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Interpretation of the corrosion process of a galvannealed coating layer on dual-phase steel
Corrosion Science 53 (2011) 2430–2436
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Interpretation of the corrosion process of a galvannealed coating layer on dual-phase steel Heon-Young Ha a,⇑, Seong-Jun Park a, Jun-Yun Kang a, Hoon-Dong Kim b, Man-Been Moon b a b
Ferrous Alloys Group, Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, South Korea Hyundai HYSCO Dangjin Works 313 Donggok, Dangjin, Chungnam 343-831, South Korea
a r t i c l e
i n f o
Article history: Received 13 September 2010 Accepted 5 April 2011 Available online 9 April 2011 Keywords: A. Zinc A. Steel A. Acid solutions B. Polarisation C. Cathodic protection
a b s t r a c t The dissolution process of a galvannealed coating layer on dual-phase steel was examined by correlating a stripping test, metallographic observations and a polarisation test in an acidified chloride solution. The galvannealed coating layer was composed of several Fe–Zn intermetallic phases, namely the gamma, delta, and zeta phases, from the substrate. The dissolution began from the outermost zeta phase and proceeded to the gamma and then the delta phase. The dissolution rates for each intermetallic phase and galvanic couples were measured and estimated through a polarisation test, and the gamma phase in the gamma-substrate galvanic couple exhibited the highest corrosion rate. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction An increase in the need for weight reduction and crash safety in the automotive industry leads to a wide application of advanced high-strength steels, such as dual-phase (DP) steel [1–4]. DP steel consists of a soft-ferrite phase and a hard martensite phase, thus it exhibits several benefits essential to automotive materials, including ease of manufacturing, superior welding characteristics, and good coating properties [5–7]. For an anti-corrosion property of DP steel, a galvannealed (GA) coating is generally used [2,4,8–11]. By annealing Zn-galvanized coating at around 500 °C, fully alloyed GA coating containing Fe–Zn intermetallic phases is formed [12–17]. The GA coating effectively protects a substrate from corrosion, and it also improves weldability and paintability. Thus, demands for the GA steel have continuously increased in the engineering industries such as automobiles, household appliances, and constructions [10–13]. The GA coating layer is composed of several distinctive Fe–Zn intermetallic compounds, which are, in order from the substrate, the gamma (C) phase (Fe3Zn10), the gamma 1 (C1) phase (Fe5Z21), the delta (d) phase (FeZn7), and the zeta (f) phase (FeZn13) [12,18–22]. The final performance, including the corrosion resistance, of the GA coating strongly depends on the composition and structure of the Fe–Zn intermetallic layers [12,16,17]. The GA Zn coating protects a steel substrate from a corrosion attack by barrier protection (primary protection) and galvanic protection ⇑ Corresponding author. Tel.: +82 55 280 3422; fax: +82 55 280 3599. E-mail address: [email protected] (H.-Y. Ha). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.04.001
(secondary protection) [2,9,12,22–25]. The Zn coating offers barrier protection by separating the steel substrate from the corrosive environment. This anti-corrosion function is the main purpose of the Zn coating. In galvanic protection, the Zn coating is sacrificially corroded prior to the steel substrate because Zn is more active (or less noble) than the steel substrate in most corrosive conditions; this phenomenon occurs when the coating and substrate are exposed to a corrosive environment together in the form of cut edges or scratches in the coating. Once the GA coating layer is damaged in corrosive environments, the dissolution of the coating is accelerated, and a corrosion rate of the multi-layered coating is continuously changed [9,26–30]. The corrosion rate of the coating is determined by electrochemical properties of the respective intermetallic layers in the coating; therefore, characterisation of the electrochemical behaviour of multi-layered GA coating is necessary for an understanding of the corrosion process of the coating. Each intermetallic layer in the GA coating exhibits different electrochemical potential [9,27,28]; thus, the layers are usually characterised by measuring a variation in the corrosion potential (Ecorr) during the dissolution of the Zn coating, which is called a chemical stripping test (in the absence of any applied external current) or an electrochemical stripping method (in the presence of an applied current) [9,26–31]. However, the stripping test method has some limitations for quantitative analysis of the GA structure because it fails to provide adequate information about the dissolution rate or corrosion process of the coating [9,29–31]. As a means of clarifying the dissolution mechanism of the GA Zn coating, the stripping test method should be used in association with other analysis method. Therefore, the aim of the
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present work is to investigate the corrosion process of the GA layer on DP steel by using a combination of a chemical stripping test, metallographic observations of the dissolution process, and an analysis of polarisation behaviour. 2. Experimental procedure The alloy under investigation was a commercial GA DP steel. The substrate composition and variables of the GA process are presented in Table 1. A GA DP steel plate with a thickness of 1 mm was used for the electrochemical tests, and the reaction area was kept at 0.13 cm2 with an electroplating tape. The morphology of the GA layer that formed on DP steel was observed by means of a scanning electron microscope (SEM) and the chemical composition was investigated using energy dispersive X-ray spectroscopy (EDX). For cross-sectional observation, the GA DP steel sample was cold-mounted with an epoxy resin, and the cross-section was polished to 1 lm with a diamond suspension. The remnant of the aqueous polishing agent on the sample was rapidly removed. The sample was then dried after each preparation step to avoid corrosion because a GA coating on DP steel usually reacts actively with an aqueous solution [16,32]. Electrochemical experiments were conducted with a conventional three-electrode cell consisting of a planar Pt counter electrode, a saturated calomel reference electrode (SCE), and a specimen that served as a working electrode. A chemical stripping test without any external current input was conducted in a 1 M NaCl + 0.01 M H2SO4 solution at 20 °C, and a potential transition curve was recorded. The Cl ions in solution help to obtain a better resolution of the potential plateaus in the stripping test result [29]. At several important potential points, the dissolution morphologies were observed with a SEM. A comparison of the stripping test results and the results of the dissolution morphology observation helps to clarify the corrosion process of the GA coating layer. The dissolution rate of each intermetallic phase was examined in a potentiodynamic test, which was conducted under the same conditions as the respective dissolution steps during the stripping test. The potential sweep rate was 2 mV s 1, and all the electrochemical tests were conducted at least three times to ensure reproducibility. 3. Results and discussion 3.1. Metallographic characterisation of the GA coating layer Fig. 1 shows SEM micrographs of the surface of the as-received GA coating on DP steel. The surface morphology shown in Fig. 1(a) is not homogeneous but has a mixture of relatively bright and dark phases. The outermost layer of the GA structure is known to be the f phase [28], and the two distinctive regions are identified as a compressed (A) f phase and a crystallised (B) f phases. The compressed part was formed when a temper rolling process (or skin
Fig. 1. (a) SEM micrographs for the surface of as-received galvannealed dual phase steel. (b) A; compressed part, B; crystallised part with cracks indicated by arrows.
pass milling process) was used as a finishing step to impart the desired surface finish [24]; the crystallised part with cracks is the primary f phase [28,32]. These cracks indicated with arrows in Fig. 1(b) were caused by the thermal stress of the annealing step [12,16,28]. A cross-sectional view of the GA coated layer is presented in Fig. 2. The coating, which is 7–10 lm thick, consists of three layers: a C phase, a d phase, and a f phase (in order from the substrate) [12,18,19,33]. An extremely thin C layer is found between the substrate and the d phase. The columnar d phase has a thickness of around 4 lm, and the thickness is more uniform than that of the f phase. Many pores and cracks are associated with the d phase; they reportedly originate along the basal plane of the d phase [12,16,28], and some of them are extended to the C or f phases. The boundaries between the several Fe–Zn intermetallic phases are not clearly revealed but the phases are identified on the basis of their morphologies and chemical composition as determined by means of an energy dispersive X-ray spectroscopy (EDX). The Fe content of the outermost f layer was about 6–7 wt% (e.g., 6.54Fe–81.58Zn– 1.00Al, in wt%). In the middle d layer, Fe content was measured to be about 10–12 wt.% (e.g., 10.21Fe–84.30Zn–1.03Mn, in wt%). The
Table 1 (a) Chemical composition of galvannealed dual phase steel substrate and (b) variables for galvannealing process. (a) Dual phase steel composition (wt%) Fe
C
Si
Mn
P
S
Al
Bal.
