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Effect of phosphating time and temperature on microstructure and corrosion behavior of magnesium phosphate coating
Electrochimica Acta 106 (2013) 1–12
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effect of phosphating time and temperature on microstructure and corrosion behavior of magnesium phosphate coating M. Fouladi, A. Amadeh ∗ School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 18 December 2012 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 21 May 2013 Keywords: Magnesium phosphate Newberyite Coating Corrosion Mild steel
a b s t r a c t In this study a novel phosphate coating, magnesium phosphate, was developed on steel surface. The formation of the coating was confirmed by X-ray diffraction method. Morphological evolution of the coating, as a function of phosphating time and temperature, was examined by scanning electron microscope. Magnetic thickness gauge was used to determine the thickness of the coating and the bath sludge weight was specified to determine the bath efficiency. Corrosion behavior of the samples was studied using potentiodynamic polarization curves. The results indicated that increasing the phosphating temperature facilitated the precipitation of coating and increased its thickness. Furthermore the best corrosion behavior was observed at 80 ◦ C. Also increasing the phosphating time, enhanced both thickness and uniformity of the coating. The best results were observed after 20 min of phosphating. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Carbon steels are widely used in various industries, due to their high strength, good hardness and proper toughness, but their low corrosion resistance limits their application in some cases. Phosphating is one of the most important processes, applied to steels, especially in automotive industries, to improve their corrosion resistance, paintability and lubrication properties [1–3]. Phosphate coatings are usually applied on carbon steel, galvanized steel, magnesium, aluminum and zinc, but in some cases when improving the paintability is required they are also applied on stainless steels [4–7]. Zinc, manganese and iron phosphate coatings are the most common types of these coatings [8–13]. Lots of research has been done to reach a good corrosion resistance in phosphate coatings. Using a double cationic phosphate coatings, post sealing of the coating with molybdate or some other compounds and using additives, such as copper ions and ethanolamine have shown to be effective for improving the corrosion resistance of these coatings [14–18]. The type and amount of accelerators has also shown to play an important role in coating quality [9,19]. Several parameters affect the corrosion resistance of a coating, e.g. thickness of coating, its porosity and the microstructure. It has shown that increasing the thickness of coating and decreasing its porosity, results in better corrosion resistance [20,21]. One of the problems of the prevalent phosphate coatings such as zinc and manganese phosphate coatings is their
∗ Corresponding author. Tel.: +98 21 82084090; fax: +98 21 88006076. E-mail address: [email protected] (A. Amadeh). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.041
low thickness. The normal thickness of zinc phosphate coating is less that 10 m and for manganese or zinc–manganese phosphate coating can reach 20 m [14]. So it seems necessary to find a way to increase the thickness of these coatings. Therefore developing a novel phosphate coating with different chemical composition can be effective. Although some research has been done to develop third and secondary magnesium phosphate on steel and magnesium respectively [22,23], but developing the secondary magnesium phosphate on steel was never been studied. So in this study, novel secondary magnesium phosphate coating is developed on steel surface to improve its efficiency. 2. Experimental procedure Mild steel sheets (50 mm × 40 mm × 1 mm) were used as the substrate. Chemical composition of the substrate is given in Table 1. The sheets were degreased in 10 wt.% NaOH solution at 60 ◦ C for 5 min. Abrading procedure was performed by 400 grit emery paper. Then the samples were rinsed with acetone and deionized water to remove any remaining grease from the surface. Afterwards the samples were acid pickled using 10 wt.% H2 SO4 solution at 60 ◦ C for 3 min to provide a proper base for nucleation of the phosphate coating. They were then rinsed with deionized water again and finally they were immersed in 350 mL volume of magnesium phosphate bath with the composition, mentioned in Table 2. To study the effect of phosphating time, the samples were phosphated for 1, 3, 5, 10, 20 and 30 min while the temperature was stayed constant at 80 ◦ C. Also to study the effect of phosphating temperature, the phosphating time was stayed constant at 20 min and phosphating was studied at 25 ◦ C, 40 ◦ C, 60 ◦ C, 80 ◦ C and 90 ◦ C. Formation of
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Table 1 The composition of steel substrate (wt.%). Fe
C
Si
Mn
P
S
Ni
Al
Mo
Cu
Balance
0.02
0.01
0.21
0.007
0.006
0.02
0.054
0.01
0.004
Table 2 Chemical composition of magnesium phosphate bath. Concentration
Bath composition
23 mL/L 8.5 g/L 0.4 g/L 6.8 g/L
H3 PO4 (85%) MgCO3 NaNO2 NaOH
the coating was confirmed by X-ray diffraction method. Scanning electron microscope was used to study the coating microstructure. A magnetic thickness gauge was used to determine the coating thickness. The results are the average of 5 measurements. The sludge amount which precipitated in the bath after the process was separated using a filter paper and weighed in order to determine the bath efficiency factor [22]. Bath efficiency factor can be defined by Eq. (1). It defines a criterion of effectiveness of the bath by changing different parameters, i.e. by increasing the coating thickness and decreasing the sludge weight, the bath efficiency would be enhanced. bath efficiency =
thickness of coating sludge weight
(1)
Potentiodynamic polarization tests were performed by suspending the samples in 3.5 wt.% NaCl solution. The counter and reference electrodes were platinum and Saturated Calomel Electrode (SCE), respectively. After about 1 h of stabilization at rest potential, polarization test commenced at a scan rate of 2 mV/s using an EG&G273 potentiostat instrument. To check the reproducibility of the tests, each sample was tested three times. Finally the corrosion rate was calculated using Eq. (2) [24,25]: corrosion rate = 0.326 × 10−2
i
corr M
ZD
(2)
where icorr is the corrosion current density, M is the molecular weight, D is the density of metal and Z is the metal capacity in oxidation state. The coating porosity percentage was also calculated according to Eq. (3) [26,27]: P=
Rps × 10−(Ecorr /ˇa ) × 100 Rp
(3)
where P is the total coating porosity percentage, Rps is the polarization resistance of bare substrate, Rp is the polarization resistance of coated substrate in, Ecorr is the difference between free corrosion potentials of coated and bare substrate, and ˇa is the anodic Tafel slope of the substrate. Furthermore corrosion protection efficiency was calculated according to Eq. (4) [15]:
Pe % =
1−
icorr
Fig. 1. XRD patterns of magnesium phosphate coating.
