- Email: [email protected]
Nano zinc phosphate coatings for enhanced corrosion resistance of mild steel
Accepted Manuscript Title: Nano Zinc Phosphate Coatings for Enhanced Corrosion Resistance of Mild Steel Author: M. Tamilselvi P. Kamaraj M. Arthanareeswari S. Devikala PII: DOI: Reference:
S0169-4332(14)02562-8 http://dx.doi.org/doi:10.1016/j.apsusc.2014.11.081 APSUSC 29126
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-10-2014 13-11-2014 16-11-2014
Please cite this article as: Nano Zinc Phosphate Coatings for Enhanced Corrosion Resistance of Mild Steel, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.11.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano Zinc Phosphate Coatings for Enhanced Corrosion Resistance of Mild Steel M.Tamilselvi1, P.Kamaraj2, M.Arthanareeswari2* and S.Devikala2 1
Department of Chemistry, Thiru Kolanjiappar Government Arts College,Virudhachalam,606001,India
2
ip t
Department of Chemistry, SRM University,Kattankulathur,603203,India
Abstract
d
M
an
us
cr
Nano crystalline zinc phosphate coatings were developed on mild steel surface using nano zinc oxide particles. The chemical composition and morphology of the coatings were analyzed by Xray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy (EDX) . The particles size of the nano zinc phosphate coating developed was also characterized by TEM analysis. Potentiodynamic polarization and electrochemical impedance studies were carried out in 3.5% NaCl solution. Significant variations in the coating weight, morphology and corrosion resistance were observed as nano ZnO concentrations were varied from 0 .25 -2g/L in the phosphating baths. The results showed that nano ZnO particles in the phosphating solution yielded phosphate coatings of higher coating weight, greater surface coverage and enhanced corrosion resistance than the normal zinc phosphate coatings (developed using normal ZnO particles in the phosphating baths). Better corrosion resistance was observed for coatings derived from phosphating bath containing 1.5g/L nano ZnO. The activation effect brought about by the nano ZnO reduces the amount of accelerator (NaNO2)required for phosphating.
te
Keywords
Ac ce p
Nano ZnO, mild steel, nano zinc phosphate coating, normal zinc phosphate coating *Corresponding author 2
Department of Chemistry, SRM University,Kattankulathur,603203,India.
Tel. : + 044 27455865
Email-id : [email protected]
Page 1 of 1 Page 1 of 24
1.Introduction
M
an
us
cr
ip t
Phosphating improves the paint adhesion by providing a surface which has many anchor points and provide a resistance barrier to the spread of corrosion under the paint film [1-3]. Recent efforts to enhance the corrosion resistance of phosphate coatings have mainly been focused on the pre-treatment methods before phosphating and the process technologies for phosphating [4–8]. Zinc phosphating is one of the promising method for enhancing the corrosion resistance of iron and steel [8]. It has been shown that addition of metal salts in the phosphating bath can greatly influence the microstructure of zinc phosphate coating and make the coatings denser and finer [9,10]. Addition of Mn2+ into the phosphating bath improves the corrosion resistance of zinc phosphate coatings on steel and zinc coated steel[11,12]. Also it has been reported that the addition of Ni2+ into the phosphating bath improves the corrosion resistance of zinc phosphate coatings on the 2024 Al alloy [5,13]. The utility of the galvanic coupling for accelerating low temperature zinc phosphating processes was established recently [1416].Nowadays, nanostructure materials have attracted considerable interest due to their importance in fundamental research and potential wide ranging applications. In the coating industry, the quality of the coatings might be enhanced using nano particles[17-20].In the present study, phosphate coating was produced by using a phosphating bath consisting of nano ZnO particles instead of normal ZnO particles. The aim of this study is to develop nano zinc phosphate coatings on mild steel and to evaluate the corrosion resistance of the phosphated panels in 3.5% NaCl solution at ambient temperature.
2. Experimental
Ac ce p
te
d
Mild steel specimens of dimensions 8.0 cm × 6.0 cm × 0.2 cm having composition C-0.16, Si0.17, Mn-0.68, P-0.027, S-0.026, Cr-0.01, Ni- 0.01, Mo-0.02, and balance iron (all in wt. percentage)were used as the substrate materials for the deposition of zinc phosphate coating. After grinding, the substrates were degreased with alkaline solution, ultrasonically cleaned in acetone. The phosphate specimens were rinsed with deionized water to remove the acid and the soluble salts left after phosphating. After rinsing, the specimens were dried using compressed air. The chemical composition of the zinc phosphating bath and its operating conditions are given in Table 1. All the reagents used in the experiments were of analytical purity. Table 1 Chemical composition, control parameters and operating conditions of the bath used for zinc phosphating
The nano ZnO was purchased from Aldrich. Phosphating was done by immersion process at room temperature(27◦C) for 30 min. The amount of iron dissolved during phosphating and the coating weight were determined in accordance with the standard procedures[15] .The normal zinc phosphate coating was developed at room temperature using optimized bath of chemical compositions : normal ZnO : 5g/L; H3PO4 :11.3mL/L; NaNO2:2g/L and pH= 2.7 [15 ]. The coatings’ surface morphology was examined by a Hitachi Scanning Electron Microscope SU1510 and the chemical composition was investigated by EDX. The phases in the phosphate coating were analyzed by XRD using Philips X’Pert pro diffractometer with Cu Kα (λ= 1.54060 Å) incident radiation. The XRD peaks were recorded in the 2θ range of 0°–100°. Page 2 of 2 Page 2 of 24
us
cr
ip t
Potentiodynamic polarization and electrochemical impedance measurements were carried out using Biologic Electrochemical Analyser (model SP 300) at the open circuit potential. The phosphated mild steel substrate formed the working electrode, whereas a saturated calomel electrode and a platinum electrode served as the reference and counter electrodes respectively. EC Lab software was used for data acquisition and analysis. Polarization technique was carried out from a cathodic potential of -2V to an anodic potential of 2V with respect to corrosion potential at a sweep rate 1 mV/s. Electrochemical impedance studies were carried out in the frequency range between 10000 and 0.01 Hz. The corrosion potential (Ecorr) and corrosion current density (icorr) were determined using Tafel extrapolation method. The charge transfer resistance (Rct) and double layer capacitance (Cdl) were determined from Nyquist plot by fitting the data using EC Lab software. All the experiments were repeated for confirming the reproducibility of the coatings.
M
an
Salt spray test was carried out by subjecting the phosphated panels to a salt mist of 5% sodium chloride solution in a salt spray chamber (ASTM B 117-03) for a specified period of time (96 hours) [16] . The edges of the substrates were sealed with paraffin wax to avoid the excessive corrosion at the edges. The extent of corrosion after 96 hours of exposure was assessed and photographed.
3.Results and Discussion
Ac ce p
te
d
During the preliminary investigations, the effect of amount of nano ZnO on the phosphate coating weight was studied by varying its concentrations in the phosphating bath. It was observed that there was no significant effect between the concentrations of 0.1- 0.25g/L of nano ZnO. But substantial increase in the coating weight was observed by varying the concentrations from 0.25 – 2g/L of nano ZnO in the phosphating bath with an immersion time of 30 minutes at room temperature (27◦C) was studied ( Fig. 1). Fig.1 Weight of phosphate coatings and iron dissolved during phosphating (*the standard deviation of above data is within 0.16g/m2) It was observed from the fig.1 that the phosphate coating weight increases with increase in the nano ZnO content in the phosphating bath from 0.25-2g/L. The increase in the coating weight was significant between the concentrations 1-1.5g/L of nano ZnO. But there was no substantial increase in the coating weight after 1.5g/L of nano ZnO. There is a steady increase in the coating weight from 0.5- 1.5g/L of nano ZnO. The phosphate coatings formed from the bath with 1.5g/L of nano ZnO is heavier than that formed from the other baths with different concentrations of nano ZnO and normal phosphate coating weight reported by the authors[14 ]. The amount of iron dissolved during phosphating was comparatively lower than the normal phosphating process, which reduces the amount of sludge formed during the process . The increase in the coating weight and decrease in the metal dissolution can be attributed to the increase in the nucleation sites and reduction in the size of the crystal clusters of nano zinc phosphate deposited on the mild steel plate[20]. Higher coating weight was resulted from the zinc phosphating bath consisting of Page 3 of 3 Page 3 of 24
1.5g/L of nano ZnO. This was confirmed by XRD, SEM, EDX ,potentiodynamic polarization and electrochemical impedance studies. Potential-Time measurements
d
M
an
us
cr
ip t
During phosphating, the potential of the mild steel coupled with saturated calomel electrode (reference electrode) is monitored continuously as a function of time for the entire duration of coating formation . The potential-time curves obtained for phosphating of mild steel using normal ZnO particles and nano ZnO particles ( Fig. 2) suggest that phosphating using nano ZnO particles shift the measured initial potential to more anodic direction. The anodic shift in potential represents the progressive build up of the phosphate coating formation [15].The shift towards cathodic direction was due to the conversion of soluble primary phosphate to insoluble tertiary phosphate (point of incipient precipitation). The time taken for attainment of incipient precipitation (induction time) is little earlier (2 minutes) for phosphating using nano ZnO particles than compared to phosphating using normal ZnO particles (6 minutes). This may probably be due to the activation effect brought by the nano ZnO particles in the phosphating bath[20]. A high negative shift displayed by normal zinc phosphating indicates the high rate of metal dissolution than compared to phosphating using nano ZnO.The plateau suggests that the reactions occurring at the interface reached the steady state and the surface is conversed. The earlier attainment of steady state in the presence of nano ZnO can be attributed to the reduction in the activation energy of the process by nano ZnO [20,21].
te
Fig. 2 The potential-time curves obtained during phosphating of mild steel using normal ZnO particles and nano ZnO particles
Effect of NaNO2 on the Coating Weight
Ac ce p
NaNO2 is the most popular accelerator used in the phosphating treatment. It is well established that phosphating reaction from unaccelerated baths tend to be slow owing to the polarization caused by hydrogen evolution at the cathode[15] . The very slow rate of recombination of hydrogen atoms to form hydrogen gas causes the formation of a very low coating weight. The effect of NaNO2 on the phosphate coating weight (at optimum concentration of nano ZnO(1.5g/L) )was studied by varying its concentration from 0 - 0.6g/L and the results have been exhibited in Fig.3. From the results, the optimum concentration of NaNO2 was found to be 0.4g/L for the present study. In the normal zinc phosphating baths reported, the concentration of NaNO2 is 2-16g/L [14,20-22]. The activation effect brought about by nano ZnO may be the reason for the requirement of small amount of NaNO2 for the present study [22].
