Comprehensive study of the effect of hot-dipping process parameters on Sn-Sb coating properties for α-brass substrate

Abstract

This study's main purpose is to achieve an optimal hot-dip coating condition of SnSb for an α-brass alloy. Therefore, the hot-dipping parameters, including pre-flux lubricants, immersion temperature, time, and withdrawal speed were investigated. ZnCl2 and SnCl2 were used as pre-flux bath additives. The temperature of the immersion bath was selected to be in the range of 250-300 °C. Also, the exposing time and withdrawal speed of the specimens during the hot-dipping process were in the range of 10-60 sec and 254-1524 mm/min, respectively. Visual inspection of the coating revealed that by using SnCl2 as a pre-flux additive, high-quality smooth coating is achieved. According to the AFM result, the initial roughness value of the substrate was 450 nm. The coating's roughness value with SnCl2 and SnCl2+ZnCl2 pre-fluxes were in the range of 300-500 and 700-900 nm, respectively. Therefore, ZnCl2 pre-flux is associated with a rougher surface. Corrosion test analysis revealed that both coating condition with different prefluxes leads to increasing corrosion resistance however better improvement in corrosion behavior is accomplished by smooth coating surface. The quantitative analysis of the polarization curve revealed that the corrosion rate of the smooth coating is decreased 712.5 times in comparison with the substrate. According to the SEM analysis, the predominant phases which were appeared at the interface of the coating and substrate were Cu3Sn and Cu6Sn5. SEM analysis revealed that the Cu3Sn intermetallic compound was this first phase, which was promoted near to the substrate vicinity during the hotdipping process. Keyword: α-brass alloy; hot-dipping coating; polarization curve; intermetallic; SEM. Corresponding author: Ali Barak, Alibarak77@gmail.com xxx Metall. Mater. Eng. Vol xx (x) 2021 p. xxx-xxx Introduction Brass alloys (Cu-Zn) have been regarded as commercial alloys. This alloy is one of the familiar candidates for electrical structure, pipe and valve industries due to its excellent thermal and electrical conductivity and appropriate corrosion resistance properties [1-3]. The previous studies demonstrated that the corrosion resistance of the brass alloys is severely dependent on the value of Zinc [4, 5]. As a result of dezincification, the corrosion resistance of the brass alloys was considerably dropped [69]. One practical method for protecting the corrosion behavior of engineering alloys, especially copper base alloy is coating [10]. Among different coating methods, the hotdipped method has been widely developed because of its convenience, time and costsaving issue [11]. The promoted coating has strongly protected the substrate against corrosion. The chemical composition of the coating was selected according to the working and environmental conditions [12]. Kebede et al. [13] reported a list of organic and inorganic inhibitors in order to increase the corrosion resistance of the copper base alloys. Nickel and Tin-Antimony are conventional chemical composition of the coating which is generally used for brass alloys. Previous studies demonstrated that the quality of the coating, especially in the hot-dipping technique, is directly related to the process parameters [11, 14, 15]. An appropriate coating should guarantee the free from defect, excellent adhesion and uniformity of thickness. Despite the limited studies about the quality of coating for copper-base alloy via hot-dipping process, the parameters were comprehensively investigated for other engineering alloys. For instance, the immersing time and pre-flux additives on microstructure evolution of coating for galvanized steel were studied by Deshmukh et al. [16]. It was revealed that when the exposure time is in the range of 5-10 minutes the high-quality coating is developed. In another research, hotdipping is optimized in order to obtain a high-quality coating for steel sheets [17]. According to the results of that research, it was clarified that the best immersion time and bath temperature of the hot-dipping process are 1 minute and 450 ° C, respectively. Also, the quality of the promoted coating is improved when the selected withdrawal speed of the specimens was in the range of 3-5 m/min. Most of the researches about the optimization of the hot-dipping process are focused on steels [18-20], aluminum [21], titanium [22], and superalloys [23, 24]. However, hot-dipping process parameters of copper-base alloys, especially α-brass alloy have not been comprehensively investigated. Due to the significant demand for α-brass alloy in various branches of industry, it is required to study the corrosion protection methods and especially the hot-dipping process of this alloy. One of the vital steps for industrializing α-brass alloys is to optimize its hotdipping process parameters. Therefore the main aim of this study is to optimize the hotdipping process of Sn-2.5%Sb on α-brass alloy and achieve