called newberyite, with the chemical formula of MgHPO4 ·3H2 O. This material is also known as magnesium phosphate dibasic and magnesium hydrogen phosphate. In XRD patterns all of the peaks are related to newberyite except the two at 2 = 44.76◦ and 65.16◦ which relate to iron and originates from steel substrate. The existence of these two peaks is because of the penetration of X-ray to the substrate. XRD pattern of the bath sludge, shown in Fig. 2, indicates that the sludge is mostly consisted of amorphous compounds. There are also some crystalline compounds in the sludge but it is not possible to determine their composition due to the amorphous background. Meanwhile it is obvious that the sludge does not include a considerable amount of newberyite phase. So it declares that the sludge consists of some materials other than newberyite and much formation of sludge, means that the reactions do not shift toward the formation of newberyite. The other possibility is that some amount of newberyite which were not able to gain the substrate would precipitate as sludge in the bath and therefore it would decrease the bath efficiency too.
3.2. Formation mechanism The reactions which lead to formation of newberyite can be as follows [28]:
0 icorr
× 100
(4)
where Pe is the corrosion protection efficiency of the coating, icorr 0 and icorr are corrosion current density of coated sample and the substrate, respectively. 3. Results and discussion 3.1. Phase analysis X-ray diffraction pattern of the sample (Fig. 1) illustrates that the coating formed on steel substrate is a single phase coating,
Fig. 2. XRD pattern of the sludge, precipitated from magnesium phosphate bath.
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Fig. 3. SEM micrographs of magnesium phosphate coating at different bath temperatures (a) 25 ◦ C, (b) 40 ◦ C, (c) 60 ◦ C, (d) 80 ◦ C, (e) 90 ◦ C.
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Fig. 4. Variation of coating thickness and sludge weight vs. bath temperature curves for magnesium phosphate coating.
Fig. 5. Variation of bath efficiency factor vs. bath temperature curve for magnesium phosphate coating.
Since in present study MgCO3 was used as the source of magnesium, so the decomposition reaction would occur as follows:
Magnesium hydroxide formed in this stage reacts with phosphoric acid as follows:
MgCO3 → MgO + CO2
Mg(OH)2 + 2H3 PO4 → Mg(H2 PO4 )2 + 2H2 O
(R1)
Then, MgO would react with H2 O molecules to form Mg(OH)2 . MgO + H2 O → Mg(OH)2
(R2)
(R3)
Mg(H2 PO4 )2 formed in this stage is the primary magnesium phosphate which is soluble in water.
Table 3 Corrosion data extracted from Fig. 6. Temperature (◦ C)
Ecorr (V vs SCE)
icorr (A/cm2 )
Rp ( cm2 )
Corrosion rate (mm/y)
Porosity (%)
Corrosion protection efficiency (%)
Substrate 40 60 80 90
−0.501 −0.400 −0.433 −0.571 −0.648
16.05 16.459 6.337 0.549 5.475
1887 1703.34 5433.03 56772.81 6281.35
0.368 0.375 0.144 0.012 0.124
– 23.62 14.61 0.37 0.69
– 10.01 65.35 97.00 70.06
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Fig. 6. Potentiodynamic polarization curves of magnesium phosphate coating applied at different temperatures.
Finally the secondary insoluble magnesium phosphate would form according to (R4): MgO + Mg(H2 PO4 )2 ·2H2 O → 2MgHPO4 ·3H2 O
(R4)
Also sodium nitrite can affect the bath by consuming H+ ions and helping the precipitation of magnesium phosphate as follows: NaNO2 → Na+ + NO2 −
(R5)
NO2 − + 2H+ + 2e− → (1/2)N2 + (1/2)O2 + H2 O
(R6)
It is due to mention that the aim of adding sodium hydroxide is to adjust the pH, considering the fact that sodium hydroxide dissolves in water and neutralizes H+ ions. NaOH → Na+ + OH−
(R7)
H+ + OH− → H2 O
(R8)
3.2.1. Effect of bath temperature To study the effect of bath temperature on coating quality, phosphating time stayed constant at 20 min. Fig. 3 shows the effect of bath temperature on magnesium phosphate coating microstructure. No noticeable coating is formed at low temperatures such as 25 ◦ C and 40 ◦ C. At these temperatures only a few crystals are emerged after 20 min, indicating that although the reactions occur at low temperatures but their kinetics is so slow. When the temperature of the bath rises to 60 ◦ C, more crystals appear, but still no dense coating is formed at this temperature (Fig. 3c). At 80 ◦ C, a dense and uniform coating is formed on steel surface. It means that 80 ◦ C is a proper temperature for the formation of magnesium phosphate coating and kinetics of the reactions is high enough at this temperature. Further increase in temperature up to 90 ◦ C does not change the morphology of the coating but causes the growth of magnesium phosphate crystals. This effect can be attributed to formation reaction of newberyite which is an endothermic reaction [28]. It means that by increasing the bath temperature the mentioned reaction shifts toward the formation of newberyite.
Fig. 7. Effect of phosphating temperature on corrosion rate of magnesium phosphate coating.
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Fig. 8. Effect of phosphating temperature on coating protection efficiency and porosity of magnesium phosphate coating.
