-cm2. Since solution resistance RQis negligible ( G O O ohm; Tait, 1989) as compared to R, in the system, a large coating resistance R, N 3 X 109 ohm-cm2is displayed which is in agreement with that obtained from the Bode-magnitude diagram of Figure 2a. This suggeststhat the chemical system of paint formula 143-117 with Hap04 = 3% is kinetically rather sluggish with no observable mass-transfer effects. The diffusion-controlled processes (Warburg impedance) are not detected in Figure 2c, indicating the formation of a nonconductive coating/phosphate/metal interfacial layer. 3. Warburg Model (Walter, 1986): Paint Formula 143-117 with H$O4 = 0.5 wt %. Phosphating has a significant effect on the paint film, including the corrosion resistance and adhesion to the substrate. When the coating/phosphate/metal interfaciallayer is not completely formed or when a paint film is damaged so that the underlying metal is exposed, and a corrosive medium is
present (water, air, salt), differential aeration results in formation of a local couple ( h u s h , 1990) at the underprotected or damaged site. The bore metal, to which oxygen has unrestricted access, becomes a local cathode while the paint-covered area, which is more or less inaccessible to oxygen, becomes a local anode. Where the metal is not protected (or not well protected) by a phosphate coating, a corrosion process can proceed as follows: local anode: local cathode:
Fe
-
+
Fez+ 2e-
0, + 2H,O
+ 4e-
-
iron hydroxide precipitation: 2Fe2++ 40Hrust formation:
4Fe(OH), + 0, + 2H,O
(6)
40H-
(7)
2Fe(OH),I (8) 4Fe(OH), (9)
In cases where there is a well-protected phosphate coating between the paint and metal, the external current circuit is broken because of the insulating nature of the phosphate coating. Current can only flow through the pores in the phosphate coating, and the corrosion process is strongly hindered. According to this model, it would be anticipated that the corrosion resistance of paint films increases with the thickness of the phosphate coating and is inversely related to the porosity. For a thin (or not well-protected) phosphate coating, the Warburg impedance is expected. The diffusion impedance (or Warburg model) in a Bode-magnitude plot is a straight line with a slope of -1/2, and in a Bode-phase plot should display a phase angle of -45O. The impedance responses for an in-situ phosphating paint film on "Q" panel coated by 143-117formula with an acid concentration = 0.5 % (a thin phosphate coating) are displayed in Figure 3. The Bode-magnitude plot in Figure 3a shows a composite of at least three frequency ranges for log 14: a slope of almost -1 for log f = 2-5, a frequency-independent range for log f = 0.5-1.5, and a slope of almost -1/2 for log f = -2 to 0.5. The medium-frequency range of slope N 0 reflects the metal/electrolyte corrosion potential, and the low-frequency range of slope N -1/2 describes the diffusion-related impedance of the rate-limiting species. This complex EIS behavior of paint formula 143-117with 0.5 5% H3P04 is also reflected on the Bode-phase diagram as shown in Figure 3b. A broad Bode-phase transition with a wide range of scattering data may be described as 0 = -90" at log f = 5, -45" at log f = 4-2, -loo at log f = 1-0, and -2OO at log f = -2. These results strongly suggest that electrochemical reactions are quite active at the metal/ coating interface for a paint film of low acid concentration (0.5 w t %) of 143-117 system coated on "Q" panel. It is presumably due to the fact that, at this low acid concentration, there is not sufficient H3PO4 (i.e., well below its optimal value of 1.5 5% ) in the paint formulas to form a thick and well-protected phosphate coating and provide a corrosion-protective barrier to the metal substrate. The Nyquist plot and ita corresponding electrical equivalent circuit for paint film applied on "Q" panel by using the 143-117system a t an acid concentration = 0.5% are shown in Figure 3c. For a Warburg model, the calculated real and imaginary impedance of the electrical equivalent circuit can be expressed as
