Role of surface nanocrystallization on corrosion properties of low carbon steel during surface mechanical attrition treatment

Surface mechanical attrition treatment (SMAT) was carried out on low carbon steel (LCS) by varying ball size from 4 to 8 mm diameter. Present work studies the effect of ball size on the electrochemical behaviour of the LCS in 3.5 wt.% NaCl solution, using open circuit potential (OCP), impedance (EIS) and anodic polarization methods. The untreated LCS shows similar to 29 nm grain size, whereas after SMAT with 4 mm ball size exhibited remarkable reduction in grain size i.e., similar to 11 nm. Reduction in grain size was achieved due to the presence of highly densified mechanical twins after SMAT as well as the formation of dislocation tangles that rearrange themselves into nanocrystallites. These factors have a direct impact on the corrosion behaviour of SMATed LCS. The increase in corrosion potential (E-corr) towards positive side and a reduction of 93% in corrosion current density (i(corr)) value were observed after SMAT using 4 mm balls as compared to the untreated LCS. Remarkable improvement in corrosion resistance was due to grain refinement after SMAT processing, hindrance of the corrosive ion, increase in the activity of charge carriers at the interface between solution and substrate, and reduction in surface roughness. Nanocrystalline surface after SMAT effectively hindered the chloride infiltration into the substrate and resisted pit formation, thus the corrosion performance is improved.