This study investigates the influence of surface roughness on the coating thickness, microstructure, and mechanical properties of hot dip galvanized medium carbon steel. Despite its widespread industrial application, the HDG process in small and medium-scale industries remains poorly optimized, particularly in terms of surface preparation. The surface roughness of steel plays a critical role in determining the formation, diffusion, and bonding quality of the zinc coating layer. In this work, the influence of surface roughness on the quality of HDG coating was evaluated.

The surface roughness was varied by mechanical polishing using sandpapers with grit numbers 100, 400, 800, and 2000. All specimens underwent standard pre-treatment, including degreasing, pickling in 32% HCl, and fluxing with zinc ammonium chloride before immersion in molten zinc at 450–455°C for 3 minutes. Experimental results showed that increasing surface roughness led to a significant rise in coating thickness and microhardness values. The highest coating thickness was observed on the specimen with the roughest surface (grit 100) reaching 246 μm, while the lowest value (73 μm) was recorded for grit 2000. The micro-Vickers hardness followed a similar trend, where higher surface roughness produces the formation of harder intermetallic phases, increasing hardness up to 67.47 HV. However, bending tests indicated that the coatings were relatively brittle due to the dominance of Fe–Zn intermetallic compounds.

Proper control of surface topography before galvanizing can improve coating adhesion, diffusion bonding, and mechanical performance, which are essential for enhancing corrosion protection and service life of galvanized components.