Fig. 4 shows the effect of bath temperature on coating thickness and sludge weight of magnesium phosphate coating. As mentioned before, by increasing the temperature, the reactions shift toward the formation of newberyite and so it is reasonable for the coating thickness to enhance. Therefore by increasing the temperature from 25 ◦ C to 80 ◦ C the thickness rises from 1 m to 30 m, but there is no difference between the thickness of the coating at 80 ◦ C and 90 ◦ C and at both conditions the thickness reaches 30 m. On the other hand, the sludge weight rises slowly by increasing the bath temperature, but there is a sharp increase between 80 ◦ C and 90 ◦ C and the sludge weight rises from 0.16 g to 0.33 g. So, despite the fact that the thickness of the coating is the same at 80 ◦ C and 90 ◦ C but due to the sharp increase in sludge weight, the bath efficiency decreases a lot at 90 ◦ C. This fact can be seen in Fig. 5. It is obvious that the greatest value of bath efficiency factor is achieved at 80 ◦ C. It means that formation of the coating is more dominant than formation of sludge at 80 ◦ C and coating thickness to sludge weight ratio is more than the same ratio at other temperatures. Hence, 80 ◦ C can be proposed as the optimum temperature for the formation of magnesium phosphate coating. This temperature is higher to some extent in comparison with formation temperature of zinc phosphate coating, which is about 45–60 ◦ C [9,19,29], but it is almost the same as the temperature applied for the formation of double cationic zinc–manganese phosphate coating [14] and it is about 10 ◦ C lower than the optimum temperature for manganese phosphate bath [21,30]. Fig. 6 shows the potentiodynamic polarization curves of the coating applied at different temperatures. Also Table 3 summarizes the data, extracted from the curves. It can be seen that by increasing the phosphating temperature from 40 ◦ C to 60 ◦ C the corrosion rate decreases about 2.6 times. On the other hand there is a considerable decrease in corrosion rate between 60 ◦ C and 80 ◦ C, i.e. the corrosion rate decreases about 11.4 times. By comparing these results with coating morphology and thickness, it is obvious that the improvement in the corrosion resistance from 40 ◦ C to 80 ◦ C can be attributed to increase of coating density and thickness. It is reasonable that a thicker and denser barrier can better protect the substrate. On the other hand as it can be seen in Fig. 7 and Table 3, the corrosion rate decreases continuously from 40 ◦ C to 80 ◦ C but it increases about 10 times between 80 ◦ C and 90 ◦ C. So the minimum
corrosion rate is achieved at 80 ◦ C. Also, according to Figs. 4 and 8, the most corrosion protection efficiency, the least porosity percentage and the heaviest thickness are all achieved at 80 ◦ C. So 80 ◦ C can be considered as optimum temperature for the formation of magnesium phosphate coating. However, despite the fact that the coating is not as favorable as 80 ◦ C at 90 ◦ C, but the samples showed less corrosion rate and porosity percentage, more protection efficiency and heavier thickness at 90 ◦ C than 40 ◦ C and 60 ◦ C (Figs. 4, 7 and 8). As it was mentioned before, by increasing the bath temperature from 80 ◦ C to 90 ◦ C the coating thickness would stay constant and the crystals would grow larger. The growth of coating crystals to larger sizes means that there is enough room for crystals to grow, but the coating morphology showed to be dense enough at 80 ◦ C and there was no room for crystal growth. Therefore there might be fewer crystals formed at 90 ◦ C than 80 ◦ C so there was more room for these crystals to grow larger. This phenomenon might be occurred because of the fact that the bath reaches its boiling temperature at 90 ◦ C, so the nucleation of phosphate crystals encounters problem and the precipitated nuclei have more room to grow. Hence because of lighter density of the coating at 90 ◦ C than 80 ◦ C the corrosion rate increases. 3.2.2. Effect of phosphating time It is clear that as time passes by, more reactions would occur, more nuclei would form and the nuclei can grow larger. Therefore, proper phosphating time is needed for complete formation of the coating. To study the effect phosphating time on coating quality, phosphating temperature stayed constant at 80 ◦ C. Fig. 9 shows the effect of phosphating time on microstructure of magnesium phosphate coating. As it is obvious, after 1 min of phosphating, as shown in Fig. 9a only a few nuclei are formed on steel surface. But after 3 min, more nuclei are emerged and the former nuclei have grown (Fig. 9b). After 5 min, almost the most portion of sample’s surface is covered by magnesium phosphate nuclei at this condition, the nucleation stage is almost completed, but the growth stage would still go on (Fig. 9c). It should be mentioned that nucleation of phosphate crystals would not end up completely at this time and it would be continued in the coating free regions and porosities. At the 10th minute of phosphating, almost a dense coating is formed but there are also some few regions without newberyite crystals. Also
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Fig. 9. SEM micrographs of magnesium phosphate coating at different phosphating times (a) 1 min, (b) 3 min, (c) 5 min, (d) 10 min, (e) 20 min, (f) 30 min.
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Fig. 10. Variation of coating thickness and sludge weight vs. phosphating time curves for magnesium phosphate coating.
Fig. 11. Variation of bath efficiency factor vs. phosphating time curve for magnesium phosphate coating.
