The effect of surface patterning and unidirectional roughness on corrosion of metals

In this study two different surface modification methods, namely surface patterning and unidirectional roughness, were applied to nickel and mild steel specimens to investigate the effect of surface roughness on the corrosion resistance. The goal is to decrease the contact area between the corrosive electrolyte and the substrate by creating different surface morphologies using different methods including surface patterns by laser ablation and unidirectional surface roughness through using SiC papers with different grits (G60- to G1200) on nickel and mild steel with different passivation behaviours. Electrochemical impedance spectroscopy measurements (EIS), potentiodynamic polarization and different surface characterization methods were performed to investigate the protection performance of the metals. In the first phase, patterns of holes with specific diameters (D) and inter-hole spacings (L) were created by laser ablation on nickel and corrosion tests were carried out in a 0.5 M H 2 SO 4 solution. The corrosion potential, E Corr , and current density (I Corr ) were determined and compared for different ratios of (D/L). Energy dispersive spectroscopy (EDS) was performed on the surface of the samples to investigate the chemical composition, specifically the oxygen content of different regions of the patterned area before and after corrosion testing. By creating such patterns we are able to produce a surface with heterogeneous wetting properties, to decrease the contact area between the liquid and the substrate. It has been shown that for a few specific patterns the corrosion resistance can be increased by orders of magnitude. In the next phase, the effect of unidirectional surface roughness on the corrosion of nickel and mild steel was investigated using EIS and potentiodynamic polarization techniques. Scanning electron micrographs were also taken and roughnesses were measured before and after corrosion testing with a profilometer. EDS also measured oxygen concentration. By decreasing the roughness, the corrosion resistance of nickel and mild steel increased and decreased respectively. The patterned sample showed the best corrosion resistance as a result of the heterogeneous wetting phenomenon that happened on the surface. Surface patterning also can achieve a much larger improvement in corrosion resistance compared to unidirectional roughness