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Effect of deposition temperature on microstructure, corrosion behavior and adhesion strength of Zn-Mg coatings on mild steel
Accepted Manuscript Effect of deposition temperature on microstructure, corrosion behavior and adhesion strength of Zn-Mg coatings on mild steel JoungHyun La, WooSung Jung, HoeKun Kim, MyeonGyu Song, SangYul Lee PII:
S0925-8388(17)34485-7
DOI:
10.1016/j.jallcom.2017.12.289
Reference:
JALCOM 44375
To appear in:
Journal of Alloys and Compounds
Received Date: 17 August 2017 Revised Date:
12 December 2017
Accepted Date: 24 December 2017
Please cite this article as: J. La, W. Jung, H. Kim, M. Song, S. Lee, Effect of deposition temperature on microstructure, corrosion behavior and adhesion strength of Zn-Mg coatings on mild steel, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.289. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of Deposition Temperature on Microstructure, Corrosion Behavior and Adhesion Strength of Zn-Mg Coatings on Mild
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Steel JoungHyun La1, WooSung Jung2, HoeKun Kim1, MyeonGyu Song1, SangYul Lee1,* 1
Center for Surface Technology and Applications, Department of Materials Engineering,
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Korea Aerospace University, Goyang-si, Gyeonggi–do 412–791, Korea 2
POSCO Technical Research Laboratories, Gwangyang-shi, Jeonnam, 545-090, Korea.
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Abstract
Zn-Mg alloys are strong candidates for the protective coating materials on steel products due to their excellent corrosion resistance compared to pure Zn. In previous research,
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sputtered Zn-Mg coatings with a high Mg content (above 27 at.%) exhibited a featureless amorphous structure that improved the corrosion resistance of the coatings. However, these Zn-Mg coatings showed a brittle fracture during deformation, resulting in a decrease in
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adhesion strength. Accordingly, in the present study, to improve the adhesion strength of Zn-
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Mg coating, the deposition temperature of the Zn-Mg coatings was controlled and the effects of the deposition temperature on the coating microstructure, corrosion resistance, and adhesion strength of the synthesized coatings were investigated. The results revealed that the deposition temperature significantly affected the microstructure as well as the coating properties. Zn-Mg coatings synthesized below 50oC showed a featureless amorphous microstructure, while porous crystalline Zn-Mg coatings were synthesized above 100oC. Even though the corrosion resistance of the Zn-Mg coatings decreased slightly as the deposition temperature increased, all of the Zn-Mg coated steel samples exhibited passivation 1
ACCEPTED MANUSCRIPT behavior in the corrosion environment. As the deposition temperature of coating increased above 100oC, the fracture and detachment phenomena of the Zn-Mg coatings during deformation decreased. These results revealed that the adhesion strength of Zn-Mg coating
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could be improved by controlling the deposition temperature.
resistance; adhesion strength *Corresponding author. (SangYul Lee)
E-mail address: [email protected]
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1. Introduction
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Tel.: +82-2-300-0166; fax: +82-2-3158-3770
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Keywords: Zn-Mg alloy; coating materials; vapor deposition; crystal structure; corrosion
For over a century, Zn coatings have been regarded as one of the most effective corrosion protective coatings for steel. However, Zn coatings provide insufficient corrosion resistance
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under severe atmospheric conditions or corrosive aqueous solutions [1,2]. Additionally, during high temperature processes such as welding and hot forming, a Zn coating is likely to
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melt due to its low melting temperature of 420˚C. In particular, the liquid Zn phase tends to penetrate into the steel substrate and copper electrode during the resistance spot welding process, which leads to brittle fractures in welded steel [3-8] and the degradation of welding electrode tips [9-12].
Over the past several decades, significant focus has been placed on developing alternative Zn-based coatings to overcome the aforementioned weaknesses. Zn-Mg alloy coatings are a strong candidate to serve as protective coatings for steel because they boast excellent corrosion resistance compared to Zn coatings [1,2,13-24]. Although the anticorrosion 2
ACCEPTED MANUSCRIPT mechanism of Zn-Mg coatings has not been fully explained, several studies in the literature have reported that a simonkolleite (Zn5(OH)8Cl2·H2O) layer stabilized using Mg ions and Mg corrosion products enhanced the corrosion resistance of Zn-Mg coatings [1,13]. Other studies
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explained the high corrosion resistance of Zn-Mg coatings with phases containing Mg such as Mg2Zn11 and MgZn2, which showed higher corrosion resistance than pure Zn phases [14-16]. In addition, previous reports have demonstrated that for Zn-Mg coatings, a transition from
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porous to dense structures with increasing Mg content leads to an improvement in corrosion resistance [17-18].
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Based upon the extensive work carried out to determine the roles of Mg ions and phases containing Mg on the corrosion resistance of Zn-Mg alloys [15-17], many studies have worked toward the development of Zn-Mg coatings. For example, Mg was added to a galvanizing bath to synthesize a Zn-Mg coating using a conventional galvanizing process,
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and the properties of the synthesized coating were evaluated [1,19,20]. However, the maximum allowable Mg content in the galvanizing bath was limited to a relatively low level (below 3wt.%Mg) due to the different densities and melting points between Zn and Mg [21].
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In order to synthesize an Zn-Mg coating with a higher Mg content, researchers have also employed a two-step process that first applies a physical vapor deposition (PVD) coating and
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then implements an annealing heat treatment. In this process, a pure Mg or Zn-Mg coating is first deposited onto galvanized steel (GI) [21] or electrogalvanized steel (EG) [22] using PVD processes such as sputtering [21] or evaporation [22]. This is followed by the annealing heat treatment to form the final Zn-Mg alloy coatings through a thermal inter-diffusion process. Recently, PVD processes such as an evaporation process [23], magnetron sputtering [24], jet PVD [25], and electro-magnetic levitation (EML)-PVD [26,27] to synthesize Zn-Mg coatings have been developed to replace traditional processes of depositing Zn coatings because these traditional processes are widely known to contribute to environmental pollution. PVD 3
ACCEPTED MANUSCRIPT processes, on the other hand, are clean, eco-friendly, and highly versatile. However, Zn-Mg coatings synthesized using the various PVD processes have produced irregular structures (e.g. amorphous [17,24], crystalline [1,2,28], and nano-crystalline [18,23] structures) depending
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on deposition processes and conditions. In previous research, a sputtered Zn-Mg coating showed a microstructure transition from a columnar structure to a featureless structure as a function of the Mg content [17, 18]. The Zn-
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Mg coatings with a high Mg content (above 27 at.%) exhibited the featureless amorphous structure, and it showed a greater ability to protect the steel from corrosion. However, the
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coating with the featureless amorphous structure was significantly vulnerable to fracturing during the deformation of the coated steel, which led to a reduction in the protection ability of the coatings [18]. Therefore, the present study endeavored to improve the adhesion strength of the Zn-Mg coating by controlling the deposition temperature. Additionally, the adhesion
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strength of the Zn-Mg coatings on the steel substrate was investigated using a hybrid test method that was proposed to evaluate the adhesion strength of the Zn-Mg coatings [18]. This evaluation was impossible using conventional scratch tests or tape tests according to ASTM
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C 1624-05 and ASTM D 3359-97 because the Zn-Mg coatings were too soft (low hardness), and the high roughness of the steel substrates interfered with the accuracy of the
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investigation.