0.075
0.170
1.750
0.018
0.001
0.560
(b) Variables for galvannealing process Sheet Thickness Zn bath composition Zn bath temperature Galvannealing temperature Temper rolling reduction ratio Coating weight
1 mm Zn–0.14 wt%Al 465 °C 510 °C 1.0% 95 g cm 2
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Fig. 2. Cross-sectional view of the galvannealed coating layer on dual phase steel.
highest Fe content was detected at the C layer adjacent to the steel substrate, which was about 20 wt%. (e.g., 19.29Fe–70.54Zn–1.06Mn in wt%). Based on the EDX result, it is able to distinguish the f phase, d phase and C phase by their Fe content [12,16,18,20,33].
3.2. Electrochemical characterisation of the GA coating layer: the chemical stripping test The GA Zn-coated DP steel was immersed in a 1 M NaCl + 0.01 M H2SO4 solution at 20 °C without any external current input, and a change in the Ecorr was recorded (Fig. 3) during the dissolution of the GA coating. The obtained potential transition curve (chronopotentiogram, Ecorr vs. time) shows a characteristic shape with good reproducibility [28,30]. Once the specimen was immersed in the solution, the open circuit potential was located at 0.907 VSCE and then lowered to about 0.968 VSCE. After immersion for 500 s, a stabilised potential region appeared; it was named potential plateau A. At about 4500 s, the Ecorr value began to increase until around 8000 s; it passed through plateau B at about 0.89 VSCE, plateau C at 0.806 VSCE, and plateau D at 0.633 VSCE. The four plateau potentials are known to correspond with the Ecorr values of the respective Fe–Zn intermetallic layers in the GA coating: A: the f phase; B: the d phase; C: the C phase; and D: the substrate [9,28–30]. The presence of the three intermetallic phases (f, d, and C) in the GA coating shown in the cross-sectional view (Fig. 2) was confirmed again by the stripping test results (Fig. 3). However, the relative duration of the plateaus was not equivalent to the relative thickness of the phases. Even though the two Fe–Zn intermetallic phases (the d and f phases have almost the same thickness, the duration of plateau B, which corresponds to the d phase, is significantly shorter than that of plateau A, which corresponds to the f phase. This difference has been reported by several authors [9,26,30,31]. They pointed out that the respective layers in the GA coating could not be quantified through a chemical or
Fig. 3. Potential transition curve with time obtained from chemical stripping test in 1 M NaCl + 0.01 M H2SO4 solution.
electrochemical stripping test because the corrosion rate and current efficiency of the dissolution could not be determined [31,34]. The reactive area constantly increased because of the development of cracks during the dissolution, and the corrosion of the single phase and galvanic couples occurred at the same time; hence, the corrosion rate could not be defined. Furthermore, because the metal dissolution proceeded concurrently with a H2 evolution reaction and the formation of corroded products such as Zn–Cl compounds, Zn-hydroxides, and Fe particles [2,35–37], the current efficiency was also difficult to determine. These problems highlight the inadequacy of relying solely on a stripping test to make an electrochemical interpretation of the dissolution process of a GA coating. A better understanding can be attained when the stripping test results are correlated with metallographic observations and the results of other electrochemical investigations. 3.3. Correlation of the chronopotentiogram and observations of the dissolution morphology The dissolution morphologies at several corrosion stages are marked with arrows at points (a–h) in Fig. 3; they were observed with a SEM and the respective micrographs are shown in Fig. 4. At point (a) in Fig. 3, the corrosion started at the compressed part in the f phase. Fig. 4(a-1) clearly shows that the selectively dissolved surface of the compressed f phase leaves tangled initiative cracks within the hollows (Fig. 4(a-2)). The higher susceptibility to corrosion of the compressed f part is thought to be due to the change in crystallographic orientations and the residual stress from the temper rolling process [24]. The potential dropping within 500 s of immersion is reportedly attributed to the fact that the surface area is increased by the developed cracks [28], which are evident in Fig. 4(b-1). At the beginning of plateau A (point (b) in Fig. 3), both the crystallised and compressed f parts were dissolved simultaneously and numerous cracks were propagated into the crystallised part. As shown in Fig. 4(b-1), some of the cracks intruded into the compressed part with tangled initiative cracks. The cracks that cover the entire surface act as a pathway that a solution can penetrate; and, if that happens, the multiple Fe–Zn intermetallic phases and the substrate are exposed to the solution simultaneously [26]. In addition, the compressed f part was dissolved in a spongy-like morphology (indicated by an arrow in Fig. 4(b-2)), whereas the crystallised f part remained in its crystal shape during dissolution. The mid-point of plateau A (point (c) in Fig. 3) shows a uniform dissolution of the f phase, expanded cracks, and some detachments of the f phase. Where the flakes of the f phase fell off, the d phase was partly exposed to the solution, as indicated by the arrows in Fig. 4(c-1) and 4(c-2). The exposed d phase with fine cracks was more compact than the f phase, as confirmed in Fig. 4(c-2). At the end of plateau A (point (d) in Fig. 3), the f phase was completely dissolved, the d phase was fully exposed to the solution (Fig. 4(d-1)), and the crevices in the d phase were much more enlarged than the cracks in the d phase shown in Fig. 4(c-2). In addition, Fig. 4(d-2) clearly shows the difference in aspects of crack propagation between the compressed and crystallised (undamaged) f parts. The relatively straight cracks that formed in the crystallised f part could not penetrate the area of the compressed f part with the tangled cracks (which is indicated by an arrow in the Fig. 4(d-2)). In contrast to the d phase fragments that were divided by straight cracks, the flakes of the d phase in the tangled crack region were electrically connected to each other. When the d and C phases and substrate corrode together, the less noble phase is preferentially dissolved. In that situation, any electrically shorted d and C islands are dissolved faster than the relatively noble continuous phases because of the establishment of a condition involving a ‘small anode-large cathode’ (i.e., d fragment-C
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Fig. 4. Dissolution morphologies at respective potential steps, (a–h) of chronopotentiogram in the Fig. 3.