Table 4 Corrosion data extracted from Fig. 12. Sample
Ecorr (V vs SCE)
icorr (A/cm2 )
Rp ( cm2 )
Corrosion rate (mm/y)
Porosity (%)
Corrosion protection efficiency (%)
Substrate 5 10 20 30
−0.501 −0.562 −0.594 −0.571 −0.671
16.05 15.745 13.023 0.549 4.932
1887 1825.44 2174.81 56772.81 6667.46
0.368 0.360 0.297 0.012 0.112
– 14.01 6.09 0.37 0.41
– 13.91 28.80 97.00 73.04
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Fig. 12. SEM cross-section micrograph of the sample phosphated at 80 ◦ C for 20 min using backscattered electron detector.
the coating is not well uniform at this time. As it is obvious in Fig. 9e a dense coating with uniform crystals is formed after 20 min. There is no noticeable coating free region in this condition. After 30 min of phosphating (Fig. 9f), the coating is almost dense enough but the uniformity of the coating is less than what was achieved at 20 min. On the other hand as it is obvious in Fig. 9e and f, the crystals grow larger over time and their size increased from what was observed after 20 min. The effect of phosphating time on coating thickness and sludge weight are shown in Fig. 10. It is obvious that as the time goes on, the thickness of the coating enhances. Also it can be seen that after 5 min of phosphating, the slope of thickness–phosphating time curve decreases and the rate of increasing the coating thickness reduces. This means that after 5 min, when the nucleation stage is almost completed, the coating formation rate decreases. Also the sludge weight–phosphating time curve shows that although the sludge weight increases over time, but there is only 0.08 g difference in sludge weight, from the first to the 30th minute. It would show that most of the sludge would form at the nucleation stage. Furthermore it is obvious in Fig. 11 that bath efficiency factor has its maximum value after 10 min of phosphating. It means that
either an increase or decrease of the phosphating duration would decrease the bath efficiency. Accordingly, it can be concluded from Figs. 9–11 that although the best bath efficiency is achieved at 10 min of phosphating but the coating formed is not uniform enough. Regarding this fact it can be concluded that despite the fact that there are 20% difference in bath efficiency factor between the coatings formed after 10 and 20 min but since the coating is thicker and more uniform after 20 min, so 20 min of phosphating can be considered as the optimum duration for the formation of magnesium phosphate coating. A comparison with other phosphate coatings reveals that the optimum phosphating time for most of the phosphate coatings is about 10 min [19,30,31]. To verify the accuracy of the magnetic thickness gauge, a SEM cross-section micrograph of the sample which was phosphate at 80 ◦ C for 20 min is provided using backscattered electron detector (Fig. 12). As it is obvious, the coating thickness is about 28–33 m which is in accordance with the data achieved by magnetic thickness gauge (30 m). Potentiodynamic polarization curves (Fig. 13) show the effect of phosphating time on corrosion behavior of the samples. Also Table 4 summarizes the data, extracted from the curves. As it can be seen
Fig. 13. Potentiodynamic polarization curves of magnesium phosphate coating applied at different phosphating times.
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Fig. 14. Effect of phosphating time on corrosion rate of magnesium phosphate coating.
in Fig. 13 and Table 4, phosphating the steel improves its corrosion resistance. Also it is obvious in Fig. 14 that by increasing the phosphating time from 5 to 20 min the corrosion rate decreases continuously. As it is obvious there is no noticeable difference between the corrosion rates of the samples after 5–10 min of phosphating, But by increasing the phosphating time from 10 to 20 min there is a considerable decrease in corrosion rate, i.e. the corrosion rate decreases from 0.297 mm/y to 0.012 mm/y. It means that by increasing the phosphating time from 10 to 20 min the corrosion rate decreases about 25 times. To clarify this behavior, it can be mentioned that by increasing the phosphating time, a denser and thicker phosphate layer would form on steel surface and thicker layer would act as better corrosion protector. On the other hand by increasing the phosphating time from 20 to 30 min the corrosion rate increases from 0.012 mm/y to
0.112 mm/y. Also there is a sharp decrease in corrosion protection efficiency between 20 and 30 min. To clarify this fact, as it is obvious in Table 4 and Fig. 15 coating porosity increases a little from 20th to 30th minutes. Furthermore it is visible in higher magnification SEM micrographs of the samples phosphated for 20 and 30 min (Fig. 16), that there is more porosity in the coating after 30 min than 20 min. Also the crystals are larger at 30th minutes of phosphating. So it can be concluded that since the nucleation stage is completed before the 30th minutes of phosphating, but the growth is still continuing at this time, so by growth of some crystals to larger sizes the surrounding crystals which may not be well adhered to the substrate would be detached from the substrate and leave some porosities instead. A novel phosphate coating, magnesium phosphate, was developed successfully on steel surface. The following conclusions can be made from this investigation:
Fig. 15. Effect of phosphating time on coating protection efficiency and porosity of magnesium phosphate coating.
M. Fouladi, A. Amadeh / Electrochimica Acta 106 (2013) 1–12
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Fig. 16. SEM micrographs of magnesium phosphate coating after (a) 20 min (500×), (b) 20 min (1000×), (c) 30 min (500×), (d) 30 min (1000×).
4. Conclusion 1. Increasing the bath temperature increased both the coating thickness and the sludge weight. The maximum value for coating thickness and bath efficiency factor was achieved at bath temperature of 80 ◦ C. There was a sharp increase in sludge weight and decrease in bath efficiency factor, between 80 ◦ C and 90 ◦ C, i.e. the sludge formed at 90 ◦ C was about 2 times more than what was formed at 80 ◦ C by weight and it caused the bath efficiency factor to be decreased. Increasing the bath temperature also caused the growth of coating crystals and increased the coating density up to 80 ◦ C. Enhancing the temperature more than 80 ◦ C, increased the crystal size but decreased the coating density. Furthermore the best corrosion behavior was achieved at 80 ◦ C, i.e. the corrosion rate was 0.012 mm/y at this temperature. It can be concluded that 80 ◦ C is the optimum temperature for the formation of newberyite film.