2. Experimental
2.1. Sample preparation Zn-Mg coatings were synthesized using an unbalanced magnetron sputtering system. For the substrates of the Zn-Mg coating, commercial mild steel (CSP3 produced by POSCO, chemical composition (wt.%) = C<0.08%, Mn<0.45%, Si<0.04%, P<0.030%, S<0.030%, Fe 4
ACCEPTED MANUSCRIPT remainder, thickness = 0.7 mm) were prepared (length: 50 mm; width: 30 mm). The steel substrates were used as received and not subjected to surface grinding or heat treatment. They were cleaned in an ultrasonic ethanol bath for 30 minutes, and the surfaces were subsequently
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degreased with an acetone. Before the deposition, the base pressure of the sputtering chamber was pumped down to less than 1.4x10-3 Pa, and the steel substrates were ion bombardment etched about 20 nm to remove any surface contaminants or oxides. Pre-sputtering was conducted for 5 minutes to remove target surface impurities before synthesizing the Zn-Mg
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coatings. A power density of 3.82 W/cm2 (type: pulsed DC, frequency: 20 kHz, duty: 75%),
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was applied to the Zn-10 wt.% Mg alloy target at a working pressure of 0.66 Pa with a 30sccm Ar flow rate to deposit the coatings. The target-to-substrate distance, substrate bias, and deposition time were 90 mm, floating, and 15 minutes, respectively. To control the microstructure of the Zn-Mg coating, the deposition temperature during the sputtering
2.2. Coating analysis
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process was controlled from 25 to 150°C.
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The crystal phases of the Zn-Mg coatings synthesized at various deposition temperatures were investigated using an X-ray diffractometer (XRD, Rigaku MiniFlex II) with Cu Kα
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irradiation (λ = 0.15456 nm). The morphology of the coatings was observed using field emission scanning electron microscopy (FE-SEM, JEOL JSM-7100F), and the chemical composition of the coatings was determined using energy dispersive X-ray spectroscopy (EDX, JEOL JED-2300).
2.3 Corrosion and adhesion tests The corrosion resistance and adhesion strength of the Zn-Mg coatings on steel substrates were investigated using a hybrid test method. The hybrid test is a corrosion and adhesion 5
ACCEPTED MANUSCRIPT evaluation method combining a potentio-dynamic polarization test with a punch stretching test. First, to investigate the corrosion resistance of the Zn-Mg coatings, the potentio-dynamic polarization studies were carried out in a 3.5% NaCl solution with conventional three-
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electrode cell system in which a saturated calomel electrode (SCE) acted as a reference electrode (RE), the platinum mesh was a counter electrode (CE) and the Zn-Mg coated steels acted as the working electrode (WE). The NaCl solution was prepared by mixing 193mL of
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distilled water and the 7g of NaCl powder with 99.0% purity, and it was used in stagnant and naturally aerated condition at room temperature. To obtain a stable open circuit potential
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(OCP), the Zn-Mg coated steel was allowed to stabilize in the electrolyte prior to evaluation. The polarization curves were measured at a scan rate of 5 mV/s, starting from an initial cathodic potential of -2.0 VSCE with the potential increasing toward the anodic side. From the electrochemical measurements, a Tafel plot (log i vs. E plot) was employed according to
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ASTM G3-14 to measure the corrosion potential (Ecorr) and corrosion current density (icorr). Second, to estimate the adhesion strength of the Zn-Mg coating, the Zn-Mg coated steel plates were deformed via a punch stretching test with approximately 35% strain.
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Subsequently, the microstructure and electrochemical properties were derived from the center of the deformed area using FE-SEM and a potentio-dynamic polarization test. The punch
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stretching test using a 19-mm diameter hemispherical punch was conducted using suitably designed tools and a hydraulic press. In all cases, the specimens (Zn-Mg coated steel) were 50 mm in length and 30 mm in width, and the punched depth was 10 mm, which was 35% stain. After the deformation. the FE-SEM and potentio-dynamic polarization results of the deformed coated steel were compared with those of the non-deformed coated steel. Further details regarding the hybrid test method used in this study are provided in [18].
3. Results and discussion 6
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3.1 Characteristics of Zn-Mg coatings on steel substrate The Zn-Mg coatings with various structures were synthesized at various deposition
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temperatures between 25 and 150oC. Hereafter, the synthesized Zn-Mg coatings are denoted as ZM-25, ZM-50, ZM-75, ZM-100, and ZM-150 according to the deposition temperature. Their chemical compositions are summarized in Table 1. The chemical compositions of the
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Zn-Mg coatings ranged from 27.73 at.% to 28.77 at.% Mg, and included a small amount of oxygen content (below 2.5 at.%). As the deposition temperature increased, the Mg content of
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the Zn-Mg coatings increased slightly. However, differences in Mg content within approximately 1 at.% were deemed insignificant, and thus the chemical composition of all of the Zn-Mg coatings could be considered identical.
The X-ray diffraction patterns of the Zn-Mg coatings are shown in Fig. 1. The XRD pattern
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of the ZM-25 coating on the steel plate showed a broad and diffused peak, which indicated the formation of an amorphous structure. In a Mg-Zn binary system, the equilibrium phases are (Zn), Mg2Zn11, and the MgZn2 phases in the Mg content range from 0 to 34 at.% [29,30].
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However, the extremely high cooling rate (higher than 106 K/s) of the sputtering process would induce the formation of an amorphous phase as a non-equilibrium metastable state
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[31-33].
Miedema et al. have proposed a theoretical semi-empirical method to calculate the formation enthalpies of amorphous phases and solid solution phases in binary alloys [34]. Generally, Gibbs free energy should be considered rather than enthalpy to determine the stability of a phase. However, the contribution of entropy is much smaller than that of enthalpy in solid states, and thus the entropy contribution to the Gibbs free energy could be neglected. Therefore, the enthalpy term was regarded as an indicator of the stability of a phase in this study. 7
ACCEPTED MANUSCRIPT According to the Miedema's model, the formation enthalpies of the solid solution phase and the amorphous phase in the binary system could be calculated as follows. ∆ = ∆ + ∆ + ∆
The formation enthalpy of the solid solution (∆HSolid
Solution)
(2)
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∆ = ∆ + ∆
(1)
consists of three terms –
∆HChemical is the enthalpy of chemical mixing, ∆HElastic is the elastic enthalpy origniating from
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the atomic size mismatch, and ∆HStructure is the enthalpy originating from the valence electrons of the transition metals and crystal structure of the solid solution. In an amorphous state, the
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structural and elastic contributions can be deemed negligible due to the absence of a crystal structure and atomic arrangement. Therefore, the formation enthalpy of the amorphous (∆HAmorphous) was represented by the sum of ∆HChemical and ∆HTopological, and ∆HTopological was the enthalpy originating from the different melting temperatures of the two materials. Detailed equations for each enthalpy in the A-B binary system are as follows.