basement). Thus, the tangled crack form is preferable to straight cracks, which can divide the continuous phase into electrically isolated fragments. During the dissolution, as shown in Fig. 3, the potential was smoothly elevated to plateau B. The surface morphology observed at the end point of plateau B (point (e) in Fig. 3) is presented in Fig. 4(e-1) and (e-2). The cleavages were further expanded and the d phase became thinner during immersion up to point (e) as seen in Fig. 4(e-2). In addition, the C phase underneath the d phase was disclosed when some of the fragments of the d phase fell off. Although the d phase was not completely dissolved until the end of plateau B, which corresponds to the d phase, the potential was suddenly raised up to plateau C. The C phase adjacent to the substrate began to corrode at plateau C but the d phase remained intact. When the cracks in the d phase were sufficiently developed, the corrosive solution directly attacked the C phase and the substrate; and the C phase dissolved more quickly than the d phase
in plateau C, the stable potential period. The corrosion of the C phase in plateau C is confirmed by the fact that the C phase beneath the d phase (Fig. 4(f-1) and (f-2)) is less compact than that shown in Fig. 4(e-2). When the dissolution of the C phase was completed, fragments of the d phase were separated and, as a result, the bare substrate was exposed. Fig. 4(g-1) and (g-2), the micrographs observed at the beginning of plateau D (point (g) in Fig. 3), clearly show the separated d fragments where the C phase beneath the d phase was completely dissolved. Finally, in plateau D (point (h) in Fig. 3), which is a stable potential region, the substrate is entirely disclosed without any residual fragments of the Fe–Zn intermetallic phases; the intergranular corrosion of the DP steel substrate can also be observed [9]. An inspection of the dissolution morphology during the stripping test revealed several significant findings. First, there is no exact correspondence between each potential plateau and each Fe–Zn intermetallic phase [9,31,34,38]. Secondly, the dissolution
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Fig. 4 (continued)
does not sequentially occur in accordance with the amount of Fe content in the Fe–Zn intermetallic layers. According to Lee et al., [27] the polarisation resistance value of the Fe–Zn intermetallic phase increased with increase in Fe content, but the multi-layered GA coating did not dissolve in order of the polarisation resistance [30]. Lastly, the dissolution process does not obey Faraday’s rule because of the detachment of the f or d phase fragments [26]. These findings confirm that the stripping method alone is incapable of quantifying each intermetallic layer in the GA coating. Fig. 5(a) combines and illustrates the potential transition curve (as obtained from the stripping test (Fig. 3)) and the observation results of the dissolution process of the respective Fe–Zn intermetallic layers. The potential transition curve is divided into three regions for the sequential dissolution of the f, d, and C phases; the three regions are depicted in grey colours in Fig. 5(a). Each Fe–Zn intermetallic phase begins to dissolve from the starting points of the respective grey zones. A mixed zone (relatively dark grey zone) occurs whenever two or three adjacent phases
(including the substrate) are simultaneously exposed to the solution, causing galvanic corrosion [25,27,30,39]. 3.4. Correlation of the chronopotentiogram and potentiodynamic responses The dissolution sequence can be quantitatively explained on the basis of the correlation among the stripping test results, observations of the dissolution morphology, and analysis of the polarisation behaviour of each Fe–Zn intermetallic layer in the GA coating. The polarisation responses of the five potential points indicated by arrows in the stripping test results were measured under the same conditions (i.e., in a 1 M NaCl + 0.01 M H2SO4 solution at 20 °C). The results of the polarisation responses are presented in Fig. 5(b). The polarisation responses to the measured potential points of 1 and 5 in Fig. 5(a) represent those to the cracked f phase and the DP steel substrate, respectively (black lines in Fig. 5(b)). The f phase
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Fig. 5. Comparison of the stripping test result with the dissolution process and anodic polarisation test results. (a) Potential transition curves (Fig. 3) associated with dissolution process, (b) polarisation responses at respective potential points indicated by arrows in the (a and c) Tafel plots for the polarisation curves in the (b). The stripping and polarisation test were conducted in 1 M NaCl + 0.01 M H2SO4 solution.
and DP steel substrate do not show passive behaviour but actively dissolve in the solution in Fig. 5(b). The f phase has an Ecorr value of 0.991 VSCE. Particularly, 1.087 VSCE at which a current wave occurs is not the Ecorr because the value of the measured current density is not changed from negative (cathodic) to positive (anodic) at the potential. This current peak at 1.087 VSCE below Ecorr indicates a presence of a temporarily formed barrier against to corrosion deposited on the f phase surface during dissolution, which is expected to be zinc-chloride hydroxide (ZnCl24Zn(OH)2 [35,39–41] or zinc hydroxysulfate (Zn4SO4(OH)64H2O and Zn4Cl2 (OH)4SO45H2O [37,42,43]) The Ecorr value for the DP steel substrate is 0.651 VSCE, which much higher than the Ecorr of the f phase [38,39]. All the electrochemical dissolution reactions of the Fe–Zn alloy in the GA coating occurred in the potential range 0.991 VSCE to 0.651 VSCE, and this localised potential difference accelerates the galvanic corrosion of GA Zn coating [30,38,39]. The other three polarisation curves measured at the end points of plateaus A, B and C (arrows 2, 3 and 4, respectively, in Fig. 5(a)) correspond to those of the f–d, d–C, and C-substrate galvanic couples (dashed lines in Fig. 5(b)). The values of Ecorr and icorr of a single phase (f and substrate each) or a galvanic coupled phase (f–d, d–C, or C-substrate) obtained from Fig. 5(b) are listed in Table 2. Individual polarisation curves of the d and C phases could not be measured separately because the d and C phases and the substrate were electrically connected in the solution due to the existence of cracks. A comparison of Fig. 5(a and b) confirms that the beginning point of each potential plateau is associated with the Ecorr value for each intermetallic phase and that the end potential point of each plateau corresponds to the galvanic coupled potential (mixed potential) of the two phases in contact. The potentiodynamic curves of the f phase and substrate were used to extract Tafel plots (which are represented by solid lines Table 2 Corrosion rates (icorr) and corrosion potentials (Ecorr) for individual Fe–Zn intermetallic phase and galvanic couples measured by potentiodynamic tests. Each phase and galvanic couples Measured f f d d d C
C C-substrate Substrate
icorr/mA cm
2
Ecorr/VSCE
Estimated 0.565 1.641 0.619 2.089 0.608 3.133 0.479
0.991 0.952 0.909 0.875 0.814 0.782 0.651
in Fig. 5(c)). In addition, Tafel plots for the d and C single phases were estimated from the measured five polarisation curves (which are represented by dashed lines in Fig. 5(b)). Under this condition, the cathodic Tafel slope (bc) was measured to be 0.122 V per decade for the DP substrate and 0.171 V per decade for the f phase. The Tafel slope values for a single d and a single C phase were applied in accordance with the value for the f phase. The icorr values for the single phases of d and C were estimated on the basis of Fig. 5(c); the results are presented in Table 2. The dissolution rate of each Fe–Zn intermetallic phase is in the range of 0.57–0.62 mA cm 2; the icorr value of the DP steel substrate (0.48 mA cm 2) is slightly lower than that [27]. However, as shown in Table 2, when the two adjacent intermetallic phases are galvanically coupled in a solution as a result of the crack formation, the icorr value for the couple underwent a threefold to fivefold increase. In the mixed potential zone between each potential plateau (the relatively dark grey region in Fig. 5(a)), the less noble phase containing a lower Fe content in the galvanic couple (e.g., the f phase in the f–d couple) is expected to corrode quickly. In this way, as shown in Fig. 5(c), the icorr value of the multi-layered GA coating should change theoretically in a zigzag shape from the icorr of the f phase to the icorr value of the substrate. However, the dissolution rate for the less noble phase in the galvanic couple becomes much faster than the single phase dissolution; thus, the overall icorr value are observed to change along the large black circles in Fig. 5(c), as confirmed by the practical polarisation curves in Fig. 5(b). The corrosion of the GA coating was accelerated from the point where the dissolution of the f phase was completed and the corrosion of the galvanic couple of the d–C-substrate started. Hence, the f phase is thought to be the most important phase for the barrier protection function, and it should be kept uniform and thick to retard the acceleration of the dissolution. In addition, the fact that the dissolution rate of the C phase is higher than that of the d phase is explained in Fig. 5(c). As the cracks widen as a result of the dissolution until the end of plateau B (arrow 3 in Fig. 5(a) and the micrographs in Fig. 4(e)), the d and C phases as well as the substrate are exposed to the corrosive environment together. From this point, the C phase begins to dissolve, the d phase still remains, and the potential suddenly increases to plateau C. The mixed icorr value for the C-substrate couple is 3.133 mA cm 2, which appears to be higher than that of the d–C couple (2.089 mA cm 2). For this reason, when the d and C phases and the substrate are exposed to the solution at the same time, the C phase dissolves before the d phase. This phenomenon is attributed to the fact that the C phase and substrate have a larger potential difference than the d–C galvanic couple.
