Improving the Reactivity and Receptivity of Alloy and Tool Steels for Phosphate Conversion Coatings: Role of Surface Mechanical Attrition Treatment

The role of surface mechanical attrition treatment (SMAT) in the phosphatability of EN8, EN19, and H11 steels using a zinc phosphating bath and the corrosion resistance of the resultant coatings are addressed. SMAT generated a uniform surface profile, increased the surface roughness, and decreased the grain size, which helped to increase the extent of metal dissolution during phosphating and the rate of growth of zinc phosphate coating. In spite of the similarity in phase composition, the phosphate crystal size is relatively large for the treated steels. The zinc phosphate coatings deposited on treated steels offer a corrosion resistance in 3.5% NaCl that is better than that of their untreated counterparts. Among them, the extent of corrosion protection is relatively higher for coatings deposited on EN8 steel. The difference in corrosion protective ability is a function of amount of phosphate coating weight, which is determined by the reactivity of steels in the phosphating bath.