Fig.3 The effect of NaNO2 on the phosphate coating weight (*the standard deviation of above data is within 0.16g/m2) TEM Page 4 of 4 Page 4 of 24
cr
ip t
Fig.4 shows TEM images of nano ZnO particles and nano zinc phosphate coating developed. The particles of the nano ZnO and nano zinc phosphate deposit have an average grain size between 30-40nm. Formation of nano crystalline zinc phosphate coating was confirmed by the TEM results.
us
Fig.4 TEM images of (a) nano ZnO and (b) nano zinc phosphate coating
an
SEM
Ac ce p
te
d
M
From the SEM micrographs( Fig.5), It was obvious that the size of the crystal clusters was reduced and the surface coverage was increased. In a comparison, the morphology of the surface coatings formed from the bath with nano ZnO 1.5g/L is more uniform and the crystal clusters bonded together more compactly than that formed from the other baths with different concentrations of nano ZnO and normal zinc phosphate coatings. It was observed that the incorporation of nano ZnO in the phosphating bath increases the degree of crystalline coverage by reducing the grain size of the phosphate coating. d
Fig.5 SEM images of phosphate coatings obtained from baths containing different contents of nano ZnO : ( a)0.25g/L,(b) 0.5g/L (c) 0.75g/L (d)1g/L, (e) 1.25g/L, (f) 1.5g/L, (g) 1.75g/L, (h) 2g/L and (i) normal ZnP coating On an average, there is an increase in the number of small sized crystals that grow when compared to few crystals growing large. The nano ZnO in the phosphating bath showed a significant effect on the reduction in the size of the zinc phosphate crystals which activate the surface of mild steel substrate and increase the number of micro cathodic sites which speeds up the hydrogen evolution reaction[21,22]. This enables the rapid consumption of free phosphoric acid and increases the interfacial pH between the mild steel substrate and phosphating solution. The increase in pH causes the conversion of soluble primary phosphate to insoluble tertiary Page 5 of 5 Page 5 of 24
cr
ip t
phosphate with the subsequent deposition of the phosphate coating on mild steel substrate[23].The optimum amount of nano ZnO increases the number of cathodic sites there by increasing the deposition of the phosphate coating[14]. However it was also observed that when the amount of nano ZnO increases beyond 1.5g/L in the phosphating bath, the coating is not compact and uniform and cracks were observed, while the degree of crystalline coverage also decreases. The possible reason is that the excessive amount of nano ZnO in phosphating solution would seal up the anodic surface by agglomeration, restraining anodic reaction. Even though the formation of well crystallized coating was observed in all the phosphating baths, denser and finer coating was observed in the phosphating solution with 1.5g/L nano ZnO. The initially deposited crystallites provide nucleation sites for further coating. In general, the smaller the size of the crystals, the higher their coverage and more effective coating was obtained [20,22 ]. EDX
Ac ce p
te
d
M
an
us
For a comparison. EDX analysis (Fig.6) of the well defined nano zinc phosphate coatings developed using 1.0g, 1.25g,1.5g and 1.75g/L of nano ZnO indicated that the phosphate coatings consists of both hopeite (Zn3(PO4)2.4H2O) and phosphophylite (Zn2Fe(PO4)2.4H2O) phases.The nano zinc phosphate deposit resulting from bath containing 1.5g/L of nano ZnO contains more zinc than from the other baths. The relative compositions of zinc phosphate coatings (wt.%) obtained by EDX analyses was given in Table 2. The composition of C and Si are not included in this table as they are insignificant in the coating phases. From the table it was observed that the ratio of Zn/P is about 1.38, 1.87, 2.20 and 1.92 for phosphate coatings developed using baths containing 1.0g, 1.25g,1.5g and 1.75g/L of nano ZnO respectively. This indicates that the content of Zn3(PO4)2.4H2O is higher than that of Zn2Fe(PO4)2.4H2O when the phosphating bath is having the optimum concentration of nano ZnO(1.5g/L). The nano zinc phosphate coating developed using 1.5g/L of nano ZnO indicates the formation of a thick coating as the composition of iron was less in this coating.
Fig.6 EDX of phosphate coatings developed using different contents of nano ZnO: (a)1g /L,(b) 1.25g/L, (c)1.5g/L,(d) 1.75g/L
Page 6 of 6 Page 6 of 24
ip t
Table 2 Relative Compositions of Zinc Phosphate Coatings (Wt.%) obtained by EDX
XRD
Ac ce p
te
d
M
an
us
cr
The phase compositions in the phosphate coatings on the mild steel were analyzed by XRD. The phase compositions of uncoated mild steel, mild steel substrates coated with normal zinc phosphate coating and phosphate coating developed using nano ZnO(1.5g/L) have been compared(Fig.7). It is shown that the phosphate coatings developed using nano ZnO mainly consisted of Zn3(PO4)2.4H2O (hopeite,JCPD file #37-0465). However , Zn2Fe(PO4)2.4H2O(phosphophyllite,JCPD file#29-1427)was also present in the coatings. The peaks of iron were due to the mild steel substrates. The peak intensities of nano crystalline ZnP coatings on mild steel specimen are stronger than those of normal ZnP coatings which clearly indicate the formation of thick phosphate crystal layer in the case of coatings produced using nano ZnO particles[20].The presence of nano ZnO particles in the phosphating bath increased the intensity of the(040)plane of hopeite phase and the intensity of (020) plane of hopeite and phosphophyllite phases. The results of XRD spectra indicated that the growth and coverage of the crystal clusters of phosphate coating increased when nano ZnO particles are used in the phosphating bath. From this study, considering the peak at degrees, average crystallite size has been estimated by using Debye-Scherrer formula [23].The calculated crystallite sizes have been presented in Table.3.The average crystallite size is less than 40 nm. The results confirm the formation of nano zinc phosphate crystals.
Fig.7 XRD patterns of phosphate coatings (a) uncoated mild steel (b) normal ZnP coating (c) phosphate coating developed using nano ZnO (1.5g/L)
Table.3 The calculated crystallite size of nano zinc phosphate coating using Scherrer calculator
Evaluation of Corrosion Performance Potentiodynamic Polarization
Page 7 of 7 Page 7 of 24
The protectiveness of the coatings wase evaluated through potentiodynamic polarization technique using 3.5% NaCl solution(Fig.8). Corrosion potential (Ecorr) ,Corrosion current density (icorr) and the corrosion rate derived from these data are presented in Table 4 .