2. Increasing the phosphating time enhanced both the coating thickness and the sludge weight. Between 5 and 10 min the nucleation stage was almost completed and the crystals formed in this stage grew larger overtime. The maximum value for bath efficiency factor was achieved after 10 min of phosphating, but a dense and uniform coating with suitable thickness was formed after 20 min. The best corrosion behavior was also observed after 20 min. Further increase in phosphating time (more than 20 min) decreased the uniformity, density and corrosion resistance of the coating. Therefore 20 min of phosphating can be considered as the optimum time for the formation of magnesium phosphate coating. References [1] M. Manna, Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: an option to simulate TMT rebars surfaces, Surface and Coatings Technology 203 (2009) 1913.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effect of phosphating time and temperature on microstructure and corrosion behavior of magnesium phosphate coating M. Fouladi, A. Amadeh ∗ School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 18 December 2012 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 21 May 2013 Keywords: Magnesium phosphate Newberyite Coating Corrosion Mild steel
a b s t r a c t In this study a novel phosphate coating, magnesium phosphate, was developed on steel surface. The formation of the coating was confirmed by X-ray diffraction method. Morphological evolution of the coating, as a function of phosphating time and temperature, was examined by scanning electron microscope. Magnetic thickness gauge was used to determine the thickness of the coating and the bath sludge weight was specified to determine the bath efficiency. Corrosion behavior of the samples was studied using potentiodynamic polarization curves. The results indicated that increasing the phosphating temperature facilitated the precipitation of coating and increased its thickness. Furthermore the best corrosion behavior was observed at 80 ◦ C. Also increasing the phosphating time, enhanced both thickness and uniformity of the coating. The best results were observed after 20 min of phosphating. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Carbon steels are widely used in various industries, due to their high strength, good hardness and proper toughness, but their low corrosion resistance limits their application in some cases. Phosphating is one of the most important processes, applied to steels, especially in automotive industries, to improve their corrosion resistance, paintability and lubrication properties [1–3]. Phosphate coatings are usually applied on carbon steel, galvanized steel, magnesium, aluminum and zinc, but in some cases when improving the paintability is required they are also applied on stainless steels [4–7]. Zinc, manganese and iron phosphate coatings are the most common types of these coatings [8–13]. Lots of research has been done to reach a good corrosion resistance in phosphate coatings. Using a double cationic phosphate coatings, post sealing of the coating with molybdate or some other compounds and using additives, such as copper ions and ethanolamine have shown to be effective for improving the corrosion resistance of these coatings [14–18]. The type and amount of accelerators has also shown to play an important role in coating quality [9,19]. Several parameters affect the corrosion resistance of a coating, e.g. thickness of coating, its porosity and the microstructure. It has shown that increasing the thickness of coating and decreasing its porosity, results in better corrosion resistance [20,21]. One of the problems of the prevalent phosphate coatings such as zinc and manganese phosphate coatings is their
∗ Corresponding author. Tel.: +98 21 82084090; fax: +98 21 88006076. E-mail address: [email protected] (A. Amadeh). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.041
low thickness. The normal thickness of zinc phosphate coating is less that 10 m and for manganese or zinc–manganese phosphate coating can reach 20 m [14]. So it seems necessary to find a way to increase the thickness of these coatings. Therefore developing a novel phosphate coating with different chemical composition can be effective. Although some research has been done to develop third and secondary magnesium phosphate on steel and magnesium respectively [22,23], but developing the secondary magnesium phosphate on steel was never been studied. So in this study, novel secondary magnesium phosphate coating is developed on steel surface to improve its efficiency. 2. Experimental procedure Mild steel sheets (50 mm × 40 mm × 1 mm) were used as the substrate. Chemical composition of the substrate is given in Table 1. The sheets were degreased in 10 wt.% NaOH solution at 60 ◦ C for 5 min. Abrading procedure was performed by 400 grit emery paper. Then the samples were rinsed with acetone and deionized water to remove any remaining grease from the surface. Afterwards the samples were acid pickled using 10 wt.% H2 SO4 solution at 60 ◦ C for 3 min to provide a proper base for nucleation of the phosphate coating. They were then rinsed with deionized water again and finally they were immersed in 350 mL volume of magnesium phosphate bath with the composition, mentioned in Table 2. To study the effect of phosphating time, the samples were phosphated for 1, 3, 5, 10, 20 and 30 min while the temperature was stayed constant at 80 ◦ C. Also to study the effect of phosphating temperature, the phosphating time was stayed constant at 20 min and phosphating was studied at 25 ◦ C, 40 ◦ C, 60 ◦ C, 80 ◦ C and 90 ◦ C. Formation of
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Table 1 The composition of steel substrate (wt.%). Fe
C
Si
Mn
P
S
Ni
Al
Mo
Cu
Balance
0.02
0.01
0.21
0.007
0.006
0.02
0.054
0.01
0.004
Table 2 Chemical composition of magnesium phosphate bath. Concentration
Bath composition
23 mL/L 8.5 g/L 0.4 g/L 6.8 g/L
H3 PO4 (85%) MgCO3 NaNO2 NaOH
the coating was confirmed by X-ray diffraction method. Scanning electron microscope was used to study the coating microstructure. A magnetic thickness gauge was used to determine the coating thickness. The results are the average of 5 measurements. The sludge amount which precipitated in the bath after the process was separated using a filter paper and weighed in order to determine the bath efficiency factor [22]. Bath efficiency factor can be defined by Eq. (1). It defines a criterion of effectiveness of the bath by changing different parameters, i.e. by increasing the coating thickness and decreasing the sludge weight, the bath efficiency would be enhanced. bath efficiency =
thickness of coating sludge weight
(1)
Potentiodynamic polarization tests were performed by suspending the samples in 3.5 wt.% NaCl solution. The counter and reference electrodes were platinum and Saturated Calomel Electrode (SCE), respectively. After about 1 h of stabilization at rest potential, polarization test commenced at a scan rate of 2 mV/s using an EG&G273 potentiostat instrument. To check the reproducibility of the tests, each sample was tested three times. Finally the corrosion rate was calculated using Eq. (2) [24,25]: corrosion rate = 0.326 × 10−2
i
corr M
ZD
(2)
where icorr is the corrosion current density, M is the molecular weight, D is the density of metal and Z is the metal capacity in oxidation state. The coating porosity percentage was also calculated according to Eq. (3) [26,27]: P=
Rps × 10−(Ecorr /ˇa ) × 100 Rp
(3)
where P is the total coating porosity percentage, Rps is the polarization resistance of bare substrate, Rp is the polarization resistance of coated substrate in, Ecorr is the difference between free corrosion potentials of coated and bare substrate, and ˇa is the anodic Tafel slope of the substrate. Furthermore corrosion protection efficiency was calculated according to Eq. (4) [15]:
Pe % =
1−
icorr
Fig. 1. XRD patterns of magnesium phosphate coating.
