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Study of the corrosion properties of zinc–nickel alloy electrodeposits before and after chromating
Journal of Materials Processing Technology 138 (2003) 63–66
Study of the corrosion properties of zinc–nickel alloy electrodeposits before and after chromating M. Heydarzadeh Sohi*, M. Jalali Department of Metallurgy and Materials, Faculty of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran
Abstract In this study, corrosion behavior of zinc–nickel alloy electrodeposits on steel sheets has been investigated using the neutral salt spray test. The performance of the coatings with various nickel contents (up to 28 wt.%) was compared with electrodeposited zinc coating. The results show that the corrosion resistance of zinc–nickel alloy coatings is superior to that of pure zinc coating and the zinc–13 wt.% nickel gives the best protection. The corrosion behavior of chromated zinc–nickel alloy coatings was also studied. The results show that chromate conversion coating increases the corrosion resistance significantly (up to 3–4 times), and the best corrosion resistance is achieved by chromating of zinc– 13 wt.% nickel alloy coating. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Zn–Ni alloys plating; Chromating; Corrosion resistance
1. Introduction Zinc electrodeposit is widely used as corrosion protective coating for steel parts. In order to enhance the corrosion resistance of this coating, it is alloyed with metals of the iron group (such as nickel, iron and cobalt) [1,2]. Amongst the zinc alloys, zinc–nickel alloys with different nickel content have been the most successful [3,4]. On the other hand, chromate conversion coating of zinc electrodeposit is also used for enhancement of its corrosion resistance [5–7]. The added corrosion resistance due to the chromate coating is dependent on the type of the chromating. In this research the effect of the nickel content and chromate conversion coating on the corrosion resistance of zinc–nickel alloy electrodeposits have been studied.
2. Experimental The base material in this study was a low carbon steel sheet that is used for car body. The dimensions of specimens used in this work were 100 mm 70 mm 1 mm. After surface preparation, the specimens were electroplated with zinc and zinc–nickel alloys. The formulations of the electrolytic bathes that were used for pure zinc and zinc–nickel * Corresponding author. E-mail address: [email protected] (M. Heydarzadeh Sohi).
alloy electrodepositions and their optimum conditions are shown in Tables 1 and 2, respectively. A number of specimens were then conversion coated by yellow and green chromating. The formulations of the electrolytes that were used for chromating and their optimized conditions are shown in Tables 3 and 4. Corrosion resistance of the coatings was measured by using neutral salt spray test (NaCl, 5%; T ¼ 35 8C). Periodical visual observation was carried out in order to evaluate the amount of the corrosion of the specimens. The corrosion resistance of the coatings was determined by observing the white rust (which was the corrosion product of the coatings) and red rust (which was the corrosion product of the steel substrate). The microstructural characterization was performed using scanning electron microscopy. The coatings analysis was performed by energy dispersive X-ray spectroscopy (EDXS).
3. Results and discussion Fig. 1(a)–(d) shows the surface morphology of a number of the Zn–Ni deposits as observed by SEM. As shown in Fig. 1(a) the morphology of the deposits with low nickel content is lamella. According to Zn–Ni binary diagram [8], deposits with up to 9 wt.% nickel should be consisted of Z phase that have hexagonal structure like pure zinc. Fig. 1(b)
0924-0136/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00050-5
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M. Heydarzadeh Sohi, M. Jalali / Journal of Materials Processing Technology 138 (2003) 63–66
Table 1 The composition and conditions of the zinc plating bath
Table 3 Composition and conditions of yellow chromating bath
Variables
Amount
Variables
Amount
Zinc oxide (g/l) NaCN (g/l) NaOH (g/l) Temperature (8C) Anode Current density (A/dm2) Time (min)
34 90 80 25 Zinc 15 5–10
Sodium dichromate (g/l) Sodium sulfate (g/l) Sodium nitrate (g/l) Nitric acid (g/l) Temperature (8C) Immersion time (s) pH
160 50 50 4 30–40 25–50 1–2
Table 2 The composition and conditions of Zn–Ni electroplating Variables
Amount
Nickel sulfate (g/l) Zinc sulfate (g/l) Sodium sulfate (g/l) Ammonium chloride (g/l) Boric acid (g/l) Temperature (8C) pH Anode Current density (A/dm2) Time (min)
9–45 170 150 14 20 40–50 3–5 Zinc 5–15 10–15
Table 4 Composition and conditions of green chromating bath Variables
Amount
Potassium chromate (g/l) Chromic acid (g/l) Nitric acid (g/l) Formic acid (g/l) Temperature (8C) Immersion time (s) pH
50 10 7.5 60 35–50 25–50 1–2
Fig. 1. SEM micrographs of the Zn–Ni alloy electrodeposits, showing the change of morphology with the deposit nickel content: (a) Zn–3.3 wt.% Ni; (b) Zn–10.2 wt.% Ni; (c) Zn–13.2 wt.% Ni; (d) Zn–19 wt.% Ni.