822 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 l-7
dependent terms in eq 13 come only from Warburg impedance contributions described in eq 12. Thus, a linear correlation of 21, and displays in Figure 3c is a characteristic of a diffusion-controlled electrode process. The in-situ phosphating paint film on "Q" panel coated by 143-117 formula with an acid concentration = 0.5% was exposed to a 3% NaCl solution. The Warburg coefficient, a, was calculated as 1.4 X 106 o h m / ~ l / ~AS. w (high-frequency limit), however, the Warburg impedance becomes unimportant in relation to Ret, and a simple restrictive charge-transfer process is obtained. The &e and 2 1,of eqs 10 and 11are reduced to those similar to eq 4, and the electrical equivalent circuit converges to that of Figure 2c. The values of Rctand cd are calculated to be 1.6 X lo6ohmcm2and 1.7 X lV F/cm2,respectively. The Nyquist plot of both Figure 2c and Figure 3c at high frequencies represent a kinetic-controlled corrosion process. B. Structures and Bondings of Phosphate Fundamentals at the Interface of Polymer/Phosphate/ Steel. In section A, the electrochemical properties of a paint film on "Q" panel finished by applying a single-step polyester-melamine enamel/HaPOd system are shown to be critically dependent on the exact amount of H3P04 admixed in the formulations. This dependence reflects the nature of chemical mechanisms of the in-situ phosphating process, where % H3P04 used should be enough only for the formation of interfacial layer of metal phosphate and simultaneously for catalyzing the polymerization. The basic understandings of phosphate fundamental at the molecular level, i.e., structures and bondings at the interface of polymer/phosphate/steel,are essential for the scientific and technological development of a single-step phosphate/paint system. A control experimenton in-situ phosphatizing paint was conducted by using hydrochloric acid in place of HsP04. The FTIR spectra illustrated that the H3P04 does not function simplyto activate the metal substrate, but rather to form a corrosion-protective metal phosphate layer. Additionally, the H3P04 provides a function of the acidcatalyzed cure of the coating which is similar to that by using p-toluenesulfonic acid (Lin et al., 1992b; Gan et al. 1989). Our recent results indicated that the ester phosphoric acid and phosphonic acids are better than H3P04 as in-situ phosphatizing reagents, in particular for a "pigmented" single-step phosphate/paint system. This is currently under investigation in our laboratory. In order to investigate the interfacial chemistry of the in-situ phosphatizing coatings, three types of interfacial layer of metal phosphate were prepared on a polished "Q" panel: (i) 85% H3P04, (ii) 143-117 system with different acid concentrations, and (iii) polyester acid phosphatemelamine enamel. All coated panels were cured at 325 OF for 10 min. The top polymer layer of each panel was removed by soaking it in the tetrahydrofuran solvent. The interfacial phosphate layer was rinsed with deionized water, dried, and characterized by FTIR spectroscopy equipped with a 80° grazing angle reflectance accessory. The results are shown in Figure 4. In a molecular graphic simulation, it was predicted (Lin et al., 1992b) that H3P04 is stable while the 143-117 system is stored in a container other than metal. Upon application on a "Q" panel, the admixed H3P04 diffuses to and reacts with the metal surface to form the iron phosphate layer IFeHP04, Fe(H2P0412, FeP04, and/or Fe3(P04)2]which is chemically bonded to the "Q" panel
--
h
I
I
I
1
ZRC (Ol-llll)
I
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I
I
4.1300EC
Figure 3. EIS measurementsand EECM analysis of 143-117system with0.5%H3P04on 'Q" panel. (a) Bode magnitude;(b) Bode phase; (c) Nyquist plot.
