OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 Characteristically, silicon reflections are observed in the X-ray spectrum of the coatings, but there are no iron silicides. This may indicate unfavorable conditions for the formation of ferrosilicon under conditions of a low-voltage electric discharge. This also explains the halo visible in the diffraction patterns of the coatings in the angle range 2θ 35 – 50°, which indicates the presence of an amorphous phase in the coatings. As consequence, according to the results of X-ray phase analysis, it is impossible to judge reliably the effect of silicon concentration in the anode mixture on the content of titanium silicide in coatings. Figure 2, a shows a cross section image of the Si2.6 coating. The coating has a darker shade compared to the substrate due to the enrichment with silicon and titanium, which have a lower atomic weight compared to AISI304 steel elements. There are no clear boundaries and longitudinal cracks between the deposited layer and the substrate, which indicates good adhesion of the Fe-Ti-Si coating to steel AISI304. According to the EDS data (Fig. 2, b), the composition of the coating is dominated by iron and chromium from the substrate, which corresponds to the of X-ray data (Fig. 1). The concentrations of titanium and silicon dissolved in the coating matrix ranged from 5 to 20 at.%. There are dark inclusions in the coating structure (Fig. 2, c). According to its EDS analysis, the ratio of titanium to silicon is 49.3 to 31.8, which corresponds to titanium silicide Ti5Si3 (Fig. 2, d). The microstructure of inclusions is represented by columnar crystallites, which corresponds to the microstructure of Ti-Si coatings on a titanium alloy [11]. With an increase in the powder charge concentration in the anode mixture from 2.6 to 6 vol.%, the average coating thickness decreased from 24.8 to 21.7 µm (Table 1). Figure 3 shows polarization diagrams of Fe-Ti-Si coatings and AISI304 steel in 3.5 % NaCl solution at room temperature. Based on these data, the corrosion current density Icorr, the corrosion potential Ecorr and the polarization resistance Rp were calculated (Table 3). Rp was calculated using the simplified expression (2): corr 2.3 3 ( ) 0 a c p a c b b R I b b = + , (2) where ba and bc are the slopes of the Tafel section of the anode and cathode curves, respectively. It follows from Table 3 that the corrosion potentials were similar for both coatings and significantly greater than for AISI304 stainless steel. This suggests that Fe-Ti-Si coatings can reduce the activity of the stainless steel surface to spontaneous corrosion. The corrosion current density of the coatings was 1.8 to 2.1 times lower than that of AISI304 stainless steel (Table 3). The Si2.6 sample showed the highest corrosion potential and Fig. 1. X-ray diffraction patterns of deposited coatings
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