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Effect of coating thickness on modifying the texture and corrosion performance of hot-dip galvanized coatings
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Current Applied Physics 9 (2009) 59–66 www.elsevier.com/locate/cap www.kps.or.kr
Effect of coating thickness on modifying the texture and corrosion performance of hot-dip galvanized coatings H. Asgari a,b,*, M.R. Toroghinejad a, M.A. Golozar a a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran b Young Researchers Club, Arak, Iran Received 9 March 2007; received in revised form 31 August 2007; accepted 17 October 2007 Available online 8 December 2007
Abstract Hot-dip galvanized zinc coating is the most frequently used among coatings to protect steel against corrosion. When coated steel sheets are subjected to a corrosive environment, its corrosion behaviour is affected by texture and microstructure variations. The aim of this research work was to study the texture and corrosion resistance of hot-dip galvanized zinc coatings affected by the coating thickness and chemical composition of the zinc bath. Texture of the coatings was evaluated employing X-ray diffraction whilst its corrosion behaviour was analyzed using Tafel polarization test. Experimental results showed that (00.2) basal texture component would be weakened by increasing the lead content of the zinc bath and coatings with strong (00.2) texture component have lower corrosion current density than the coatings with weak (00.2) texture component. Furthermore, it was inferred that by increasing the thickness of the coatings with the same content of lead in the zinc bath, the relative intensity of (00.2) texture component and corrosion resistance of the coatings would be decreased and conversely, relative intensity of (20.1) high angle pyramidal planes and (10.0) prism planes would be increased due to establish a balance between surface and strain energies. Besides, five types of morphology were observed on the surface of hot-dip galvanized coatings in dull and bright spangles. Finally, it was recognized that the main corrosion product of the salt spray test is Simonkolleite. Ó 2007 Elsevier B.V. All rights reserved. PACS: 68.55.jm; 68.55.jd; 81.40.wx Keywords: Texture; Corrosion resistance; Coating thickness; Surface/strain energy; Hot-dip galvanizing; Tafel polarization test
1. Introduction Hot-dip galvanized coatings have been used in industrial fields such as automobile, electrical home applications or construction due to their excellent corrosion performance [1,2]. The corrosion protection of galvanized coatings arises from the barrier action of a zinc layer, the secondary barrier action of the zinc corrosion products and the cathodic protection of zinc on unintentionally exposed part of the steel, with the coating acting as a sacrificial anode [1–3]. In a large * Corresponding author. Address: Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran. Tel.: +98 861 2776316; fax: +98 311 3912752. E-mail address: [email protected] (H. Asgari).
1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.10.090
number of continuous hot-dip galvanizing lines, lead is usually incorporated into the zinc bath [1,4]. This addition not only causes an increase in the bath fluidity and a decrease in its surface tension, but also in small concentrations (0.04– 0.2 wt%) improves the zinc coating uniformity and its adhesion to steel sheet [4,5]. Texture is an important factor which affects the coating properties and depends strongly on external factors such as cooling rate gradient, surface condition of steel substrate during the coating solidification process and bath chemical composition [5,6]. Concerning the coating corrosion resistance, this depends in particular on the zinc layer chemical composition and also affected by the crystallographic orientation [1,4–6]. It should be noted that in hot-dip galvanized coatings, it is the texture of eta layer which is
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considered as the coating texture because this layer consists of about 98 wt% of zinc and only about 2 wt% of iron and has an important role in determining the texture and corrosion resistance of the coating [5]. Generally, there are many models and theories about the texture development in coatings which every one has its advantages and disadvantages. Among these models, two models are more useful and applicable, namely surface energy minimization and strain energy minimization models. It has been shown that texture is determined by a competition between two thermodynamical parameters, i.e. surface and strain energies [7,8]. During the formation and development of a texture component, there should be a balance between these energies [7,8]; for example, when the coating is thick and consequently, its strain energy is much higher than its surface energy, texture develops in the way in which the intensity of high surface energy planes would be increased (strain energy minimization model) [7,8]. Therefore, coating thickness influences on the texture of the coatings and since the corrosion resistance is affected by its texture, the thickness plays an important role in determining the corrosion resistance of the coatings [5,7–9]. The aim of this study was to assess the effect of coating thickness and zinc bath chemical composition variations on some properties of hot-dip galvanized coatings such as texture and corrosion resistance. It should be noticed that here, only (00.2), (20.1) and (10.0) texture components were considered because theses components are more effective than other components in changing the corrosion performance of hot-dip galvanized coatings; i.e, corrosion resistance decreases as the intensity of (20.1) and (10.0) components increases, respectively. Due to this fact, these components were selected for comparison. Table 1 Chemical composition of steel substrate Grade
%C
%Ti
%S
%Mo
%P
%V
%Si
%Al
JIS G3302
0.038
0.001
0.005
0.002
0.007
0.001
0.009
0.048
2. Experimental procedure 2.1. Materials Studies were carried out three times on commercially available steel sheets hot-dip coated with Zinc in Mobarakeh Steel Company and the average of the results considered as the final report. Also, production conditions such as rolling finishing temperature, coiling temperature, cold work percentage and annealing conditions are the same for all of the steel sheets used in this study. Table 1 gives the chemical composition of the substrate, whilst the galvanizing production parameters of the samples are summarized in Tables 2 and 3. All of the samples were degreased with alkaline detergent and toluene; they were kept in a dessicator up to starting the tests. 2.2. Texture determination The crystallographic orientation of the coatings was determined using X-ray diffraction (Philips XL Model 30, Cu Ka radiation, step size of 0.03°, counting time of 1 s). A 2-theta scan was performed between 20° and 140° and the integrated intensities of several reflections were determined, these are termed Ihkil. It should be noted that integrated intensities require background subtraction. Each Ihkil is normalized by dividing it by its structure factor or random intensity, I 0hkil , giving I nhkil . The values of I 0hkil were obtained from powder zinc pattern. It should be noted that in this study, only I n00:2 , I n20:1 and I n10:0 were considered. 2.3. Corrosion behaviour analysis Tafel polarization tests were conducted in a 5% NaCl solution at room temperature using an EG&G Princton Applied Research Model 263A potentiostat, a standard corrosion cell kit with the working electrode, two graphite counter electrodes and a saturated calomel reference elec-