M. Heydarzadeh Sohi, M. Jalali / Journal of Materials Processing Technology 138 (2003) 63–66
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Fig. 2. SEM micrographs of yellow chromated deposits with different nickel content: (a) pure zinc; (b) Zn–6.5 wt.% Ni; (c) Zn–17 wt.% Ni.
Fig. 3. SEM micrographs of the green chromated deposits with different nickel content: (a) pure zinc; (b) Zn–6.5 wt.% Ni; (c) Zn–l7 wt.% Ni.
shows the morphology of deposits with 10.2 wt.% nickel that should be consisted of Z and g phases. As can be seen, this deposit has a granular structure with some pores in it. Fig. 1(c) shows the SEM image of a deposit with 13.2 wt.% nickel with a dense plate like structure that should be consisted of g phase. Fig. 1(d) shows the morphology of a deposit with 19 wt.% nickel that has a granular structure with some pores in it. According to the phase diagram, this deposit should also be consisted of g phase. Fig. 2(a)–(c) shows the morphology of a number of yellow chromated deposits as observed by SEM. Almost all the deposits show a superficial network of cracks. These cracks seem to be smaller in the deposits with higher nickel content. The presence of cracks in the chromated deposits is probably due to stresses that develop in the chromate coatings after drying [7].
Fig. 3(a)–(c) shows the SEM image of a number of olive green chromated coatings. These coatings also contain a superficial network of cracks, but in comparison with the yellow chromated film, they are smaller and the surface of the coatings is smoother. Figs. 4 and 5 show the change of time for the white and red rust appearance with nickel content of the deposits, respectively. The results show that the corrosion resistance of zinc– nickel alloy coatings is superior to that of pure zinc coating and zinc–13 wt.% nickel gives the best protection. This is in agreement with the results of the other researchers [9]. The superior corrosion resistance observed for the zinc–nickel alloy coatings compared to the zinc coating could be explained by the barrier protection mechanism theory. During corrosion, zinc dissolves preferentially, leaving a
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M. Heydarzadeh Sohi, M. Jalali / Journal of Materials Processing Technology 138 (2003) 63–66
The results also showed that chromate conversion treatment improves the corrosion resistance significantly and the best protection is achieved by chromating of zinc–13 wt.% nickel alloy coating. The results also show that, on the whole, green chromating shows better protection than yellow chromating. The protective action of the passive chromate films is due both to the barrier effect of the film and the inhibiting action of chromium and its compounds with respect to dissolution of the metal of the substrate and to the reduction of the oxygen [6]. The higher protective nature of green chromate films in comparison with yellow chromate film is due to smaller size of the cracks in the green chromate coatings. Fig. 4. Change of time for the appearance of white rust with the nickel content for the Zn–Ni alloy deposits.
4. Conclusions Zinc–nickel alloy electrodeposits have better corrosion resistance than pure zinc deposit and zinc–13 wt.% nickel deposit has the best corrosion resistance. The chromated zinc and zinc–nickel alloy deposits show a superficial network of cracks. Chromate conversion treatment of zinc and zinc–nickel alloy deposits improves the corrosion resistance of the coatings up to 3–4 times. Among chromated specimens, chromated zinc–13 wt.% nickel coating has the highest corrosion resistance.
Fig. 5. Change of time for the appearance of red rust with nickel content for the Zn–Ni alloy coatings.
top layer enriched with nickel. This layer acts as a barrier to further attack. The reason for higher corrosion resistance of zinc–13 wt.% nickel deposit compared with the zinc–10.2 and 28 wt.% nickel alloy coatings could be explained by the absence of the porosity in this coating (Fig. 2) and its single g phase structure and hence the absence of local cells between different phases that would be present in the case of the dual phase coatings like zinc–10.2 wt.% Ni.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
T.J. Natorski, Met. Finish. 90 (1992) 15. H. Ohtsubo, H. Sogo, K. Nakai, Y. Ohmori, ISIJ Int. 37 (5) (1997) 512. R. Fratesi, G. Roventie, Surf. Coat. Technol. 82 (1996) 158. A.M. Alfantazi, U. Erb, Corros. Eng. 52 (November 1996) 880. M.P. Gigandet, J. Faucheu, M. Tachez, Surf. Coat. Technol. 89 (1997) 285. V. Ivanova, G. Raichevsky, St. Vitkova, M. Nikolova, Surf. Coat. Technol. 82 (1996) 232. N.M. Martyak, Surf. Coat. Technol. 88 (1996) 139. Metals Handbook, vol. 3, 9th ed., ASM International, 1992. D.E. Hall, Plat. Surf. Finish. 70 (11) (1983) 59.
