Development of nano SiO2 incorporated nano zinc phosphate coatings on mild steel

Applied Surface Science 332 (2015) 12–21

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Development of nano SiO2 incorporated nano zinc phosphate coatings on mild steel M. Tamilselvi a , P. Kamaraj b , M. Arthanareeswari b,∗ , S. Devikala b , J. Arockia Selvi b a b

Department of Chemistry, Thiru Kolanjiappar Government Arts College, Virudhachalam 606001, India Department of Chemistry, SRM University, Kattankulathur 603203, India

a r t i c l e

i n f o

Article history: Received 29 November 2014 Received in revised form 22 January 2015 Accepted 22 January 2015 Available online 30 January 2015 Keywords: Nano ZnO Nano SiO2 Phosphate coating Mild steel

a b s t r a c t This paper reports the development of nano SiO2 incorporated nano zinc phosphate coatings on mild steel at low temperature for achieving better corrosion protection. A new formulation of phosphating bath at low temperature with nano SiO2 was attempted to explore the possibilities of development of nano zinc phosphate coatings on mild steel with improved corrosion resistance. The coatings developed were studied by Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Electrochemical measurements. Significant variation in the coating weight, morphology and corrosion resistance was observed as nano SiO2 concentrations varied from 0.5–4 g/L. The results showed that, the nano SiO2 in the phosphating solution changed the initial potential of the interface between mild steel substrate and phosphating solution and reduce the activation energy of the phosphating process, increase the nucleation sites and yielded zinc phosphate coatings of higher coating weight, greater surface coverage and enhanced corrosion resistance. Better corrosion resistance was observed for coatings derived from phosphating bath containing 1.5 g/L nano SiO2 . The new formulation reported in the present study was free from Ni or Mn salts and had very low concentration of sodium nitrite (0.4 g/L) as accelerator. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Phosphating is the most widely used metal pretreatment process for the surface treatment and finishing of ferrous and nonferrous metals. Due 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]. Majority of the phosphating baths reported in literature require very high operating temperatures ranging from 90 to 98 ◦ C. The main drawback associated with high temperature operation is the energy demand, which is a major crisis in the present day scenario [2]. One possible way of meeting the energy demand and eliminating the difficulties encountered due to scaling of heating coils and, over heating of the bath, is through the use of low temperature phosphating baths. Though known to be in use since the 1940s [3], the low temperature phosphating processes have become more significant today due to the escalating energy costs. However, low temperature phosphating processes are very slow and need to be accelerated by some

∗ Corresponding author. Tel.: +91 044 27455855. E-mail address: [email protected] (M. Arthanareeswari). http://dx.doi.org/10.1016/j.apsusc.2015.01.177 0169-4332/© 2015 Elsevier B.V. All rights reserved.

means. The use of nitrites as the accelerator is most common in low temperature operated phosphating baths. However, a higher concentration of nitrite is required to increase the rate of deposition of phosphate coatings at low temperatures. The environmental protection agency (EPA) has classified nitrite as toxic in nature and hence use of nitrite as accelerator could cause disposal problems [4]. 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 [5,6]. Zinc phosphating is one of the promising method for enhancing the corrosion resistance of iron and steel [7]. 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 [8–10]. The utility of the galvanic coupling for accelerating low temperature zinc phosphating processes was established recently [11,12]. 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 [13–17]. Development of nano zinc phosphate coatings for enhanced corrosion resistance of mild steel was reported recently [18]. Nickel plays a major role in achieving an acceptable corrosion resistance of the coating as

M. Tamilselvi et al. / Applied Surface Science 332 (2015) 12–21

well as accelerating the process chemistry. More recent developments have created nickel-free processes that can compete with the nickel containing processes [7,17–19]. In the present study, it was aimed to develop a phosphating bath at low temperature and with low concentration of accelerator, to explore the formation of nano zinc phosphate coatings with improved corrosion resistance using nano SiO2 and nano ZnO. Phosphate coatings of enhanced corrosion resistance was developed, characterized and evaluated.