Table 2 Production parameters of samples A, B and C Sample
A B C
Chemical composition of the zinc bath %Pb
%Fe
%Al
0.010 0.045 0.065
0.023 0.027 0.021
0.195 0.191 0.188
Jet wiper distance from sheet surface (mm)
Sheet thickness (mm)
Bath temperature (°C)
Strip entry temperature (°C)
Galvanizing line speed (m/min)
Coating thickness (lm)
Eta layer thickness (lm)
100 100 100
0.5 0.5 0.5
462 ± 1 462 ± 1 462 ± 1
466 ± 1 466 ± 1 466 ± 1
100 100 100
46 46 46
13 13 13
Table 3 Production parameters of samples B1, B2, C1 and C2 Sample
B1 B2 C1 C2
Chemical composition of zinc bath %Pb
%Fe
%Al
0.045 0.045 0.065 0.065
0.027 0.027 0.021 0.021
0.191 0.191 0.188 0.188
Jet wiper distance from sheet surface (mm)
Sheet thickness (mm)
Bath temperature (°C)
Strip entry temperature (°C)
Galvanizing line speed (m/min)
Coating thickness (lm)
Eta layer thickness (lm)
105 125 105 125
0.5 0.5 0.5 0.5
462 ± 1 462 ± 1 462 ± 1 462 ± 1
466 ± 1 466 ± 1 466 ± 1 466 ± 1
55 35 55 35
60 74 60 74
18 24 18 24
H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
2.4. Microstructure study Cross sections of the coatings were studied using conventional metallography method, scanning electron microscopy and optical microscopy. Because of high sensitivity of zinc to water, absolute alcohol was used for grinding and polishing of samples. Composition of the coating layers was determined using EDS analysis. 2.5. Morphological study The morphological study of corrosion products was performed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM studies were carried out using a Philips XL Model 30 and XRD studies were done employing a Philips Xpert-MPD Model 3040. Composition of the corrosion products was detected by means of EDS analysis. The surface morphology was studied using Nomalski differential interference method and employing an optical microscopy equipped with polarized light. 3. Results Fig. 1 shows a typical cross section of zinc coated sample B; four distinct coating layers of hot-dip galvanized steel can be identified in this micrograph. EDS analysis of coating layers indicated that coatings consisted of four layers; gamma, delta, zeta and eta. Also, EDS analysis detected no aluminum within the layers. Comparison between the relative intensities of some texture components Gamma Delt
Zeta
of samples A–C is indicated in Fig. 2. It is clear that by increasing the lead content of the zinc bath, the relative intensity of (00.2) basal texture component is decreased and conversely, the relative intensities of (20.1) high angle pyramidal and (10.0) prismatic texture components are increased. It means that as the lead content of the zinc bath increases, the (00.2) basal texture component of the coating is weakened and transforms to the (20.1) and (10.0) texture components. (Fig. 3) illustrates the corrosion current densities (icorr) of the samples. It is discerned that corrosion current density increases as the lead content of the zinc bath increases and difference between the corrosion current density of sample A and C is about 14 lA cm 2. Increasing in the corrosion current density of the samples indicates the decreased corrosion resistance of the coatings due to the addition of lead to the zinc bath. After 24 h of exposing two samples in the salt spray (samples A and C), some pseudo-hexagonal plane crystals called platelets having a preferential facing (approximately normal to the substrate plane) could be observed on the galvanized steel samples (Fig. 4a). These crystals grow forming islands which spread over the entire surface as the exposure time increases (Fig. 4b); many of them nucleate heterogeneously on the surface defects but the majority
A
B
C
40 35
I (hkil)/I 0(hkil)
trode (SCE). The volume of the electrolyte for each test was 500 ml potentiodynamic scanning was performed by stepping the potential at a scan rate of 1 mV/s from 250 mV (SCE) to 500 mV (SCE). The edges of samples were covered by a plastic lacquer to prevent the corrosion of steel. Salt spray tests were performed under the criteria established by ASTM B117 to study the corrosion products of galvanized samples after exposure in salt spray.
61
30 25 20 15 10 5 0
(00.2)
(20.1)
(10.0)
Texture components Fig. 2. Relative intensities of (00.2), (20.1) and (10.0) components of the samples.
45
Eta
C
i corr (micro A/cm2)
40 35
B
30 25 20 15
A
10 5 0
0.01
10 m Fig. 1. Optical micrograph of cross section of sample B.
0.045
0.065
Wt.% of Pb in zinc bath Fig. 3. Relationship between lead content of the zinc bath and icorr of the samples.
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H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
Fig. 4. (a) Pseudo-hexagonal plane crystals of Simonkolleite on the surface of samples. (b) Islands of Simonkolleite on the surface after 24 h exposure in salt spray.
C
40
C1
C2
I(hkil) /I 0(hkil)
35 30 25 20 15 10 5 0
(00.2)
(20.1)
(10.0)
Texture components Fig. 6. Relative intensities of (00.2), (20.1) and (10.0) components of the samples with different thicknesses and equal zinc bath chemical composition.
45 40
icorr (micro A/cm2 )
does it on NaCl crystals that, in turn, precipitate on the metallic surface. XRD analysis indicated that Simonkolleite (Zn6(OH)8Cl2 H2O) was the major component of the corrosion products, whilst EDS results confirmed that chlorine, zinc and oxygen were the main elements present in the pseudo-hexagonal crystals. Some amounts of Zincite (ZnO) and Hydrozincite (Zn5(CO3)2(OH)6) were also detected. In addition, after 72 h of exposure in salt spray, the surface of sample C was covered with a relatively thick layer of corrosion products whereas the surface of sample A presented not only less voluminous corrosion products but also a partially attacked area as a result of better corrosion resistance. Figs. 5 and 6 show the influence of coating thickness on the relative intensity of texture components. It should be noticed that although the production parameters of these samples are the same (Tables 2 and 3), only by increasing the coating thickness and consequently, increasing the eta layer thickness due to decreasing the galvanizing line speed, (00.2) basal texture component is weakened and other components such as (20.1) and (10.0) are strengthened. In (Figs. 7 and 8), effect of coating thickness on the corrosion
35 30
B
B2
B1
25 20 15 10 5
B
40
B1
B2
0 46
I (hkil) /I0(hkil)
35
60
74
Coating thickness (micrometer)
30 25
Fig. 7. Relationship between coating thickness and icorr of the samples with different thicknesses and equal zinc bath chemical composition.
