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Polarization and impedance studies on zinc phosphate coating developed using galvanic coupling.

Abstract Galvanic coupling technique is capable of producing coatings of desired thickness. Good quality coatings can be produced at low temperature. Galvanic coupling of mild steel (MS) with the other cathode materials such as titanium (Ti), copper (Cu), brass (BR), nickel (Ni), and stainless steel (SS) accelerates iron dissolution, enables quicker consumption of free phosphoric acid and facilitates an earlier attainment of point of incipient precipitation, resulting in a higher amount of coating formation. In the present investigation, potenliodynamic polarization and electrochemical impedance spectra on MS substrates phosphated using galvanic coupling are studied. This study reveals that MS substrates phosphated under galvanically coupled condition possess better corrosion resistance than the substrates phosphated under uncoupled condition.

Keywords Cathode materials, Phosphatability, Corrosion resistance. Mild steel, Galvanic coupling

Introduction

Chemical surface conversion treatment of metals to modify their intrinsic properties and to provide new physical or physico-chemical characteristics is well known. Among the many possibilities available, phosphating has always occupied a position of prime importance. Phosphating is the most widely used metal pretreatment process for the surface treatment and finishing of ferrous and nonferrous metals. Owing to its economy, speed of operation, and ability to afford excellent corrosion resistance, wear resistance, adhesion, and lubricative properties, it plays a significant role in the automobile process and appliance industries. (1-3) The majority of the zinc-phosphating baths require very high operating temperatures on the order of 90[degrees]C or more. Overheating of the bath solution causes an early conversion of the primary phosphate to tertiary phosphate before the metal has been treated and the free acidity of the bath increases, which consequently delays the precipitation of the phosphate coating. The decisive role of overheating the phosphating baths was discussed by Sankara Narayanan et al. (4) Escalating energy costs and the persistent processing problems, such as overheating of the bath and the difficulty in maintaining the heating coils, warrant the development of phosphating baths that are capable of operating at relatively lower temperatures.

The low temperature phosphating processes have become more significant today because of economic reasons. However, the limitation of low temperature processes is that these processes involve greater consumption of chemicals and longer process time, in addition to limited process flexibility. Using galvanic coupling technique, these low temperature processes have been developed to perform as well as the high temperature processes with other associated advantages, such as shorter start-up time, minimal evaporation losses, less wear on equipment, a better working environment, and avoidance of the use of heating coils. (5-7) The utility of galvanic coupling on the phosphatabihty of mild steel (MS) with the other cathode materials such as titanium, copper, brass, nickel, and stainless steel was established recently. (8-11) It is understandable that galvanic coupling accelerates the initial metal dissolution reaction and enables an earlier attainment of the point of incipient precipitation (PIP) i.e., the point at which saturation of metal dissolution occurs and higher coating weight results. (8), (9), (12), (13) Chemical methods of testing the phosphate coatings obtained under galvanically coupled and uncoupled conditions have been reported elsewhere. (12)

The electrochemical methods for testing the phosphate coating mainly involve the anodic polarization studies in 0.6 M ammonium nitrate (10), (11) and AC impedance measurements in 3% sodium chloride solution. (14-16)

The anodic polarization studies in 0.6 M ammonium nitrate solution is based on the active-passive transition with applied potential. During anodic polarization in 0.6 M ammonium nitrate solution, at potentials more negative than -0.33 V, phosphated steel undergoes active dissolution. Above -0.33 V, the first passivation region occurs because of the adsorption of hydroxide ions at the electrode surface. The occurrence of the second active region is due to the replacement of hydroxide ions by phosphate ions available at the electrode/solution interface. Replacement of the adsorbed phosphate ions by nitrate causes the occurrence of the second passive region. Hence it is clear that these active and passive regions are the result of the competitive and potential-dependent adsorption of anions at the electrode surface. It should be noted here that the appearance of this second current density maximum is specific to phosphated steel and it is not observed for uncoated steel when tested under similar conditions. Hence, the value of the second current density maximum can be used to evaluate the corrosion resistance of different phosphate coatings. (10), (11)

