Analysis of Blackening Reaction of Zn-Mg-Al Alloy-Coated Steel Prepared by Water Vapor Treatment

Abstract

In the context of high-temperature water vapor treatment, Zn-Mg-Al alloy-coated steel sheets exhibit the emergence of a black surface. This study aims to explore the factors and mechanisms contributing to surface blackening by inducing black surfaces on Zn-Mg-Al alloy-coated steel sheets, which were fabricated through molten coating subjected to water vapor treatment at 150 degrees Celsius. The surface composition was predominantly identified as zinc oxide (ZnO) film validated through X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Morphological analysis of the surface and cross-section post-water vapor treatment revealed a disrupted lamellar structure with diffused features, resulting from the formation of an oxide film. Optical properties analysis demonstrated an increased absorbance and a decreased bandgap energy after water vapor treatment, which is indicative of an augmented blackening effect. Consequently, the high-temperature water vapor treatment led to the formation of oxides on the surface with the highly reactive Mg and Al extracting oxygen from the predominantly present ZnO surface. This process resulted in the creation of an oxygen-deficient oxide, ultimately causing surface blackening.

1. Introduction

In the contemporary landscape of industries, where sectors such as automotive, architecture, and electronics thrive on the pursuit of distinctive designs and unique color schemes [1], the significance of black surfaces has emerged as a prevailing trend. Black products, often associated with a luxurious, urban, and modern image, have witnessed a notable surge in demand across diverse age demographics [2]. Beyond the aesthetic allure, black surfaces confer functional advantages, particularly in terms of superior heat absorption [3,4]. The conventional approach to achieving a black appearance involves coating surfaces with black pigments, which is a process frequently applied to steel following zinc (Zn) or zinc-alloy plating with magnesium (Mg) and aluminum (Al) [5,6]. However, this method is accompanied by challenges such as cost implications, limitations in additional forming processes like welding, and environmental concerns, including volatile organic compound (VOC) emissions [7]. Moreover, relying solely on coating for blackening imposes constraints on achieving performance improvements beyond aesthetics with potential issues arising from coating damages and subsequent corrosion resistance problems [8,9]. To address these challenges, recent research has explored alternative, environmentally friendly approaches such as water vapor treatment and anodizing for achieving blackening on steel surfaces, moving away from traditional coating methods applied to Zn-Mg-Al alloy-plated steel sheets [5,6,10,11]. This study focuses explicitly on the water vapor treatment method for blackening, which is applied to Zn-Mg-Al alloy-plated steel produced through the hot-dipping method. Distinguishing itself from ongoing research in the field of Zn-Mg-Al alloy-plated steel, this study places emphasis on two crucial aspects. Firstly, it accentuates the blackening effect, prioritizing the enhancement of surface characteristics by delving into the formation of the blackening oxide layer. While existing studies often concentrate on corrosion resistance or physical properties, this research uniquely prioritizes visual and optical effects through changes in the blackening layer [12,13,14]. Secondly, by unraveling the mechanism behind the blackening effect, this study aims to explore novel applications for Zn-Mg-Al alloy-plated steel, surpassing the realm of corrosion resistance. The objective is to innovate both aesthetically and functionally, extending the potential applications of blackening. In summary, this research, distinct in its approach from existing studies, seeks to innovatively enhance the surface characteristics of Zn-Mg-Al alloy-plated steel through the blackening effect and broaden the applicability of blackening effects. Deemed crucial, this study could pave the way for new horizons in future surface treatment technologies and material applications.