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∆ = { ∆ ! "# $% + ∆ $ "# !%}
(3)
where CA is the chemical composition of the A material in the A-B binary system, and
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∆HInterface is the interfacial enthalpy originating from the contact between the A and B atoms. In the case of the Mg-Zn binary system, ∆HChemical was measured and calculated in previous
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studies, and the results of the Mg-Zn binary system were summarized by P. Ghosh et. al. [35]. ∆ = { ∆ ! "# $% + ∆ $ "# !%} ∆ ! "# $% =
'() *+ ∙ -) .-+ %/ 0() -+ 12*+ -)
(4) (5)
where K is the bulk modulus, G is the shear modulus, and V is the molar volume. For the Mg-Zn binary system, the K, G, and V values are summarized in Table 2 [36]. ∆ = 3.5 × 10.0 ∙ 9, + 9, %
(6)
where Tm,A is the melting temperature of the A material, and Tm,Zn and Tm,Mg are also 8
ACCEPTED MANUSCRIPT summarized in Table 2. In a Mg-Zn binary system, the structural contribution to the enthalpy (∆HStructure) has a minor effect compared to the chemical and elastic contribution because the ∆HStructure originates from the valence electrons of the transition metals [37], but Mg is a non-
contribution to the total enthalpy [38,39]. ∆ ≈ 0
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transition metal. In previous studies, ∆HStructure has also been neglected due to its minor
(7)
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By comparing the enthalpy difference between the formation enthalpy of the solid solution (∆HSolid Solution) and that of the amorphous phase (∆HAmorphous), the glass-forming range can be
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estimated. In the range in which the formation enthalpy of the amorphous phase was lower than that of the solid solution, the amorphous phase is preferentially formed. Fig. 2 shows the calculated enthalpies of the amorphous phase and the solid solutions in the Mg-Zn binary system. The predicted glass-forming composition range in the Mg-Zn binary system
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according to the Miedema's model is 21 -87 at.% Zn
Meanwhile, T. Egami et.al. investigated the correlation between the atomic size ratio of the constituent elements and the glass formability for metal-metal alloy systems, and they
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suggested the topological model to predict the minimum solute composition to form metallic glasses [40]. The glass forming experiments were conducted by rapid quenching from the
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melt, while the quenching rate was within a range of 10-5 to 10-6 K/s, and it was found empirically that there is a close relationship between the experimental data of the minimum solute composition to form metallic glasses (Cmin) and the volume mismatch (Vmis). Detailed equations for the volume mismatch calculation in the A-B binary system and the relationship between the experimental data (Cmin) and the volume mismatch are as follows. 0
< = =+ > − 1
(8)
C × |< | ≅ 0.1
(9)
)
9
ACCEPTED MANUSCRIPT where rA and rB are atomic radius of the matrix and the solute, respectively. In the case of the Mg-Zn binary system, rMg is 0.160nm and rZn is 0.138nm [40]. Consequently, the value of Vmis is equal to -0.358 at the Mg rich side, but is equal to 0.559 at the Zn rich side, and the
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value of Cmin is 0.279 and 0.179 at the Mg rich side and the Zn rich side, respectively. Therefore, the predicted glass-forming composition range in the Mg-Zn binary system according to the topological model is approximately 28 -82 at.% Zn.
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In this study, the Zn contents of the Zn-Mg coatings ranged from 68.73 at.% to 70.40 at.%, and it was possible to form the amorphous Zn-Mg coatings according to both model
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suggested by Miedema and Egami. The formation of the amorphous phase in the Zn-Mg coatings with similar Mg contents has previously been reported [2,17,18,28]. In addition, the distorted Zn-based hcp lattice with the increasing Mg content in the (Zn) due to the different atomic spaces between Zn and Mg was verified via molecular dynamics simulations [41].
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As the deposition temperature increased to 50oC, crystallized peaks originating from the Zn phase (JCPDS card 4-831) started to appear in the XRD pattern, as shown in Fig. 1, and the diffuse peak was still present, suggesting that the crystalline Zn was embedded in the
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amorphous matrix. The crystalline size of the Zn was calculated from the Zn peak in the XRD pattern around 36.5° with the Scherrer equation (t=0.9λ/Bcosθ), which increased
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gradually from 10.92 nm to 24.17 nm as the deposition temperature increased from 50oC to 100oC. As the deposition temperature increased to 100oC, the equilibrium crystalline MgZn2 phase (JCPDS card 34-457) was observed. Similar crystallization behaviors of amorphous Zn-Mg based alloys in the temperature range from 75oC to 120oC have previously been reported [42-44]. When the deposition temperature increased further, the ZM-150 coating showed clear crystalline peaks originating from the Zn and MgZn2 phases. In addition, the diffused peak disappeared. Depending on the deposition temperature, the plane-view as well as the cross-sectional 10
ACCEPTED MANUSCRIPT microstructure of the coatings also varied, as shown in Fig. 3. The ZM-25, ZM-50, and ZM75 coatings showed featureless microstructures. As the deposition temperature increased above 100°C, the microstructure of the Zn-Mg coatings changed drastically to a sponge-like
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structure. A. F. Jankowski et. al. suggested a new transition Zone S between Zone T and Zone 2 in the classic structure zone model, and the morphology of the coating deposited within Zone S was a sponge-like structure with continuous porosity on the sub-micron scale
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[45]. The homologous deposition temperature (Th) required to stabilize the metallic sponge was approximately 0.5, which can be described as follows.
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9 = ≈ '
(10)
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C
where T is the deposition temperature, and Tm is the melting temperature of the coating. The sponge-like structures have previously been observed in various metallic coatings such as gold, silver, and nickel deposited in the Th range from 0.48 to 0.57 [45]. In this study, the
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Zn-Mg coatings with the sponge-like structure were observed when Th was higher than 0.44, i,e, near 0.5, as shown in Figs. 3 d-1 and e-1.
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3.2 Corrosion resistance and adhesion strength of Zn-Mg coated steels The corrosion resistances of the Zn-Mg coated steel samples were evaluated using an
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electrochemical test. Fig. 4 shows the potentio-dynamic polarization curves for the Zn-Mg coated steel measured in a 3.5-wt.% NaCl solution at room temperature. The ZM-25 coated steel showed a relatively positive corrosion potential and low corrosion current density compared to other coated steel samples. The detailed corrosion parameters of the Zn-Mg coated steel are summarized in Table 3. The minimum corrosion current density was measured to be 5.355 µA/cm2 with the ZM-25 coated steel, which also exhibited the highest corrosion potential (-1.295 VSCE) among the 11
ACCEPTED MANUSCRIPT Zn-Mg coated steel samples. As the deposition temperature of the Zn-Mg coating increased to 75°C, the corrosion potential of the Zn-Mg coated steel showed a similar value (-1.297 VSCE), and the corrosion current density also slightly increased from 12.810 µA/cm2, which
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occurred due to the similar featureless microstructures of the ZM-25, ZM-50, and ZM-75 coatings. However, when the deposition temperature increased above 100°C, the corrosion potential of the coated steel decreased to -1.327 VSCE, and the corrosion current density
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increased to 71.041 µA/cm2. The corrosion rate could be calculated according to ASTM G 102-89 based on Faraday’s law. Detailed equations for the corrosion rate is as follows. (F GHII J K
(11)
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CR =
where CR is corrosion rate in mpy, K1 is a constant of 0.1288 mpy·g/µA·cm, EW is the equivalent weight, and ρ is density in g/cm3. The corrosion rates of Zn-Mg coatings were summarized in Table 3, which indicates that they increased from 3.315 mpy to 59.377 mpy
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with increasing the deposition temperature. Although the MgZn2 phase was present in the ZM-100 and ZM-150 coatings, these coatings showed lower corrosion resistance than the coatings deposited at lower temperatures. This is evidence that the microstructure of the
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coating was a parameter that determined the corrosion protection ability of the coating to a greater extent than the intermetallic compounds in the Zn-Mg coatings. Several previous
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studies concluded that amorphous coatings exhibited better corrosion resistance than those coatings that contained many nano-crystalline phases [46,47]. This high corrosion resistance was attributed to their chemical homogeneity and lack of structural defects such as dislocations or grain boundaries. The homogeneous coating phase led to the formation of a uniform passive layer, which was able to obstruct the direct pathway between the corrosive environment and the substrate, resulting in the improvement of corrosion resistance. However, all of the Zn-Mg coated steels exhibited passivation behavior in corrosive environments due 12
ACCEPTED MANUSCRIPT to the formation of insoluble corrosion products on the corroding surface. In previous research, the formation of the simonkolleite phase was observed on the surface of the Zn-Mg coating with a similar Mg content corrosion product after a corrosion test in a 3.5% NaCl
diffusion of corrosive species as well as the corrosion reaction.