AC impedance studies involve the measurement of the charge transfer resistance ([R.sub.ct]), the double layer capacitance ([C.sub.d1]) and the Warburg impedance ([Z.sub.w]), the parameters that constitute the proposed equivalent electrical circuit model in correlation with the behavior of coated system in a corrosive environment. Literature reports on the evaluation of phosphate coatings (17-19) suggest that the corrosion behavior of phosphate coatings in contact with the corrosive medium (3.5% NaCl) can be explained on the basis of a porous film model since the electrolyte/coating-metal interface approximates such a model. Accordingly, the phosphated substrates are considered as partially blocked electrodes when they come in contact with 3.5% NaCl solution. This implies that the metal substrate is corroding in the same way when protected in a much smaller area where coverage is lacking. Since the capacitive and resistive contributions vary directly and indirectly, respectively, with regards to the area, based on these measured parameters, predictions on the corrosion rate of different phosphate coatings can be easily made. High values of charge transfer resistance and a low value of double layer capacitance signifies a coaling of better performance. The appearance of Warburg impedance helps in predicting whether or not the process is diffusion controlled. (20), (21)

Experimental

Mild steel specimens of dimensions 8.0 cm x 6.0 cm x 0.2 cm were used as the substrate materials for the deposition of zinc phosphate coating. Titanium, copper, brass, nickel, and stainless steel (AIS1 304 grade) substrates were utilized to create the galvanic couple with MS substrate with varying anodic-to-cathodic area ratio. (12) The chemical composition of the MS substrate and the cathodic materials used have been reported.12 The schematic of the experimental setup used for the phosphating process is shown in Fig. 1. The chemical composition of the zinc phosphaling bath and its operating conditions are given in Table 1. Phosphating was done by immersion process at room temperature (27[degrees]C) for 30 min. The amount of iron dissolved during phosphating and coating weight were determined in accordance with the standard procedures1 and reported.
Table 1: Chemical composition, control parameters, and operating
conditions of the bath used for zinc phosphating by galvanic coupling

Chemical composition [Z.sub.n]O            5g/L

  [H.sub.3]P[O.sub.4]                 11.3 mL/L
  [N.sub.a]N[O.sub.2]                      2g/L

Control parameters

  pH                                       2.70
  Free acid value (FA)               3 pointage
  Total acid value (TA)             25 pointage
  FA:TA                                  1:8:33

Operating condition

  Temperature                      27[degrees]C
  Time                                   30 min


Potentiodynamic polarization and electrochemical impedance studies of MS panels phosphated using galvanic coupling were carried out using a polentiostat/ galvanostat/frequency response analyzer of ACM Instruments (Model: grill AC). A 3.5% sodium chloride solution was used as the electrolyte. The solution temperature was maintained at 27 [+ or -] 1[degrees]C. Only 1 cm2 area of the phosphated MS was exposed to the electrolyte. The remaining areas were sealed by a masking tape. Potentiodynamic polarization and electrochemical impedance measurements were carried out at the open circuit potential. The corrosion cell consisted of four electrodes. The phosphated MS substrate formed the working electrode, whereas a saturated calomel electrode and a platinum electrode served as the reference and counter electrodes, respectively. The fourth electrode was a platinum electrode, which was connected to the reference electrode, to reduce the noise during the electrochemical measurement. Potcntiodynamic polarization measurements were made at a potential scan rate of 100 mV/min. The corrosion potential ([E.sub.corr]) and corrosion current density ([i.sub.corr]) were determined using Tafel extrapolation method. Electrochemical impedance studies were carried out in the frequency range between 10000 and 0.01 Hz, The charge transfer resistance ([R.sub.ct]) and double layer capacitance ([C.sub.d1]) were determined from Nyquist plot by fitting the data using Boukamp software. Electrochemical impedance studies were also carried out for phosphated and painted MS panels at their respective open circuit potential.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Results and discussion

The effect of galvanic coupling of MS substrate with titanium, copper, brass, nickel, and stainless steel substrates on the amount of iron dissolved during phosphating and coating weight have been reported. (12) From the reports, it was evident that the extent of metal dissolution and coating formation is higher for MS substrates phosphated under galvanically coupled condition than the one coated without coupling. When the MS substrate is galvanically coupled to cathodic substrates, the surface sites for hydrogen evolution reaction is changed from MS to cathodic substrate, making more surface sites available on the MS substrate for coating formation. This has resulted in the formation of higher coating weight.