cr
ip t
Fig.8 Polarisation curves of mild steel samples coated with (a) normal zinc phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO :( b)0.25g/L,(c) 0.5g/L (d) 0.75g/L (e)1g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Ac ce p
te
d
M
an
us
It is evident from the fig.8 that, for the substrates coated using nano ZnO particles(optimum range 1-1.5g/L ),the corrosion potentials have been shifted towards less negative values. The extent of shift in potential is largely a function of phosphate coating weight and the porosity of the coating[15]. A larger shift of Ecorr in the positive direction was observed when nano ZnO particles are used in the phosphating bath which indicates the improved corrosion resistance. Among the substrates studied, the substrate with phosphate coatings prepared from baths containing nano ZnO(1.5 g/L) has shown the more positive corrosion potential, lowest corrosion current density and the lowest corrosion rate which could be attributed to the more uniform and compact outer crystal layer. This is in good agreement with the results obtained from weight-loss measurements. The polarization current of phosphate coatings developed using nano ZnO particles in the range 1-1.5g/L show a marked decrease compared to that of the normal phosphate coating. It is observed that the dissolution takes place during anodic polarization and cathodic polarization is a diffusion controlled process. The depolarization of oxygen plays a major role in the corrosion failure of the coatings. The cathodic current primarily depends on the amount of oxygen arriving at the cathodic zone per unit area in unit time [22,23 ].The transport of oxygen to the substrate is hindered by the protective phosphate film between the substrate and the electrolyte due to which the average polarization current decreases considerably. Phosphate coatings are generally porous, which will favor adhesion of paint film on the surface. At the same time, the porosity favors the diffusion of the electrolyte which will ultimately result into corrosion. The decrease in the corrosion current for the coatings developed using nano ZnO particles (concentrations ranging from 1-1.5g/L) clearly indicate that the coating is more uniform and less porous than the normal zinc phosphate coating. Presence of nano ZnO particles in the phosphating bath caused better surface coverage , more homogeneous coating which lead to decrease in the rate of corrosion of these substrates. However, when the content of nano ZnO exceeds 1.5g/L there is a small increase in the polarization current and the corrosion potential is little less positive. The nano ZnO (optimum amount) in the phosphating bath leads to compact nucleation and growth of zinc phosphate crystals results in a denser morphology containing more crystal clusters[24]. But beyond optimum concentration, the increase in the nano ZnO content ,may lead to agglomeration of nano particles which disfavor the formation of intact phosphate coatings. Table 4 Polarisation parameters of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution Page 8 of 8 Page 8 of 24
Electrochemical Impedance Characteristics
M
an
us
cr
ip t
Comparison of Nyquist plots of phosphate coatings developed using nano ZnO particles and the normal zinc phosphate coatings in 3.5% NaCl have been shown in Fig.9 and the results have been presented in Table 5. To account for the corrosion behavior of phosphated panels in 3.5% sodium chloride solution, an equivalent circuit model (Fig.10) is proposed. The electrolyte/coating-metal interface approximates such a model [20 ] where R1 is the solution resistance ,R2 is the coating resistance, R3 is the charge transfer resistance, C1is the coating capacitance and Cdl is the double layer capacitance. The semicircles obtained are not well defined. The semicircles obtained are flattened capacitive semicircles. The flattened capacitive semicircles are related to frequency dispersion due to defects in the phosphate coating [25,26 ]. The shape of the EIS plots for mild steel substrates with nano zinc phosphate coatings is almost identical, but the size of the plots is increased with nano-ZnO content from 0.25 to 1.5 g/L. However, when the content of nano-ZnO exceeds 1.5 g/L or more, there is a drop.
te
d
Fig.9 EIS of mild steel samples coated with (a) normal phosphate coating and phosphate coatings obtained from baths containing different contents of nanoZnO : ( b)0.25g/L(c) 0.5g/L (d) 0.75g/L (e)1g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Ac ce p
A high charge transfer resistance and low double layer capacitance values were obtained for coatings developed using nano ZnO particles. The impedance studies confirmed that the corrosion behavior of phosphated panels using nano zinc oxide particles is a much more diffusion controlled process and thereby offering a higher corrosion resistance than the normal phosphate coating[16]. As discussed earlier, nano zinc oxide particles increases the surface coverage, uniformity and the thickness of the coating and decreases the porosity which improves the corrosion resistance of the phosphate coatings more than that of the normal zinc phosphate coating[20]. But beyond the optimum concentration(1.5g/L), the increase in the nano ZnO content in the phosphating bath results in micro cracks which lead to small decrease in the charge transfer resistance and increase in the double layer capacitance. This is in good agreement with the results of weight loss measurements and SEM photographs of these coatings.
Fig.10 Equivalent circuit model proposed to account for the corrosion of phosphate coatings obtained from baths containing nanoZnO.
Page 9 of 9 Page 9 of 24
ip t
Table 5 Charge transfer resistance and double layer capacitance of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution
Salt Spray Test
M
an
us
cr
Salt spray test measures the ability of various types of coatings to withstand in a corrosive-cum-humid atmosphere. As paint pre-treatment coating, the phosphate coating is expected to improve the adhesion of the paint coating and prevents the spreading of underfilm corrosion. The ability to prevent underfilm corrosion is best measured by assessing the spreading of corrosion from the X-scratch made on the phosphated and painted mild steel substrate[ 16 ].The spreading of corrosion from the scribe is less in the case of nano zinc phosphate coatings than normal zinc phosphate coatings on mild steel specimens after 96 hours of salt spray test ( Fig.11). During salt spray test, chloride ions attack the base metal, forming iron (II) chloride followed by hydrolysis and rust formation at the anodic areas (exposed bare steel in the scribed region). The corresponding cathodic reaction is the reduction of oxygen to form hydroxide ions which dissolve the phosphate at the cathodic areas [ 16 ].
Ac ce p
te
d
Fig:11 The corrosion behaviour of (a) nano zinc phosphate coating and (b) normal zinc phosphate coating on mild steel specimens subsequently finished with a paint coating (DFT: 50 µm) after subjecting them to salt spray test for 96 hours.
Conclusions
The paper reports the development of nano zinc phosphate coatings on mild steel substrates. The nano zinc phosphate coating weight increased with increase in the concentration of nano ZnO particles from0.25-2g/L in the phosphating baths . The concentration of nano ZnO in the phosphating bath was optimized to 1.5g/L .The nano zinc phosphate coating consists of more of hopeite phase than the phosphophylite phase. The nano ZnO acts as a nucleating agent, increases the number of nucleation sites for deposition and reduces the size of the phosphate crystal clusters formed which enabled higher coating weight. The decrease in the induction time, earlier attainment of steady state for the formation of new phases ,activation effect brought about by the nano ZnO decreases the amount of accelerator (NaNO2) required for phosphating than the normal phosphating process. The results of potentiodynamic polarization , impedance studies and salt spray test confirmed that the nano zinc phosphate coating offers better corrosion protection than normal zinc phosphate coating. Hence, it can be concluded that nano zinc phosphate coatings can substitute normal phosphate coatings due to its better corrosion resistance and less pollutant nature. Page 10 of 10 Page 10 of 24
References Rausch, W, ThePhosphating of Metals. Finishing PublicationsLtd., London, 1990 J.B. Bajat, V.B. Miskovic-Stankovic, J.P. Popic, D.M. Drazic ,Adhesion characteristicsand corrosion stability of epoxy coatings electrodeposited on phosphate hot-dip galvanized steel , Prog. Org. Coat. 63 (2008) 201–208 [3] W. H. Kok, X. Sun, L. Shi, K. C. Wong, K. A. R. Mitchell, Formation of zinc phosphate coatings on AA6061aluminum alloy, Journal of Materials Science 36 (2001) 3941 – 3946 [4] B.L. Lin, J.T. Lu, K. Gang, Synergistic corrosion protection for galvanized steel byphosphating and sodium silicate post-sealing, Surf. Coat. Technol. 202 (2008) 1831– 1838 [5] A.S. Akhtar, D. Susac, P. Glaze, K.C. Wong, P.C. Wong, K.A.R. Mitchell, The effect of Ni2+ on zinc phosphating of 2024-T3 Al alloy, Surf. Coat. Technol. 187 (2004) 208–215 [6] P.K. Sinha,, R. Feser, Phosphate coating on steel surfaces by an electrochemical method, Surf. Coat. Technol. 161(2002) 158–168 [7] Y.K. Song, F. Mansfeld, Development of a molybdate–phosphate-silane-silicate(MPSS) coating process for electrogalvanized steel, Corros. Sci. 48 (2006) 154–164 [8] M. Wolpers, J. Angeli ,Activation of galvanized steel surfaces before zinc phosphatingXPS and GDOES investigations, Appl. Surf. Sci. 179 (2001) 281–291, [9] S.-L. Zhang, H.-H. Chen, X.-L. Zhang, M.-M. Zhang, The growth of zinc phosphate coatings on 6061-Al alloy, Surf. Coat. Technol. 202(2008) 1674–1680 [10] G. Li, L. Niu, J. Lian, Z. Jiang , A black phosphate coating for C 1008 steel, Surf.Coat. Technol. 176 (2004) 215–221 [11] A.S. Akhtar, K.C. Wong, P.C. Wong, K.A.R. Mitchell ,Effect of Mn2+ additive onthe zinc phosphating of 2024-Al alloy. Thin Solid Films 515 (2007)7899–7905 [12] A.S. Akhtar, K.C. Wong, K.A.R. Mitchell, The effect of pH and role of Ni2+ in zincphosphating of 2024-Al alloy. Part 1. Macroscopic studies with XPS and SEM, Appl. Surf. Sci. 253 (2006) 493–501.
Ac ce p
te
d
M
an
us
cr
ip t
[1] [2]
[13] S. Palraj, M. Selvaraj, P. Jayakrishnan , Effect of phosphate coatings on the performanceof epoxy poly amide red oxide primer on galvanized steel, Prog. Org. Coat. 54 (2005) 5–9 [14] M. Arthanareeswari, T. S. N. Sankara Narayanan, P. Kamaraj, and M. Tamilselvi, Polarization and impedance studies on zinc phosphate coating developed using galvanic coupling, Journal of Coatings Technology Research 9 (2012) 39–46 [15] M. Arthanareeswari, T. S. N. Sankara Narayanan, P. Kamaraj, and M. Tamilselvi ,Influence of galvanic coupling on the formation of zinc phosphate coating, Indian Journal of Chemical Technology 17 (2010) 167–175 [16] M. Arthanareeswari, P. Kamaraj and M.Tamilselvi, Anticorrosive performance of zinc phosphate coatings on mild steel developed using galvanic coupling, Journal of Chemistry, 2013 (2013) 1-8 [17] Xiuzhi Zhang, Fuhui Wang, Du. Yuanlong , Effect of nano-sized titanium powder addition on corrosion performance of epoxy coatings, Surf. Coat. Technol. 201 (2007) , 7241-7245.