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solution [17]. the simonkolleite phase was insoluble corrosion products that hindered the
After the deformation via the punch test, the corrosion potential increased in all cases due to
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the exposure of the steel substrate, which had a relatively positive corrosion potential compared to those of the Zn-Mg coatings (dashed black line in Fig 4). Therefore, the
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adhesion strength between the Zn-Mg coating and steel substrate was estimated by analyzing this corrosion potential variation as a function of the deformation of the coated steel. In previous work, the corrosion potential difference (∆CP) between the non-deformed coating and deformed coating was calculated and used to estimate the adhesion strength of the Zn-Mg
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coatings [18]. The ∆CPs of the Zn-Mg coated steels with various deposition temperatures were also calculated, as shown in the insert in Fig. 5. The results are summarized in Table. 3. However, to exclude the influence of the type of substrate material, the ratio of corrosion
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potential differences (RCP) value was used in this study. The RCP value is one of the
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parameters that determine coating adhesion strength, and it was calculated as follows. L M %% =
MO.P . MO.Q
MRS . MO.P
× 100
(12)
where CPC,0 is the corrosion potential of the Zn-Mg coating before deformation, CPC,D is the corrosion potential of the coating after deformation, and CPFe is the corrosion potential of the steel substrate. When the area of exposed steel increased, the RCP would also increase. Fig. 5 shows the RCP of the Zn-Mg coated steel specimens with various deposition temperatures. In the case of the ZM-25 coating, the RCP and ∆CP were 82.7% and 0.31 VSCE, respectively, which were the highest values among the Zn-Mg coated steel samples. The fracture and 13
ACCEPTED MANUSCRIPT detachment area from the coated steel samples after deformation were shown as the dark area in the SEM observations in Fig. 5, and this revealed that the fracture and detachment area from the ZM-25 coating was significant, while the ZM-150 coatings showed a small fracture
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after deformation. The fracture and detachment of the ZM-25 coating could be explained via the intrinsic brittleness [48] and high hardness [49] of the amorphous coatings. The ZM-150 coating showed an RCP value that was approximately two times lower than that of the ZM-25
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coating. This indicated that the adhesion strength of the Zn-Mg coating could be improved by forming the porous crystalline microstructure and increasing the deposition temperature. It is
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believed that this study will serve as a fundamental starting point for further research related to the industrial application of Zn-Mg coatings.
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4. Conclusion
In this study, Zn-Mg coatings on steel substrates were synthesized at various substrate temperatures using the sputtering system, and the microstructure, the corrosion properties,
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and the adhesion strength of the Zn-Mg coated steel were investigated. The conclusions can be summarized as follows.
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1. The calculated glass-forming composition ranges in the Mg-Zn binary system were 2187 at.% Zn and 28 -82 at.% Zn according to two models suggested by Miedema and Egami, and the amorphous Zn-Mg coatings with approximately 70 at.% Zn were synthesized using a magnetron sputtering process at deposition temperatures below 50°C. 2. As the deposition temperature increased, the structure of the Zn-Mg coating changed from an amorphous structure to a crystalline structure consisting of a MgZn2 intermetallic phase. 14
ACCEPTED MANUSCRIPT 3. The Zn-Mg coatings synthesized at elevated temperatures above 100°C showed that the microstructures had continuous porosity. 4. The Zn-Mg coatings deposited below 50°C showed enhanced corrosion resistance due to
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the dense microstructure compared to the coatings deposited above 100°C, but all of the Zn-Mg coated steels exhibited passivation behavior during the corrosion test.
5. The adhesion strength of the Zn-Mg coating with a featureless amorphous structure was
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unsatisfactory. However, by increasing the deposition temperature of the coating, the
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adhesion strength between the Zn-Mg coating and steel substrate could be improved.
Acknowledgement
This work was financially supported by the Smart Coating Steel Development Center
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operating for the execution of the World Premier Materials (WPM) Program funded by the
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Ministry of Trade, Industry and Energy, Republic of Korea.
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[49]
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ACCEPTED MANUSCRIPT Table 1. Chemical composition of the Zn-Mg coatings as a function of deposition temperature.
Sample
Zn composition
Mg composition
temperature (oC)
classification
(at.%)
(at.%)
(at.%)
25oC
ZM-25
70.40
27.73
1.87
50oC
ZM-50
69.94
27.94
2.12
75oC
ZM-75
69.84
28.61
1.55
100oC
ZM-100
69.97
28.76
1.27
150oC
ZM-150
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68.73
21
O composition
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Deposition
28.77
2.50
ACCEPTED MANUSCRIPT Table. 2 Parameters used in the formation enthalpy calculations and elastic enthalpies [36].
V [cm3/mol]
K [GPa]
G [GPa]
Tm [K]
Mg
13.98
45
17
923
Zn
9.16
108
43
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Element
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V: molar volume, K: bulk modulus, G: shear modulus, Tm: melting temperature
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ACCEPTED MANUSCRIPT Table 3. Corrosion parameters of the Zn-Mg coated steel with various deposition temperatures.
Sample
Ecorr
icorr
Corrosion
Tafel
classification
(VSCE)
(µA/cm2)
rate (mpy)
slopes
Cathodic
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Anode
Tafel
∆CP
slopes
(VSCE)
(V/decade) (V/decade) 5.355
3.315
ZM-50
-1.295
12.810
7.931
ZM-75
-1.297
32.517
ZM-100
-1.327
71.041
ZM-150
-1.338
95.899
0.195
-0.073
0.3114
0.158
-0.061
0.3097
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-1.295
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ZM-25
0.133
-0.060
0.2898
43.986
0.105
-0.133
0.2648
59.377
0.091
-0.107
0.1824
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20.133
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ACCEPTED MANUSCRIPT Figure captions
Fig. 1 X-ray diffraction patterns of the Zn-Mg coatings synthesized at various deposition
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temperatures. The diffraction peaks of Zn (JCPDS card 4-831) and MgZn2 (JCPDS card 34-457) are plotted for reference.
Fig. 2 Calculated enthalpies of the amorphous phase and solid solution phase in the Mg-Zn
the Mg-Zn binary system. (21 -87 at.% Zn)
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binary system. The dashed black lines indicate the glass-forming composition range in
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Fig. 3 Cross sectional and plane-view micrographs of the Zn-Mg coatings synthesized at various deposition temperatures: (a) 25oC; (b) 50oC; (c) 75oC; (d) 100oC; (e) 150oC. -1 indicates the cross sectional micrographs and -2 indicates the plane-view micrographs. Fig. 4 Potentio-dynamic polarization curves of the Zn-Mg coated steel samples synthesized at
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various deposition temperatures. The solid lines show the potentio-dynamic polarization curves of the Zn-Mg coatings and the dashed black line shows the that of steel substrate as a reference.
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Fig. 5 Ratio of corrosion potential differences (RCP) of Zn-Mg coated steels, and typical SEM observations of deformed Zn-Mg coatings. Insert shows the calculation method of ∆CP.
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The dark area in the SEM observations is the fracture and detachment area from the coated steel samples after deformation.
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ACCEPTED MANUSCRIPT Highlights: Zn-Mg
coatings were synthesized at various deposition temperatures on steel substrates.