20 15 10 5 0
(00.2)
(20.1)
(10.0)
Texture components Fig. 5. Relative intensities of (00.2), (20.1) and (10.0) components of the sample with different thicknesses and equal zinc bath chemical composition.
current density of the samples is exhibited. As can be recognized, increasing the coating thickness gives rise to increase the corrosion current density of the coatings so that this increasing is completely vivid for thicknesses about 74 lm. It should be mentioned that the surface of hot-dip galvanized coating contains dull and bright spangles and these
H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
45
icorr (micro A/cm2 )
40
C
C1
C2
35 30 25 20 15 10 5 0 46
60
74
Coating thickness (micrometer) Fig. 8. Relationship between coating thickness and icorr of the samples with different thicknesses and equal zinc bath chemical composition.
spangles have different morphologies. Microscopic analysis was performed on both dull and bright spangle areas. In the dull area, three different surface morphologies were
63
found whereas the bright area showed almost uniform surface morphology. In addition, a feathery type of morphology was observed in the area where the dull and bright spangles were present alternatively. Hence, the five types of spangle patterns were observed in hot-dip galvanized coatings at high magnifications. Macroscopically, dullappearing areas are subdivided into three microscopic different forms. The dull spangles areas are: Dimpled: The dimpled structure is characterized by globular lead particles, which were segregated across the zinc surface. Fig. 9a shows the dimpled area. The lead element does not form the solute solution with zinc but exists in pure form at the surface and it is dense in the dull spangle areas. Ridged: Small hills and valleys characterize the ridged morphology. In this morphology, the lead particles were found at certain points. The amount of lead particles present in this morphology is less than that of in dimpled morphology (Fig. 9b).
Fig. 9. (a) Dimpled morphology, (b) ridged morphology, (c) orthogonal morphology, (d) shiny morphology and (e) feathery morphology.
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H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
(00.2) component
(20.1) component
(10.0) component
Corrosion current density
45
45
A
40
C
35
I (hkil) /I 0(hkil)
30
35 30
B
25
25
20
20
15
15
10
10
5
icorr(micro A/cm2)
40
5
0 0
0.02
0.04
0.06
0 0.08
Wt.% of Pb in zinc bath Fig. 10. Relationship between lead content of the zinc bath, texture components and icorr of the samples.
Orthogonal dendritic: The orthogonal dendrite spangles are characterized by the secondary dendritic arms morphology as illustrated in Fig. 9c. In this area, the lead particles were also found at some places. The bright spangles areas are: Shiny: The shiny regions display a smooth surface structure in comparison with the dull area. The surface morphology is indicated in Fig. 9d. The shiny area contains lead particles, but in much smaller amounts and sizes with respect to the dull area. Feathery: The feathery structure consists of a lath-like pattern with alternate shiny dendritic branches and dull areas (Fig. 9e). The above morphologies, observed in this work, are in good agreement with the morphologies obtained by other researchers [10–12]. 4. Discussion It can be seen in Fig. 1 that increasing the lead content of the zinc bath affects the intensity of the texture components. This is due to the fact that lead decreases the nucleation sites of basal planes and conversely, increases the nucleation sites of other planes such as low angle pyramids, high angle pyramids and prism planes [13]. Therefore, when the lead content of zinc bath is increased, less nuclei of basal planes form and consequently, less basal planes would be formed and lower area of the surface would be covered by these planes which are parallel with the surface [5,13,14]. In Fig. 10, it is evident that (00.2) basal texture component has a key role in determining the corrosion resistance of the coatings. When the relative intensity of (00.2) basal plane texture is high, it can be concluded that the surface is
mainly covered with basal planes which are parallel to sheet surface [5,13,14]. The basal planes have the highest binding energy of the surface atoms and thus the total energy involved in the breaking of the bonds and the subsequent dissolution of atoms is highest for these planes [15]. Also, surface becomes electrochemically less active because surface energy, that its value is inversely proportional to the atomic spacing (d) and directly proportional to the number of broken bonds per atom (a), is lowest for basal plane with respect to other planes in hcp structure (for example: 1/d = 0.41 and 0.89 for (00.2) and (20.1) planes, respectively) [5,16]. In (Figs. 5 and 6), it was observed that by increasing the coatings thickness, (00.2) basal texture component weakened and conversely, (20.1) and (10.0) texture components strengthened. These effects would be probably ascribed to the surface and strain energies. There are many models and theories that explain the changes of texture due to the variations of the coating thickness; however, the nature of these changes and relations is not completely clear yet. Among these models, the most general ones are usually used: surface energy minimization and strain energy minimization models. As it was mentioned before, texture is determined by a competition and balancing between surface and strain energies; i.e. at small thicknesses, the coating shows an orientation corresponding to that with lowest surface energy and highest strain energy [7–9]. In other words, when the coating is thin, the strain energy of the coating structure is very low and negligible and conversely, the surface energy is high and important because surface energy does not vary with the coating thickness [7,8]; therefore, the texture of the coating consists of planes which have the lowest surface energy and highest strain energy to com-
H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