called newberyite, with the chemical formula of MgHPO4 ·3H2 O. This material is also known as magnesium phosphate dibasic and magnesium hydrogen phosphate. In XRD patterns all of the peaks are related to newberyite except the two at 2 = 44.76◦ and 65.16◦ which relate to iron and originates from steel substrate. The existence of these two peaks is because of the penetration of X-ray to the substrate. XRD pattern of the bath sludge, shown in Fig. 2, indicates that the sludge is mostly consisted of amorphous compounds. There are also some crystalline compounds in the sludge but it is not possible to determine their composition due to the amorphous background. Meanwhile it is obvious that the sludge does not include a considerable amount of newberyite phase. So it declares that the sludge consists of some materials other than newberyite and much formation of sludge, means that the reactions do not shift toward the formation of newberyite. The other possibility is that some amount of newberyite which were not able to gain the substrate would precipitate as sludge in the bath and therefore it would decrease the bath efficiency too.
3.2. Formation mechanism The reactions which lead to formation of newberyite can be as follows [28]:
0 icorr
× 100
(4)
where Pe is the corrosion protection efficiency of the coating, icorr 0 and icorr are corrosion current density of coated sample and the substrate, respectively. 3. Results and discussion 3.1. Phase analysis X-ray diffraction pattern of the sample (Fig. 1) illustrates that the coating formed on steel substrate is a single phase coating,
Fig. 2. XRD pattern of the sludge, precipitated from magnesium phosphate bath.
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Fig. 3. SEM micrographs of magnesium phosphate coating at different bath temperatures (a) 25 ◦ C, (b) 40 ◦ C, (c) 60 ◦ C, (d) 80 ◦ C, (e) 90 ◦ C.
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Fig. 4. Variation of coating thickness and sludge weight vs. bath temperature curves for magnesium phosphate coating.
Fig. 5. Variation of bath efficiency factor vs. bath temperature curve for magnesium phosphate coating.
Since in present study MgCO3 was used as the source of magnesium, so the decomposition reaction would occur as follows:
Magnesium hydroxide formed in this stage reacts with phosphoric acid as follows:
MgCO3 → MgO + CO2
Mg(OH)2 + 2H3 PO4 → Mg(H2 PO4 )2 + 2H2 O
(R1)
Then, MgO would react with H2 O molecules to form Mg(OH)2 . MgO + H2 O → Mg(OH)2
(R2)
(R3)
Mg(H2 PO4 )2 formed in this stage is the primary magnesium phosphate which is soluble in water.
Table 3 Corrosion data extracted from Fig. 6. Temperature (◦ C)
Ecorr (V vs SCE)
icorr (A/cm2 )
Rp ( cm2 )
Corrosion rate (mm/y)
Porosity (%)
Corrosion protection efficiency (%)
Substrate 40 60 80 90
−0.501 −0.400 −0.433 −0.571 −0.648
16.05 16.459 6.337 0.549 5.475
1887 1703.34 5433.03 56772.81 6281.35
0.368 0.375 0.144 0.012 0.124
– 23.62 14.61 0.37 0.69
– 10.01 65.35 97.00 70.06
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Fig. 6. Potentiodynamic polarization curves of magnesium phosphate coating applied at different temperatures.
Finally the secondary insoluble magnesium phosphate would form according to (R4): MgO + Mg(H2 PO4 )2 ·2H2 O → 2MgHPO4 ·3H2 O
(R4)
Also sodium nitrite can affect the bath by consuming H+ ions and helping the precipitation of magnesium phosphate as follows: NaNO2 → Na+ + NO2 −
(R5)
NO2 − + 2H+ + 2e− → (1/2)N2 + (1/2)O2 + H2 O
(R6)
It is due to mention that the aim of adding sodium hydroxide is to adjust the pH, considering the fact that sodium hydroxide dissolves in water and neutralizes H+ ions. NaOH → Na+ + OH−
(R7)
H+ + OH− → H2 O
(R8)
3.2.1. Effect of bath temperature To study the effect of bath temperature on coating quality, phosphating time stayed constant at 20 min. Fig. 3 shows the effect of bath temperature on magnesium phosphate coating microstructure. No noticeable coating is formed at low temperatures such as 25 ◦ C and 40 ◦ C. At these temperatures only a few crystals are emerged after 20 min, indicating that although the reactions occur at low temperatures but their kinetics is so slow. When the temperature of the bath rises to 60 ◦ C, more crystals appear, but still no dense coating is formed at this temperature (Fig. 3c). At 80 ◦ C, a dense and uniform coating is formed on steel surface. It means that 80 ◦ C is a proper temperature for the formation of magnesium phosphate coating and kinetics of the reactions is high enough at this temperature. Further increase in temperature up to 90 ◦ C does not change the morphology of the coating but causes the growth of magnesium phosphate crystals. This effect can be attributed to formation reaction of newberyite which is an endothermic reaction [28]. It means that by increasing the bath temperature the mentioned reaction shifts toward the formation of newberyite.
Fig. 7. Effect of phosphating temperature on corrosion rate of magnesium phosphate coating.
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Fig. 8. Effect of phosphating temperature on coating protection efficiency and porosity of magnesium phosphate coating.