Study of the corrosion properties of zinc–nickel alloy electrodeposits before and after chromating M. Heydarzadeh Sohi*, M. Jalali Department of Metallurgy and Materials, Faculty of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran
Abstract In this study, corrosion behavior of zinc–nickel alloy electrodeposits on steel sheets has been investigated using the neutral salt spray test. The performance of the coatings with various nickel contents (up to 28 wt.%) was compared with electrodeposited zinc coating. The results show that the corrosion resistance of zinc–nickel alloy coatings is superior to that of pure zinc coating and the zinc–13 wt.% nickel gives the best protection. The corrosion behavior of chromated zinc–nickel alloy coatings was also studied. The results show that chromate conversion coating increases the corrosion resistance significantly (up to 3–4 times), and the best corrosion resistance is achieved by chromating of zinc– 13 wt.% nickel alloy coating. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Zn–Ni alloys plating; Chromating; Corrosion resistance
1. Introduction Zinc electrodeposit is widely used as corrosion protective coating for steel parts. In order to enhance the corrosion resistance of this coating, it is alloyed with metals of the iron group (such as nickel, iron and cobalt) [1,2]. Amongst the zinc alloys, zinc–nickel alloys with different nickel content have been the most successful [3,4]. On the other hand, chromate conversion coating of zinc electrodeposit is also used for enhancement of its corrosion resistance [5–7]. The added corrosion resistance due to the chromate coating is dependent on the type of the chromating. In this research the effect of the nickel content and chromate conversion coating on the corrosion resistance of zinc–nickel alloy electrodeposits have been studied.
2. Experimental The base material in this study was a low carbon steel sheet that is used for car body. The dimensions of specimens used in this work were 100 mm 70 mm 1 mm. After surface preparation, the specimens were electroplated with zinc and zinc–nickel alloys. The formulations of the electrolytic bathes that were used for pure zinc and zinc–nickel * Corresponding author. E-mail address: [email protected] (M. Heydarzadeh Sohi).
alloy electrodepositions and their optimum conditions are shown in Tables 1 and 2, respectively. A number of specimens were then conversion coated by yellow and green chromating. The formulations of the electrolytes that were used for chromating and their optimized conditions are shown in Tables 3 and 4. Corrosion resistance of the coatings was measured by using neutral salt spray test (NaCl, 5%; T ¼ 35 8C). Periodical visual observation was carried out in order to evaluate the amount of the corrosion of the specimens. The corrosion resistance of the coatings was determined by observing the white rust (which was the corrosion product of the coatings) and red rust (which was the corrosion product of the steel substrate). The microstructural characterization was performed using scanning electron microscopy. The coatings analysis was performed by energy dispersive X-ray spectroscopy (EDXS).
3. Results and discussion Fig. 1(a)–(d) shows the surface morphology of a number of the Zn–Ni deposits as observed by SEM. As shown in Fig. 1(a) the morphology of the deposits with low nickel content is lamella. According to Zn–Ni binary diagram [8], deposits with up to 9 wt.% nickel should be consisted of Z phase that have hexagonal structure like pure zinc. Fig. 1(b)
0924-0136/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00050-5
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M. Heydarzadeh Sohi, M. Jalali / Journal of Materials Processing Technology 138 (2003) 63–66
Table 1 The composition and conditions of the zinc plating bath
Table 3 Composition and conditions of yellow chromating bath
Variables
Amount
Variables
Amount
Zinc oxide (g/l) NaCN (g/l) NaOH (g/l) Temperature (8C) Anode Current density (A/dm2) Time (min)
34 90 80 25 Zinc 15 5–10
Sodium dichromate (g/l) Sodium sulfate (g/l) Sodium nitrate (g/l) Nitric acid (g/l) Temperature (8C) Immersion time (s) pH
160 50 50 4 30–40 25–50 1–2
Table 2 The composition and conditions of Zn–Ni electroplating Variables
Amount
Nickel sulfate (g/l) Zinc sulfate (g/l) Sodium sulfate (g/l) Ammonium chloride (g/l) Boric acid (g/l) Temperature (8C) pH Anode Current density (A/dm2) Time (min)
9–45 170 150 14 20 40–50 3–5 Zinc 5–15 10–15
Table 4 Composition and conditions of green chromating bath Variables
Amount
Potassium chromate (g/l) Chromic acid (g/l) Nitric acid (g/l) Formic acid (g/l) Temperature (8C) Immersion time (s) pH
50 10 7.5 60 35–50 25–50 1–2
Fig. 1. SEM micrographs of the Zn–Ni alloy electrodeposits, showing the change of morphology with the deposit nickel content: (a) Zn–3.3 wt.% Ni; (b) Zn–10.2 wt.% Ni; (c) Zn–13.2 wt.% Ni; (d) Zn–19 wt.% Ni.