Page 11 of 11 Page 11 of 24
d
M
an
us
cr
ip t
[18] AlkaPhanasgaonkar, V.S. Raja, Influence of curing temperature, silica nanoparticles- and cerium on surface morphology and corrosion behaviour of hybrid silane coatings on mild steel .Surf. Coat. Technol. 203 (2009) 2260-2271 [19] A.N. Khramov, V.N. Balbyshev, N.N. Voevodin,M.S.Donley, Nanostructured Sol–gel Derived Conversion Coatings Based on Epoxy- and Amino-silanes, Prog. Org. Coat.47(2003) 207-213 [20] S.M.A. Shibli , Francis Chacko, Development of nano TiO2-incorporated phosphate coatings on hot dip zinc surface for good paintability and corrosion resistance, Applied Surface Science 257 (2011) 3111–3117 [21] Zhang Shenglin ,Study on phosphating treatment of aluminium alloy : role of yttrium oxide, Journal of rare earths 27 (2009) 469-473 [22] Minqi Sheng,Yi Wang,Qingdong Zhong,Hongyan Wu,Qiongyu Zhou,Hai Lin , The effects of nano-SiO2 additive on the zinc phosphating of carbon steel, Surf. Coat. Technol. 205 (2011) 3455-3460 [23] B. D. Hall, D. Zanchet and D. Ugarte , Estimating nanoparticle size from diffraction measurements , J.Appl. Cryst., 33 (2000) 1335-1341 [24] T. S. N. Sankara Narayanan and M. Subbaiyan ,Effect of Surfactants on the Growth and Crystal Habit of Zinc-Phosphate Coating, Trans.Inst. Met.Finish.71(1993) 37-40 [25] Susac D, Sun X, Li R Y, Wong K C, Wong P C, Mitchell K AR, Champaneria R, Microstructural effects on the initiation of zinc phosphate coatings on 2024-T3 aluminum alloy, Surf.Coat. Technol.,239 (2004) 45-59 [26] Weng, D, Jokiel, P, Uebleis, A, Boehni, H ,Corrosion and Protection Characteristics of Zinc and Manganese Phosphate Coatings, Surf. Coat. Technol. 88 (1997) 147–156
Ac ce p
te
[27] E.M.A. Martini, S.T. Amaral, I.L. Muller , Electrochemical behaviour of Invar in phosphate solutions at pH=6.0,Corros. Sci. 46(2004) 2097-2115
Page 12 of 12 Page 12 of 24
Table 1 Chemical composition, control parameters and operating conditions of the bath used for zinc phosphating Chemical Composition 0.25- 2g/L 2.3ml/L 0.4g/L
ip t
Nano ZnO H3PO4 NaNO2 Control Parameters
2.70 3pointage 25pointage 1: 8.33 27◦C 30 minutes
an
us
cr
pH Free acid value(FA) Total Acid Value(TA) FA:TA Temperature Time
Table 2 Relative Compositions of Zinc Phosphate Coatings (Wt.%) obtained by EDX Zn
P
Zn/P
28.40
41.13
16.05
11.55
39.91
32.72
15.66
8.38
27.55
36.87
20.18
9.16
36.56
34.60
14.00
7.30
1.38 1.87 2.20 1.92
M
O
d
Fe
Ac ce p
te
Nano zinc phosphate coatings developed using nano ZnO (g/L) 1 1.25 1.5 1.75
Table.3 The calculated crystallite size of nano zinc phosphate coating using Scherrer calculator No. 1 2 3 4 5 6 7
B obs. [°2Th]
B std. [°2Th]
0.492 0.394 0.394 0.590 0.394 0.394 0.394
0.180 0.180 0.180 0.180 0.180 0.180 0.180
Peak pos. [°2Th] 9.953 19.457 19.973 26.310 31.502 40.531 64.210
B struct. [°2Th] 0.312 0.214 0.214 0.410 0.214 0.214 0.214
Crystallite size [nm] 26 37 37 20 38 39 43
Page 13 of 13 Page 13 of 24
an
us
cr
ip t
Table 4 Polarisation parameters of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution SYSTEM STUDIED Ecorr (mV) VS SCE Icorr(µA/cm2) CORROSION RATE -611 16.03 11.37 a) normal Zinc phosphate coated mild steel substrate Phosphated mild steel substrates from baths containing different contents of Nano Zinc Oxide b) 0.25 g/L -610 28.68 13.29 c) 0.5 g/L -553 12.13 5.62 d) 0.75 g/l -539 10.57 4.90 e) 1.0 g/L -509 9.83 4.56 f) 1.25 g/L -488 6.92 3.21 g) 1.5 g/L -465 6.60 3.06 h) 1.75 g/L -488 7.84 3.64 i) 2.0 g/L -494 7.83 3.63
te
d
M
Table 5 Charge transfer resistance and double layer capacitance of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution Rct System studied (Ω cm2) Cdl (F) x 10-6 550 3.56 a) normal Zinc phosphate coated mild steel substrate
Ac ce p
Phosphated substrates from baths containing different contents of Nano Zinc Oxide 87 32.68 b) 0.25 g/L c) 0.5 g/L 289 4.51 432 2.88 d) 0.75 g/l 651 1.42 e) 1.0 g/L 654 1.38 f) 1.25 g/L 830 0.95 g) 1.5 g/L 758 1.32 h) 1.75 g/L 637 1.41 i) 2.0 g/L
Page 14 of 14 Page 14 of 24
M
an
us
cr
ip t
Figures
Ac ce p
te
d
(*the standard deviation of above data is within 0.16g/m2)
Page 15 of 15 Page 15 of 24
phosphating using normal ZnO phosphating using nano ZnO
-430 -440 -450 -460
ip t
-480 -490 -500 -510
cr
-520 -530 -540 -550 -560 -570 -580 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
an
Time in minutes
us
potential(mV vs SCE)
-470
Ac ce p
te
d
M
Fig. 2 The potential-time curves obtained during phosphating of mild steel using normal ZnO particles and nano ZnO particles
Fig.3 The effect of NaNO2 on the phosphate coating weight (*the standard deviation of above data is within 0.16g/m2) Page 16 of 16 Page 16 of 24
ip t cr an
us
b
aa
Ac ce p
te
d
M
Fig.4 TEM images of (a) nano ZnO and (b) nano zinc phosphate coating
Page 17 of 17 Page 17 of 24
ip t
b
C
e
f
h
i
Ac ce p
te
d
M
d
an
us
cr
a
g
Fig.5 SEM images of phosphate coatings obtained from baths containing different contents of nano ZnO : ( a)0.25g/L, (b) 0.5g/L, (c) 0.75g/L, (d)1.0g/L, (e) 1.25g/L, (f) 1.5g/L, (g) 1.75g/L, (h) 2g/L and (i) normal ZnP coating
Page 18 of 18 Page 18 of 24
b
us
cr
ip t
a
d
d
M
an
c
Ac ce p
te
Fig.6 EDX of phosphate coatings developed using different contents of nano ZnO: (a)1.0g /L,(b) 1.25g/L, (c)1.5g/L,(d) 1.75g/L
Page 19 of 19 Page 19 of 24
o # (110)
o #
*
*
c
cr
#
* o (241)
*
Fe
ip t
* (040)
#
Intensity
* # (020)
Zn3(PO4)2.4H2O # Zn2Fe(PO4)2.4H2O
us
b
a
20
40
60
80
an
0
100
2theta
Ac ce p
te
d
M
Fig.7 XRD patterns of phosphate coatings (a) uncoated mild steel, (b) normal ZnP coating, (c) phosphate coating developed using nano ZnO (1.5g/L)
Page 20 of 20 Page 20 of 24
g h
ip t
f
i -0.5
e Ewe/V
-0.55
d -0.6
c
us
a
an
-0.65
-0.7
-2 log (|/mA|)
b
-1
M
-3
cr
-0.45
Ac ce p
te
d
Fig.8 Polarisation curves of mild steel samples coated with (a) normal zinc phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO :( b)0.25g/L,(c) 0.5g/L, (d) 0.75g/L, (e)1.0g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Page 21 of 21 Page 21 of 24
ip t an
us
cr
Im(Z)/Ohm
Re(Z)/Ohm
Ac ce p
te
d
M
Fig.9 EIS of mild steel samples coated with (a) normal phosphate coating and phosphate coatings obtained from baths containing different contents of nanoZnO : ( b)0.25g/L,(c) 0.5g/L, (d) 0.75g/L, (e)1.0g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Fig.10 Equivalent circuit model proposed to account for the corrosion of phosphate coatings obtained from baths containing nanoZnO.
Page 22 of 22 Page 22 of 24
ip t cr
an
b
us
a
Ac ce p
te
d
M
Fig:11 The corrosion behaviour of (a) nano zinc phosphate coating and (b) normal zinc phosphate coating on mild steel specimens subsequently finished with a paint coating (DFT: 50 µm) after subjecting them to salt spray test for 96 hours.
Page 23 of 23 Page 23 of 24
Highlights of the paper Nano zinc phosphate coating on mild steel was developed Nano zinc phosphate coatings on mild steel showed enhanced corrosion resistance.
ip t
The nano ZnO increases the number of nucleating sites for phosphating.
Ac ce p
te
d
M
an
us
cr
Faster attainment of steady state during nano zinc phosphating.