Potentiodynamic polarization study

The polarization curves obtained for panels phosphated under coupled and uncoupled condition are shown in Figs. 2-7. The corrosion potential ([E.sub.corr]) and corrosion current density ([i.sub.corr]) calculated using Tafel extrapolation method are given in Table 2.
Table 2: Corrosion potentials ([E.sub.corr]), corrosion current
densities ([i.sub.corr]), and corrosion rates of zinc phosphate-coated
mild steel substrates under galvanically coupled and uncoupled
conditions in 3.5% sodium chloride solution

System studied           [E.sub.corr]  [i.sub.corr]  Corrosion
                         (mV) vs SCE    ([mu]A/cm)     rate
                                                       (mpy)

Uncoupled mild steel             -685         21.70       9.92

Mild steel coupled with          -649         16.51       7.54
stainless steel (area
ratio-MS:SS-1:1)

Mild steel coupled with          -645         14.17       6.47
stainless steel (area
ratio-MS:SS-1:2)

Mild steel coupled with          -627         10.83       4.94
stainless steel (area
ratio-MS:SS-1:3)

Mild steel coupled with          -644         13.80       6.29
nickel (area
ratio-MS:Ni-1:1)

Mild steel coupled with          -629         11.22       5.12
nickel (area
ratio-MS:Ni-1:2)

Mild steel coupled with          -623          6.76       3.08
nickel (area
ratio-MS:Ni-1:3)

Mild steel coupled with          -621          6.72       3.06
brass (area
ratio-MS:brass-1:1)

Mild steel coupled with          -611          5.69       2.60
brass (area
ratio-MS:brass-1:2)

Mild steel coupled with          -578          1.14       0.52
brass (area
ratio-MS:brass-1:3)

Mild steel coupled with          -615          5.78       2.64
copper (area
ratio-MS:Cu-1:1)

Mild steel coupled with          -587          2.56       1.17
copper (area
ratio-MS:Cu-1:2)

Mild steel coupled with          -575          1.02       0.47
copper (area
ratio-MS:Cu-1:3)

Mild steel coupled with          -607          4.36       1.99
titanium (area
ratio-MS:Ti-1:1)

Mild steel coupled with          -580          1.28       0.59
titanium (area
ratio-MS:Ti-1:2)

Mild steel coupled with          -573          0.90       0.41
titanium (area
ratio-MS:Ti-1:3)


It is evident from the figures and table previously mentioned that the [E.sub.corr] shift toward less negative values for panels phosphated under galvanically coupled condition. The extent of shift in potential is largely a function of phosphate coating weight and the porosity of the coating. Higher coating weights and low porosity of the coatings were reported for the MS substrates phosphated by coupling with more noble metals by the authors elsewhere. (12) Since the coating weight increases with increase in galvanic potential and the cathode-to-anode area ratio, a larger anodic shift in corrosion potential is observed for panels phosphated using the MS-Ti couple with 1:3 area ratio. The corrosion current density is lower for panels phosphated under galvanically coupled condition than for panels phosphated without galvanic coupling. This could be attributed to the decrease in porosity of the coatings obtained under galvanically coupled conditions. For panels phosphated using different galvanic couples, the corrosion current density is less for those obtained using the MS-Ti couple with 1:3 area ratio.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Electrochemical impedance spectroscopy study

Comparison of Nyquist plots of phosphated MS substrates under galvanically coupled and uncoupled conditions with varying area ratios (1:1, 1:2, and 1:3) in 3.5% NaCl are shown in Figs. 8-12. Usually, the Nyquist plot of MS phosphated by conventional chemical method consists of a semicircle in the high-frequency region followed by a diffusion tail in the low frequency region. This implies that the corrosion of phosphated steel is diffusion controlled. (22)In the present study also, the Nyquist curves obtained appear to be similar; a semicircle in the high-frequency region followed by a diffusion tail, characteristic of Warburg impedance behavior, in the low-frequency region. (23), (24)

To account for the corrosion behavior of phosphated panels in 3% sodium chloride solution, an equivalent electrical circuit model (Fig. 13) is proposed, since the electrolyte/coating-metal interface approximates such a model. This model is similar to the one proposed by Meszaros et al. (25) and Weng et al. (17) to study the porosity of the phosphate coating.