2. Materials and Methods

The test specimens of Zn-Mg-Al were obtained by purchasing PosMAC steel plates from the South Korean company POSCO, which were produced using a hot dipping method [15,16]. The nominal composition of the alloy coating is 94.5 wt.% zinc, 3.0 wt.% magnesium, and 2.5 wt.% aluminum. Prior to water vapor treatment, impurities on the surface were removed by degreasing using sodium hydroxide (NaOH) and ultrasonic cleaning in ethanol (C₂H₅OH). The specimen was then washed with ultrapure water with a resistance value of 18.2 MΩ·cm and dried completely in air. To form a black coating layer, the prepared coating was subjected to water vapor treatment for 1 h and 30 min in a saturated steam atmosphere at 150 °C, according to the process shown in Figure 1. This treatment involves the infiltration of water molecules into an extremely thin naturally formed oxide layer on the metal plating surface in a high-temperature steam environment. During this process, electrons released from the Zn-Mg-Al alloy plating combine with water molecules, forming hydroxide ions (OH⁻). The interaction between hydroxide ions and metal ions leads to the continuous formation of oxides in the high-temperature steam environment. Based on these reactions, a black surface layer is formed on the surface [17]. The appearance of the test specimen with the black surface formed through water vapor treatment can be observed in Figure 2. In this paper, the specimen before water vapor treatment is referred to as Bare, and the specimen after water vapor treatment is referred to as Bare ST. To elucidate the reaction of black coating formation, the surface and cross-sectional morphology of the specimens were observed using a field emission-scanning electron microscope (FE-SEM, MIRA 3, Tescan, Brno, Czech Republic) with an accelerating voltage of 10 kV and transmission electron microscope (TEM, JEM-2100F, JEOL, Tokyo, Japan) with an accelerating voltage of 200 kV. Elemental compositions of iron (Fe), Zn, O, Al, and Mg in the depth direction were analyzed using a glow discharge mass spectrometer (GD-MS, Element GD, Thermo-fisher, Waltham, MA, USA), and surface elemental compositions were analyzed through energy-dispersive spectroscopy (EDS) attached to FE-SEM equipment. X-ray diffraction (XRD, SmartLab, Rigaku, Tokyo, Japan) using Cu kα radiation (λ = 1.5418 Å, 40 kV, 200 mA) and a scanning speed of 1° min⁻¹ at a 2θ range of 10–90° was applied to characterize the crystalline composition of the coatings. The surface chemistry analyzed by X-ray photoelectron spectroscopy (XPS, AXIS SUPRA, KRATOS Analytical Ltd., Manchester, UK) using Al Kα radiation (1486.6 eV, 20 kV, 15 mA). Binding energies were calibrated by the C1s of 284.8 eV. A UV-Vis/NIR spectrometer (V-770, JASCO, Tokyo, Japan) was used for optical property analysis, including absorbance and bandgap. UV-Vis/NIR spectra were analyzed within the range of 200–2500 nm.

3. Results and Discussion

3.1. Material Characterization

Figure 3 illustrates the surface morphology of test specimens for both Bare and Bare ST. In Figure 3a, the surface SEM image of Bare was analyzed at three different points, each presumed to exhibit distinct metal structures, with confirmed elemental compositions. For Bare ST, the surface closely resembled point 3 in Figure 3b, and additional surfaces corresponding to points 1 and 2 were identified. Elemental composition analysis for these points was conducted using EDS. As observed in Figure 3a, a lamellar structure consistent with previous research on Zn-Mg-Al was noted, indicating the presence of the Zn-matrix phase, Zn-MgZn₂, and Zn-Mg₂Zn₁₁ binaries as well as Zn-MgZn₂-Al ternary phases [18]. In Figure 3a, point 1 lacked a lamellar structure in the SEM image, and elemental composition analysis revealed a predominant presence of Zn. This implies a structure of Zn-Al with a Zn-matrix phase or an Al eutectic. For points 2 and 3, a decrease in Zn elemental composition and an increase in the Mg and Al ratio were observed through

Table 1. EDS quantitative analysis at each point marked on Figure 3a.

Point	Zn (wt.%)	Mg (wt.%)	Al (wt.%)	O (wt.%)
1	94.12	1.03	2.58	1.53
2	89.31	4.95	6.58	0.64
3	83.96	3.92	6.12	4.51

. Consequently, it was inferred that these points might correspond to Zn-MgZn₂ binary, Zn-Mg₂Zn₁₁ binary, or Zn-MgZn₂-Al ternary phases, as suggested by the elements presented in the surface composition of the Bare specimen in Table 1. Specifically, the examination of the bare specimen’s surface in Figure 3a speculated the presence of the Zn-matrix phase and other binary, ternary phases.

In Figure 3b of Bare ST, the destruction of the lamellar structure is depicted, and it is replaced by a diffusion-like shape. Point 3, characterized by the highest oxygen (O) content and a rough surface, is presumed to have undergone the most significant oxide formation during water vapor treatment. This observation suggests that light scattering due to surface roughness may not be the primary factor in blackening, but it could contribute to the development of a blackened appearance [19,20]. On the other hand, points 1 and 2, retaining the lamellar structure while undergoing diffusion, exhibit relatively less oxide formation, as indicated by the elements present in the surface composition of the Bare ST specimen in

Table 2. EDS quantitative analysis at each point marked on Figure 3b.

Point	Zn (wt.%)	Mg (wt.%)	Al (wt.%)	O (wt.%)
1	71.45	5.89	8.47	14.19
2	65.98	6.83	10.13	17.06
3	59.16	6.15	13.13	21.56

. In the case of point 3, which occupies the largest surface area on the surface of Bare ST, the proportion of Al increases. This is attributed to the high reactivity of Al in the presence of high-temperature water vapor, leading to oxidation and the formation of a surface layer. It is presumed that this process may assist in facilitating the oxidation of Zn in MgZn₂ in the depth direction, contributing to the enhancement of blackening [8]. Analysis of the O composition in Bare ST suggests an association between the advancement of blackening and an increase in the ratio of O elements.