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4. Conclusion This analysis of the dissolution process of a GA coating on DP steel is based on a correlation of the results of a chemical stripping test, observations of the dissolution morphology, and polarisation tests in an acidified chloride solution (1 M NaCl + 0.01 M H2SO4). The dissolution process of the GA coating in the 1 M NaCl + 0.01 M H2SO4 solution was observed in relation to morphological changes during a stripping test. The results show that the corrosion sequence of the Fe–Zn intermetallic phase failed to correspond with the Fe content in the respective intermetallic phases. The corrosion sequence was as follows: the dissolution of the compressed f phase ? crack initiation and propagation in the f phase ? the f phase dissolution and detachment ? the d phase dissolution and crack development ? the prior dissolution of the C phase to the d phase ? the d phase detachment ? the disclosure of the substrate (intergranular corrosion). In addition, the variation in the corrosion rate during the stripping test was measured and estimated by means of polarisation tests at the important dissolution stages. The corrosion rate was accelerated when the outermost f phase was completely dissolved and the galvanic couple of the d–C-substrate was exposed to the solution. The C phase dissolved before the d phase because the corrosion potential difference between the C phase and the substrate was larger than that of the d–C galvanic couple. Thus, the f phase should be kept uniform and thick to protect the substrate from corrosion. References [1] D.K. Matlock, J.G. Speer, Processing opportunities for new advanced highstrength sheet steels, Mater. Manuf. Processes 25 (2010) 7–13. [2] A. Amirudin, D. Thierry, Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings, Prog. Org. Coat. 28 (1996) 59–76. [3] S.H. Han, S.H. Choi, J.K. Choi, H.G. Seong, I.B. Kim, Effect of hot-rolling processing on texture and r-value of annealed dual-phase steels, Mater. Sci. Eng. A-Struct. 527 (2010) 1686–1694. [4] E. De Moor, P.J. Gibbs, J.G. Speer, D.K. Matlock, J.G. Schroth, Strategies for thirdgeneration advanced high-strength steel development, Iron Steel Technol. 7 (2010) 133–144. [5] D.T. Llewellyn, D.J. Hillis, Dual phase steels, Ironmak. Steelmak. 23 (1996) 471– 478. [6] R.G. Davies, Influence of martensite composition and content on the properties of dual phase steels, Metall. Mater. Trans. A 9 (1978) 671–679. [7] Y.I. Son, Y.K. Lee, K.-T. Park, C.S. Lee, D.H. Shin, Ultrafine grained ferrite– martensite dual phase steels fabricated via equal channel angular pressing: microstructure and tensile properties, Acta Mater. 53 (2005) 3125–3134. [8] F.E. Goodwin, E.A. Silva, An overview of North American zinc-based sheet steel coatings production: status and opportunities, in: Proceedings of the Galvatech ‘07, International Conference on Zinc and Zinc Alloy Coated Steel, November 18–22, 2007, Osaka, Japan, pp. 6–13. [9] X.G. Zhang, I.C. Bravo, Electrochemical stripping of galvannealed coatings on steel, Corrosion 50 (1994) 308–317. [10] J. Bian, Y. Zhu, X.-h Liu, G.-d. Wang, Development of hot dip galvanized steel strip and its application in automobile industry, J. Iron. Steel Res. Int. 13 (2006) 47–50. [11] S. Fujita, D. Mizuno, Corrosion and corrosion test methods of zinc coated steel sheets on automobiles, Corros. Sci. 49 (2007) 211–219. [12] A.R. Marder, The metallurgy of zinc-coated steel, Prog. Mater. Sci. 45 (2000) 191–271. [13] S. Feliu Jr., V. Barranco, XPS study of the surface chemistry of conventional hotdip galvanised pure Zn, galvanneal and Zn–Al alloy coatings on steel, Acta Mater. 51 (2003) 5413–5424. [14] C.S. Lin, M. Meshii, C.C. Cheng, Phase evolution in galvanneal coatings on steel sheets, ISIJ Int. 35 (1995) 503–511. [15] S. FeliuJr, M.L. Perez-Revenga, Correlation between the surface chemistry of annealed if steels and the growth of a galvanneal coating, Acta Mater. 53 (2005) 2857–2866.
[16] C.E. Jordan, K.M. Goggins, A.R. Marder, Interfacial layer development in hot-dip galvanneal coatings on interstitial free (IF) steel, Metall. Mater. Trans. A 25 (1994) 2101–2109. [17] N.-Y. Tang, X.B. Yu, F.N. Coady, Communications: phase transition in Galvanneal coatings, Metall. Mater. Trans. A 34 (2003) 879–881. [18] S. Dionne, The characterization of continuous hot-dip galvanized and galvannealed steels, JOM 58 (2006) 32–40. [19] K.-K. Lee, I.-H. Lee, C.-R. Lee, H.-K. Ahn, In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn–Fe coating layers, Surf. Coat. Technol. 201 (2007) 6261–6266. [20] J.M. Long, D.A. Haynes, P.D. Hodgson, Characterisation of galvanneal coatings on strip steel, Mater. Forum 27 (2004) 62–67. [21] F.H. Kao, W.C. Li, C.Y. Chen, C.Y. Huang, J.R. Yang, S.H. Wang, Cross-sectional observation of the intermetallic phase in a galvannealed steel, Mater. Sci. Eng. A-Struct. 499 (2009) 45–48. [22] T.K. Rout, N. Bandyopadhyay, T. Benugopalan, D. Bhattacharjee, Mechanistic interpretation of electrochemical behaviour of galvannealing coating in saline environment, Corros. Sci. 47 (2005) 2841–2854. [23] X.G. Zhang, Corrosion performance of zinc-coated steels - mechanism, life prediction and corrosivity mapping, in: Proceedings of the Galvatech ‘07, International Conference on Zinc and Zinc Alloy Coated Steel, November 18– 22, 2007, Osaka, Japan, pp. 694–698. [24] H. Asgari, M.R. Toroghinejad, M.A. Golozar, On texture, corrosion resistance and morphology of hot-dip galvanized zinc coatings, Appl. Surf. Sci. 253 (2007) 6769–6777. [25] A.P. Yadav, H. Katayama, K. Noda, H. Masuda, A. Nishikata, T. Tsuru, Surface potential distribution over a zinc/steel galvanic couple corroding under thin layer of electrolyte, Electrochim. Acta 52 (2007) 3121–3129. [26] A. Besseyrias, F. Dalard, J.J. Rameau, H. Baudin, A study of galvanic corrosion during coulometric dissolution of galvannealed steel, Corros. Sci. 37 (1995) 587–595. [27] H.H. Lee, D. Hiam, Corrosion resistance of galvannealed steel, Corrosion 45 (1989) 852–856. [28] F.M. Queiroz, I. Costa, Electrochemical, chemical and morphological characterization of galvannealed steel coating, Surf. Coat. Technol. 201 (2007) 7024–7035. [29] C. Xhoffer, H. Dillen, B.C. De Cooman, A. Hubin, Quantitative phase analysis of galvannealed coatings by coulometric stripping, J. Appl. Electrochem. 29 (1999) 209–219. [30] A.P. Yadav, H. Katayama, K. Noda, H. Masuda, A. Nishikata, T. Tsuru, Effect of Fe–Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments, Corros. Sci. 49 (2007) 3716–3731. [31] T.M.C. Nogueira, U.R. Seixas, P.R. Rios, Application of voltammetric stripping to a galvannealed coating on an interstitial free steel sheet, ISIJ Int. 38 (1998) 775–777. [32] C.E. Jordan, K.M. Goggins, A.O. Benscoter, A.R. Marder, Metallographic preparation technique for hot-dip galvanized and galvannealed coatings on steel, Mater. Charact. 31 (1993) 107–114. [33] A.T. Alpas, J. Inagaki, Effect of microstructure on fracture mechanisms in galvannealed coatings, ISIJ Int. 40 (2000) 172–181. [34] M. Arai, T. Takayama, Y. Matano, T. Usuki, Analysis of phase structure in coatings of galvannealed steel sheets, CAMP-ISIJ 5 (1992) 662. [35] I. Suzuki, The behavior of corrosion products on zinc in sodium chloride solution, Corros. Sci. 25 (1985) 1029–1034. [36] E. Tada, H. Kaneko, Optical visualization of concentration field of Zn2+ during galvanic corrosion of a Zn/steel couple, Corros. Sci. 52 (2010) 3421–3427. [37] Y.Y. Chen, S.C. Chung, H.C. Shih, Studies on the initial stages of zinc atmospheric corrosion in the presence of chloride, Corros. Sci. 48 (2006) 3547–3564. [38] A. Besseyrias, F. Dalard, J.J. Rameau, H. Baudin, Electrochemical behaviour of zinc–iron intermetallic compounds in an aqueous solution containing NaCl and ZnSO4, Corros. Sci. 39 (1997) 1883–1896. [39] D.-L. Zhang, W. Wang, Y. Li, An electrode array study of electrochemical inhomogeneity of zinc in zinc/steel couple during galvanic corrosion, Corros. Sci. 52 (2010) 1277–1284. [40] P. Volovitch, C. Allely, K. Ogle, Understanding corrosion via corrosion product characterization: I. Case study of the role of Mg alloying in Zn–Mg coating on steel, Corros. Sci. 51 (2009) 1251–1262. [41] R.P. Edavan, R. Kopinski, Corrosion resistance of painted zinc alloy coated steels, Corros. Sci. 51 (2009) 2429–2442. [42] I. Odnevall, M. Westdahl, Zinc chlorohydroxosulfates: Newly-discovered corrosion products on zinc. Structure determination of NaZn4Cl(OH)6SO46H2O and X-ray study of Zn4Cl2(OH)4SO45H2O, Corros. Sci. 34 (1993) 1231–1242. [43] D. de la Fuente, J.G. Castano, M. Morcillo, Long-term atmospheric corrosion of zinc, Corros. Sci. 49 (2007) 1420–1436.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Interpretation of the corrosion process of a galvannealed coating layer on dual-phase steel Heon-Young Ha a,⇑, Seong-Jun Park a, Jun-Yun Kang a, Hoon-Dong Kim b, Man-Been Moon b a b