The resistance and capacitance were included in the model to account for the changes taking place at the interface which will give a quantitative value to the resistance and capacitance of the coating. The diffusion impedance, [Z.sub.w] is introduced in the model, since it is known that the transport matter can have considerable influence on the kinetics of the bare metal electrode. (26)

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The corrosion potential ([E.sub.corr]), a thermodynamic parameter which indicates the tendency of the metal to corrode, is also included among the parameters, because the shift in this potential gives an idea about the diffusion of water and oxygen in the metal/coating interface and the corrosion rate.

The values of the corrosion potential ([E.sub.corr]), charge transfer resistance ([R.sub.ct]), and double-layer capacitance ([C.sub.d1]) obtained for panels coated in the uncoupled and galvanically coupled conditions arc presented in Table 3.
Table 3: Change transfer resistance([R.sub.ct]) and double layer
capacitance ([C.sub.d1]) of zinc phosphate-coated mild steel substrates
under uncoupled and gaivanically coupled conditions in 3.5% sodium
chloride solution at their respective open circuit potentials

System studied           OCP   [R.sub.ct](  [C.sub.d1] (F) x
                         (mV)    [OMEGA]      [10.sup.-5]
                          vs   c[m.sup.2])
                         SCE

Uncoupled mild steel     -685         1057              34.8

Mild steel coupled with  -649         1235             13.80
stainless steel (area
ratio-MS:SS-1:1)

Mild steel coupled with  -645         6336              5.87
stainless steel (area
ratio-MS:SS-1:2)

Mild steel coupled with  -627        12980              4.59
stainless steel (area
ratio-MS:SS-1:3)

Mild steel coupled with  -644         1380             10.90
nickel (area
ratio-MS:Ni-1:1)

Mild steel coupled with  -629         6795              5.60
nickel (area
ratio-MS:Ni-1:2)

Mild steel coupled with  -623        13450              4.45
nickel (area
ratio-MS:Ni-1:3)

Mild steel coupled with  -621         2242              8.34
brass (area
ratio-MS:brass-1:1)

Mild steel coupled with  -611         7044              5.48
brass (area
ratio-MS:brass-1:2)

Mild steel coupled with  -578        15340              3.29
brass (area
ratio-MS:brass-1:3)

Mild steel coupled with  -615         2308              8.03
copper (area
ratio-MS:Cu-1:1)

Mild steel coupled with  -587         7148              5.12
copper (area
ratio-MS:Cu-1:2)

Mild steel coupled with  -575        20180              2.67
copper (area
ratio-MS:Cu-1:3)

Mild steel coupled with  -607         5679              6.74
titanium (area
ratio-MS:Ti-1:1)

Mild steel coupled with  -580        14030              3.86
titanium (area
ratio-MS:Ti-V.2)

Mild steel coupled with  -573        20210              2.42
titanium (area
ratio-MS:Ti-1:3)


From Table 3, it is evident that when compared with the MS panels phosphated in the uncoupled condition, substrates coated using galvanic coupling technique prove to be good in improving the corrosion resistance of the resultant phosphate coatings following their high values of [R.sub.ct] and low values of [C.sub.d1]. Among all the couples studied, the MS-Ti couple with area ratio (1:3) possesses a much better corrosion resistance.