Additionally, to examine the surface morphology in more detail, a higher magnification of ×35,000 was employed to confirm that in Bare test specimens, the lamellar structure was maintained, which is consistent with the observations at ×5000. However, in the case of Bare ST test specimens, the lamellar structure could not be observed. Hence, due to the high-temperature water vapor treatment in the Bare, a distinct surface was formed in Bare ST. Through surface morphology and elemental composition, this is presumed to be an oxide layer, and it is identified as the primary factor contributing to surface blackening.

Similarly to the surface morphology results observed in Figure 3, when examining the cross-sectional morphology of the Bare specimen in Figure 4, it was confirmed that a lamellar structure is formed between the intermetallic compounds. Additional TEM analysis with EDS mapping revealed predominantly formed Zn, and when confirming the point element composition in the region where Mg elements were mapped, it was determined to be 23.99 at% Mg and 71.75% Zn. This suggests the formation of the intermetallic compound MgZn2. Furthermore, Al was found to be distributed around Zn and Mg, which is similar to the results observed on the surface. The oxide layer, as seen in the mapping, was found to be extremely thin near the surface, indicating the presence of a naturally formed native oxide layer on the metal surface.

In contrast, Figure 5 shows the examination of cross-sectional morphology and EDS mapping of Bare ST, where it was observed that the lamellar structure, akin to the surface morphology, was disrupted and appeared to be diffusing. This phenomenon was attributed to surface treatment occurring due to high-temperature steam. Through EDS mapping to confirm the positions of each element, it was evident that the boundaries were clearly defined in Bare, while in Bare ST, all elements were diffused. Particularly, oxygen (O) was found to be formed approximately 5 μm deep from the surface. This led to the conclusion that an oxide layer formed through steam treatment on the surface.

Figure 6 presents the depth-wise elemental composition analysis results from the surface analyzed through GD-MS. When examining the depth-wise elemental composition of Bare, it was observed that Zn predominantly occupied the surface, which is consistent with the composition of the test specimen. Small amounts of Mg and Al were also detected, and Fe showed an increase from a depth of 10 μm, intersecting with the decreasing Zn, indicating the formation of an approximately 10 μm alloy coating on the specimen.

Unlike Bare, in the case of Bare ST, a higher ratio of O was observed on the surface, with the ratio gradually decreasing to a depth of about 5 μm. Additionally, the depth of the coating layer was approximately 14 μm, with a surface oxide layer of 5 μm thickness. This observation indicates an increase in the thickness of the oxide layer. This observation aligns with the results obtained through TEM’s oxygen mapping, supporting the hypothesis that the main factor contributing to blackening is the formation of an oxide layer on the surface.

Next, we analyzed the crystal structure and phase information of the thin film using XRD. As seen in Figure 7, peaks for mainly Zn and MgZn₂ were detected in the Bare specimen. In contrast, in Bare ST, peaks for the oxide, ZnO, were additionally confirmed, and peaks related to reactions with high-temperature water vapor, such as Mg₂Zn₁₁, were observed [21]. The formation of oxides on the surface was further confirmed, which was consistent with elemental composition analysis through EDS. Based on the ZnO peak from the XRD analysis, the crystal size and interplanar spacing were analyzed using Bragg’s law in Equations (1) and (2). As shown in Figure 8, the interplanar spacing of ZnO increased in Bare ST [22]. This is presumed to result from a chemical state change or oxygen vacancy, as illustrated in Figure 9 [23].

$$\mathrm{L}=\frac{0.9\lambda }{FWHM·\cos \theta }$$

(1)

(L: lattice interplanar spacing of the crystal, λ: wavelength of the characteristic X-ray, ϴ: X-ray incidence angle).

$$\mathrm{D}=\frac{n·\lambda }{\sin \theta }$$

(2)

(D: spacing of the crystal layers, n = integer, λ: wavelength of the X-ray, θ: the angle between incident ray and the scatter plane).

In addition, the crystal structure was verified through XPS spectra analysis, as illustrated in Figure 10 and Figure 11. Following water vapor treatment, discernible peaks emerged in the binding energy of the oxide in the Zn 2p, Mg 1s, and Al 2p results, corroborating the formation of oxide, which is in line with the findings from XRD.

Upon examining the Zn LMM spectra for the oxygen Auger peak in the Bare specimen, a peak indicative of Zn²⁺ ion binding was observed. Conversely, in the Bare, a peak corresponding to Zn¹⁺ binding was identified. This observation suggests a shift in the ZnO peak in Bare ST, signifying a transformation from ZnO to ZnO_1−x. The alteration is likely attributed to the change in oxidation state during the transformation from ZnO to ZnO_1−x [24]. Furthermore, as depicted in Figure 11, the O1s binding energy peak was subjected to fitting, revealing three distinct peaks with binding energies of approximately 530.2, 530, and 528 eV, respectively. The lowest binding energy is associated with the metal–oxygen bond (M-O), such as ZnO or MgO; the intermediate energy corresponds to oxygen vacancy, and the highest energy is linked to the bond with moisture on the metal surface (OH⁻) [25]. Notably, oxygen vacancy was not detected in Bare, but its occurrence was confirmed in Bare ST. Therefore, the observed increase in the interplanar distance of ZnO, as evidenced in the XRD results, is attributed to the occurrence of oxygen vacancy in ZnO, leading to the formation of an oxygen-deficient oxide.