pensate the shortage of strain energy and make a balance between surface and strain energies [7–9]. Eta layer has about 98 wt% of zinc and in hcp structure of this element, (00.2) planes have the lowest surface energy and the highest strain energy [5]. Thus, when the coating thickness (and eta layer thickness) is small, basal planes would increase and become the predominant texture component of the coating and other planes, such as (20.1) and (10.0) with much higher surface energy and lower strain energy than basal planes would have a low intensity. In contrast, as the coating thickness increases, the strain energy would be increased and became much higher than surface energy. In this situation, texture of the coating tries to establish a balance between surface and strain energies by means of decreasing the strain energy and increasing the surface energy [7–9]; hence, the intensity of high strain energy (00.2) basal planes would be decreased whilst the intensity of (20.1) and (10.0) planes with much higher surface energy would be increased. In (Figs. 7 and 8), it was illustrated that increasing the coatings thickness was detrimental to the corrosion resistance of the coatings. As it was mentioned above, it can be attributed to the low corrosion resistance of (20.1) and (10.0) planes which strengthened in intensity as the coatings thickened [15]; consequently, the corrosion resistance of the thick coating (with thicker eta layer) would be decreased. Therefore, the outstanding result of this research is the fact that although increasing the thickness of hot-dip galvanized coatings results in the increasing of the coatings lifetime [1], this thickening would not lead to better corrosion resistance or texture of the coating and would have detrimental effect on these properties. Therefore, considering the relationship between texture, corrosion resistance and coating thickness in any application of these coatings is necessary. It is possible by more accurate studies to attain an optimized coating thickness in which the texture, corrosion resistance and microstructure of the coating are in the best situation. This optimized coating thickness is not only the best ones in terms of corrosion resistance, but also the most economical; thus, increasing the thickness of hot-dip galvanized coatings up to this optimized thickness for sensitive applications in which the quality and corrosion resistance considered as determining factor is not suggested. Different morphologies and segregated lead on the surfaces in Fig. 9 could be attributed to the fact that in zinc solidification, the fast growth crystallographic direction is [1 0 1 0] that drops in the basal plane [2]. When the dendrites grow parallel to the sheet surface, that means the basal plane of zinc is parallel to it, the solutes are rejected towards the grain boundary, leaving the surface relatively pure and bright [2,5,17]. However, when the basal plane of the grain is at angle with the substrate, the fast growth direction is towards the coating surface and the solutes are rejected towards it and cause the surface to be darkened. These facts explain the different segregation observed in bright and dull spangles [2,5,17]. In addition, segregation
65
of lead in the spangles increases as the lead content of the bath increases [2,5]. It is worthy of note here that to some extent, it was expected that with concentrations of aluminum up to 0.1 wt% aluminum in the zinc bath, (0.2 wt% in this study), Fe2Al5 layer would be formed and suppressed the formation of delta and eta phases [1,18]. However, microscopic investigations confirmed that four layers have been formed (Fig. 1) and EDS analysis detected no aluminum within the layers. It is probably due to the fact that for Fe2Al5 layer to be formed, in addition to the required amount of aluminum concentration in the bath [1,19], low temperature of the zinc bath [20] and rather high difference between the strip entry temperature and bath temperature [21] should be prepared. The low temperature of the bath can slow down the reaction rate of zinc–iron and provides a suitable condition for the reaction of aluminum and iron [22–24]. Also, as the temperature of the zinc bath decreases, the incubation period for Fe–Zn phases formation increases and their formation becomes more difficult [22–24]. Besides, the high temperature difference between the strip and galvanizing bath (e.g. 15 °C) can effectively increase Al–Fe reaction rate [19,20]. In the other words, the rate of Al uptake in zinc coatings to form Fe2Al5 depends on the difference between the temperatures of the zinc bath and strip entry temperature, that is, the Al content in the interface increases approximately linearly with the difference between the strip entry temperature and the bath temperature [19,20]. Note that iron in Fe2Al5Znx and Fe2Al5 belongs to the galvanizing bath and steel substrate, respectively. In this work, temperature of the zinc bath was enough high (462 °C) [20] and the difference between the bath temperature and strip entry temperature (466 °C) was small [21]; thus, the formation of Fe2Al5 was inhibited. 5. Conclusions 1. Increasing the lead content of the zinc bath results in the weakening of (00.2) basal texture component and strengthening of (20.1) high angle pyramidal and (10.0) prism texture components. 2. Coatings with strong (00.2) texture component and weak (20.1) and (10.0) texture components have higher corrosion resistance than the coatings with weak (00.2) texture component and strong (20.1) and (10.0) texture components. 3. As the coating thickness increases, the relative intensity of (00.2) basal planes decreases and conversely, the relative intensity of (20.1) and (10.0) planes increases; thus, corrosion resistance of the coating decreases. 4. Simonkolleite is the major corrosion product which would be formed on the surface of hot-dip galvanized coatings during the salt spray test. 5. Five types of morphology would be observed on the surface of hot-dip galvanized coatings in dull and bright spangles.
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6. Increasing the bath temperature (462 °C) and decreasing the difference between the bath temperature and strip entry temperature would inhibit the formation of Fe2Al5 inhibition layer. Acknowledgement The authors would like to thank Mobarakeh Steel Company laboratories staff and R&D unit engineers for helping with the samples. Assistance received from SEM and XRD laboratories staff of Materials Engineering Department of Isfahan University of Technology is also appreciated. References [1] A.R. Marder, The metallurgy of zinc coated steel, Prog. Mater. Sci. 45 (2000) 191–271. [2] M. Zapponi, A. Quiroga, T. Perez, Segregation of alloying elements during the hot-dip coating solidification process, Surf. Coat. Technol. 22 (1999) 18–20. [3] N. Katiforis, G. Papadimitriou, Influence of copper, cadmium and tin additions in the galvanizing bath on the structure, thickness and cracking behavior of the galvanized coatings, Surf. Coat. Technol. 78 (1999) 185–195. [4] D.I. Cameron, G.J. Harvey, J. Ormay, Aust. Inst. Metals 10 (1965) 225. [5] P.R. Sere, C.I. Culcasi, J.D. Elsner, A.I. Di Sarli, Relationship between texture and corrosion resistance in hot-dip galvanized steel sheets, Surf. Coat. Technol. 122 (1999) 143–149. [6] S. Chang, J.C. Shin, Corr. Sci. 36 (8) (1994) 1425. [7] J. Pelleg, L.Z. Zervin, S. Lungo, N. Croitoru, Thin Solid Films 197 (1991) 117. [8] U.C. OH, J.H. JE, J. Appl. Phys. 74 (1993) 1692. [9] L. Hultman, W. Munz, J. Musil, K. Kadlec, I. Petrov, J. Vac. Sci. Technol. A9 (1991) 434.