Fig. 4 shows the effect of bath temperature on coating thickness and sludge weight of magnesium phosphate coating. As mentioned before, by increasing the temperature, the reactions shift toward the formation of newberyite and so it is reasonable for the coating thickness to enhance. Therefore by increasing the temperature from 25 ◦ C to 80 ◦ C the thickness rises from 1 m to 30 m, but there is no difference between the thickness of the coating at 80 ◦ C and 90 ◦ C and at both conditions the thickness reaches 30 m. On the other hand, the sludge weight rises slowly by increasing the bath temperature, but there is a sharp increase between 80 ◦ C and 90 ◦ C and the sludge weight rises from 0.16 g to 0.33 g. So, despite the fact that the thickness of the coating is the same at 80 ◦ C and 90 ◦ C but due to the sharp increase in sludge weight, the bath efficiency decreases a lot at 90 ◦ C. This fact can be seen in Fig. 5. It is obvious that the greatest value of bath efficiency factor is achieved at 80 ◦ C. It means that formation of the coating is more dominant than formation of sludge at 80 ◦ C and coating thickness to sludge weight ratio is more than the same ratio at other temperatures. Hence, 80 ◦ C can be proposed as the optimum temperature for the formation of magnesium phosphate coating. This temperature is higher to some extent in comparison with formation temperature of zinc phosphate coating, which is about 45–60 ◦ C [9,19,29], but it is almost the same as the temperature applied for the formation of double cationic zinc–manganese phosphate coating [14] and it is about 10 ◦ C lower than the optimum temperature for manganese phosphate bath [21,30]. Fig. 6 shows the potentiodynamic polarization curves of the coating applied at different temperatures. Also Table 3 summarizes the data, extracted from the curves. It can be seen that by increasing the phosphating temperature from 40 ◦ C to 60 ◦ C the corrosion rate decreases about 2.6 times. On the other hand there is a considerable decrease in corrosion rate between 60 ◦ C and 80 ◦ C, i.e. the corrosion rate decreases about 11.4 times. By comparing these results with coating morphology and thickness, it is obvious that the improvement in the corrosion resistance from 40 ◦ C to 80 ◦ C can be attributed to increase of coating density and thickness. It is reasonable that a thicker and denser barrier can better protect the substrate. On the other hand as it can be seen in Fig. 7 and Table 3, the corrosion rate decreases continuously from 40 ◦ C to 80 ◦ C but it increases about 10 times between 80 ◦ C and 90 ◦ C. So the minimum
corrosion rate is achieved at 80 ◦ C. Also, according to Figs. 4 and 8, the most corrosion protection efficiency, the least porosity percentage and the heaviest thickness are all achieved at 80 ◦ C. So 80 ◦ C can be considered as optimum temperature for the formation of magnesium phosphate coating. However, despite the fact that the coating is not as favorable as 80 ◦ C at 90 ◦ C, but the samples showed less corrosion rate and porosity percentage, more protection efficiency and heavier thickness at 90 ◦ C than 40 ◦ C and 60 ◦ C (Figs. 4, 7 and 8). As it was mentioned before, by increasing the bath temperature from 80 ◦ C to 90 ◦ C the coating thickness would stay constant and the crystals would grow larger. The growth of coating crystals to larger sizes means that there is enough room for crystals to grow, but the coating morphology showed to be dense enough at 80 ◦ C and there was no room for crystal growth. Therefore there might be fewer crystals formed at 90 ◦ C than 80 ◦ C so there was more room for these crystals to grow larger. This phenomenon might be occurred because of the fact that the bath reaches its boiling temperature at 90 ◦ C, so the nucleation of phosphate crystals encounters problem and the precipitated nuclei have more room to grow. Hence because of lighter density of the coating at 90 ◦ C than 80 ◦ C the corrosion rate increases. 3.2.2. Effect of phosphating time It is clear that as time passes by, more reactions would occur, more nuclei would form and the nuclei can grow larger. Therefore, proper phosphating time is needed for complete formation of the coating. To study the effect phosphating time on coating quality, phosphating temperature stayed constant at 80 ◦ C. Fig. 9 shows the effect of phosphating time on microstructure of magnesium phosphate coating. As it is obvious, after 1 min of phosphating, as shown in Fig. 9a only a few nuclei are formed on steel surface. But after 3 min, more nuclei are emerged and the former nuclei have grown (Fig. 9b). After 5 min, almost the most portion of sample’s surface is covered by magnesium phosphate nuclei at this condition, the nucleation stage is almost completed, but the growth stage would still go on (Fig. 9c). It should be mentioned that nucleation of phosphate crystals would not end up completely at this time and it would be continued in the coating free regions and porosities. At the 10th minute of phosphating, almost a dense coating is formed but there are also some few regions without newberyite crystals. Also
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Fig. 9. SEM micrographs of magnesium phosphate coating at different phosphating times (a) 1 min, (b) 3 min, (c) 5 min, (d) 10 min, (e) 20 min, (f) 30 min.
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Fig. 10. Variation of coating thickness and sludge weight vs. phosphating time curves for magnesium phosphate coating.
Fig. 11. Variation of bath efficiency factor vs. phosphating time curve for magnesium phosphate coating.