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carried out in the frequency range between 10,000 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 with multiple samples for confirming the reproducibility of the coatings. 3. Results and discussion

2. Experimental details Mildsteel specimens of dimensions 8.0 cm × 6.0 cm × 0.2 cm having composition C-0.16, Si-0.17, Mn-0.68, P-0.027, S-0.026, Cr0.01, Ni- 0.01, Mo-0.02, and balance iron (all in wt.%) were used as the substrate materials for the deposition of zinc phosphate coating. The specimens were abraded with a series of emery papers up to 400 grits and degreased with alkaline solution and rinsed in deionised water. The chemical composition of the zinc phosphating bath and its operating conditions are given in Table 1. The nano ZnO and nano SiO2 were purchased from Aldrich. The particles of the nano ZnO and nano SiO2 has an average grain size between 30 and 40 nm. All the other chemicals used for the study were of analytical grade. Nano zinc oxide was dissolved in water in the presence of phosphoric acid. Nano SiO2 was added to the phosphating solution with vigorous stirring and NaNO2 was added just before the phosphating process. The pH of the bath was adjusted to 3 by adding NaOH. Phosphating was done by immersion process at room temperature (27 ◦ C) for 30 min. Then the phosphated specimens were rinsed with deionised water to remove the acid and the soluble salts left after phosphating. After rinsing, the specimens were subjected to drying by using compressed air. The amount of iron dissolved during phosphating and coating weight were determined in accordance with the standard procedures [7]. 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 ˚ incident radiation. The XRD peaks were Cu K␣ (␭ = 1.54060 A) recorded in the 2 range of 0◦ –100◦ . 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 −250 mV(SCE) to an anodic potential of +250 mV(SCE) with respect to corrosion potential at a sweep rate 1 mV/s. Electrochemical impedance studies were

3.1. Effect of concentration of nano SiO2 on the phosphate coating weight The effect of amount of nano SiO2 on the coating weight was studied by varying its concentrations (0.5–4 g/L) in the phosphating bath with an immersion time of 30 min at room temperature is shown in Fig. 1. At concentrations less than 0.1 g/L of nano SiO2 , there is no significant effect on the coating weight. It was observed that the phosphate coating weight increases linearly from 0.5–1.5 g/L of nano SiO2 in the phosphating bath. The phosphate coatings obtained from bath containing 1.5 g/Lof nano SiO2 is the heaviest. There is no substantial increase in the coating weight beyond 1.5 g/L of nano SiO2 . From the Fig. 1, it was observed that the increase in coating weight with increase in the concentration of nano SiO2 in the phosphating bath exhibits linearity only from 0.5 to 1.5 g/L while it followed a Boltzmann fit for the entire range. The adj. R-square value given in the fitting parameter gives an estimate of goodness of fit of the function to the data [20]. This is 0.997 for the coating weight, which means that 99.7% of the variation of the ‘independent’ variable (concentration of SiO2 in g/L) can be explained by the variation of the ‘dependent’ variable (coating weight obtained). The iron dissolved during phosphating had followed the LogNormal fit, which is the best fit for entire range. The adj. R-square value for this fit is 0.995. Presence of nano SiO2 reduces the crystal size of nano zinc phosphate, increases the compactness of the coating which results in higher coating weight than compared to nano zinc phosphate coatings developed in the absence of nano SiO2 [18]. Presence of nano SiO2 at optimum concentration (1.5 g/L), confirmed by further analysis, accelerate the formation of new phosphate crystal nucleation and thereby increasing the phosphate coating weight. But further increase in the concentrations of nano SiO2 reduce

Table 1 Chemical composition, control parameters and operating conditions of the bath used for zinc phosphating. Chemical composition Nano ZnO Nano SiO2 H3 PO4 NaNO2

1.5 g/L 0–4 g/L 2.3 ml/L 0.4 g/L Control parameters

pH Free acid value (FA) Total acid value (TA) FA:TA Temperature Time

3 ± 0.1 3 pointage 25 pointage 1: 8.33 27 ◦ C ± 3 ◦ C 30 min

Fig. 1. The weight of phosphate coatings obtained and iron dissolved during phosphating of mild steel using baths containing different contents of nano SiO2 .