Microstructure of coatings Amorphization
of the Zn-Mg coating was verified through theoretical calculations.
the Zn-Mg coatings exhibited passivation behavior during the corrosion test. of Zn-Mg coatings could be improved by deposition temperature control.
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Adhesion
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varied depending on the deposition temperature.
S0925-8388(17)34485-7
DOI:
10.1016/j.jallcom.2017.12.289
Reference:
JALCOM 44375
To appear in:
Journal of Alloys and Compounds
Received Date: 17 August 2017 Revised Date:
12 December 2017
Accepted Date: 24 December 2017
Please cite this article as: J. La, W. Jung, H. Kim, M. Song, S. Lee, Effect of deposition temperature on microstructure, corrosion behavior and adhesion strength of Zn-Mg coatings on mild steel, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.289. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of Deposition Temperature on Microstructure, Corrosion Behavior and Adhesion Strength of Zn-Mg Coatings on Mild
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Steel JoungHyun La1, WooSung Jung2, HoeKun Kim1, MyeonGyu Song1, SangYul Lee1,* 1
Center for Surface Technology and Applications, Department of Materials Engineering,
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Korea Aerospace University, Goyang-si, Gyeonggi–do 412–791, Korea 2
POSCO Technical Research Laboratories, Gwangyang-shi, Jeonnam, 545-090, Korea.
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Abstract
Zn-Mg alloys are strong candidates for the protective coating materials on steel products due to their excellent corrosion resistance compared to pure Zn. In previous research,
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sputtered Zn-Mg coatings with a high Mg content (above 27 at.%) exhibited a featureless amorphous structure that improved the corrosion resistance of the coatings. However, these Zn-Mg coatings showed a brittle fracture during deformation, resulting in a decrease in
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adhesion strength. Accordingly, in the present study, to improve the adhesion strength of Zn-
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Mg coating, the deposition temperature of the Zn-Mg coatings was controlled and the effects of the deposition temperature on the coating microstructure, corrosion resistance, and adhesion strength of the synthesized coatings were investigated. The results revealed that the deposition temperature significantly affected the microstructure as well as the coating properties. Zn-Mg coatings synthesized below 50oC showed a featureless amorphous microstructure, while porous crystalline Zn-Mg coatings were synthesized above 100oC. Even though the corrosion resistance of the Zn-Mg coatings decreased slightly as the deposition temperature increased, all of the Zn-Mg coated steel samples exhibited passivation 1
ACCEPTED MANUSCRIPT behavior in the corrosion environment. As the deposition temperature of coating increased above 100oC, the fracture and detachment phenomena of the Zn-Mg coatings during deformation decreased. These results revealed that the adhesion strength of Zn-Mg coating
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could be improved by controlling the deposition temperature.
resistance; adhesion strength *Corresponding author. (SangYul Lee)
E-mail address: [email protected]
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1. Introduction
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Tel.: +82-2-300-0166; fax: +82-2-3158-3770
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Keywords: Zn-Mg alloy; coating materials; vapor deposition; crystal structure; corrosion
For over a century, Zn coatings have been regarded as one of the most effective corrosion protective coatings for steel. However, Zn coatings provide insufficient corrosion resistance
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under severe atmospheric conditions or corrosive aqueous solutions [1,2]. Additionally, during high temperature processes such as welding and hot forming, a Zn coating is likely to
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melt due to its low melting temperature of 420˚C. In particular, the liquid Zn phase tends to penetrate into the steel substrate and copper electrode during the resistance spot welding process, which leads to brittle fractures in welded steel [3-8] and the degradation of welding electrode tips [9-12].
Over the past several decades, significant focus has been placed on developing alternative Zn-based coatings to overcome the aforementioned weaknesses. Zn-Mg alloy coatings are a strong candidate to serve as protective coatings for steel because they boast excellent corrosion resistance compared to Zn coatings [1,2,13-24]. Although the anticorrosion 2
ACCEPTED MANUSCRIPT mechanism of Zn-Mg coatings has not been fully explained, several studies in the literature have reported that a simonkolleite (Zn5(OH)8Cl2·H2O) layer stabilized using Mg ions and Mg corrosion products enhanced the corrosion resistance of Zn-Mg coatings [1,13]. Other studies
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explained the high corrosion resistance of Zn-Mg coatings with phases containing Mg such as Mg2Zn11 and MgZn2, which showed higher corrosion resistance than pure Zn phases [14-16]. In addition, previous reports have demonstrated that for Zn-Mg coatings, a transition from
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porous to dense structures with increasing Mg content leads to an improvement in corrosion resistance [17-18].
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Based upon the extensive work carried out to determine the roles of Mg ions and phases containing Mg on the corrosion resistance of Zn-Mg alloys [15-17], many studies have worked toward the development of Zn-Mg coatings. For example, Mg was added to a galvanizing bath to synthesize a Zn-Mg coating using a conventional galvanizing process,
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and the properties of the synthesized coating were evaluated [1,19,20]. However, the maximum allowable Mg content in the galvanizing bath was limited to a relatively low level (below 3wt.%Mg) due to the different densities and melting points between Zn and Mg [21].
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In order to synthesize an Zn-Mg coating with a higher Mg content, researchers have also employed a two-step process that first applies a physical vapor deposition (PVD) coating and
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then implements an annealing heat treatment. In this process, a pure Mg or Zn-Mg coating is first deposited onto galvanized steel (GI) [21] or electrogalvanized steel (EG) [22] using PVD processes such as sputtering [21] or evaporation [22]. This is followed by the annealing heat treatment to form the final Zn-Mg alloy coatings through a thermal inter-diffusion process. Recently, PVD processes such as an evaporation process [23], magnetron sputtering [24], jet PVD [25], and electro-magnetic levitation (EML)-PVD [26,27] to synthesize Zn-Mg coatings have been developed to replace traditional processes of depositing Zn coatings because these traditional processes are widely known to contribute to environmental pollution. PVD 3
ACCEPTED MANUSCRIPT processes, on the other hand, are clean, eco-friendly, and highly versatile. However, Zn-Mg coatings synthesized using the various PVD processes have produced irregular structures (e.g. amorphous [17,24], crystalline [1,2,28], and nano-crystalline [18,23] structures) depending
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on deposition processes and conditions. In previous research, a sputtered Zn-Mg coating showed a microstructure transition from a columnar structure to a featureless structure as a function of the Mg content [17, 18]. The Zn-
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Mg coatings with a high Mg content (above 27 at.%) exhibited the featureless amorphous structure, and it showed a greater ability to protect the steel from corrosion. However, the
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coating with the featureless amorphous structure was significantly vulnerable to fracturing during the deformation of the coated steel, which led to a reduction in the protection ability of the coatings [18]. Therefore, the present study endeavored to improve the adhesion strength of the Zn-Mg coating by controlling the deposition temperature. Additionally, the adhesion
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strength of the Zn-Mg coatings on the steel substrate was investigated using a hybrid test method that was proposed to evaluate the adhesion strength of the Zn-Mg coatings [18]. This evaluation was impossible using conventional scratch tests or tape tests according to ASTM
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C 1624-05 and ASTM D 3359-97 because the Zn-Mg coatings were too soft (low hardness), and the high roughness of the steel substrates interfered with the accuracy of the
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investigation.
