Corrosion Inhibition of Sodium Phosphate for Coarse and Near Ultrafined-Grain Mild steel surface

An ultrafine grain surface layer with average crystallite size of 28 nm was produced on annealed mild steel through a wire brushing process. The effects of grain size reduction on the inhibition performance of sodium phosphate were investigated using polarization and electrochemical impedance spectroscopy (EIS) measurements. The crystal grain size of wire brushed surface was analyzed by X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM). The electrochemical tests were conducted in artificial sea water (ASW) in the presence and absence of 250 mg/lit sodium phosphate (SP). The wire brushed surface indicated considerable deformed plastic flows and high surface roughness. Due to the accumulated strains, a deformed layer with thickness of 20±5 μm was produced and the crystal grain size of severe deformed zone was about 28 nm. Wire brushed surface increased uniform corrosion rate of mild steel due to enhanced surface roughness and preferential sites to adsorption of corrosive ions. However, the wire brushed surface showed a positive effect on inhibition performance of sodium phosphate. The electrochemical results revealed that average inhibition efficiency increased from 65 to about 80 percent in ASW solution containing 1.5 mM Na3PO4 for coarse grained samples in comparison to that of ultra-fined grain samples respectively .The wire brushing process encouraged passivity on the surface in SP-containing solution due to a high density of nucleation sites which increased the adsorption of phosphate ions leading to a high fraction of passive layers and low corrosion rates.