## 3 Results and Discussion
### Analysis of Deposited Zinc Mass and Zinc Coating Quality
#### 3.2.2 Hot-Dip Zinc Coating  
As illustrated in Figure 12, the hot-dip zinc coating process differs significantly from electrolytic zinc plating. Based on the approximation function curves, the hot-dip galvanizing process can be divided into two distinct phases. The first phase occurs immediately after sample immersion in molten zinc, where the highest zinc mass increase was recorded for the ground substrate. This phenomenon can be attributed to the following factors:  
* Higher surface roughness. * Lower subsurface hardness. * A favourable residual stress state, which strongly facilitates Fe/Zn atomic diffusion [40; 41]. In contrast, the steel grit-blasted surface exhibited the least favourable conditions for diffusion, despite its statistically comparable roughness to the ground surface. This suggests that high subsurface hardness plays a dominant role, as it induces significant compressive stresses, which act as a barrier to diffusion [42]. However, with material recrystallization, these compressive stresses gradually decrease, accelerating diffusion processes [43]. This marks the transition to the second phase, where diffusion is primarily supported by high dislocation densities and fine-grained structures [44; 45]. As a result, the synthetic corundum-blasted substrate exhibited the highest zinc deposition rates after 175 s and 210 s of immersion in the molten zinc bath. Similar to electrolytic zinc coating, the influence of localized mechanical pre-treatment was briefly examined. In this case, steel grit blasting was again identified as the most suitable variant, regardless of the corrosion-mechanical properties of the hot-dip zinc coatings. This conclusion is based on the statistically comparable zinc deposition rates recorded for the reference sample after approximately 140-210 s of immersion, which aligns well with technical practice. By contrast, the other two mechanical pre-treatment methods appear less suitable for localized application. However, this does not apply when combining two mechanical pre-treatment techniques. The most effective combination was grinding followed by steel grit blasting, as evidenced by the higher zinc mass deposition observed after 175 s and 210 s. These time intervals correspond well with standard industrial practices [2]. Figure 11: Appearance of metallographic specimens with electrolytic zinc coating (immersion time: 4500 s): (**a**) blasted with synthetic corundum, (**b**) blasted with steel grit, (**c**) ground, and (**d**) reference sample. The roughness analysis revealed a statistically significant leveling effect of hot-dip zinc coating (Figure 13). The most pronounced decrease in average Ra and Rz values was observed for coatings applied to synthetic corundum-blasted substrates. This phenomenon can be attributed to the faster heating of sharp surface protrusions compared to the bulk material, which leads to earlier initiation of Fe/Zn diffusion processes in these regions [46]. This effect represents a fundamental difference from electrolytic zinc deposition [47]. After 210 s of immersion, the Ra and Rz values became statistically comparable, as indicated by their standard deviations. This can be further explained by the higher zinc deposition rate observed on substrates with greater initial roughness. In contrast, the reference sample exhibited the least reduction in roughness values, likely due to the manufacturing process of the semi-finished product. Since cold rolling inherently produces surfaces with an optimized microgeometry, minimal surface leveling occurs during hot-dip galvanizing [4; 20]. Metallographic analysis (Figure 14) further revealed the negative effects of blasting technologies on coating homogeneity. On the steel grit-blasted surface, pronounced zeta-phase outgrowths were observed, surrounded by an eta-phase matrix. However, these outgrowths remained mostly continuous with the lower part of the zeta phase.