[10] J. Strutzenberger, J. Faderl, Solidification and spangle formation of hot-dip galvanized zinc coatings, Met. Mater. Trans. 29A (1998) 631. [11] D. Jaffrey, J.D. Brown, T.J. Howard, Metall. Trans. A. Phys. Metall. 12 (1980) 631–635. [12] A.K. Singh, G. Jha, S. Chakrrabarti, Spangle formation on hot-dip galvanized steel sheet and its effects on corrosion resistant properties, Corrosion 59 (2) (2003) 189–196. [13] V. Rangarajan, C.A. Natalie, D.K. Matlock, G. Krauss, Deformation behavior of hot-dip galvanized coatings in complex sheet metal forming, J. Mater. Shaping Technol. 8 (1990) 101–105. [14] H. Asgari, M.R. Toroghinejad, M.A. Golozar, F. Ashrafizadeh, K. Raeissi, The role of (00.2) basal texture component and morphology in modifying the corrosion resistance of hot-dip galvanized zinc coatings, in: 45th Conference of Metallurgists, Proceedings of the Conference, Canada, Montreal, 2006. [15] J.C. Scully, The Fundamentals of Corrosion, Pergamon Press, 1990. [16] J.P. Zhao, X. Wang, Z.Y. Chen, S.Q. Yang, T.S. Shi, J. Appl. Phys. 30 (1997) 5. [17] L.E. Helwing, in: Proc. Galvanizers. Comm., Zinc Inst. Inc., 1981, pp. 22–51. [18] Y. DeAbreu, A. Dasilva, A. Ruiz, R. Requiz, N. Angula, Study of zinc coating on steel substrate attained by two different techniques, Surf. Coat. Technol. 120–121 (1999) 682–686. [19] T. Kanamaru, M. Nakayama, Alloying reaction control in production of galvannealed steel, Mater. Sci. Res. Int. 1 (1995) 150. [20] J. Mackowiak, N.R. Short, Metallurgy of galvanized coating, Int. Met. Rev. 1 (1979) 345. [21] J. Fadrel, M. Pimminger, L. Schonberger, Influence of Steel Grade and Surface Topography on the Galvannealing Reaction, Galvatech 92, Amsterdam, 1992, p. 194. [22] M. Isobe, Initial alloying behaviour in galvannealing process, Camp. ISIJ (1992) 1629. [23] N.-Y. Tang, G.R. Adams, P.S. Kolisnyk, On Determining Effective Aluminum in Continuous Galvanizing Bath, Galvatech 95, Chicago, 1995, p. 777. [24] N.-Y. Tang, Modelling of enrichment on galvanized coatings, Met. Mater. Trans. 26A (1995) 1669.
Current Applied Physics 9 (2009) 59–66 www.elsevier.com/locate/cap www.kps.or.kr
Effect of coating thickness on modifying the texture and corrosion performance of hot-dip galvanized coatings H. Asgari a,b,*, M.R. Toroghinejad a, M.A. Golozar a a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran b Young Researchers Club, Arak, Iran Received 9 March 2007; received in revised form 31 August 2007; accepted 17 October 2007 Available online 8 December 2007
Abstract Hot-dip galvanized zinc coating is the most frequently used among coatings to protect steel against corrosion. When coated steel sheets are subjected to a corrosive environment, its corrosion behaviour is affected by texture and microstructure variations. The aim of this research work was to study the texture and corrosion resistance of hot-dip galvanized zinc coatings affected by the coating thickness and chemical composition of the zinc bath. Texture of the coatings was evaluated employing X-ray diffraction whilst its corrosion behaviour was analyzed using Tafel polarization test. Experimental results showed that (00.2) basal texture component would be weakened by increasing the lead content of the zinc bath and coatings with strong (00.2) texture component have lower corrosion current density than the coatings with weak (00.2) texture component. Furthermore, it was inferred that by increasing the thickness of the coatings with the same content of lead in the zinc bath, the relative intensity of (00.2) texture component and corrosion resistance of the coatings would be decreased and conversely, relative intensity of (20.1) high angle pyramidal planes and (10.0) prism planes would be increased due to establish a balance between surface and strain energies. Besides, five types of morphology were observed on the surface of hot-dip galvanized coatings in dull and bright spangles. Finally, it was recognized that the main corrosion product of the salt spray test is Simonkolleite. Ó 2007 Elsevier B.V. All rights reserved. PACS: 68.55.jm; 68.55.jd; 81.40.wx Keywords: Texture; Corrosion resistance; Coating thickness; Surface/strain energy; Hot-dip galvanizing; Tafel polarization test
1. Introduction Hot-dip galvanized coatings have been used in industrial fields such as automobile, electrical home applications or construction due to their excellent corrosion performance [1,2]. The corrosion protection of galvanized coatings arises from the barrier action of a zinc layer, the secondary barrier action of the zinc corrosion products and the cathodic protection of zinc on unintentionally exposed part of the steel, with the coating acting as a sacrificial anode [1–3]. In a large * Corresponding author. Address: Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran. Tel.: +98 861 2776316; fax: +98 311 3912752. E-mail address: [email protected] (H. Asgari).
1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.10.090
number of continuous hot-dip galvanizing lines, lead is usually incorporated into the zinc bath [1,4]. This addition not only causes an increase in the bath fluidity and a decrease in its surface tension, but also in small concentrations (0.04– 0.2 wt%) improves the zinc coating uniformity and its adhesion to steel sheet [4,5]. Texture is an important factor which affects the coating properties and depends strongly on external factors such as cooling rate gradient, surface condition of steel substrate during the coating solidification process and bath chemical composition [5,6]. Concerning the coating corrosion resistance, this depends in particular on the zinc layer chemical composition and also affected by the crystallographic orientation [1,4–6]. It should be noted that in hot-dip galvanized coatings, it is the texture of eta layer which is
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considered as the coating texture because this layer consists of about 98 wt% of zinc and only about 2 wt% of iron and has an important role in determining the texture and corrosion resistance of the coating [5]. Generally, there are many models and theories about the texture development in coatings which every one has its advantages and disadvantages. Among these models, two models are more useful and applicable, namely surface energy minimization and strain energy minimization models. It has been shown that texture is determined by a competition between two thermodynamical parameters, i.e. surface and strain energies [7,8]. During the formation and development of a texture component, there should be a balance between these energies [7,8]; for example, when the coating is thick and consequently, its strain energy is much higher than its surface energy, texture develops in the way in which the intensity of high surface energy planes would be increased (strain energy minimization model) [7,8]. Therefore, coating thickness influences on the texture of the coatings and since the corrosion resistance is affected by its texture, the thickness plays an important role in determining the corrosion resistance of the coatings [5,7–9]. The aim of this study was to assess the effect of coating thickness and zinc bath chemical composition variations on some properties of hot-dip galvanized coatings such as texture and corrosion resistance. It should be noticed that here, only (00.2), (20.1) and (10.0) texture components were considered because theses components are more effective than other components in changing the corrosion performance of hot-dip galvanized coatings; i.e, corrosion resistance decreases as the intensity of (20.1) and (10.0) components increases, respectively. Due to this fact, these components were selected for comparison. Table 1 Chemical composition of steel substrate Grade