a high-quality coating. The studied parameters are as follows: 1) chemical composition of the pre-flux, 2) temperature of the bath, 3) immersion time, and 4) withdrawal speed of the specimens. After optimizing the promoted coating, the corrosion properties of the alloy was evaluated. Also, the uniformity of the thickness and the appeared phases on the interface of the substrate (i.e. α-brass) and coating (Sn-2.5% Sb) was characterized by SEM and XRD techniques. A. Barak et al.A Comprehensive Study of the Effect of Hot-Dipping Process Parameters ... xxx Experimental The chemical composition of the α-brass alloy, which was used as a base metal, has been determined using OES analysis. The result revealed that the chemical composition of the base metal is 30 % Zn70 % Cu. The specimens with a dimension of 3×6 cm were provided. The initial thickness of the specimens was approximately 2.5 mm. At the first step of preparation, the surface of the specimens was properly degreased. For this purpose, the specimens were cleaned in the alkali solution under a magnet heater at the temperature of 70 ° C for 3 minutes. The chemical composition of the alkali solution was reported in table 1. Table 1. The chemical composition of the alkali solution. Compound NaOH Na2CO3 Na3PO4 Na2SiO3 Distilled waster Amount 6 g 8 g 6 g 10 g 200 ml In the second step, the alkali solution components were completed wiped out from the specimens' surface via the pickling process. Hence, the specimens were completely dried at 100 ° C and immediately pickled using HCl (30 %) –H2O (70 %) solution. The pickling time was minimized in order to inhibit from pitting phenomenon. Eventually, the specimens were washed and dried at 100 °C. At this step, the specimens have been ready for the hot-dipped coating process. The effect of pre-flux baths on coating quality is going to be investigated in this research. During the hot dipping process, various lubricants, including SnCl2, ZnCl2, and NH4Cl was added to the melting bath. Experimental results revealed that appropriate coating was achieved as a result of using SnCl2 and ZnCl2 lubricants. The chemical composition of the used pre-flux baths was reported in table 2. The temperature of the melt bath was selected to be in the range of 250-320 ° C. The exposure time and withdrawal speed of the specimens via the hot-dipping process were 254-1524mm/min and 10-60 sec, respectively. Table 2. The chemical composition of the used flux. Pre-flux baths number The component of the flux Pre-flux baths 1 12 g ZnCl2+ 80 cc distilled water ZnCl2 2 6 g ZnCl2 + 6 g SnCl2 + 80 cc distilled water SnCl2 + ZnCl2 3 10 g SnCl2+ 80 cc distilled water SnCl2 In the following steps after the hot-dipped coating process, the thickness of the coatings has been measured using optical microscopy (Reichert – Jung/micro-dromat 4000E). The metallographic preparation has been fulfilled based on the conventional metallographic method. In this method, the cross-section of the specimens was prepared using SiC abrasive papers and mechanical polishing with alumina powder. The thickness of the specimens was measured using Clemex software. The roughness of the coating was determined using the atomic force microscopy (AFM) technique. Also, the appeared phases via hot-dipped coating process were detected using low angle X-ray diffraction and Gazing methods (Xpert pro MPD PANalytical). Finally, the microstructure, thickness and the appeared phase in the coating were characterized using scanning electron microscopy (TESCAN, VEGA/XMU). After optimizing the hot-dipping parameters, a high-quality coating of Sn-Sb on α-brass base metal was achieved subsequently the corrosion behavior of the optimal coating was measured via polarization test. The xxx Metall. Mater. Eng. Vol xx (x) 2021 p. xxx-xxx corrosion polarization test was carried out by AutoLab PGSTAT (potentiostatgalvanostat) machine at the temperature of 25 °C. The slope of the Tafel plot has been derived using potentiostatic technique in a 3.5% NaCl solution. The reference and counter electrode was Ag-AgCl and Pt, respectively. The dimension of the working electrode was 1×1 cm and the corrosion test has been done on high-quality coated specimens with an area size of 1 cm. The potential value of the solution has been measured using Cyclic Voltammetry (CV) test. The potential range and scanning rate were selected to be 0.8511 mv and 0.5 mW/sec, respectively. Finally, the extracted curves have been interpreted using NOVA 17.8 software. Results and discussion The effect of pre-flux additives on the quality of the coating In this section, the effect of pre-flux additives on the quality of the coating was investigated. The effect of pre-flux type on the quality of the promoted coating was visually illustrated in figure 1. For an exact evaluation of the pre-flux effect on the coating, the other hot dipping parameters including immersion temperature, exposure time and withdrawal speed were invariable. Visual inspection of the obtained coating indicates that the quality of