Ferrous Alloys Group, Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, South Korea Hyundai HYSCO Dangjin Works 313 Donggok, Dangjin, Chungnam 343-831, South Korea
a r t i c l e
i n f o
Article history: Received 13 September 2010 Accepted 5 April 2011 Available online 9 April 2011 Keywords: A. Zinc A. Steel A. Acid solutions B. Polarisation C. Cathodic protection
a b s t r a c t The dissolution process of a galvannealed coating layer on dual-phase steel was examined by correlating a stripping test, metallographic observations and a polarisation test in an acidified chloride solution. The galvannealed coating layer was composed of several Fe–Zn intermetallic phases, namely the gamma, delta, and zeta phases, from the substrate. The dissolution began from the outermost zeta phase and proceeded to the gamma and then the delta phase. The dissolution rates for each intermetallic phase and galvanic couples were measured and estimated through a polarisation test, and the gamma phase in the gamma-substrate galvanic couple exhibited the highest corrosion rate. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction An increase in the need for weight reduction and crash safety in the automotive industry leads to a wide application of advanced high-strength steels, such as dual-phase (DP) steel [1–4]. DP steel consists of a soft-ferrite phase and a hard martensite phase, thus it exhibits several benefits essential to automotive materials, including ease of manufacturing, superior welding characteristics, and good coating properties [5–7]. For an anti-corrosion property of DP steel, a galvannealed (GA) coating is generally used [2,4,8–11]. By annealing Zn-galvanized coating at around 500 °C, fully alloyed GA coating containing Fe–Zn intermetallic phases is formed [12–17]. The GA coating effectively protects a substrate from corrosion, and it also improves weldability and paintability. Thus, demands for the GA steel have continuously increased in the engineering industries such as automobiles, household appliances, and constructions [10–13]. The GA coating layer is composed of several distinctive Fe–Zn intermetallic compounds, which are, in order from the substrate, the gamma (C) phase (Fe3Zn10), the gamma 1 (C1) phase (Fe5Z21), the delta (d) phase (FeZn7), and the zeta (f) phase (FeZn13) [12,18–22]. The final performance, including the corrosion resistance, of the GA coating strongly depends on the composition and structure of the Fe–Zn intermetallic layers [12,16,17]. The GA Zn coating protects a steel substrate from a corrosion attack by barrier protection (primary protection) and galvanic protection ⇑ Corresponding author. Tel.: +82 55 280 3422; fax: +82 55 280 3599. E-mail address: [email protected] (H.-Y. Ha). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.04.001
(secondary protection) [2,9,12,22–25]. The Zn coating offers barrier protection by separating the steel substrate from the corrosive environment. This anti-corrosion function is the main purpose of the Zn coating. In galvanic protection, the Zn coating is sacrificially corroded prior to the steel substrate because Zn is more active (or less noble) than the steel substrate in most corrosive conditions; this phenomenon occurs when the coating and substrate are exposed to a corrosive environment together in the form of cut edges or scratches in the coating. Once the GA coating layer is damaged in corrosive environments, the dissolution of the coating is accelerated, and a corrosion rate of the multi-layered coating is continuously changed [9,26–30]. The corrosion rate of the coating is determined by electrochemical properties of the respective intermetallic layers in the coating; therefore, characterisation of the electrochemical behaviour of multi-layered GA coating is necessary for an understanding of the corrosion process of the coating. Each intermetallic layer in the GA coating exhibits different electrochemical potential [9,27,28]; thus, the layers are usually characterised by measuring a variation in the corrosion potential (Ecorr) during the dissolution of the Zn coating, which is called a chemical stripping test (in the absence of any applied external current) or an electrochemical stripping method (in the presence of an applied current) [9,26–31]. However, the stripping test method has some limitations for quantitative analysis of the GA structure because it fails to provide adequate information about the dissolution rate or corrosion process of the coating [9,29–31]. As a means of clarifying the dissolution mechanism of the GA Zn coating, the stripping test method should be used in association with other analysis method. Therefore, the aim of the
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present work is to investigate the corrosion process of the GA layer on DP steel by using a combination of a chemical stripping test, metallographic observations of the dissolution process, and an analysis of polarisation behaviour. 2. Experimental procedure The alloy under investigation was a commercial GA DP steel. The substrate composition and variables of the GA process are presented in Table 1. A GA DP steel plate with a thickness of 1 mm was used for the electrochemical tests, and the reaction area was kept at 0.13 cm2 with an electroplating tape. The morphology of the GA layer that formed on DP steel was observed by means of a scanning electron microscope (SEM) and the chemical composition was investigated using energy dispersive X-ray spectroscopy (EDX). For cross-sectional observation, the GA DP steel sample was cold-mounted with an epoxy resin, and the cross-section was polished to 1 lm with a diamond suspension. The remnant of the aqueous polishing agent on the sample was rapidly removed. The sample was then dried after each preparation step to avoid corrosion because a GA coating on DP steel usually reacts actively with an aqueous solution [16,32]. Electrochemical experiments were conducted with a conventional three-electrode cell consisting of a planar Pt counter electrode, a saturated calomel reference electrode (SCE), and a specimen that served as a working electrode. A chemical stripping test without any external current input was conducted in a 1 M NaCl + 0.01 M H2SO4 solution at 20 °C, and a potential transition curve was recorded. The Cl ions in solution help to obtain a better resolution of the potential plateaus in the stripping test result [29]. At several important potential points, the dissolution morphologies were observed with a SEM. A comparison of the stripping test results and the results of the dissolution morphology observation helps to clarify the corrosion process of the GA coating layer. The dissolution rate of each intermetallic phase was examined in a potentiodynamic test, which was conducted under the same conditions as the respective dissolution steps during the stripping test. The potential sweep rate was 2 mV s 1, and all the electrochemical tests were conducted at least three times to ensure reproducibility. 3. Results and discussion 3.1. Metallographic characterisation of the GA coating layer Fig. 1 shows SEM micrographs of the surface of the as-received GA coating on DP steel. The surface morphology shown in Fig. 1(a) is not homogeneous but has a mixture of relatively bright and dark phases. The outermost layer of the GA structure is known to be the f phase [28], and the two distinctive regions are identified as a compressed (A) f phase and a crystallised (B) f phases. The compressed part was formed when a temper rolling process (or skin
Fig. 1. (a) SEM micrographs for the surface of as-received galvannealed dual phase steel. (b) A; compressed part, B; crystallised part with cracks indicated by arrows.