The Nyquist plot shows only one semicircle in all the cases studied, indicating that the process involves a single time constant. For a clear analysis of this factor, the Bode plots ([absolute value of Z] vs log w) and phase angle vs log w) were made. The appearance of a single inflection point in the plot of [absolute value of Z] vs log w and a single maximum in the plot of phase angle vs log w) confirm that the process involves only one time constant. In electrochemical reactions, as a precipitated film, phosphate coating acts as a mechanical barrier between the substrate and the aggressive solution. Appearance of Warburg impedance confirms that the mechanical barrier hinders the penetration of the aggressive species (here, 3.5% NaCl solution) and contributes to the protection behavior of phosphated panels. The reaction resistance increases with the thickness of the phosphate coatings. (16) The impedance studies confirmed that the corrosion behavior of the phosphated MS substrates under galvanically coupled condition in 3.5% sodium chloride solution is a diffusion-controlled process following the appearance of a Warburg impedance. The high values of charge transfer resistance and low values of double layer capacitance obtained for panels coated under galvanically coupled condition showed their better ability in preventing the on-set of corrosion.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

The performance of the phosphate coatings in 3.5% NaCl solution with respect to the cathode materials coupled to MS substrate during phosphating is in the following order:

Ti > Cu > BR > Ni > SS

Conclusion

From the study, it can be concluded that the corrosion resistance of MS substrates phosphated under a galvanically coupled condition is better than the one coated under an uncoupled condition. The galvanic coupling of MS with metals that are nobler than steel such as titanium, copper, brass, nickel, and stainless steel substrates during phosphating proved to be beneficial in producing coatings with improved corrosion resistance. Among the different couples studied, the phosphate coating developed using MS-Ti couple with 1:3 area ratio offered a better corrosion protection. The higher coating weight of panels phosphated under galvanically coupled conditions is one of the reasons for the observed improvement in corrosion resistance. The easy adaptability of this methodology for producing zinc phosphate coatings with better corrosion resistance using low temperature zinc phosphating processes makes it a positively beneficial and cost-effective approach.

References

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(2.) Freeman. DB, Phosphating and Metal Pretreatmenl--A Guide to Modern Processes and Practice. Industrial Press Inc., New York, 1986

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(13.) Arthanareeswari, M, Ravichandran, K, Sankara Narayanan. TSN, Rajeswari, S, "Effect of Cathode Materials on the Phosphatability and Corrosion Resistance of Mild Steel." Indian Surf. Finish., 1 (1) 80-88 (2004)

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(16.) Sankara Narayanan, TSN, Subbaiy in, M, "Sodium Diethyl-dithiocarbamate: An Effective Complexone for Cold Zinc Phosphating." Trans. Inst. Met. Finish., 75 (12) 81-83 (1992)

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(23.) Walter. GW, "A Critical Review of d.c. Electrochemical Tests for Painted Metals." Corros. Sci. 26 (9) 39-47 (1986)

(24.) Picaud, Th, Duprat. M, Dahosi. F. "Electrochemical Methods in Corrosion Research." In: Duprat, M (ed.) Materials Science Forum, Vol. 8, pp. 303-307. Trans Tech Publications Ltd., Switzerland, 1986

(25.) Meszaros, L, Lendvay-Gyorik, G. Lengyel, B. "Study of the Coverage of Phosphated Steel Surfaces by Impedance Method." Mater Chem. Phys., 23 267-271 (1989)

(26.) Erdey-Gruz, T, Kinetics of Electrode Process, pp. 97-101. Adam Hilger, London, 1972

[c] ACA and OCCA 2011

M. Arthanareeswari *, P. Kamaraj

Department of Chemistry, Faculty of Engineering & Technology, SRM University, Chennai 603203, India

e-mail: marthanareeswari@gmail.com

T. S. N. Sankara Narayanan

National Metallurgical Laboratory, Madras Centre. CSIR Complex, Taramani, Chennai 113, India

M. Tamilselvi

Department of Chemistry, Thiru Kolanjiappar Government Arts College. Virudhachalam 606 001, India

DOI 10.1007/s11998-011-9339-9
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Author:Arthanarceswari, M.; Narayanan, T.S.N. Sankara; Kamaraj, P.; Tainilselvi, M.
Publication:JCT Research
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Date:Jan 1, 2012
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