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b

-440

Potential (mV vs SCE)

-460

-480

a

-500

-520

-540

-560 0

5

10

15

20

25

30

Time in minutes Fig. 2. The potential-time curves obtained during nano zinc phosphating of mild steel (a) without the presence of nano SiO2 and (b) with the presence of nano SiO2 .

Fig. 3. The variations of phosphate coating weight as a function of time during phosphating.

the zinc phosphate nucleation rate and inhibit subsequent coalescence and growth. Hence there was no substantial increase in the coating weight after optimum concentrations of nano SiO2 . One significant observation is the reduction in the amount of iron dissolved during phosphating when compared to nano zinc phosphate coatings developed in the absence of nano SiO2 . The possible reason is that the nano SiO2 activates the mild steel surface and restrains the anodic dissolution reaction [17]. Incorporation of SiO2 in the phosphating bath results in increasing the density of nucleation sites. The nano zinc phosphate crystals nucleate on these sites. The amount and distribution of these nucleation sites affect the number of phosphate crystals that are initiated [19]. Due to high density of nucleation sites, large number of nano zinc phosphate crystals would nucleate during phosphating resulting in finer, denser and uniform coatings. Zhang Shenglin [19] reported that the additive Y2 O3 (metal oxide electro negativity is 5.406) in the phosphating bath, being strongly electronegative, they can easily be polarized, metamorphosed and adsorbed on the substrate and become nucleation sites. This could be extended to the present work also. As the nano SiO2 (metal oxide electronegativity is 6.190) is a strong electronegative, this could have been adsorbed on mild steel surface and become nucleation sites.

to phosphating without nano SiO2 . After attaining the most anodic potential, the potential shifts in the cathodic direction until it stabilized practically at a constant value. The plateau suggests that the reactions occurring at the interface reached the steady state and the surface is conversed and the phosphate coating become dense. The earlier attainment of steady state in the presence of nano SiO2 than without it can be attributed to reduction in the activation energy of the process by nano SiO2 [16,17]. The initial potentials are also different for phosphating from baths with and without nano SiO2 . It is reported that [19] when different concentrations of Y2 O3 is present in the phosphating bath, the change in the open circuit potentials of the substrate is due to the adsorption of Y2 O3 on the substrate, which results in change of double electrode layer at the interface between the substrate and phosphating solution, and causes potential shift. This may be the reason for the shift in the initial open circuit potentials during phosphating of mild steel with the presence of nano SiO2 . Achieving point of incipient precipitation and steady state potential quickly by the phosphating process from the bath containing nano SiO2 than without it confirmed the acceleration of conversion of soluble primary phosphates to insoluble tertiary phosphates by the cathodic depolarizing ability of nano SiO2 [22].

3.2. Potential-time measurements

3.3. Effect of phosphating time on the coating weight

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 with and without nano SiO2 particles (Fig. 2) suggest that phosphating using nano SiO2 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 [21]. The shift towards cathodic direction was due to the conversion of soluble primary phosphate to insoluble tertiary phosphate (point of incipient precipitation). The shift of potential to the anodic direction is rapid for both phosphating processes (1 min for bath with nano SiO2 and 2 min for bath without nano SiO2 ) during the initial period. This confirms the activation effect brought about by both the nano ZnO and nano SiO2 particles in the phosphating bath [22]. It also implied that the phosphate coating formed quickly. A less anodic shift displayed by zinc phosphating with nano SiO2 indicates the decrease in the rate of metal dissolution than compared