pass milling process) was used as a finishing step to impart the desired surface finish [24]; the crystallised part with cracks is the primary f phase [28,32]. These cracks indicated with arrows in Fig. 1(b) were caused by the thermal stress of the annealing step [12,16,28]. A cross-sectional view of the GA coated layer is presented in Fig. 2. The coating, which is 7–10 lm thick, consists of three layers: a C phase, a d phase, and a f phase (in order from the substrate) [12,18,19,33]. An extremely thin C layer is found between the substrate and the d phase. The columnar d phase has a thickness of around 4 lm, and the thickness is more uniform than that of the f phase. Many pores and cracks are associated with the d phase; they reportedly originate along the basal plane of the d phase [12,16,28], and some of them are extended to the C or f phases. The boundaries between the several Fe–Zn intermetallic phases are not clearly revealed but the phases are identified on the basis of their morphologies and chemical composition as determined by means of an energy dispersive X-ray spectroscopy (EDX). The Fe content of the outermost f layer was about 6–7 wt% (e.g., 6.54Fe–81.58Zn– 1.00Al, in wt%). In the middle d layer, Fe content was measured to be about 10–12 wt.% (e.g., 10.21Fe–84.30Zn–1.03Mn, in wt%). The
Table 1 (a) Chemical composition of galvannealed dual phase steel substrate and (b) variables for galvannealing process. (a) Dual phase steel composition (wt%) Fe
C
Si
Mn
P
S
Al
Bal.
0.075
0.170
1.750
0.018
0.001
0.560
(b) Variables for galvannealing process Sheet Thickness Zn bath composition Zn bath temperature Galvannealing temperature Temper rolling reduction ratio Coating weight
1 mm Zn–0.14 wt%Al 465 °C 510 °C 1.0% 95 g cm 2
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Fig. 2. Cross-sectional view of the galvannealed coating layer on dual phase steel.
highest Fe content was detected at the C layer adjacent to the steel substrate, which was about 20 wt%. (e.g., 19.29Fe–70.54Zn–1.06Mn in wt%). Based on the EDX result, it is able to distinguish the f phase, d phase and C phase by their Fe content [12,16,18,20,33].
3.2. Electrochemical characterisation of the GA coating layer: the chemical stripping test The GA Zn-coated DP steel was immersed in a 1 M NaCl + 0.01 M H2SO4 solution at 20 °C without any external current input, and a change in the Ecorr was recorded (Fig. 3) during the dissolution of the GA coating. The obtained potential transition curve (chronopotentiogram, Ecorr vs. time) shows a characteristic shape with good reproducibility [28,30]. Once the specimen was immersed in the solution, the open circuit potential was located at 0.907 VSCE and then lowered to about 0.968 VSCE. After immersion for 500 s, a stabilised potential region appeared; it was named potential plateau A. At about 4500 s, the Ecorr value began to increase until around 8000 s; it passed through plateau B at about 0.89 VSCE, plateau C at 0.806 VSCE, and plateau D at 0.633 VSCE. The four plateau potentials are known to correspond with the Ecorr values of the respective Fe–Zn intermetallic layers in the GA coating: A: the f phase; B: the d phase; C: the C phase; and D: the substrate [9,28–30]. The presence of the three intermetallic phases (f, d, and C) in the GA coating shown in the cross-sectional view (Fig. 2) was confirmed again by the stripping test results (Fig. 3). However, the relative duration of the plateaus was not equivalent to the relative thickness of the phases. Even though the two Fe–Zn intermetallic phases (the d and f phases have almost the same thickness, the duration of plateau B, which corresponds to the d phase, is significantly shorter than that of plateau A, which corresponds to the f phase. This difference has been reported by several authors [9,26,30,31]. They pointed out that the respective layers in the GA coating could not be quantified through a chemical or
Fig. 3. Potential transition curve with time obtained from chemical stripping test in 1 M NaCl + 0.01 M H2SO4 solution.