and
Z, = (2/~)'/~a (12) where 2, is the Warburg impedance which is related to the frequency, w , and Warburg coefficient, a. Warburg impedance is also called mass-transfer impedance and described as the diffusion-related impedance. For simplicity, let us consider the limiting behavior at high and low frequencies. As w 0 (low-frequency limit), eqs 10 and 11 approach the limiting forms
-
ZR,= R ,
+ R,, + aU-1/2, 21,= 2a2Cd +
(13) and can be combined to give eq 14 by the elimination of w. A plot of 2 1 , versus 2 , in eq 14 should be a straight line with a unit slope of +1,as shown in Figure 3c at the low-frequency range. It is noted that the frequencyaW-'/2
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 823 MlCraU
7.0
8.0
9.0
IJ
Figure 4. FTIR spectra of interfacial iron phosphate layer on "4"panel prepared by using (a) 85% H3P04, (b) polyester acid phosphatemelamine, (c) 143-117 system with 0.5% HsP04, and (d) 143-117 system with 1.5% H3P04.
and should promote a strong adhesion with polymer resin. Recently, there has been some growing evidence (Gorecki, 1991) that the composition of iron phosphate films (in particular, those films coated by 85% H3P04 on "Q" panel) is a mixture of ferrous (Fe3(P04)2) and ferric (FeP04) phosphates and oxides with little or no FeHP04 and Fe(HzP04)~present. The chemistry in the reported "amorphous" iron phosphate fiis (Gorecki, 1991)is clearly different from the commonly understood iron phosphate reaction (Hare, 1978). Fe
-
+ 2H3P04
-
Fe(H2P04),
Fe(H,PO,), + 2H primary iron phosphate
FeHPO, + H,P04 secondary iron phosphate
In this case, a fine crystalline layer of Fe(H2P04)2/FeHP04 on "Q"panel is expected. Incidentally, [P043-l, [HP042-l, and [HzP04-1 can be expressed (Satoet al., 1991;Bellamy, 1980;Colthup et al., 1964) as point groups Td, CsU,and C2u,respectively. The IR spectra of H3P04, HzP04-, HP0d2-,and Pod3-are wellknown. The fingerprint regions are 2800- and 2400-cm-' peaks for H 8 0 4 , 540- and 450-cm-1 peaks for HzP04-, 1230-and 1076-cm-' peaks for HP04%,and 1080-cm-1peak for PO4%. Pod3- is a tetrahedron structure (Sato et al., 1991;Bellamy, 1980;Colthup et al., 1964)with phosphorus as the center atom. The regular tetrahedron structure has four basic vibration modes: V I is the symmetrical stretching vibration AI, v2 is the double-degeneracy vibration E with deformation, v3 is the triple-degeneracy vibration F2 of stretching type, and v4 is the tripledegeneracy vibration F2 of deformationtype. Only US (9001200 cm-') and u4 (640 cm-') are active (Osaka et al., 1984;
Sato and Minami, 1989; Carter et al., 1986) in IR absorption. It is noted that, in the case of a solid crystal, the symmetry will become distorted by interaction with the surrounding crystalline structure. As a result, the degeneracy of the vibration modes will be united and split. Figure 4 shows the FTIR spectra of interfacial phosphate layer on polished "Q" panel prepared by (a) 85% H3P04, (b) polyester acid phosphate-melamine enamel, (c) 143117 system admixed with 0.5% by weight H3P04, and (d) 143-117 system admixed with 1.5% by weight H3P04 (an optimal acid concentration). The 3% H3P04 143-117 system has the same FTIR spectra as the 1.5% system shown in Figure 4d. In spectrum a, one strong and one weak broad band centered at 1078 and 612 cm-l are observed and respectively can be assigned to the v3 and v4 vibration modes in Pod3-,separately. This assignment indicates (Osaka et al., 1984) the presence of amorphous FePO$Fe3(P04)2 on the surface layer which is in agreement with the report of Gorecki (1991). The spectrum in Figure 4b is quite broad, but four spectral peaks are clearly identified at 1196,949,612, and 499 cm-'. The spectral assignment should be similar to that reported (Carter et al., 1986,1987)for epoxy acid phosphate thin film on steel surface. Both polyester and epoxy