The variations of phosphate coating weight as a function of time is shown in Fig. 3 The variations of phosphate coating weight with phosphating time represent the growth rate of phosphate coating. Fig. 3 shows that during the initial period of phosphating, the coating weight increased proportionally with respect to time. This is also confirmed by potential-time measurements (Fig. 2). When the majority of the substrate was coated, the growth rate of the coating decreases and saturation level was achieved. Fig. 3 also shows that the nano zinc phosphate coating obtained from bath containing nano SiO2 is heavier than the coating developed without nano SiO2 . The nano zinc phosphate coating weight increases linearly from 0 to 15 min (Fig. 3), then it followed a Boltzmann fit for the entire range. The adj. R-square value (0.999) indicated the goodness of fit. Weng et al. [23] reported that, between coating thickness and coating weight, there is a relation of about1 ␮m corresponding to 1.5–2 g/m2 for most of the phosphate coatings. Hence it can be confirmed that the nano SiO2 in the phosphating bath produces a thick nano zinc

M. Tamilselvi et al. / Applied Surface Science 332 (2015) 12–21

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Fig. 4. The effect of NaNO2 on the phosphate coating weight.

Fig. 6. SEM image of nano SiO2 .

phosphate coating than from the bath without it. The SiO2 in the bath caused an activating effect to achieve rapid conversion of insoluble phosphates and subsequent deposition on the substrate, yielding phosphate coatings with greater thickness.

the reason for the requirement of small amount of NaNO2 for the present study.

3.4. Effect of NaNO2 on the coating weight

Fig. 5 shows TEM images of nano SiO2 particles and nano zinc phosphate coating developed from bath containing nano SiO2 (1.5 g/L). The particles of the nano SiO2 and nano zinc phosphate deposit have an average grain size between 30 and 40 nm. Formation of nano crystalline zinc phosphate coating was confirmed by the TEM results.

Some experiments were performed without sodium nitrite to understand whether the nano SiO2 alone could accelerate the low temperature phosphating process. But in the absence of NaNO2 , with the immersion time of 30 min, no significant phosphate coating weight was observed and iron dissolution was more pronounced. 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 [21]. The very slow rate of recombination of hydrogen atoms to form hydrogen gas causes the formation of a very low coating weight [7]. The effect of NaNO2 on the phosphate coating weight developed using nano SiO2 (1.5 g/L) has been exhibited in Fig. 4. The nano zinc phosphate coating weight increases linearly with increase in the concentration of NaNO2 from 0 to 0.4 g/L, then it followed a Boltzmann fit for the entire range (Fig. 4). The adj. R-square value (0.997) indicated the goodness of fit. From the results, the optimum concentration of NaNO2 was found to be 0.4 g/L for the present study. In the normal zinc phosphating baths reported, the concentration of NaNO2 is 2–16 g/L [7,16,17,21]. The decrease in the activation energy of the phosphating process by the nano SiO2 and nano ZnO may be

3.5. TEM

3.6. SEM The SEM image of nano SiO2 was shown in Fig. 6. The morphology of the nano zinc phosphate coatings obtained from baths with and without nano SiO2 was demonstrated in Fig. 7. From the SEM micrographs, it can be seen that the diameter of the crystals of the phosphate coating is around 1 ␮m. The visible crystals are phosphate compounds of the coatings and they are usually of more than 10 ␮m in a normal phosphating process. Well-crystallized coatings were observed from the baths containing with and without nano SiO2 . It is observed that the crystal clusters of the coatings were more compact with the presence of nano SiO2 than compared without it. It was evident that the incorporation of nano SiO2 in the phosphating bath increases the degree of crystalline coverage by reducing the grain size of the phosphate coating.

Fig. 5. TEM images of (a) nano SiO2 and (b) nano zinc phosphate coating developed from bath containing nano SiO2 (1.5 g/L).

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Fig. 7. SEM images of nano zinc phosphate coatings developed from baths containing different contents of nano-SiO2 ; (a) 0 g/L, (b) 0.5 g/L, (c) 1.0 g/L, (d) 1.5 g/L, (e) 2.0 g/L, (f) 2.5 g/L, (g) 3.0 g/L, (h) 3.5 g/L, (i) 4 g/L.