electrochemical stripping test because the corrosion rate and current efficiency of the dissolution could not be determined [31,34]. The reactive area constantly increased because of the development of cracks during the dissolution, and the corrosion of the single phase and galvanic couples occurred at the same time; hence, the corrosion rate could not be defined. Furthermore, because the metal dissolution proceeded concurrently with a H2 evolution reaction and the formation of corroded products such as Zn–Cl compounds, Zn-hydroxides, and Fe particles [2,35–37], the current efficiency was also difficult to determine. These problems highlight the inadequacy of relying solely on a stripping test to make an electrochemical interpretation of the dissolution process of a GA coating. A better understanding can be attained when the stripping test results are correlated with metallographic observations and the results of other electrochemical investigations. 3.3. Correlation of the chronopotentiogram and observations of the dissolution morphology The dissolution morphologies at several corrosion stages are marked with arrows at points (a–h) in Fig. 3; they were observed with a SEM and the respective micrographs are shown in Fig. 4. At point (a) in Fig. 3, the corrosion started at the compressed part in the f phase. Fig. 4(a-1) clearly shows that the selectively dissolved surface of the compressed f phase leaves tangled initiative cracks within the hollows (Fig. 4(a-2)). The higher susceptibility to corrosion of the compressed f part is thought to be due to the change in crystallographic orientations and the residual stress from the temper rolling process [24]. The potential dropping within 500 s of immersion is reportedly attributed to the fact that the surface area is increased by the developed cracks [28], which are evident in Fig. 4(b-1). At the beginning of plateau A (point (b) in Fig. 3), both the crystallised and compressed f parts were dissolved simultaneously and numerous cracks were propagated into the crystallised part. As shown in Fig. 4(b-1), some of the cracks intruded into the compressed part with tangled initiative cracks. The cracks that cover the entire surface act as a pathway that a solution can penetrate; and, if that happens, the multiple Fe–Zn intermetallic phases and the substrate are exposed to the solution simultaneously [26]. In addition, the compressed f part was dissolved in a spongy-like morphology (indicated by an arrow in Fig. 4(b-2)), whereas the crystallised f part remained in its crystal shape during dissolution. The mid-point of plateau A (point (c) in Fig. 3) shows a uniform dissolution of the f phase, expanded cracks, and some detachments of the f phase. Where the flakes of the f phase fell off, the d phase was partly exposed to the solution, as indicated by the arrows in Fig. 4(c-1) and 4(c-2). The exposed d phase with fine cracks was more compact than the f phase, as confirmed in Fig. 4(c-2). At the end of plateau A (point (d) in Fig. 3), the f phase was completely dissolved, the d phase was fully exposed to the solution (Fig. 4(d-1)), and the crevices in the d phase were much more enlarged than the cracks in the d phase shown in Fig. 4(c-2). In addition, Fig. 4(d-2) clearly shows the difference in aspects of crack propagation between the compressed and crystallised (undamaged) f parts. The relatively straight cracks that formed in the crystallised f part could not penetrate the area of the compressed f part with the tangled cracks (which is indicated by an arrow in the Fig. 4(d-2)). In contrast to the d phase fragments that were divided by straight cracks, the flakes of the d phase in the tangled crack region were electrically connected to each other. When the d and C phases and substrate corrode together, the less noble phase is preferentially dissolved. In that situation, any electrically shorted d and C islands are dissolved faster than the relatively noble continuous phases because of the establishment of a condition involving a ‘small anode-large cathode’ (i.e., d fragment-C
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Fig. 4. Dissolution morphologies at respective potential steps, (a–h) of chronopotentiogram in the Fig. 3.
basement). Thus, the tangled crack form is preferable to straight cracks, which can divide the continuous phase into electrically isolated fragments. During the dissolution, as shown in Fig. 3, the potential was smoothly elevated to plateau B. The surface morphology observed at the end point of plateau B (point (e) in Fig. 3) is presented in Fig. 4(e-1) and (e-2). The cleavages were further expanded and the d phase became thinner during immersion up to point (e) as seen in Fig. 4(e-2). In addition, the C phase underneath the d phase was disclosed when some of the fragments of the d phase fell off. Although the d phase was not completely dissolved until the end of plateau B, which corresponds to the d phase, the potential was suddenly raised up to plateau C. The C phase adjacent to the substrate began to corrode at plateau C but the d phase remained intact. When the cracks in the d phase were sufficiently developed, the corrosive solution directly attacked the C phase and the substrate; and the C phase dissolved more quickly than the d phase
in plateau C, the stable potential period. The corrosion of the C phase in plateau C is confirmed by the fact that the C phase beneath the d phase (Fig. 4(f-1) and (f-2)) is less compact than that shown in Fig. 4(e-2). When the dissolution of the C phase was completed, fragments of the d phase were separated and, as a result, the bare substrate was exposed. Fig. 4(g-1) and (g-2), the micrographs observed at the beginning of plateau D (point (g) in Fig. 3), clearly show the separated d fragments where the C phase beneath the d phase was completely dissolved. Finally, in plateau D (point (h) in Fig. 3), which is a stable potential region, the substrate is entirely disclosed without any residual fragments of the Fe–Zn intermetallic phases; the intergranular corrosion of the DP steel substrate can also be observed [9]. An inspection of the dissolution morphology during the stripping test revealed several significant findings. First, there is no exact correspondence between each potential plateau and each Fe–Zn intermetallic phase [9,31,34,38]. Secondly, the dissolution
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Fig. 4 (continued)
does not sequentially occur in accordance with the amount of Fe content in the Fe–Zn intermetallic layers. According to Lee et al., [27] the polarisation resistance value of the Fe–Zn intermetallic phase increased with increase in Fe content, but the multi-layered GA coating did not dissolve in order of the polarisation resistance [30]. Lastly, the dissolution process does not obey Faraday’s rule because of the detachment of the f or d phase fragments [26]. These findings confirm that the stripping method alone is incapable of quantifying each intermetallic layer in the GA coating. Fig. 5(a) combines and illustrates the potential transition curve (as obtained from the stripping test (Fig. 3)) and the observation results of the dissolution process of the respective Fe–Zn intermetallic layers. The potential transition curve is divided into three regions for the sequential dissolution of the f, d, and C phases; the three regions are depicted in grey colours in Fig. 5(a). Each Fe–Zn intermetallic phase begins to dissolve from the starting points of the respective grey zones. A mixed zone (relatively dark grey zone) occurs whenever two or three adjacent phases
(including the substrate) are simultaneously exposed to the solution, causing galvanic corrosion [25,27,30,39]. 3.4. Correlation of the chronopotentiogram and potentiodynamic responses The dissolution sequence can be quantitatively explained on the basis of the correlation among the stripping test results, observations of the dissolution morphology, and analysis of the polarisation behaviour of each Fe–Zn intermetallic layer in the GA coating. The polarisation responses of the five potential points indicated by arrows in the stripping test results were measured under the same conditions (i.e., in a 1 M NaCl + 0.01 M H2SO4 solution at 20 °C). The results of the polarisation responses are presented in Fig. 5(b). The polarisation responses to the measured potential points of 1 and 5 in Fig. 5(a) represent those to the cracked f phase and the DP steel substrate, respectively (black lines in Fig. 5(b)). The f phase
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Fig. 5. Comparison of the stripping test result with the dissolution process and anodic polarisation test results. (a) Potential transition curves (Fig. 3) associated with dissolution process, (b) polarisation responses at respective potential points indicated by arrows in the (a and c) Tafel plots for the polarisation curves in the (b). The stripping and polarisation test were conducted in 1 M NaCl + 0.01 M H2SO4 solution.