2. Experimental
2.1. Sample preparation Zn-Mg coatings were synthesized using an unbalanced magnetron sputtering system. For the substrates of the Zn-Mg coating, commercial mild steel (CSP3 produced by POSCO, chemical composition (wt.%) = C<0.08%, Mn<0.45%, Si<0.04%, P<0.030%, S<0.030%, Fe 4
ACCEPTED MANUSCRIPT remainder, thickness = 0.7 mm) were prepared (length: 50 mm; width: 30 mm). The steel substrates were used as received and not subjected to surface grinding or heat treatment. They were cleaned in an ultrasonic ethanol bath for 30 minutes, and the surfaces were subsequently
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degreased with an acetone. Before the deposition, the base pressure of the sputtering chamber was pumped down to less than 1.4x10-3 Pa, and the steel substrates were ion bombardment etched about 20 nm to remove any surface contaminants or oxides. Pre-sputtering was conducted for 5 minutes to remove target surface impurities before synthesizing the Zn-Mg
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coatings. A power density of 3.82 W/cm2 (type: pulsed DC, frequency: 20 kHz, duty: 75%),
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was applied to the Zn-10 wt.% Mg alloy target at a working pressure of 0.66 Pa with a 30sccm Ar flow rate to deposit the coatings. The target-to-substrate distance, substrate bias, and deposition time were 90 mm, floating, and 15 minutes, respectively. To control the microstructure of the Zn-Mg coating, the deposition temperature during the sputtering
2.2. Coating analysis
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process was controlled from 25 to 150°C.
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The crystal phases of the Zn-Mg coatings synthesized at various deposition temperatures were investigated using an X-ray diffractometer (XRD, Rigaku MiniFlex II) with Cu Kα
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irradiation (λ = 0.15456 nm). The morphology of the coatings was observed using field emission scanning electron microscopy (FE-SEM, JEOL JSM-7100F), and the chemical composition of the coatings was determined using energy dispersive X-ray spectroscopy (EDX, JEOL JED-2300).
2.3 Corrosion and adhesion tests The corrosion resistance and adhesion strength of the Zn-Mg coatings on steel substrates were investigated using a hybrid test method. The hybrid test is a corrosion and adhesion 5
ACCEPTED MANUSCRIPT evaluation method combining a potentio-dynamic polarization test with a punch stretching test. First, to investigate the corrosion resistance of the Zn-Mg coatings, the potentio-dynamic polarization studies were carried out in a 3.5% NaCl solution with conventional three-
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electrode cell system in which a saturated calomel electrode (SCE) acted as a reference electrode (RE), the platinum mesh was a counter electrode (CE) and the Zn-Mg coated steels acted as the working electrode (WE). The NaCl solution was prepared by mixing 193mL of
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distilled water and the 7g of NaCl powder with 99.0% purity, and it was used in stagnant and naturally aerated condition at room temperature. To obtain a stable open circuit potential
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(OCP), the Zn-Mg coated steel was allowed to stabilize in the electrolyte prior to evaluation. The polarization curves were measured at a scan rate of 5 mV/s, starting from an initial cathodic potential of -2.0 VSCE with the potential increasing toward the anodic side. From the electrochemical measurements, a Tafel plot (log i vs. E plot) was employed according to
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ASTM G3-14 to measure the corrosion potential (Ecorr) and corrosion current density (icorr). Second, to estimate the adhesion strength of the Zn-Mg coating, the Zn-Mg coated steel plates were deformed via a punch stretching test with approximately 35% strain.
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Subsequently, the microstructure and electrochemical properties were derived from the center of the deformed area using FE-SEM and a potentio-dynamic polarization test. The punch
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stretching test using a 19-mm diameter hemispherical punch was conducted using suitably designed tools and a hydraulic press. In all cases, the specimens (Zn-Mg coated steel) were 50 mm in length and 30 mm in width, and the punched depth was 10 mm, which was 35% stain. After the deformation. the FE-SEM and potentio-dynamic polarization results of the deformed coated steel were compared with those of the non-deformed coated steel. Further details regarding the hybrid test method used in this study are provided in [18].
3. Results and discussion 6
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3.1 Characteristics of Zn-Mg coatings on steel substrate The Zn-Mg coatings with various structures were synthesized at various deposition
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temperatures between 25 and 150oC. Hereafter, the synthesized Zn-Mg coatings are denoted as ZM-25, ZM-50, ZM-75, ZM-100, and ZM-150 according to the deposition temperature. Their chemical compositions are summarized in Table 1. The chemical compositions of the
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Zn-Mg coatings ranged from 27.73 at.% to 28.77 at.% Mg, and included a small amount of oxygen content (below 2.5 at.%). As the deposition temperature increased, the Mg content of
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the Zn-Mg coatings increased slightly. However, differences in Mg content within approximately 1 at.% were deemed insignificant, and thus the chemical composition of all of the Zn-Mg coatings could be considered identical.
The X-ray diffraction patterns of the Zn-Mg coatings are shown in Fig. 1. The XRD pattern
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of the ZM-25 coating on the steel plate showed a broad and diffused peak, which indicated the formation of an amorphous structure. In a Mg-Zn binary system, the equilibrium phases are (Zn), Mg2Zn11, and the MgZn2 phases in the Mg content range from 0 to 34 at.% [29,30].
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However, the extremely high cooling rate (higher than 106 K/s) of the sputtering process would induce the formation of an amorphous phase as a non-equilibrium metastable state
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[31-33].
Miedema et al. have proposed a theoretical semi-empirical method to calculate the formation enthalpies of amorphous phases and solid solution phases in binary alloys [34]. Generally, Gibbs free energy should be considered rather than enthalpy to determine the stability of a phase. However, the contribution of entropy is much smaller than that of enthalpy in solid states, and thus the entropy contribution to the Gibbs free energy could be neglected. Therefore, the enthalpy term was regarded as an indicator of the stability of a phase in this study. 7
ACCEPTED MANUSCRIPT According to the Miedema's model, the formation enthalpies of the solid solution phase and the amorphous phase in the binary system could be calculated as follows. ∆ = ∆ + ∆ + ∆
The formation enthalpy of the solid solution (∆HSolid
Solution)
(2)
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∆ = ∆ + ∆
(1)
consists of three terms –
∆HChemical is the enthalpy of chemical mixing, ∆HElastic is the elastic enthalpy origniating from
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the atomic size mismatch, and ∆HStructure is the enthalpy originating from the valence electrons of the transition metals and crystal structure of the solid solution. In an amorphous state, the
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structural and elastic contributions can be deemed negligible due to the absence of a crystal structure and atomic arrangement. Therefore, the formation enthalpy of the amorphous (∆HAmorphous) was represented by the sum of ∆HChemical and ∆HTopological, and ∆HTopological was the enthalpy originating from the different melting temperatures of the two materials. Detailed equations for each enthalpy in the A-B binary system are as follows.