The nano SiO2 acts as a nucleation agent and increases the nucleation sites for nano zinc phosphate coating. The nano SiO2 in the phosphating bath showed a significant effect on the reduction in the size of the nano 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. 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 phosphate with the subsequent deposition of the phosphate coating on mild steel substrate. This results into denser and finer coatings than that in the phosphating bath without nano SiO2 . Instead of a few crystals growing large, many grows to a smaller size on average. Although well-crystallized coatings were observed for all the concentrations of SiO2 , the coatings obtained from the baths containing SiO2 ranging from 1 to 2.5 g/L is denser and finer than others. In a comparison, the microstructure of the surface coatings formed in the bath with nano SiO2 1.5 g/L is the most uniform and the crystal clusters bonded together more compactly than that formed in other baths. The optimum amount of nano SiO2 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 SiO2 increases beyond 2.5 g/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 SiO2 in phosphating solution would seal up the anodic surface by agglomeration, restraining anodic reaction [17]. 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 is obtained [24]. The incorporation of nano SiO2 in to the phosphating bath caused structural refinement of the crystal and also helped to achieve maximum surface coverage. 3.7. EDX EDX analysis (Fig. 8) confirm that SiO2 is present in the coating. The zinc phosphate coating resulting from bath containing 1.5 g/L of nano SiO2 has more of hopeite phase (contain more zinc than the others). The relative compositions of zinc phosphate coatings (wt.%) obtained by EDX analyses was given in Table 2.

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Fig. 8. EDX of nano zinc phosphate coatings developed from baths containing different contents of nano-SiO2 ; (a) 0.5 g/L, (b) 1.0 g/L, (c) 1.5 g/L, (d) 2 g/L, (e) 2.5 g/L, (f) 3 g/L, (g) 3.5 g/L, (h) 4 g/L.

The composition of C was not included in this table as they are insignificant in the coating phases. It was observed that the ratio of Zn/P is about 2.20, 2.02, 3.63, 2.70, 2.31, 2.23, 2.04, 1.78 for phosphate coatings developed using baths containing 0.5 g, 1.0 g, 1.5 g, 2.0 g, 2.5 g, 3.0 g, 3.5 g, and 4.0 g, respectively. This indicates

that the content of Zn3 (PO4 )2 ·4H2 O (hopeite) is higher than that of Zn2 Fe(PO4 )2 ·4H2 O (phosphophyllite)when the phosphating bath is having the optimum concentration of nano SiO2 (1.5 g/L) [25]. Between the concentrations of 1.0–2.0 g of nano SiO2 , the composition of iron in the coating is less whereas the compositions of

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Table 2 Relative compositions of zinc phosphate coatings (wt.%) developed from baths with different contents of nano SiO2 obtained by EDX. Nano zinc phosphate coatings developed from baths with different contents of nano SiO2 (g/L)

Fe

O

Zn

P

Si

Zn/P

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4

27.55 26.50 18.25 12.23 13.37 25.26 30.07 28.61 32.46

36.87 37.80 40.27 44.37 43.80 38.08 35.68 39.55 37.60

20.18 23.43 28.00 29.49 26.99 22.47 23.13 20.18 14.34

9.16 9.56 9.55 10.56 9.98 9.72 10.36 9.86 8.04

0.09 0.30 0.46 1.02 0.61 0.63 0.76 0.70 0.48

2.20 2.45 2.93 3.63 2.70 2.31 2.23 2.04 1.78

zinc, phosphorous and oxygen are high. This indicates that the surface coverage of the coating is good and more uniform coating is obtained. 3.8. XRD The phase compositions in the phosphate coatings on the mild steel were analyzed by XRD. The phase compositions of zinc phosphate coatings developed with and without nano SiO2 in the phosphating baths is presented in Fig. 9. It is shown that the phosphate coatings developed using nano SiO2 mainly consist of Zn3 (PO4 )2 ·4H2 O (hopeite, JCPD file #37-0465). However, Zn2 Fe(PO4 )2 ·4H2 O (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 zinc phosphate coatings on mild steel specimen are stronger which indicate the formation of thick phosphate crystal layer in the case of coatings produced using nano SiO2 than that of nano zinc phosphate coating developed without it. The presence of nano SiO2 particles in the phosphating bath increased the intensity of the (0 4 0) plane of hopeite phase and the intensity of (0 2 0) plane of hopeite and phosphophyllite phases. The intensity of the peak due to iron (1 1 0) plane diminishes when the concentration of silica increases from