Page 24 of 24 Page 24 of 24
S0169-4332(14)02562-8 http://dx.doi.org/doi:10.1016/j.apsusc.2014.11.081 APSUSC 29126
To appear in:
APSUSC
Received date: Revised date: Accepted date:
10-10-2014 13-11-2014 16-11-2014
Please cite this article as: Nano Zinc Phosphate Coatings for Enhanced Corrosion Resistance of Mild Steel, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.11.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano Zinc Phosphate Coatings for Enhanced Corrosion Resistance of Mild Steel M.Tamilselvi1, P.Kamaraj2, M.Arthanareeswari2* and S.Devikala2 1
Department of Chemistry, Thiru Kolanjiappar Government Arts College,Virudhachalam,606001,India
2
ip t
Department of Chemistry, SRM University,Kattankulathur,603203,India
Abstract
d
M
an
us
cr
Nano crystalline zinc phosphate coatings were developed on mild steel surface using nano zinc oxide particles. The chemical composition and morphology of the coatings were analyzed by Xray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy (EDX) . The particles size of the nano zinc phosphate coating developed was also characterized by TEM analysis. Potentiodynamic polarization and electrochemical impedance studies were carried out in 3.5% NaCl solution. Significant variations in the coating weight, morphology and corrosion resistance were observed as nano ZnO concentrations were varied from 0 .25 -2g/L in the phosphating baths. The results showed that nano ZnO particles in the phosphating solution yielded phosphate coatings of higher coating weight, greater surface coverage and enhanced corrosion resistance than the normal zinc phosphate coatings (developed using normal ZnO particles in the phosphating baths). Better corrosion resistance was observed for coatings derived from phosphating bath containing 1.5g/L nano ZnO. The activation effect brought about by the nano ZnO reduces the amount of accelerator (NaNO2)required for phosphating.
te
Keywords
Ac ce p
Nano ZnO, mild steel, nano zinc phosphate coating, normal zinc phosphate coating *Corresponding author 2
Department of Chemistry, SRM University,Kattankulathur,603203,India.
Tel. : + 044 27455865
Email-id : [email protected]
Page 1 of 1 Page 1 of 24
1.Introduction
M
an
us
cr
ip t
Phosphating improves the paint adhesion by providing a surface which has many anchor points and provide a resistance barrier to the spread of corrosion under the paint film [1-3]. Recent efforts to enhance the corrosion resistance of phosphate coatings have mainly been focused on the pre-treatment methods before phosphating and the process technologies for phosphating [4–8]. Zinc phosphating is one of the promising method for enhancing the corrosion resistance of iron and steel [8]. It has been shown that addition of metal salts in the phosphating bath can greatly influence the microstructure of zinc phosphate coating and make the coatings denser and finer [9,10]. Addition of Mn2+ into the phosphating bath improves the corrosion resistance of zinc phosphate coatings on steel and zinc coated steel[11,12]. Also it has been reported that the addition of Ni2+ into the phosphating bath improves the corrosion resistance of zinc phosphate coatings on the 2024 Al alloy [5,13]. The utility of the galvanic coupling for accelerating low temperature zinc phosphating processes was established recently [1416].Nowadays, nanostructure materials have attracted considerable interest due to their importance in fundamental research and potential wide ranging applications. In the coating industry, the quality of the coatings might be enhanced using nano particles[17-20].In the present study, phosphate coating was produced by using a phosphating bath consisting of nano ZnO particles instead of normal ZnO particles. The aim of this study is to develop nano zinc phosphate coatings on mild steel and to evaluate the corrosion resistance of the phosphated panels in 3.5% NaCl solution at ambient temperature.
2. Experimental
Ac ce p
te
d
Mild steel specimens of dimensions 8.0 cm × 6.0 cm × 0.2 cm having composition C-0.16, Si0.17, Mn-0.68, P-0.027, S-0.026, Cr-0.01, Ni- 0.01, Mo-0.02, and balance iron (all in wt. percentage)were used as the substrate materials for the deposition of zinc phosphate coating. After grinding, the substrates were degreased with alkaline solution, ultrasonically cleaned in acetone. The phosphate specimens were rinsed with deionized water to remove the acid and the soluble salts left after phosphating. After rinsing, the specimens were dried using compressed air. The chemical composition of the zinc phosphating bath and its operating conditions are given in Table 1. All the reagents used in the experiments were of analytical purity. Table 1 Chemical composition, control parameters and operating conditions of the bath used for zinc phosphating
The nano ZnO was purchased from Aldrich. Phosphating was done by immersion process at room temperature(27◦C) for 30 min. The amount of iron dissolved during phosphating and the coating weight were determined in accordance with the standard procedures[15] .The normal zinc phosphate coating was developed at room temperature using optimized bath of chemical compositions : normal ZnO : 5g/L; H3PO4 :11.3mL/L; NaNO2:2g/L and pH= 2.7 [15 ]. The coatings’ surface morphology was examined by a Hitachi Scanning Electron Microscope SU1510 and the chemical composition was investigated by EDX. The phases in the phosphate coating were analyzed by XRD using Philips X’Pert pro diffractometer with Cu Kα (λ= 1.54060 Å) incident radiation. The XRD peaks were recorded in the 2θ range of 0°–100°. Page 2 of 2 Page 2 of 24
us
cr
ip t
Potentiodynamic polarization and electrochemical impedance measurements were carried out using Biologic Electrochemical Analyser (model SP 300) at the open circuit potential. The phosphated mild steel substrate formed the working electrode, whereas a saturated calomel electrode and a platinum electrode served as the reference and counter electrodes respectively. EC Lab software was used for data acquisition and analysis. Polarization technique was carried out from a cathodic potential of -2V to an anodic potential of 2V with respect to corrosion potential at a sweep rate 1 mV/s. Electrochemical impedance studies were carried out in the frequency range between 10000 and 0.01 Hz. The corrosion potential (Ecorr) and corrosion current density (icorr) were determined using Tafel extrapolation method. The charge transfer resistance (Rct) and double layer capacitance (Cdl) were determined from Nyquist plot by fitting the data using EC Lab software. All the experiments were repeated for confirming the reproducibility of the coatings.
M
an
Salt spray test was carried out by subjecting the phosphated panels to a salt mist of 5% sodium chloride solution in a salt spray chamber (ASTM B 117-03) for a specified period of time (96 hours) [16] . The edges of the substrates were sealed with paraffin wax to avoid the excessive corrosion at the edges. The extent of corrosion after 96 hours of exposure was assessed and photographed.
3.Results and Discussion
Ac ce p
te
d
During the preliminary investigations, the effect of amount of nano ZnO on the phosphate coating weight was studied by varying its concentrations in the phosphating bath. It was observed that there was no significant effect between the concentrations of 0.1- 0.25g/L of nano ZnO. But substantial increase in the coating weight was observed by varying the concentrations from 0.25 – 2g/L of nano ZnO in the phosphating bath with an immersion time of 30 minutes at room temperature (27◦C) was studied ( Fig. 1). Fig.1 Weight of phosphate coatings and iron dissolved during phosphating (*the standard deviation of above data is within 0.16g/m2) It was observed from the fig.1 that the phosphate coating weight increases with increase in the nano ZnO content in the phosphating bath from 0.25-2g/L. The increase in the coating weight was significant between the concentrations 1-1.5g/L of nano ZnO. But there was no substantial increase in the coating weight after 1.5g/L of nano ZnO. There is a steady increase in the coating weight from 0.5- 1.5g/L of nano ZnO. The phosphate coatings formed from the bath with 1.5g/L of nano ZnO is heavier than that formed from the other baths with different concentrations of nano ZnO and normal phosphate coating weight reported by the authors[14 ]. The amount of iron dissolved during phosphating was comparatively lower than the normal phosphating process, which reduces the amount of sludge formed during the process . The increase in the coating weight and decrease in the metal dissolution can be attributed to the increase in the nucleation sites and reduction in the size of the crystal clusters of nano zinc phosphate deposited on the mild steel plate[20]. Higher coating weight was resulted from the zinc phosphating bath consisting of Page 3 of 3 Page 3 of 24
1.5g/L of nano ZnO. This was confirmed by XRD, SEM, EDX ,potentiodynamic polarization and electrochemical impedance studies. Potential-Time measurements
d
M
an
us
cr
ip t
During phosphating, the potential of the mild steel coupled with saturated calomel electrode (reference electrode) is monitored continuously as a function of time for the entire duration of coating formation . The potential-time curves obtained for phosphating of mild steel using normal ZnO particles and nano ZnO particles ( Fig. 2) suggest that phosphating using nano ZnO particles shift the measured initial potential to more anodic direction. The anodic shift in potential represents the progressive build up of the phosphate coating formation [15].The shift towards cathodic direction was due to the conversion of soluble primary phosphate to insoluble tertiary phosphate (point of incipient precipitation). The time taken for attainment of incipient precipitation (induction time) is little earlier (2 minutes) for phosphating using nano ZnO particles than compared to phosphating using normal ZnO particles (6 minutes). This may probably be due to the activation effect brought by the nano ZnO particles in the phosphating bath[20]. A high negative shift displayed by normal zinc phosphating indicates the high rate of metal dissolution than compared to phosphating using nano ZnO.The plateau suggests that the reactions occurring at the interface reached the steady state and the surface is conversed. The earlier attainment of steady state in the presence of nano ZnO can be attributed to the reduction in the activation energy of the process by nano ZnO [20,21].
te
Fig. 2 The potential-time curves obtained during phosphating of mild steel using normal ZnO particles and nano ZnO particles
Effect of NaNO2 on the Coating Weight
Ac ce p
NaNO2 is the most popular accelerator used in the phosphating treatment. It is well established that phosphating reaction from unaccelerated baths tend to be slow owing to the polarization caused by hydrogen evolution at the cathode[15] . The very slow rate of recombination of hydrogen atoms to form hydrogen gas causes the formation of a very low coating weight. The effect of NaNO2 on the phosphate coating weight (at optimum concentration of nano ZnO(1.5g/L) )was studied by varying its concentration from 0 - 0.6g/L and the results have been exhibited in Fig.3. From the results, the optimum concentration of NaNO2 was found to be 0.4g/L for the present study. In the normal zinc phosphating baths reported, the concentration of NaNO2 is 2-16g/L [14,20-22]. The activation effect brought about by nano ZnO may be the reason for the requirement of small amount of NaNO2 for the present study [22].