and DP steel substrate do not show passive behaviour but actively dissolve in the solution in Fig. 5(b). The f phase has an Ecorr value of 0.991 VSCE. Particularly, 1.087 VSCE at which a current wave occurs is not the Ecorr because the value of the measured current density is not changed from negative (cathodic) to positive (anodic) at the potential. This current peak at 1.087 VSCE below Ecorr indicates a presence of a temporarily formed barrier against to corrosion deposited on the f phase surface during dissolution, which is expected to be zinc-chloride hydroxide (ZnCl24Zn(OH)2 [35,39–41] or zinc hydroxysulfate (Zn4SO4(OH)64H2O and Zn4Cl2 (OH)4SO45H2O [37,42,43]) The Ecorr value for the DP steel substrate is 0.651 VSCE, which much higher than the Ecorr of the f phase [38,39]. All the electrochemical dissolution reactions of the Fe–Zn alloy in the GA coating occurred in the potential range 0.991 VSCE to 0.651 VSCE, and this localised potential difference accelerates the galvanic corrosion of GA Zn coating [30,38,39]. The other three polarisation curves measured at the end points of plateaus A, B and C (arrows 2, 3 and 4, respectively, in Fig. 5(a)) correspond to those of the f–d, d–C, and C-substrate galvanic couples (dashed lines in Fig. 5(b)). The values of Ecorr and icorr of a single phase (f and substrate each) or a galvanic coupled phase (f–d, d–C, or C-substrate) obtained from Fig. 5(b) are listed in Table 2. Individual polarisation curves of the d and C phases could not be measured separately because the d and C phases and the substrate were electrically connected in the solution due to the existence of cracks. A comparison of Fig. 5(a and b) confirms that the beginning point of each potential plateau is associated with the Ecorr value for each intermetallic phase and that the end potential point of each plateau corresponds to the galvanic coupled potential (mixed potential) of the two phases in contact. The potentiodynamic curves of the f phase and substrate were used to extract Tafel plots (which are represented by solid lines Table 2 Corrosion rates (icorr) and corrosion potentials (Ecorr) for individual Fe–Zn intermetallic phase and galvanic couples measured by potentiodynamic tests. Each phase and galvanic couples Measured f f d d d C
C C-substrate Substrate
icorr/mA cm
2
Ecorr/VSCE
Estimated 0.565 1.641 0.619 2.089 0.608 3.133 0.479
0.991 0.952 0.909 0.875 0.814 0.782 0.651
in Fig. 5(c)). In addition, Tafel plots for the d and C single phases were estimated from the measured five polarisation curves (which are represented by dashed lines in Fig. 5(b)). Under this condition, the cathodic Tafel slope (bc) was measured to be 0.122 V per decade for the DP substrate and 0.171 V per decade for the f phase. The Tafel slope values for a single d and a single C phase were applied in accordance with the value for the f phase. The icorr values for the single phases of d and C were estimated on the basis of Fig. 5(c); the results are presented in Table 2. The dissolution rate of each Fe–Zn intermetallic phase is in the range of 0.57–0.62 mA cm 2; the icorr value of the DP steel substrate (0.48 mA cm 2) is slightly lower than that [27]. However, as shown in Table 2, when the two adjacent intermetallic phases are galvanically coupled in a solution as a result of the crack formation, the icorr value for the couple underwent a threefold to fivefold increase. In the mixed potential zone between each potential plateau (the relatively dark grey region in Fig. 5(a)), the less noble phase containing a lower Fe content in the galvanic couple (e.g., the f phase in the f–d couple) is expected to corrode quickly. In this way, as shown in Fig. 5(c), the icorr value of the multi-layered GA coating should change theoretically in a zigzag shape from the icorr of the f phase to the icorr value of the substrate. However, the dissolution rate for the less noble phase in the galvanic couple becomes much faster than the single phase dissolution; thus, the overall icorr value are observed to change along the large black circles in Fig. 5(c), as confirmed by the practical polarisation curves in Fig. 5(b). The corrosion of the GA coating was accelerated from the point where the dissolution of the f phase was completed and the corrosion of the galvanic couple of the d–C-substrate started. Hence, the f phase is thought to be the most important phase for the barrier protection function, and it should be kept uniform and thick to retard the acceleration of the dissolution. In addition, the fact that the dissolution rate of the C phase is higher than that of the d phase is explained in Fig. 5(c). As the cracks widen as a result of the dissolution until the end of plateau B (arrow 3 in Fig. 5(a) and the micrographs in Fig. 4(e)), the d and C phases as well as the substrate are exposed to the corrosive environment together. From this point, the C phase begins to dissolve, the d phase still remains, and the potential suddenly increases to plateau C. The mixed icorr value for the C-substrate couple is 3.133 mA cm 2, which appears to be higher than that of the d–C couple (2.089 mA cm 2). For this reason, when the d and C phases and the substrate are exposed to the solution at the same time, the C phase dissolves before the d phase. This phenomenon is attributed to the fact that the C phase and substrate have a larger potential difference than the d–C galvanic couple.
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4. Conclusion This analysis of the dissolution process of a GA coating on DP steel is based on a correlation of the results of a chemical stripping test, observations of the dissolution morphology, and polarisation tests in an acidified chloride solution (1 M NaCl + 0.01 M H2SO4). The dissolution process of the GA coating in the 1 M NaCl + 0.01 M H2SO4 solution was observed in relation to morphological changes during a stripping test. The results show that the corrosion sequence of the Fe–Zn intermetallic phase failed to correspond with the Fe content in the respective intermetallic phases. The corrosion sequence was as follows: the dissolution of the compressed f phase ? crack initiation and propagation in the f phase ? the f phase dissolution and detachment ? the d phase dissolution and crack development ? the prior dissolution of the C phase to the d phase ? the d phase detachment ? the disclosure of the substrate (intergranular corrosion). In addition, the variation in the corrosion rate during the stripping test was measured and estimated by means of polarisation tests at the important dissolution stages. The corrosion rate was accelerated when the outermost f phase was completely dissolved and the galvanic couple of the d–C-substrate was exposed to the solution. The C phase dissolved before the d phase because the corrosion potential difference between the C phase and the substrate was larger than that of the d–C galvanic couple. Thus, the f phase should be kept uniform and thick to protect the substrate from corrosion. References [1] D.K. Matlock, J.G. Speer, Processing opportunities for new advanced highstrength sheet steels, Mater. Manuf. Processes 25 (2010) 7–13. [2] A. Amirudin, D. Thierry, Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings, Prog. Org. Coat. 28 (1996) 59–76. [3] S.H. Han, S.H. Choi, J.K. Choi, H.G. Seong, I.B. Kim, Effect of hot-rolling processing on texture and r-value of annealed dual-phase steels, Mater. Sci. Eng. A-Struct. 527 (2010) 1686–1694. [4] E. De Moor, P.J. Gibbs, J.G. Speer, D.K. Matlock, J.G. Schroth, Strategies for thirdgeneration advanced high-strength steel development, Iron Steel Technol. 7 (2010) 133–144. [5] D.T. Llewellyn, D.J. Hillis, Dual phase steels, Ironmak. Steelmak. 23 (1996) 471– 478. [6] R.G. Davies, Influence of martensite composition and content on the properties of dual phase steels, Metall. Mater. Trans. A 9 (1978) 671–679. [7] Y.I. Son, Y.K. Lee, K.-T. Park, C.S. Lee, D.H. Shin, Ultrafine grained ferrite– martensite dual phase steels fabricated via equal channel angular pressing: microstructure and tensile properties, Acta Mater. 53 (2005) 3125–3134. [8] F.E. Goodwin, E.A. Silva, An overview of North American zinc-based sheet steel coatings production: status and opportunities, in: Proceedings of the Galvatech ‘07, International Conference on Zinc and Zinc Alloy Coated Steel, November 18–22, 2007, Osaka, Japan, pp. 6–13. [9] X.G. Zhang, I.C. Bravo, Electrochemical stripping of galvannealed coatings on steel, Corrosion 50 (1994) 308–317. [10] J. Bian, Y. Zhu, X.-h Liu, G.-d. Wang, Development of hot dip galvanized steel strip and its application in automobile industry, J. Iron. Steel Res. Int. 13 (2006) 47–50. [11] S. Fujita, D. Mizuno, Corrosion and corrosion test methods of zinc coated steel sheets on automobiles, Corros. Sci. 49 (2007) 211–219. [12] A.R. Marder, The metallurgy of zinc-coated steel, Prog. Mater. Sci. 45 (2000) 191–271. [13] S. Feliu Jr., V. Barranco, XPS study of the surface chemistry of conventional hotdip galvanised pure Zn, galvanneal and Zn–Al alloy coatings on steel, Acta Mater. 51 (2003) 5413–5424. [14] C.S. Lin, M. Meshii, C.C. Cheng, Phase evolution in galvanneal coatings on steel sheets, ISIJ Int. 35 (1995) 503–511. [15] S. FeliuJr, M.L. Perez-Revenga, Correlation between the surface chemistry of annealed if steels and the growth of a galvanneal coating, Acta Mater. 53 (2005) 2857–2866.
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