M. Heydarzadeh Sohi, M. Jalali / Journal of Materials Processing Technology 138 (2003) 63–66
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Fig. 2. SEM micrographs of yellow chromated deposits with different nickel content: (a) pure zinc; (b) Zn–6.5 wt.% Ni; (c) Zn–17 wt.% Ni.
Fig. 3. SEM micrographs of the green chromated deposits with different nickel content: (a) pure zinc; (b) Zn–6.5 wt.% Ni; (c) Zn–l7 wt.% Ni.
shows the morphology of deposits with 10.2 wt.% nickel that should be consisted of Z and g phases. As can be seen, this deposit has a granular structure with some pores in it. Fig. 1(c) shows the SEM image of a deposit with 13.2 wt.% nickel with a dense plate like structure that should be consisted of g phase. Fig. 1(d) shows the morphology of a deposit with 19 wt.% nickel that has a granular structure with some pores in it. According to the phase diagram, this deposit should also be consisted of g phase. Fig. 2(a)–(c) shows the morphology of a number of yellow chromated deposits as observed by SEM. Almost all the deposits show a superficial network of cracks. These cracks seem to be smaller in the deposits with higher nickel content. The presence of cracks in the chromated deposits is probably due to stresses that develop in the chromate coatings after drying [7].
Fig. 3(a)–(c) shows the SEM image of a number of olive green chromated coatings. These coatings also contain a superficial network of cracks, but in comparison with the yellow chromated film, they are smaller and the surface of the coatings is smoother. Figs. 4 and 5 show the change of time for the white and red rust appearance with nickel content of the deposits, respectively. The results show that the corrosion resistance of zinc– nickel alloy coatings is superior to that of pure zinc coating and zinc–13 wt.% nickel gives the best protection. This is in agreement with the results of the other researchers [9]. The superior corrosion resistance observed for the zinc–nickel alloy coatings compared to the zinc coating could be explained by the barrier protection mechanism theory. During corrosion, zinc dissolves preferentially, leaving a
66
M. Heydarzadeh Sohi, M. Jalali / Journal of Materials Processing Technology 138 (2003) 63–66
The results also showed that chromate conversion treatment improves the corrosion resistance significantly and the best protection is achieved by chromating of zinc–13 wt.% nickel alloy coating. The results also show that, on the whole, green chromating shows better protection than yellow chromating. The protective action of the passive chromate films is due both to the barrier effect of the film and the inhibiting action of chromium and its compounds with respect to dissolution of the metal of the substrate and to the reduction of the oxygen [6]. The higher protective nature of green chromate films in comparison with yellow chromate film is due to smaller size of the cracks in the green chromate coatings. Fig. 4. Change of time for the appearance of white rust with the nickel content for the Zn–Ni alloy deposits.
4. Conclusions Zinc–nickel alloy electrodeposits have better corrosion resistance than pure zinc deposit and zinc–13 wt.% nickel deposit has the best corrosion resistance. The chromated zinc and zinc–nickel alloy deposits show a superficial network of cracks. Chromate conversion treatment of zinc and zinc–nickel alloy deposits improves the corrosion resistance of the coatings up to 3–4 times. Among chromated specimens, chromated zinc–13 wt.% nickel coating has the highest corrosion resistance.
Fig. 5. Change of time for the appearance of red rust with nickel content for the Zn–Ni alloy coatings.
top layer enriched with nickel. This layer acts as a barrier to further attack. The reason for higher corrosion resistance of zinc–13 wt.% nickel deposit compared with the zinc–10.2 and 28 wt.% nickel alloy coatings could be explained by the absence of the porosity in this coating (Fig. 2) and its single g phase structure and hence the absence of local cells between different phases that would be present in the case of the dual phase coatings like zinc–10.2 wt.% Ni.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
T.J. Natorski, Met. Finish. 90 (1992) 15. H. Ohtsubo, H. Sogo, K. Nakai, Y. Ohmori, ISIJ Int. 37 (5) (1997) 512. R. Fratesi, G. Roventie, Surf. Coat. Technol. 82 (1996) 158. A.M. Alfantazi, U. Erb, Corros. Eng. 52 (November 1996) 880. M.P. Gigandet, J. Faucheu, M. Tachez, Surf. Coat. Technol. 89 (1997) 285. V. Ivanova, G. Raichevsky, St. Vitkova, M. Nikolova, Surf. Coat. Technol. 82 (1996) 232. N.M. Martyak, Surf. Coat. Technol. 88 (1996) 139. Metals Handbook, vol. 3, 9th ed., ASM International, 1992. D.E. Hall, Plat. Surf. Finish. 70 (11) (1983) 59.