Fig.3 The effect of NaNO2 on the phosphate coating weight (*the standard deviation of above data is within 0.16g/m2) TEM Page 4 of 4 Page 4 of 24
cr
ip t
Fig.4 shows TEM images of nano ZnO particles and nano zinc phosphate coating developed. The particles of the nano ZnO and nano zinc phosphate deposit have an average grain size between 30-40nm. Formation of nano crystalline zinc phosphate coating was confirmed by the TEM results.
us
Fig.4 TEM images of (a) nano ZnO and (b) nano zinc phosphate coating
an
SEM
Ac ce p
te
d
M
From the SEM micrographs( Fig.5), It was obvious that the size of the crystal clusters was reduced and the surface coverage was increased. In a comparison, the morphology of the surface coatings formed from the bath with nano ZnO 1.5g/L is more uniform and the crystal clusters bonded together more compactly than that formed from the other baths with different concentrations of nano ZnO and normal zinc phosphate coatings. It was observed that the incorporation of nano ZnO in the phosphating bath increases the degree of crystalline coverage by reducing the grain size of the phosphate coating. d
Fig.5 SEM images of phosphate coatings obtained from baths containing different contents of nano ZnO : ( a)0.25g/L,(b) 0.5g/L (c) 0.75g/L (d)1g/L, (e) 1.25g/L, (f) 1.5g/L, (g) 1.75g/L, (h) 2g/L and (i) normal ZnP coating On an average, there is an increase in the number of small sized crystals that grow when compared to few crystals growing large. The nano ZnO in the phosphating bath showed a significant effect on the reduction in the size of the zinc phosphate crystals which activate the surface of mild steel substrate and increase the number of micro cathodic sites which speeds up the hydrogen evolution reaction[21,22]. This enables the rapid consumption of free phosphoric acid and increases the interfacial pH between the mild steel substrate and phosphating solution. The increase in pH causes the conversion of soluble primary phosphate to insoluble tertiary Page 5 of 5 Page 5 of 24
cr
ip t
phosphate with the subsequent deposition of the phosphate coating on mild steel substrate[23].The optimum amount of nano ZnO increases the number of cathodic sites there by increasing the deposition of the phosphate coating[14]. However it was also observed that when the amount of nano ZnO increases beyond 1.5g/L in the phosphating bath, the coating is not compact and uniform and cracks were observed, while the degree of crystalline coverage also decreases. The possible reason is that the excessive amount of nano ZnO in phosphating solution would seal up the anodic surface by agglomeration, restraining anodic reaction. Even though the formation of well crystallized coating was observed in all the phosphating baths, denser and finer coating was observed in the phosphating solution with 1.5g/L nano ZnO. The initially deposited crystallites provide nucleation sites for further coating. In general, the smaller the size of the crystals, the higher their coverage and more effective coating was obtained [20,22 ]. EDX
Ac ce p
te
d
M
an
us
For a comparison. EDX analysis (Fig.6) of the well defined nano zinc phosphate coatings developed using 1.0g, 1.25g,1.5g and 1.75g/L of nano ZnO indicated that the phosphate coatings consists of both hopeite (Zn3(PO4)2.4H2O) and phosphophylite (Zn2Fe(PO4)2.4H2O) phases.The nano zinc phosphate deposit resulting from bath containing 1.5g/L of nano ZnO contains more zinc than from the other baths. The relative compositions of zinc phosphate coatings (wt.%) obtained by EDX analyses was given in Table 2. The composition of C and Si are not included in this table as they are insignificant in the coating phases. From the table it was observed that the ratio of Zn/P is about 1.38, 1.87, 2.20 and 1.92 for phosphate coatings developed using baths containing 1.0g, 1.25g,1.5g and 1.75g/L of nano ZnO respectively. This indicates that the content of Zn3(PO4)2.4H2O is higher than that of Zn2Fe(PO4)2.4H2O when the phosphating bath is having the optimum concentration of nano ZnO(1.5g/L). The nano zinc phosphate coating developed using 1.5g/L of nano ZnO indicates the formation of a thick coating as the composition of iron was less in this coating.
Fig.6 EDX of phosphate coatings developed using different contents of nano ZnO: (a)1g /L,(b) 1.25g/L, (c)1.5g/L,(d) 1.75g/L
Page 6 of 6 Page 6 of 24
ip t
Table 2 Relative Compositions of Zinc Phosphate Coatings (Wt.%) obtained by EDX
XRD
Ac ce p
te
d
M
an
us
cr
The phase compositions in the phosphate coatings on the mild steel were analyzed by XRD. The phase compositions of uncoated mild steel, mild steel substrates coated with normal zinc phosphate coating and phosphate coating developed using nano ZnO(1.5g/L) have been compared(Fig.7). It is shown that the phosphate coatings developed using nano ZnO mainly consisted of Zn3(PO4)2.4H2O (hopeite,JCPD file #37-0465). However , Zn2Fe(PO4)2.4H2O(phosphophyllite,JCPD file#29-1427)was also present in the coatings. The peaks of iron were due to the mild steel substrates. The peak intensities of nano crystalline ZnP coatings on mild steel specimen are stronger than those of normal ZnP coatings which clearly indicate the formation of thick phosphate crystal layer in the case of coatings produced using nano ZnO particles[20].The presence of nano ZnO particles in the phosphating bath increased the intensity of the(040)plane of hopeite phase and the intensity of (020) plane of hopeite and phosphophyllite phases. The results of XRD spectra indicated that the growth and coverage of the crystal clusters of phosphate coating increased when nano ZnO particles are used in the phosphating bath. From this study, considering the peak at degrees, average crystallite size has been estimated by using Debye-Scherrer formula [23].The calculated crystallite sizes have been presented in Table.3.The average crystallite size is less than 40 nm. The results confirm the formation of nano zinc phosphate crystals.
Fig.7 XRD patterns of phosphate coatings (a) uncoated mild steel (b) normal ZnP coating (c) phosphate coating developed using nano ZnO (1.5g/L)
Table.3 The calculated crystallite size of nano zinc phosphate coating using Scherrer calculator
Evaluation of Corrosion Performance Potentiodynamic Polarization
Page 7 of 7 Page 7 of 24
The protectiveness of the coatings wase evaluated through potentiodynamic polarization technique using 3.5% NaCl solution(Fig.8). Corrosion potential (Ecorr) ,Corrosion current density (icorr) and the corrosion rate derived from these data are presented in Table 4 .
cr
ip t
Fig.8 Polarisation curves of mild steel samples coated with (a) normal zinc phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO :( b)0.25g/L,(c) 0.5g/L (d) 0.75g/L (e)1g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Ac ce p
te
d
M
an
us
It is evident from the fig.8 that, for the substrates coated using nano ZnO particles(optimum range 1-1.5g/L ),the corrosion potentials have been shifted towards less negative values. The extent of shift in potential is largely a function of phosphate coating weight and the porosity of the coating[15]. A larger shift of Ecorr in the positive direction was observed when nano ZnO particles are used in the phosphating bath which indicates the improved corrosion resistance. Among the substrates studied, the substrate with phosphate coatings prepared from baths containing nano ZnO(1.5 g/L) has shown the more positive corrosion potential, lowest corrosion current density and the lowest corrosion rate which could be attributed to the more uniform and compact outer crystal layer. This is in good agreement with the results obtained from weight-loss measurements. The polarization current of phosphate coatings developed using nano ZnO particles in the range 1-1.5g/L show a marked decrease compared to that of the normal phosphate coating. It is observed that the dissolution takes place during anodic polarization and cathodic polarization is a diffusion controlled process. The depolarization of oxygen plays a major role in the corrosion failure of the coatings. The cathodic current primarily depends on the amount of oxygen arriving at the cathodic zone per unit area in unit time [22,23 ].The transport of oxygen to the substrate is hindered by the protective phosphate film between the substrate and the electrolyte due to which the average polarization current decreases considerably. Phosphate coatings are generally porous, which will favor adhesion of paint film on the surface. At the same time, the porosity favors the diffusion of the electrolyte which will ultimately result into corrosion. The decrease in the corrosion current for the coatings developed using nano ZnO particles (concentrations ranging from 1-1.5g/L) clearly indicate that the coating is more uniform and less porous than the normal zinc phosphate coating. Presence of nano ZnO particles in the phosphating bath caused better surface coverage , more homogeneous coating which lead to decrease in the rate of corrosion of these substrates. However, when the content of nano ZnO exceeds 1.5g/L there is a small increase in the polarization current and the corrosion potential is little less positive. The nano ZnO (optimum amount) in the phosphating bath leads to compact nucleation and growth of zinc phosphate crystals results in a denser morphology containing more crystal clusters[24]. But beyond optimum concentration, the increase in the nano ZnO content ,may lead to agglomeration of nano particles which disfavor the formation of intact phosphate coatings. Table 4 Polarisation parameters of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution Page 8 of 8 Page 8 of 24
Electrochemical Impedance Characteristics
M
an
us
cr
ip t
Comparison of Nyquist plots of phosphate coatings developed using nano ZnO particles and the normal zinc phosphate coatings in 3.5% NaCl have been shown in Fig.9 and the results have been presented in Table 5. To account for the corrosion behavior of phosphated panels in 3.5% sodium chloride solution, an equivalent circuit model (Fig.10) is proposed. The electrolyte/coating-metal interface approximates such a model [20 ] where R1 is the solution resistance ,R2 is the coating resistance, R3 is the charge transfer resistance, C1is the coating capacitance and Cdl is the double layer capacitance. The semicircles obtained are not well defined. The semicircles obtained are flattened capacitive semicircles. The flattened capacitive semicircles are related to frequency dispersion due to defects in the phosphate coating [25,26 ]. The shape of the EIS plots for mild steel substrates with nano zinc phosphate coatings is almost identical, but the size of the plots is increased with nano-ZnO content from 0.25 to 1.5 g/L. However, when the content of nano-ZnO exceeds 1.5 g/L or more, there is a drop.