Table 4 Corrosion data extracted from Fig. 12. Sample
Ecorr (V vs SCE)
icorr (A/cm2 )
Rp ( cm2 )
Corrosion rate (mm/y)
Porosity (%)
Corrosion protection efficiency (%)
Substrate 5 10 20 30
−0.501 −0.562 −0.594 −0.571 −0.671
16.05 15.745 13.023 0.549 4.932
1887 1825.44 2174.81 56772.81 6667.46
0.368 0.360 0.297 0.012 0.112
– 14.01 6.09 0.37 0.41
– 13.91 28.80 97.00 73.04
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Fig. 12. SEM cross-section micrograph of the sample phosphated at 80 ◦ C for 20 min using backscattered electron detector.
the coating is not well uniform at this time. As it is obvious in Fig. 9e a dense coating with uniform crystals is formed after 20 min. There is no noticeable coating free region in this condition. After 30 min of phosphating (Fig. 9f), the coating is almost dense enough but the uniformity of the coating is less than what was achieved at 20 min. On the other hand as it is obvious in Fig. 9e and f, the crystals grow larger over time and their size increased from what was observed after 20 min. The effect of phosphating time on coating thickness and sludge weight are shown in Fig. 10. It is obvious that as the time goes on, the thickness of the coating enhances. Also it can be seen that after 5 min of phosphating, the slope of thickness–phosphating time curve decreases and the rate of increasing the coating thickness reduces. This means that after 5 min, when the nucleation stage is almost completed, the coating formation rate decreases. Also the sludge weight–phosphating time curve shows that although the sludge weight increases over time, but there is only 0.08 g difference in sludge weight, from the first to the 30th minute. It would show that most of the sludge would form at the nucleation stage. Furthermore it is obvious in Fig. 11 that bath efficiency factor has its maximum value after 10 min of phosphating. It means that
either an increase or decrease of the phosphating duration would decrease the bath efficiency. Accordingly, it can be concluded from Figs. 9–11 that although the best bath efficiency is achieved at 10 min of phosphating but the coating formed is not uniform enough. Regarding this fact it can be concluded that despite the fact that there are 20% difference in bath efficiency factor between the coatings formed after 10 and 20 min but since the coating is thicker and more uniform after 20 min, so 20 min of phosphating can be considered as the optimum duration for the formation of magnesium phosphate coating. A comparison with other phosphate coatings reveals that the optimum phosphating time for most of the phosphate coatings is about 10 min [19,30,31]. To verify the accuracy of the magnetic thickness gauge, a SEM cross-section micrograph of the sample which was phosphate at 80 ◦ C for 20 min is provided using backscattered electron detector (Fig. 12). As it is obvious, the coating thickness is about 28–33 m which is in accordance with the data achieved by magnetic thickness gauge (30 m). Potentiodynamic polarization curves (Fig. 13) show the effect of phosphating time on corrosion behavior of the samples. Also Table 4 summarizes the data, extracted from the curves. As it can be seen
Fig. 13. Potentiodynamic polarization curves of magnesium phosphate coating applied at different phosphating times.
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Fig. 14. Effect of phosphating time on corrosion rate of magnesium phosphate coating.
in Fig. 13 and Table 4, phosphating the steel improves its corrosion resistance. Also it is obvious in Fig. 14 that by increasing the phosphating time from 5 to 20 min the corrosion rate decreases continuously. As it is obvious there is no noticeable difference between the corrosion rates of the samples after 5–10 min of phosphating, But by increasing the phosphating time from 10 to 20 min there is a considerable decrease in corrosion rate, i.e. the corrosion rate decreases from 0.297 mm/y to 0.012 mm/y. It means that by increasing the phosphating time from 10 to 20 min the corrosion rate decreases about 25 times. To clarify this behavior, it can be mentioned that by increasing the phosphating time, a denser and thicker phosphate layer would form on steel surface and thicker layer would act as better corrosion protector. On the other hand by increasing the phosphating time from 20 to 30 min the corrosion rate increases from 0.012 mm/y to
0.112 mm/y. Also there is a sharp decrease in corrosion protection efficiency between 20 and 30 min. To clarify this fact, as it is obvious in Table 4 and Fig. 15 coating porosity increases a little from 20th to 30th minutes. Furthermore it is visible in higher magnification SEM micrographs of the samples phosphated for 20 and 30 min (Fig. 16), that there is more porosity in the coating after 30 min than 20 min. Also the crystals are larger at 30th minutes of phosphating. So it can be concluded that since the nucleation stage is completed before the 30th minutes of phosphating, but the growth is still continuing at this time, so by growth of some crystals to larger sizes the surrounding crystals which may not be well adhered to the substrate would be detached from the substrate and leave some porosities instead. A novel phosphate coating, magnesium phosphate, was developed successfully on steel surface. The following conclusions can be made from this investigation:
Fig. 15. Effect of phosphating time on coating protection efficiency and porosity of magnesium phosphate coating.
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Fig. 16. SEM micrographs of magnesium phosphate coating after (a) 20 min (500×), (b) 20 min (1000×), (c) 30 min (500×), (d) 30 min (1000×).
4. Conclusion 1. Increasing the bath temperature increased both the coating thickness and the sludge weight. The maximum value for coating thickness and bath efficiency factor was achieved at bath temperature of 80 ◦ C. There was a sharp increase in sludge weight and decrease in bath efficiency factor, between 80 ◦ C and 90 ◦ C, i.e. the sludge formed at 90 ◦ C was about 2 times more than what was formed at 80 ◦ C by weight and it caused the bath efficiency factor to be decreased. Increasing the bath temperature also caused the growth of coating crystals and increased the coating density up to 80 ◦ C. Enhancing the temperature more than 80 ◦ C, increased the crystal size but decreased the coating density. Furthermore the best corrosion behavior was achieved at 80 ◦ C, i.e. the corrosion rate was 0.012 mm/y at this temperature. It can be concluded that 80 ◦ C is the optimum temperature for the formation of newberyite film.
2. Increasing the phosphating time enhanced both the coating thickness and the sludge weight. Between 5 and 10 min the nucleation stage was almost completed and the crystals formed in this stage grew larger overtime. The maximum value for bath efficiency factor was achieved after 10 min of phosphating, but a dense and uniform coating with suitable thickness was formed after 20 min. The best corrosion behavior was also observed after 20 min. Further increase in phosphating time (more than 20 min) decreased the uniformity, density and corrosion resistance of the coating. Therefore 20 min of phosphating can be considered as the optimum time for the formation of magnesium phosphate coating. References [1] M. Manna, Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: an option to simulate TMT rebars surfaces, Surface and Coatings Technology 203 (2009) 1913.
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