0 to 3 g/L, which indicates that the growth and coverage of phosphate crystal clusters increases in the presence of nano silica. From this study, considering the peak at degrees, average crystallite size has been estimated by using Debye-Scherrer formula [26]. The calculated crystallite size were presented in Table 3. The average crystallite size is less than 35 nm. The results confirm the formation of nano zinc phosphate crystals with reduction in the crystallite size when compared to nano zinc phosphate coatings developed in the absence of nano SiO2 [18]. 3.9. Evaluation of corrosion performance 3.9.1. Potentiodynamic polarization The protectiveness of the coatings evaluated through potentiodynamic polarization technique in 3.5% NaCl solution is shown in Fig. 10. Corrosion potential (Ecorr ), corrosion current density and (icorr ) and the corrosion rate derived from these data are given in Table 4. It is evident from Fig. 10 that, for the zinc phosphate coatings developed in the presence of silica, the corrosion potential 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

Fig. 9. XRD patterns of phosphate coatings developed from baths with and without nano SiO2 (a) 0 g of nano SiO2 /L, (b) 0.5 g/L, (c) 1.0 g/L, (d) 1.5 g/L, (e) 2 g/L, (f) 2.5 g/L, (g) 3 g/L, (h) 3.5 g/L, (i) 4 g/L.

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Table 3 The calculated crystallite size of nano zinc phosphate coating (developed from bath containing 1.5 g/L of nano SiO2) using Scherrer calculator. No.

B obs. (◦ 2Th)

B std. (◦ 2Th)

Peak pos. (◦ 2Th)

B struct. (◦ 2Th)

Crystallite size (nm)

1 2 3 4 5 6 7

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

9.953 19.457 19.973 26.310 31.502 40.531 64.210

0.312 0.214 0.214 0.410 0.214 0.214 0.214

26 37 37 20 38 39 43

Fig. 10. Polarisation curves of mild steel samples deposited with phosphate coatings developed from baths with and without nano SiO2 in 3.5% NaCl (a) 0 g of nano SiO2 /L, (b) 0.5 g/L, (c) 1.0 g/L, (d) 1.5 g/L, (e) 2 g/L, (f) 2.5 g/L, (g) 3 g/L, (h) 3.5 g/L, (i) 4 g/L.

direction was observed when we increase the concentrations of SiO2 in the phosphating baths from 0 to 2 g/L. Among the substrates studied, the substrate with phosphate coatings prepared from baths containing nano SiO2 (1.0–2.0 g/L) has shown the most 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 in the presence of nano SiO2 ranging from 0.5 to 2 g/L show a marked decrease than compared to coatings developed in the absence of nano SiO2 . This indicates that these phosphate coatings prevent as much as possible reactions at its interface in 3.5% NaCl. 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

Table 4 Polarization parameters of mild steel samples coated with nano zinc phosphate coating developed from baths containing different contents of nano SiO2 in 3.5% NaCl solution. Nano zinc phosphate coatings on mild steel developed from baths with different contents of nano SiO2 (g/L)

Ecorr (mV) VS SCE

Icorr (␮A/cm2 )

Corrosion rate (mpy)

(a) 0 (b) 0.5 (c) 1.0 (d) 1.5 (e) 2.0 (f) 2.5 (g) 3.0 (h) 3.5 (i) 4.0

−465 −494 −493 −481 −487 −498 −506 −524 −524

6.60 6.401 5.63 4.08 6.32 6.89 7.42 8.93 10.53

3.06 2.96 2.60 1.89 2.93 3.19 3.43 4.13 4.87

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3500

3000

a b c d e f g h i

- Im(Z)/ ohm

2500

0.1Hz

2000

1500 0.01Hz

1000

500

4 10 Hz

0 0

500

1000

1500

2000

2500

3000

3500

Re(Z)/ ohm Fig. 11. EIS of mild steel samples deposited with phosphate coatings developed from baths with and without nano SiO2 in 3.5% NaCl (a) 0 g/L, (b) 0.5 g/L, (c) 1.0 g/L, (d) 1.5 g/L, (e) 2 g/L, (f) 2.5 g/L, (g) 3 g/L, (h) 3.5 g/L, (i) 4 g/L.

