LAUSR

Abstract In this research, the effects of Surface Mechanical Attrition Treatment (SMAT) as a pretreatment process of cold worked steel on the growth of phosphate coatings, micro-structure, morphological features and corrosion resistance of the resultant coatings have been studied. SMAT decreases the grain size and increases the grain boundaries density and the crystal nucleation potential of coating. SMAT of mild steel was performed using 3 mm diameter zirconia balls with 50Hz frequency for 0, 20, 40 and 60 min. SEM observation and X-ray analysis showed that SMATed mild steel enabled the formation of a higher Vertical growth related to Lateral growth leading to an increase in the iron enriched phosphate phase (phosphophyllite) as a stable phase compared to the zinc-enriched phosphate phase (hopite) and increased the size of phosphophyllite crystallites and decreased the size of hopite crystallites. The results obtained from the polarization curves revealed that the corrosion resistance of the pre-treated SMATed and phosphate samples for 60 min, increased significantly by 75%. Furthermore, the results of electrochemical impedance spectroscopy (EIS) showed that n (ideality of capacitor) decreases by increasing the SMAT time; that means increasing the roughness of coating; which can be related to crystal growth mechanism changes of phosphate coatings from two dimensional to three dimensional and consequently reduced porosity of coatings. In addition, Nyquist curves confirmed that the values of charge transfer resistance increased by the SMAT time.