%C
%Ti
%S
%Mo
%P
%V
%Si
%Al
JIS G3302
0.038
0.001
0.005
0.002
0.007
0.001
0.009
0.048
2. Experimental procedure 2.1. Materials Studies were carried out three times on commercially available steel sheets hot-dip coated with Zinc in Mobarakeh Steel Company and the average of the results considered as the final report. Also, production conditions such as rolling finishing temperature, coiling temperature, cold work percentage and annealing conditions are the same for all of the steel sheets used in this study. Table 1 gives the chemical composition of the substrate, whilst the galvanizing production parameters of the samples are summarized in Tables 2 and 3. All of the samples were degreased with alkaline detergent and toluene; they were kept in a dessicator up to starting the tests. 2.2. Texture determination The crystallographic orientation of the coatings was determined using X-ray diffraction (Philips XL Model 30, Cu Ka radiation, step size of 0.03°, counting time of 1 s). A 2-theta scan was performed between 20° and 140° and the integrated intensities of several reflections were determined, these are termed Ihkil. It should be noted that integrated intensities require background subtraction. Each Ihkil is normalized by dividing it by its structure factor or random intensity, I 0hkil , giving I nhkil . The values of I 0hkil were obtained from powder zinc pattern. It should be noted that in this study, only I n00:2 , I n20:1 and I n10:0 were considered. 2.3. Corrosion behaviour analysis Tafel polarization tests were conducted in a 5% NaCl solution at room temperature using an EG&G Princton Applied Research Model 263A potentiostat, a standard corrosion cell kit with the working electrode, two graphite counter electrodes and a saturated calomel reference elec-
Table 2 Production parameters of samples A, B and C Sample
A B C
Chemical composition of the zinc bath %Pb
%Fe
%Al
0.010 0.045 0.065
0.023 0.027 0.021
0.195 0.191 0.188
Jet wiper distance from sheet surface (mm)
Sheet thickness (mm)
Bath temperature (°C)
Strip entry temperature (°C)
Galvanizing line speed (m/min)
Coating thickness (lm)
Eta layer thickness (lm)
100 100 100
0.5 0.5 0.5
462 ± 1 462 ± 1 462 ± 1
466 ± 1 466 ± 1 466 ± 1
100 100 100
46 46 46
13 13 13
Table 3 Production parameters of samples B1, B2, C1 and C2 Sample
B1 B2 C1 C2
Chemical composition of zinc bath %Pb
%Fe
%Al
0.045 0.045 0.065 0.065
0.027 0.027 0.021 0.021
0.191 0.191 0.188 0.188
Jet wiper distance from sheet surface (mm)
Sheet thickness (mm)
Bath temperature (°C)
Strip entry temperature (°C)
Galvanizing line speed (m/min)
Coating thickness (lm)
Eta layer thickness (lm)
105 125 105 125
0.5 0.5 0.5 0.5
462 ± 1 462 ± 1 462 ± 1 462 ± 1
466 ± 1 466 ± 1 466 ± 1 466 ± 1
55 35 55 35
60 74 60 74
18 24 18 24
H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
2.4. Microstructure study Cross sections of the coatings were studied using conventional metallography method, scanning electron microscopy and optical microscopy. Because of high sensitivity of zinc to water, absolute alcohol was used for grinding and polishing of samples. Composition of the coating layers was determined using EDS analysis. 2.5. Morphological study The morphological study of corrosion products was performed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM studies were carried out using a Philips XL Model 30 and XRD studies were done employing a Philips Xpert-MPD Model 3040. Composition of the corrosion products was detected by means of EDS analysis. The surface morphology was studied using Nomalski differential interference method and employing an optical microscopy equipped with polarized light. 3. Results Fig. 1 shows a typical cross section of zinc coated sample B; four distinct coating layers of hot-dip galvanized steel can be identified in this micrograph. EDS analysis of coating layers indicated that coatings consisted of four layers; gamma, delta, zeta and eta. Also, EDS analysis detected no aluminum within the layers. Comparison between the relative intensities of some texture components Gamma Delt
Zeta
of samples A–C is indicated in Fig. 2. It is clear that by increasing the lead content of the zinc bath, the relative intensity of (00.2) basal texture component is decreased and conversely, the relative intensities of (20.1) high angle pyramidal and (10.0) prismatic texture components are increased. It means that as the lead content of the zinc bath increases, the (00.2) basal texture component of the coating is weakened and transforms to the (20.1) and (10.0) texture components. (Fig. 3) illustrates the corrosion current densities (icorr) of the samples. It is discerned that corrosion current density increases as the lead content of the zinc bath increases and difference between the corrosion current density of sample A and C is about 14 lA cm 2. Increasing in the corrosion current density of the samples indicates the decreased corrosion resistance of the coatings due to the addition of lead to the zinc bath. After 24 h of exposing two samples in the salt spray (samples A and C), some pseudo-hexagonal plane crystals called platelets having a preferential facing (approximately normal to the substrate plane) could be observed on the galvanized steel samples (Fig. 4a). These crystals grow forming islands which spread over the entire surface as the exposure time increases (Fig. 4b); many of them nucleate heterogeneously on the surface defects but the majority
A
B
C
40 35
I (hkil)/I 0(hkil)
trode (SCE). The volume of the electrolyte for each test was 500 ml potentiodynamic scanning was performed by stepping the potential at a scan rate of 1 mV/s from 250 mV (SCE) to 500 mV (SCE). The edges of samples were covered by a plastic lacquer to prevent the corrosion of steel. Salt spray tests were performed under the criteria established by ASTM B117 to study the corrosion products of galvanized samples after exposure in salt spray.
61
30 25 20 15 10 5 0
(00.2)
(20.1)
(10.0)
Texture components Fig. 2. Relative intensities of (00.2), (20.1) and (10.0) components of the samples.
45
Eta
C
i corr (micro A/cm2)
40 35
B
30 25 20 15
A
10 5 0
0.01
10 m Fig. 1. Optical micrograph of cross section of sample B.
0.045
0.065
Wt.% of Pb in zinc bath Fig. 3. Relationship between lead content of the zinc bath and icorr of the samples.
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H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
Fig. 4. (a) Pseudo-hexagonal plane crystals of Simonkolleite on the surface of samples. (b) Islands of Simonkolleite on the surface after 24 h exposure in salt spray.
C
40
C1
C2
I(hkil) /I 0(hkil)
35 30 25 20 15 10 5 0
(00.2)
(20.1)
(10.0)
Texture components Fig. 6. Relative intensities of (00.2), (20.1) and (10.0) components of the samples with different thicknesses and equal zinc bath chemical composition.