acid phosphates have functional groups, polymer -[C-O-PO(OH)21. The spectral peaks at 1196, 949, 612, and 499 cm-' are assigned (Carter et al., 1986,1987)asP4stretchingmode ofP=O/ Fe complex, P-0 stretching mode of P-0-C linkage, u4 vibration mode in P04%,and the skeletal deformation of the PO4group. The skeletal deformation of the PO4 group seems also active in Figure 4a of 85 % H3P04 treated "Q" panel. The infrared spectral activities of the interfacial metal phosphate layer prepared by using a single-stepphosphate/ paint system are expected to be a combination of those in
824 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
Figure 4a,b. For the 143-117 system with 0.5% H3P04 (Figure 4c), the spectral peak a t 940 cm-l is definitely due to the formation of P-0-C linkage whereas that at 1123 cm-1 can be assigned to the v3 vibration mode in Pod3-. A shift to higher frequency at 1123 cm-l in Figure 4c as compared to that at 1078cm-’ in Figure 4a may be caused (Osaka et al., 1984) by an increase in the bond order of P-0 bonds due to a larger ionicity of the stronger counterpart P=O/Fe complex bonds. In Figure 4d, the 143-117 system with an optimal acid concentration 1.5% H3P04,the spectral peak at 1195 cm-l corresponding to the P-0 stretching mode of P=O/Fe complex is clearly observed. The degeneracy of both v3 and v4 vibration modes in Pod3- becomes united and is observed to split into 1147/1105 and 625/521 cm-l, respectively. The splitting of degeneracy resulted from the symmetry distortion by interaction with the surrounding crystalline structure. In fact, the observation of 625/521-cm-l doublet may be attributed (Osaka et al., 1984)to the formation of crystalline iron phosphate layer (e.g., or HP04’-) on “Q” panel surface. This enhances the corrosionprotective barrier. Similar to Figure 4c, a strong covalent bond P-0-C linkage is evidenced at 934 cm-l, providing a coating adhesion at the polymer/phosphate interface. C. Technological, Economical, and Environmental Impacts. A single-step phosphate/paint system incorporates the phosphatizing reagents (e.g., H3P04) directly in the coating formula to produce a stable and compatible “one-pack”paint composition. Upon application on metal substrate (“Q” panel or aluminum), the phosphate chemistry and polymer chemistry in the unicoat system are expected to proceed independently and simultaneously. The “simultaneous” chemical reaction of in-situ phosphatizing reagent with metal substrate and polymer resin should provide an excellent corrosion-protective barrier (described in section A) by the formation of crystalline metal phosphate layer (describedin section B)and enhance the coating adhesion by the covalent bonding of P-0-C linkage (described in section B). The present state-of-the-art paint coating involves a multistep process: surface phosphating, primer and top coat applications,and considerableenergy, material, labor, and control. The “clear”single-stepformulation combines the first two, and the “pigmented” unicoat system incorporates all three in one application. It is expected that in coexist system reactions the advantages of the combined system in a single-step process after often greater than the sum of those of the systems’components in a multistep process. Technologically and economically, the singlestep phosphate/paint system will provide increased finish quality (adhesion and corrosion protection) as has been illustrated in the “real-world”testing (Meldrum and Lin, 1993), without the capital and operating expenses of a separate phosphating line; Le., it will save energy, time, labor, resources and money. More importantly, the unicoat system uses only a required amount of phosphatizing reagents, thus preventing the waste of valuable natural resources. The single-stepphosphate/coating technique eliminates the phosphatizing bath and phosphating line. Hexavalent chromium is highly toxic and is used currently in the multistep process to seal the surface of phosphatized metal components with a chromic acid rinse prior to painting. Environmentally, the elimination of phosphatizing bath will result in no toxic sludge generation. Moreover, the phosphatizing reagents used in the unicoat system are hydrophilic in nature; thus water is formulated as one of the solvent components. The single-step coating design