te
d
Fig.9 EIS of mild steel samples coated with (a) normal phosphate coating and phosphate coatings obtained from baths containing different contents of nanoZnO : ( b)0.25g/L(c) 0.5g/L (d) 0.75g/L (e)1g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Ac ce p
A high charge transfer resistance and low double layer capacitance values were obtained for coatings developed using nano ZnO particles. The impedance studies confirmed that the corrosion behavior of phosphated panels using nano zinc oxide particles is a much more diffusion controlled process and thereby offering a higher corrosion resistance than the normal phosphate coating[16]. As discussed earlier, nano zinc oxide particles increases the surface coverage, uniformity and the thickness of the coating and decreases the porosity which improves the corrosion resistance of the phosphate coatings more than that of the normal zinc phosphate coating[20]. But beyond the optimum concentration(1.5g/L), the increase in the nano ZnO content in the phosphating bath results in micro cracks which lead to small decrease in the charge transfer resistance and increase in the double layer capacitance. This is in good agreement with the results of weight loss measurements and SEM photographs of these coatings.
Fig.10 Equivalent circuit model proposed to account for the corrosion of phosphate coatings obtained from baths containing nanoZnO.
Page 9 of 9 Page 9 of 24
ip t
Table 5 Charge transfer resistance and double layer capacitance of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution
Salt Spray Test
M
an
us
cr
Salt spray test measures the ability of various types of coatings to withstand in a corrosive-cum-humid atmosphere. As paint pre-treatment coating, the phosphate coating is expected to improve the adhesion of the paint coating and prevents the spreading of underfilm corrosion. The ability to prevent underfilm corrosion is best measured by assessing the spreading of corrosion from the X-scratch made on the phosphated and painted mild steel substrate[ 16 ].The spreading of corrosion from the scribe is less in the case of nano zinc phosphate coatings than normal zinc phosphate coatings on mild steel specimens after 96 hours of salt spray test ( Fig.11). During salt spray test, chloride ions attack the base metal, forming iron (II) chloride followed by hydrolysis and rust formation at the anodic areas (exposed bare steel in the scribed region). The corresponding cathodic reaction is the reduction of oxygen to form hydroxide ions which dissolve the phosphate at the cathodic areas [ 16 ].
Ac ce p
te
d
Fig:11 The corrosion behaviour of (a) nano zinc phosphate coating and (b) normal zinc phosphate coating on mild steel specimens subsequently finished with a paint coating (DFT: 50 µm) after subjecting them to salt spray test for 96 hours.
Conclusions
The paper reports the development of nano zinc phosphate coatings on mild steel substrates. The nano zinc phosphate coating weight increased with increase in the concentration of nano ZnO particles from0.25-2g/L in the phosphating baths . The concentration of nano ZnO in the phosphating bath was optimized to 1.5g/L .The nano zinc phosphate coating consists of more of hopeite phase than the phosphophylite phase. The nano ZnO acts as a nucleating agent, increases the number of nucleation sites for deposition and reduces the size of the phosphate crystal clusters formed which enabled higher coating weight. The decrease in the induction time, earlier attainment of steady state for the formation of new phases ,activation effect brought about by the nano ZnO decreases the amount of accelerator (NaNO2) required for phosphating than the normal phosphating process. The results of potentiodynamic polarization , impedance studies and salt spray test confirmed that the nano zinc phosphate coating offers better corrosion protection than normal zinc phosphate coating. Hence, it can be concluded that nano zinc phosphate coatings can substitute normal phosphate coatings due to its better corrosion resistance and less pollutant nature. Page 10 of 10 Page 10 of 24
References Rausch, W, ThePhosphating of Metals. Finishing PublicationsLtd., London, 1990 J.B. Bajat, V.B. Miskovic-Stankovic, J.P. Popic, D.M. Drazic ,Adhesion characteristicsand corrosion stability of epoxy coatings electrodeposited on phosphate hot-dip galvanized steel , Prog. Org. Coat. 63 (2008) 201–208 [3] W. H. Kok, X. Sun, L. Shi, K. C. Wong, K. A. R. Mitchell, Formation of zinc phosphate coatings on AA6061aluminum alloy, Journal of Materials Science 36 (2001) 3941 – 3946 [4] B.L. Lin, J.T. Lu, K. Gang, Synergistic corrosion protection for galvanized steel byphosphating and sodium silicate post-sealing, Surf. Coat. Technol. 202 (2008) 1831– 1838 [5] A.S. Akhtar, D. Susac, P. Glaze, K.C. Wong, P.C. Wong, K.A.R. Mitchell, The effect of Ni2+ on zinc phosphating of 2024-T3 Al alloy, Surf. Coat. Technol. 187 (2004) 208–215 [6] P.K. Sinha,, R. Feser, Phosphate coating on steel surfaces by an electrochemical method, Surf. Coat. Technol. 161(2002) 158–168 [7] Y.K. Song, F. Mansfeld, Development of a molybdate–phosphate-silane-silicate(MPSS) coating process for electrogalvanized steel, Corros. Sci. 48 (2006) 154–164 [8] M. Wolpers, J. Angeli ,Activation of galvanized steel surfaces before zinc phosphatingXPS and GDOES investigations, Appl. Surf. Sci. 179 (2001) 281–291, [9] S.-L. Zhang, H.-H. Chen, X.-L. Zhang, M.-M. Zhang, The growth of zinc phosphate coatings on 6061-Al alloy, Surf. Coat. Technol. 202(2008) 1674–1680 [10] G. Li, L. Niu, J. Lian, Z. Jiang , A black phosphate coating for C 1008 steel, Surf.Coat. Technol. 176 (2004) 215–221 [11] A.S. Akhtar, K.C. Wong, P.C. Wong, K.A.R. Mitchell ,Effect of Mn2+ additive onthe zinc phosphating of 2024-Al alloy. Thin Solid Films 515 (2007)7899–7905 [12] A.S. Akhtar, K.C. Wong, K.A.R. Mitchell, The effect of pH and role of Ni2+ in zincphosphating of 2024-Al alloy. Part 1. Macroscopic studies with XPS and SEM, Appl. Surf. Sci. 253 (2006) 493–501.
Ac ce p
te
d
M
an
us
cr
ip t
[1] [2]
[13] S. Palraj, M. Selvaraj, P. Jayakrishnan , Effect of phosphate coatings on the performanceof epoxy poly amide red oxide primer on galvanized steel, Prog. Org. Coat. 54 (2005) 5–9 [14] M. Arthanareeswari, T. S. N. Sankara Narayanan, P. Kamaraj, and M. Tamilselvi, Polarization and impedance studies on zinc phosphate coating developed using galvanic coupling, Journal of Coatings Technology Research 9 (2012) 39–46 [15] M. Arthanareeswari, T. S. N. Sankara Narayanan, P. Kamaraj, and M. Tamilselvi ,Influence of galvanic coupling on the formation of zinc phosphate coating, Indian Journal of Chemical Technology 17 (2010) 167–175 [16] M. Arthanareeswari, P. Kamaraj and M.Tamilselvi, Anticorrosive performance of zinc phosphate coatings on mild steel developed using galvanic coupling, Journal of Chemistry, 2013 (2013) 1-8 [17] Xiuzhi Zhang, Fuhui Wang, Du. Yuanlong , Effect of nano-sized titanium powder addition on corrosion performance of epoxy coatings, Surf. Coat. Technol. 201 (2007) , 7241-7245.