45 40
icorr (micro A/cm2 )
does it on NaCl crystals that, in turn, precipitate on the metallic surface. XRD analysis indicated that Simonkolleite (Zn6(OH)8Cl2 H2O) was the major component of the corrosion products, whilst EDS results confirmed that chlorine, zinc and oxygen were the main elements present in the pseudo-hexagonal crystals. Some amounts of Zincite (ZnO) and Hydrozincite (Zn5(CO3)2(OH)6) were also detected. In addition, after 72 h of exposure in salt spray, the surface of sample C was covered with a relatively thick layer of corrosion products whereas the surface of sample A presented not only less voluminous corrosion products but also a partially attacked area as a result of better corrosion resistance. Figs. 5 and 6 show the influence of coating thickness on the relative intensity of texture components. It should be noticed that although the production parameters of these samples are the same (Tables 2 and 3), only by increasing the coating thickness and consequently, increasing the eta layer thickness due to decreasing the galvanizing line speed, (00.2) basal texture component is weakened and other components such as (20.1) and (10.0) are strengthened. In (Figs. 7 and 8), effect of coating thickness on the corrosion
35 30
B
B2
B1
25 20 15 10 5
B
40
B1
B2
0 46
I (hkil) /I0(hkil)
35
60
74
Coating thickness (micrometer)
30 25
Fig. 7. Relationship between coating thickness and icorr of the samples with different thicknesses and equal zinc bath chemical composition.
20 15 10 5 0
(00.2)
(20.1)
(10.0)
Texture components Fig. 5. Relative intensities of (00.2), (20.1) and (10.0) components of the sample with different thicknesses and equal zinc bath chemical composition.
current density of the samples is exhibited. As can be recognized, increasing the coating thickness gives rise to increase the corrosion current density of the coatings so that this increasing is completely vivid for thicknesses about 74 lm. It should be mentioned that the surface of hot-dip galvanized coating contains dull and bright spangles and these
H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
45
icorr (micro A/cm2 )
40
C
C1
C2
35 30 25 20 15 10 5 0 46
60
74
Coating thickness (micrometer) Fig. 8. Relationship between coating thickness and icorr of the samples with different thicknesses and equal zinc bath chemical composition.
spangles have different morphologies. Microscopic analysis was performed on both dull and bright spangle areas. In the dull area, three different surface morphologies were
63
found whereas the bright area showed almost uniform surface morphology. In addition, a feathery type of morphology was observed in the area where the dull and bright spangles were present alternatively. Hence, the five types of spangle patterns were observed in hot-dip galvanized coatings at high magnifications. Macroscopically, dullappearing areas are subdivided into three microscopic different forms. The dull spangles areas are: Dimpled: The dimpled structure is characterized by globular lead particles, which were segregated across the zinc surface. Fig. 9a shows the dimpled area. The lead element does not form the solute solution with zinc but exists in pure form at the surface and it is dense in the dull spangle areas. Ridged: Small hills and valleys characterize the ridged morphology. In this morphology, the lead particles were found at certain points. The amount of lead particles present in this morphology is less than that of in dimpled morphology (Fig. 9b).
Fig. 9. (a) Dimpled morphology, (b) ridged morphology, (c) orthogonal morphology, (d) shiny morphology and (e) feathery morphology.
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H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
(00.2) component
(20.1) component
(10.0) component
Corrosion current density
45
45
A
40
C
35
I (hkil) /I 0(hkil)
30
35 30
B
25
25
20
20
15
15
10
10
5
icorr(micro A/cm2)
40
5
0 0
0.02
0.04
0.06
0 0.08
Wt.% of Pb in zinc bath Fig. 10. Relationship between lead content of the zinc bath, texture components and icorr of the samples.
Orthogonal dendritic: The orthogonal dendrite spangles are characterized by the secondary dendritic arms morphology as illustrated in Fig. 9c. In this area, the lead particles were also found at some places. The bright spangles areas are: Shiny: The shiny regions display a smooth surface structure in comparison with the dull area. The surface morphology is indicated in Fig. 9d. The shiny area contains lead particles, but in much smaller amounts and sizes with respect to the dull area. Feathery: The feathery structure consists of a lath-like pattern with alternate shiny dendritic branches and dull areas (Fig. 9e). The above morphologies, observed in this work, are in good agreement with the morphologies obtained by other researchers [10–12]. 4. Discussion It can be seen in Fig. 1 that increasing the lead content of the zinc bath affects the intensity of the texture components. This is due to the fact that lead decreases the nucleation sites of basal planes and conversely, increases the nucleation sites of other planes such as low angle pyramids, high angle pyramids and prism planes [13]. Therefore, when the lead content of zinc bath is increased, less nuclei of basal planes form and consequently, less basal planes would be formed and lower area of the surface would be covered by these planes which are parallel with the surface [5,13,14]. In Fig. 10, it is evident that (00.2) basal texture component has a key role in determining the corrosion resistance of the coatings. When the relative intensity of (00.2) basal plane texture is high, it can be concluded that the surface is
mainly covered with basal planes which are parallel to sheet surface [5,13,14]. The basal planes have the highest binding energy of the surface atoms and thus the total energy involved in the breaking of the bonds and the subsequent dissolution of atoms is highest for these planes [15]. Also, surface becomes electrochemically less active because surface energy, that its value is inversely proportional to the atomic spacing (d) and directly proportional to the number of broken bonds per atom (a), is lowest for basal plane with respect to other planes in hcp structure (for example: 1/d = 0.41 and 0.89 for (00.2) and (20.1) planes, respectively) [5,16]. In (Figs. 5 and 6), it was observed that by increasing the coatings thickness, (00.2) basal texture component weakened and conversely, (20.1) and (10.0) texture components strengthened. These effects would be probably ascribed to the surface and strain energies. There are many models and theories that explain the changes of texture due to the variations of the coating thickness; however, the nature of these changes and relations is not completely clear yet. Among these models, the most general ones are usually used: surface energy minimization and strain energy minimization models. As it was mentioned before, texture is determined by a competition and balancing between surface and strain energies; i.e. at small thicknesses, the coating shows an orientation corresponding to that with lowest surface energy and highest strain energy [7–9]. In other words, when the coating is thin, the strain energy of the coating structure is very low and negligible and conversely, the surface energy is high and important because surface energy does not vary with the coating thickness [7,8]; therefore, the texture of the coating consists of planes which have the lowest surface energy and highest strain energy to com-
H. Asgari et al. / Current Applied Physics 9 (2009) 59–66