allows for a reduction in total volatile organic contents (VOCs) to be less than the EPA 3.0 lb/gal mandate. The present design in our laboratory is at 2.8 with 2.5 lb/gal a very real possibility. IV. Conclusion The singlestep polyester-melamine enamel/lb% HiPo4 coating on “Q“ panel displays a superior surface finish as has been tested from a “real-world” performance and behaves like a pure capacitance. At 0.5 % H3P04, the EIS measurements andEECM analysis indicate that “Warburg type” electrochemical reactions are quite active at the metal/coating interface. At this low acid concentration, there is not sufficient H3P04 in the paint formulas to diffuse to and react with “Q” panel surface, and provide a corrosion-protective barrier to the substrate. A kinetic control EECM paint f i i is detected for the 143-117system with 3% H3P04, suggesting that the excess amount of H3P04 admixed in the paint formula may have been entrapped in the paint film as electrolytes. 85% H3P04 treated “Q” panel surface comprises amorphous iron phosphates [e.g., FeP04/Fe3(P04)2Is FTIR studies of polymer/phosphate/substrateproduced by insitu phosphating coatings on “8” panel show that (1) crystalline iron phosphates [e.g., FeHP04/Fe(HzP04)21 are evidenced a t the phosphate/substrate interface and (2) P-O-C linkage of covalent bonds are formed at the polymer/phosphate interface. A conversion electron Mijssbauer spectroscopy and X-ray diffraction technique are currently being used in our laboratory to further confirm these findings. These results together with the technological, economical,and environmental impacts are used to draw the benefits of a single-step phosphate/paint process over those of a multistep technique. Acknowledgment Financial support from the Illinois Department of Commerce and Community Affairs, a State of Illinois Technology Challenge Grant, Finishes Unlimited, Inc., Cargill and Northern Illinois University Graduate School, College of Liberal Arts and Sciences, and Technology Commercialization Center is gratefully acknowledged. Literature Cited Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980. Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; Chapman and Hall: New York, 1980; Vols. 1 and 2. Carter 111, R. 0.; Gierczak, C. A.; Dickie, R. A. The Chemical Interaction of Organic Materials with Metal Substrates. Part I 1 FT-IR Studies of Organic Phosphate Films on Steel. Appl. Spectrosc. 1986,40, 649-655. Carter 111, R. 0.;Parsons, J. L.; Holubka, J. W. Synthesis and Characterization of Epoxy Phosphate Steel Surface Modifiers. Znd. Eng. Chem. Res. 1987,26, 1518-1523. Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1964. Epelbion, I.; Gabrielli, C.; Keddam, M.; Takenouti, H. Alternating Current ImpedanceMeasurementa Applied to Corrosion Studies and CorrosionRate Determination. InElectrochemical Corrosion Testing; Mansfeld, F., Bertocci, U., Eds.; Am. Soc. Test. Mater. (ASTM): Philadelphia, 1981; STP 727, p 150. Gan, S.; Solimeno, R. D.; Jones, F. N., Hill, L. W. Recent Studies of the Curing of Polyester-Melamine Enamels. Possible Causes of Overbake Softening. Proceedings of the Sixteenth Water-Borne and Higher-Solid Coatings Symposium, New Orleans;University of Southern Mississippi: Hattiesburg, MS, 1989; pp 87-108. Gerrish, H. H.; Dugger, W. E. Transistor Electronics; GoodheartWillcox Co.: South Holland, 1981; pp 144-152.
