Performance of phosphate–alumina pigments in waterborne paints for protection of cold-rolled steel

Abstract

Core–shell theory presents a new easy method to obtain high performance, economic, and eco-friendly anticorrosive mixed pigments. The core–shell pigments in this work are prepared by depositing a surface layer of an expensive efficient anticorrosive pigment (phosphates) on a bulk of less expensive extender pigment (alumina). The combination of these two compounds led to the production of new pigments with improved properties different from each of its individual components; and consequently these improved properties led to changing the efficiency of protection properties of paint films containing these new pigments. The new pigments were characterized using XRD, SEM, TEM, and EDAX analysis to elucidate their structure and prove the presence of phosphate ions on the surface of alumina. These pigments were also evaluated according to ASTM measurements. Then they were incorporated in three groups of emulsion paint formulations based on styrene acrylic emulsion copolymer, and comprising different concentrations of Zn, Mg, and 1Zn·1Mg phosphates/alumina core–shell pigments; in addition to a control formulation free of these pigments, to evaluate their efficiencies in anticorrosive paints for protection of cold-rolled steel. Chemical, physical, and mechanical measurements were carried out, besides corrosion resistance tests and weight loss of steel panels under paint films which were determined in 5% NaCl solution for 28 days. The results proved that the prepared formulations containing the prepared pigments could protect steel efficiently from corrosion compared to the control specimen. Groups II and III containing higher weight percentages of prepared pigments showed the best performance in corrosion protection, and the best performance among these two groups was paint films containing magnesium phosphate/alumina pigments.