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 [27]. 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 SiO2 particles (ranging from 0.5 to 2 g/L) clearly indicate that the coating is more uniform and less porous than the zinc phosphate coatings without nano SiO2 The nano SiO2 in optimum concentration (1.5 g/L) in the phosphating bath leads to compact nucleation and growth of zinc phosphate crystals results in a denser morphology containing more crystal clusters and the formation of intact phosphate coatings. The polarization curve (d) emphasizes the effect of nano SiO2 on the corrosion resistance of nano zinc phosphate coatings developed on mild steel substrate. The protecting effect of phosphate coating is greater when optimum concentration of nano SiO2 is included in the phosphating bath by supporting the formation of the more compact phosphate coating. The available anodic surface must have decreased [21]. However, when the content of nano SiO2 exceeds 2 g/L, the polarization current for the coatings increases and the corrosion potential is little less positive. This is due to the intersecting grain boundaries and the defects on the phosphate layer due to agglomeration, which may allow the corroding medium to pass through it and due to the reaction between the corroding medium and the substrate. 3.9.2. Electrochemical impedance characteristics Comparison of Nyquist plots of phosphate coatings developed with and without the incorporation of nano SiO2 in 3.5% NaCl have been shown in Fig. 11. The semi circles obtained are not well defined [17]. The size of the curves is increased with increase in the concentration of nano SiO2 content in the phosphating bath from 0 to 1.5 g/L. This indicates the significant increase in the polarization

resistance of these phosphate coatings (Fig. 11b–d) in 3.5% NaCl, which means that the corrosion resistance of these phosphated samples is improved [23]. However, when the concentration of nano SiO2 , exceeds 1.5 g/L in the phosphating bath, the diameter of the semicircles decreased showing the decrease in the barrier protection. The corrosion attack on the phosphate coatings on mild steel takes place on the exposed surface of the phosphate coating itself. As both reactant and corrosion product need to be transported during the electrochemical corrosion reaction, the reaction resistance increases with the thickness of the barrier film [22]. But beyond the optimum concentration (1.5 g/L), the increase in the nano SiO2 content in the phosphating bath results in micro cracks which lead to corrosive attack at the phosphate layer and subsequently the substrate is attacked, which results in decrease in the corrosion resistance. This is in good agreement with the results of weight loss measurements and SEM photographs of these coatings. Shibli et al. [17] reported that the incorporation of nano TiO2 in the phosphating bath caused an increase in the coating thickness which ultimately yielded coatings with better barrier protection in 0.5 M NaCl solution. Minqi Sheng et al. [16] reported the effects of nano SiO2 in the phosphating bath (at 40 ◦ C) of carbon steel using different bath formulation. In the present work, phosphating was carried out at low temperature (27 ◦ C) using minimum concentration of chemicals and the phosphate coatings obtained are showing corrosion protection characteristics comparable with that of the phosphate coatings reported by them. 4. Conclusion The paper reports the development of nano zinc phosphate coatings on mild steel specimens with nano SiO2 in the zinc phosphating baths for improving the corrosion resistance of mild steel. Nano SiO2 adsorbed on the mild steel surface, reduce the iron dissolution during phosphating and enhance the amount of coating deposition by providing large number of nucleation sites for the formation of nano zinc phosphate crystals. Many small sized crystals grow in large number than compared to nano zinc phosphate coatings developed without nano SiO2 . The nano SiO2 acts as a

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