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∆ = { ∆ ! "# $% + ∆ $ "# !%}
(3)
where CA is the chemical composition of the A material in the A-B binary system, and
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∆HInterface is the interfacial enthalpy originating from the contact between the A and B atoms. In the case of the Mg-Zn binary system, ∆HChemical was measured and calculated in previous
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studies, and the results of the Mg-Zn binary system were summarized by P. Ghosh et. al. [35]. ∆ = { ∆ ! "# $% + ∆ $ "# !%} ∆ ! "# $% =
'() *+ ∙ -) .-+ %/ 0() -+ 12*+ -)
(4) (5)
where K is the bulk modulus, G is the shear modulus, and V is the molar volume. For the Mg-Zn binary system, the K, G, and V values are summarized in Table 2 [36]. ∆ = 3.5 × 10.0 ∙ 9, + 9, %
(6)
where Tm,A is the melting temperature of the A material, and Tm,Zn and Tm,Mg are also 8
ACCEPTED MANUSCRIPT summarized in Table 2. In a Mg-Zn binary system, the structural contribution to the enthalpy (∆HStructure) has a minor effect compared to the chemical and elastic contribution because the ∆HStructure originates from the valence electrons of the transition metals [37], but Mg is a non-
contribution to the total enthalpy [38,39]. ∆ ≈ 0
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transition metal. In previous studies, ∆HStructure has also been neglected due to its minor
(7)
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By comparing the enthalpy difference between the formation enthalpy of the solid solution (∆HSolid Solution) and that of the amorphous phase (∆HAmorphous), the glass-forming range can be
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estimated. In the range in which the formation enthalpy of the amorphous phase was lower than that of the solid solution, the amorphous phase is preferentially formed. Fig. 2 shows the calculated enthalpies of the amorphous phase and the solid solutions in the Mg-Zn binary system. The predicted glass-forming composition range in the Mg-Zn binary system
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according to the Miedema's model is 21 -87 at.% Zn
Meanwhile, T. Egami et.al. investigated the correlation between the atomic size ratio of the constituent elements and the glass formability for metal-metal alloy systems, and they
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suggested the topological model to predict the minimum solute composition to form metallic glasses [40]. The glass forming experiments were conducted by rapid quenching from the
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melt, while the quenching rate was within a range of 10-5 to 10-6 K/s, and it was found empirically that there is a close relationship between the experimental data of the minimum solute composition to form metallic glasses (Cmin) and the volume mismatch (Vmis). Detailed equations for the volume mismatch calculation in the A-B binary system and the relationship between the experimental data (Cmin) and the volume mismatch are as follows. 0
< = =+ > − 1
(8)
C × |< | ≅ 0.1
(9)
)
9
ACCEPTED MANUSCRIPT where rA and rB are atomic radius of the matrix and the solute, respectively. In the case of the Mg-Zn binary system, rMg is 0.160nm and rZn is 0.138nm [40]. Consequently, the value of Vmis is equal to -0.358 at the Mg rich side, but is equal to 0.559 at the Zn rich side, and the
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value of Cmin is 0.279 and 0.179 at the Mg rich side and the Zn rich side, respectively. Therefore, the predicted glass-forming composition range in the Mg-Zn binary system according to the topological model is approximately 28 -82 at.% Zn.
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In this study, the Zn contents of the Zn-Mg coatings ranged from 68.73 at.% to 70.40 at.%, and it was possible to form the amorphous Zn-Mg coatings according to both model
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suggested by Miedema and Egami. The formation of the amorphous phase in the Zn-Mg coatings with similar Mg contents has previously been reported [2,17,18,28]. In addition, the distorted Zn-based hcp lattice with the increasing Mg content in the (Zn) due to the different atomic spaces between Zn and Mg was verified via molecular dynamics simulations [41].
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As the deposition temperature increased to 50oC, crystallized peaks originating from the Zn phase (JCPDS card 4-831) started to appear in the XRD pattern, as shown in Fig. 1, and the diffuse peak was still present, suggesting that the crystalline Zn was embedded in the
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amorphous matrix. The crystalline size of the Zn was calculated from the Zn peak in the XRD pattern around 36.5° with the Scherrer equation (t=0.9λ/Bcosθ), which increased
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gradually from 10.92 nm to 24.17 nm as the deposition temperature increased from 50oC to 100oC. As the deposition temperature increased to 100oC, the equilibrium crystalline MgZn2 phase (JCPDS card 34-457) was observed. Similar crystallization behaviors of amorphous Zn-Mg based alloys in the temperature range from 75oC to 120oC have previously been reported [42-44]. When the deposition temperature increased further, the ZM-150 coating showed clear crystalline peaks originating from the Zn and MgZn2 phases. In addition, the diffused peak disappeared. Depending on the deposition temperature, the plane-view as well as the cross-sectional 10
ACCEPTED MANUSCRIPT microstructure of the coatings also varied, as shown in Fig. 3. The ZM-25, ZM-50, and ZM75 coatings showed featureless microstructures. As the deposition temperature increased above 100°C, the microstructure of the Zn-Mg coatings changed drastically to a sponge-like
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structure. A. F. Jankowski et. al. suggested a new transition Zone S between Zone T and Zone 2 in the classic structure zone model, and the morphology of the coating deposited within Zone S was a sponge-like structure with continuous porosity on the sub-micron scale
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[45]. The homologous deposition temperature (Th) required to stabilize the metallic sponge was approximately 0.5, which can be described as follows.
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9 = ≈ '
(10)
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C
where T is the deposition temperature, and Tm is the melting temperature of the coating. The sponge-like structures have previously been observed in various metallic coatings such as gold, silver, and nickel deposited in the Th range from 0.48 to 0.57 [45]. In this study, the
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Zn-Mg coatings with the sponge-like structure were observed when Th was higher than 0.44, i,e, near 0.5, as shown in Figs. 3 d-1 and e-1.
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3.2 Corrosion resistance and adhesion strength of Zn-Mg coated steels The corrosion resistances of the Zn-Mg coated steel samples were evaluated using an
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electrochemical test. Fig. 4 shows the potentio-dynamic polarization curves for the Zn-Mg coated steel measured in a 3.5-wt.% NaCl solution at room temperature. The ZM-25 coated steel showed a relatively positive corrosion potential and low corrosion current density compared to other coated steel samples. The detailed corrosion parameters of the Zn-Mg coated steel are summarized in Table 3. The minimum corrosion current density was measured to be 5.355 µA/cm2 with the ZM-25 coated steel, which also exhibited the highest corrosion potential (-1.295 VSCE) among the 11
ACCEPTED MANUSCRIPT Zn-Mg coated steel samples. As the deposition temperature of the Zn-Mg coating increased to 75°C, the corrosion potential of the Zn-Mg coated steel showed a similar value (-1.297 VSCE), and the corrosion current density also slightly increased from 12.810 µA/cm2, which
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occurred due to the similar featureless microstructures of the ZM-25, ZM-50, and ZM-75 coatings. However, when the deposition temperature increased above 100°C, the corrosion potential of the coated steel decreased to -1.327 VSCE, and the corrosion current density
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increased to 71.041 µA/cm2. The corrosion rate could be calculated according to ASTM G 102-89 based on Faraday’s law. Detailed equations for the corrosion rate is as follows. (F GHII J K
(11)
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CR =
where CR is corrosion rate in mpy, K1 is a constant of 0.1288 mpy·g/µA·cm, EW is the equivalent weight, and ρ is density in g/cm3. The corrosion rates of Zn-Mg coatings were summarized in Table 3, which indicates that they increased from 3.315 mpy to 59.377 mpy
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with increasing the deposition temperature. Although the MgZn2 phase was present in the ZM-100 and ZM-150 coatings, these coatings showed lower corrosion resistance than the coatings deposited at lower temperatures. This is evidence that the microstructure of the
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coating was a parameter that determined the corrosion protection ability of the coating to a greater extent than the intermetallic compounds in the Zn-Mg coatings. Several previous
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studies concluded that amorphous coatings exhibited better corrosion resistance than those coatings that contained many nano-crystalline phases [46,47]. This high corrosion resistance was attributed to their chemical homogeneity and lack of structural defects such as dislocations or grain boundaries. The homogeneous coating phase led to the formation of a uniform passive layer, which was able to obstruct the direct pathway between the corrosive environment and the substrate, resulting in the improvement of corrosion resistance. However, all of the Zn-Mg coated steels exhibited passivation behavior in corrosive environments due 12
ACCEPTED MANUSCRIPT to the formation of insoluble corrosion products on the corroding surface. In previous research, the formation of the simonkolleite phase was observed on the surface of the Zn-Mg coating with a similar Mg content corrosion product after a corrosion test in a 3.5% NaCl
diffusion of corrosive species as well as the corrosion reaction.