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 825 Gorecki, H. Corrosion 91;National Association of Corrosion Engineers, Inc.; Houston, TX, 19911;conference paper 381. Hall, W. S. Theory and Practice of Coating by Autodeposition. J. Water Borne Coat. 1978,Aug, 2-10. Hare, C. H. Corrosion and the Preparation of Metallic Surfacesfor Painting,Federations Series on Coating Technology;Federation of Societies for Paint Technology: Philadelphia, PA, 1978;Unit 26,pp 5-50. Kaelble, D. H. Physical Chemistry of Adhesion; Wiley: New York, 1971;Chapter 5, p 180. Kendig, M. W.; Mansfeld, F.; Tsai, S. Determination of the Long Term Corrosion Behavior of Coated Steel with AC Impedance Measurements. Corros. Sci. 1983,23,317. Leidheiser, Jr., H. Electrical and Electrochemical Measurements as Predictors of Corrosion at the Metal Organic Interface. Prog. Org. Coat. 1979,7,79. Lin, C. T.; Lin, P.; Hsiao, M. W.; Burd, S. L.; Meldrum, D. A. Factorial Designs of a Single-Step Phosphate/Pain System. Proceedings of the Nineteenth Annual Water-Borne & Higher Solid Coatings Symposium, New Orleans; University of Southern Mississippi: Hattisburg, MS, 1992a;pp 431-452. Lin, C. T.; Lin, P.; Hsiao, M. W.; Meldrum, D. A,; Martin, F. L. Chemistry of a Single-Step Phosphate/Paint System. Ind. Eng. Chem. Res. 1992b,31,424-430. Manefield, F.; Kendig, M. Evaluation of Protective Coatings with Impedance Measurements. Proceedings of the International Congress Metallic Corrosion Toronto, Canada; NRCC, Ottawa, 1984,pp 74-84. Meldrum, D. A,; Lin, C. T. AC Impedance Analysis and Factorial Designs of an In-Situ Phosphatizing Coating. J. Coat. Technol. 1993,in press. Osaka, A.; Takahashi, K.; Ikeda, M. Infrared Study of Trivalent Cations B and Fe in Amorphous and Crystalline Phosphates. J. Mater. Sci. Lett. 1984,3, 36-38.
Rausch, W. The Phosphating of Metals; ASM International: Metal Park, OH, and Finishing Publications LTD: Teddington, Middlesex, England, 1990. Sato, N.; Minami, T. Influence of Metal Components in Hopeite Films on IR and Laser Raman Spectra. J. Mater. Sci. 1989,24, 3375-3379. Sato, N.;Watanabe, K.; Minami, T. Study on Attribution of Laser Raman Spectroscopy for Hopeite Crystal Films. J. Mater. Sci. 1991,26,1383-1386. Scantlebury, J. D.; Ho, K. N.; Eden, D. A. Impedance Measurements on Organic Coatings on Mild Steel in Sodium Chloride. In Electrochemical Corrosion Testing; Mansfeld, F., Bertocci, U., Eds.; Am. SOC.Test. Mater. (ASTM): Philadelphia, 1981;STP 727,p 187. Scully, J. “Electrochemical Impedance Spectroscopy for Evaluation of Organic Coating Deterioration and under Film Corrosion, A State of the Art Technical Review”; David W. Taylor Research Center Report SME-86/006, September 1986. Strivens, T. A., Taylor, C. C. An Assessment of AC Impedance as a Basic Research and Routine Testing Method for Studying Corrosion of Metals under Paint Films. Mater. Chem. 1982,7 , 199. Tait, W. S. Using Electrochemical Impedance Spectroscopy to Study Corrosion Behavior of Internally Coated Metal Containers. J. Coat. Technol. 1989,61,57-61. Utrilla, F.; Morcillo, M. Rev. Zberoam. Corros. Prot. 1983,14,251. Wake, W. C. Adhesion and the Formulation of Adhesives; Applied Science: Barking, Essex, England, 1976;Chapter 5, p 65. Water, G. W. Corros. Sci. 1986,26,39. Received for review August 31, 1992 Revised manuscript received January 25, 1993 Accepted January 30,1993
