## 3 Results and Discussion
### Analysis of Corrosion Resistance  
Due to the significant difference in the amount of deposited zinc between the two coating types (consistent with technical practice), the electrolytic zinc coating underwent 7 degradation cycles, while the hot-dip zinc coating endured 33 cycles. As indicated by the corrosion test results (Figure 18), when comparing the corrosion resistance of mechanically pre-treated and untreated samples, two main factors must be considered:  
* Coating structure quality. * The mass of deposited zinc, which represents the quantitative aspect. For the electrolytically galvanized reference sample, no structural defects were observed in the coating. However, the highest degree of degradation was recorded during corrosion testing. This can be attributed to the lower zinc surface mass, as the reference sample contained approximately 1.5\(\times\) less zinc than the synthetic corundum-blasted substrate. When evaluating zinc coatings in a salt spray environment, the relatively rapid reduction in zinc layer thickness must also be considered [54]. This was further confirmed by the results, where the first signs of red rust in the electrolytic zinc coating appeared uniformly after six cycles, with corrosion spots typically up to 3 mm in diameter. The only exception was the reference sample, where corrosion spots reached diameters of up to 5 mm. Corrosion testing was terminated after the following cycle (Figure 19) due to rapid degradation of the coating, specifically its thin layer [37]. For all three types of mechanical pre-treatment, the corrosion degradation rate was statistically comparable. However, based on these findings, synthetic corundum blasting can be considered the least suitable method, while grinding appears to be the most effective. The key factor influencing these differences is the varying zinc deposition on different substrates. The fastest coating degradation occurred on the synthetic corundum-blasted substrate, primarily due to coating discontinuities, which were confirmed through metallographic specimen analysis. A similar trend was observed in hot-dip zinc coatings, where inhomogeneities within the coating contributed to faster corrosion degradation. Once again, synthetic corundum blasting proved to be the least suitable pre-treatment method. In this case, the zeta phase exhibited high heterogeneity, accompanied by numerous microcracks in the delta phase. This intermetallic phase structure not only promotes salt solution penetration to the steel substrate but also accelerates red rust formation [2; 44]. The first signs of red rust appeared in synthetic corundum- and steel grit-blasted samples after 30 cycles, with corrosion spots generally up to 4 mm in diameter. In the remaining two sample types, similar corrosion patterns emerged after 31 cycles. Exposure in the salt spray environment was terminated after 33 cycles (Figure 20). This number of degradation cycles is comparable to those used in testing organic coatings [18; 50; 55]. As indicated by the extent of corrosion damage in the reference sample and ground substrate, coating quality in hot-dip zinc coatings is more critical than zinc mass. For this reason, grinding can once again be identified as the most suitable mechanical pre-treatment method. Another factor supporting this conclusion is the relatively favourable microgeometry of the steel substrate, which reduces the adhesion of corrosive agents to the zinc coating surface [38; 39]. If blasting with angular abrasives is necessary, additional measures should be implemented to enhance the corrosion resistance of zinc coatings. In technical practice, such measures commonly include the following:  
* Application of conversion layers [56; 57]. * Use of organic coatings [58]. * A combination of both methods [59].