pensate the shortage of strain energy and make a balance between surface and strain energies [7–9]. Eta layer has about 98 wt% of zinc and in hcp structure of this element, (00.2) planes have the lowest surface energy and the highest strain energy [5]. Thus, when the coating thickness (and eta layer thickness) is small, basal planes would increase and become the predominant texture component of the coating and other planes, such as (20.1) and (10.0) with much higher surface energy and lower strain energy than basal planes would have a low intensity. In contrast, as the coating thickness increases, the strain energy would be increased and became much higher than surface energy. In this situation, texture of the coating tries to establish a balance between surface and strain energies by means of decreasing the strain energy and increasing the surface energy [7–9]; hence, the intensity of high strain energy (00.2) basal planes would be decreased whilst the intensity of (20.1) and (10.0) planes with much higher surface energy would be increased. In (Figs. 7 and 8), it was illustrated that increasing the coatings thickness was detrimental to the corrosion resistance of the coatings. As it was mentioned above, it can be attributed to the low corrosion resistance of (20.1) and (10.0) planes which strengthened in intensity as the coatings thickened [15]; consequently, the corrosion resistance of the thick coating (with thicker eta layer) would be decreased. Therefore, the outstanding result of this research is the fact that although increasing the thickness of hot-dip galvanized coatings results in the increasing of the coatings lifetime [1], this thickening would not lead to better corrosion resistance or texture of the coating and would have detrimental effect on these properties. Therefore, considering the relationship between texture, corrosion resistance and coating thickness in any application of these coatings is necessary. It is possible by more accurate studies to attain an optimized coating thickness in which the texture, corrosion resistance and microstructure of the coating are in the best situation. This optimized coating thickness is not only the best ones in terms of corrosion resistance, but also the most economical; thus, increasing the thickness of hot-dip galvanized coatings up to this optimized thickness for sensitive applications in which the quality and corrosion resistance considered as determining factor is not suggested. Different morphologies and segregated lead on the surfaces in Fig. 9 could be attributed to the fact that in zinc solidification, the fast growth crystallographic direction is [1 0 1 0] that drops in the basal plane [2]. When the dendrites grow parallel to the sheet surface, that means the basal plane of zinc is parallel to it, the solutes are rejected towards the grain boundary, leaving the surface relatively pure and bright [2,5,17]. However, when the basal plane of the grain is at angle with the substrate, the fast growth direction is towards the coating surface and the solutes are rejected towards it and cause the surface to be darkened. These facts explain the different segregation observed in bright and dull spangles [2,5,17]. In addition, segregation
65
of lead in the spangles increases as the lead content of the bath increases [2,5]. It is worthy of note here that to some extent, it was expected that with concentrations of aluminum up to 0.1 wt% aluminum in the zinc bath, (0.2 wt% in this study), Fe2Al5 layer would be formed and suppressed the formation of delta and eta phases [1,18]. However, microscopic investigations confirmed that four layers have been formed (Fig. 1) and EDS analysis detected no aluminum within the layers. It is probably due to the fact that for Fe2Al5 layer to be formed, in addition to the required amount of aluminum concentration in the bath [1,19], low temperature of the zinc bath [20] and rather high difference between the strip entry temperature and bath temperature [21] should be prepared. The low temperature of the bath can slow down the reaction rate of zinc–iron and provides a suitable condition for the reaction of aluminum and iron [22–24]. Also, as the temperature of the zinc bath decreases, the incubation period for Fe–Zn phases formation increases and their formation becomes more difficult [22–24]. Besides, the high temperature difference between the strip and galvanizing bath (e.g. 15 °C) can effectively increase Al–Fe reaction rate [19,20]. In the other words, the rate of Al uptake in zinc coatings to form Fe2Al5 depends on the difference between the temperatures of the zinc bath and strip entry temperature, that is, the Al content in the interface increases approximately linearly with the difference between the strip entry temperature and the bath temperature [19,20]. Note that iron in Fe2Al5Znx and Fe2Al5 belongs to the galvanizing bath and steel substrate, respectively. In this work, temperature of the zinc bath was enough high (462 °C) [20] and the difference between the bath temperature and strip entry temperature (466 °C) was small [21]; thus, the formation of Fe2Al5 was inhibited. 5. Conclusions 1. Increasing the lead content of the zinc bath results in the weakening of (00.2) basal texture component and strengthening of (20.1) high angle pyramidal and (10.0) prism texture components. 2. Coatings with strong (00.2) texture component and weak (20.1) and (10.0) texture components have higher corrosion resistance than the coatings with weak (00.2) texture component and strong (20.1) and (10.0) texture components. 3. As the coating thickness increases, the relative intensity of (00.2) basal planes decreases and conversely, the relative intensity of (20.1) and (10.0) planes increases; thus, corrosion resistance of the coating decreases. 4. Simonkolleite is the major corrosion product which would be formed on the surface of hot-dip galvanized coatings during the salt spray test. 5. Five types of morphology would be observed on the surface of hot-dip galvanized coatings in dull and bright spangles.
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6. Increasing the bath temperature (462 °C) and decreasing the difference between the bath temperature and strip entry temperature would inhibit the formation of Fe2Al5 inhibition layer. Acknowledgement The authors would like to thank Mobarakeh Steel Company laboratories staff and R&D unit engineers for helping with the samples. Assistance received from SEM and XRD laboratories staff of Materials Engineering Department of Isfahan University of Technology is also appreciated. References [1] A.R. Marder, The metallurgy of zinc coated steel, Prog. Mater. Sci. 45 (2000) 191–271. [2] M. Zapponi, A. Quiroga, T. Perez, Segregation of alloying elements during the hot-dip coating solidification process, Surf. Coat. Technol. 22 (1999) 18–20. [3] N. Katiforis, G. Papadimitriou, Influence of copper, cadmium and tin additions in the galvanizing bath on the structure, thickness and cracking behavior of the galvanized coatings, Surf. Coat. Technol. 78 (1999) 185–195. [4] D.I. Cameron, G.J. Harvey, J. Ormay, Aust. Inst. Metals 10 (1965) 225. [5] P.R. Sere, C.I. Culcasi, J.D. Elsner, A.I. Di Sarli, Relationship between texture and corrosion resistance in hot-dip galvanized steel sheets, Surf. Coat. Technol. 122 (1999) 143–149. [6] S. Chang, J.C. Shin, Corr. Sci. 36 (8) (1994) 1425. [7] J. Pelleg, L.Z. Zervin, S. Lungo, N. Croitoru, Thin Solid Films 197 (1991) 117. [8] U.C. OH, J.H. JE, J. Appl. Phys. 74 (1993) 1692. [9] L. Hultman, W. Munz, J. Musil, K. Kadlec, I. Petrov, J. Vac. Sci. Technol. A9 (1991) 434.
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