3.2. Optical Characterization

Figure 12 illustrates the analysis of optical properties using UV-Vis-nir to measure reflectance across the wavelength range of 200–2500 nm. Upon measuring reflectance, it was observed that the black specimen exhibited low reflectance, which is akin to the lightness depicted in Figure 2. For the conversion of reflectance to absorbance, representing the extent of light absorption, the Kubelka–Munk equation (Equation (3)) was applied [26]. Upon examining the absorbance results, a sharp increase was observed around 390 nm in the visible light range for the Bare ST, suggesting the absorption of visible light by a certain factor, as depicted in Figure 12.

$$\mathrm{F}(\mathrm{R})=\frac{{\left(1-R\right)}^2}{2R}=\frac{a}{s}$$

(3)

(F(R): absorbance, R: diffuse reflectance, a: absorption coefficient, s: scattering coefficient).

This is attributed to the absorption of visible light. The absorption is a result of ZnO defects caused by an increase in interfacial distance due to oxygen vacancies, as revealed in the preceding XRD and XPS analyses [27,28]. Furthermore, the bandgap was measured based on the obtained absorbance using the Tauc plot. The Tauc plot graph can be obtained by using photon energy and optical energy as the x- and y-axes, respectively, and the formulas for obtaining photon energy and optical energy are shown in Equations (4) and (5) [29].

$$\mathrm{E}=\frac{hc}{\lambda }$$

(4)

(E: photon energy, h: Planck constant, c: speed of light, λ: wavelength of light).

$${\left(\mathrm{ahv}\right)}^2=2.303·\mathrm{F}(\mathrm{R})$$

(5)

E(F(R): absorbance, E: photon energy).

Utilizing the absorbance data, the calculated bandgaps for Bare and Bare ST were found to be approximately 4.2 eV and 2.9 eV, respectively, as illustrated in Figure 13. The optical bandgap energy of Bare, estimated at 4.2 eV, corresponds to the optical bandgap energy of MgO [30]. This led to the conjecture that MgO was naturally formed in small quantities. In the case of Bare ST, a noticeable decrease in the optical bandgap energy to approximately 2.9 eV was observed. This is attributed to the thickening of the oxide layer on the surface, specifically the formation of the oxide layer of ZnO, as confirmed by cross-sectional morphology and GD-MS [31]. Bare ST’s bandgap energy is lower than the typical optical bandgap of ZnO, which is 3.4 eV, indicating that the bandgap energy decreases with the ratio of oxygen in ZnO [32]. In another study, it was observed that by adjusting the oxygen deficiency in the ZnO coating, the bandgap energy decreased to approximately 2.5 eV at an oxygen vacancy concentration of 4.8% and ZnO composition of 0.952 [33,34]. Therefore, the bandgap measurement implies the presence of oxygen deficiency in ZnO. As observed in Figure 14, the reactive Mg and Al ions combine with oxygen to form ZnO_1−x, an oxygen-deficient oxide, by extracting oxygen from ZnO. Based on these results, it is inferred that the ZnO_1−x structure, formed through the absorption of visible light, contributes to the development of a blackened surface [35,36].

4. Conclusions

This study investigates the reaction of blackening on Zn-Mg-Al alloy-coated steel sheets resulting from high-temperature water vapor treatment via the hot-dip galvanizing method. The key findings can be summarized as follows: After water vapor treatment, the steel sheet exhibited increased surface roughness and slightly enlarged particle size, with cross-sectional analysis revealing the formation of an oxide layer, primarily comprising ZnO. Confirmed oxides in Bare ST indicated the creation of oxygen vacancies, leading to an increased lattice spacing of ZnO and the formation of an oxygen-deficient oxide. Bare ST’s optical bandgap measurement revealed a value of approximately 2.9 eV, significantly lower than ZnO’s typical 3.4 eV, suggesting the formation of a non-stoichiometric structure (ZnO_1−x) due to defects, with visible light absorption identified as the cause of blackening. With a composition of Zn 94.5 wt.%, Mg 3.0 wt.%, Al 2.5 wt.%, future research should explore blackening under varied Mg and Al ratios and investigate factors influencing blackening at lower energy levels, contributing to a comprehensive understanding. In conclusion, this study provides valuable insights into the intricate process of blackening, paving the way for further exploration and the optimization of conditions in future research.