Page 11 of 11 Page 11 of 24
d
M
an
us
cr
ip t
[18] AlkaPhanasgaonkar, V.S. Raja, Influence of curing temperature, silica nanoparticles- and cerium on surface morphology and corrosion behaviour of hybrid silane coatings on mild steel .Surf. Coat. Technol. 203 (2009) 2260-2271 [19] A.N. Khramov, V.N. Balbyshev, N.N. Voevodin,M.S.Donley, Nanostructured Sol–gel Derived Conversion Coatings Based on Epoxy- and Amino-silanes, Prog. Org. Coat.47(2003) 207-213 [20] S.M.A. Shibli , Francis Chacko, Development of nano TiO2-incorporated phosphate coatings on hot dip zinc surface for good paintability and corrosion resistance, Applied Surface Science 257 (2011) 3111–3117 [21] Zhang Shenglin ,Study on phosphating treatment of aluminium alloy : role of yttrium oxide, Journal of rare earths 27 (2009) 469-473 [22] Minqi Sheng,Yi Wang,Qingdong Zhong,Hongyan Wu,Qiongyu Zhou,Hai Lin , The effects of nano-SiO2 additive on the zinc phosphating of carbon steel, Surf. Coat. Technol. 205 (2011) 3455-3460 [23] B. D. Hall, D. Zanchet and D. Ugarte , Estimating nanoparticle size from diffraction measurements , J.Appl. Cryst., 33 (2000) 1335-1341 [24] T. S. N. Sankara Narayanan and M. Subbaiyan ,Effect of Surfactants on the Growth and Crystal Habit of Zinc-Phosphate Coating, Trans.Inst. Met.Finish.71(1993) 37-40 [25] Susac D, Sun X, Li R Y, Wong K C, Wong P C, Mitchell K AR, Champaneria R, Microstructural effects on the initiation of zinc phosphate coatings on 2024-T3 aluminum alloy, Surf.Coat. Technol.,239 (2004) 45-59 [26] Weng, D, Jokiel, P, Uebleis, A, Boehni, H ,Corrosion and Protection Characteristics of Zinc and Manganese Phosphate Coatings, Surf. Coat. Technol. 88 (1997) 147–156
Ac ce p
te
[27] E.M.A. Martini, S.T. Amaral, I.L. Muller , Electrochemical behaviour of Invar in phosphate solutions at pH=6.0,Corros. Sci. 46(2004) 2097-2115
Page 12 of 12 Page 12 of 24
Table 1 Chemical composition, control parameters and operating conditions of the bath used for zinc phosphating Chemical Composition 0.25- 2g/L 2.3ml/L 0.4g/L
ip t
Nano ZnO H3PO4 NaNO2 Control Parameters
2.70 3pointage 25pointage 1: 8.33 27◦C 30 minutes
an
us
cr
pH Free acid value(FA) Total Acid Value(TA) FA:TA Temperature Time
Table 2 Relative Compositions of Zinc Phosphate Coatings (Wt.%) obtained by EDX Zn
P
Zn/P
28.40
41.13
16.05
11.55
39.91
32.72
15.66
8.38
27.55
36.87
20.18
9.16
36.56
34.60
14.00
7.30
1.38 1.87 2.20 1.92
M
O
d
Fe
Ac ce p
te
Nano zinc phosphate coatings developed using nano ZnO (g/L) 1 1.25 1.5 1.75
Table.3 The calculated crystallite size of nano zinc phosphate coating using Scherrer calculator No. 1 2 3 4 5 6 7
B obs. [°2Th]
B std. [°2Th]
0.492 0.394 0.394 0.590 0.394 0.394 0.394
0.180 0.180 0.180 0.180 0.180 0.180 0.180
Peak pos. [°2Th] 9.953 19.457 19.973 26.310 31.502 40.531 64.210
B struct. [°2Th] 0.312 0.214 0.214 0.410 0.214 0.214 0.214
Crystallite size [nm] 26 37 37 20 38 39 43
Page 13 of 13 Page 13 of 24
an
us
cr
ip t
Table 4 Polarisation parameters of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution SYSTEM STUDIED Ecorr (mV) VS SCE Icorr(µA/cm2) CORROSION RATE -611 16.03 11.37 a) normal Zinc phosphate coated mild steel substrate Phosphated mild steel substrates from baths containing different contents of Nano Zinc Oxide b) 0.25 g/L -610 28.68 13.29 c) 0.5 g/L -553 12.13 5.62 d) 0.75 g/l -539 10.57 4.90 e) 1.0 g/L -509 9.83 4.56 f) 1.25 g/L -488 6.92 3.21 g) 1.5 g/L -465 6.60 3.06 h) 1.75 g/L -488 7.84 3.64 i) 2.0 g/L -494 7.83 3.63
te
d
M
Table 5 Charge transfer resistance and double layer capacitance of mild steel samples coated with normal phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO in 3.5% NaCl solution Rct System studied (Ω cm2) Cdl (F) x 10-6 550 3.56 a) normal Zinc phosphate coated mild steel substrate
Ac ce p
Phosphated substrates from baths containing different contents of Nano Zinc Oxide 87 32.68 b) 0.25 g/L c) 0.5 g/L 289 4.51 432 2.88 d) 0.75 g/l 651 1.42 e) 1.0 g/L 654 1.38 f) 1.25 g/L 830 0.95 g) 1.5 g/L 758 1.32 h) 1.75 g/L 637 1.41 i) 2.0 g/L
Page 14 of 14 Page 14 of 24
M
an
us
cr
ip t
Figures
Ac ce p
te
d
(*the standard deviation of above data is within 0.16g/m2)
Page 15 of 15 Page 15 of 24
phosphating using normal ZnO phosphating using nano ZnO
-430 -440 -450 -460
ip t
-480 -490 -500 -510
cr
-520 -530 -540 -550 -560 -570 -580 0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
an
Time in minutes
us
potential(mV vs SCE)
-470
Ac ce p
te
d
M
Fig. 2 The potential-time curves obtained during phosphating of mild steel using normal ZnO particles and nano ZnO particles
Fig.3 The effect of NaNO2 on the phosphate coating weight (*the standard deviation of above data is within 0.16g/m2) Page 16 of 16 Page 16 of 24
ip t cr an
us
b
aa
Ac ce p
te
d
M
Fig.4 TEM images of (a) nano ZnO and (b) nano zinc phosphate coating
Page 17 of 17 Page 17 of 24
ip t
b
C
e
f
h
i
Ac ce p
te
d
M
d
an
us
cr
a
g
Fig.5 SEM images of phosphate coatings obtained from baths containing different contents of nano ZnO : ( a)0.25g/L, (b) 0.5g/L, (c) 0.75g/L, (d)1.0g/L, (e) 1.25g/L, (f) 1.5g/L, (g) 1.75g/L, (h) 2g/L and (i) normal ZnP coating
Page 18 of 18 Page 18 of 24
b
us
cr
ip t
a
d
d
M
an
c
Ac ce p
te
Fig.6 EDX of phosphate coatings developed using different contents of nano ZnO: (a)1.0g /L,(b) 1.25g/L, (c)1.5g/L,(d) 1.75g/L
Page 19 of 19 Page 19 of 24
o # (110)
o #
*
*
c
cr
#
* o (241)
*
Fe
ip t
* (040)
#
Intensity
* # (020)
Zn3(PO4)2.4H2O # Zn2Fe(PO4)2.4H2O
us
b
a
20
40
60
80
an
0
100
2theta
Ac ce p
te
d
M
Fig.7 XRD patterns of phosphate coatings (a) uncoated mild steel, (b) normal ZnP coating, (c) phosphate coating developed using nano ZnO (1.5g/L)
Page 20 of 20 Page 20 of 24
g h
ip t
f
i -0.5
e Ewe/V
-0.55
d -0.6
c
us
a
an
-0.65
-0.7
-2 log (|/mA|)
b
-1
M
-3
cr
-0.45
Ac ce p
te
d
Fig.8 Polarisation curves of mild steel samples coated with (a) normal zinc phosphate coating and phosphate coatings obtained from baths containing different contents of nano ZnO :( b)0.25g/L,(c) 0.5g/L, (d) 0.75g/L, (e)1.0g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Page 21 of 21 Page 21 of 24
ip t an
us
cr
Im(Z)/Ohm
Re(Z)/Ohm
Ac ce p
te
d
M
Fig.9 EIS of mild steel samples coated with (a) normal phosphate coating and phosphate coatings obtained from baths containing different contents of nanoZnO : ( b)0.25g/L,(c) 0.5g/L, (d) 0.75g/L, (e)1.0g/L, (f) 1.25g/L, (g) 1.5g/L, (h) 1.75g/L, (i) 2g/L
Fig.10 Equivalent circuit model proposed to account for the corrosion of phosphate coatings obtained from baths containing nanoZnO.
Page 22 of 22 Page 22 of 24
ip t cr
an
b
us
a
Ac ce p
te
d
M
Fig:11 The corrosion behaviour of (a) nano zinc phosphate coating and (b) normal zinc phosphate coating on mild steel specimens subsequently finished with a paint coating (DFT: 50 µm) after subjecting them to salt spray test for 96 hours.
Page 23 of 23 Page 23 of 24
Highlights of the paper Nano zinc phosphate coating on mild steel was developed Nano zinc phosphate coatings on mild steel showed enhanced corrosion resistance.
ip t
The nano ZnO increases the number of nucleating sites for phosphating.
Ac ce p
te
d
M
an
us
cr
Faster attainment of steady state during nano zinc phosphating.
Page 24 of 24 Page 24 of 24