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solution [17]. the simonkolleite phase was insoluble corrosion products that hindered the
After the deformation via the punch test, the corrosion potential increased in all cases due to
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the exposure of the steel substrate, which had a relatively positive corrosion potential compared to those of the Zn-Mg coatings (dashed black line in Fig 4). Therefore, the
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adhesion strength between the Zn-Mg coating and steel substrate was estimated by analyzing this corrosion potential variation as a function of the deformation of the coated steel. In previous work, the corrosion potential difference (∆CP) between the non-deformed coating and deformed coating was calculated and used to estimate the adhesion strength of the Zn-Mg
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coatings [18]. The ∆CPs of the Zn-Mg coated steels with various deposition temperatures were also calculated, as shown in the insert in Fig. 5. The results are summarized in Table. 3. However, to exclude the influence of the type of substrate material, the ratio of corrosion
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potential differences (RCP) value was used in this study. The RCP value is one of the
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parameters that determine coating adhesion strength, and it was calculated as follows. L M %% =
MO.P . MO.Q
MRS . MO.P
× 100
(12)
where CPC,0 is the corrosion potential of the Zn-Mg coating before deformation, CPC,D is the corrosion potential of the coating after deformation, and CPFe is the corrosion potential of the steel substrate. When the area of exposed steel increased, the RCP would also increase. Fig. 5 shows the RCP of the Zn-Mg coated steel specimens with various deposition temperatures. In the case of the ZM-25 coating, the RCP and ∆CP were 82.7% and 0.31 VSCE, respectively, which were the highest values among the Zn-Mg coated steel samples. The fracture and 13
ACCEPTED MANUSCRIPT detachment area from the coated steel samples after deformation were shown as the dark area in the SEM observations in Fig. 5, and this revealed that the fracture and detachment area from the ZM-25 coating was significant, while the ZM-150 coatings showed a small fracture
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after deformation. The fracture and detachment of the ZM-25 coating could be explained via the intrinsic brittleness [48] and high hardness [49] of the amorphous coatings. The ZM-150 coating showed an RCP value that was approximately two times lower than that of the ZM-25
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coating. This indicated that the adhesion strength of the Zn-Mg coating could be improved by forming the porous crystalline microstructure and increasing the deposition temperature. It is
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believed that this study will serve as a fundamental starting point for further research related to the industrial application of Zn-Mg coatings.
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4. Conclusion
In this study, Zn-Mg coatings on steel substrates were synthesized at various substrate temperatures using the sputtering system, and the microstructure, the corrosion properties,
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and the adhesion strength of the Zn-Mg coated steel were investigated. The conclusions can be summarized as follows.
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1. The calculated glass-forming composition ranges in the Mg-Zn binary system were 2187 at.% Zn and 28 -82 at.% Zn according to two models suggested by Miedema and Egami, and the amorphous Zn-Mg coatings with approximately 70 at.% Zn were synthesized using a magnetron sputtering process at deposition temperatures below 50°C. 2. As the deposition temperature increased, the structure of the Zn-Mg coating changed from an amorphous structure to a crystalline structure consisting of a MgZn2 intermetallic phase. 14
ACCEPTED MANUSCRIPT 3. The Zn-Mg coatings synthesized at elevated temperatures above 100°C showed that the microstructures had continuous porosity. 4. The Zn-Mg coatings deposited below 50°C showed enhanced corrosion resistance due to
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the dense microstructure compared to the coatings deposited above 100°C, but all of the Zn-Mg coated steels exhibited passivation behavior during the corrosion test.
5. The adhesion strength of the Zn-Mg coating with a featureless amorphous structure was
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unsatisfactory. However, by increasing the deposition temperature of the coating, the
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adhesion strength between the Zn-Mg coating and steel substrate could be improved.
Acknowledgement
This work was financially supported by the Smart Coating Steel Development Center
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operating for the execution of the World Premier Materials (WPM) Program funded by the
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Ministry of Trade, Industry and Energy, Republic of Korea.
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ACCEPTED MANUSCRIPT Table 1. Chemical composition of the Zn-Mg coatings as a function of deposition temperature.
Sample
Zn composition
Mg composition
temperature (oC)
classification
(at.%)
(at.%)
(at.%)
25oC
ZM-25
70.40
27.73
1.87
50oC
ZM-50
69.94
27.94
2.12
75oC
ZM-75
69.84
28.61
1.55
100oC
ZM-100
69.97
28.76
1.27
150oC
ZM-150
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68.73
21
O composition
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Deposition
28.77
2.50
ACCEPTED MANUSCRIPT Table. 2 Parameters used in the formation enthalpy calculations and elastic enthalpies [36].
V [cm3/mol]
K [GPa]
G [GPa]
Tm [K]
Mg
13.98
45
17
923
Zn
9.16
108
43
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Element
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V: molar volume, K: bulk modulus, G: shear modulus, Tm: melting temperature
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ACCEPTED MANUSCRIPT Table 3. Corrosion parameters of the Zn-Mg coated steel with various deposition temperatures.
Sample
Ecorr
icorr
Corrosion
Tafel
classification
(VSCE)
(µA/cm2)
rate (mpy)
slopes
Cathodic
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Anode
Tafel
∆CP
slopes
(VSCE)
(V/decade) (V/decade) 5.355
3.315
ZM-50
-1.295
12.810
7.931
ZM-75
-1.297
32.517
ZM-100
-1.327
71.041
ZM-150
-1.338
95.899
0.195
-0.073
0.3114
0.158
-0.061
0.3097
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-1.295
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ZM-25
0.133
-0.060
0.2898
43.986
0.105
-0.133
0.2648
59.377
0.091
-0.107
0.1824
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20.133
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ACCEPTED MANUSCRIPT Figure captions
Fig. 1 X-ray diffraction patterns of the Zn-Mg coatings synthesized at various deposition
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temperatures. The diffraction peaks of Zn (JCPDS card 4-831) and MgZn2 (JCPDS card 34-457) are plotted for reference.
Fig. 2 Calculated enthalpies of the amorphous phase and solid solution phase in the Mg-Zn
the Mg-Zn binary system. (21 -87 at.% Zn)
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binary system. The dashed black lines indicate the glass-forming composition range in
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Fig. 3 Cross sectional and plane-view micrographs of the Zn-Mg coatings synthesized at various deposition temperatures: (a) 25oC; (b) 50oC; (c) 75oC; (d) 100oC; (e) 150oC. -1 indicates the cross sectional micrographs and -2 indicates the plane-view micrographs. Fig. 4 Potentio-dynamic polarization curves of the Zn-Mg coated steel samples synthesized at
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various deposition temperatures. The solid lines show the potentio-dynamic polarization curves of the Zn-Mg coatings and the dashed black line shows the that of steel substrate as a reference.
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Fig. 5 Ratio of corrosion potential differences (RCP) of Zn-Mg coated steels, and typical SEM observations of deformed Zn-Mg coatings. Insert shows the calculation method of ∆CP.
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The dark area in the SEM observations is the fracture and detachment area from the coated steel samples after deformation.
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ACCEPTED MANUSCRIPT Highlights: Zn-Mg
coatings were synthesized at various deposition temperatures on steel substrates.
Microstructure of coatings Amorphization
of the Zn-Mg coating was verified through theoretical calculations.
the Zn-Mg coatings exhibited passivation behavior during the corrosion test. of Zn-Mg coatings could be improved by deposition temperature control.
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varied depending on the deposition temperature.